Rough running engine

[Mike McKown]

I bought this car over thirty years ago and it runs the same today as it did then. I mention this only to ward off suggestions that the problem is from sitting idle. It is a 327/250 hp. The engine idles good. It pulls strong at anywhere from one third to wide open throttle. The problem is just off idle. Any gear. It runs rough, bucks, jumps and lurches. You either have to get your foot into the gas, let off the gas completely or slip the clutch or you will be embarrassed if there is a crowd around.
A long time ago, I changed plugs, wires, cap, rotor, points, condenser, the whole distrubutor and then finally ran a ground wire to the battery from the distributor. No luck. I have left the car parked for many years and when I knocked the cob webs off the other day and started it up, it ran just like it did when i parked it.
Sometime before I purchased the car, the original owner put a re-built WCFB carburetor on it. Tag shows the right number for my car. Problem is, in this area at least, re-built carbs were notoriously unreliable. I have come to the conclusion the metering rod spring is not correct tension and the off idle roughness is caused by a lean condition. I don't how to check for this and it has been almost 40 years since I have had a WCFB apart. "new,rebuilt ones" are expensive to be guessing about the problem. Can anyone offer a suggestion?

Thanks,
Mike

 

[cntrhub]

Check float level in the carb. Sounds like fuel starvation at partial throttle.

 

[Mike McKown]

I agree with the part throttle fuel starvation. Will check. Thanks.

Mike

 

[maxrevs85]

Could be the excelerater pump. That will cause it to hesitate when you excellerate.

 

[Mike McKown]

Thanks for the reply. But this is not a "tip in" problem. It will idle for example @ 700 rpm in neutral. If you speed the engine to say 1000/1500 and hold it steady, that's where the problem is.

Thanks,
Mike

 

[maxrevs85]

I would tend to believe it to be a float problem too. .....Fuel filter? :confused

 

[cntrhub]

Pop the cap, and make sure the advancer has a... full lock/full close operation. Then, recheck that points gap to specs. (Dwell the points if you have the equipment.) Take a timing light and move the throttle up to that "1000/1500" rpm window, and make sure you can see the advance move off that 700 rpm idle. Get that variable out of the way, before you chase a possible fuel problem.

 

[Mike McKown]

I'll check this again too, just to make sure. Remember in my first post, I said I changed the whole distributor and then made sure it was grounded. No change. And I changed the points (you can assume I reset the gap). No change. But, I'll re-check.

The roughness could be timing jumping around at low speed but it would have to be a lot (15 degrees or so). But it would really have to be jumping. The engine will sputter and buck, then smooth out, sputter, buck smooth out back and forth, rapidly. That's why I think it is a lean mis-fire.

But I thank you guys for your answers. I will check. If anyone else has ideas, feel free to post.

I will re-post if I get the answer.

Thanks,
Mike

 

[vigman]

The distrib has VAC advance?

Going to the correct port? If it was ME... I would throw another carb on it to verify the problem.... If it runs good @ WOT...it's not a fuel pump fuel filter problem.. and I would venture not float level.

I'm leaning with the metering rod theory.... but I do not KNOW Carter's that well.

But having the VAC advance @ the wrong port might do the same thing.

What happens ( with a timing light ) when you wing the motor how many degrees adv with & without the VAC hooked up?


I'm assuming STOCK distrib...right.... no "tricks" right?


Vig!

 

[Mike McKown]

First of all, thanks to all who had input. I found/fixed the problem based on your input.

First, a little history. Any v-8 Chevvy I ever had made in the '50's or '60's got an automatic 10-12 degrees lead on the timing. That's where they seemed to run best.

I had the initial timing set on this car at 12 degrees. should have been okay. but it wasn't. If you add it all up, there was 24 degrees in the centrifugal, 16 in vacuum and 12 on initial, that's 52 dgrees total. This I might add is only two degrees plus to the '65 shop manual. I was suprised to see it gave a range on timing of 4-10 degrees advance. I used a dial back timing light to verify the above numbers.

Anyway, I cut the intial timing back to 0 and it runs fine.

The timing marks on the balancer and tab are unverified as to their actual location to design intent.

Thanks again to everyone.

 

[Subfixer]

Mike, I had a similar problem and had to reset my initial timing to 0* also. Read my post "Corvette Maintenance Day" and see what I found with the mechanical advance bushing. I'm now set at 8* initial and she runs great up to redline either WOT or just cruising.

 

[Mike McKown]

Thanks.

Good write-up. Since I am reading 24 degrees mechanical advance as it calls for, I probably don't have the bushing problem but it'll only take a minute to check.

Mike