1970 LS5 Fuel issue
Thread Starter
Intermediate
Joined: Jul 2007
Posts: 43
Likes: 0
From: Elgin Ontario
1970 LS5 Fuel issue
I have a 1970 LS5 completed a complete frame off 4000 miles ago.I have to run a fan clutch eliminator, if I don't the car will cough and sputter and kick back through the carb until it stops and I have to wait about 15 minutes for it to cool then it will start. Acts like vapor lock. It is the original carb rebuilt, I also replaced everything new including the tank . Temperture with the fan clutch is 180 and with the eliminator it runs 160. It is a new fan clutch. I also put an aluminum gasket under the carb that extends under the fuel bowl. Car runs great as long as the temperture stays down which is all the time as long as the eliminator is used. I would like to get rid of the extra noise caused by the eliminate speeding the fan and have considered an electric fan but I am trying to keep the car all original.
thanks from Ontario Canada
thanks from Ontario Canada
Melting Slicks
Joined: Mar 2010
Posts: 2,270
Likes: 375
From: Randolph nj
is the fuel line close to the block ,,also check your pick-up in the tank for debris,but it really sounds like a like your chokes not released fully when the motors hot ,,good luck Terry
Last edited by forman; Oct 11, 2011 at 06:43 PM.
Safety Car
Joined: Sep 2007
Posts: 4,458
Likes: 331
From: Indy Indiana
sounds like your carb is getting too hot and percolating
make sure you run a 160 degree thermostat.
check your exhaust cold bypass flapper valve under your exhaust manifold, it may be stuck closed and sending heat across your intake manifold, causing it to overheat
aluminum gasket just passes heat, get you one of those phenolic carb spacer/insulator. don't go past a 1/4 inch so your hood will still close. Your carb should run lots cooler
at speed, the radiator is cooled by the ram air affect much more than the fan.. the fan takes over its duties while idleing or being slow in traffic where the ram air effect is weakened. the clutch eliminator is just using horsepower that shouldn't be required to use.
make sure you run a 160 degree thermostat.
check your exhaust cold bypass flapper valve under your exhaust manifold, it may be stuck closed and sending heat across your intake manifold, causing it to overheat
aluminum gasket just passes heat, get you one of those phenolic carb spacer/insulator. don't go past a 1/4 inch so your hood will still close. Your carb should run lots cooler
at speed, the radiator is cooled by the ram air affect much more than the fan.. the fan takes over its duties while idleing or being slow in traffic where the ram air effect is weakened. the clutch eliminator is just using horsepower that shouldn't be required to use.
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
LS5s came from the factory with a 180 degree thermostat and ran just fine at that temp. They still do.
Make sure that you didn't downgrade your car with a two port fuel pump, instead of the proper three port. Fuel vapour lock or percolation is unheard of with the correct pump and return line.
Make sure that you didn't downgrade your car with a two port fuel pump, instead of the proper three port. Fuel vapour lock or percolation is unheard of with the correct pump and return line.
Melting Slicks
Joined: May 2009
Posts: 3,079
Likes: 315
2024 C3 of the Year Finalist - Unmodified
When I had my intake manifold off, I was told to ream and tap each side of the exhaust crossover under the front of the carb and install screw-in plugs (with Loc-Tite) that I made from threaded rod. This stops the heat from even getting to the stainless plate and gasket that I have under the Q-Jet.
My exhaust heat riser has been tack welded open. Then again, I live in Miami and don't worry about warming up the engine.
I run the recommended thermostat. Your cooling system was designed to run at that temperature.
Check your timing...sounds like it may be retarded. You should be running 12-16 degrees advanced at idle, no vacuum advance.
Make sure you are running full manifold vacuum, not ported...which is for emissions and really hurts your idling/shutoff/startup characteristics.
My exhaust heat riser has been tack welded open. Then again, I live in Miami and don't worry about warming up the engine.
I run the recommended thermostat. Your cooling system was designed to run at that temperature.
Check your timing...sounds like it may be retarded. You should be running 12-16 degrees advanced at idle, no vacuum advance.
Make sure you are running full manifold vacuum, not ported...which is for emissions and really hurts your idling/shutoff/startup characteristics.
Thread Starter
Intermediate
Joined: Jul 2007
Posts: 43
Likes: 0
From: Elgin Ontario
LS5s came from the factory with a 180 degree thermostat and ran just fine at that temp. They still do.
Make sure that you didn't downgrade your car with a two port fuel pump, instead of the proper three port. Fuel vapour lock or percolation is unheard of with the correct pump and return line.
Make sure that you didn't downgrade your car with a two port fuel pump, instead of the proper three port. Fuel vapour lock or percolation is unheard of with the correct pump and return line.
