Repairing stripped Idle Mixture screw hole in throttle body

[Reaper19]

Well, I had the car running almost smoothly for the last month or so after adjusting my rebuilt E4ME that a local carb builder completed. Today I was taking my 81 Corvette out for a ride and the next thing I know the idle jumped up to 1500 and running rough. After checking all connections for gas or vacuum leaks I found one of the idle mixture screws popped out of the throttle body. Come to find out it will not screw back in because it is stripped. I also found the secondaries did not close all the way everytime leaving about a 1/16 inch gap, which could explain some of my rough idle issues.

Anyone know how to repair a striped screw hole? Or will the throttle body need replacement.

Thanks

[Ethan1]

 You could either find a used one or send it to Cliff. What carb # are we talking about? Maybe a Helicoil?

[Reaper19]

It is a 17081228 carburetor. The base is 707225. It is for a 1981 corvette with the black M/C solenoid. Do you know the hole size and what Helicoil might work? Do you know of anyone who sells the Helicoils?

Thanks

Randy

[Ethan1]

It is a 17081228 carburetor. The base is 707225. It is for a 1981 corvette with the black M/C solenoid. Do you know the hole size and what Helicoil might work? Do you know of anyone who sells the Helicoils?

Thanks

Randy

 I cannot remember what thread pitch was used for the mixture screws on the later carbs. I know the 10-32 was widely used. Any vendor that sells helicoils, should be fine. Do you have a thread pitch gauge?

[Reaper19]

Thanks for the replies. I do not have a thread pitch gauge, but probably can borrow one from my tranny mechanic. I decided to see if the carb builder would repair since I've had multiple issues with the carb since the rebuild. Took the carb back to the local carb builder who is looking it over. He mentioned possibly replacing the base with another one from the same model carb, so I'm waiting to see if he decides to repair or replace the base or come back and tell me he will not repair. He is determining why the idle screw would have been stripped, but after all the other issues with the rebuild and taking it back 7 times I am beginning to lose faith in their abilities on electric carbs. Wish I would have found this site first and just sent the carb to Cliff to rebuild in the first place.

I'll let you know how it turns out.

[Cliff Ruggles]

Easy fix, take them up to 10-32 threads on both sides and use the pre-1979 style mixture screws.....Cliff

[Ethan1]

Easy fix, take them up to 10-32 threads on both sides and use the pre-1979 style mixture screws.....Cliff

 I would go with Cliffs recommendation.

[Reaper19]

Thank you Ethan and Cliff. I was able to talk to the local carb rebuilder that did my carb and he agreed this is something they should have caught in their rebuild, so they replaced the baseplate. Problem now after installing the new base plate the throttle shaft has a lot of play in/out as well as side-to-side causing the idle to jump around, TPS  voltage to jump around and not stable. Looks like the baseplate was not re-bushed at all. Also after testing on the car, the carb is now leaking fuel through the throttle shaft onto the intake, so it must also be flooding. So to say I am not satisfied is beyond description. I have taken this carb back 10 times to this place and every time the original issue is not repaired, worse or another issue arises. So I am taking this carb back one more time on Monday and give them one more chance to get it right, otherwise I will ask for a refund and send it out.

Cliff, these are my specs. Hopefully my engine builder did not steer me wrong and I can still get the E4ME running with the build. Would you be able to rebuild the E4ME to run with this setup?

1981 Corvette Specifications
Carburetor – Model 17081228 with Black M/C solenoid and electric choke

Motor: Chevrolet 350
-   Bored over 0.30
-   Pistons EngineTech P15334 Flat Top
-   Cam EngineTech ES1014R – These are the numbers that are close. The builder did not provide exact specs and these were pulled from the internet for the Cam with make and model that was installed
o   Stage 1
o   Hydraulic flat followers
o   0.50 Duration
   204 Dur. Int.
   214 Dur. Exh.
o   Cam Lift .280/.295
o   Lob center
   C/L Int 107
   C/L Exh 117
-   Heads - Chevy 333882
-    Intake – Original factory Aluminum 14033058
-   Exhaust – Original factory design 2-1-2 with Magnaflow High Flow Catalytic Converter
-   Transmission TH350C
-   Rear Differential gear ratio 2.87

If you can, what is the turn around time or approximate cost? If you rather discuss on the phone I can contact you during the week.

