Carb problem or Fuel Pump Problem
 

ok. so here is my problem that I need help with... I have a '69 C3 that I pulled out of a barn in October. (See my other post https://www.corvetteforum.com/forums...-corvette.html).

The engine is not original, it is a mishmosh of different parts, having been rebuild by previous owners at least twice. As far as I know the heads and carb are original, the lower block has been replaced with a mid 70's pick-up small block 350, with an RV cam. (This is all what I was told by previous owners of the car that I tracked down and spoke with.) It has a Rochester 4bbl Quadrajet (#7029207, which fits with the original year and make.) She ran when I got her, although a little rough. The only thing I have done to the engine itself is change the oil, spark plugs, and plug wires. I still need to do the rest of the ignition, cap, rotor, points, coil, etc. I also know that I will need to rebuild the carb, if nothing else, just on general principle. I have replaced the gas tank, and when I did, I found the tank that was in the car did not have a fuel return line, so that had been capped off at the frame. I reconnected it when I replaced the tank.

So the new problem that I have is that it runs ok at low RPMs, up to about 3k, but once I get up to about 3500 RPM, if I have my foot all the way into the pedal, she will start to cut out. Power will drop for a half-second then return, drop and return, and will continue to run like that. Once I let off the gas, and drop to 3k RPMs, she runs ok again.

So, I am assuming I either have a carb problem, and need to rebuild it; or I have a fuel delivery problem. Possibly the fuel pump, or maybe even a bad fuel filter, although the fuel filter is reportedly brand new, and looks brand new.

So... any thoughts from the gear heads?

Seems more ignition related to me. I would do the ignition components that you mentioned that haven't been replaced yet and see how it runs.

I am in the same boat as you are on my 79. Waiting for parts to progress. All ignition apparatus, including distributor and carburetor are brand new - in my case I will start with fuel pressure measurements to see what is happening. My problem shows up when I am driving and reaching over 3K RPM. At idle, revving up the engine, I have no issue. It will be interesting to monitor your progress.

I'm inclined to agree with Sunflower-- go ahead and change the points, condenser, cap and rotor. I would also change the primary lead between the coil and distributor. IT moves every time the vacuum advance does. And they have been known to break.

Not to cause a commotion, but have you checked the valve springs for a broken one or a soft one.

Last summer I helped a friend who had a similar problem. Pulled the valve covers and one spring was broken. The car ran fine up to 3300 and then start to act up.

Also be sure the gas cap is vented and not clogged. Sealed caps started in mid / late 71

You say the gas filter looks new, are you aware there is a second filter in the carburetor fuel inlet, or at least there was one originally.

I figured that out yesterday, as I was looking at rebuilding the carb. Thanks for pointing that out!

Anything is possible... but I want to start with simple first. But pulling the valve covers, cleaning them, and replacing gaskets is easy, and probably a good idea anyway. Thanks for the suggestion.

Gas cap looks ok, but is there a way to test it?

Good point. I also picked up a new coil, just to be safe, and I will definitely check that lead.

Agreed.... shared knowledge will make things go faster. :willy:

In any situation like yours, you always check and verify ignition and timing first, and then move on to the fuel system: Timing and ignition will affect the carb operation, but carb will not affect ignition operation. So make sure you have a good set of points (I recommend running the NAPA/Echlin CS786) along with a fresh condenser (use the RR175). Verify your vacuum advance is working correctly - if it's that old, it will probably be ruptured or seized, so replace it with part number VC1765 and hook it up to manifold vacuum. Verify/set initial timing in the low-teen range (about 12 - 14 degrees as an initial starting point), and then verify that the centrifugal advance system is working by rev'ing the engine up and verifying that the timing advances correctly. Re-adjust the timing by setting this total pegged-out timing (with vacuum advance disconnected) to 36 degrees.

If the problem still persists, you likely have a fuel delivery problem. Even though the in-line filter appears new, I would install a new one. Also, remove and replace the in-carb filter. This filter is likely the cause of any fuel starvation problem. NAPA part number 3051 for a good filter.

If the problem still persists, it is likely that your carb's secondary airvalve is set too loose. Either send me the carb for setup, or set the airvalve spring tension to 3/4 turn to prevent premature opening of the secondary airvalve. If the airvalve opens too quickly, the car will fall on its face when tipping into the secondaries.

It's unlikely that you have issues with valves or springs - you have a tuning and/or fuel supply problem. Take care of the items noted and the car will run great. :thumbs:

Lars

Great info! I am a backyard mechanic, and I can hold my own with surficial items, but I tend to get lost when we start digging deeper, so this helps out a lot. I will do as you recommend, and start with finishing the tune up. I will replace all of the ignition parts, and both fuel filters, amd then check the timing and vacuum advance on the distributor.

Did you say to send you the carb? Would that be for a rebuild, or just adjustments? I am considering rebuiling the carb myself (that would be a first for me, but there always has to be a first time), and if I screw it up, I can buy a new one.

Can you recommend a good "how to" article, book, or video?

You can't buy a new Q-Jet carb. Any commercially rebuilt Q-Jet is complete junk. If you drop me an e-mail request I'll send you some info.

Lars
V8FastCars@msn.com

e-mail sent!

Info sent! :thumbs:

Well... all I can say is... HOLY CRAP! You were all absolutely dead-on right! This was absolutely an igition problem!

After reading the info you guys provided, I decided to go ahead and finish the tune-up. I had already replaced the plugs, and plug wires, but now it was time to do the rest.

So, I replaced both fuel filters (the "new" inline filter and the carb filter), distributor cap & rotor, points, condensor, and for good measure I replaced the coil as well. The only points that were immediately available were unipoints, so I wasn't able to use a matchbook cover or feeler gauge to set the point gap, but I eyeballed it close enough to get her running, and then made some adjustments to the dwell through the distributor capwindow, as best I could without a meter.

All of this made her run very well... at low idle. But anything over 3k RPM, and I had the same trouble.

I don't have a timing light or a dwell meter. (Before you say anything, yes I will end up buying both, eventually.) But I also don't have much experience adjusting either of those either. So instead of buying the equipment and futzing with it for weeks to get it right, I took it to my trusted mechanic and left it with him for the day.

He not only adjusted the dwell and the timing, but the carb as well. He also adjusted/fixed my throttle cable/linkage.

WOW, what a difference! I enjoyed this car before, but honestly, was a little dissapointed with the power and responsiveness. I thought my engine was sad and tired. Now, this cat ROARS down the road. My challenge now is how to avoid getting arrested!

Thanks to everyone, ESPECIALLY Lars, who was very patient when explaining carbs and ignition to me, and providing a BUNCH of great material to read. You all saved me a lot of money, and more valuable time, chasing down non-existent problems.

I can't wait to head out to Camarillo, CA on Sunday and workout with Ruthless Racing Events! (https://www.corvetteforum.com/forums/southern-california/4088864-ruthless-racing-autox-sunday-feb-18-a.html)

Good job taking care of the issues - hope you have some fun with the car!! :thumbs:

Lars

Thank you sir!

e-mail sent !

What e-mail?