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Thread: Flat spot under light throttle application.

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    Default Flat spot under light throttle application.

    hi guys, I have a 2.9 stroker motor in my z. I have a flat spot under light throttle with load, it sings under WOT and idles and revs perfectly fine. the problem is a consistent flat spot where the car feels like it doesn't want to go any faster, it lacks response and torque unless you push the pedal through the trouble area. unfortunately, this flat spot is almost exactly where the car cruises on the highway, so it hates highway trips unless i'm climbing a hills and use more pedal than normal cruise.

    -Upgraded 240z dizzy points to electronic pickup, running stock 240z coil at 9v
    -F54 block with flat top pistons
    -N42 head

    Timing is set to 36 degrees total with vac advance disabled. Total advance is all in at 2800rpm. initial is 10 degrees.

    -z therapy SU's
    -I have synched and resynched the carbs
    -checked float bowl levels
    -seated the needles against the seats
    -atf oil in the dampers

    Highest idle achieved at 3 turns out on fuel screws.

    I read somewhere that the stroker motor is supposed to have a different needle profile? is this true? if the problem is just the needles i'l happily buy a pair. I have no idea what needles are in the car now.
    Last edited by azriel_strife; 08-21-2014 at 08:16 AM.

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    Assuming SU's?
    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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    yes sorry, fixed original post.

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    Try thicker oil in the carb dampers.

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    I am running ATF in there now. the problem is a consistent flat spot where the car feels like it doesn't want to go any faster, it lacks response and torque unless you push the pedal through the trouble area.

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    Registered User siteunseen's Avatar
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    When you say a stock coil do you mean a stock ZX coil? or your stock 240 coil? There's a difference.
    1972 240Z #918 New Sight Orange
    1977 280Z #305 Light Blue Metallic
    1972 240Z #110 Persimmons Red

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    The needles should be labeled . You need to pull them off the piston because they are stamped around the head.
    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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    'Running stock coil at 9v'????
    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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    Yeah when you swap to the electronic dizzy it does away with the ballast resistor giving it 12V.
    1972 240Z #918 New Sight Orange
    1977 280Z #305 Light Blue Metallic
    1972 240Z #110 Persimmons Red

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    Your paperwork from us (ZTherapy) will tell you what needles were installed. More than likely SMs.
    Bruce Palmer
    Salem Or
    Sales@ztherapy.com
    www.ztherapy.com
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    Sorry guys, its the stock 240 dizzy with an electronic conversion. it's also a stock 240 1.5 ohm coil with the ballast resistor in tact from what I can tell, but i don't know where the resistor is, the engine bay was cleaned up.

    I don't have the paperwork for the carbs because I bought the car from someone else, but I do have the "just SU" dvd.
    Last edited by azriel_strife; 08-21-2014 at 08:18 AM.

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    So the comments about the needles and the damper oil are about richening the fuel-air mixture, right? On an EFI engine with a potentiometer on the coolant temperature circuit you would just turn the knob to add resistance, leading to more fuel, if you had this problem. Of course, that would add fuel everywhere.

    Anyway, if it sings under WOT and idles and revs fine, it's not likely a spark problem. Timing looks about right too, doesn't it?

    So, change needles to a set that add fuel at part-throttle? Just trying to learn a little carb stuff.
    siteunseen likes this.
    1976 280Z, with some minor modifications

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    I had a similar problem, verified on the dyno as fuel related. Raised the float levels slightly higher than stock (one was way low) and was able to rev higher but there was still bogging on acceleration from cruise at freeway speed.
    Switching from 20wt 3-in-one oil to 30 wt. helped noticeably, now it was put-putt- putting where it was totally bogging before. Then I installed some modified (by someone who knows what they're doing, not me) needles and it made a big difference. Almost there but I think the needles need a little more polishing to be perfect. The head's not stock, guess it needs more gas at mid-range. Got some good advice from members along the way, thank you.
    Maybe try thicker oil first, it's easy.

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    I'm not sure where I read it but I think ATF is like 20 weight?

    I'm not sure thicker oil will help, because it feels like it's constantly bogging on the highway. not sure how to explain it. it;s like the engine just isn't smooth and happy when its in that range. A bit more throttle and the engine perks up and moves along just fine.

    the slides are stationary during cruise unless i change throttle position, so I'm not sure if adding thicker oil will make any difference. the throttle response is great, it never stumbles or sputters when i put it to the mad hard, i believe this would indicate that the oil is fine?

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    If it's the cam you could get smaller tires. So you'd have slightly higher rpm cruising around town at 35mph or so. I had the maybe the same problem and going to 65 series instead of 70 seems to have fixed it. Less top speed I guess, but when would I go 125 anyway.
    Last edited by Stanley; 08-21-2014 at 11:27 PM.

