427/435 tripower hesitation on full throttle

[Kenneth K.]

Can anyone tell me what the vacuum should be for a L-71 automatic at idle? In neutral my car's manifold vacuum fluctuates between 9 and 10 inch Hg. In drive the vacuum fluctuates between 6 and 7 inch Hg. I am thinking this is to low because it is so close to the power value opening of 6.5 inch Hg.

The carbs idle okay. Although, I can backout the idle mixture screws almost all the way without much change in vacuum or rpm. However, the engine will die if I turn the idle screws in all the way to about a 1/2 turn from bottom.

I am thinking this low vacuum is related to the bad hesitation/sag when I punch the throttle from a dead stop. The vacuum drops to zero and the car stumbles for a few seconds, spits a bit, and then takes off. Generally leaves a small puff of black smoke after it takes off. The car runs great otherwise. Vacuum runs steady between 12 and 14 inch Hg on the road at speeds of around 30 to 50 mph. While curising at these speeds the vacuum will jump to 15 to 17 inch Hg if you let off the throttle and drops to zero at WOT.

One intersting observation is there is NO hesitation from a dead start if the car is warmed up only long enough to close the choke plate but not warm enough to reach full operating temperature. Also, there is no hesitation if you punch the throttle from a moving start.

I know these tripower carbs are hard to tune so before I installed the tripower I tuned the car using a single 4-bbl Holley 750 cfm. I followed Ken Anderson's power tuning procedure he outlined in Vol 22, No. 2, Fall 1995 Restorer. After tuning based on Ken's article my car ran better than it had ever ran before.

I then installed the tripower after I rebuilt all 3 carbs using original Holley rebuild kits and stock sizes for jets, power valve, accelerator nozzle, etc. Here are the list numbers for the carbs:

3659 ends
4055 center

Note the 4055 is stock for the manual transmission. For the automatic option on my 427/435 the 4056 was the stock center carb. I am running the 4055 because I have not yet located a 4056. I don't know if this can cause my problem? The only difference I can tell is the 4055 is jetted richer using 63 rather than 61 jets but uses a smaller shooter, a 21 rather than a 25. Therefore, the center carb after I rebuilt it had a small shooter (21) but larger jets (63) than the 4056 used on the automatic.

I have also tested the car with the linkage disconnected to the end carburetors and still get the hesitation. This indicates to me the problem is with the center carb. I double checked the 6.5 power valve and made sure the acclerator pump was adjusted properly. No improvement.

Thinking the problem is a lean stumble I have tried the following:

increased the shooter from a stock 21 to a 28
richened up the idle circuit by 1/4 to 1/2 turn
(the jets are already at 63 from the stock 61)
used a red accelerator cam on the no. 2 position (Anderson's recommendation)

Thinking the problem is an air leak (vacuum leak) I checked for leaks by spraying carb cleaner. I also closed off the choke plate to see if the idle increased. It did not. In fact, I could almost completely close the choke plate and would not see any difference in idle speed or vacuum. I also replaced all the vacuum lines on the car just to make sure.

I can't find any vacuum leaks, I have rebuilt the carbs using genuine Holley parts, tuned according to Ken Anderson's procedures, but nothing has made any significant improvement. Several facts seem key; 1) the car accelerates perfectly from a dead stop when it is slightly cold, 2) vacuum seems to low at idle, and 3) the car acclerates perfectly from a rolling start above 15 mph after it it warmed up.

Sorry so being so long winded, but I am hoping someone can piece these clues together and give me some suggestions to solve this problem. I am at a loss for ideas.

Thanks,
Ken

[Clem Z.]

Re: 427/435 tripower hesitation on full throttle

you could also have rich condition because if it was lean it should spit back thru the carb. try a leaner setting on the pump cam. when we had to run the 350 CFM holley 2 barrel carb because of track rules on a V-8s we had to cut back on the pump volume or we would have a bog caused by a rich pump shot

[Timothy B.]

Re: 427/435 tripower hesitation on full throttle

Ken,

At idle the car needs to run on the idle circuit and it's not as the mixture screws don't effect RPM as you mention. The engine vacuum is finding air somewhere else other then through the idle circuit and transfer slots when accellerating, it's a internal vacuum leak, could be throttle shaft bore or alignment of throttle plates to carburetor bore.

This could also be on either the front or rear carburetors as well. Remove the carburetors and drain the gas then hold them up to a light with the plates closed all the way and look for light through the bores. If you can see light, there is misalignment in the throttle plates, if not I suspect throttle shaft wear.

I have the same problem with my 3461 AFB and used a sealer around the throttle plates. The car ran perfect and finally idles correctly so I am sending this carburetor to Bob Kunz to be repaired properly. The problem drove me nuts but he knew exactly what it was after explaining my problem to him.

