hesitation while driving

[abiddle]

I need some ideas. My 69 SS350 with original engine, transmission and rear with approx 74k miles on it. While driving, there is a noticeable hesitation every few seconds when accelerating or allowing the engine to deccelerate while in gear - but "at pace" it doesn't seem to do it as much. The issue has been getting worse, and it does feel underpowered. It dramatically lunges at low speed.

My first thoughts may have bee way off base. I was thinking transmission, because I also had an oil leak that was minor until after a 18 month sit in a paint shop. Now the oil pan leak is much worse, so I was also thinking I had developed a rear main seal leak - which might have been letting oil pass to the transmission. Not knowing what the result of this could be, I just put these two things together.

But maybe this is a carb issue? I had it rebuilt, and I reinstalled it myself a few years ago. And the it sat on the car unused while there were other issues. It was off the engine during a freshening of the paint. So could this be a vacuum leak issue? Maybe as simple as a gas filter? I did install a new fuel pump that had been leaking, but then that fuel pump sat for the 18 months.

I have the car at a shop getting new springs front and rear. I asked them to look at this when fixing the oil pan. Too much of a mess underneath to decide if there is a rear main seal leak. The mechanic suggested it may be the carb, says he doesn't like rochesters and suggested replacing it with a holley?!?! or having the carb rebuilt. I don't think I need to have it rebuilt again, but it could have something to do with the removal at the paint shop. Original carb.

Where would ya'll start? Everything is original equipment, except the fuel pump which is a recent replacement. Its an original fuel filter. The only known issue I have is the gas tank was removed and now the seal on top of the gas tank leaks if I fill it past 3/4.

Thanks - Andy

[BULLITT65]

Well for me, I would take care of all the known issues. Get the oil pan leak fixed, pop out your gas tank and either replace the seal or use a little grease and make sure it is seated correctly. There is no oil that goes from the motor to the trans, so you can rule that out. Even a rear main seal leak is not going to cause hesitation.
Now I can see a hesitation on acceleration, but you also say there is a hesitation when you let off the gas?
I would lean towards your carburetor or a vacuum leak. There is no need to switch to a Holley. Could be as simple as the carb gasket, or a vacuum hose leak. Just make sure you solve all known issues first. It is hard for me to solve multiple problems at once, always easier when you can narrow it down.

[abiddle]

Thanks Austin. I'm hoping someone has an Ah-ha moment based on my description and their experience with the same problem. I will definitely work on the known issues.

[BULLITT65]

Sorry, Andy there is no "ah-ha " moment coming in form of a comment, thats why you don't have 3 guys chiming in, saying "all you have to do is this..."

This is the part where if you have the money, you have to pay to have someone figure it out, or if you have been around these cars long enough you start solving problems and eliminating possible issues, and then narrow it down. The "ah-ha" moment will actually come from you , when you solve it.

We can help guide you in the right direction, based on your investigation, but without being in front of the car hearing what it is doing and how it responds to different tweaks, it is all a shot in the dark for us.

So to put you in the best possible scenario, solve the issues you know about first, then go from there.
It sounds like a fuel issue initially to me, so start by fixing the sender so it is sealed in the tank properly. Hell you may pull the sender out and realize the pickup is blocked, or functioning incorrectly. ( I would say small chance, but you have to fix the seal any ways, so might as well fix the seal and check off the seal and sender as possible issues). And thats the thing* you have to start eliminating possibilities. It is great to find the problem right away first try, but even the mechanics have to use a process of elimination on the tough fixes.

Good luck, and keep us (me) up to date .

[JKZ27]

Sorry, Andy there is no "ah-ha " moment coming in form of a comment, thats why you don't have 3 guys chiming in, saying "all you have to do is this..."

This is the part where if you have the money, you have to pay to have someone figure it out, or if you have been around these cars long enough you start solving problems and eliminating possible issues, and then narrow it down. The "ah-ha" moment will actually come from you , when you solve it.

We can help guide you in the right direction, based on your investigation, but without being in front of the car hearing what it is doing and how it responds to different tweaks, it is all a shot in the dark for us.

