Tri-Power Idle

[cutiger]

I'm having a tough time getting the center carb to idle at the correct (reasonable) RPM all the time. In the shop, I can warm the engine and set the idle and it runs great. While driving, when I come to a stop, the engine idles up about 500 RPM above the initial setting. If I tap the accelerator and sit there long enough (sometimes 2-3 minutes) it will eventually idle back down. My first thought is that it was a idle return spring tension problem. The spring is correct. I took a couple of wire ties and compressed several coils of the spring (I'm up to about 5 now) to increase the spring tension, but it hasn't worked. When I manually return the idle arm on the carb to its set idle position (while the engine is revving 500 over) it seems to take a lot of force to move it very little. I suppose I could put a wicked return spring and solve the problem, but I'm afraid that it will significantly affect the accelerator action and feel. Any suggestions?

[cutiger]

I'm having a tough time getting the center carb to idle at the correct (reasonable) RPM all the time. In the shop, I can warm the engine and set the idle and it runs great. While driving, when I come to a stop, the engine idles up about 500 RPM above the initial setting. If I tap the accelerator and sit there long enough (sometimes 2-3 minutes) it will eventually idle back down. My first thought is that it was a idle return spring tension problem. The spring is correct. I took a couple of wire ties and compressed several coils of the spring (I'm up to about 5 now) to increase the spring tension, but it hasn't worked. When I manually return the idle arm on the carb to its set idle position (while the engine is revving 500 over) it seems to take a lot of force to move it very little. I suppose I could put a wicked return spring and solve the problem, but I'm afraid that it will significantly affect the accelerator action and feel. Any suggestions?

[Blackcat]

Cutiger,
What year is your setup? I have had lots of experience with tri-powers. Have you checked to see if the secondary carbs are shutting all the way. If they leak air the idle will go up.

[TinInjun]

cutiger:
Sounds like the steel throttle shaft has worn out the aluminum throttle body. It wears it out in an oval shape usually in the direction the throttle return spring is pulling. When this happens, the throttle plate will hit the side of the throttle bore keeping it from returning to the idle position. To fix this you have to get the throttle body repaired by installing a bushing in the throttle shaft hole. Its an easy fix, and most any carb shop can do it. To check and see if this is your problem, grab the throttle linkage and move it fore and aft. If it moves alot, there is your problem. Don't add a heavier springs as this makes the problem worse. Good luck with that pretty goat.

6-0 and still going
GO TIGERS

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"Run wild with an Indian, not with the crowd in a Chevy"
Check out my restoration at http://members.aol.com/jetmech15/gto

[cutiger]

Thanks TinInjin, I'll check this out and let you know. If this is the problem, is this something that I can fix myself or does it require boring to the size of the bushing and then pressing in the bushing?
Blackcat, it is a 66 tri-power that the previous owner installed on the 67 400 block. I have not checked to see if the secondary carbs are shutting all the way. I will check this out also.

[Blackcat]

Cutiger,
Check out TinInguns idea. Most of my idle problems came from vaccume leaks. The next big one was the secondary carbs leaking down into the intake. Use a flash light and look down the carb when idleing and watch for gas dropping onto the throttle blades.
Hope this helps!

[Ccass]

Also, Rochester introduced a throttle plate sealant called DAG-213 that was applied around the edge of the plates to solve the very problem you mention. It might be as simple as that. Now finding the stuff....hmmm. Carbking, you sell that stuff?

[george kujanski]

I've done the throttle shaft fix on Qjets several times. What you need is a 11/32" reamer to ream out the hole, some brass tubing (11/32"outside, 5/16" inside), a 5/16 reamer to finish the new bore, and preferably a drill press. Most throttle shafts are 5/16" dia.

I got the original kit with precut bushings and the reamers from The Carb Shop, I believe, but I think the brass tube is standard in those dimensions from a hardware store. If so, all you need are the reamers.

Remove the throttle plates from the shaft. You may need to grind down the ends of the screws because the factory swages them after assembly to keep them from loosening up and falling into the manifold.

The important thing is to line up the body with the press carefully so the new hole is not misaligned.

I line up the throttle body in a vise on the drill press and line up the holes with a 5/16 drill. Tighten the body in the vise and change the drill bit to the 11/32" reamer and have at it.

