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Thread: Z therapy SU carbs not running right.

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    Default Z therapy SU carbs not running right.

    hi guys, i have a set of brand new SU's carbs on my car, they ran good for a while, but now they have started acting up.

    The problem i have is that the car will run great until operating temperature or a bit above. I notice it the most when I get on the highway, or sustain rpm above 2000 for any length of time. as long as my revs are up, this problem persists, if I stop and let the car idle rough for a couple minutes it smartens up and runs nice again.

    at first i thought it was the stock mechanical pump not being able to keep up with the 2.9 stroker. I saw a lot of air inside the lines, it was boiling the fuel in the lines so I decided to install a carter pusher pump at the tank. this resolved the fuel boiling issue but my car still isn't running right. I've thrown a good bit of money at it now and nothing has changed.

    I have checked and rechecked the carbs for dirt, engine bay fuel filter was just replaced, and last chance filters are spotless, fuel pressure is right, bowls are staying full, and float levels are correct (I checked them right after it started acting strangely the last time)

    2 things that i have not had the ability to check are carb synch and timing. But I can't see those being intermittent problems. It runs so happily when it decides to run right. I did back off the timing a bit to see if that would help with the backfires (lean) but it didn't do much.

    I'm contemplating selling these things and just getting a holley manifold and 390 carb. I can't stand being so frustrated with this beautiful machine.
    Last edited by azriel_strife; 06-05-2014 at 11:16 AM.

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    back fire from the tail pipe is a sign of being rich

    What do the plugs look like. If you can post a picture that would help.

    It could also be ignition. What ignition system are you running? What is your timing set to?
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    Check for vacuum leaks as well. Tighten the carbs down, check the manifold bolts too.

    Are both enrichment knobs turned out the same, and approx 2 1/2 turns?

    Don't blame the carbs until all other aspects of the tune are corrected first. Timing, valve lash, vacuum leaks, bad plugs, low compression, bad wires, cap etc are all out of ztherapy's control.
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    Quote Originally Posted by Blue View Post
    back fire from the tail pipe is a sign of being rich

    What do the plugs look like. If you can post a picture that would help.

    It could also be ignition. What ignition system are you running? What is your timing set to?
    It's backfiring through one the carbs. I believe afterfire is when the exhaust has popping (i love the sound as as far as i can tell it's perfectly normal)

    I'm not entirely sure of the plugs, I suppose i should take a look, they are new, gapped to .35 so I didn't check them. I believe I am running the stock 240z ignition, but given the problem is intermittent I was looking at things that could work and then not work.

    Quote Originally Posted by zKars View Post
    Check for vacuum leaks as well. Tighten the carbs down, check the manifold bolts too.

    Are both enrichment knobs turned out the same, and approx 2 1/2 turns?

    Don't blame the carbs until all other aspects of the tune are corrected first. Timing, valve lash, vacuum leaks, bad plugs, low compression, bad wires, cap etc are all out of ztherapy's control.
    I have sprayed a bit of ether around the manifolds and carb isolators where i suspected leaks but no change in rpm that i could discern.

    Both knobs are 4.5 turns out, since this engine is a good bit larger than the l24. I achieve highest idle at 4.5 turns. it runs like a top at idle and low rpm around town, then acts like a pile of junk when i actually try and drive it. it's upsetting.

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    There is your problem right there. 4.5 turns is a million too many. Rich rich rich. Your plugs are loading up and this is causing your poor performance during actual use.

    Please, just humour us, put them at 2.5 and spend a day with it. I will gladly be wrong if this is the not the issue.

    From there, please get access to a wide band O2 and a vacuum gauge, and stop guessing about what the problem is.
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    It won't idle well at 2.5 turns out, I tried what the z therapy video said and it wouldn't run right at all after setting them up how it instructed me to. highest idle is at 4.5 turns.

    if i try to blip the throttle at lower mixture settings it stumbles or hesitates. Carb backfire is a lean condition, it does this at higher RPM when i try to give it moderate gas to accelerate. seems much better if i go WOT. certainly isn't acting rich.

    I'll give it a shot at 2.5 turns out and try to bring up my idle with the set screws. maybe it will surprise me, I hope you are right.

    Wish i could locate someone in my area to help me figure this thing out, i dont have much experience with these carbs.

