Carb stumble/hesitation

[Bills money pit]

Recently had the carb on the 81 rebuilt by a mechanic who only works on corvettes. When I drove it home, it ran well but had a rough idle. I returned it and he fixed the rough idle but created a significant stumble off idle.

Second issue, now the choke stays on way to long and the rpm at idle is not consistant.

Third comment, I timed the engine by removing the ecm connection and set the initial at 6 degrees. Then tried 8 degrees and finally, settled on 10. It seemed to run the best at 10 but this did not take out the hesitation.

Do you have any suggestions to fix the carb issues on my own so I will not need to return once again the the mechanic.

Thanks for your suggestions!

 

[Nut]

Replace the carb or have it rebuilt properly. I've got 238K miles on my original carb and only rebuilt it once in 25 years. But I also clean the carb it least once a month. 78SilverAnniv has driven my '81 and will attest that the throttle response is perfect. However..... a rough idle could also be a vacuum problem too. Check ALL the hoses in/out for cracks as well as a bad seal.

Good luck ............ nut

 

[gmjunkie]

However..... a rough idle could also be a vacuum problem too. Check ALL the hoses in/out for cracks as well as a bad seal.

Good luck ............ nut

A vacuum leak can cause a Heck of a Hesitation or Stumble Too!!

 

[Tom Bryant]

One thing I keep saying when drivability problems show up on these L81s is to disconnect the battery for 15 minutes to erase the ECM memory when you do any tuning. Your ECM is remembering the settings it was using before you tuned the car or rebuilt the carb. It needs to be erased and needs to relearn new info based on the current state of tune. This is the first thing I do when I start to get a rough idle like first time out in the spring.

I'd have him readjust it to like it was when the choke worked right , erase the ECM and start from there. The rough idle may have been all ECM related. He definately went the wrong way last change so you need to return to the prior settings before trying something new or else you just keep getting futher away.

 

[gmjunkie]

One thing I keep saying when drivability problems show up on these L81s is to disconnect the battery for 15 minutes to erase the ECM memory when you do any tuning. Your ECM is remembering the settings it was using before you tuned the car or rebuilt the carb. It needs to be erased and needs to relearn new info based on the current state of tune. This is the first thing I do when I start to get a rough idle like first time out in the spring.

I'd have him readjust it to like it was when the choke worked right , erase the ECM and start from there. The rough idle may have been all ECM related. He definately went the wrong way last change so you need to return to the prior settings before trying something new or else you just keep getting futher away.

Very Good Point,Tom!!

 

[Bills money pit]

Replace the carb or have it rebuilt properly. I've got 238K miles on my original carb and only rebuilt it once in 25 years. But I also clean the carb it least once a month. 78SilverAnniv has driven my '81 and will attest that the throttle response is perfect. However..... a rough idle could also be a vacuum problem too. Check ALL the hoses in/out for cracks as well as a bad seal.

Good luck ............ nut

Thank you for your response. I was starting to wonder if I smelled bad or had a breath problem.

How do you clean your carb each month.

Bill

 

[Bills money pit]

One thing I keep saying when drivability problems show up on these L81s is to disconnect the battery for 15 minutes to erase the ECM memory when you do any tuning. Your ECM is remembering the settings it was using before you tuned the car or rebuilt the carb. It needs to be erased and needs to relearn new info based on the current state of tune. This is the first thing I do when I start to get a rough idle like first time out in the spring.

I'd have him readjust it to like it was when the choke worked right , erase the ECM and start from there. The rough idle may have been all ECM related. He definately went the wrong way last change so you need to return to the prior settings before trying something new or else you just keep getting futher away.

Tom:
Sounds like you are the man with the plan. I took the 81 to him today and he said he could easly cure the hesitation but did not say how. In addition, he said the choke is not pulling of completely. If he corrects the problems and the car runs well, should I still erase the ecm memory.

Thanks for your recommendations.
Bill

 

[Bills money pit]

A vacuum leak can cause a Heck of a Hesitation or Stumble Too!!

Thanks for your suggestion. I asked the mech to check for a vacuum leak as I could hear what sounded like an exhaust or vacuum leak. He said I may have given it to much advance and the noise I am hearing under load is detonation??????

Thanks
Bill

 

[Bills money pit]

One thing I keep saying when drivability problems show up on these L81s is to disconnect the battery for 15 minutes to erase the ECM memory when you do any tuning. Your ECM is remembering the settings it was using before you tuned the car or rebuilt the carb. It needs to be erased and needs to relearn new info based on the current state of tune. This is the first thing I do when I start to get a rough idle like first time out in the spring.

I'd have him readjust it to like it was when the choke worked right , erase the ECM and start from there. The rough idle may have been all ECM related. He definately went the wrong way last change so you need to return to the prior settings before trying something new or else you just keep getting futher away.

Tom:
I am new to the corvette world and continually read about finding a build sheet on the gas tank. Please explain to me what is meant and how do I find the build sheet and what will it tell me.

Also, I remember reading a post about emailing your vin to someone who will give you build sheet/born on dates.

Thanks for taking the time to educate this corvette rookie..
Bill

 

[Tom Bryant]

Re: the ECM. Yes, I'd still erase and get a fresh start even if he gets the hesitation out of it.

Re: build sheet. The build sheet that shows all of the equipment to be installed on a car is glued to the top of the fuel tank on the assembly line before the body drop. The best way to remove it is to drop the tank out so you can do it carefully where you can see what you are doing. If your car is a Bowling Green car you can get a copy of the build sheet by contacting the National Corvette Museum. Just fill in the form and pay the fee. This will save you some busted knuckles removing the tank. Who know? One day having an old Corvette with the tank sticker still in place may add a couple hundred grand to the selling price.

Tom

 

[bill81vette]

welcome to CAC!!! ,bill,nice to see another L-81!be sure to check out the l-81 registry and register yout 81!!!!enjoy the vette and the forums!!!!!
be sure to post some pics when you get a chance!!!!

 

[Nut]

How do you clean your carb each month.

Bill

Bill.... I hit it with carb cleaner every time I change the oil. Keep the linkage lubed and clean as well as the air cleaner. You'd be amazed how much crap comes through that air cleaner element.

Good luck. and yes, Tom is absolutely right about reseting the ECM. Could also be a bad ECM too.

nut

 

[Bills money pit]

Thanks for your help. Wish we lived closer, bet you could fix the vett as well as any mech locally.

Bill

 

[Bills money pit]

Nut, your dog pic reminds me of one of my old girfriends. Wonder how she got to Annapolis.

Bill

 

[Bills money pit]

Thanks for the welcome. I am still trying to figure out how to use the website.
How would I post photos? Just learning computer stuff.....

Thanks
Bill