Most in-tank pumps are low-pressure. I'd suspect the filter's being plugged or a loose connection before condemning the existing pump.

quote:

Originally posted by K&E:
QUESTION:



QUESTION: What specs should I look at on an electric pump to move the fuel 30 feet but still be able to feed a 750cfm Edelbrock if I open up all 4 barrels???? What pressure and how many GPH????

My 502 is happy with a Carter rotary vane electric pump. 6 psi and 72 gph. That is pulling a heavy 4WD up some very steep grades.

My ski boat has a really hot 502 with an 850 douple pumper and runs fine on the same pump.

Both are monitored with a fuel pressure gauge, and show o signs of insufficient flow.

quote:

Interesting thing, when I took off the EFI unit I was looking to find a quadrajet type of split bore but it had a square bore and not like the one in the Edelbrock book that looked like 2 long ovals but it was square with rounded corners. The Edelbrock bolted right up and I didnt have to use the GM adapter....

Is your manifold aluminum or iron?

It's not an intank pump...it'e externally mounted outside of the tank...what concerned me was it was high enough pressure that it blew apart a connection at an inline fuel filter I had installed thinking that it was a lower pressure pump..

Yes, I think the intake is aluminum....the previous owners did a lot to try to increase the horsepower...what do you think it would be?


What do you think of this idea...when I took off the EFI, it had what appears to be a loop system (the fuel came up to the EFI and looped back to another return line with some kind of a vacuum switch that was plumbed into same vacuum circuit as the vacuum advance on distributor. Maybe to shut off the return flow of fuel under hard acceleration to increase available fuel????) that supplied a continous supply of fuel to the EFI and returned it to the tank....I had thought of as last resort, to plumb back the higher pressure loop system and do a tee-off of it and install my fuel pressure regulator to feed the carb....

Fuel Injection systems operate without a reservoir so they circulate high pressure fuel (130 psi) to feed the system. The constant feed reduces the pulse effect of the injectors opening and closing so you have a constant supply. So long as you can regulate the carb supply to 4-7 psi the float bowl will have sufficient fuel for all operations of the engine. A "T" will work as long as your regulator can hold back the high pressure and uniformly reduce pressure to the lower psi. If you exceed the 7psi at the carb you will overwhelm the float and needle and flood the carb. You might try installing a low pressure electric fuel pump inline and disconnect the high pressure pump. The in-tank pumps are generally rotary vane type and you can suck fuel theough them without any damage and still get the low pressure to the carb. If it we me, I'd find a hot rod mechanic and see how to fix the EFI. Apparently it worked before so something has failed. Good luck!

It is interesting to note how many different fuel systems we find on just the GM gassers. Apparently the factory recognized vapor lock problems by late 70's and began to cobble field fixes. By 84 we have external rear pumps. For 85-89 we have in tank pump recirculating systems. After 90 they were TBI, with pressures around 40 psi. Then there's the aftermarket stuff..

I have the 85-89 version in mine. I'm thinking of removing the engine pump since it seems redundant and is a potential leak. I'm not sure if or how I'll run the return line then. The tank pumps supply a steady 5-6 psi with or without the mechanical pump.

CHEV P3(X) OWNERS TAKE NOTE!!!

The 1995 Chev Motor Home Chassis guide has an Appendix dedicated to this. Evidently, this was omitted by GM in the 1997 version which is on the Barth CD. This might help locate parts you need to upgrade/retrofit. Here is what I have:

39a.) Chevy P32 Vaporlock Issues - 742 KB ~ Original post located here.

Still some problems...

I got the Edelbrock 1411 750 cfm put on.I have a low pressure fuel pump like Bill has, 6psi and 72 gph.

1. Coaches idles well but will die suddenly, restarts easily.It revs well and sounds good.

2. When driving, it will die, stumbles some on acceleration and worse it will backfire through the carb and die.


It doesn't smell like it's flooding. There are no adjustments for mixture except at idle.

