Chasing vacuum leak, carb cleaner kills engine?
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Thread: Chasing vacuum leak, carb cleaner kills engine?

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    Chasing vacuum leak, carb cleaner kills engine?

    I have been chasing a nasty vacuum leak for weeks in my 1969 GT 1.9L with Weber 32/36. I have done the usual stuff (replace manifold gasket, carb gasket, pull carb, rebuild carb, replace) but no luck solving it. Hopefully this is a new facet to an old question: when I spray carb cleaner it at carb base and intake manifold it will actually slow the engine if not outright stall it. I suppose this is essentially flooding the engine instead of making it race, but the end result is that it is indicating the same, major leaks everywhere.

    Could carb cleaner stalling the engine be indicative of anything else, the reason I ask is that all my repairs seem to have yielded absolutely no progress on the leak. I am a hack mechanic, but still I thought I would see a little progress swapping out the old gaskets and rebuilding the carb.

    Thanks in advance.

    Matt

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    Bikini Inspector Frozen Tundra GT's Avatar
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    check throttle shaft bushings, use a thick gasket with sealer. ditch the heat shield for this exercise.
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    Über Genius First opel 1981's Avatar
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    Check the vacuum tee below the carb.
    Opel GTs are not GM products
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    Senior Member The Cub's Avatar
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    I had the stove (stock manifold) once I got it hot the carb cleaner coming out of the can it contacts the aluminum at a very cold temperature even directed nowhere near any gasketed or threaded areas just a gentle spray on the top of the runner would stall the engine, a very slow drop in idle speed, no misfires until it would stall if you keep the spray coming. There were 3 ways easy around this. I could spray anywhere while the manifold was cold you will not get this effect. Better yet using either WD-40 or a dry gas like propane you will not get this effect even when it’s running hot. If what the other guys said doesn’t work give it a try. Gas boils off rapidly on the hot runner with carb cleaner in my experience. I don’t know why other than the stove set up maybe? HTH

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    Member Michael A. Smith's Avatar
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    I have been chasing the same thing and found a few things that I had not thought about. First, what is the condition of the brake booster hose? If old, it may be leaking, and if new the fittings may require tightening. Second, look at the vacuum tree underneath the carburetor. My car's previous owner had blocked off some of the ports with rubber caps that had deteriorated. I also found something with my 19 71 GT and its 1969 engine: it has a manifold from a 1973 or 1974 engine, with the port for the valve cover tube on the inside of the manifold between the carburetor and the block. Previous models had this port on the vacuum tree. My previous owner had blocked off this port as well with a rubber cap that had deteriorated.

    The advice to remove the heat shield is well taken. When I went to reinstall mine, I found that it did not sit exactly properly on the manifold and I was unsure that tightening down the carburetor attachment would solve this.

    I will assume for the moment that the carburetor is new or relatively new, as mine is, and for the moment will rule out things such as shaft bushings. If otherwise, then look there as well.

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    Quote Originally Posted by Frozen Tundra GT View Post
    check throttle shaft bushings, use a thick gasket with sealer. ditch the heat shield for this exercise.
    Thanks for the excellent advice. I thought the carb cleaner was giving me false positives. I will check with propane and focus on the throttle shaft bushings. The carb is not new and the throttle linkage in general has lots of play in it. I would love to completely replace the carb but I hesitant until I ID the prob. I would hate drop money on a new carb still have the same issues to content with.

    Matt

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    Just Some Dude in Jersey The Scifi Guy's Avatar
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    For me, for 30+ years, the leaks always developed at the 6 intake/exhaust-to-head bolts. Usually just an 1/8 to 1/4 of a turn of those bolts did the trick. You have to tighten them about once a year when driving daily. Don't overtighten them or the heads will snap off when the exhaust gets red hot. The factory torque spec on those is about 10lbs lower than would be normal for bolts that size because of that exhaust expansion problem. The low tightness contributes to them unscrewing. If you end up removing them, add a little red Loktite to them upon reinsertion. The heat will burn off the Loktite, but it will help to gum up the threads and dissuade them from unscrewing so easily. There's a headpipe-to-block bracket that many Opeler's cars are missing that helps keep the intake/exhaust from wiggling up and down and encouraging the bolts to unscrew. It looks like this:

    Exhaust to block brace 3.jpg

    I'm not a fan of used carbs. You have no idea how many miles are on them and what rig jobs the previous owners did to them. For peace of mind, put a new one on, then you rule out every one of the 25 different carburetor-related problems that can cause you grief.

    If your carb throttle shaft is leaking bad enough that it's affecting your engine, the carb is probably shot.

