carburetor woes

[Keith_M]

I just finished rebuilding the carbs on my '65 Sprite. Everything seems fine except that I can't get the idle below about 1200 RPM's. I've checked the butterfly valves, and they seem to be seating correctly; i.e., with the carbs off the car, I can rotate the throttle shaft until the valve is completely closed. The angled surfaces of the valve are correctly oriented so that they sit flush on the sides of the throat. The slow running adjustment screws are backed out completely, and the throttle cable is loose so that the throttle shaft definitely is rotated to fully closed.

The carbs behave normally in that I can adjust the mixture to be lean or rich, and I can hear the air entering the mouth of the carb with a listening tube, so the airflow is clearly coming past the jet. This rules out a vacuum leak, I think...

I think this means that air is somehow getting past the butterfly valve despite the fact that it is apparently seated correctly. I just don't see how that is possible. Anybody have any suggestions?

 

[David_Doan]

try putting your hand over the air input of each carb. If it does not slow down, you have air leaking in somewhere else.

 

[Keith_M]

try putting your hand over the air input of each carb. If it does not slow down, you have air leaking in somewhere else.

Hi David,
I'll try it this evening. I'm 99% sure it will have a huge effect on the engine because I can hear the air (with a tube) rushing past the piston and jet at the mouth of the carb. Nevertheless, it will be a good diagnostic whatever the outcome.

Thanks,
Keith

 

[SaxMan]

I had a similar situation where I kept trying to slow down the idle and nothing happened. I went back into the car, pressed down on the accelerator and released it. Suddenly, the car was loping along at only 400 RPM. I guess the good news was that the carbs were synched well enough that the car didn't stall at that low of an engine speed. The problem never reoccurred.

 

[drooartz]

There's definitely extra air getting in somewhere, Keith. I had that problem on my previous MGB after the carbs were rebuilt. One of the butterflies had slipped ever-so-slightly and wasn't sealing properly. Luckily that was an easy fix -- unscrew the screws on the butterfly and reseat it.

The old trick is to spray some WD40 or similar around the carbs at likely leak points and see if it changes the engine speed at all. Also disconnect the linkage between the two carbs and see if you can isolate the problem. With my issue last year I could stall the engine with the front carb but not the rear, so at least I knew which one to look into.

 

[Keith_M]

Thanks for all the suggestions. Here are some updates.

David - cutting off the air flow at the front of the carb had a huge effect on engine speed, plus I could feel the suction. I'm pretty sure most of the air is going through the carb, past the jet and piston, and somehow through the butterfly valve.

Saxman- I've completely disconnected the throttle linkage and manually pushed the throttles as closed as they go. With the carbs off the car, this closed position corresponds to completely closed butterfly valves.

Drew - I've reseated the butterfly valves 3 times; even changed them back to the originals (as opposed to those in the rebuild kit). Each time the fit seems perfect. I can't see anywhere for air to get past. I've been spraying WD40 as well, and I can't find a vacuum leak.

I'm convinced that the air is somehow getting past the butterfly valve. Could it be going between the valve and throttle shaft? (i.e., where the valve slips through the shaft?). I'm thinking of a smearing a little gasket sealant over that junction to see if it changes things. At this point, I'm an expert in removing and refitting the carbs. :smile-new:

 

[David_Doan]

Thanks for all the suggestions. Here are some updates.

David - cutting off the air flow at the front of the carb had a huge effect on engine speed, plus I could feel the suction. I'm pretty sure most of the air is going through the carb, past the jet and piston, and somehow through the butterfly valve.

So if redistricting the airflow to the front carb had a bigger effect than the rear, that gives you some info....

Front carb butterfly is not closing

OR

Rear carb has an air leak

 

[JPSmit]

how are the shafts? did you replace them as part of your rebuild? leaky shafts are the main reason these things give problems

 

[jlaird]

Shafts, no question in my mind. With engine running spray a bit of carb cleaner where the shafts go into the carb, if rpm increased you found it.

 

[John Kuzman]

You may have already addressed these, but are you certain that BOTH throttle connection clamps are absolutely loose? I think they take a 5/16" socket.

 

[Keith_M]

These are all good suggestions.

David - I blocked flow at the front of the carbs, not just the front carb. Blocking flow has the same large effect for both carbs.

JP and Jack - Shafts are new, they fit tight, and spraying WD40 on them as no effect.

John - the throttle shaft connection was one of my first thoughts, but even with everything disconnected and the slow running screws all the way out on both carbs, the idle is still high. The throttle linkages are not sitting on the choke idle screws either. The only thing I didn't disconnect is the choke linkage. I suppose it's possible that it is somehow holding the butterfly valves open. I'll try that tonight.

From everything I've done and all the suggestions, I'm pretty sure that air is going in the front of the carbs, past the jets and pistons and through the butterfly valve. This means that either 1) the butterfly valves are somehow not closing, or 2) air is leaking past or through the closed valves.

I'll pull them yet again tonight and see if I can see how that might be possible.

Thanks again for all the suggestions.

 

[John Kuzman]

Keith -

The only reason I suggested the throttle shaft clamps is from watching the late Spritenut in action at the 2008 LoTO. A young couple with a 70s Midget was struggling with the same problem. Frank not only loosened the two clamps, but also took them completely out of the equation by sliding them both inward on the throttle shaft. Problem solved and carbs could be balanced and idle set. Then, set up the connecting clamps as called for in the books.

 

[JPSmit]

choke linkage could account for this too

 

[Glen Byrns]

Also check for intake manifold gasket leaks.

 

[BWA]

A 5/16 socket will not fit. It is annoyingly close, but, unless the socket is really worn out, it just won't fit. I think the correct size is 2BA (British Association). Snap On sells 2BA sockets.


If you can feel good suction through the carb when you block them with your hand, it would seem that the bores are worn where the butterflies rest. This is fairly common around the shaft area, about half an inch above and below where the shaft goes through.

Retarding the timing at idle, and, richening the carbs a bit will help bring the idle down.

 

[Silverghost]

I may be wrong but I seem to recall reading about this or a very similar problem before. In that case I believe the fix was that the butterflys had been installed upside down and/or face reversed. As I recall, there is a very slight offset to the screw holes and putting the plates in upside down results in them not seating correctly?? It's a very fine point but can make a difference? Again, I may be wrong but worth a check.

 

[jlaird]

Butterflies could indeed be in backwards or the needle seat is to low or the needle its self mornted too high?? Any or all the above.

How about a broken anti backfire valve in the butterfly in fact if you have em might look for butterflies without or solder em shut.

Man we really getting down to serious stuff now.

 

[Keith_M]

Here's an update. The butterfly valves are definitely not in backwards. I know because I put them in backwards the first time, but I immediately saw that they didn't seal correctly to the throat and reversed them. :victorious:

What's interesting is that the problem is slowly going away. In other words the idle is getting slower as I drive the car, and it is now to the point where I have to screw in the slow-running screws a little to keep the idle up to about 1000. The only thing I can figure is that the new butterfly valves are slowly wearing to the shape the carburetor throat and are sealing better with time.

Thanks for all the suggestions. I guess there's a moral here somewhere.