427/435 tripower hesitation on full throttle

[Kenneth K.]

Re: 427/435 tripower hesitation on full throttle

At operating temperature in neutral and at about 900 rpm the manifold vacuum fluctuates between 9 and 10 inch Hg. In drive, the rpm set at 750 rpm (per spec) 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.

I bought the car when it was 5 years old. At that time it had a piston slap. Eventhough I was told it was not a problem to worry about I went ahead and had it fixed. At the same time they replaced the cam with an OE. Also, before I installed the tripowers I adjusted the valve lash according to Ken Anderson's article in the Fall 1995 Restorer. I think Ken's lash settings are based on an OE cam (please correct me if I got this wrong). Because the car ran so good with these lash settings also suggests to me the cam is OE.

The 3.08 rearend and converter are also OE. This combination won't smoke the tires off like a 4 speed could do, but when I had the Holley 750 on the car I didn't experience any bog. Because the car ran so good before I installed the tripower it seem the tripower carbs are causing the problem. Another data point to consider is when the car is cold it does not bog and will actually turn the tires loose. As soon as the car heats up to operating temp (about 200 to 210) the bog returns.

Timothy Barbieri's post has a good explaination that 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." I took the carbs off and held them up to a light as he suggested and I could see light coming around the throttle plates. Is this much gap common for old carbs? Would the combination of worn throttle shafts and worn throttle plates drop the vacuum to the low levels I am seeing?

Thanks,
Ken

[Timothy B.]

Re: 427/435 tripower hesitation on full throttle

Ken,

The most likely reason the car will not bog when cold is because the choke plate is still somewhat closed forcing the engine to pull a richer mixture than when it's warmed and the choke is opened.

When you open the throttle plates, the low air pressure area in the intake manifold goes to atmospheric pressure and the fuel and air mixture suddenly changes because fuel drops out of the air onto the intake manifold. The accellerator pump injects fuel to cover this up until the engine recovers to once again pull mixture from the booster venturies. That is the purpose of accelerator pump but I don't think it's your problem.

If your car will idle with the idle adjustment screw all the way out then the problem is like I suggested earlier. You mention the car ran great with a single carburetor, do you recall engine vacuum? It should not have changed with the three carbs. Check with a carb guy to get a pro's opinion.

[Kenneth K.]

Re: 427/435 tripower hesitation on full throttle

I checked that the choke plate was fully opened before I tested it cold. After I started the car I then removed the air cleaner lid and made sure the engine was warmed up enough so that the choke remained open but was still less than operating temperature. I replaced the lid and the car ran great until the temperature increased to the 200 to 210 range and the bog returned. Maybe I need to wire the choke plate open in case it is closing after I replace the lid?

Unfortunately, I can't find my notes of the vacuum readings I took when it was running a single 4 bbl but I am confident the vacuum was closer to "the OE SHP cam should yield about 14 at 900 in neutral" that Duke mentioned in his post. I ran the test Duke suggested and the vacuum was only about 10 inch Hg at 900 rpm in neutral.

Thanks everyone, and have a great 4th,

Ken

[Ken A.]

Re: 427/435 tripower hesitation on full throttle

Are you still using the .028 squirter? If so go to a .031 & see if it improves.

[Kenneth K.]

Re: 427/435 tripower hesitation on full throttle

Ken, yes I am still using the .028 nozzle. I will try a .031 as soon as I can find one. I have tried both .021 and .028 nozzles but don't see any difference. Since the stock configuration came with a .025 wouldn't you expect the car to at least perform without the bog using the .028? Are you thinking a .031 will cover up the worn throttle plates and shafts?

Thanks for helping,
Ken

[Ken A.]

Re: 427/435 tripower hesitation on full throttle

Yes, the key is the puff of black smoke on launch. A nice big puff indicates the right size nozzles. no puff = too small.

[Kenneth K.]