    Based on what you said about your spray test, it's more likely that you're leaking at the carb-to-manifold gasket. You can regasket and/or you can try a quicky fixy by smearing Permatex High Tack sealer all around the carb at that gasket to plug the leak. If it works, then that was your problem and you now know you simply need to do a commonly needed regasketing. When I drove daily in my GT for 18 years, I would need to redo the gasket about every 5 years.

    A somewhat far-fetched cause could be a leaking brake booster or hose. They're just a giant rubber diaphragm inside a tin can. And they're 50 years old. Not too many rubber things can last 50 years without degrading. It's a pain to remove the hose to test them though, you would either need to disconnect the hose from the one way valve and try blowing into the hose to see if it leaks or remove the hose at the booster and fasten on a temporary hose to blow through. A simpler test would be to clamp some vice grips on the hose immediately after the vacuum tree and see if that changes the engine behavior. Or try just aiming your carb spray at the vacuum tree to see if that makes the engine misbehave.

    If you have an automatic you may have a leak in it's 1/4" hose also. Same deal, clamp the hose near the vacuum tree and see if that changes anything.

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    tomking tomking's Avatar
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    I found that some intake manifolds dont properly seal with a weber carb but do seal with the solex. Had to make a phenolic spacer to get a good seal.
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    TMK

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    Mike's Opel Shop Site Supporter opellane's Avatar
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    In the Past I've always found Vacuum leaks under the Heat shield and I've had intake rubber plugs and hoses crack too
    You know you have one, when your GT will not Idle and takes long time to crank engine, engine races and stalls, hard re-starts
    loss of power, back fire up through carb..
    I would spray the carb. spray under heat shield. look for change in Idle or stall..Look for change,, better yet use brake cleaner as I hazard carb cleaner will desolve gasket sealer
    I use the 38/38 Weber setup , and found the thin gasket under heat shield must be sealed

    I use a product called Red High Tack Gasket sealer , b at Napa stores

    Permatex High Tack Gasket Sealant #58981 download.png

    This sealer resist Fuel and seals, seems that when the gaskets are not sealed, or carb. nuts not tight or loosen
    the gaskets get soaked with Gasoline and the Gasket Expands and then will never seal again.

    Using the High Tack sealer, it really seals the Heat shield , very sticky and gummy,, messy stuff but I Have to tell you guys since I;ve been using this product ...haven't had that much Fun chasing Vacuum leaks

    Also If your using a Sprint Manifold, the two manifolds can't touch make sure you grind down the bump tab a bit,,on top . I've seen that can loosen up Intake manifold over time, from vibration and heat
    Just my Opinion..... Good Luck ..Hope you find the Gremlin
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    ---------------------------------------------------
    1972 Opel GT, 1969 Opel GT, 1973 MGB,

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    Following up on this thread. Interestingly enough, the propane had the same result as the carb cleaner, the engine did not race, but the revs dropped and almost stalled out the car. This happened with spraying on the carb base and intake manifold.

    The brake booster theory is interesting. The check valve and hose are new, but nothing else. Carb cleaner/propane had no effect, although I understand that this often doesn’t show an issue that far from the intake. I will clamp off and see what happens. I noticed a different feel in the brakes. Before they were nice and firm and now pretty soft and spongy.

    I could not get my torque wrench to fit when reattaching the manifold, this could be the culprit. I will revisit this.

    Matt




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    Just plug the line from the booster to the carb base and repeat the testing. If the booster and line are not leaking then the engine ought to start up and idle as before. There is no steady airflow out of the booster.

    This sounds more like what has been posted in post #2: Throttle shaft bushings, or carb base gaskets.

