sonny_burnett

My car runs great when cold, or as long as I don't shut it off. But if I shut it off and come back in say 10 minutes, it won't start. I'm pretty sure I have a fuel boiling issue and need a carb spacer, but the spacers I see on jegs say they are not for intakes with a heat crossover. Did the early 70s LT-1 intakes have a heat crossover? I think mine is blocked but I really don't know, is there a way to tell without removind the intake?

bobs77vet

what carb are you running? maybe a heat shield will help if you have a holley

mr.beachcomber

I've seen hot fuel percolate out of the booster venturis on my race Vettes due to heat soak, but never on a street car. Have you observed this with your Vette?

If not, take the air cleaner off, manually open the throttle,and check to see if you have a stream of fuel coming out of the accelerator pump nozzles. If you have enough liquid fuel for the pump shot, check your fuel level on the front bowl of your Holley to make sure it's at the correct level.

If you can't pass both of these tests, you may have a fuel leak somewhere in the carb that is allowing raw fuel to puddle in the intake manifold causing an over rich condition making the car hard to fire. Do you see black smoke coming out of the exhaust once the car fires?

All the stock LT1/Z28 intake manifolds had exhaust heat crossovers to preheat the intake air/fuel mixtures. To find out if yours is completely blocked off on both sides, you really need to remove the intake manifold. A quick check on a cold engine would be start the engine and see if either crossover on the intake manifold becomes hotter than the surrounding area. (I'd use a meat thermometer rather than my fingers just is case!)

Good luck!

Rmorgan&11

Needle & seats may be bad also allowing pressurized fuel to continue to pass into the intake untill psi bleeds off ??

Rich's'78

When you say it won't start, presumably it is cranking just fine. If not, the problem is probably starter heat soak.

James

This is a issue that drove me crazy on a Holley 4160 Vac Secondary carb I had on 1968 Camaro. After driving the car I would shut it off, pull off the air cleaner, and like clock work in a matter of seconds I would start see fuel drip out of the boosters. I replaced the needle and seats, checked float levels, check fuel pressure, lowered the float levels, tried heat shields with spacers, and still had the issue. I finally pulled the carb off and soaked the entire carb in solvent and blew out every orifice I could. What I finally discovered was that air bleeds on the secondaries were partially plugged on one and totally plugged on the other. Hit them with some carb cleaner and hi pressure air and "Pop" they opened up. Problem solved no more boiling fuel out of the carb into the manifold after I shut the car off. All I can think of is some kind of vacuum was created when those air bleeds were plugged that caused fuel to be pulled out of the bowls and into the carb when the car got turned off. Maybe others might chime in?

Easy Mike

My '74 would do it, almost always when I didn't want her to.

sonny_burnett

The engine cranks ok, I will probably change to a high torque starter later, but that doesn't seem to be the problem. The car has always been hard to start when hot, originally it had dual quads and ran like garbage. I put an LT-1 intake and a 4555 on and I think it was ok for the most part but I hardly ever drove it. The 4555 became clogged with crap due to no filter on the fuel line. I bought a 4150 hp and swapped it on, car runs great now, except won't start after it it heats up and I shut it off for 10 minutes or so. I took the air cleaner off and I can hear hissing and see fuel pouring slowly down the barrels into the intake. The car runs hot, and the percolating fuel seems to fit, although the air bleeds could be a problem I guess.

I was asking about the heat crossover because all the carb spacers I see on jegs say not to use them on a manifold with a heat crossover, but don't say why. I also thought about wrapping my headers to reduce the under hood temp, but header insulation is more money than I thought, so I think I'll be putting that off and trying something cheaper first.

The air bleeds are right on top of the carb, and just unscrew right? I probably still need to take the carb off to avoid dropping them down the intake though.

SH-60B

I would definitely block off the heat crossover. Have you tried a fuel filter with a return line? I've seen the on early 70s smallblocks.

doctorgene

G.M. sold a heat shield for the Q-Jet also, it stops the carb from boiling. Have a nice day Gene

sonny_burnett

I just took it for a spin, before I left I sprayed some carb cleaner into the air bleeds for good luck and took some temps when I came back. The manifold on the drivers side got up to 230F in the middle, but was about 180F everywhere else or cooler. The carb only got up to 135F on the primary fuel bowl, the secondary and main body were cooler than 135. Water temp in the car was cooler than usual, it stayed under 195 the whole time (thermostat is 195), usually will run slightly over 200 even when it's cold outside and will run up to 220 or so on a warm day, nearly overheated when I took it over the pass on a 95 degree day this summer.

I'm going to go check and see if the carb has started hissing and pouring fuel like it did before.

sonny_burnett

Yep, still hissing and flooding. My IR thermometer is high on crack too, it reads the same temp on the carb flange and main body, I tested that out with a finger, the flange is way hotter. The fuel bowls and main body are pretty warm, but not burn your hand hot, the flange will burn you if you leave your finger there for more than just an instant. Fuel line is warm, but not too bad and doesn't seem like it is too close to anything.