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    Quote Originally Posted by azriel_strife View Post
    I'm not sure thicker oil will help
    the throttle response is great, i believe this would indicate that the oil is fine?
    I would agree with your assessment. The oil is there only for transients. If you're keeping a steady foot, the oil doesn't do much of anything. Might apply some "anti-oscillation" damping for the loop, but other than that, it's a non-factor.

    Also, if you can punch it and it doesn't sputter or bog, then that's another data point indicating that your oil is of a suitable weight.

    As for the problem you're having... Are you sure it's not an ignition issue? Timing slipping around from sloppy gears or bushings or something?

    How do the plugs look? Can you tell if you've got a mixture issue one way or the other?
    Last edited by Captain Obvious; 08-22-2014 at 08:30 AM.

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    So what needles do you have? Stock needles might be too small.
    You could experiment with slightly raising the needles in the piston. If the shoulder is not flush with the groove, you could push it in further or closer to being flush. This might put a smaller part of the needle in the transition zone for more fuel.
    How do the spark plugs look?
    When you synched the carbs did you do that at 2500rpm and idle?
    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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    Possibly slack in the linkage?

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    Quote Originally Posted by Captain Obvious View Post
    As for the problem you're having... Are you sure it's not an ignition issue? Timing slipping around from sloppy gears or bushings or something?

    How do the plugs look? Can you tell if you've got a mixture issue one way or the other?
    My timing used to jump all over + or - 5 degrees with the points, but now that i have electronic the timing marks are rock solid when i use the timing light. the stock distributor is solid, bushings are good.

    every time i have had the plugs out they looked to be a nice golden brown. but to get a real condition under that situation i would have to quickly shut the car down after a minute or so of that rpm/load range to get an accurate indication, once it settles back down to in town speeds the car runs and drives great, so the plugs would indicate proper mixture after a minute or so in those conditions.

    Quote Originally Posted by madkaw View Post
    So what needles do you have? Stock needles might be too small.
    You could experiment with slightly raising the needles in the piston. If the shoulder is not flush with the groove, you could push it in further or closer to being flush. This might put a smaller part of the needle in the transition zone for more fuel.
    How do the spark plugs look?
    When you synched the carbs did you do that at 2500rpm and idle?
    I have no idea what needles i have, I guess i should go have a look at one. the needles are properly seated as in the z therapy SU DVD.

    I first synched the carbs at idle, then at 3000rpm since that is where the car sits on the highway. they are dead on. I am convinced this is a needle/fuel problem.

    Quote Originally Posted by Stanley View Post
    Possibly slack in the linkage?
    linkage is all brand new, there is some slack, but you sync the carbs with the slack taken up anyway, so it shouldn't be an issue.

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    OK, so i pulled a needle out, it says SM on it, no numbers that I can see.

    What am i after for a 2.9 stroker?

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    I would think SM's would be pretty close for a stroker.
    How does the engine do in a heavy load low rpm scenerio- let's say pulling a hill with some moderate throttle in a higher gear?
    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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    seems fine, maybe a little bit jerky on the throttle.

    there's no cam in the car from what i can tell, maybe it needs one.

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    Well, I checked the black wire with white stripe on my coil (which i can only assume is my coil voltage feed/tachometer wire) an it is feeding 12v to the coil with the key on.

    My coil reads 1.4 ohms across the positive and negative terminals. So I have to assume this is a factory 1.5 ohm coil without the ballast resistor in place. I'm pretty sure I should have a 3 ohm coil if i'm running it at 12v shouldn't i?

    I don't think this will cause poor running though, if anything the coil will generate a hotter spark with the extra voltage. Will the coil deliver crappy ignition if it is getting more voltage than it's supposed to have? It doesn't seem to get hot, I can place my hand on it after a long drive and it won't burn me, but it is warm.

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    I bought a Crane PS20, 1.5 OHMS., when I put the ZX dizzy on my 240. Zed Head had a FSM for a ZX that showed that to be the right OHMS. Arne, a 240 guru suggested the Crane. Mine runs way better after doing what they suggested.
    1972 240Z #918 New Sight Orange
    1977 280Z #305 Light Blue Metallic
    1972 240Z #110 Persimmons Red

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    If you didn't update your coil with your electronic ignition then you need to. If you went pertronix you need a coil meant for that specifically . Probably measure near or less then 1 ohm. From what I have read the electronic coils are meant to have 12 volts continuous , but a points coil is not
    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 have a hotspark conversion, it says to never use a a coil under 1.5 ohms.

    This is the unit I got. http://www.hot-spark.com/Installing-...rk-Hitachi.pdf

    It says I should have a 3 ohm coil with no ballast resistor. I overlooked this because I checked the voltage at the coil while the car was running and got around 9 volts, so I assumed the resistor was still in place.