[Clem Z.]

Re: 427/435 tripower hesitation on full throttle

with the idle speed screw set where you idle the engine remove the carb and check to see how far the idle transfer slots are open. they should look like a square,about .020X.020. if these I-T-S are closed the engine is pulling air at idle from someplace else not thru the carb butterflies causing your problem. if the idle transfer slots are not feeding fuel at idle you will have a bog when you open the throttle because the engine is waiting for the fuel from the squirters to get thru the carb into the manifold

[Kenneth K.]

Re: 427/435 tripower hesitation on full throttle

Thanks everyone for your suggestions. I have tried a few and have a few questions.

I tried a leaner setting on the accelerator cam but it didn't help. I could drop the jets down from the 63 to the stock 61 if you think this is worth trying?

I'll remove the center carb and check for a misalignment in the throttle plates and also check to see how far the I-T-S are open.

What was the sealer you used and where did you apply it around the throttle plates?

I think it is possilbe that I have an internal leak on one of the end carbs. If I open the throttle plates and look at the bottom of the intake manifold after the car has cooled down I can see a little gas pooled at the base on one of the end carbs. I have been told this indicates a warpped metering plate. I have another end carb I will try and see if this helps. I didn't think I had to worry about the end carbs causing my problem because I ran a test with the linkage disconnected on the end carbs and still got the bog.

If I have a missaligned throttle plate or an internal leak on the end carb could this be causing a low vacuum at idle?

Ken

[Clem Z.]

Re: 427/435 tripower hesitation on full throttle

Thanks everyone for your suggestions. I have tried a few and have a few questions.

I tried a leaner setting on the accelerator cam but it didn't help. I could drop the jets down from the 63 to the stock 61 if you think this is worth trying?

I'll remove the center carb and check for a misalignment in the throttle plates and also check to see how far the I-T-S are open.

What was the sealer you used and where did you apply it around the throttle plates?

I think it is possilbe that I have an internal leak on one of the end carbs. If I open the throttle plates and look at the bottom of the intake manifold after the car has cooled down I can see a little gas pooled at the base on one of the end carbs. I have been told this indicates a warpped metering plate. I have another end carb I will try and see if this helps. I didn't think I had to worry about the end carbs causing my problem because I ran a test with the linkage disconnected on the end carbs and still got the bog.

If I have a missaligned throttle plate or an internal leak on the end carb could this be causing a low vacuum at idle?

Ken

if one of the end carbs has the butterfly open slightly you are pulling air thru there on idle and the center carb is not open enough to expose the I.T.S

[Timothy B.]

Re: 427/435 tripower hesitation on full throttle

Ken,
Install the factory accellerator cam and squirter and be certain it's correctly adjusted to give a good pump shot and .015 +- between lever and arm as the book say's. The jets only effect the main circuit of the carburetor and are changed for tuning so if it's apart install the factory size until everything is straigtened out, your problem is internal vacuum leak effecting idle circuit and transfer slots.

Just because your test was done with the linkage unhooked does not mean anything. The vacuum leak is still present and engine vacuum is finding air where it should not, throttle shaft or throttle plates, and the problem could be on any one of three carbs.

Don't worry about gas pooling at this time, I will try to find the email address of the man that sells the shaft sealer treatment but if the carburetors are correct dated for your car it may be time to send to a pro to be corrected.

Does the car idle down with the idle screw or does it find it's own idle with screw backed all the way out (throttle plates closed)? If you remove any one of the carburetors drain the gas and hold it up to a light to see if the throttle plates are sealing, watch your eyes.

[Kenneth K.]

Re: 427/435 tripower hesitation on full throttle

I pulled the center carb and found out the idle screw was backed out all the way. Also, I held it up to a light and I could see light between the throttle plates. It also looked to me like the throlltle plates may not be centered because I could see light only on one side not the other.

I examined the position of the throttle plate to the I-T-S and it looks like a "square" to me. The width at the top of the slot was about the same amount showing as the length. I also notice a small hole to the right of one of the transfer slots. The top of this hole is about the same as the top of the slot but is present in only one barrel. The throttle plate covers about 3/4 of this hole.

I also pulled the end carb that was leaking gas into the manifold and held it up to a light. I could see light between the throttle plates but not as much as the center carb. I did the same with the spare end carb I have and it looked better than the other two. I could see only a little diffuse light near the throttle shaft.

I replaced the center carb and the end carb using the spare. I set the idle mixture screws 1 1/2 turns out from bottom. I also increased the idle for about 600 rpm to 800 rpm so I know the idle screw was engaged and opening the throttle plate a little more to expose more of the slot and more of the hole next to the slot.