Adding a line back to return seems like a good idea, I have hard lines from the pump to the carb though, so adding a filter after the pump will have to wait until the weekend at least.

sonny_burnett

I took the air cleaner off and opened the throttle 3x, each time the accelerator nozzles sprayed fluid, seemed like I had enough coming out. I removed the sight screw on the front bowl, I can't tell where the fuel is, it isn't at the threads though. I checked it again after attempting to start the car and still not at the bottom of the threads.

I'm not sure if it blows black smoke when I start it cold, I'll check tomorrow, the one time I had the door open and started it when it was warmed up it blew a small amount of black smoke for a second or so.

noonie

I block the crossover and also always use one of these heat shield, but I am in hot weather. Low 90's today.
Hard to see but this is a thin ss heatshield from Advance auto.
Also use the return line to keep consistant low fuel pressure, but it helps with heat too.

Paul L

Noonie,
Here is another variation on that. The intake is a stock 1974 cast iron painted with high-heat cast aluminum enamel. But I believe the OP is using a Holley.

mr.beachcomber

Does you new carb have a fuel filter installed in it or in the fuel line? It's your number one protection against debris holding the inlet needle & seat open.

With the engine shut off, could you tell where was the raw fuel coming from: the booster venturis; the idle air bleeds; or the fuel bowl vent? These are the three main areas for excess fuel to leak under hot soak conditions. If you can't tell, is the fuel leaking from the gasket line between the main body and the throttle body? (Just trying to pin down the leaky source of the problem.) If this leakage is occurring while the engine is running, you could have a stuck float or a needle & seat that can't close.

The main air bleeds and the idle air bleeds are pressed in and cannot be unscrewed. Cleaning them w/carb cleaner is a good idea though as they tend to collect gum and varnish over time.

In another post you mentioned the temperatures you took on the intake manifold and the carburetor it self. From your description of the temperatures, it appears that the driver's side exhaust crossover is open, but the passenger side is either blocked or greatly reduced in size. The other temperatures you quoted don't sound unusual for a closed hood environment. Were these temperatures taken immediately after shutting the engine off, or did you wait ten minutes to see what they were after heat soaking for that period of time?

You primary fuel bowl level is too low. With the carb running at idle with the engine fully warmed up, the fuel level should be even with the lower edge of the sight plug hole as a general rule of thumb. If it isn't, make sure that there isn't a fuel leaking from the front bowl before your raise your fuel level. (That would just add more fuel to the present problem.)

Hope this info helps!

sonny_burnett

I added an inline filter before I swapped the 4150 on, and the metering blocks on the 4555 are hosed, talking to holley there may be additional issues with my 4555 that are unrelated to this problem. I also have the sintered brass filters in the fuel bowls, so I shouldn't get much crap into the carb, although I probably should have put the filter after the pump instead of near the tank.

I'll have to check and see if I can tell where the fuel is coming from when I shut it off, I seem to recall seeing it stream down the sides of the barrels, which makes me think it's coming out of the accelerator nozzles, but I need to check again to be sure.

When I check the float level the car should be running or does it matter?

I checked the temperatures immediately after shutting down, and again after a few minutes, they didn't seem to change much, but the IR thermometer doesn't read correctly, probably because of the different materials I'm trying to measure temp on. I'd believe the 135F on the carb, I could put my finger on the carb anywhere but the flange and not burn myself, and the fuel line wasn't overly hot either. Also, if it makes a difference, it doesn't immediately start dumping fuel into the manifold, I popped the lid off the air cleaner right after I shut down and everything seemed ok, came back in about 5 minutes and there was fuel puddled up on top of the butterflies and it was hissing and there were gas vapors coming up out of the carb.

mds3013

Hello Sonny. I had the same problem on my '77 with a Holley intake, Holey 3310 carb. and Holley air cleaner. After driving car fuel would boil out of accelerator pump nozzels and down over venturis. This caused flooding and hard to start. I blocked heat crossover, added mulit layer insulator of gasket-aluminum plate- gasket- plate- gasket- plate about six layers worth. This solved the problem on all but the hottest days. Carb. was brand new at time. good luck, mds...

schmegeggie

Same here. Already had heat crossovers blocked, carb was new, and it was happening to my newly rebuilt 383. Put in a spacer and problem was solved. Still have 3/8" hood clearance...whew!:o

Faster Rat

Been using a stainless heat shield under my quadrajet with moderate success. When the intake manifold was off during a recent rebuild, I tapped (not completely thru) the heat crossover openings on each side and screwed in plugs (with Loctite) that I made out of a couple of cut-off bolts. Hopefully this will prevent exhaust gases from even getting near the base of the carb. Too damn hot down here in Florida.