Another weird idle problem

[spandit]

Took this short video of my truck idling today:

http://youtu.be/tOAK3mCBlNQ

It's not pretty, but it is running without me feathering the gas pedal.

I then took the top cover off and attempted to balance the carbs with one of those air meter tool things:



However, when I placed it over the top of the carb, the engine started accelerating. This happened on both carbs and it took the needle off the top of the scale so made it impossible to make any adjustments.

My guess would be that it was running too lean and that restricting the airflow would richen it up enough to start racing like that (I hadn't wound the throttle stops all the way out because I just couldn't get it to idle like that...)

[M Wehrman]

Loose-cracked intake manifolds? Under the tin,shoot some carb spray and look for idle changes. Are the carbs tight between t-plates and body?

[M Wehrman]

Sorry,didn't look at vid,still ck for vac leak,or maybe it is simply that the mix screws are too far in!

[spandit]

I'll tighten everything up (and might make some more gaskets for the bit between the carb top and the throttle body). Mixture screws have been the bane of my life - replaced them all as had broken all the old ones but in doing so, had broken through the carb wall. Glued up with JB Weld but difficult to tell how accurate the metering is now. Roll on EFI!!

[Jim LaGuardia]

Spandit, you have to start out with proper tooling or you will never diagnose it. Your syncro meter looks too small and if so, cannot flow the volume of air needed.(meters measure and do not restrict flow in the process)
As I have posted before, good carbs balance between 6-10 Kg/h on a healthy engine. If you are not in that window you have leaks, worn parts. One other item you seem to have overlooked is the throttle shafts. If they are brass, I would suspect twisted/worn shafts(can cause unstable idle). Brass ones can be replaced with new steel ones as long as the throttle body shaft bores are not worn out.

[spandit]

I'll borrow a bigger one! Throttle shafts all move freely & look like they seal properly when closed

[undysworld]

Robin,

I believe that what Jim's referred to there is wear between the throttle shafts and the throttle base itself. Although the butterflies may be properly aligned and close properly, you may have air leaking into the engine around the throttle shafts, if things are worn. When the butterflies are fully closed, this worn shaft/base will allow air to slip around the throttle shafts, thus leaning things out too much.

Early throttle shafts were brass and they can wear to the point where they are too small for the bores they move in. Newer shafts are steel, so the base itself probably wears faster.

[spandit]

I'm not convinced my throttle shafts are worn but I'll have a look. They're steel shafts. It's a moot point anyway as I'll be installing EFI soon