L78 idling issues

**JRSully** · #1 07-11-2009, 05:38 PM

Have spoke to the experts but figured I would get as many opionions out there as possible. My 70 L78 Nova has developed poor idling/stalling traits after driving it for 15-20 minutes or so. Specs: dead stock L78, except for headers and recurved distrib 11:1, orig Holley, 569 intake, heads, cam. One change I made recently was to swap the intake gasket to allow the heat cross over to function (stock set up) previous gasket blocked it (I wanted to now use the stock choke stove set up) That seems to be when the problem started. It will run fine until it gets warm and runs about 180-185 with no overheating issues. Once warm it will slowly die/stall out at idle and I will have to "feather it" to keep it going etc. When I first start it and drive it it idles fine
First inclination is that the fuel is getting "boiled' after it is warm as the intake itself is hotter now with the heat crossover and the general "flat design of the 569 intake". I have moved the fuel line away from the headers so I don't think that is an issue. I beleive that if I install the 70 factory Holley heat insulator plate uner the carb (do these things even work.?), that will cool the fuel bowls and also install also a 160 thermostat, that will cool the engine itself, hopefully that will cure the problem. Any opions are welcome, Thx for the tips Eric as well. SULLY

sYc · #2 07-11-2009, 05:54 PM

My guess would be the problem is with the choke.

wrench · #3 07-11-2009, 06:10 PM

Check for fuel inside the floats.I have found several floats with fuel inside lately. A small amount takes a while to act up.

ss427copo · #4 07-11-2009, 06:42 PM

I have a NOS of the Holley Heat Shield. Soulds like a perculation problem if it only happens after running....?

Jeff x 3

Keith Tedford · #5 07-11-2009, 08:01 PM

I went the opposite direction and blocked off the heat cross over and double gasketed the carb base. I'm still going to go with the aluminum heat shield that Jeff has pictured above. For summer driving, I feel that the carb and intake are getting too much heat. Now the carb isn't dry of gas after sitting a few days like it was before. A few years ago I had a brass float that took on gas. To see the crack, I had to remove the float to see the back side. When the float sank the car just plain flooded out right on the spot. Luckily I was at home.

**JRSully** · #6 07-11-2009, 08:05 PM

addressing the opinions, Rob, I currently have the choke wired wide open, so that one is probably out. Will have to check the floats if the 160 stat and heat shield install don't solve it. Is anybody running one of those ugly Holley shields currently and do they have any noticable effect.? I guess Chevy installed these for a reason. The 569 intake sucks and lays the Holley right on top of the intake runners, I should just throw a 163 on it and shelf the 569. Thx all for the tips, keep them coming

**SuperNovaSS** · #7 07-11-2009, 08:10 PM

Or put the blocking gasket back in.

Chevy454 · #8 07-11-2009, 08:14 PM

Have you looked for a vacuum leak, around the carb and/or intake...?

**azcamaros** · #9 07-11-2009, 08:22 PM

Two things come to mind. Have you adjusted the valves recently? If you have you may want to recheck them. I recently helped a friend with the same symtoms and discovered he had recently set his valves and got them a little too tight. As soon as our summer heat set in the car started giving him similar problems to yours. His coolant temp said 190 but the ambient temp. did make a differance.. Another thought is the rear power-valve. I have chased idle issues for days and overlooked the rear powervalve on my LT-1 vette. I don't no why it went bad but after a day of troubleshooting fun I replaced the re powervalve and I was back in action. I run my cars in 110 degree heat and my idle issues have never been related to boiling fuel, just other weird stuff.. Good luck

**Mr. Chevy** · #10 07-11-2009, 09:30 PM

Sully, I had a problem with my 70 L78 Nova when driving it. When it would get hot it would stall/shut off. I put the heat shield, GM one pictured above, on and never had a problem again.. I figured it was because the intake and carb. were soo close together it was boiling the fuel in the primary bowl. That shield really does work!!

It does not look that bad either under the carb... Rich

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