Fuel mixture screws

[Muleskinner]

I have a 1980 GMC Sierra classic with a 350 cid motor. It has a Rochester four barrel carb, number 17063425. There are silver buttons where the idle mixture screws are supposed to be. Does anyone know what the deal is and how I access the mixture screws? Motor runs rough at low idle and backfires bad at full throttle, otherwise it runs pretty smooth in between until its under load above fifty mph. Then occasional backfire. Choke seems to work right, secondary plates are opening at full throttle. It gets bad gas mileage, less than ten mpg. I hope someone can help.

 

[legopnuematic]

Those are plugs that were installed when manufactured to keep from tampering with Idle mixture screws, there are various methods out there to remove the plugs to access the screws behind. Most are best done with carb off engine.

With your running issues, what is the ignition timing like? Retarded timing can cause symptoms similar to what you describe. As the timing set wears, it retards cam timing, since the distributor is cam driven, it retards ignition timing as well.

 

[Copakekid]

Vacuum Leak ?

 

[fast 99]

Those plugs are hardened. Carb will need to be removed and turned over. Using a small wheel make slots on both sides of the plugs. Only about 1/4" long, 1/8"th deep. Then with a small pin punch break out that piece. Should be able to get behind the plugs and tap them out.

One problem doing this is any dirt inside will be dislodged from being inverted. If the plugs are still in there it likely has never been rebuilt. Doing it now would be recommended.

 

[Muleskinner]

Thank you for your reply. I have been thinking fuel /air ratio and also looking for leaky vacuum lines. I had not considered ignition timing. I haven't done compression testing either, 70,000 miles seems very unlikely for a 44 year old vehicle.

 

[ali_c20]

I think 170,000 miles is more realistic.
Check the carb base and also the throttle plate shafts for vacuum leak.

 

[Buck69]

Idle mixture is just that and will be out of the picture once the throttle plates have opened. They will not have much effect to the overall mileage and the loaded performance you are experiencing.
In addition to what has been mentioned, a cylinder not firing can give similar symptoms as well.
If doing a compression test, take a close look at the cap, plugs and wires. Have a look at the ignition components with it running in the dark. Bad wires are usually pretty easy to spot when the spark isn't making it to where it should be going. Lay your plugs out and post a pic of them as well.

 

[Muleskinner]

This is all good advice and I appreciate it. I don't post often but I do enjoy following the conversations and questions. Owning this truck is causing me to relearn things that I had forgotten or never knew, we didn't have Google, YouTube, or informative forums like this fifty years ago. I hope all of you realize how important you are.

 

[Muleskinner]

Been working on 1980 GMC Sierra concerning rough idle and high RPM backfiring out the carb. I pulled out all the spark plugs and did compression test in numerical order. The compression numbers ranged from 140 to 90 (one cylinder). The low compression spark plugs all had wet carbon and smelled like gas. I replaced the distributor cap and rotor, fixed a vacuum leak at the distributor vac advance and replaced the dried but not cleaned spark plugs. It ran better and for a while stopped backfiring. I am sure that I have stuck lifters and open valves. Would it help to do a quality engine flush and clean?

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

 

[75gmck25]

Since you already have the plugs out, I would pull the valve covers and adjust the valves. You might also find that you have a really loose or bent pushrod, or other problems in the valve train.

Rerun your compression test, but squirt a little oil in the low cylinder. If compression comes up on that cylinder, usually it means the rings are the problem.

If you have or can borrow a piston stop, I would also take the time to verify TDC and ensure the timing pointer and balancer are right, and nothing has slipped. No sense setting the timing and then finding out the pointer is way off.

There used to be a special tool that was used to remove the tamperproof mixture screws. Kind of like a hollow drill bit that cut the casting out around the plug. Once you get the plug off you will probably find that the screws have a weird double D head shape. You can buy the mixture tool with the right socket to fit, or just replace the mixture screws with aftermarket. You can also try crimping down some small metal tubing (1/4”?) to get the right shape, and try using it to turn the screws.

 

[Matt69olds]

Since you already have the plugs out, I would pull the valve covers and adjust the valves. You might also find that you have a really loose or bent pushrod, or other problems in the valve train.

Rerun your compression test, but squirt a little oil in the low cylinder. If compression comes up on that cylinder, usually it means the rings are the problem.

If you have or can borrow a piston stop, I would also take the time to verify TDC and ensure the timing pointer and balancer are right, and nothing has slipped. No sense setting the timing and then finding out the pointer is way off.

There used to be a special tool that was used to remove the tamperproof mixture screws. Kind of like a hollow drill bit that cut the casting out around the plug. Once you get the plug off you will probably find that the screws have a weird double D head shape. You can buy the mixture tool with the right socket to fit, or just replace the mixture screws with aftermarket. You can also try crimping down some small metal tubing (1/4”?) to get the right shape, and try using it to turn the screws.

Or remove the mixture screws, and use a small cutoff wheel and cut a slot in them. Then you can use a regular screwdriver.

 

[Muleskinner]

Hey, really appreciate all the great advice on the issues with my 1980 square body. It is running better but still a little rough and still back fires out the carb at high RPM. I still need to pull the valve covers to check for sticking valves or bent rods. And maybe do a quality engine flush. Problem is for a retired person I am so unbelievably busy and it is hard to stick with it. I just wanted to let you know that you are appreciated. Thank you.

 

[Muleskinner]

I would like to ask a question. Can connecting the wrong tachometer to the tach connection on an electronic ignition distributor damage the ignition or the voltage regulator in the alternator. I connected an old SunPro Super tach II and it didn't work. I found out that you need an adapter for that hook up. After that, the voltage regulator in a four month old alternator went bad as well as it's replacement as soon I turned on the ignition. I also read that a bad battery can do that. Does anyone have any experience with this?

 

[Ricko1966]

Run a wet compression test as already mentioned. If the compression comes up on a wet compression test,NOTHING!!! is going to make this engine run right, or get decent gas milage. The numbers you have are all engine mechanical, not carbureation,ignition,vacuum leaks etc. The engine doesn't have acceptable compression. If it comes up wet,it needs rings and rod bearings bare minimum.

 

[Muleskinner]

You are right. I had forgotten that I was going to do that. I have been getting different advice from different people and I had forgotten. I get only a little time to work on it. But I will get around to it.