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    While the engine is cold i'm going to set my valves (.010 intake and .012 exhaust?), then i'll set up the carbs for idling at 2.5 turns out, then i'll try and set my timing, then reset my idle.

    i need to get my hands on a carb sync tool, but local auto parts stores want 100 bucks for one. but i imagine if i do all of the above, it should reduce my issue if not resolve it. lets hope!
    Last edited by azriel_strife; 06-05-2014 at 02:55 PM.

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    Quote Originally Posted by azriel_strife View Post

    Wish i could locate someone in my area to help me figure this thing out, i dont have much experience with these carbs.
    I'm also located in Chilliwack, I'm not much of a mechanic and luckily I've never had a problem with the SU's on my '70, but I will send a message to a friend and see if he can help.

    Nice to here about another local 240, if you want PM me and maybe we can compare rides sometime.

    Mike

    I seem to have lost the contact info for the guy I was thinking of, but you could try Dan at the ZShop in Surrey, I've bought parts from him in the past and I have heard good things about his service as well.

    ZShop.ca
    Last edited by CanTechZ; 06-05-2014 at 04:39 PM.
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    Sounds exactly like my 260Z with round tops when the valve lash was out of whack. You want to have your car completely tuned up before you try and tune your carbs. If your valve lash isnt set properly your car will run differently when cold and when fully warmed up. If youve got decent ears you can use a length of hose to sync the carbs up. Put one end to your year and the other end at the mouth(?) of the carb and adjust the vacuum screw until they sound identical. Personally I believe the 390cfm is a step up from stock hitachis but id definitely get the SU's running right first before you make that choice
    Late 71' 240Z: HLS3040666

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    You can put your knuckles in the throat to "feel" the air flow between your fingers. Place your hand (just use the same one) and knuckles in the same location with the same finger spacing on each carb, you can balance fairly close.

    btw you need to balance at two points:

    1. At idle using the throttle valve stops
    2. At ~ 3000 rpm using the linkage balance adjuster (if this is not balanced one carb will do all the work. )
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
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    ok. I spent the afternoon on her today and if anything it's running worse than before.

    I set the valves cold to .008 intake and .010 exhaust, set mixture screws to 2.5 turns out, set idle speed, topped up the carbs with atf to make sure i was getting proper accelerator pump action.

    The car is running worse now than before, backfiring every time I touch the throttle, it runs progressively better each time I turn up the enrichment screws. It is most definitely leaning out somewhere, I may just be masking the real problem by adding fuel though.

    I ordered a carb sync tool, and I will try that tomorrow, but I don't know if the carbs being a little out of synch would cause performance loss of this magnitude.

    Thank you all for the input, it's nice to have a community like this, so tight and helpful.
    Last edited by azriel_strife; 06-05-2014 at 09:21 PM.

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    sure sounds like timing...
    '78 280z - Daily driver/work in progress...

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    If it's burping out the carbs, that would indicate too far retarded?

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    Burping out of the carbs is usually timing and if your timing is set properly then that means you are too lean. I dont know how much the "2.5 turns" applies to you since youre running a 2.9L stroker but IIRC 2.5 turns was a baseline setting that should get your car started so you can tune it further. I wouldnt get so hung up on the # of turns but keep turning out until the car runs properly and make sure you have no vacuum leaks ( which can lean out your mixture)
    Last edited by porkbun; 06-06-2014 at 05:39 AM.
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    Stupid question......Oil in the carbs? 3 in One or a light 20 weight. Also make sure you're nozzles aren't stuck in the open position after choking the engine....put your hands directly under each carb and push up on the nozzle.
    Last edited by Diseazd; 06-06-2014 at 06:12 AM.
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    Have you reset the needle height in the dashpot piston per the Z Therapy video? This wouldn't necessarily change how it runs, but if the needles are too low they would require the mixture nuts to be lower to compensate.
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    Quote Originally Posted by Diseazd View Post
    Stupid question......Oil in the carbs? 3 in One or a light 20 weight. Also make sure you're nozzles aren't stuck in the open position after choking the engine....put your hands directly under each carb and push up on the nozzle.
    Not so stupid!
    I recently bought a '72 that wasn't running for cheap. Set the valves still no go, bought 3-in-one oil blue bottle for $3. the plungers wouldn't even get wet with oil they were so low. It started then. That $3 saved me thousands off the asking price.
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    carbs are working sweet on the mechanical aspect of things, not getting stuck, dropping at the same rate, nozzles are at correct height and not binding, no vacuum leaks, needle height in pistons is set.

    topped up the atf yesterday, i'll advance the timing a bit at a time and listen for knock, I'll also try unplugging the vac advance, it might be doing some funny things.

