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Thread: Suspected Fuel Issue - Now I'm not that sure. 74 260Z w/ Flat Tops

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    Default Suspected Fuel Issue - Now I'm not that sure. 74 260Z w/ Flat Tops

    I need some suggestions of what to try next on my 74 260Z that starts fine but has little to NO power on the road. I have working on fixing the issue for 10 solid days and have ran out of ideas.

    Quick history - I finished restoring and finally started the car on Dec 14. The first drive was 12/23/11 and she did great. Drove her almost 400 miles over a few days without incident. Plenty of power and fun as hell to drive. Only twice in the first week did she kinda studder on take-off. Didn't think much of it until on New Year's day she almost stranded us 30 miles from home. Started making a different engine sound (a "bauhhhh" ipo a "brrrhhh") and sputtering real bad. Gimped home at 40mph and much slower going up hills but made it home.

    After 10 days of trying any and everything I think it could be, exactly same as it was. I need some new suggestions. She starts fine. Idles OK even though the RPM's fluctuate some. Just zero power.

    Here's what I've tried:

    Fuel:
    1. New filter near mech FP, new filter near elec FP, neither were real dirty but replaced anyway.
    2. Confirmed power to elec FP while running w/ test light wired into trunk, getting 12V.
    3. Re-tore carbs apart. Yes they are flat-tops. Front carb bowl was kinda dirty with rust-colored debris. Rear was clean. Rechecked float heights and installed new diaphragms/gaskets on both power valves. I had put a new kit (needle/seat and other gaskets during restoration but failed to do power valves.
    4. Removed fuel lines and blew them out. Checked with empty Mason jar that I'm getting plenty of gas when starting and running. I haven't hooked up a pressure gauge in-line yet as some have suggested.
    5. Removed and cleaned all spark plugs. Front 3 were dirty and appeared to be wet or fuel fouled. Ran same after reinstallation of clean plugs.
    6. Checked both sides of exhaust manifold after running awhile (ouch) and both sides were hot.
    7 Disconnect charcoal cannister and cracked fuel cap for venting. Ran the exact same.
    8. Ran with mech FP removed from series but ran about the same.

    Began thinking electrical at this point.

    Electrical:
    1. Coil is a Crane PS20 and is seeing 12V while running. Replaced with the old coil and ran the same way.
    2. I am going to check the air gaps in the dizzy today and run a test on vacuum advance that many posts mention but was hoping someone else had other ideas.

    Before we start a whole series of "boat-anchor" discussion about the flat-tops, I personally think its something else. She ran great for 400 miles first and the weather here is cool and not causing vapor-locking. I do not run coolant to the flat-tops per suggestions that overheating can occur.

    Anticipated thanks.

  2. #2
    Boat Anchor Repairman Captain Obvious's Avatar
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    The list of fuel related things you've tried looks pretty good, and should rule out fuel delivery issues to the carbs themselves. However, just because fuel delivery to the carbs is OK, it doesn't mean that fuel delivery to the float bowls is OK. Couple of quick easy things to check:

    Have you verified that your float levels are correct? I'm not talking about on the bench. I'm talking about checking through the sight glasses with the engine running.

    Did you check the fuel filter screens that are located in the carbs themselves? Maybe they are partially plugged and will pass enough fuel for idle, but not to move the car. If you haven't checked them, it's easy. You can pull them out of the carb bodies without having to take the carbs off the car.

    Let's start there because after those two things, it's gonna get more complicated.

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    Thanks Captain. Yesterday I had a hard time viewing fuel level through the float windows using a small mirror. That's really why I took the carbs completely off and re-verified the float set-up per the rebuild kit manual. I'll try wiping the windows and try again.

    Are you sure my flat-top carbs (model hmb 46W) have filter screens in them. I'm looking at the rebuild kit's exploded view and can't find a filter or screen? I'm assuming they would be directly inside the main fuel input fitting but is that where they are?

    Other work to eliminate fuel issues today that yielded no improvement:

    1. Hooked up and hung a small lawnmower tank and fed both carbs (with gravity) from the lawnmower tank. It idled OK but when I gunned it, I still heard the "baughhh"instead of "brrrhhh". I couldn't drive it this way but pretty sure it would have no power.
    2. Earlier today, I had drained the tank, removed electric fuel input hose (exit of tank) and blew air into it with the 19mm tank drain plug out and gas cap on. A few small flakes blew out of the tank but hopefully the screen is clean for awhile. I was concerned that I had a build-up on the metal screen inside the tank. Hooked back up the pump and drove it, No better.

    I am pretty convinced now the problem is after input to the carbs as you have mentioned. I will remove the fuel input fitting and look for a screen.

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    It sounds like weak spark. Check the plug wires, cap, rotor, etc.

