Hello Forum,

I purchased a 1969 tri power Vette last year and have been working on it ever since (not complaining!). I solved many an issue thanks to the people on this forum. The flow of information is amazing, and so much better that when I was last in the hobby as a youngster back in the 80s.

So, I'm trying to understand something that I don't like about the throttle response of the car. I'm not taking about what one might normally be concerned about - does the engine respond quickly when you open the throttle, it does. I'm talking about what happens when you release the throttle. Currently, when the throttle is released, the rpm drops back to the idle setting slowly rather than quickly as it seems it should. This happens whether you're just snapping the throttle by hand in the driveway, or actually driving the car (so independent of load, throttle cable, secondaries, etc.).

The throttle return spring is fine. The throttle returns to the idle stop quickly via the throttle spring, but the drop in rpm significantly lags (about 2 seconds). The rpm does not hang up at the high level then drop suddenly to idle; it just drops at a slow consistent rate, back to the idle rpm.

To trouble shoot I've disconnected the outboard carb return linkage, and tied the outboard carb throttle blades closed, but the problem continues. It seems tied to the center carb, unless the outboard cards are pulling fuel through the transfer slots or something.

I assume that his can't be how the tri power system operates, but am I wrong? Any insights/ideas much appreciated.

Thanks,

Keith

 

Is the engine totally stock?
You checked for vacuum leaks, disconnected the feed to headlights and wiper and HVAC?

 

Yes, the engine is totally stock except that I've given it considerably more advance at idle (vacuum advance connected to manifold source not ported) to keep the throttle blades in the correct position, etc.

I did not think to look for vacuum leaks. I will defiantly put it on the trouble shooting short list.

Your response also just got me thinking that it could be advance related, either mechanical or vacuum, somehow hanging up a little bit.

Thanks for the reply.

 

That except about the distributor could be your answer or the vacuum leak.

 

It sounds like it could mechanical advance sticking or combo of weak Springs and sticky weights on the advance.

 

Do this and stick a vacuum gauge on it.......rev it a few times.....does the needle bounce?
Go around at idle with an unlit propane torch to "sniff" out any vacuum leaks......if there is, the vacuum will suck the propane in and idle will change.

Jebby

 

I did modify the vacuum advance to limit it due to running so much initial timing, I may have bungled it.

I will investigate both the advance system and the vacuum leak theories, thanks for the quick replies!

I assume that based on your relies that having the lag I'm experiencing when I release the throttle is not normal for a tri power setup.

Keith

 

It's not normal for any engine

 

Your on the right track. Did you get the paper from Lars? I have been
battling this for a couple of years. But it is worlds better. It will be
worth your time.

 

I have read several white papers from Lars. I set up the timing using his suggestions. I also read his article on how to tune a Holley. I have never found a post or article by Lars addressing the issue I'm having regarding the slow return to idle RPM when the throttle is closed. I love to hear more about you journey if you had this issue.

I just confirmed that I have no significant vacuum leaks by plugging the primary vac hose at the engine. No change in idle quality, and no change in throttle response. I still need to check for leaks at the carb/manifold.

I also disconnected the vacuum advance, no change in throttle response. still need to pull the distributor to check if the mechanical advance is sticking.

Thanks for the ideas.

Keith

 

You are sure BOTH outboard carbs throttles have snapped closed and revs still dropping lazily? They are vacuum actuated like holley back barrels but not as well connected to the center carb to be mechanically closed in sync with the center carb.

 

Another guy with worse version of same issue, i think. https://www.corvetteforum.com/forums...ould-i-be.html

 

Thanks for the reply. Yes I am sure both end carbs are closed. I have the issue when I just snap the throttle in the driveway with no secondary operation.

 

Thanks for the link to the "worse version," thank goodness I don't have that issue, I'm totally sure the throttle blade return linkage for the end carbs is working.

 

You would do best to email Lars and ask him that question. That way you will
get the correct information. Good luck.

 

Good idea for sure, I emailed him yesterday, hopefully he'll have time to reply. Thanks.

 

How heavy is your flywheel? Sounds like an issue with inertia to me.

 

Only thing I can think of to add to the above would be to look for a leaky/worn out throttle shaft.
They frequently need re-bushed. Spray the propane around the base of the carb.

 

I am clueless whether the flywheel is relatively heavy or not. That's an interesting angle.

 

Leigh, I have looked over the throttle shafts, the center carb is tight (but moves freely), The end carbs are fairly sloppy. I am going to get some propane and do a leak check as you and others suggest.

Thanks.