TR2/3/3A SU Carb. Vacuum Leak-tr3a

[midnight]

I have a vacuum leak in one of my SU Carbs, I installed new shaft & bushings (fitted to .002) & new rubber & seals - what am I missing - where else can it leak?

 

[NutmegCT]

Miles - welcome to BCF.

If you're already sure the leak is in one carb, try this:

spray water (like from a mister or windex sprayer) in small areas around the carb. one area at a time.

when the engine stumbles, you've found the leak spot.

Lots of other guys here will have better suggestions, but that's what I'd do first, if I were sure the leak is in the carb.

Tom

 

[TR3driver]

Although I might use WD-40 or carb cleaner instead of water.

Why do you think the carb is leaking vacuum? My experience has been that even pretty badly worn throttle shaft/bushings won't leak enough to make any major difference, and as you've noted there isn't much else to leak except maybe the vacuum advance fitting under the front carb.

Of course, one of the gaskets between the carb, spacer and intake manifold could have failed (but it's not a common problem).

 

[Geo Hahn]

Once I was tidying up the engine with a paper towel and it got itself stuck on something -- then I saw that the towel was sucked against the intake manifold right at the gasket to the head. Engine also sped up a bit as this happened. Probably never would have noticed that leak but for the paper towel -- though the WD40 or carb cleaner is the preferred method.

 

[midnight]

Randell - thank you for your thoughts - I have replaced all manifold gaskets - surfaced all mating faces, rebuilt both carbs, ran a compression check, sprayed every possible location with carb cleaner - found no tell tell signs of a leak - my last test: I switched the carbs (front to back/back to front) the original back carb still has very low vacuum - at this point everything I have check leads to a defect of some dimension in the carb - I guess I will eventually find it - thank you for your experience - Miles

 

[Don Elliott]

If you swopped carbs, how did you connect the fitting for the vacuum advance to the rear carb (formerly the front one) which is a tiny tube from the front of your distributor to an inlet on the bottom of the normal front carb ?

 

[Brinkerhoff]

Miles, what do you mean it has low vacuum? How did you check it? If you've switched carbs , you've isolated the problem to one carb.? Or do you mean you can't adjust the mixture in one of your carbs so you think its a vacuum problem? Kevin

 

[TR3driver]

Miles, what do you mean it has low vacuum? How did you check it?

 

[AEW]

Oh well just chiming in...

So in this conversation is there any chance of converting terms over to "manifold pressure" (lack/excess there of)? It makes it easier to compare opinions without double negatives. I mean in the descriptions/advice of diagnostic results/expectations. :wink:

The thought about the distributor line is a good one. Made me think about the piston lift pin possibly missing and the fast idle cam screw (I think that's what it is, I have at least one that is drilled through into the throat.) If the carb bodies are reversed there may be a hole in the back of the aft unit. ( I think)


As far as test tools, there are sonic probes that locate leaks based on the air sound. They can even "see" a leak on the underside of a manifold runner on a 'V' engine. I also remember seeing an unlit propane torch with aquarium tube used to induce an RPM jump. Agree though, WD40 and carb spray are most common.

Smart money is still on a manifold gasket leak. There aren't very many holes in these SU things, and they're small...Good thing there's no homemade PCV here or this would be really complicated

 

[Minesweeper]

Randell - thank you for your thoughts - I have replaced all manifold gaskets - surfaced all mating faces, rebuilt both carbs, ran a compression check, sprayed every possible location with carb cleaner - found no tell tell signs of a leak - my last test: I switched the carbs (front to back/back to front) the original back carb still has very low vacuum - at this point everything I have check leads to a defect of some dimension in the carb - I guess I will eventually find it - thank you for your experience - Miles

Miles -- I think you need to clarify for us where you have "very low vacuum." Most of us are probably assuming you mean low manifold vacuum (downstream of the carb). However, if you mean low vacuum in the carb suction chamber and/or low air flow through the carb throat ... well that can be an altogether different issue.

