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This is a discussion on Stall within the Open General Discussions forums, part of the CHIT-CHAT category; On my 73 240 I have triple Webers and headers, and no heat shield. I have been having problems with ...


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    Registered User tanny's Avatar
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    Question Stall

    On my 73 240 I have triple Webers and headers, and no heat shield. I have been having problems with the engine stalling occasionally and not able to be restarted for approximately one hour. The fuel pumps(electric and mechanical) both work well, the fuel filters(including the one in the electric pump are all new and fine. No obstructions in gas line or tank. There is good spark to all plugs(I am certain the problem is not electrical in nature). The cooling system is clean and working well. The last time it stalled, I popped off a fuel line and there was plenty of fuel pressure so I fed a little fuel into the top of two of the Webers and it started up and kept running. There doesn't seem to be any correlation to outside temperature(happens on hot days, cool nights, etc.) What might be the problem? If fuel is getting up to the carbs, could it somehow still be vapor lock? It would seem that if fuel was getting up to the carbs and the floats are working properly(which they apparently are because the car runs fine the rest of the time), vapor lock wouldn't be an issue. Or would it? Any ideas would be appreciated. Thanks, Victor.

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    Not a webbers expert I'm afraid , but do you have the vented style bonnet or non vented?

    You mention no heat shield which is most likely your problem if you ask me. Alot of heat sink will occur and disrupt the operation of the carbs if you ask me.

    Try some header wrap if you can't get a suitable heat shield that works with the webbers and see if that helps.

    All the best,

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    With vapor lock the car will idle.It just runs like two spark plugs have no plug wire attached when you try to move.It surges.Are these you symtoms?

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    No, the engine runs really well up until the moment it dies(it doesn't die immediately as in an electrical fault, but acts like it's running out of gas a couple seconds before it quits.). It sometimes will restart for a few seconds, but then quits for about an hour. I don't believe it's the coil because it did the same thing with two different coils. Thanks, Victor.

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    From the sound of it I really think it's vapour lock. If anything installing the heat shield and header wrap will improve performance anyhow but don't write off any other possiblities.

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    I've read where the header wraps can be harmful to the headers. Myth or fact? Thanks, Victor.

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    Had a look at your gallery and you have no vents in the bonnet and no heat shield under your extractors, I believe this is a big part of the problem.

    Your fuel lines are also not insulated.

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    Fact header wrap will reduce the life of the headers mainly because it's keeping all that heat inside the headers and puts more stress on them. Could cause them to eventually crack and warp I guess but to me this is a minor thing considering how much I believe it will help my 240z perform.

    I don't know about you guys but at night when it's real cold my car is a whole different beast.

    BTW how long have you been running these carbs on the car? How long has the problem been around? Does it happen in traffic while driving?

    Does it cough and splutter backfire at all or just die?

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    I did insulate a portion of the fuel lines, but not all. Any suggestions on insulating material? I think you're right about it being vapor lock. I'll try insulating all of the engine compartment gas line first, then try the header wrap. Those wraps are pretty expensive, but if it works would be worth it. Victor.

    The stall has been around since I got the car running. Cool and hot weather makes no difference. Happens while driving around town and while on highway.
    Last edited by tanny; 08-06-2003 at 05:57 AM.

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    I'm open to any other suggestions, but the cheapest and easiest would be to install the heat shield first you could grab one from any wrecker but I was gonna go and get mine made up becaue the stock one is useful but has it's flaws.

    This is my inspiration .
    Attached Thumbnails Attached Thumbnails Stall-h.jpg  

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    Looks like a small amount of effort and money has been put into that engine compartment. Beautiful. Thanks, Victor.

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    One of my favorite Z's around if not the best.

    http://www5a.biglobe.ne.jp/~satotats/zhp/
    Is the website alot of time any money went into it. More the time I'd say as it was painstakingly and cleverly assembled.

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    FWIW, I would use a header blanket instead of the wrap unless you have a ceramic coated header. The wrap is tightly wound around the tubes and if it gets wet, it holds a lot of moisture and will cause the header to rust out that much quicker. The blanket wraps around the whole header, thereby giving it some dead air space.

    Most hot rod shops (or Jeg's or Summit Racing) should have some type of gas line heat shield that you can slip over the gas lines, usually it is sold in bulk lenghts and you can then cut it to fit and it will look a lot neater than the factory type shield.

    I agree that it sounds more like vapor lock, but the coil overheating and breaking down the spark is also a good possibility as well. I'd insulate the header first one way or the other and see if that helps, even if it doesn't cure the problem, it should help the car run better simply because it is not building up so much heat under the hood and the carbs will get a little cooler air.
    "If it weren't for fools, the rest of us could not succeed." Mark Twain.