QUESTION: Do you think I'm "over-carbeuratored"?

Edelbrock told me this carb was what I needed. Their installation CD seems to say only a 600cfm is needed. They said evidence of over-carburation is stumbling and poor performance. Why am I having backfiring? Timing issue?

What type of timing advance on the distributor? Did that get changed or modified with the F/I?

Assuming it ran well when the F/I worked, the timing must be OK. Normally I'd suspect a stretched timing chain, but not if it was fine prior.

Have you read the plugs? They should tell a story.

Are you sure all the vacuum lines are in proper order and condition?

Air cleaner? New, aftermarket, clogged?

Backfiring is a lean condition symptom (on quick deceleration). Too big a carb = too much air = equals lean condition; so, yes, I believe it could be too big. I forget, 454 or 350? Sounds really big for the small block. Did they know it was an RV application?

Sounds like fuel starvation... That carb's been sitting a while, be sure the float isn't sticking.

Bill is on to something... The stock GM manifold is a cast iron 2 plane design. The one you describe sounds like a single plane style. The single plane style gives more high end with some low end loss. Some of these may be desirable some not. What works on a street rod may be useless in a motorhome.

Given these changes I wonder if there is a trick cam or something else. Bottom line: Final tuning may be a bit voodoo.

I would think a 600 to 650 carb would be better, esp if you have a single plane manifold. Shoot for hp/torque around 3200 rpm. Good luck!!

quote:

Originally posted by Steve VW:
I'm thinking of removing the engine pump since it seems redundant and is a potential leak.

My 502 does not have a boss for an engine pump, so the elderly OEM Carter in back does it all.

Are you still in Texas? Detonation and stumbling also suggest a crossed spark plug wire as in cylinder 3 and 4. There was a tech note on the 454 that advised to not run the 3 and 4 through the same wire loom to stop magnetic crossover inductance. However if you are still in the region I suggest calling Steve Zepeda at S. Z. Carburetors. His number is (281) 484-6648. If it is a flow or jet issue he can fix you up. If you are already further along you might talk to him about setting up your carb after get home.
Good luck.

quote:

1. Coaches idles well but will die suddenly, restarts easily.It revs well and sounds good.

2. When driving, it will die, stumbles some on acceleration and worse it will backfire through the carb and die.

Slightly different approach: When I experienced ALL of these same symptoms, they were ALL cured by replacing ignition module in distributor (If you do, don't forget to add the grease heat sink.)

If it's fuel starvation, a fuel pressure gauge will tell. A vacuum gauge can be used as a temporary trouble-shooting measure. They usually have a few pounds of fuel pressure read by the back side of the needle in the fine print on the dial.

Another possibility is a poor 12 volt supply to the distributor. Driving it with a volt meter attached to the dist 12V lead will tell you if the voltage drops when the engine acts up.

There are small flexible wires in the distributor that flex with the vacuum advance. They can short out or develop an intermittent open. Up in the top side, the cap can get carbon tracks or dust inside can make the ignition weak, as can a failed resistor in the rotor, if so equipped. A weak coil can cause some of these symptoms.

I thought I had it figured out....this afternoon, I took the Edelbrock off. When I first put the carb on, when I took off the EFI unit, there was a square bore plate and the Edelbrock bolted right up to it..even had a perfect square bore gasket that fit it perfect also. I looked more and realized this square bore flange was actually an adapter plate. I took it off and the bolts were not very tight. I'm guessing that I might have been getting a vacuum leak causing the dieing. Once I took off this adapter plate, my manifold looks EXACTLY like the picture of SteveVW's he had posted a few days ago. So, I got a new gasket, used the Edelbrock adapter plate(which I had not initially) on the manifold and bolted everything right up. It seemed to run better but I'm still getting dieing. It will start, run fine and just die. I don't know if it's because of the choke letting off and going lean. The dash mounted vacuum guage doesn't give indications of low vacuum. My next thought is to disconnect all vacuum lines and see if that stops the dieing..

That's an update...