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    Should you be using SM needles on a stroker?
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    I've brought this up before with the SU's and that's to check the hoses from the float bowls to the carb. I had hoses that would collapse with heat and thus make the engine run lean. Probably check while engine is warm and feel the hose for flat spots.
    Timing check is a must.
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    Inspite of being sure the float level is right, 4 1/2 turns down would be neccesary if the fuel level was too low. You said the bowls were full. The level has to be near perfect. Just a thought that hadn't been brought up.
    Have you tried pulling on the choke to see if anything changes. It's a handy way to check the mixture especially if it's lean.

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    Call me and I'll be happy to give you the number at the shop and you can talk to Steve Epperly the owner of ZTherapy. Steve is a very talented mechanic and diagnostician. My number is 503-587-9800.

    Our ZTV01 Tune up dvd covers a multitude of these engine related issues that can drive you nuts.....
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    I have the "just SU" dvd. it was very informative, 4 hours of carbs is quite the long watch!

    I got my sych tool and synched the carbs, they were out a bit, about half an inch on the beads on idle, and about the same on high idle sych.

    it seems to be running much better now, doesn't fire through the carbs when I blip the throttle anymore, it's sitting pretty happy at 3 turns out for mixture, I have yet to drive it and see if my crappy drivability still persists.

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    Well, the car seems much better after the synch, it does stumble here or there if i stab the throttle, i can get around that by being smooth with my throttle application.

    I believe there is still some mixture fine tuning, i'll try 1/4 turn richer, and if that doesn't improve it, i'll try a bit leaner than it is now.

    Glad I grabbed that uni-syn tool.

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    Try a thicker oil in the carbs. ATF is about 7 weight. Thin oil will allow it to lean out on throttle opening. Try 10W30 engine oil. It works better for me that ATF.

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    So I found something else contributing to my issues. The car was running good for a few days, but then it cooled off a bit and it started backfiring through the carbs again. It only backfires around town at 2000rpm or below when I slowly apply throttle. I put 91 octane in just to be sure it wasn't the 87 i have been running. The fuel made no difference.

    (still running the points system from the 240 engine, my engine is an f54 with flat tops stroked to 2.9L with an N42 head, pretty high compresion)

    I decided to check the vacuum advance line to the dizzy. The line itself is fine, I checked the vac advance by pulling off the Cap and watching the advance work as i sucked on the hose, i blocked the hose with my tonuge and there are no leaks in the distributor or line. I decided to disable the vacuum advance so I blocked off the carb side with a bolt in the line connected to the carb.

    This completely solved all of my backfiring issues around town, the car runs amazingly, idles nicely, and runs flawlessly under all conditions at speeds below 80km/h. My guess is that it was getting too much ignition advance for the compression ratio of my engine build.

    When I take it on the highway however, it will hesitate (no backfiring) if you lightly apply throttle. Hesitation appears at sustained engine speeds of 2500rpm or more in any gear, but it seems to be just fine under WOT.

    Carb oil levels are good, plugs and wires are good, cap and rotor look good, Timing is set. I'm at a loss, should I be running different timing than stock for this Block/head combo?
    Last edited by azriel_strife; 06-15-2014 at 11:55 AM.

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    Sounds like the diaphragm in your vacuum advance might have a leak, thus leaning out your mixture.
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    Quote Originally Posted by azriel_strife View Post
    The line itself is fine, I checked the vac advance by pulling off the Cap and watching the advance work as i sucked on the hose, i blocked the hose with my tonuge and there are no leaks in the distributor or line.
    That is what made me think of the distributor, but I have already checked that.

