Modified tri-power center carb stumble when accelerating
 

I am having an annoying issue with stumbling/hesitation on the center carb when accelerating. It only happens when driving, not when free-reving.

Hoping for an easy fix without drilling out the carb if possible.
The car will start on the key, the first turn, and idles great.

Is there a way to increase or decrease the pump shot without drilling out the tubes?
I am surprised this 455 carb wouldn't already have enough shot for this smaller motor.

Anything else come to mind?

Center Carb: Original 455 2bbl NOS rebuilt, stock jetting - untouched, jets 61?
End Carbs: Stock '65, bores opened up on a machine, jets 74

Here is the rest of my combo
433CI
11:1 compression
Ross flattop pistons .005 in hole.
Ported #77 '65 389 heads, Ported intake
230-236 HR Extreme energy on 110 LS, 1.6 rockers
4 speed, 3.73 gears.
Distributor has 8 initial and 34 total timing, in by 3000. Vaccum advance limited to 6 degrees.
Full exhaust.

Are you sure you even have a pump shot? Is it possible someone dropped the wrong size check valve BB into one end of the acc pump circuit?

Yep, the pump shot is there.

On Tri powers Cliff reccomends a 7 number spread from the center carburetors to the outboards. I run an original 66 unit on my 67 and I have 64 in the center and 70's in the outboards without issue. Throttle response and idle is terrific, no stumble anyplace. FWIW. My jetting is fat compared to factory jets.

Tim john---

Might have to drill out the squirter a bit. ;)

How much of the idle transfer slots is exposed in the center carb at idle? If the throttle blades are open too far into or past the slots you will get an off-idle stumble.

I have a '66 tripower setup on my 463 with a big cam and run a 65 jet in the center and 74's on the ends. I might drop to a 64 in the center after some dyno tuning as it is slightly rich right now. I drilled out the idle tubes a few thousandths (stock is 0.032" and I went to 0.038") and also opened up the mixture openings to 0.073". Also had to adjust the accelerator pump linkage rods a little bit so that the pump starts squirting right away. Without adjusting the acc. pump linkages there was play that kept them from squirting which caused a stumble.

I'm rather certain the blades are near closed but will check this weekend. And it looks like I might not be too far off on the end jets. I'm guessing the center has 61s for fuel economy in 1972, the date of this carb.

Am thinking about getting a LM2 O2 sensor to dial it in. Are they worth the expense?

If your end carb throttle blades weren't fully closed, the engine idle would not be good.

Based on your symptoms, I believe the problem is in the power valve actuating rod and spring that's attached to the airhorn.

Cams more radical than the original grinds on these engines produce less vacuum at normal operating conditions than the stock cams. The spring on the above-mentioned rod needs to be weaker to keep the power valve from opening when vacuum is low. I've had luck cutting two coils off the end of the power valve rod spring. Do not attempt to modify the spring in the actual power valve in the bottom of the float bowel.

Let us know how this works out. I don't believe it's necessary to modify your idle tubes.

Thanks! I will look at that before drilling anything. Isn't there a spring kit available also? Is that better than cutting the spring? Anyone got a picture of the spring I'm supposed to cut?

FYI, Jon at the Carburetor Shop sells various tension power valve springs for tripower carbs, and can set you up with the right spring based on your idle vacuum.

Idle screws are out 2.5 turns each on the stock NOS carb. Idled great. If I turn them in past 2 turns, the idle quality goes out the window. Timing at idle is 6-8 degrees, there is 13 degrees in the distributor. The throttle blades are almost fully closed.

Does the idle screws need to be 1 to 1.5 turns out max?

Any slop in the pump linkage??

I would open up the bottom orifices of the idle tubes on the center carburetor .004".

Sounds like a lean hesitation, which can be caused by a lean idle after a RA-IV or larger camshaft is installed. The holes need to be opened up with a precision numbered drill set using a hand operated "pin vice" drill holder.

Tom Vaught

Turning in the idle mixture screws really helped to eliminate most of the stumble.

Now we are encountering overheating and a leaky heater core due to the pressure. Could too lean of primary jets (61s) cause such overheating after a short drive?

For what you would use an O2 meter for, probably the NGK would be your best bet.

http://www.ngk.com/afx/ Summit sells them

My 10.5:1 433 w/ 240/244 HFT had no hesistation problems with an original 66 trips set up. My guess is something is wrong with that carb.

421 Engines typically had at least 64 jets on the center carb and many put 66-68 jets in the primary and 72 jets in the end carbs. I am amazed that it would even run on a 433 cid engine with 61 jetting.

Tom Vaught

Jon at the Carb shop also said to try turning in the idle screws and go to a richer jet before drilling anything or changing the springs. I will try with 74 or 76 main jets and see if that helps before doing anything else.

The carb is a NOS '71 2bbl 455 carb - factory setting out of the box was 61 jets and idle screws 2.5-3 turns out!

I think I will like this large center carb - as is, it gets rubber in first gear and some of second just on the center carb alone. And that is with huge 275/60/15 tires.

Am not sure if my smaller stock '65 center carb would do that - but it would be interesting to do a back to back test of both.

I hope the "74 or 76 main jets" for the center carb is a typo!

We discussed 64~66 main jets when you called.

Jon.

Sorry Jon, yep, 64-66 are numbers you gave me.