I sold my grandson an '84 Z28 305HO last summer that I had recently purchased. I replaced the cat before selling it to him to pass emissions. It has had apparent fuel starvation problems ever since he bought it. I initially thought it was vapor lock due to summer temps so we installed a Carter P4070 elec fuel pump wired with a switch, with filter and relay below the gas tank. With this pump on, it seemed to flood the carb even though it is supposed to be rated for carb use. Since then we have replaced the manual pump with a new Carter M6626, replaced the carb with a quality rebuilt exact replacement, replaced the distributor control module (Holley LX315), rotor and coil. We verified the return line isn't plugged and I made a vented gas cap. Gas seems to gravity flow out of the tank with no problem. Today in 40 degree weather, it will start and idle but when revving in place, it will rev a couple times but trying to rev it a 3rd time causes it to die. It restarts but not easily. We checked all vacuum lines and replaced a few. I'd love to toss all the emissions stuff but here they do a visual of all emissions equipment to be sure it is as it came from the factory. Stupid but I have no control on that. My son thinks I sold my grandson a lemon. It's frustrating that we can't fix it.

 

Did you replace your fuel filter? Being carbureted, you don't need very high pressure but good flow. Anything above 6 or 7 psi fuel pressure is not good for a carb. Good flow through your fuel lines? Get a fuel pressure gauge and see what pressure you have. If it dies due to lack of fuel with 3 stabs of the throttle, it should be easy to see your fuel pressure drop if that is the culprit.

 

Why Did You Do This,I Believe They Use A Pressure-Vacuum Cap.

 

Non-vented gas caps are for emissions, they prevent raw fuel vapors escaping to atmosphere. By venting the cap, you eliminate the possibility of vacuum buildup in the tank. Vacuum buildup can prevent good fuel flow and put undue stress on the fuel pump. You could have just cracked the gas cap open, to prevent vacuum buildup, as a test but you should probably get a new non-vented cap, especially if your emissions rules are strict.

 

I tried the vented cap after most everything else failed to fix the problem. I did it in case the carbon canister was plugged up. But stock cap or vented cap--no difference.
As for fuel pressure, that's why I installed the elec pump in the first place. It was reviewed as suitable for carb use, 5-7 PSI. Many reviewers said they used it on carbureted cars. But it flooded my carb. I believed the return line should have handled the excess fuel but it didn't. After that is when I replaced the mechanical pump. We tried both ways--with and without the elec pump. Then I replaced the carb not fully understanding what computer controls are in it and not knowing how to test it. Still seems to run out of gas.

 

Replace the fuel filter. That's all it is, most likely.



It's inside the big nut in the front of the carb where it's labeled "<---FILTER", behind that blue fitting in the pic. You'll need a 1" open-end wrench to hold the big nut still while you use a 5/8" crow's foot wrench to break the fuel line nut loose from it and then a regular 5/8" open-end to back the nut the rest of the way out; then use the 1" to remove the big nut; and the filter will probably come out with it. DO NOT attempt to remove the fuel line nut without supporting the big nut: you WILL destroy the fuel line!!! Pay careful attention to the filter's orientation, and put the new filter back in the same direction, spring first. Put the big nut back in BY HAND and tighten it BY HAND first, then finish tightening it with the 1", then start the fuel line nut into the big one BY HAND and tighten it BY HAND as far as possible, AT LEAST one full turn, then tighten it with the 5/8" open-end while holding the big one still with the 1". Be DOUBLE EXTRA TRIPLE SUPER CAREFUL not to strip any threads: that's an unrecoverable mistake that will disable the car until repaired.

 

[QUOTE=sofakingdom;5993106]Replace the fuel filter. That's all it is, most likely.



It's inside the big nut in the front of the carb where it's labeled "

 

I did something and my reply didn't show so I'll try again. This is a new Q-Jet and I purposely left the in-carb filter off to avoid problems. I installed a filter below the tank and before the elec pump months ago (car hasn't run enough to go anywhere since).
Fuel pressure check is a good idea but I will have to find a gauge. But when the elec pump is hooked up, carb floods, old and new carb. I have had the elec pump bypassed since the new carb was installed, assuming new mechanical pump should do the job.
Why wouldn't return line at mechanical pump handle excess flow, if any, from elec pump? That's one reason I replaced mechanical pump, in case return circuit was plugged or something.

 

There's your problem, right there.

Get rid of that. Fuel pumps can't suck. All you have done, is put a massive blockage, (2 of them actually) in a place that the pump can't overcome it.

Do away with the electric pump entirely. It is totally unnecessary. Do away with the filter on the suction side of the system.

Put the correct filter back where it goes.

99.9999% odds, that will put your car back running again just fine.

 

Sofakingdom, I respect your knowledge and I will try your suggestion, even though I had a good gravity flow out of the filter back when I installed it. Unfortunately, with Denver's current weather (snow on ground for 2 weeks now), it may be a while before I can get under the car and remove the filter. I will report back after I have finally made the change with the results.

 

the 84 HO 'should' have an in-tank pump to assist with fuel flow in addition to the mechanical pump on the block.

I'd verify it is present and working.

 

Naf, my research shows that was a voluntary dealer replacement back in the day. Mine doesn't have one as shown by the blank spot in the fuse box.

 

 

gt4373, thanks for the input. Your source is wrong in one respect. All small block V8 Chevys have the mechanical fuel pump on the right front not left front. Maybe they mean as you look at it from the front of the car.

I fail to understand why when I run the elec pump excess fuel isn't returned to the tank. As I said before, that's why I replaced the manual pump. And I figured if the float was bad on the old carb causing it to flood, the new one surely wouldn't flood but it does with the elec pump on. I have rebuilt old school Q-Jets; maybe I should pull the top off and check the float to be sure. But neither carb flooded with just the mechanical pump so...

