My daily driver (’57 BelAir 2dr hardtop) is all “original”. 283 with Rochester 4G 4-barrel carb and 2-speed powerglide. Over the past few years I’ve been driving it even though the carb was having issues, choke didn’t work well and was frequently idling high, accelerator pump didn’t work so it had a hesitation from a stop. Well, I finally took the time to replace the carb with an Edelbrock 1403 (500 CFM with electric choke). Of course the choke works well and the hesitation from a stop has disappeared but I did discover a possible new problem. When traveling between 30-40 mph and having my foot on the pedal on an every so slight downgrade (just where you let off the gas a little to keep your speed) it starts to miss. If I give it any pedal at all it immediately stops. Doesn’t do this at highway speeds. With the old carb the car would slightly miss at about 40-45 but you could barely feel it.

I’ve replaced the plugs, wires, coil, cap/rotor about three years (15,000 miles) ago and I just checked the timing (5 degrees advanced). I unplugged the vacuum advance and drove it, no change. I sprayed starter fluid around the base of the carb, etc and no vacuum leaks.

This could have been an issue before I changed carbs as the last carb had issues that may have been masking other issues.

Thoughts on where to start?

How is it at wide open throttle?

Just fine. As soon I as give it any pedal at all (5% increase to floored) it runs like a top.

Seems to be limited to a very slight downgrade that allows you to remove 5% - 10% of the throttle to keep your speed.

We had a chevelle with an original 283/glide that started running like crap, cam was wiped

I was asking about WOT because I was wondering about cam or burnt exhaust valve but his is okay at WOT.

Did you plug vacuum port on carb?

too much vacuum advance sounds like..

15,000 miles on a set of plugs with a carb is a lot of miles. If this is a recent development, I think I would start with at least checking the gap on the plugs, I suspect the gap has grown larger. I would probably change at least the plugs out. Wouldn't hurt to look at the condition of the point contacts either.

Back in the days of leaded fuel, and point ignitions, tune ups were done regularly around 10,000 miles or less. The unleaded fuel helped extend the time frame, but few made it past 12,000 miles before they needed attention. Gene

Don't think it's a wiped cam (hope not). It's always started instantly and runs like a top except for this.

I have the vacuum advance plugged into the ported vacuum on the carb. When I unplugged the vacuum advance I plugged the port on the carb to avoid vacuum leak. Since I unplugged the vacuum advance I would assume that I can count that out.

Forgot to mention that is has aftermarket electronic ignition under the cap (no points).

Check the diaphragm for the vacuum advance in the distributor.

[quote=CYACOP]Check the diaphragm for the vacuum advance in the distributor.
[/quote

Just went out and checked. The diaphram holds 20" vacuum. Looks to be fine.

Checked timing while I was out there.

With vacuum advance disconnected:
~12-14 degrees advanced at idle
~23-24 degrees advanced all in at 1800-1900 rpms

I plugged the advance back in and it jumped to ~33 degrees at 2000 rpms and went higher from there as the rpms increased (I think I saw 42 degrees around 2800 rpms).

I want to spend some time this weekend working on the car.

Any suggestions on what I can look at?? Thanks all.

Im with POORBOY, I would throw another set of plugs in it.

leave it disconnected and drive it... i bet its fixed..

I did this today. Unplugged the vacuum advance, capped the ported vacuum on the carb and drove it. Same problem...

Good though, I can try that this weekend.

Keep all other ideas coming!!

If new plugs doesn't fix it I'm at a loss...

hmmm thought that would do it.. ok did you use a SOFT carb base gasket on it?.. if not you may have a vacuum leak there...but now you must ck the plugs first..and the wires.. good luck

OK, so if I'm reading & understanding correctly:
Happens at a light decelleration, butterflies partially/mostly closed.
At that point, should have a pretty high vacuum reading, not pulling much fuel, no load.
Maybe it's a bit too lean @ cruise, or maybe advance weights are sticking (not sure what ignition you have).
When you say "missing", does it feel like chugging, or dropping the occasional spark, or like a burble from the exhaust ?

It has a metal adapter with regular carb gaskets top/bottom. I did spray starting fluid all around the base of the carb with no indication of a vacuum leak.

Thanks!

Happens when I pull back very slightly on the accelerator due to going down a very slight downgrade. I just pull back ever so slight on the pedal. I'm only pulling back about 5-10%.

Seems like it's loosing spark. The first time it did it I thought I was running out of gas for a second.

Here's my WAG......

How old is the timing chain?

In my mind, I'm picturing when the engine is under load, you've taken up the slack in the chain, but when you 'let up' you're into the slack for a moment.

I had a 360 years ago with high miles on its original chain set, and it had several hesitation/stumble issues that went away with a new chain.

[quote][/quote]Forgot to mention that is has aftermarket electronic ignition under the cap (no points).

Is this a conversion of the old distributor? If so make sure the ground wire from the breaker plate to the housing is intact and good. I had troubles with an Oldsmobile and the breaker plate would intermittently loose ground at various vacuum conditions causing a stumble. As the vacuum advance was working the plate it would loose ground. If it was removed add one and see if it helps.

had a Chrysler electronic ignition pickup coil break the wire internally once, that was a fun one to find

Old distributor with conversion.

Thanks for all the ideas guys. I had to help my daughter move into her first house today so I'll attack this tomorrow.

How confident are you the old and new carbs are set up correctly? .. if you aren't confident, I'm tempted to suspect the carbs..

Can you repeat the problem with the car sitting in your driveway and you manipulate the throttle?
If you can, watch the carb throats and see what the fuel is doing.. Flooding or leaning out?
Or do you HAVE to be on the road.. ?
- Art

lLol. Not laughing at you but with you. Had the Cyclone's engine rebuilt and bought a really nice Quickfuel carb. Nope, that seemed to just go dead at about 2500rpm. Installed a dyno mule Holley that we knew was right, boom, same freaking thing. Finally went with a Holley Sniper EFI system and that took care of the problem. Where was the problem? Not a clue as we replaced the billet distributor, AL box, coil, etc all with Holley gear. Cost plenty to finally get the car running right. Hate to say it but carbs are going to be a thing of the past. Why? Fewer and fewer folks know much about them. Seems like only yesterday that I bought the barracuda and Bee but coming up on 20 years now. I'm will to wager the number of folks that understand(I do) carbs and can fix them(not me apparently) has taken a 50% drop in that time.

I used to rebuild a lot of carbs back in the day, I was the carb rebuild guy in the shop. Its a lot harder to find a good quality carb to rebuild these days because most are getting quite old, throttle bushings are getting bad on most cores. Many of the "new" carbs are not up to the quality the ones from the 60s -early 80s were (and those 80s carbs were very good either), and the current kits leave a lot to be desired as well.

The last vehicle I owned with a carb on it was wrecked in 2011 (nothing to do with the carb). Everything is now EFI and I won't be going back to carbs. Gene