    I checked without the car running today and I have 11.95 volts at the coil power wire. So i know now that i don't have a ballast resistor.

    I don't know for sure if this is a 12v coil or not, but it was working with the stock points when I got the car. So it is probably the factory coil made to run with the resistor.

    I know running a coil with too low of a resistance is bad for the coil, but the coil doesn't seem to get very hot, and it's within spec for the electronic ignition i installed. I can't see this causing a flat spot only at a certain RPM/load.

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    Actually--it says to have a 1.5 ohm coil for a 6 cylinder. Might double check all your ohm readings again just so you can cross this off your list.
    It's also confusing to me that the instructions imply that you should see charging voltage at the coil--12-14volts, and you say you read 9volts running.
    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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    yeah 1.5 ohm for the 6, but thats also assuming you have the stock resistor in place.

    it says: "Check the voltage reading at the coil's + terminal, engine running, at 2,500+ RPM. If the voltage measures more than +14
    volts, you'll need to replace the voltage regulator, install a coil with 3.0 Ohms or more internal primary resistance or install a
    1.4 Ohm external ballast resistor between the ignition switch and the coil's + terminal."

    I think i see 9 volts because the coil is constantly discharging, that and the coil is getting an intermittent signal because of the electronic ignition box sending pulses. I also never tried this at 2500rpm, just at idle.

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    1.5 ohm is measuring the primary resistance of the coil- nothing to do with any resistor
    What's the secondary resistance read?
    The only running voltage test I see in the instructions is at 2500rpm which you haven't done.
    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 just know the coil is getting 12v with the engine off and probably closer to 14 with the engine running., which means i don't have a ballast resistor.

    secondary resistance is from one of the power leads to the spark output lead i assume? I never did check that, I just assumed the coil was good cause the car runs well aside from the flat spot.

    I went to the local parts store today and they don't carry aftermarket coils that are 3 ohms. I guess i'll have to order one online. the only ones they have are like the blaster 2 with .7 ohms, far too low. Insurance is up on the car on the 30th anyway, it'll give me lots of time to give her the attention she needs, maybe new plugs and wires too, even though they look new.

    Here are some pictures of her for reference.

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    You are really getting distracted by the coil resistance. If things were getting hot from too much current you would not see the results in the middle of the RPM range, and the results of overheating are usually complete failure or temporary complete failure which fixes itself when things cool down.

    1.4 ohms is close enough. 0.1 ohms is not going to make a difference.

    Electronic Ignition Conversion Kits, Distributors, Ignition Coils for VW, Volkswagen, Porsche, Audi, BMW, Volvo-Penta, Mercedes, Saab, Bosch, Ford, Autolite, Mallory, Prestolite, Delco, GM, Chevrolet, Honda, Nissan, Datsun
    1976 280Z, with some minor modifications

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    What rpm when it happens? Only at mid-range? Good acceleration from about 4000 to 5000 rpm and from 1500 to 2500?

    I don't understand how ignition could cause a problem with light acceleration that would go away when accelerating at higher rpm or up a hill.

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    I would try to beg/borrow/buy a tail pipe AFR gauge and determine if the bog is the result a lean condition or not.
    That will give a reason to pursue fuel or spark with better confidence.

    Feels like either weak spark/bad plugs or fuel metering problem to me.
    Last edited by zKars; 08-26-2014 at 12:49 PM.
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    Quote Originally Posted by Zed Head View Post
    You are really getting distracted by the coil resistance. If things were getting hot from too much current you would not see the results in the middle of the RPM range, and the results of overheating are usually complete failure or temporary complete failure which fixes itself when things cool down.

    1.4 ohms is close enough. 0.1 ohms is not going to make a difference
    I agree, but if the coil is not supposed to be getting 12v because it is a stock coil, this could be a problem. I know the coil is getting 12v, and this coil was on the stock points distributor, so I should be getting a 3 ohm coil regaurdless, just because that is what should be int here with no ballast resistor.

    Quote Originally Posted by Stanley View Post
    What rpm when it happens? Only at mid-range? Good acceleration from about 4000 to 5000 rpm and from 1500 to 2500?

    I don't understand how ignition could cause a problem with light acceleration that would go away when accelerating at higher rpm or up a hill.
    I don't think it is an ignition problem. But I was asking if it could cause this problem.

    I moved my needles up about 4mm in the slides (so the seats of the needles are flush with the slide bottom, and set my fuel screws to 2 & 3/4 turns out, the bog is definitely reduced.

    It occurs right around 3000rpm and it will remain there until I give it more pedal. the bog doesn't go away really. But to go any faster than 3000rpm on the highway, i need more pedal anyway. I can feel the bog come on in every gear if i let the car accelerate without increasing the throttle input. it just doesn't seem happy above 3000rpm unless i have it at 3/4 throttle or more.