Also, I purposely titled my post "Apparent" fuel starvation because I'm not sure that is what the problem is. That's why I replaced parts in the distributor to see if the problem was spark. I understand old tech engines well, having worked on them for 40 years or so. But throw in the patched on emissions junk this car has from the factory and I'm lost for potential conflicts. I'd love to toss everything and get back to a basic engine but next emission test, they'd fail the car on visual inspection.

As potential useful info, I will describe what my grandson devised last summer. When driving, he could feel the car start to die out. At that point, he'd turn on the elec pump briefly. When the car resumed normal running, he'd turn the pump off and drive awhile. That's why I first thought it was vapor lock and the elec pump should have fixed it but then the flooding happened if the pump was on too long. It has steadily gotten worse to the point that he doesn't even try to drive it although he misses the car terribly. I don't understand why it is worse now in the middle of winter than last summer. It won't run long enough to move the car 50 feet.

 

I understand your hesitation with the emissions equipment but, none of the emissions equipment should cause a starvation problem. You say you are not sure if that is the problem. This is why you need to plumb in a pressure gauge and see what the fuel system is doing. JMO!

 

when the car dies after three revs, check the carburetor for fuel by actuating the booster shot and observing for fuel squirting into the primaries.

if I understand correctly you first installed an electric helper pump near the tank

second installed a canister filter between the new electric pump and tank

third replaced the carburetor with a reman unit

fourth replaced the fuel pump with a new M6626

all of this is still hooked up? otherwise you wouldn't be able to turn on the electric pump to clear it?

you've already verified that the return line is clear by blowing air through it? and observing fuel return from the M6626 by removing the rubber return line while the motor is running?

your symptoms are consistent with either a failing fuel pump or blocked feed/return line. there is no piece of emissions related equipment that I could conceive as causing, or contributing to, your problem (since the vented cap had no effect).

 

This is my car. There is no "massive blockage". I wired the switch myself so that I can flick it when apparent fuel starvation happens, and it saves the car. That's when it was hot and we thought it had vapor locking issues. The best thing I can think of right now is to put a regulator on, and make sure the return line works correctly.

 

Especially when there is two pumps sucking fuel from the engine.

 

GLWT

I used to wonder why a system that works perfectly fine in thousands of other applications suddenly, sometimes, requires some alternate engineering to operate correctly in a specific instance. I just chalk it up to the operator now and move on.

 

When we purchased our 84 Z28 HO, it had sat for several years without being run. Before starting car we rebuilt the stock carb and replaced fuel filter. Mechanical block mounted fuel pump was bad so replaced it. Started car and it run fine. On 100 mile trip home it started sputtering and died. Next day it started and run fine for awhile then same problem. Again rebuilt carb which was already very dirty inside and replaced fuel filter.

Car drove great again for month or so (not very many miles). Replaced plugs, wires, vacuum hoses, fuel line hoses (sometimes a collapsed hose will cause this) everything I could think of. Car still did the same.

Started to check into somebody to work on it for me and mechanic friend said the first thing they will do is replace the fuel filter even if customer said they did. So I went ahead and changed fuel filter again. Car ran fine.

I cut open the filter and it was completely plugged after driving a month or so.

Removed gas tank and it was rusting out. Purchased new tank and sending unit as gauge did not work. Old one had a 3-5 psi electric in tank pump (in addition to block mechanical) and pickup was broken above it. After researching it, I was under impression that all HO's had the additional electric pump. You can tell by how many wires are going into fuel tank. We ended up eliminating the in tank pump.

I would get proper new fuel filter in place to see if that helps and cut open old filter.

Older cars sometimes need a lot of tlc to get them working good. I know we have replaced a lot of parts.

 

Not much I can add that others haven't touched on, above. Agreed, you need to get a fuel pressure gague on that thing, at least temporarily, and see under what conditions (if any) you are losing fuel pressure at the carb. My guess? Given you've replaced the entire fuel system at least twice and tried two different carbs.... maybe the problem isn't with the carb or fuel system. Yeah, I know that sounds stupid, but I have a nasty habit of using logic.

However, the "fuel return line" system on a mechanical pump is IN NO WAY THE SAME as the return system on a typical EFI regulator or stand-alone bypass-style regulator. The return line on a mechanical pump is nothing but a small bypass hole inside the pump (I'm gonna say it's probably no bigger than ~.090") and is NOT used in pressure regulation. It's just there to make sure there is SOME flow through the fuel lines in low-demand situations so fuel can't sit in a line near something hot (exhaust) and slowly heat up, causing vapor lock. Don't believe me? Block it off completely- your pressure won't change.

I only mention this because if you've got an electric pump out back that is over-pressure and you're depending on the little bypass line on the mechanical pump to bleed it off to a carb-friendly pressure for you.... ain't gonna happen. It's just not big enough, nor does it function in the way an EFI or stand-alone bypass-style fuel pressure regulator does.

BTW- I use that same Carter electric pump IN ADDITION TO a Carter street mechanical pump (no regulator) on my Malibu (signature, below). Aside from the pump being a noisy bastard, the whole setup is bulletproof. I don't think I've touched it in more than 10 years. So, not sure why you're having such difficulty.

 

To all those who contributed--thank you. I replaced the rebuilt factory QJet with a Summit carb (non emissions) and I replaced the factory distributor with an aftermarket non-emissions HEI. The Z28 now runs well. I also found that the fuel filter quickly clogged. Like dlinger, it appears that our tank is rusting from a previous several-year period of inoperation, so we will have to replace the tank in the near future. In the meantime, we will keep an eye on the filters (we installed another one just before the carb). I just replaced the one below the tank yesterday, it was already filling with rust particles.