My 68 has been having a persistent problem with poor cold start up ( need to really pump the accelerator pedal to get it to fire up) fluctuating idle speed and inconsistent running reliability. Some times it runs okay sometimes it doesn't. I gave up on trying to locate the problem and took it to a local "hot rod" mechanic who figured it was a carburetor problem and took my carburetor apart numerous times and finally replaced it with a new carburetor. The car still has the same issues and almost left me stranded on my last outing. I have checked for vacuum leaks with no success. Mechanical fuel pump is new and I have a back up electric pump installed so the car is getting fuel. I'm stumped! Any help on sorting this out?

 

You didnt mention what carburetor, a Quadrajet?
If a Quadrajet how did you mechanic manage to install a new one, sure it isnt a rebuild?
Do you have a divorced choke set up, coil on the intake manifold with a rod connecting to the choke linkage?
Do you know how to adjust a divorced choke?
Do you have an electric choke?
What is the cold fast idle speed ?

 

The carb was a holley replaced with my original Q jet that was just refurbished by a reputable carb shop to stock specs. The carb has a divorced choke which was adjusted by the speed shop when they installed it. Idle speed is running around 600 rpm which fluctuates up or down. Adjusting idle to 750 rpm causes idle to fluctuated between 600 and 1200. This is the initial problem I had with the holley so a carb swap was tried to solve the issue which it did not. Last evening the car was running okay for a 15' run, 2 blocks from home started to sputter and lost power had to feather the pedal to limp home. Symptoms were like I was running on empty in the tank (not). I notice I have an issue with my ammeter reading "+" when driving (I did replace battery). Can a faulty voltage regulator effect engine operation? At this point I'm back to thinking its not a carb related problem.

 

FOR WHAT THIS IS WORTH:

When I work on the 1968-1982 Corvettes. I have learned a long time ago that I disconnect the vacuum at the intake fitting that operates all of the headlight/wiper/HVAC vacuum controls can cap that fitting off. In some cases I also remove the vacuum to the power brake booster just to make sure it is not causing for a vacuum leak. Keep in mind I know how to CAREFULLY drive a car where the power brake booster has been disabled and it does not take me long to confirm of the vacuum for the booster is causing the problem or not.

***DISCLAIMER**** If a person does what I just described they are taking all responsibility and all the risks and ALL LIABILITY due to I do not know if that person knows how to carefully do what I know how to do.

I them begin to make sure the distributor and ignition parts are all god regardless if they are points or an HEI. I also verify the vacuum advance is good and that the advance weights in the distributor are good and that the advance will works as designed..( because I have had some distributors where the advance portion of the distributor was sticking and not freely working)...( and the advance weights had lost the bushing that keeps them as designed on those distributors that have the plastic bushings for the advance weights).

I also make sure the fuel hoses ( supply and return)are correct and not pinched/kinked....or when and IF they are getting warm/ hot..they get sucked shut.

Keep in mind that it may be required that you allow the engine to cycle a few second to prime the bowl in the Quarda-Jet and thus...allowing the accelerator pump to actually pump fuel in the carb when you go to start it. You and you alone will have to determine how many days it takes for when you go in and take the air cleaner off...and look down in the primaries and while holding the choke plate open...then you move your accelerator linkage..DOES the carb IMMEDIATELY pump fuel. You might find that three days is when it does not squirt good stream of fuel....or it may be 7 days. WE have gotten so used to fuel injected cars that require nothing more than turning the key...that often times we forget that sometimes a different procedure is required to get the car to crank when it has been sitting for some time. And comparing your car to your friends Corvette (for example) is a pointless situation due to the numerous variables that make each of them so unique.


CURIOUS....What is your 'fast idle RPM's' at when you crank it up when it is cold?

Automatic or manual transmission???