Scott

 

[midnight]

Gentlemen - thank all of you for your input - Clarification; I have low air flow through the throat of the carb - After switching the carbs (FRONT TO BACK) I used a "Uni-Syn" Model-A" to test air flow - Adjusted (at idle) so float reached the third bar - tested the other carb, and I get Zero lift of the float - (Note:The
port for the dist.vacuum bleed was temporarily blocked while testing) Also, there is no lift of the dash pot at idle - it will lift, somewhat, at higher RPM - I have tried a different piston in the dash pot, to no avail. Somewhere, this carb has a substantial leak - The carb's performance is unchanged irrespective if its mounted at the front ports or the rear ports of the intake manifold -Thank all of you for your incite/suggestions - Miles

 

[TFB]

Miles,have you lifted or removed pistons and looked to see if throttle butterflies are both moving in sync.Just a thought,
tom

 

[Minesweeper]

That's what I suspected. Check that the butterfly valve is actually opening when you operate the throttle. Second, if you lift the bad carb's piston with you finger, does it bind? How does it compare to that of the good carb?

Scott

 

[Twosheds]

Oh well just chiming in...

So in this conversation is there any chance of converting terms over to "manifold pressure" (lack/excess there of)? It makes it easier to compare opinions without double negatives. I mean in the descriptions/advice of diagnostic results/expectations. :wink:

I can't tell from your emoticon if you are serious or not, but I agree. Coming from aviation, I have the hardest time thinking in terms of vacuum instead of manifold pressure.

But, alas, I doubt that any auto mechanic can be convinced to think in terms of manifold pressure when discussing a car problem. I had a hard enough time getting students with an auto mechanic background to understand manifold pressure when talking about an aircraft engine! It was actually easier to teach manifold pressure to someone with no mechanic background.

 

[TR3driver]

Clarification; I have low air flow through the throat of the carb

Ah, almost certainly not a leak, IMO. A leak that big would keep the engine from running at all.

Lack of flow implies a restriction, most likely the throttle plate not moving for whatever reason.

Pistons & domes are a matched set, swapping them around can lead to problems. If it weren't for the Uni-syn results, I might suggest that someone has managed to swap the pistons front to rear without swapping the domes.

So, what happens if you try to adjust the synchronization ?

 

[TR3driver]

Coming from aviation, I have the hardest time thinking in terms of vacuum instead of manifold pressure.

Oh come on, John! This conversion is even easier than converting degrees F to degrees C. The units are the same, only the reference is different.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]I had a hard enough time getting students with an auto mechanic background to understand manifold pressure when talking about an aircraft engine! [/QUOTE]
Manifold pressure is just 1 atmosphere minus the manifold vacuum. The only tricky part is that, for an aircraft, "1 atmosphere" can change considerably (based on altitude).

I'd also argue that manifold vacuum is more relevant to how the engine is performing ... does 11" MAP mean you are idling at sea level or full power at 25,000 feet? ... but I doubt anyone here wants to hear about it

 

[Mkutz]

Are your jets centered? Could be (as aluded to in other posts) that perhaps the needle is binding in the jet, which is not allowing the piston to travel as it should? Push the piston all the way up, then see if it drops all the way down on its own with a metallic "clunk". Also, make sure the pistons are identical in terms of the holes at the base in terms of size and location.

Just to be sure here -- you are loosening the coupling between the SU's and independently adjusting them when your trying to synchronize for airflow -- right?

 

[TexasKnucklehead]

Turning the idle adjustment screw on the 'dead' SU should increase the UNI-SYN float for that carb (as well as the idle).

I recently had an issue with one drawing too much:
https://www.britishcarforum.com/bcforum/u...ting#Post735212

 

[midnight]

Gentlemen - thank all of you for your help - It has been 40+ years since a last worked on SU carbs - obviously I had forgotten how simple they are - several of you got it right - Without any vacuum passages or idle jet nothing moves when the butter flies are closed - there in was my problem - while the front carb had the butter fly cracked the rear carb was closed - I re-set everything, balanced the carbs, adjusted the mixture,locked the throttle shafts together - runs beautifully - Thanks so much for getting me to remember what I had long forgotten - P.S. On aircraft I always referred to manifold pressure in inches of mercury - in racing they call it LBS of boost - Miles

 

[Mkutz]

all's well that ends with proper SU synchronization!