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    Quote Originally Posted by azriel_strife View Post
    That is what made me think of the distributor, but I have already checked that.
    Didn't you say it ran better without the vacuum advance?
    Now without it you won't get the ignition advance you normally get at cruise with light throttle advances. Not sure what numbers you are running but you might want to advance your timing with the vacuum part disconnected and see how it does.
    Steve
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    Measure the advance with an instrument if you want to tune, otherwise it is guessing.
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


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    Quote Originally Posted by madkaw View Post
    Didn't you say it ran better without the vacuum advance?
    Now without it you won't get the ignition advance you normally get at cruise with light throttle advances. Not sure what numbers you are running but you might want to advance your timing with the vacuum part disconnected and see how it does.
    Yes it runs much better without the vac advance hooked up, because I believe it was getting too much advance for my compression ratio. thus backfiring through the carbs on detonation. This is what makes me believe this distributor is not right for my engine.

    Quote Originally Posted by Blue View Post
    Measure the advance with an instrument if you want to tune, otherwise it is guessing.
    The timing is set to stock 240 specs because I am using the 240z distributor, i thought that to be the best option, since stock timing spec is based on the distributors advance amount.

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    Can you watch the timing with a timing light. It is easy to see the timing at any rpm with a light, especially one that has an adjustable offset.

    Check at idle, 2000 rpm, 3000rpm and at max advance. (probable ~ 3000)
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


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    To Hell with stock specs - you aren't running a stock motor. Any pinging you might have had might have been from vacuum advance. Unplug it and play around with advance. If you don't have a timing light- you shouldn't own a Z
    Steve
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    Quote Originally Posted by Blue View Post
    Can you watch the timing with a timing light. It is easy to see the timing at any rpm with a light, especially one that has an adjustable offset.

    Check at idle, 2000 rpm, 3000rpm and at max advance. (probable ~ 3000)
    So I should set timing based on full advance and not at idle? it's set for idle as the FSM states. But timing is more important at driving rpm.

    Quote Originally Posted by madkaw View Post
    To Hell with stock specs - you aren't running a stock motor. Any pinging you might have had might have been from vacuum advance. Unplug it and play around with advance. If you don't have a timing light- you shouldn't own a Z
    Who said i don't have a timing light? It's set properly. I know my engine isn't stock, I know the distributor isn't even for this size of engine. It's got plenty of torque, i don't need the vac advance at all. I guess I'll just set it up for total advance instead of factory spec.

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    You should check your timing at all rpms up to max advance to understand what it is doing and when. Try with and without the vac advance to see what is going on.


    The neat thing with the distributor is that it is independent of load. It only depends on rpm.
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


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    Guess I sounded snarky there - sorry.
    Maybe not the best dizzy and they are all different as far as mechanical advance.
    How much timing all in?
    Might consider a ZX dizzy or electronic ignition .
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    Quote Originally Posted by Blue View Post
    You should check your timing at all rpms up to max advance to understand what it is doing and when. Try with and without the vac advance to see what is going on.


    The neat thing with the distributor is that it is independent of load. It only depends on rpm.
    I'll have a look when I get some extra time here soon. I would think that i can only have one or the other if the advance range doesnt meet my engine needs, correct? so if i have to choose, i should tune for operation RPM, not idle?

    Quote Originally Posted by madkaw View Post
    Guess I sounded snarky there - sorry.
    Maybe not the best dizzy and they are all different as far as mechanical advance.
    How much timing all in?
    Might consider a ZX dizzy or electronic ignition .
    I am sitting at 32 degrees total advance @ 3000rpm without advance plugged in.

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    So you are advanced now already?
    What dizzy model are you running and what's your static timing
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    I don't know how to tell, i'm assuming it's a stock 240z dizzy, it's a points unit. I read that 32 degrees was a good place to be with locked timing. just trying things at this point because I don;t have any information pertinent to my particular setup.

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    Try taking measurements and filling in the list below to help us help you


    Advance measured with VAC connected:
    Idle:____________ (degrees)
    1000:___________ (degrees)
    2000:___________ (degrees)
    3000:___________ (degrees)

    Idle RPM:______________
    Max Advance RPM:_______




    Advance measured with VAC disconnected:
    Idle:____________ (degrees)
    1000:___________ (degrees)
    2000:___________ (degrees)
    3000:___________ (degrees)

    Idle RPM:______________
    Max Advance RPM:_______
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    Quote Originally Posted by Blue View Post
    Try taking measurements and filling in the list below to help us help you


    Advance measured with VAC connected:
    Idle:____________ (degrees)
    1000:___________ (degrees)
    2000:___________ (degrees)
    3000:___________ (degrees)

    Idle RPM:______________
    Max Advance RPM:_______




    Advance measured with VAC disconnected:
    Idle:____________ (degrees)
    1000:___________ (degrees)
    2000:___________ (degrees)
    3000:___________ (degrees)

    Idle RPM:______________
    Max Advance RPM:_______
    Idle: 10-11 degrees (mark jumps around)
    1000: 15 degrees
    2000: 25ish degrees
    3000: 30ish degrees

    The vac hose being connected didn't make any difference to the readings, so all of this is with vac disconnected.