DUB

 

Manual transmission and cold idle is around 600 rpm but I may have located the problem. or at least one of them. Driving the car today the car suddenly started to sputter and stall. I noticed the RPMs would increase to 1200 or so just before this situation starts. I flipped on my auxiliary fuel pump and within about 30 seconds the car was operating okay. I repeated this turning off the pump until the car sputtered then flip on the electric pump and it seemed to proved out the mechanical pump was not delivering enough fuel. parked at idle I could also duplicate this, RPMs would increase to 1200 or so with mechanical pump, switch on electric pump and they gradually drop to 600 RPM. The odd thing is I can drive with the mechanical pump and the car will go fine for awhile then start acting up. I'm going to swap out the mechanical pump and cross my fingers I have found the culprit. Not sure if the cold start problem will be fixed though as I have to pump the pedal a lot to get it started and I can smell the fuel so the mechanical pump must be delivering at start up.

 

so wait, you have a mechanical fuel pump and an electric pump, but you shut the electric pump off while driving?


I'd say that's at least part of your problem. why would you shut it off?

 

I WONDER if becasue you have an electric fuel pump...It is starving the mechanical fuel pump for fuel.

Secondly...(just my opinion and something that I would have to correct) If your idle is 600 RPM's when you crank it up and it is cold... your carb is not set correctly and you should be at fast idle which is about 1350-1500 RPMS.

DUB

 

There is no need for me to use the electric pump if the mechanical is working. The electric was an auxiliary that was put in place years ago for a rochester fulie unit that was installed on the car at that time. no need to have 2 pumps running now at once. Fortunately the electric was on there cause it helped me pin point this problem.

 

yup i know the idle is low. When the car was running properly with the Holley it idled around 1200 with the choke on. I'll be checking into that once I put the new mechanical pump in. Never had a problem before with the electric limiting fuel to the mechanical pump and its been in there for MANY years.

 

Do you still have your original 327? I had the idle go goofy on me, can't remember what it did, probably too low of rpm,s. I finally figured out that the 68 oil fill tube cap was leaking and throwing the engine vacuum out of wack.
DUB, have you run across this on a 68 327? Lou.

 

It is the electric a flow thru design?

Some pumps will not free flow.

You say flipping the pump on corrects the problem and earlier I believe you said your mechanical pump was new?

I'm just taking this at the simplest level- car sputters and dies(fuel starving) turning pump on corrects that.

I'd decide on using one or the other- electric or mechanical and remove the other.

Is the electric pump easily accessible. I'd just remove and replace with a piece of hose for temp.

 

Yes its the original 327. Never heard of the fill tube cap throwing vacuum off. My fill cap gasket doesn't fit very snug. didnt know that would effect vacuum. Anyone have an opinion on this?

 

Dont know the type of electric pump as its many years ago it was installed but I would imagine that it free flow as its been working fine for years without difficulty.. The mechanical pump is about 2 1/2 years old.

 

Anything is possible but it doesnt sound likely unless the fill tube had the hose to the carb. I do know that some of the fill valves had a hose to the carb as a PCV type of thing, if that was open then it would create a leak, so yes it is possible to create a vacuum leak in that circumstance. I think that was C1 or C2s, maybe 283s was going to buy one for my car cause it looked cool. Guess I won't now

 

if I'm understanding your fuel pump set up you are forcing the mechanical pump to suck fuel through the electric pump? If so before replacing the mechanical run a piece of hose in the back to take the electric out of the feed. Your electric may have internal filters that are clogged.

 

Not meaning to get this thread off of what it was intended to help on..

I have had a few issues on the oil fill tubes that have the PCV in them and the cap not sealing well. Thus the filtered fresh air that the PCV is supposed to be pulling into the crankcase is not doing its job due to there is an open area to the outside air that the PCV can draw from that is right next to it.

DUB

 

I was wondering if that effects engine vacuum? Mine doesn't have a PCV in the tube ,my PCV is into the valve cover. Never really thought a loose seal on the fill tube cap would effect anything.

 

only with the pcv in the tube connected to the carb. On its own you are correct, it will effect nothing

 

Rescue Rogers, you are wrong. A new oil fill cap solved my low idle problem. 68's did not have the PCV tube on them. Only the mid years did. Greggome, close off your oil fill tube with some clear plastic wrap and you will see that I am right. I could not believe it was the problem either. I have had my 68 coupe for 45 years in July. Lou.

 

really, wow. That makes no sense at all. Very surprising. And you didnt have a hose going to your carb that would affect the vacuum from the fill tube. I still dont see how positive crankcase pressure would affect vacuum inside the carb and intake. Can you post pics of your motor please. Thanks