So to put you in the best possible scenario, solve the issues you know about first, then go from there.
It sounds like a fuel issue initially to me, so start by fixing the sender so it is sealed in the tank properly. Hell you may pull the sender out and realize the pickup is blocked, or functioning incorrectly. ( I would say small chance, but you have to fix the seal any ways, so might as well fix the seal and check off the seal and sender as possible issues). And thats the thing* you have to start eliminating possibilities. It is great to find the problem right away first try, but even the mechanics have to use a process of elimination on the tough fixes.

Good luck, and keep us (me) up to date .

X2

[arocars]

It sounds like a gas issue to me. I would replace the gas filters first. Not always, but usually there's one in the fuel line and one in the carb. It's cheap and easy.
There is no reason to swap from Rochester to Holley.

[Kelley W King]

My car has a Holley but for what it is worth it was the needle and seat for the float.

[abiddle]

The mechanic says he can see the carb leaking and that it needs rebuilt. It was recently rebuilt maybe 4 years ago, and has very few miles on it, but I imagine this can happen when a car sits for an extended time. I'll ask him to show me when I pick it up.

I can't find my receipt or any communication I had with whoever rebuilt it last. Suggestions on a reputable company?

Side note - all my angst about having new front and rear leaf springs and shocks is gone. Thoughts on this stance? I did expect more height in the back replacing the 5 leaf springs.

[BULLITT65]

Andy, if you are a little daring you may want to try and rebuild it yourself. While the Holley may be an easier rebuild, than most, your still doing the same thing (taking apart blowing out all the ports, inspecting, cleaning with carb cleaner, or similar product, and then reassembling with the rebuild kit)
This way if it starts to act up you will have some knowledge of what us going on.

Worst case scenario- you do a terrible rebuild and have to bring the carb to the mechanic anyway, so you would just be out the cost of the rebuild kit, and your time.
Best case- you learn a little bit about the rochester, and become a resource on the CRG for having rebuilt one, and your car either runs better or you eliminate a possible issue on your car.

Suspension- If you recently just replaced the springs, front and rear, there is some lag time for the springs to settle. In your pic it looks like the front sits a tad higher than the rear. The fronts will settle and come down over the first year. Some of this is dependent on what type of springs were used. You may want to start a separate thread for that.

[jdv69z]

Just my opinion, but I concur with the fuel issue idea. My first instinct says it's not getting enough fuel, so it hesitates when accelerating or when the demand for fuel increases. But sill has marginally enough supply at cruise speeds so that it runs ok.

[TRLAND]

The mechanic says he can see the carb leaking and that it needs rebuilt. It was recently rebuilt maybe 4 years ago, and has very few miles on it, but I imagine this can happen when a car sits for an extended time. I'll ask him to show me when I pick it up.

I can't find my receipt or any communication I had with whoever rebuilt it last. Suggestions on a reputable company?

Where is it leaking?  May be just a stuck float but leaking fuel onto a hot manifold is a fire hazard.  Get that fixed ASAP. Lars Grimsrud, Henry Olsen, and Cliff Ruggles are 3 of the best experts on Q-Jets and tuning. Lars rebuilt mine and it operates perfectly.

[jeff68]

Suggestions on a reputable company?

Cliff Ruggles (Cliff's High Performance) did mine.  He jetted mine for the 10% ethanol blend gas we have around here.  I couldn't be happier.  If it weren't for those secondaries opening & giving you a big kick in the butt, you'd swear the car was fuel injected.  The only issue is turn-around time.  I think mine took 3 months.

[JKZ27]

I overhauled my Quadrajet (207. still hunting a correct 203) about 12 years ago and it works wonderful. A few noteworthy things I remember are the replacement accelerator pumps can be junky and don't always fit the bore and, if I remember, not all the kits come with a garter spring for the plunger. I put epoxy on the baseplate plugs that are known to leak and I had to replace my float with an all brass because the foam/fiber float swelled from ethanol and would stick at the bottom of the bowl. Chem Dip, 2+2 and compressed air are gold!

[abiddle]

I've found a leak under the idle mixture screw. It doesn't appear that bad, but it may be the culprit. I guess any leak is bad, and who knows what it looks like while actually cruising down the road.