Regards, George

[carbking]

Chris, no, we have no current source for the DAG-213, but we have been looking for years, and still are looking. If one is careful with the throttle bodies, it isn't necessary (my 1964 with dogmatic trans idles in drive at 550 RPM the hottest day of the Missouri summer). When doing the throttle bodies, remember the design specifications on throttle shaft to throttle body clearance are 0.004 - 0.006 inch. Our own internal shop tolerances are 0.004 to 0.005. Measure this clearance with a dial indicator in the direction perpandicular to the shaft. If the clearance is greater, remove the shaft and mic the shaft for being oval. If the shaft is round, but undersize, it can be 'flash-chromed' to bring into tolerance (or bushings can be installed). If the shaft is out of round it should be replaced (Chris - haven't checked. Do you carry these?). If bushings are necessary, it is important NEVER to ream oversize clear through the throttle body wall. Drilling, or reaming clear through will leave a small triangle of a vacuum leak where the square edge of the bushing meets the round curvature of the throttle bore. Instead, center the throttle body in a mill using a pilot shaft (this can be accomplished on a good drill press equiped with a indexing milling vise). Then mill (or drill from one side stopping to leave about .060 to .070 wall thickness protecting the curvature of the throttle bore. Press in the bushing, repeat from the opposite side, and THEN ream through with the standard size reamer.

I think cutigers problem may be related to sticking outer throttle shafts, but more likely the choke setting, as he states that the idle will eventully come back to normally if he taps the throttle. Assuming the choke plate is not in a vertical condition, the fast idle cam could be engaged on the lowest step, causing an idle around 1000 RPM. Idling at this RPM for a couple of minutes could result in sufficient heat opening the choke fully, which would kill the fast idle when the throttle is depressed. Will post if I can locate a source for the DAG-213, which allows for less precision when doing the end carbs. Jon.

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Good carburetion is fuelish hot air.

[Old Man Taylor]

The same idling symptoms will be experienced if the bell crank on the center carb is no longer staked onto the throttle shaft well. It starts wobbling and will not return to the same place. You can test this by testing for "wiggle" and re-seating of the idle position. This is a common occurence on carbs that still have the original factory assembly (staking).

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Jim

[hozy]

One of my end carbs also leaks air. It runs fine just idles a little rough. Will it hurt anything other than the idle if I keep it that way??

[Blackcat]

Honzy,
The car will be running lean. I would take the carb and work on it untill it gets the best seal. Take off the base and hold up to a light and try to get the least amount of light showing around the edges of the throttle blades.

[Old Man Taylor]

Hozy - I will e-mail you a procedure I have posted before.

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Jim

[llucas]

CarbKing, you can purchase DAG 213 from Acheson Colloids Company. They are located in Port Huron, Michigan. Their phone number is (810) 948-5581. The cost is $114.66 per quart. Now, how about posting a tutorial on how to use this stuff to improve the idle on my Tri-Power?

[Old Man Taylor]

I tried the hand application of the DAG 213 when it was available from PY. I was not happy with my ability to get it on right. You paint it around the butterflies of the end carburetors to assure no air leaks by the butterflies. It just goes on the end carburetors, and it goes on both the butterfly and the throttle body. I have been happy with very carefully adjusting the fit of the end butterflies to minimize these air leaks. I believe I mailed that procedure to you. I do not know what process the factory used to apply this material to the carbs.

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Jim

[carbking]

llukus - thanks for posting the source. We don't use the stuff. We center the throttle plates (see Old Man Taylor's post) and have not found it necessary, but I am glad to know where to send questions for it. However, tripower idle is no different than idle on anything else. Often the tripower gets the blame, when in fact a vacuum leak or other problem) exists elsewhere. Our suggestion is always to eliminate unknowns until you find the problem. With tripower, simply cut out two steel blockoff plates that you can slip under the front and rear carburetors. NOW try to adjust the idle. If the idle will adjust, you need to work with the end carbs. If it still won't adjust, find the vacuum leak and fix it. Once you have a smooth idle on the center only, remove the blockoff plates. If the end carbs are the cause of the problem, often it has nothing to do with the plates, rather, the front to rear rod is not adjusted so the 'slot' end is located in the center of the 'slot'. Make this adjustment. If this doesn't do the trick, then use the blockoff plates on one end carb at a time to determine which is faulty, and concentrate on that one. For what it is worth, my 1964 (see the picture on my profile) with tripower will idle in drive (yes, it has a dogmatic transmission - darn) at 550 RPM the hottest summer day we have in Missouri, and there is no sealer on the throttle bodies. If all of the above fails, give me a call during normal office hours. Jon.

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Good carburetion is fuelish hot air.