Torque Converter Question

[gag1025]

My 1953 Chevy truck has a 350 engine with a mild cam (I bought the truck with the engine installed so I have no information on the cam) The rear differential is a1976 Camaro 10 Bolt with a 2:56 ratio. At low speed the carburetor (Holley 4150 570ccfm) loads up. Once you accelerate it is OK. I was told that I need to replace the torque converter to correct the problem. My question is how do I determine which torque converter to replace it with. I am running a Chevy turbo 350 transmission. All ideas will be greatly appreciated.

[octanejunkie]

Do you know what your torque converter stall speed is now?
Does your truck want to stall stopping at traffic lights or when shifting from park/neutral into gear?
What do you mean by your "carburetor loads up" at low speed?

[OrrieG]

Sounds like a carb or timing problem to me.. What is the idle speed set to? If the carb and timing are right it should idle at 700 rpm in gear, that is what my Chevelle and truck are set for. Any time I have had the problem you are describing it has been the carb.

[gag1025]

Thanks. I will answer some of these questions now and the others later. First the carb loading up question. Cruising along at a steady speed and all of a sudden the engine stumbles like it is getting too much fuel. When I accelerate it goes away and is fine as long as I continue to accelerate. Slow down and stay at a steady speed and it again loads up.

What is the idle speed set to? I don't know but will find out out Sat. when I go over to work on it some more.

Do you know what your torque converter stall speed is now? Don't know but will check it out Sat.

Any time I have had the problem you are describing it has been the carb. What was the solution to your similar problem?

[Bad Apache]

Agreed, sure sounds like a carb problem. Even if the cam is "big" and the converter is "tight", you should be able to tune enough to make it drivable.

[Bad Apache]

Definitely not the torque converter. This will be tough to diagnose without feeling it. Are you certain it loads up? Does it give a puff of black smoke or misfire when you accelerate through it? It could be a misfire or lean stumble as well. If it's loading up on the cruise circuit of a Holley, I'd check several things. float level, power valve rating and fuel pressure.

[gag1025]

Bad Apache, there is no puff of smoke or misfire. You will be cruising along and it will start stumbling but when you accelerate it smooths out. Myu first thought is that it is a carb issue and needs to be adjusted. I will check those three things. Thanks.

[Bad Apache]

A couple other questions. Do you have a mechanical or electric fuel pump? Is this the avenger series carb? Does it occur when the engine is hot, cold or anytime? And what ignition system/distributor are you using?

[gag1025]

Bad Apache. I actually have a 80670 carb. I wrote down the wrong information in the beginning. Actually if I had the 80570 carb it may not have the same issue. To answer your questions it has a Holley electric fuel pump. This is the Holley Avenger 80670 series carb. It is a stock Chevy distributor. The engine is a 4 bolt main 350. It cranks fine, idles fine and is OK until I hit a steady speed (say 45 mph) and all of a sudden it stumbles and when I accelerate it smooths out fine. The problem only occurs when I hold it at a steady speed. By the time I drive it up the road it has heated up so I have no way of knowing how it reacts when it is cold.

[gag1025]

I need to correct a mistake. We came up with the 2:56 ratio after counting the teeth. Apparently we were 1 number off (16 instead of 15 which would make the number 2:56 correct). I did some research and found out that the Camaro only offered two differentials that year. A 2:73 and an optional 3:08. Mine has to be a 2:73. I am going to run the codes to make sure.

[Speedbumpauto]

In spite of what racers may tell you, Holley's don't like 7 pounds of fuel pressure very well, especially on the street. You need to make sure to regulate the fuel pressure to around 5 to 6 psi. Sounds like your float level is marginally wrong (high) or the pump pressure is slightly over riding the needle and seat and in periods of low fuel use, like cruise, it has enough time to raise the fuel level in the float bowl enough to where the main circuit starts bleeding fuel in addition to the transition slots where the engine wants to run. As soon as you pound it a little, the fuel level drops slightly due to increased demand and everything cleans up until the next time you cruise too long and it happens again. Make sure you set the float level by the sight plug correctly and use a regulator and gauge to keep the fuel pressure in check.

[gag1025]

Thanks Speedbumpauto. I have eliminated the torque converter theory and have now pretty much come to the conclusion that it is a carb or power valve problem. I will definitely check the pressure and float settings because this would be the simplest fix for the problem. Thanks for this information. It is very helpful since I know very little about Holley carbs. I look forward to sharing what I find with you.

[Bad Apache]

Ok. The carb size and gear ratio shouldn't have any effect as far as this problem is concerned. I'll assume the distributor is an HEI and in good working order. As mentioned by SpeedBump (love that name) fuel pressure should be 4-6psi with an accurate gauge. You can check the fuel levels and for a leaking needle and seat by doing the following. Park the truck on level ground, remove the sight plugs, lay some rags under them to catch any spillage, with engine off turn the fuel pump on, watch the sight holes. The fuel should be right at the edge and not continuously spilling out. If the float is set too high or the needle and seat are leaking, it'll spill constantly.

With all that said, I suspect this is not going to be a float level or pressure problem. The engine is either going lean or rich under a cruise condition only. This leads me to believe it's internal to the carb. Is the carb 100% stock? Do you have a vacuum gauge or access to one? Do you have a wideband O2 sensor on the truck?

Sorry for all the questions. Just trying to figure out how to diagnose this and make the best possible suggestions.

[gag1025]

Bad Apache. I appreciate all of your questions. There will never be too many questions. I was going to check out thee fuel pressure but had to take the console I made as well as the seats to the upholstery guy. We stayed over at his place looking at his newly acquired 1972 Chevy Impala he just purchased. We were late getting back to the shop and did not have time to start it up. I will check next sat. We are also thinking it may be the power valve. Again thanks for all your questions and comment. You can never have enough advice. There are too many things (large and small) that can create the same problem.

[gregg pellicer]

Before you take anything apart, try disconnecting the vacum advance and drive it and see if problem goes away .I had what seems like a similar like a simalar problem on my model A with a 350/350 .It ran fine at idle and accelerating but at steady speed would start running bad 40/45 mph.The timing was advancing too far. Might not be your problem but is easy to check
Gregg

[gag1025]

Thanks gregg. I will do this and let you know what I find out. It will be a couple of weeks because the seats are at the upholstery shop and I will have to get the drivers side seat back before I can drive it.

[Bad Apache]

The power valve/valves are always suspect for two reasons. One- the diaphragms can rupture due to weakening from age and fuel composition (ethanol), and two- I have seen a few of the power valve gaskets split and leaking. I don't know if there was a change in the gasket material or what. I can't remember seeing this before 5 or so years ago. I'll keep watching the thread for your progress. Hope those seats turn out well for you!

[OrrieG]

Used to be just about any back fire thru a Holley would bust the power valve, but I thought they had corrected that with better material???

[gregg pellicer]

Any update's?

[gag1025]

The seats and console (I made the console) are at the upholstery shop. He does all the upholstery work for a company here called Race Tech (they build drag cars from pro Stock to Top Fuel Dragsters) so he is working me in his daily routine. As soon as we get the truck back we are going to install the 4 link rear suspension and bed wood. Then we are going to tackle this problem. As soon as we solve the problem I will post the results. Hopefully this will be in a month. Thanks for asking.