    There are replacement shaft bushings/seals sold for your carb (and for other carbs too) in case you have leaks there:
    https://www.pegasusautoracing.com/pr...asp?RecID=4130
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    Senior Member The Cub's Avatar
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    Strange, I’ve never killed the idling of an engine with a vaporized gas I use acetylene only because it’s a finer tip to pinpoint the area. I don’t see why it would make a difference though. I wonder how you have your idle mixture and idle speed adjusted? Assuming that your choke speed adjustment screw is set properly to where it lets the idle speed screw rest on its stop. Do you have the idle cut off solenoid? Let us know the details on how it’s set up. It sounds like it’s out of adjustment to compensate. Gordo brings up a good point on the brake booster. If you do find it elsewhere you’re done. What are the symptoms you’ve been having? Throttle shaft leaks are usually accompanied by flat spots while accelerating or driving. If the main shaft is wobbly then the carburetor is done, buying a new genuine Weber for a little over $200 is money well worth spending. I tried reviving old Weber’s like that using various methods, don’t waste your time and money. That’s the #1 suspect, #2 suspect in my experience is the gasketing between the carburetor and intake plenum. I’ve tried everything over the years. The best I’ve found is taking my time making absolutely certain that all mating surfaces under the carburetor including the carburetor itself are machined flat, to achieve that a here’s a foolproof home remedy I’ve found works: I had a very thick piece of flat glass made about an inch wider than the carburetor and about 9” long. I purchase some 80 grit sanding cloth 3M Wetordry Sandpaper, 9-Inch by 11-Inch, Assorted Grit, 5-Sheet https://www.amazon.com/dp/B001449TPS..._JkduDb3JQCWEW I remove the carburetor and mounting studs using the double nut method make sure everything is bone dry inside the intake plenum, stuff a clean rag in there and resurface the top of the intake carburetor mounting surface if it’s in good shape to begin with it’ll only take a little more than a few passes. While you’re at it (optional but recommended) if it needs it, I used the Weber mounting gasket I mention as a template and my Dremel and port match the intake plenum, giving the secondary proper flow. When done, carefully removing the rag, I THOROUGHLY vacuum everything out from the inside of the plenum, that’s why it’s critical for it to be clean & dry to begin you don’t want any oily residue trapping the little tiny metallic flakes, if you use carburetor cleaner and clean it out before you begin you won’t have any problems with that. I really urge you to buy the new carburetor. If you think you still have a good Weber you can use the same method with the glass & sand paper to resurface the bottom of the carburetor it’s important that you don’t skip this on a used Weber, thoroughly cleaning the base when done. For assembly I use this: https://www.pegasusautoracing.com/pr...SABEgJn-fD_BwE
    for the gasketing under the heat shield, between the heat shield and carburetor something like this phenolic spacer: https://www.pegasusautoracing.com/pr...asp?RecID=3585
    I purchased a new heat shield from OGTS it’s properly ported for the Weber made from stainless steel which is superior to the factory regular steel for minimizing heat conduction or heat transfer, it’s perfectly flat. The factory heat shield is not ported properly for the Weber put it on the shelf it’s only made for the Solex. I’ve also used a custom made aluminum spacer but the heat shield what I have now. Again it’s important that the phenolic spacer be properly supported the 4 factory slots on the intake plenum to separate the intake from the exhaust will cause vacuum leaks if not done properly using solid flat metal, they also make other spacers but they have to have the support that metals have not plastic or whatever else is out there because of those 4 groves previously mentioned . Finally the best I’ve found to bolt down the carburetor are these self locking nuts Micro Trader 12pcs Exhaust... https://www.amazon.com/dp/B07GJHDLQY...p_mob_ap_share use a flat washer and the nut and your done! Everyone has there own recipes. This is the bulletproof method that works best for me. I’ve tried sealers to mask imperfections and many eventually fail in my experience. If your surfaces are truly flat and the phenolic spacer is probably supported by the again, perfectly flat heat shield then sealers are not needed and they just make a mess anyway. Many will say that what I just posted is overkill, I say it works! Vacuum leaks at the carburetor base are commonplace and subtle, you may not even know that you have them and these engines are very reactive to small vacuum leaks. My #3 source would be the intake to cylinder head gasket. The bracket shown in Gordo’s post is what I have along with a good Felpro gasket and it seems to keep mine leak free at this gasket area. Good luck and let us know what you come up with.
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    Thanks for the detailed reply. No idle cut off solenoid. No heat shield, just carb on new thick gasket.

    While I always suspected a vacuum leak, the vacuum leak became apparent when I tried to reduce the idle revs to combat some nasty dieseling I was experiencing.

    Previously the idle speed set screw was all the way in, just to get the idle to about 850-900, completely zeroed out with the small spring around the screw almost distorted it was in so far. I backed this off a few turns and the idle dropped to about 500 rpm. Car became not drivable. Adjusting the idle mixture screw to compensate had no effect, wouldn’t even stall the engine when fully screwed in.

    At this point I replaced the manifold gasket, swapped the old carb base gasket with a thick gasket and pulled the carb and cleaned and replaced all the O rings in the carb. I was liberal with the permeatex grey and the high tack sealer.

    I checked the timing and it seems to be dead on. The vacuum gauge on the distributor advance port reads 3cm of Hg at idle. Plus I swear can hear a hiss of air at idle.

    I was able to get the car back on the road by tweaking the fast idle screw and screwing the idle speed screw all the way in again to get the idle back up to 900. The dieselling is as bad as ever at this setting.

    I am with you on the logic of an outright replacement of the carb if the throttle plate bushings prove to be the culprit.

