Carburetor questions

[Bob NH]

Looking for some advice on Holly 94 carb. As I have mentioned in past posts, truck back fires going down hill which seems like excessive extra fuel not being used up ? Plus it gave me real trouble starting when warm a couple days ago. I have the carb off and apart, jets are 50's I have others but don't know where I put them. Have several power valves and do not know how you would test them ?Please don't tell me to send it out,I should be able to handle it with some advise.

[KiWinUS]

Do you have a mill to correct surfaces? Do you know how to machine power valves to make the fit correctly? Yes these are basic carbs but it’s experience that gets the running correctly without the trial and error.

[51504bat]

Does the throttle shaft need to be bushed?

[miniceptor86]

So back firing is on the exhaust side or out the carb? Out the exhaust maybe more correctly after fire but that’s not used much. So if out the exhaust it’s caused by a lean condition which won’t ignite in the combustion chamber. After some accumulation in the muffler/ exhaust pipes it becomes rich enough to ignite. At least on motorcycles some of which have a vacuum operated air cut off valve for the emulsion tube which makes the mixture rich enough to ignite properly in the combustion chamber. I don’t no about your carb I would think no system like that exists a you are just running lean.

[1952henry]

Vintage Speed, Daytona Carburetor, and I’ve hear Car Quest have correct mating surface power valves, so this shouldn’t be an issue. I have used Car quest parts before that worked. As to above post, I find that to be true. Exhaust backfire, lean condition. At least on single cylinder AtVs. I have modified a couple in my life, and had to change jets in carburetors to match airflow after changes to exhaust/air boxes or elimination thereof. Even found the “wisdom” on websites regarding jet sizes didn’t match real life.

[scicala]

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Originally Posted by Bob NH

Looking for some advice on Holly 94 carb. As I have mentioned in past posts, truck back fires going down hill which seems like excessive extra fuel not being used up ? Plus it gave me real trouble starting when warm a couple days ago. I have the carb off and apart, jets are 50's I have others but don't know where I put them. Have several power valves and do not know how you would test them ?Please don't tell me to send it out,I should be able to handle it with some advise.

Number 50 jets should be fine. If you use a Walker brand carb kit, the power valve in the kit will work as is. It doesn't have a step on the gasket surface.
Walker kits are available through O'Reilly Auto Parts.
You need to make sure the gasket surface on the bottom of the bowl is flat and straight where it mates with the throttle body (base). If you don't have a mill to machine the bottom of the bowl flat, you can use a flat file to carefully make it flat if needed. It can be a pretty good vacuum leak there if not flat and will effect the power valve operation.

Sal

[flatjack9]

Retarded timing can also cause the exhaust to backfire.

[Brian]

Exhaust leak[s] can also cause backfiring like in your example of going down a hill

[Bob NH]

WOW ! never would have thought being lean would cause these issues. Yes, it is in the out going exhaust. I do have good mated surfaces and good seating on the power valve. I met a guy at a car show today who lives about two miles from me with a couple of old flat head race coupes, asked him if he had a spare moment to stop by and take a look. Said "two heads are better than one sometimes" Looking forward to his visit. I'll try to let y'all know how it goes and thanks much to all for the input !

[flatjack9]

A lean mixture is slower burning. Combustion may not be complete when the exhaust valve opens.

[1948F-1Pickup]

Originally Posted by Bob NH

Looking for some advice on Holly 94 carb. As I have mentioned in past posts, truck back fires going down hill which seems like excessive extra fuel not being used up ? Plus it gave me real trouble starting when warm a couple days ago. I have the carb off and apart, jets are 50's I have others but don't know where I put them. Have several power valves and do not know how you would test them ?Please don't tell me to send it out,I should be able to handle it with some advise.

As others have indicated, the power valves are available through Walker. The Autolite 2100/4100 PV will also work with its correct PV gasket.
There's no foolproof easy/cheap method of testing PVs. A Mityvac will get you halfway there but the test fitting doohickey they provide sucks and will not yield reliable/repeatable results.
The correct item for the job is (unfortunately) stupidly expensive.
I borrowed that tool once and checked a bunch of PVs that were all
brand new, including one or two from a well known supplier of Holley 94 parts and several were leakers..... much to my surprise.

Moroso 62295

[petehoovie]

Originally Posted by 1948F-1Pickup

As others have indicated, the power valves are available through Walker. The Autolite 2100/4100 PV will also work with its correct PV gasket.
There's no foolproof easy/cheap method of testing PVs. A Mityvac will get you halfway there but the test fitting doohickey they provide sucks and will not yield reliable/repeatable results.
The correct item for the job is (unfortunately) stupidly expensive.
I borrowed that tool once and checked a bunch of PVs that were all
brand new, including one or two from a well known supplier of Holley 94 parts and several were leakers..... much to my surprise.

Moroso 62295

[ford38v8]

Originally Posted by Bob NH

Looking for some advice on Holly 94 carb. As I have mentioned in past posts, truck back fires going down hill which seems like excessive extra fuel not being used up ? Plus it gave me real trouble starting when warm a couple days ago. I have the carb off and apart, jets are 50's I have others but don't know where I put them. Have several power valves and do not know how you would test them ?Please don't tell me to send it out,I should be able to handle it with some advise.

Easy peasy testing a power valve. If it holds to your tongue when you suck on it, it's good. If the diaphram is busted, it won't hold. All you need to bust the diaphram is one backfire through the carb. Also, a good power valve can show bad results if the gasket can't seat correctly. New carbs that use these power valves have a different mating surface, so check that both surfaces mate correctly. Several outfits now machine the new valves to fit the old carbs, or if you have a lathe, you can machine the mating surface yourself.

[1948F-1Pickup]

Originally Posted by ford38v8

Easy peasy testing a power valve. If it holds to your tongue when you suck on it, it's good. If the diaphram is busted, it won't hold. All you need to bust the diaphram is one backfire through the carb. Also, a good power valve can show bad results if the gasket can't seat correctly. New carbs that use these power valves have a different mating surface, so check that both surfaces mate correctly. Several outfits now machine the new valves to fit the old carbs, or if you have a lathe, you can machine the mating surface yourself.

Although your entire post is true, I'm not testing anything that's been around ethanol like that.