Thanks again for your reply and yes I have the 3 port, all new lines, new tank, proper routing of lines, correct rebuilt carb. One question. When I release the fuel cap it seams like there is either pressure or vacuum present, is the normal?
Thread Starter
Intermediate
Joined: Jul 2007
Posts: 43
Likes: 0
From: Elgin Ontario
When I had my intake manifold off, I was told to ream and tap each side of the exhaust crossover under the front of the carb and install screw-in plugs (with Loc-Tite) that I made from threaded rod. This stops the heat from even getting to the stainless plate and gasket that I have under the Q-Jet.
My exhaust heat riser has been tack welded open. Then again, I live in Miami and don't worry about warming up the engine.
I run the recommended thermostat. Your cooling system was designed to run at that temperature.
Check your timing...sounds like it may be retarded. You should be running 12-16 degrees advanced at idle, no vacuum advance.
Make sure you are running full manifold vacuum, not ported...which is for emissions and really hurts your idling/shutoff/startup characteristics.
My exhaust heat riser has been tack welded open. Then again, I live in Miami and don't worry about warming up the engine.
I run the recommended thermostat. Your cooling system was designed to run at that temperature.
Check your timing...sounds like it may be retarded. You should be running 12-16 degrees advanced at idle, no vacuum advance.
Make sure you are running full manifold vacuum, not ported...which is for emissions and really hurts your idling/shutoff/startup characteristics.
Thanks
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Race Director
Joined: Jul 2001
Posts: 15,892
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A certain amount is normal- is your charcoal canister hooked up properly? That's what controls how much pressure is present in the gas tank.
Thread Starter
Intermediate
Joined: Jul 2007
Posts: 43
Likes: 0
From: Elgin Ontario
sounds like your carb is getting too hot and percolating
make sure you run a 160 degree thermostat.
check your exhaust cold bypass flapper valve under your exhaust manifold, it may be stuck closed and sending heat across your intake manifold, causing it to overheat
aluminum gasket just passes heat, get you one of those phenolic carb spacer/insulator. don't go past a 1/4 inch so your hood will still close. Your carb should run lots cooler
at speed, the radiator is cooled by the ram air affect much more than the fan.. the fan takes over its duties while idleing or being slow in traffic where the ram air effect is weakened. the clutch eliminator is just using horsepower that shouldn't be required to use.
make sure you run a 160 degree thermostat.
check your exhaust cold bypass flapper valve under your exhaust manifold, it may be stuck closed and sending heat across your intake manifold, causing it to overheat
aluminum gasket just passes heat, get you one of those phenolic carb spacer/insulator. don't go past a 1/4 inch so your hood will still close. Your carb should run lots cooler
at speed, the radiator is cooled by the ram air affect much more than the fan.. the fan takes over its duties while idleing or being slow in traffic where the ram air effect is weakened. the clutch eliminator is just using horsepower that shouldn't be required to use.
Thanks
Thread Starter
Intermediate
Joined: Jul 2007
Posts: 43
Likes: 0
From: Elgin Ontario
Thread Starter
Intermediate
Joined: Jul 2007
Posts: 43
Likes: 0
From: Elgin Ontario
Thanks for the feedback
Race Director
Joined: Jul 2001
Posts: 15,892
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Melting Slicks
Joined: Apr 2007
Posts: 3,189
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From: Tolland CT
Mine is an early build, March 3, 1970..
Team Owner
Joined: Jul 1999
Posts: 26,545
Likes: 46
From: Huntersville NC
LS5s came from the factory with a 180 degree thermostat and ran just fine at that temp. They still do.
Make sure that you didn't downgrade your car with a two port fuel pump, instead of the proper three port. Fuel vapour lock or percolation is unheard of with the correct pump and return line.
Make sure that you didn't downgrade your car with a two port fuel pump, instead of the proper three port. Fuel vapour lock or percolation is unheard of with the correct pump and return line.
Team Owner
Joined: Jul 2008
Posts: 23,419
Likes: 22
From: In limbo
St. Jude Donor '13-'15, '17 thru '22
When I had my intake manifold off, I was told to ream and tap each side of the exhaust crossover under the front of the carb and install screw-in plugs (with Loc-Tite) that I made from threaded rod. This stops the heat from even getting to the stainless plate and gasket that I have under the Q-Jet.
Melting Slicks
Joined: May 2009
Posts: 3,079
Likes: 315
2024 C3 of the Year Finalist - Unmodified
I don't think '70 had the exhaust crossover. I know I don't have one on my car, which in and of itself proves little. But I am pretty sure my manifold checked out as proper date code and part number, and I don't see how it could have a crossover in it. I am pretty sure I could say the same about my exhaust manifolds (I don't remember if they have date codes or not).