    Matt

















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    Site Founder Gary's Avatar
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    Back in the '70's when my GT was my daily driver, I spent a LOT of time tracking down a vacuum leak. Finally narrowed it down to a fitting at the base of the intake. The '69's had a dual can dizzy and one of the lines ran to a fitting on the engine side of the intake. It was either porous or was leaking by the threads.

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    Just Some Dude in Jersey The Scifi Guy's Avatar
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    A few years back I also had a hidden leak caused by a cracked plastic T-fitting I had spliced into a vac line. The crack was on the bottom of a black plastic fitting where I couldn't see it and a black crack on a black fitting was hard to detect.

    Don't buy plastic fittings if metal ones are available.


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    ILLUMINATI
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    Vacuum leak

    If the carb cleaner is a chlorinated solvent it may not be combustible and you may be diluting the air/fuel. If it is a high boiling aromatic hydrocarbon it may not combust well. Either way you may be screwing up the A/F ratio.

    Even if it is combustible, you may have the carb set rich to run with an air leak, thn wind up with a rich mixture when spraying carb cleaner.


    Quote Originally Posted by mbasura View Post
    I have been chasing a nasty vacuum leak for weeks in my 1969 GT 1.9L with Weber 32/36. I have done the usual stuff (replace manifold gasket, carb gasket, pull carb, rebuild carb, replace) but no luck solving it. Hopefully this is a new facet to an old question: when I spray carb cleaner it at carb base and intake manifold it will actually slow the engine if not outright stall it. I suppose this is essentially flooding the engine instead of making it race, but the end result is that it is indicating the same, major leaks everywhere.

    Could carb cleaner stalling the engine be indicative of anything else, the reason I ask is that all my repairs seem to have yielded absolutely no progress on the leak. I am a hack mechanic, but still I thought I would see a little progress swapping out the old gaskets and rebuilding the carb.

    Thanks in advance.

    Matt

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    Just Some Dude in Jersey The Scifi Guy's Avatar
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    I don't think he's spraying the cleaner INTO the carb. He's spraying it at various locations at the hoses, manifold, and OUTSIDE the carb in order to find out where it's leaking. Therefore, he would want something that doesn't combust and WILL gag the engine if it inhales it. He could spray Windex if he wanted to, as long as it's presence in the air/fuel mixture, should it get sucked in through a leak spot, disrupts the combustion in a noticable manner.

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    bent carb

    Quote Originally Posted by mbasura View Post
    I have been chasing a nasty vacuum leak for weeks in my 1969 GT 1.9L with Weber 32/36. I have done the usual stuff (replace manifold gasket, carb gasket, pull carb, rebuild carb, replace) but no luck solving it. Hopefully this is a new facet to an old question: when I spray carb cleaner it at carb base and intake manifold it will actually slow the engine if not outright stall it. I suppose this is essentially flooding the engine instead of making it race, but the end result is that it is indicating the same, major leaks everywhere.

    Could carb cleaner stalling the engine be indicative of anything else, the reason I ask is that all my repairs seem to have yielded absolutely no progress on the leak. I am a hack mechanic, but still I thought I would see a little progress swapping out the old gaskets and rebuilding the carb.

    Thanks in advance.

    Matt
    check the bottom of the carb and make sure its flat, I had one long ago that got torqued down too tight and it warped it.
    set it on a piece of glass or other flat surface....
    And as others have said to check vacuum tree fitting, as it can be cracked too, and may not be obvious till you take it out and get it in your hand...
    i always ran the heat shield, but gasket sequence is critical,

    paper gasket
    heat shield
    phenolic spacer with paper gaskets on both sides
    then the carb,

    good luck

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    Smoke it?

    I've had a number of vacuum leak issues with my Mini. A couple of times I got some good information by taking it to the one shop in the area that has a smoke machine. I'm not sure how well it would work on the Opel with a carb, but if it really is a vacuum leak, running a smoke test might give you a clue. I'd also thought that there might be some way to rig a "smoke in a can" (the type used for smoke detector testing) for a crude DIY smoke test, but haven't had the need for a smoke test since the last time I had it done with the commercial rig.

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    Chasing vacuum leak, carb cleaner kills engine?

    Thanks for the assistance on this. Need more help with a strange, Coulter intuitive result.

    I fashioned a smoke machine and it revealed a leak in the tree below the carb. I over tightened the clamp and it cut through the cap, essentially leaving the fitting wide open.

    After I capped the leak, the vacuum gauge showed an INCREASE in vacuum at the distributor advance port from 1-2” of mercury to about 6” of mercury.

    The smoke did not show any other leaks. The screwing in or letting out the idle mixture screw had no effect on the engine timing.

    Matt






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