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Discussion Starter · #1 ·
I have a 68 Chevelle SS with a 66 corvette 427 motor with GM 502 aluminum performance heads .030 over bore, speed pro pistons and 1.7 roller aluminum rockers. I recently put in a 533/533 lift-313/313 duration cam and had to do a lot of fine tuning with timing, rejetting my holley 800 dual feed and internal distributor work. Electrical now is great. Car runs strong. I also installed a new mini high torq starter, new battery, recurved the distributor and put in lighter springs and a new holley 80 gpm mechanical fuel pump and new rubber on the gas line. When I start the car fuel pressure is around 9 lbs. After idleing or running car around 10 minutes fuel pressure start to drop to 3 or 4 lbs. I have a glass fuel inline filter and it shows about half filled, no regulator just a pressure guage. I was told maybe the pickup sock in the gas tank is blocked or maybe I have a bad fuel pump that when it gets hot the spring is weak. Any ideas.
 

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Are you having problems related to the fuel pressure? What type of guage do you have, and where is it? If it's one of those little liquid filled ones installed in the engine bay, I'd suspect the guage is giving false readings wehn warm. They are notorious for that. You can try to "burp" the guage (peel back the little rubber nipple on the back of it), or replace it with something better. Electric, or a mechanical isolator with a mech guage in the car. I replaced my little liquid filled one with an electric one, and I discovered that I didn't really have any pressure problems. It was all those stupid little guages. I really don't know why they keep making and selling them :mad:

The filter being half full is not a problem, they just do that. I'd replace the glass one with a metal or plastic one though, just for safety's sake. I don't like glass holding fuel anywhere near my engine.... just seems like trouble waiting to happen.
 

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Discussion Starter · #3 ·
I appreciate your quick reply. I made a mistake, I do not have a glass one, I changed to the plastic about a month ago because the gas was always leaking. For the gauge, I do have one of the liquid filled one connected to my dual feed arm next to the carb but I really don't believe that to be my problem. I just put in my third fuel pump and this one looked promising for about twenty minutes. I started of with almost 8-1/2 lbs and my filter was almost 3/4 filled,went for a 10 minute ride and felt it starving for gas got it home and I was down to almost 2 lbs and my filter was almost empty, a few drop to keep it idleing. I am running out of ideas and it's getting very expensive. I even opened my gas cap thinking maybe a vapor lock but no difference. Thanks God for an understanding wife. There aren't too many of them around.
 

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Ahhh..... sounds like you have something besides a guage issue.... :D

I'm with Dean. Sounds like something restricted or plugged upstream of the pump. What kind of shape is the rest of the fuel line to the tank in? No soft rubber lines, or pinched or kinked hard lines? Maybe it is the sock in the tank.... but that's more work to check.

Hehe, I remember one time with my old beater chev pickup..... same thing. Starving for fuel, and cound;'t figure it out. I replaced the filter in the engine compartment, and still, no good. A few days later, it finally died on the side of the road, and I couldn't get it started again. While looking EVERYTHING over, I found a second fuel filter inline, up in the frame rail near the tank. Sure enough.... it was really, really plugged :D :D
 

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Before you go to far, try running a long piece of fuel hose from the inlet on the fuel pump to a can of gas, if the pressure stays up at 9lbs then it's in the tank. If it still drops it might be the pump.
 

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Discussion Starter · #7 ·
Thanks for all your answers but here is where I stand now. I am on my third fuel pump changed all the rubber in the fuel line with heavy gauge gas line, put in a new gas tank and sending unit, blew compressed air while everything was off. Still have my problem. As soon as it get to operation temperature it starts to drop from 9 lbs and works it way down. After 10 minutes of idle time it's already down to 5 lbs. and if it runs longer it drop all the way down. I don't believe it's only a gauge misreading because if I go for ride when hot it is actually starving for gas unless the liquid filled gauge is preventing gas from getting thru the line. I really thought it was my gas tank and sending unit but that didn't help. Now I'm really stumped.
 

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a liquid gauge will show a loss in pres-also, a mech pump will lose pres as it warms up-are you using a gasket bet the pump and block? could you be getting vapor lock
when i tried one of the howard stewart mech race pumps, i found out a lot about heat and mech pumps, and under hood gauges
i have a liq gauge on my carb, just for setting idle f/p, and a hi dollar elect gauge in the car, i see a 3-4 lb loss in the liq gauge after warm up
 

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Paul,i dont see anywhere where you state your having any fuel delievery issues when driving the car or when getting on it unless i missed it.

If thats the case dont overthink it,drive and be happy.

But if your experiencing top end fuel delivery issues then yes the strainer /sock on the fuel pickup in the tank could be partially clogged causing the problem but keep in mind there are other things that can cause this type problem too..

Also,i have seen where the heavy stock gm f/pump rod can cause pump arm float at 5k rpms and above even with some hi perf f/pumps too so the stock heavy pump rods can float pump arms and not just @ 6k rpm and above as advertised .

And BTW,setups with mech f/pumps that had fuel psi gauges i have worked on over the yrs always dropped in fuel psi when they heated up but ran fine. It's when you experience fuel delivery issues esp on top end that it becomes an issue to deal with IMHO.

Scott
 

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Discussion Starter · #10 ·
Thanks Scott and everyone else for all your info and yes I was having fuel delivery issues even after I changed all the rubber gas lines and also the line from the pump to the carb and three fuel pumps. I just changed the tank and sending unit and I took it on its first ride yesterday and it seem to run better even though the pressure dropped from 9 lbs. to 3 lbs. but I am not concerned with that as long as fuel delivery is better. I think I'm going to change the guage to correct that. But since I changed the tank and sending unit it runs better. My only problem now is engine temp is about 220 when idleing and running around 180, and with gm 502 aluminum heads that is too hot. Last year I was running 170 and 195 idleing which was great.I changed the thermostat with a 165 but that didn't help. I think I am going to put in a electric fan to cure that)hopefully). I developed these problems I think since I put in a bigger cam over the winter. Hopefully my fine twicking is just about finished. I would like to enjoy it now that the warm weather is finally here.
 

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Paul,you stated you installed a lerge cam and it now runs hotter,are you running enough base timing like 18deg btdc which many aftermarket pref cams require?

If your running much less then 18 base timing then that can be contributing to your increased engine temps .

you also need to be running approx 18 deg mech in the dist all in by approx 2800rpm so then 18 base + 18 mech in dist = 36 in by 2800 rpm which is a goos starting place for total timing.

Also,dont forget if its street motor to run a vacuum adv thats fully activated at 2" below the lowest idle vacuum (auto trans in gear) hooked to full intake vacuum all the time that add's an additional 12-14 deg timing for when at low load/part throttle cruise.

Retarded timing can and will make an engine run hotter so keep that in mind.

Scott
 
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