Hi everybody,

I have a 425 cui Oldsmobile Super Rocket motor (with a bigger cam from Crane cams) in my car. I bought my car a month ago. The car drives great, but it's running a bit rich and idles a bit too high (at about 900 RPM in neutral).

I wanted to adjust the idle, so I decided to turn the idle adjustment screw so that the engine would idle at a lower RPM. When turning the screw the engine started to shake/bog. Then I turned the idle mixture screws so that it would idle smoother. The car idled at about 700 RPM in neutral, 600 RPM in drive. When I was satisfied with the result, I started driving around. After a while I pressed the throttle to about 3/4 to all the way down. When I stopped the car I noticed that the idle was 900 RPM again.

The next cold morning the car started only with choke opened, else it would not start and I drove about a mile at 700 RPM before the car started to idle at 2000 RPM in neutral, 1100 RPM in drive (the engine was still cold). When I was driving I had the feeling the throttle was stuck. It would bring the idle back to 700 RPM. So I tried turning the engine off when it was warmed up. That wasn't possible. When I turned off the ignition and removed the key the engine would not stall, but kept bogging. When I put the key back in and put it on ignition it started to idle at 2000 RPM again. I looked under the hood and I'm guessing that my throttle return spring isn't that great anymore and that I should replace it with a newer and stronger on.

My questions are:
- How do you properly adjust a Rochester Quadrajet carb?
- When do you adjust the idle adjustment screw?
- Is the throttle return spring causing the high idle that randomly occurs?
- Is it bad when the engine shakes that much?

Regards,

Robert

 

Sounds like you need an exorcist. Start with the basics first though, make sure the throttle is being closed you won't get anywhere until that is established. Make sure the throttle shafts aren't real sloppy in the baseplate that could be a pretty good vacuum leak if they are.

 

Check out a thread I started (help me tune the quadrajet on my 455), another member put a link there that was really helpful.

 

Make sure your choke is engaging correctly when the engine is cold, one full pump of the gas pedal will set it. Then adjust your hi idle speed as required, there are multiple steps, and one adjustment screw. Each step becomes available as the engine warms, but you have to tap on the gas pedal to get it to drop down to the next one.


Once the engine warms make sure the choke is completely disengaged and then you can st your curb idle.


Make sure your linkage is not binding.

 

425...
what kind of choke- coil in black plastic round housing on the carb, or remote coil mounted in the intake?

It sounds like you used to have the intake mtd [divorced] choke but either it's not hooked up right [or at all] or the coil spring is weak. As a result, the Fast Idle cam is coming on and off randomly instead of as designed, changing your idle accordingly. You need a carb 101 class maybe? There's only like 6 systems in the carb, so it's possible to learn what each and every gadget on there does, and how, and therefor assess whether it is behaving correctly or not.

Photos of your carb's left and especially RH side would be helpful.

 

I forgot to mention that the choke is manual with a cable running to the dash. I will be posting photo's later today.

I think the throttle cable/shaft/return spring mechanism isn't working properly anymore. Because when i gave it a good wack under the hood, the idle dropped back to 700 RPM. I didn't even touch the choke.

 

Sounds like the fast idle cam is sticky, or maybe some part of the manual choke cable is interfering with it.

Check for binding, give it a good spray with GumOut, and see if it works better.

Also, if you want to know how to adjust the carb, why not read the section on carburetor adjustment in the Chassis Service Manual?

- Eric

 

Sounds like you have either a cable or linkage binding issue. Agree with THe above to give it a good cleaning and inspect.

 

Everybody thanks for the replies. I've added some pictures of my carb.

 

Now that's some engineering.


First your choke cable sleeve seems to floating, it really needs to be clamped down to something. I'd use the rear carb mounting stud. You should not need a return spring on the chock side.


Disconnect your throttle cable and make sure it's not binding and make sure the throttle is returning to idle without it connected. It looks like you have some built in friction with the guide plate.

 

Thanks for the replies. I will look into it.

What are your opinions about the air cleaner? Should I replace it with a paper pan type cleaner? I have heard that the foam can drop into the carb. I don't know if I have enough hood clearance. For that I also have to look into.

 

I don't know why they still sell that air cleaner, IMHO it's junk and you are correct the foam deteriorates and goes into the carb.


I use this one in limited clearance situations, you may want to do some measuring first.


http://www.ss396.com/mm5/merchant.mv...ode=AIRCLEANER

 

I agree with Oldcutlass. You should not have a spring on the choke. I think when you have that high idle the spring pulls the choke close and it sets the idle cam to high idle and it closes the choke and that will make it run rich. Take the spring off and see how it works.

 

For those who are wondering, this is the carb number: 7026250.

I got an air filter from a buddy of mine. I'll take a look in the weekend if it will fit. I'm guessing it would, but need to check to be sure.

If it fits how much clearance do you need between the hood and the air filter? Regarding the tilting of the engine.

 

FYI, I have had one of those foam air cleaners catch fire (carb pop/backfire) and nearly burned my car up. I have also read similar stories of others who have had them catch fire.


I would never run one again.. Not to mention the deterioration issue.

 

What Oldsmobile is the engine installed in, does your application require a low profile air cleaner? An Oldsmobile Q-Jet would clean up the install.

 

7026250 is an olds application number, '66 400/425 exc, ccc/a.i.r.


bill

 

It's not really an Oldsmobile. It's a 1979 Pontiac Firebird with a different hood. But because of the engine and carb I posted my question on this forum.