Hello, So I have a 1972 oldsmobile cutlass with a 455 in it. Everything seems to be great with the motor. But lately I noticed that if I cruise steady at around 3,000 rpms in 1st, 2nd, or 3rd it chugs and jerks a lot. I cant figure it out because I can put it to the floor and it takes off just great, it only when I hold it steady that it chugs really bad. I got all new NGK spark plugs and gapped them at .45. I got a brand new Coil, Cap and rotor. And 8mm Plug wires. I have played with the carb mixture and set it to what I think was the best idle. I played around with idle so many times. So now I'm lost and have absolutely no idea what the problem is. My next step is to test the compression and see if anything mechanically is wrong. Does anyone have any suggestions because I am so lost! Thanks in advance!

 

Adjusting the idle mixture does not effect cruising mixtures. Its only for the idle. I'm assuming you have an HEI distributor and running a vacuum advance canister. What is your timing with vacuum connected at 3000 and 4000rpm?

 

I have set the timing about 4 times to what everyone has said on threads, and also by ear, either is runs super good and has no power, or rust worse and has more power. And I'll try it again and probably set it at 34 degrees.

 

What size/style carb? Have you made any adjustments to the primary circuit? I had issues with cruise rpm tug/jerk and my issue with too small a primary (size 72 -> size 80 corrected the issue).

 

I have an edelbrock 750 with automatic choke. And no I haven't, I honestly don't know a thing about carburetors. So I'm confused on what you mean by 72cc, and also how I go about fixing that! Thanks!

 

Have you checked this?

 

This is a video from Ebrock on the fundamentals of tuning a carb.

A way to sum things up for air/fuel delivery- An electronic fuel injection system uses a computer (which is constantly being fed information) to access the best time to discharge fuel into the intake manifold.

A carb does the same thing but relies on the correct combination of static circumstances to produce the needed fuel/air.

A jet is a fuel passageway. Your carb houses a constant amount of fuel that is determined by the level of the floats (again, a static condition) from those fuel bowls fuel is sucked through jets in the main carb body to deliver fuel into the intake manifold.

So, when you're crusing at a given RPM and you're getting a surge it is possible that the mix of air/fuel in the combustion chamber is light on fuel..that feel is essentially you running out of gas one millisecond at a time. In the event of a computerized system the CPU just says "oh, i'm getting information here that the mix is off, deliver more fuel" and presto. But, with a carb there is no computer and the fuel delivery is the responsibility of a constant: the size of the jets.

-When you "put it to the floor" the secondaries open up which give the motor the fuel needed to achieve those higher RPMs. Secondaries are, essentially, a fuel/air reservoir that can be called upon during moments of greater air/fuel delivery needs. Make sense?

I hope this helps and as Oldcutlass stated, you need to work out the timing first as the amount of fuel delivered has a tremendous impact based on when your plugs are firing.

 

Is it an HEI?

 

72cc jets?


Cannot let that one go...


cc is short for cubic centimeters, a measure of volume.


Jets are typically measured by the hole diameter, in 0.001" in this country [mm elsewhere], so a "72" jet [usually having a GM part number ending in '72' in a brief moment of PN-assigning cleverness at GM] will have an ID of 0.072".


One might measure accelerator pump output in cc of fuel.


All that clarified, I agree that it sounds like a lean misfire if timing changes only shift the issue from one RPM band to another.


In my limited experience, you want the needle and jet to be about 30 different- 49 needle, 79 jet. Give or take a little.


My 403 carb started out with a diff of about 20-25, can't remember, and when I changed to more like 30, it was much better.

 

Thank you for the clarity, Octania.