Thanks Randy

[Cliff Ruggles]

Good news on the cam choice, it's the standard Melling/Speed Pro "RV" cam for the small block Chevy.  Very good cam for 350cid engines around 9 to 1 compression or so.  It will make PLENTY of vacuum at idle and low speeds to keep the computer happy.

If they can't get the carb working correctly, call the shop and we can discuss options, time frame, etc.....tks...Cliff

[Reaper19]

Thank you Cliff for confirming the new parts installed during the engine rebuild will work properly with the CCC!  i have kept telling the carburetor re-builder something is wrong with the carb and the engine re-builder new what he was doing having a drag racing background and building tons of these engines over the last 25 years. But now after returning it 12 times to the carb re-builder and each time having it come back with a different issue I have finally told him that was it, I was finding someone that can help get the car running properly.

The carb is the correct original model for the year and engine for the car, it just needs to be properly gone through and set up correctly. After fighting this thing since the beginning of March 2015, I read more on rebuilding and adjusting than ever. I have some of the tools required for adjusting and rebuilding, and borrowed others from a mechanic friend, the only one I do not have is the gauge for setting up the lean screw. But probably can try to find one online or create a gauge for the proper height adjustment. I may, with your suggestions try to rebuild the carb again from scratch myself, unless you feel it would be best to send it in. 

I also wanted to mention I have gone through checking all sensors, switches, wiring, looked for vacuum leaks, changed out all vacuum lines, vapor canister, TPS, M/C, EGR, CAT, thermal switches, replaced all ignition items, except HEI coil, removed the air pump and have an OTC Monitor 85 to check all systems. All are checking out good with no codes. THe car runs great at WOT and cruising, the only issue is the engine will not idle correctly, bounces up/down, rough, not steady or smooth. Also, when adjusting the idle mixture screws they have no affect on the RPM's at all. So there is definitely something going on with the carb.

I called the shop and left my number, look forward to hearing from you.

Randy

[Cliff Ruggles]

Randy, call volume was INSANE last couple of weeks, might want to try again next week, hopefully things will slow down just a tad. 

I missed a lot of calls, and didn't get all the messages as it's often difficult to hear them on the answering machine.....Cliff

[Reaper19]

Thanks Cliff, I called back and you have been very helpful listening to the problems and advising steps to try and correct. I went ahead and so far took out the mixture screws, IAB, and removed all vacuum hoses from the carb and sprayed carb cleaner through all openings before popping the top. I did notice some gunk come out of the mixture screw holes. I reconnected everything and set the mixture screws starting at 4 1/2 turns out to start and the IAB at bench setting.  After warming the engine up I set the timing again, this time a little higher from 6 to 8 BTDC which is where the engine builder felt the new cam liked best and set the dwell at 30 degrees using only the IAB for fine tuning. The car runs better at idle, and I was able to get that set around 675-700 RPM's with less of a miss or roughness from the engine, cruising and WOT are more responsive also. The idle is not silky smooth but better, so I'm going to see how things go, if the idle tubes may have a little gunk in them as you suggested, I'm hoping they may clear without popping the top.

If the engine idle does not keep getting better as I drive, I'll try to clean again and gradually turn the mixture screws out a 1/4 turn at a time if necessary up to 5 1/2 to try and smooth the idle out more. I'll get in touch again to order a new rebuild kit if things don't keep getting better.

Thanks again, what a great site, forum and service you are doing for us gear heads. Way to go Cliff!!!!!!

[Cliff Ruggles]

Good news!....Cliff