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    Yes,the wrap will kill a header,done that.Wrapping the fuel line is good----until it gets hot in the wrap then it holds the heat IN!!My experience with Vapor lock alway occured after sitting for a while on a hot day.If I could get a nonstop run down a highway for a short while it would clear up.I NEVER had the problem while the car was moving over25 or 30mph.I could run interstate all day with the outside temp at full summer,no trouble.If I got stuck in traffic the car would act sick but,it didnot shut off.Idont think thats your trouble
    Last edited by Daniel; 08-06-2003 at 06:35 AM.

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    I just happened to recall I had a friend with a maxima that had that problem.His was intermit bad electric fuel pump.Perhaps you could by-pass yours for a test. One other easy and cheap test is replace the capactor.Its rare but I have also seen those give intermit trouble before they fail .I once had a dist.cap with a hair line crack that gave me that EXACT trouble.
    Last edited by Daniel; 08-06-2003 at 06:55 AM.

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    Default Don't give up on 'em..........

    Hi Victor,
    Sorry to hear that you are having problems.........

    I remember a while back you were asking for information on getting the Webers dialled in and jetted properly for your high altitude. Did you ever get the engine running correctly?

    I think I remember pointing out to you that a trip to a rolling-road dyno for a proper set-up was probably the best way for you to get near to the jetting that you need. If the dyno was down at a lower altitude than your normal driving environment then some compromise would have to be made - but a good technician would be able to give a pretty good estimate of what was needed if he knew the normal range of barometric pressure for your home area.

    I don't want to be devil's advocate here, but did you really satisfy yourself that the carbs were correctly assembled and had all the right parts inside? I seem to remember that they might have been something of an unknown quantity ( no? ).

    Your mention that pouring fuel directly into the float bowls will help the engine to start almost immediately is very strange. That would suggest to me that you have a very simple float-sticking or inlet valve-sticking problem that could be temperature-related. Indeed, as everyone else has suggested, it could also be related to a vapour-lock situation.

    I'm also wondering just how your plumbing is connected up. Are you still using the stock fuel rail and the return-feed to the tank? Are you running an electric pump, a mechanical pump, or both?

    As has been suggested, at least a proper heat-shield would help a great deal once you got the car working right. You can make a good one yourself out of ally sheet. I personally use header wrap and an ally heat shield, and I've never found that the header wrap caused me any problem - although the ambient temperatures here in the UK are nothing like those in America or Australia ( although curiously its 36 Degrees here in central London today! ). Any vapour lock or percolation problems that I ever suffer from are due to diff, transmission and engine heat transferring into the fuel line as it runs up the centre of the car. When I'm running at a normal speed ( anything other than a crawl in fact ) I find that the fuel runs up the line fast enough to avoid being too heated by the time it arrives in the actual engine bay. If I were to stop in traffic for a while I might find the engine starts to stumble and shows symptoms of running out of fuel. Greatest danger of vapour lock and percolation is when stopping for more fuel after a good run; I can get back in and start up but after just a few feet the engine can cut out dead. Quick twist of the key with no foot on the throttle and it clears instantly. This is evidence of vapour lock and fuel vapourisation / percolation in the carbs.............

    Its very frustrating to think that I might be able to diagnose your problem or at least make it a little better if I could only have it in front of me. Trying to diagnose this kind of thing over the web is notoriously difficult, and in the end you really could find a more simple problem is causing the same symptoms ( it could even be electrical! ).

    Sorry I can't be of more help, but I really would suggest that you might like to think through your plumbing and fuel feed / return just to eliminate any possibilities there.

    Above all, I don't want to see you chuck in the towel and damn the Webers as the instruments of the Devil. I've seen this happen too many times, when in fact it was incorrect installation / tuning / jetting / set-up / plumbing that was to blame. I know, I've been there and made all those mistakes myself........

    All the best,
    Alan T.

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    Alan, thanks for all the suggestions. I'm afraid I can't give up on the Webers because it is now a battle of wills, and also my only other cheap option is my 73 boat anchors. I did have an excellent mechanic( Jeff Winter of Rally/Sport) in Denver give it a spin and he suggested richer idle jets, which did cure my bogging problem accelerating from a stop. Unfortunately he is 350 miles away and I can't get to him easily. I did rebuild these Webers(thorough disassembly, cleaning, new gaskets, and new inlet valves) and I adjusted float levels as per specs. The floats are the older brass type but are not perforated and weigh exactly what they are supposed to. I really don't think it's electrical. I've run two different coils with no change in the problem and I have cleaned all the connections I can get at in the ignition system. I also tested for spark at all the plugs immediately after it cut out on me the last time and they were firing perfectly. That's when I put some gas directly into the carbs and it started right up and ran find. I don't believe it is a float sticking because if only one float were sticking(more than one sticking at exactly the same time would be almost an impossibility), I think the engine would still run, although badly. I'm almost certain it is fuel related due to the way it dies and then can be restarted after a certain period of time. One thing I am wondering: are the Webers supposed to have an insulator between the carbs and the manifold? Mine just have a normal gasket. Thanks, Victor.
    Last edited by tanny; 08-06-2003 at 12:18 PM.

