69 Tri Power idle issue

[Grahame M.]

Thanks Duke,

I have the CSM, AIM, Haynes Holley book and a Holley booklet that goes through Ford, Chevrolet and Chrysler Tri Power setups. I haven't set up the vacuum secondary rods yet because i need to get idle and mixture right first. I have done this before on the old set of Tri Power carbs i removed.

I do have some good guides that outline the rod adjustments but may need to disconnect the VAC pods from the front and rear carbs shafts to make sure they're not causing them to stay open a bit.

I also have a smoke machine and plenty of carb cleaner to try and find the problem area. This weekends job.

Cheers and thanks again.
Grahame

[Duke W.]

The external throttle linkage does not fully determine the fully closed position of the the butterfly valves. This adjustment is at the bottom of the carburetor, and usually must be done with the carburetor off the car and upside down. This adjustment might not be in the CSM and certainly not in the AIM , but should be in the COM. It might also be in the other docs you mentioned.

If you don't have a '69 COM maybe someone scan that section of the COM on carburetor overhaul.

Some years ago I helped a friend with a Cosworth Vega correct an issue on the EFI's dual throttle body when the idle speed screw did not have enough adjustment to reduce idle speed to spec. We slightly unstaked and loosened the screws that attach the butterfly valves to the shaft so that the butterflies were just slightly loose. Then with the idle speed screw fully backed off we opened the throttles full with a strong return spring and quickly released the lever to let them snap shut. This "seated" the both butterfly valves against the throttle bores to essentially fully close both valves.

We restaked the screws installed the TB and were able to adjust the idle speed to spec.

If whoever refurbished your carbs removed the throttle valves from the shaft this could be an issue, so doing the above and then properly adjusting the internal throttle stop screw per the COM on each carb should solve the problem.

Duke

[Grahame M.]

Great, thanks Duke. I was unaware of this but still have my old Tri Power carbs on the bench to have a good look at. I'll get some pics to make sure i understand this correctly.

Also, i always thought COM meant Chevrolet Operating Manual and was similar to the CSM, is this not the case and if so where can i get a copy. I try to do all my own work on the car other than refurbed things like carbs where they need to be replated or coloured.

Cheers
Grahame

[Duke W.]

CSM stands for Chevrolet Service Manual. COM is Chevrolet Overhaul Manual. The 1963 Corvette Shop Manual has ALL service and overhaul procedures in one book, and there were supplements published for '64 and '65 Corvettes.

Beginning in 1966 Chevrolet split the manuals into service and overhaul books and each covered the entire Chevrolet passenger car line. This arrangement was, perhaps, better for those who needed service and overhaul information for all models lines like dealers, but less convenient for those of us who are usually only interested in Corvette.

Service includes maintenance, on-the-car adjustments, R & I (removal and installation procedures) for components like carburetors, starters, alternators, engines, transmissions axles.... Then to overhaul them you need the appropriate COM for '66-up model years.

It's good that you're a hands on guy because I doubt if there are too many down under mechanics that are competent to work on vintage Corvettes, and once you have all the manuals, CSM, COM, and body manual you have a complete set for your car. Also the AIM is handy as is the vehicle information package pdf that you can download from www.gmheritage.com.

Type "1969 Chevrolet Chassis Overhaul Manual" into any search engine and you will find numerous sources, both new (reprints) and used, and it's amazing how inexpensive they are. It will pay for itself the first time you use it.

Duke

[Terry M.]

Great, thanks Duke. I was unaware of this but still have my old Tri Power carbs on the bench to have a good look at. I'll get some pics to make sure i understand this correctly.

Also, i always thought COM meant Chevrolet Operating Manual and was similar to the CSM, is this not the case and if so where can i get a copy. I try to do all my own work on the car other than refurbed things like carbs where they need to be replated or coloured.

Cheers
Grahame

Grahm
I thought COM is Chassis Overhaul Manual. I too will await Duke's response.

[Grahame M.]

Thanks Duke, this is the one? $47AUD

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[Duke W.]

That's the one!

The best USA price is about 30 bucks, and given that 1 AUD = .6413 USD, it's still a pretty good deal for you. We are fortunate to have such reasonably priced manuals for our decades old cars, and everyone should have a complete set for their car even if you don't work on it yourself because they'll be real handy for any mechanic you hire to do necessary work, and they may not be around forever.

Duke

[Grahame M.]

Thanks Duke, i ordered this for only $49USD and $13USD shipping to Oz off eBay, has all the books i need and some extras

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Thanks AGAIN for you help, i'll get into this issue more over the weekend and let you know what i find.

Cheers
Grahame

[Grahame M.]

Hi All,

Just did a quick video attached to show you whats going on before i start to pull things apart. It appears to be binding when the engine is running as you'll see in the video, but when the engines off it doesn't feel the same.

I also made sure the choke was off and car was warm, secondary rods were not attached or binding as well as removing accelerator cable.

Cheers
Grahame

[Grahame M.]

Hi All,

Just checking if anyone has any further ideas as to what might be causing this issue now I've attached a video?

Many thanks
Grahame

[Dan M.]

Grahame,
Just watched your video. I know there as been many comments. I'll simply ad that to me it looks like it is blinding up on either the butterfly's or the shaft is not thru and may need to be honed. May want to ask many questions of the person and restoration process used on your carburetors. My car is a 69 L-71 and the butterfly's move freely with little to no effort. Duke is very knowable and his comments are spot on.

Dan

[Grahame M.]

Thanks Dan,

I appreciate your comments and that is the next check i'll do, pull all carbs off and make sure everything works freely. As Duke said it may be an off centre butterfly.

I did further checks for vacuum leaks through all the circuits and manifold using my smoke machine, with only a couple of very minor ones where hoses were not pushed on all the way, propane and disconnecting all additional parts on the centre carb like choke and outside carb rods etc and appears to run free when the engine is off.

Is it possible i have the wrong manifold gaskets installed? Long shot i know.

Cheers
Grahame

[Dan M.]

Grahame,

As to the wrong gaskets anything is possible and I would need a PIC to answer that question. I think it would really need to be off. Let me get back to your video when you apply hand pressure to the middle carb the throttle returns to the hard stop you should not need any additional pressure for that to happen the spring is all you need. The spec on a L-71 4 speed car is 750. I think something is not aligned correctly on you middle carburetor; either the shaft or the butterflys.

Dan

[Grahame M.]

Thanks Dan,

I'm of the same opinion so i'll come back to this post with pics of the gaskets once i have the carbs off and also ones of the butterfly positions from the base. I'll check all three at the same time just to make sure. I probably won't get a chance to do this until next weekend as i'm away with work this weekend.

Thanks again for you help.

Cheers
Grahame