The car still bogs. Also, the throttle does not return every time. I often have to "blip" the throttle to get it to back down. The carbs are correctly dated for my car so maybe it is time to have them professionally rebuilt. I don't know what else to try.

Ken

[Timothy B.]

Re: 427/435 tripower hesitation on full throttle

Ken,

The small hole next to the transfer slot and partially covered by the throttle plate is ported vacuum as it's not exposed to full engine vacuum until throttle plate is opened. I am not sure but this port may be used for your vacuum advance and if it is you will see a marked improvement in engine performance and engine idle temperature by hooking the vacuum advance can to full time engine vacuum.

Did the car idle with the idle screw all the way out. If so I suspect throttle shaft is wore and engine vacuum is finding gas above the throttle plate at transfer slot. The other two carburetors are leaking vacuum also if you can see light. I am not sure what vacuum your engine is supposed to have but low vacuum as you report will render the vacuum advance inoperative also. I would call Bob Kunz and have him restore these if they are correct for your car. As I said that's where my AFB is going to have the same problem corrected.

[Kenneth K.]

Re: 427/435 tripower hesitation on full throttle

Yes, the car idles just fine with the idle screw backed out all the way. I tuned the car according to Ken Anderson's method of 10 to 12 degrees of mechanical advance. This method disables the vacuum advance cannister and is the same way the L88 is timed. The engine runs strong but the car does idle very hot using this timing method which you mention would be improved if I used full time vacuum advance.

Sounds like the throttle shafts are shot and will need to be rebuilt. Can you give me the phone number for Bob Kunz? Do you know if he rebuilds tripowers? The 3659 end carbs are correct and dated for the car. The center carb is a 4055. I need the 4056 for the automatic. I''ll see if Bob Kunz has one he would trade for. I really don't want to restore the 4055 since it is not correct.

I am still bothered by the low vacuum, especially at idle. The 7 inch Hg at idle seems to close to the 6.5 power valve opening. I read somewhere that the idle vacuum should be a minimum of several inch Hg above the power valve opening.

Do you think the loose throttle plates could be causing the low vacuum?

Thanks,
Ken

[Timothy B.]

Re: 427/435 tripower hesitation on full throttle

Ken,
The low vacuum is caused by the leakage within the carburetors. Chech archives for Bob's # as we are not supposed to use vender names on the D/B. I think there are several rebuilders but I spoke to him and he KNOWS his stuff, seemed nice and honest also.

The reason the car runs with the idle adjustment screw all the way out is the engine vacuum is finding gas above the throttle plates because of poor seal or throttle shaft wear. As a result when you accellerate the vacuum is not present at the transfer slots to pull mixture and keep the engine from hesitation while the transition from idle circuit to the main metering circuit.

Additional problem with low vacuum is that no vacuum advance can will pull at 7" vacuum so idle timing is just initial (10-12*) so the engine runs hot. Total idle timing with vacuum advance should be 10-14 initial + 15 vacuum advance so you can see what a difference the functioning vacuum advance will make. One thing at a time though.

[Ken A.]

Re: 427/435 tripower hesitation on full throttle

Can anyone tell me what the vacuum should be for a L-71 automatic at idle? In neutral my car's manifold vacuum fluctuates between 9 and 10 inch Hg.Fluctuation is normal, but gage may be reading low. In drive the vacuum fluctuates between 6 and 7 inch Hg. I am thinking this is to low because it is so close to the power value opening of 6.5 inch Hg.

The carbs idle okay. Although, I can backout the idle mixture screws almost all the way without much change in vacuum or rpm. However, the engine will die if I turn the idle screws in all the way to about a 1/2 turn from bottom. Always set for highest reading with a vacuum gage

I am thinking this low vacuum is related to the bad hesitation/sag when I punch the throttle from a dead stop. The vacuum drops to zero and the car stumbles for a few seconds, spits a bit, You may have blown out the PV. Check by applying vacuum (mouth) and install anti blow out kit. and then takes off. Generally leaves a small puff of black smoke (This is good)after it takes off. The car runs great otherwise. Vacuum runs steady between 12 and 14 inch Hg on the road at speeds of around 30 to 50 mph. While curising at these speeds the vacuum will jump to 15 to 17 inch Hg if you let off the throttle and drops to zero at WOT. Normal

One intersting observation is there is NO hesitation from a dead start if the car is warmed up only long enough to close the choke plate but not warm enough to reach full operating temperature.Sounds lean Also, there is no hesitation if you punch the throttle from a moving start.

I know these tripower carbs are hard to tune so before I installed the tripower I tuned the car using a single 4-bbl Holley 750 cfm. I followed Ken Anderson's power tuning procedure he outlined in Vol 22, No. 2, Fall 1995 Restorer. After tuning based on Ken's article my car ran better than it had ever ran before.