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    Yes, I'm positive your flat tops (are supposed to) have filter screens in them. I don't have any pics handy, but they're not hard to find. On the cooling fan side of the carbs, there are two brass hex head plugs. If you replaced the float valves, that means you had the lower plug out in order to pull the float pin. There's another similar plug almost directly above the float pin plug, and behind it is supposed to be a filter screen.

    The filter is not inside the main fuel input, but is about halfway down the carb body between the main fuel input at the top and the float pin plug near the bottom.

    That said, in my experience, every one of those filter screens that I've come across was bent and mangled by a previous owner's rough handling, and probably wasn't sealing at the ends. In other words, just because it's in there, doesn't mean it's doing anything. And by the way... No, replacement filters do not come in the rebuild kits, which might be why they don't call it out on the diagrams.

    As for the float bowl sight glasses... An inspection mirror with an adjustable head in your left hand. A strong flashlight with a tight beam in your right hand. Leaning over the driver's side fender ever so slightly, you can get a pretty good look at the sight glasses. Took me a while to figure out the correct mirror, light, and eyeball positions, but now that I've got it, it takes me ten seconds to check both carbs. If you still can't get a good line of sight, pull the air cleaner cover and backing plate off and remove the air tube that goes from the bottom of the front carb to the bottom of the rear carb. With that stuff off, it should be "almost easy".

    I don't believe the problem is getting fuel to the carb, and your float levels are probably OK, but you have to start there to rule out the obvious. (I'm Captain Obvious) As Pete has already alluded to, it might not be carbs at all.

    You mentioned earlier that you think the front three plugs were wet. Can you run the engine for a few minutes and then pull them again for a fresh look?

    Also, can you describe the order of events leading up to now? It was running fine, you were out for a drive and all of a sudden out of the blue it just started acting up? Or it ran fine one day, and the next day now it doesn't? Was it running when the problem started?

    Quote Originally Posted by mshandor View Post
    I was concerned that I had a build-up on the metal screen inside the tank.
    That's what I've got.

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    Registered User Rainman's Avatar
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    The transistorized ignition in the 260Z has been problematic for me.. Had trouble with my early 260Z many years ago but it had different symptoms than you're having. My car would simply die after a random period of time, most notably in traffic when the engine bay would heat up from sitting there for example. It didn't over heat, just got heat soaked.. I let it sit for twenty minutes with the hood open while I checked things for spark, fuel etc.. The spark was absent initially but it would return after a short time, then start up and run again.. Until it finally quit all together.

    Since you've been through the fuel system, even checking for bad gas with a fresh fuel charge in the lawn mower tank.. I would move the front three spark plugs to different cylinders and see if the fouling follows. If the fowling continues in the same cylinders, try moving the plug wires to different cylinders and see if the trouble follows.. On second thought, these items are cheap enough to simply replace so that may be an exercise in futility, you're call on that..

    I am suspicious of the transistorized pick up in the distributor. It takes the place of the mechanical points and sits next to a little magnetic collar on the distributor shaft. The problem my 260Z had was caused by the transistorized pick up. I replaced it and it's been fine since. Unfortunately I can't offer any way to test it and I can't explain why it would act any differently under acceleration versus idle... It seems that if it were the cause of the problem, it would behave the same at idle...??

    I've talked you into a complete circle here... I hope this helps and maybe someone else will be able to point you in the right direction as far as troubleshooting this electronic part...??
    Last edited by Rainman; 01-12-2012 at 02:08 AM.


    Nissan Monterey Blue Early '74 260Z
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    Captain, you ARE the MAN!

    Found the cylindrical metal screens in my flat-top carbs and one of them was totally clogged. Removed them, cleaned them, and my Zepp is running like a scalded dog again. Wish I had posted this 10 days ago and avoided all of the other failed attempts.

    Thanks for all the other ideas guys.

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    Haha! Excellent! Sure sounds like smoking gun to me.

    In the meantime, you have bought yourself a stay of execution and now have some time to practice with that swivel mirror and flashlight, right? You still need to verify that the float levels are on target while running. And once you figure out what you're looking for and how to do it, it will be so easy that it will be the first thing you'll check if you ever have issues in the future.

    Out of curiosity, when you rebuilt the power valve the second time you had the carbs off, what did you find inside. How bad were they? Were the old sealing washers OK, or had they turned to goo?

    Boat Anchors = I'm officially out of the closet.

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    Captain, I did not find any tears in the power valves diaphragms or gaskets. They looked OK, just 38 years old. I cleaned everything up and replaced them and hope they last another 38 years. I'd like to send you a picture of the carb filters so you could see what I found but I don't know how to post a picture in the blog-o-sphere. Talk about CRUD!!!

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    Sounds like the filters were a sight to behold. The flat top filters I've messed with have been clean. Only because everything was flowing around them instead of through them. But they were clean.

    So now that the immediate crisis has been averted, how do your plugs look? And have you figured out the correct yoga position to view the sight glasses?

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