Looking for help fixing engine hesitation. I'm lost.

[WisconsinCutlass]

I have never tuned a motor or carb before so this is all new to me. I've done a lot of reading and think I have a good idea of whats going on but don't know the order or what I should be looking for. My problem is that from a stop if stomp the gas it hesitates for a second and then takes off like a bat out of hell once the rpms get up to about 2000. It also hesitates if I'm at a cruising speed and give it 1/4 throttle or more. I'm running a 72 olds 350 with cam, intake, headers, and some headwork. Petronix Hei flame thrower distributor and a 1800 stall torque converter, and holley 3310 750cfm carb. I tried a couple things this morning and either made it worse or the same. Here's what I tried.

1. Larger accelerator pump squirter. There was a .028 in it and I went up to a .031 This made it way worse. Almost to the point of stalling. Put the .028 back in.

2. Checked float level according to holley tuning video. Float level was good.

3. Tried changing timing a bit. Advanced timing seemed to make it worse and retard timing seemed same but smell a lot like gas. Put it back to where it was. On this motor counter clockwise is advance and clockwise is retard correct?

4. Was going to mess with vaccum advance and noticed that the vacuum line is hooked to the bottom of the carb which I believe would have a all time manifold vacuum. The holley tuning video says to hook the vacuum advance up to the timed vacuum port on the metering block if the carb. Which should this be?

5. Don't know if this is related but pulled a spark plug from both sides of the motor. Drivers side was definitely black and the passenger side was a slightly darker shade of black. They aren't good looking at all.

Sorry for the long post but would like to get this little issue figured out. Is the 750 to much carb for the motor and where should the vaccum advance be ran from? Anything else that could cause this?

 

[pontiacgp]

if you go to the holley site for your set up they suggest from 600 cfm carb to a 670. I ran a 670 holley on my 355 for a while just to make a buddy happy and then went back to my quadrajet when he wasn't looking. The holley seemed to run alright for my engine but........

 

[WisconsinCutlass]

I figured that the carb was to large but other than the hesitation and the black plugs it works great. I dont have the funds available to get a different carb at the moment so I'm trying to make do with what I have.

 

[pontiacgp]

what size jets do you have

 

[WisconsinCutlass]

I dont know what size jets I have. I didnt know if I could take the carb apart and put it back together without new gaskets so I didnt do anything with it yet.

 

[Texas82GP]

On timing, if the engine speeds up when you turn the distributor, you have advanced the timing. If it slows down, you have retarded the timing.

750 is too much carb for that Olds 350 in my opinion. If it was a 750 Quadrajet, then no but the unique design of the Quadrajet only provides as much flow as the engine needs. The plugs say it is running pretty rich. Also, the fact that when you went to a larger accelerator pump squirter and it made it worse, supports that it is too much fuel. You don't have to replace the carb to get the car running good. I am biased to the Quadrajet on these cars, especially one that has been custom tuned to the setup in the car but you can make the car run good on that Holley. It won't be as fuel efficient as a well tuned Quadrajet but it will run good nonetheless.

You can get the common sizes of jets at some auto parts stores like O'Reilly and the last time I went looking for them, they had the float bowl and metering block gaskets in stock. The blue gaskets are supposed to be reusable but that's not always the case. Be sure to match up your gaskets before you throw the old ones away. You really need to know what size jets you have in it so that you have a starting point on what jets to buy to jet it down. While you are in there, note what 'size' power valve you have in there. The number is stamped on the brass 'button' on the end of the spring, or otherwise described as the end of the power valve that inserts into the metering block. I think your hesitation could be power valve related. When you open the throttle, manifold vacuum drops and the power valve allows enrichment. I'm thinking since the larger accelerator pump squirters made it worse, that adding fuel only aggravated an overly rich condition. I think by re-jetting the carb and selecting the right power valve, you can get to where you want to be. I'm not a Holley expert but I've grown up around them. My old man is pretty good with them. He's always been a Holley man. He has a 600 Holley on his 390 in his 62 Galaxie and has it tuned as good as you can get it in my view. It is also possible that some tuning of your vacuum secondary is needed but one thing at a time.

You could buy the Holley jet kit and have any conceivable jet you would want but that costs around $60 and you'll have jets you will never use. Find out what size the primary jets are and go down two sizes. See what 'size' the power valve is while you are in there. Change one thing at a time and see how it does. Also, when you go to put the float bowl back on it, for the love of god don't over-tighten it. It is a very common mistake on Holley's and it causes warpage of the main body and the metering block. It causes all types of problems like post nasal drip.

Take a look and report back. We'll help you get there.

 

[pontiacgp]

I dont know what size jets I have. I didnt know if I could take the carb apart and put it back together without new gaskets so I didnt do anything with it yet.

the bowl gaskets aren't expensive, most holley owners have a few spare for each side just in case.

 

[axisg]

pretty sure you are running way rich. When you accelerate hard its dumping more fuel than can burn and causing the hesitation. Then once the fire is lit the car will accelerate. Stepping down a couple jet sizes is easy on a Holley, maybe then try a different accelerator pump. Do it in steps so you know what is working and what is not.
And don't feel bad as I chased this issue myself for a couple years. Car ran great at idle and good vacuum. Slow steady or from a roll it was great. Off the line it would sometimes burp and hesitate. I installed an AF Gauge in my car and I would have never guessed it was running pig rich at everything but idle. Different car now with just a few turns of the screws adjusting the AF ratio.

 

[WisconsinCutlass]

Thanks for all the advice. Saturday I'm going on a couple hour cruise with some friends in the morning and then I will be heading home to tear into the carb. Hopefully I'll have some progress to report saturday night or sunday. Knowbody has answered the question yet as to which vacuum port the vacuum advance should be ran from. Does it make a big difference? It's currently not ran from where the holley tuning video suggests.

 

[pontiacgp]

Keep your eyes open for a ail/fuel mixture gauge and when your budget allows get one. That gauge takes all the guesswork out of setting up a carb.