Flat spot when accelerating from idle

Asked by Jul 10, 2017 at 10:02 AM about the 1966 Chevrolet Chevelle

Question type: Maintenance & Repair

I have a 1966 chevelle with a 327 4 speed. The car starts fine, idles good and operates normal when accelerating except when it is warm from an idle. car has lots of power when driving, just a problem from idle. I replaced the fuel filter and carb and still has same problem.

12 Answers

98,565

It sounds like an accelerator pump problem. The little pump should deliver a squirt of fuel even with the slightest movement of the accelerator, small movement small squirt, large movement large squirt. You may have to adjust the linkage to get the proper stroke, most carbs still need setting up right out of the box. Another thing to check is the vacuum advance, it also should respond with throttle movement, but the change there is it will slowly advance with slow and even throttle, it will not under a wide open throttle until vacuum comes back, during those times the centrifugal advance part of the distributor handles advance. Hope this helps.

1 of 1 people found this helpful.

Thanks, I was wondering if it could be a vacuum issue. I was sure it was the accelerator on the Q jet, so I borrowed an Edelbrock from a buddy I know was good as he had it on his 63 327 Impala. Car acts exactly the same with the different carb. I bought the car in the winter and am just getting around to driving it and getting the bugs out now. Car is actually unsafe when entering a roadway.

98,565

While your in the distributor make sure the centrifugal advance weights are working, I can't remember just how many I found that were locked up or so badly worm they didn't operate. When everything is working right you should have anywhere from 38 to 42 degrees total advance at 2500 rpm, some timing lights wont read that high but that is where you want to be stock.

98,565

Ohh forgot the timing chain, no more than 6 degrees of slop. You wouldn't notice it driving down the road but you would during low rpm's. You might but not always hear some slight rattle inside the timing chain cover. HTH

1 of 1 people found this helpful.
1,915

none of the fuel lines are rusted are they ? The ones going over the rear axle ?

Thanks guys, fuel lines are good and nothing showing up in the fuel filter. I will check the centrifugal weights and timing chain. I guess i will have to pull the cover. Would a loose timing chain show up with timing light? Would the light be bouncing around?

98,565

You would have some trouble setting the base timing, you can really tell it when you bring the engine up on 10 degrees #1 take the distributor cap off so you can see the rotor and then turn it backwards and see just how far back the engine turns before the rotor moves and then see where the pointer is on the degree wheel, if its over 6 degrees then it's timing chain and gear time, now there are some different ways to go with that, stock chain and gears, dual row chain and gears, and if you don't mind the gear whine go gear driven timing set, this set will give you the most accurate crank to cam timing, but they are noisy. Now distributors are another thing, if your points are bouncing it can also give you a false flash on the timing light, so make sure the breaker plate is solid and not moving when it comes time to open the points, also the dwell will be off and had to set properly, if your not into keeping it stock you can go to an HEI setup with adjustable vacuum advance, or full centrifugal advance, no vacuum advance, it all depends on how deep your pocket book is, your local speed shop can get you some really nice components.

Well the weather here has been rain, rain, rain. Got a bit of a break yesterday and after chatting with a buddy who has a 63 Impala with a 327 4 speed like myself, he told me he disconnected his vacuum advance and his car ran much better. I plugged mine and took it for a ride and it ran much much better. I didn't get it real warm before it started spitting again. So, what does this tell me? Thanks again.

Forgot to mention that I have an HEI ignition on the car.

98,565

HEI will still have centrifugal advance, so that will still need to be checked, have you check total engine vacuum at idle? What is your base timing set at?

I am hoping to check the timing today or tomorrow. Should the base timing be around 10 deg. BTC ?

98,565

More like 6 to 8 degrees, or you can try power timing, just keep advancing the base timing by 2 degrees until you hear an engine ping during acceleration, then back off 2 degrees until it stops. But everything must be operating properly as far as the distributor to do this correctly. Do you know the compression ratio on the engine? You may need to run hi test fuel with some octane booster with this so called fuel today. I had an old Chrysler and when this fuel came out it would not run on it, had to go hi test add a bottle of 104 booster and a mix of StaBil marine stabilizer in the tank just to make it run as it use to run on premium gasoline. 440 with dual 4bbl carbs.

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