Re: 427/435 tripower hesitation on full throttle

The car leaves a puff of black smoke with the 0.028 nozzle. Do you think this indicates the .028 size it okay or are you still thinking I need a .031 nozzle? After the car is warmed up and I punch the throttle the car hesitates/bogs for a bit and then gains rpm and takes off. When it takes off it leaves the puff of black smoke that I can see in the rear view mirror. I don't remember seeing this puff when the car is cold.

Ken

[Clem Z.]

Re: 427/435 tripower hesitation on full throttle

The car leaves a puff of black smoke with the 0.028 nozzle. Do you think this indicates the .028 size it okay or are you still thinking I need a .031 nozzle? After the car is warmed up and I punch the throttle the car hesitates/bogs for a bit and then gains rpm and takes off. When it takes off it leaves the puff of black smoke that I can see in the rear view mirror. I don't remember seeing this puff when the car is cold.

Ken

you need a richer mixture when the engine is cold that is why i think you may a too rich condition. you may have a internal fuel leak in the carb or a incorrect or leaking PV

[Ken A.]

Re: 427/435 tripower hesitation on full throttle

The car leaves a puff of black smoke with the 0.028 nozzle. Do you think this indicates the .028 size it okay or are you still thinking I need a .031 nozzle? After the car is warmed up and I punch the throttle the car hesitates/bogs for a bit and then gains rpm and takes off. When it takes off it leaves the puff of black smoke that I can see in the rear view mirror. I don't remember seeing this puff when the car is cold.

Ken

Go for the .031

[Clem Z.]

Re: 427/435 tripower hesitation on full throttle

do you have the correct springs in the front and rear carb vacuum diaphragm ?? if not they could be opening too soon.

[Kenneth K.]

Re: 427/435 tripower hesitation on full throttle

I rebuilt all 3 carbs with new Holley kits and made sure I used the correct color springs. I also disconnected the linkage on both front and end carbs so the throttle plate was closed and it did not help the hesitation. I also checked and replaced the PV and it is okay.

I still need to find a .031 nozzle.

Thanks,
Ken

[Clem Z.]

Re: 427/435 tripower hesitation on full throttle

I rebuilt all 3 carbs with new Holley kits and made sure I used the correct color springs. I also disconnected the linkage on both front and end carbs so the throttle plate was closed and it did not help the hesitation. I also checked and replaced the PV and it is okay.

I still need to find a .031 nozzle.

Thanks,
Ken

someone who has a set of number drills can drill one out for you. #68 is .031. the 4055 carb uses a #240 red pump cam and a 4056 uses a 218 white pump cam. you can increase the pump shot on either cam by moving the screw to the #2 hole. the white cam goes from 17 cc to 19.5 cc and the red cam goes from 18.5 cc to 20 cc. per 10 strokes. when you go to a larger squirter you will need to increase the pump stroke(volume).

[Kenneth K.]

Re: 427/435 tripower hesitation on full throttle

I have the red cam installed and set on the no. 2 hole (per Ken Anderson's recommendation), but I don't know if it is a #240. I have also tried the no. 1 hole but don't see any real difference. I talked to a speed shop trying to find a .031 shooter and they said they could drill out my .028. I have two .028 shooters and could sacrifice one to drill out, but I hate to modify an original shooter so I am still trying to find a .031.

Thanks,

Ken

[Clem Z.]

Re: 427/435 tripower hesitation on full throttle

I have the red cam installed and set on the no. 2 hole (per Ken Anderson's recommendation), but I don't know if it is a #240. I have also tried the no. 1 hole but don't see any real difference. I talked to a speed shop trying to find a .031 shooter and they said they could drill out my .028. I have two .028 shooters and could sacrifice one to drill out, but I hate to modify an original shooter so I am still trying to find a .031.

Thanks,

Ken

the .031 should be available from holley. straight type #121-131 and the tube type #121-31

[Kenneth K.]

Re: 427/435 tripower hesitation on full throttle

Good idea. I'll get on the Holley web page. I believe I need the tube discharge type.

Thanks,
Ken