Carb Hesitation

I've got a big off idle hesition after rebuilding my 2G carb. I've searched here and other places but nothing I've tried so far has fixed it. Apparently this is a symptom of a lean idle?

Things I've tried so far:

- Accelerator pump, yes it's working. I even tried switching back to the old one.
- Idle mixture screws. I've tried moving them out 1/4 turn at a time from 1 3/4 turns to 3 turns.
- Float level. I raised the float to specs. Also set drop.
- Vacuum leak. Tried both WD40 and starting fluid to look for a vacuum leak but didn't find any.
- Checked timing.
- Checked compression and put in new spark plugs.


The carb kit was from NAPA. Buick 225 Odd Fire w/ HEI. Runs great except for this big stumble.

Help!

 

have you checked your timing? an engine out of time can sometimes cause this problem.also check the mech. advance weights in the distributer to see if they move freely.

 

How was it running before the carb rebuild?

 

Yep, as mentioned I checked the timing. Also, I revved it up with the timing light on and it looked like the timing advanced smoothly. I'm only using centrifugal advance now. The vacuum line from the carb is plugged.

If it was a timing issue, which way would I go for improvement?

 

It ran fine before the carb rebuild. I probably could have just ignored it but it's been on there quite awhile, like 10 years. One of the mounting bolts broke when I took it off - it was necked down from corrosion.

 

OK, I need to get a vacuum gauge. What numbers should I be looking for? Am I checking vacuum at the line for the distributer advance or somewhere else? I have a vacuum pump but the gauge on it is probably not the right range. Sears?

There's a line out the back side that goes to the PCV valve. Could that be a problem? I guess I could pull the line and plug it to check.

 

Got it. Thanks Sparky. Off to NAPA after work for a new tool!

 

OK. This discussion is giving me something to go on.

The vacuum advance canister on my distrubuter (HEI, old) is leaking, however, I can order one at NAPA. I looked into this once years ago and found that there is a very large number of canisters available - so one can use this as a tuning device to pull the vacuum advance in at differing amounts of vacuum. I'll research some of Rich Motts stuff to try and find out what pressure range would work best.

From memory (dangerous) the '70 2G has a ported vacuum connection, meaning that the opening inside the carb throat is above the throttle plate when it's closed (idle). So, no vacuum and no distributer advance at idle? Is this correct?

Also, normally the books say to disconnect and plug the vacuum line when setting the timing. Correct?

If the vacuum advance somehow advances the timing (quickly) right at tip in then that could be the problem. Am I grasping at straws?

I thought the vacuum advance was more for gas milage while cruising on the highway. Like Sparky said: "Vacuum advance wouldn't cause a stumble off-idle would it? It shouldn't kick in until the RPM rises quite a bit if it's properly set."

 

OK. I also hooked up my (brand new) vacuum gauge to the distributer line off the carb. Zero at idle, went to 10" just off idle, then up to 15" and on to about 20".
I don't understand timing advance really well so I'm not going to try and explain it - I'd just confuse myself. It's enough for me to say that the engine 1. needs advance with rpm (mechanical) and 2. works better with more advance as vacuum increases.

I also used tried again to find a vacuum leak but no luck.

My base timing was set at 8 degrees - seems reasonable. It seems to help the hesitation if I advanced the base timing a bunch, but I'm not comfortable with 20 degrees of base timing.

I got to see up close what it looks like when you get a backfire though the carb. Glad my face was not close to the top of the carb when it happened. I never did it, but I have been tempted to look down the carb and see if the accelerator pump was really working while the engine was running. I'm not tempted to do that any more.

 

Distributor vacuum advance - the diaphram is no good on mine so no worky. I'm leaning towards ordering one from NAPA in the pressure range that will work best, however, I haven't figured out what would be best yet. If I could find the adjustable type I would try that. I do not suspect vacuum advance (or lack of) is the reason for the stumble but I could be wrong.

Manifold vacuum - yes, I took off the line to the PCV valve and plugged it to see if that made a difference. It didn't. The new vacuum gauge didn't come with much for adapters so I wasn't able to check the manifold vacuum reading last night. I'll keep working on that.

During my research I found that this problem has come up quite a bit. The solutions were 1. fix vacuum leak or 2. open up the idle mixture screws. Neither of these seems to be the problem in my case.

In the Project Hatari JP Mag CJ6 they rebuilt the carb and said it "didn't take". Then they sent the carb out to a rebuilder and everything was wonderful. I could do this if I have to but I'm not there yet.

Thanks again for the help guys.

 

I discovered a vacuum leak in the hose that went to the heater controls. Also, When i went to the HEI I had a hesitation when the timing was advanced too much. I don't have the vacuum advance on the HEI hooked up and i have lots of power and pickup.