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    Hi Victor,
    Glad you won't ( can't ) give up on the Webers.

    I guess you would be right that it would be most unusual if all the floats all stuck at the same time - but seeing as you rebuilt / refurbished them and set them all up the same way, then it is always possible that the same problem might affect all three carbs.

    I have never had a problem with using the original brass floats, as long as they are in good condition - like you found that yours were.
    What's foxing me is that the problem was cured when you manually put fuel into the float bowls. That would suggest a fuel-feed problem stopping fuel from flowing normally into the float bowls. If all else is correct, then the floats and inlet valves are what control this. When I say all else correct, I mean that fuel is flowing as far as the inlet valves at the correct pressure and the right volume. If its not, then it could be due to any combination of the possibilities previously discussed.

    What's the schematic of your plumbing? I think this is an area that can cause any number of problems. Nissan's own Sports Option parts included a special hard line fuel supply rail to replace the one for the stock SU-type carbs. This special line was just a single pipe ( fed by the main fuel supply - ie any correct pump but preferably the electric pump ) and it had three branches off it - one to go to each carb. There was no return to the tank, and you needed to block off the return line to the tank down by the stock fuel filter ( otherwise you would risk overflow from the tank under extremes of braking / cornering and overfilling ). The Webers and Mikunis usually seem most happy with a good constant feed of around 4 to 5 psi and with the necessary volume to sustain that pressure at full load in top gear. I'm just wondering whether your plumbing is allowing vapour lock or percolation that is blocking fuel feed to the carbs and the fuel is being pumped straight back into the tank through the return line? Blocking the return can sometimes alleviate vapour lock.

    You do indeed need some kind of insulation between the carb bodies and the inlet manifold. On Webers ( which are prone to frothing under extreme vibration ) it is usually recommended to go with an o-ringed type of gasket. This allows a certain amount of flex between the carb body and manifold, and the carb is never fully tight and hard against the manifold. The nuts that hold the carb bodies to the manifold usually have rubber or spring washers under them to keep this flexibility. Without the proper gaskets you run the risk of significant temperature bleed from the head / inlet manifold, and fuel frothing in the carbs.

    If you want to take this further, can you describe or illustrate your plumbing layout? I still think this might be at least partly to blame.

    All the best,
    Alan T.

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    Alan, the fuel goes from the tank, to an inline filter, to the stock electric fuel pump, to the stock filter on the passenger side fender, to the stock mechanical fuel pump, to a fuel pressure regulator, then serialy into each of the three Webers starting with the most forward carb.. The return line is not hooked up and I did cap it off. Yesterday I insulated all of the fuel line from the front passenger side wheel well to the last carb inlet, and tomorrow I will fabricate and install a metal heat shield between the header and the intake/carbs. If that doesn't work, I'll probably have to invest the $80 and try the header blanket. I hate shotgunning this thing, but unless I have a revelation(the gray matter is way to ossified for that to happen), I'll just keep trying stuff. Thanks Alan. Victor.

    ps I think I'll order some of those carb gaskets and washers.

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    Hi Victor,
    Well, the plumbing sounds correct - so that should eliminate my questions on that issue. As far as I am concerned, blocking the return feed to the tank is important - so that's good news.
    Getting proper "O"ringed gaskets between the carb bodies and the manifold, and their correct rubber or spring washers, is going to help with insulating them from heat soak. Keeping them slightly 'loose' ( but still leak-tight ) against the manifold is going to protect from frothing which can occur at particular frequencies depending on the characteristics of your engine.
    Hopefully that insulation your are carrying out will make a great difference.
    Good luck,
    Alan T.

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    Before you do any more work trying to solve your carb issues, add a heat shield between the carbs and the header. This is not an optional thing. If you do it, you will be absolutely amazed at what this seemingly small change will do.

    This is a difficult problem to diagnose, because for me, in the garage, or a legal drive around the block, the headers just don't get hot enough to duplicate the conditions where the carbs get to hot.

    It is hot as blazes here in Texas and I made a heat shield out of a 10" X 24" piece of 24 gauge aluminum and took the car to a 2 day autox south of Houston the next day. Carb problems that were dogging my car have completely disappeared. Hot flooding and stalling, hot restart problems - all gone. Insulating header wrap is not required.

    I have pretty much an identical fuel system setup to the one in your picture gallery, but no mechanical pump. I found that I ran better without it.
    --John B
    '73 FP 240Z

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    Thanks for the info. I did install the heat shield(made mine out of 24 gage sheet metal: probably would have been easier to use the aluminum like you did) and the gas line insulation, but that didn't solve the problem. So yesterday I hooked the gas return line back up and I will see if that has any effect. If not, I'm afraid I'll have to admit I was wrong and start searching for electrical problems. Just doesn't point to electrical, but no other choice. Victor.

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