    Max advance is right around 2800rpm, I cant see any difference after that no matter how high i rev.

    My good timing light kicked the bucket, so my replacement doesn't do offset.. I can't see beyond 15 degrees on the indicator for the pulley. so anything beyond 15 degrees is a best guess based on distance.

    If I advance any farther i get knock at low RPM high load. it;s likely my increased compression ratio from the F54 with flat top pistons and the N42 head.

    The amount of advance i am getting is quite high, wich leads me to believe i have a crappy emissions distributor, maybe I should consider getting a 280zx distributor instead.

    If anyone has an e12 80 ZX distributor available for purchase I will buy it right away.
    Last edited by azriel_strife; 06-24-2014 at 09:59 PM.

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    So i was looking and I didn't see it before, the flywheel and block have been painted so the markings for timing on the flywheel and block were almost invisible.

    To my surprise each tooth on the block for timing is 2 degrees, what i thought was 10 degrees initial timing at idle was actually 20. it seems that my timing is way too far advanced. it also seems that the spot i thought was zero on the toothed indicator on the block is not zero at all. i need to go have a look again.

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    ok, so i have set my total timing to 34 degrees at 3000rpm, and the car seems to hesitate under light throttle (vacuum) at 3000+rpm. Seems like it's getting too much vac advance. Other than that it is running and idling happily.

    If I unhook vac advance, it doesn't seem to do it anymore, but I lose a noticeable amount of torque on initial pedal press.

    I may have to check and make sure my balancer hasn't spun on the rubber. is there a good method of doing this?

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    Great stuff.

    You may be able to remove the vac advance but give it more initial advance to compensate.... say ~ 17 to 19 at idle. This should help the mid RPM problem (from too much vacuum advance). However the small print side effects of no vac advance are: It may be more difficult to start and, you will loose fuel efficiency at cruise.

    The vacuum advance canisters are adjustable but you need to chip off the epoxy blob to access the limit screw.
    Last edited by Blue; 06-28-2014 at 09:57 AM.
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


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    http://ZSportCanada.com


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    Quote Originally Posted by Blue View Post
    Great stuff.

    You may be able to remove the vac advance but give it more initial advance to compensate.... say ~ 17 to 19 at idle. This should help the mid RPM problem (from too much vacuum advance). However the small print side effects of no vac advance are: It may be more difficult to start and, you will loose fuel efficiency at cruise.

    The vacuum advance canisters are adjustable but you need to chip off the epoxy blob to access the limit screw.
    It does run better without the vac advance plugged in, but it seems like it's still lacking timing under light load with no vac advance. If i advance the timing up to 38-40 degrees total advance it seems happy. This is what makes me think my balancer may have spun on the crank.

    Should i use the old screwdriver in the #1 cylinder to find tdc trick and compare it to the marks on the balancer?

    My distributor is an old points unit, so i'm not sure if i can adjust the vac advance on it.

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    A non-points distributor is a great upgrade....especially at high rpms'

    yes get the piston at TDC then check the marks
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


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    ZCars in Eastern Canada seaport ready for shipment to Europe

    http://ZSportCanada.com


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    I ordered a hot spark electronic conversion kit for my distributor today. my car doesn't have the ballast resistor either, so it should be an easy swap over.

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    Ugh i really dislike the stock ignition.. some weird things today.

    -set the points, they were well above 25 thou, set them to 19 thou

    -reset timing to 34 degrees total, no vac. I lost almost 10 degrees of advance from setting the points.

    -Checked coil voltage, there is no ballast resistor on my fender, but coil voltage is 8.8 volts roughly.

    -Checked Coil resistance, 1.6 ohms.

    All seems to be in order, but the car is running worse than before, i get misfire and very jerky throttle response in low rpm. Advancing the timing seems to resolve this. I might just give up on it until my electronic upgrade conversion arrives.

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