I then installed the tripower after I rebuilt all 3 carbs using original Holley rebuild kits and stock sizes for jets, power valve, accelerator nozzle, etc. Here are the list numbers for the carbs:

3659 ends
4055 center

Note the 4055 is stock for the manual transmission. For the automatic option on my 427/435 the 4056 was the stock center carb. I am running the 4055 because I have not yet located a 4056. I don't know if this can cause my problem? The only difference I can tell is the 4055 is jetted richer using 63 rather than 61 jets but uses a smaller shooter, a 21 rather than a 25. Therefore, the center carb after I rebuilt it had a small shooter (21) but larger jets (63) than the 4056 used on the automatic.

I have also tested the car with the linkage disconnected to the end carburetors and still get the hesitation. This indicates to me the problem is with the center carb. I double checked the 6.5 power valve and made sure the acclerator pump was adjusted properly. No improvement.

Thinking the problem is a lean stumble I have tried the following:

increased the shooter from a stock 21 to a 28 I would try a 31 & make sure it is a TUBE shooter and not the BLOCK
richened up the idle circuit by 1/4 to 1/2 turn Set with VG. Has no effect on launch.
(the jets are already at 63 from the stock 61) You can go to 64's, but it will not affect launch.
used a red accelerator cam on the no. 2 position (Anderson's recommendation) MAKE SURe YOU ADJUST PUMP BY THE BOOK!

Thinking the problem is an air leak (vacuum leak) I checked for leaks by spraying carb cleaner. I also closed off the choke plate to see if the idle increased. It did not. In fact, I could almost completely close the choke plate and would not see any difference in idle speed or vacuum. I also replaced all the vacuum lines on the car just to make sure.

I can't find any vacuum leaks, I have rebuilt the carbs using genuine Holley parts, tuned according to Ken Anderson's procedures, but nothing has made any significant improvement. Several facts seem key; 1) the car accelerates perfectly from a dead stop when it is slightly cold, 2) vacuum seems to low at idle, and 3) the car acclerates perfectly from a rolling start above 15 mph after it it warmed up. ALSO SET FLOATS 1/8 inch BELOW SIGHT PLUG

Sorry so being so long winded, but I am hoping someone can piece these clues together and give me some suggestions to solve this problem. I am at a loss for ideas.

Thanks,
Ken

Ken, please review comments. I'll think they will help. Ken

[Kenneth K.]

Re: 427/435 tripower hesitation on full throttle

Thanks everyone for all your suggestions and insights. It is starting to make sense to me that if the throttle plates/shafts are not sealing tight then it could have the same effect as an air leak causing the low vacuum readings. Especially if all 3 have internal leaks.

I have two vacuum gages and they both read the same 6 to 7 inch Hg at idle. It is good to know that this range in fluctuation is normal as I was concerned that there was also an internal engine problem like a burned valve. It is good to know also that the vacuum will drop to zero at WOT.

I set the idle at highest vacuum. PV appears okay. I also double checked the accelerator pump setting and made sure it was adjusted so that the level would respond instantaneously to throttle movement with 0.015 inch clearance at WOT. Still no change in performance.

I need to test with the floats set 1/8-inch below the sight plugs and find a 31 shooter. The 28 shooter on the carb now is a tube discharge type. I also didn't observe any improvement when I increased the shooter from the 21 to the 28. I think it would be worthwhile trying the sealing treatments to the throttle shafts.

Thanks,
Ken

[Duke W.]

Re: 427/435 tripower hesitation on full throttle

The OE SHP cam should yield about 14" at 900 in neutral. Try that test. An auto will be lower vacuum/revs in Drive. I don't have any data your configuration, but vacuum shouldn't be that low.

Also remember that idle vacuum is a partial function of idle speed, so you need to quote both to have full context and whether it is idlling in neutral (manual trans) or an auto idling in Drive.

I suspect the engine might have a non-OE cam with much more overlap that the OE cam - like an L-88 cam.

Duke

[Ken A.]

Re: 427/435 tripower hesitation on full throttle

Ken,
The leaky shafts are normal in old carbs & only impact the idle circuit, which you have "cured". When you nail the accelerator you create a huge air opening and the engine must have enough fuel to give the proper A/F ratio. That's the job of the accelerator pump & squirter. That's also why the end carbs are controlled by the center carb's airflow. A "bog" can also be caused by the tires hooking up too quickly. If you're running a stock rearend & no modified converter, you may never totally cure the "hesitation". A 4 speed car can launch at basically any RPM you need to begin the sequence of tire hookup & acceleration. If you have a 3:36 rearend & a stock converter, then you can only compromise. You may need to have the tranny modified for "drag racing launches", but that's another topic.
Ken