I just got an 85 Caprice Classic with a 305. It is stumbling on heavy acceleration. If I slowly give it gas, it will work fine. Even when in park, if you give it Wide Open Throttle, it will lose power for a second then rev up just fine.
Some one has eliminated the computer from this car and installed a non computerized carb and distributor.
I checked base timing and it is spot on. It also advances with vacuum to the vacuum advance.
I put a vacuum gauge on the engine and it holds a steady 18 in. The vacuum drops all the way to 0 with Wide Open Throttle then comes back up when the engine revs ok.
I guessing a carb problem, but I dont know jack about carbs. Any ideas would be great.
Thanks.

Check your timing (too retarded could cause this), eliminating my PCV valve fixed the stumbling problem for me, so you could give that a shot by replacing the PCV valve with a breather and blocking the vaccume line off. It could also be that your carb needs a rebuild.

Could also be a camshaft with a flatspotted lobe. This was common during this era.

I will try the PCV trick. I checked the timing and it was about 8 deg btdc. I tried advancing a little at a time until I couldnt go any farther. This helped a bit, but now it diesels upon shut down. So I put the timing back where it was.

I have heard about flat lobes on cams. I thought that if you had a flat lobe, that you would get a misfire on that cylinder. Unless.... it was just flat enough to not open the valve all the way. But even then, it would have to be on several cylinders. Is this possilbe?

I really dont want to rebuild my carb since I dont know how. :smile:

For the am, I think the problem was resolved in the 83 year, but I could be wrong. As for you're timing, it should be 6 degrees OTDC, and it makes sense that it would deisel if you advanced it as far as you could go. Anything more advanced that 5 degrees OTDC could cause that.

I have heard about flat lobes on cams. I thought that if you had a flat lobe, that you would get a misfire on that cylinder. Unless.... it was just flat enough to not open the valve all the way. But even then, it would have to be on several cylinders. Is this possilbe?



Not necessarily. I once had a 1972 Impala with the 165-hp 2bbl 350, it had 3 flat lobes on it and it never misfired - but getting it to speed was an excersize in patience and frustration. Many times I stalled it in traffic because a backfire ignited all of the air/fuel mix in the intake, and on more than one occasion even setting the carb on fire ... yeah the memories ... getting out in a cold, driving thunderstorm in shorts and a T-shirt with a towel to put out the fire in the carburetor while stalled across two lanes of traffic during rush hour ... haha ... Boy, talk about the stench that thing made when it would backfire, too...

It started as a light hesitation which I was more or less able to dial out by adjusting the idle mix screws on the carb, but it got to the point where the screws were adjusted so rich that they were held into the throttle body with one thread ... there was a particular speed on the car that if you held the throttle in a certain position, you'd kill the engine and set the carb on fire .. LOL I can laugh thinking about it now, but back when it was happening I was livid over it. Once I parked the car over it, there were two working positions on the throttle - full off and wide open. Replacing the cam solved the problem entirely, and it was like driving a new car.



For the am, I think the problem was resolved in the 83 year, but I could be wrong.



The problem persisted up to the time when they began using roller lifters - 'tis one of the reasons for the roller lifters ... Basically, the cams didn't have a problem UNLESS you deferred your oil changes - if the interval between oil changes went long enough, you'd have an excessive metal buildup in suspension in the oil, which would grind the cam, putting more metal into suspension thus accelerating the process. Once the wear started, there was nothing that would stop it, either. The hardness on the lobes didn't go too deep into the lobe and once the soft stuff became exposed, that's all she wrote.