    Quote Originally Posted by zKars View Post
    I would try to beg/borrow/buy a tail pipe AFR gauge and determine if the bog is the result a lean condition or not.
    That will give a reason to pursue fuel or spark with better confidence.

    Feels like either weak spark/bad plugs or fuel metering problem to me.
    I tend to agree. I think it is fuel related because moving my needles up in the slides has helped the issue, but it is a bit rich at idle now. I was hoping someone had a suggestion for other needles, because I have heard bad things about the SM needles i am running now.
    Last edited by azriel_strife; 08-26-2014 at 09:07 PM.

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    This is from the page I linked to in Post 31. Hot fire ignition.
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    1976 280Z, with some minor modifications

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    I see that, but it also states that if the coil is getting more then 14v when the engine is running that you need to either install a 3 ohm coil, or a ballast resistor. It is definitely getting more than 14v, since my battery terminal voltage is over 14v when the car is running, and the coil supply wire is switched battery power.

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    anyway. I'm pretty sure this is fuel related. can anyone else recommend good needles for a 2.9 stroker build with a free flowing exhaust?

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    The SM needles should run richer than stock and there's no reason you should be running lean with them. They should be capable of delivering more than enough fuel for your motor.

    Can you get your hands on a wideband to help nail down if it's a fuel problem or not?

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    Could it be the cam's power band, or lack of?
    1972 240Z #918 New Sight Orange
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    1972 240Z #110 Persimmons Red

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    I would have to buy a wideband, I don't know of anyone that will loan me one.

    If i remember correctly the cam is a factory n42 cam, making it an "a" grind. I want to get a stage 2 cam for this car if i can't find a turbo motor, but I dont want to spend that money if there's some other underlying issue.

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    Well the symptoms say "lean miss", but the SM needles say "richer than necessary" at almost all points. Only place they are leaner is way way up by the shank. I would expect that you're above that area at pretty much any spot except maybe idle?

    In your original post, you said that you "seated the needles against the seats". What do you mean by that?

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    the z thereapy dvd says to set the needles against the jets (at zero turns out) by holding up on the choke plunger and pushing the slide down until it stops, then completely tighten the needle set screw. basically it ensures the the needles are set perfectly on the slides. once this is done you set at 2.5 turns out and go from there.

    not sure if i explained that clearly?

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    I've been reading and it seems that this flat top block and n42 head bump the compression up too much for a stock cam, this might be my issue, as the mechanical advance maxes out around 3000rpm where i start to see this lack of power appear.

    thoughts?

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    What makes your engine a stroker--crank? What compression are you running? I don't buy the stock cam is the culprit.
    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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    A stumble at cruise SU's and timing


    I'm still talking ignition
    Last edited by madkaw; 08-30-2014 at 06:24 AM.
    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 madkaw View Post
    What makes your engine a stroker--crank? What compression are you running? I don't buy the stock cam is the culprit.
    it has an LD crank in it. it's definitely a stroker motor.

    Quote Originally Posted by madkaw View Post
    A stumble at cruise SU's and timing


    I'm still talking ignition
    wish i could nail it down, advancing or retarding timing doesnt seem to have any effect on the flat spot. but retarding does kill my torque and idle.

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    Wasn't sure but did you say you do or do not run with vac advance?

    Have you experimented with float adjustments?

    Spark plug gap?
    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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    floats have been rechecked twice, they are exactly on spec. plug gap is at .35 across the board, i run with no vac advance

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    I just ordered a 1.5ohm 12v petronix 40,000v coil, it's made for electronic ignition so it should be fun running at 12v on the hot spark module.

    Also gapped my plugs up to .45 to take advantage of the hotter spark i'll have when the coil is in. I really hope this will do the trick.

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    I guess you can cross that off your list and shut me up at the same time- lol
    You might just go .040 on the gap.
    Ever consider tweaking he floats a little richer?
    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 figured the floats were fine with the SM needles being so rich. I could go a half turn richer on the fuel screws too i guess. but i want to change one thing at a time until it goes away, to be sure what the problem was.

    but from what i have been reading, my static compression is over 10:1 at least. and the compression in the chambers with the stock cam will reach around 200psi. this leads to knock or poor burning. a hotter cam will bring the compression down to around 160 or so, and should move my power band farther up.

    It's possible my old stock coil was just old and tired, and couldn't produce a strong spark past 3000rpm anymore. so we shall see!

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    Maybe try it with vacuum advance hooked up?

    http://www.classiczcars.com/forums/e...m-advance.html

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    It still does it with vac advance enabled, but i got ping under low rpm high load conditions with it in use, so i disconnected it. probably the high compression combo causing this as well.

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