# Tri-power RPM returns very slowly when shifting/letting off throttle



## JesseLyon (Sep 13, 2020)

Just getting my fresh tripower setup running this weekend for the first time since swapping from 4bbl. Its finally running pretty well, idles ok'ish, and have sorted out overheating issues (thanks lust4pseed in other thread!)

Issue now is that when accelerating hard and letting off throttle at higher RPMs (ie. when shifting), the RPM falls back down very slowly, making it difficult to shift gears. Accelerator pedal, cable, and linkage all operate fine with smooth/quick movement, so its pointing to a carb or fuel delivery issue. I am running a fuel pressure regulator before the filter, currently set to 4psi.

Thoughts?


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## Lemans guy (Oct 14, 2014)

Sounds like something ani’s slamming shut,...like outside carb throttles? You say the spring is strong enough? Maybe try a stronger throttle return spring, that will close them stronger and cuts off both fuel and air.

Is distributor vacumn timing sticking? Or weights and springs in distributor hanging up? Are they free to snap back and do they...?


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## JesseLyon (Sep 13, 2020)

Lemans guy said:


> Sounds like something ani’s slamming shut,...like outside carb throttles? You say the spring is strong enough? Maybe try a stronger throttle return spring, that will close them stronger and cuts off both fuel and air.
> 
> Is distributor vacumn timing sticking? Or weights and springs in distributor hanging up? Are they free to snap back and do they...?


Throttle linkage returns just fine and very quickly, so its not the spring. Im running HEI ignition/distributer, and wasent getting this issue before I swapped over from 4bbl to tri-power. could a vaccum leak somewhere on the new intake be contributing to this? maybe on the break booster or PVC grommet?


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## PontiacJim (Dec 29, 2012)

JesseLyon said:


> Just getting my fresh tripower setup running this weekend for the first time since swapping from 4bbl. Its finally running pretty well, idles ok'ish, and have sorted out overheating issues (thanks lust4pseed in other thread!)
> 
> Issue now is that when accelerating hard and letting off throttle at higher RPMs (ie. when shifting), the RPM falls back down very slowly, making it difficult to shift gears. Accelerator pedal, cable, and linkage all operate fine with smooth/quick movement, so its pointing to a carb or fuel delivery issue. I am running a fuel pressure regulator before the filter, currently set to 4psi.
> 
> Thoughts?


You may have a couple issues going on.
Who rebuilt the tri-power? You or a reputable tri-power rebuilder? Not the best of idle at this time is suspect to an issue of some sort.


You may have a couple issues going on.


First, what trans? Muncie 4-speeds use synchronizers to shift from one gear to the other, should be no issue at any RPM in shifting. So could be a trans issue, or even an adjustment on the clutch - meaning there is not enough of a gap between the clutch face and pressure plate/flywheel and the clutch is not fully disengaging. Linkage could be worn or even flexing if an aftermarket HD pressure plate is used. Clutch disc could be hanging on the input shaft splines.


If TKO 5 speed, the older units don't like to shift at higher RPM's by design, but assume you have a Muncie.


Not sure why the engine RPM is "slow" in dropping. Vacuum operated outboard carbs, or straight linkage? Vacuum operated end carbs could cause this IF the diaphragm is not exhausting quickly to shut the throttle plates. Assuming you have mechanical linkage, so I would look into the carbs not snapping shut - ie maybe you need a stronger spring to snap the outboard carbs closed faster, even the primary carb, when you let off the gas BUT you say your springs are good.

Disconnect the outboard carbs and try it, see if any difference.

A heavy flywheel and clutch set-up can also effect how quickly an engine RPM drops. The flywheel/clutch builds inertia and heavier set-ups build more and will keep RPM's up longer. Throw in an aluminum flywheel and the RPM's will drop like a stone as they don't build much inertia - especially when trying to get the car moving from a dead stop.


Let us know if that helps or you need more suggestions.


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## JesseLyon (Sep 13, 2020)

PontiacJim said:


> You may have a couple issues going on.
> Who rebuilt the tri-power? You or a reputable tri-power rebuilder? Not the best of idle at this time is suspect to an issue of some sort.
> 
> 
> ...


Thanks Jim! LIke I said, everything was behaving fine before the tri-power swap, so seems to be pointing to the new tri-power intake/carbs and not anything else. It was rebuilt by a reputable guy up here just outside Vancouver, I was referred to him by a friend who had his '65 tripower rebuilt and is working fine. Transmission is an M21, which was shifting fine before the intake swap, and seems to shift fine now. 

It revs up nice and quick, but its just the slow RPM drop I mentioned when coming off throttle, which even happens just revving the engine in neutral, its not just in gear (hence the "during shifts" part when clutch is disengaged. 

Im noticing this when only engaging the center carb as well (light throttle), so dont think its related to the outer cabs (unless its a vaccum leak on the outers, but not sure that would cause this issue anyway). I will remove the outer carb linkage though just to rule that out (yes, carbs are linkage not vaccum operated).

Ive contacted the carb rebuilder as well, so will see what he has to say. Thanks again for the suggestions! I will update.

JL


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## Lemans guy (Oct 14, 2014)

Since you can do it in neutral just clamp off the brake booster vacumn, then PCV, line even vac advance or dash vac whatever and see if any of this make it go away...then investigate further the area that it effects


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## Jetzster (Jan 18, 2020)

Always Check for no air thru the outers when it’s idleing Like Dave 7222:


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## JesseLyon (Sep 13, 2020)

Jetzster said:


> Always Check for no air thru the outers when it’s idleing Like Dave 7222:


Ive watched all this guys videos a few times through, they are great! Havent managed to do this test yet since getting the idle set, as its still overheating, but thats my next step for sure.


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## Scott06 (May 6, 2020)

I had an issue recently on my 65 tripower that was similar, wasnt always returning the throttle to idle, only coming down to a fast idle. I had just replaced the accelrator pump push linkage rods... Had to bend the center carb rod lengthening it as it was pulling the accelerator pump shaft so far down the pump was binding on the linkage inside the carb. just straightening out the rod to lengthen it slightly worked.


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## JesseLyon (Sep 13, 2020)

Scott06 said:


> I had an issue recently on my 65 tripower that was similar, wasnt always returning the throttle to idle, only coming down to a fast idle. I had just replaced the accelrator pump push linkage rods... Had to bend the center carb rod lengthening it as it was pulling the accelerator pump shaft so far down the pump was binding on the linkage inside the carb. just straightening out the rod to lengthen it slightly worked.


Interesting, I’ll take a look at that myself. It could also be the choke on mine, as I was also adjusting the choke rod by bending it as well.


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