Rough idle when hot, fine for most other times

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TheBigE

70 Duster - Slant 6
Joined
Dec 4, 2008
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London, Ontario, Canada
Hi gang.

I'm having trouble with my 225 slant 6 in my 70 Duster. It is a completely stock engine with the Holley 1920 carb on it.

It starts instantly when hot or cold, picks up speed well and idles well when cold while the choke is on.

But when it gets warmed up, the idle surges then almost dies, then isfine for a few seconds. So if you're in gear at a stop light you go between almost stalling and having to hold the brake hard not to move. It does a similar thing when you're just coasting at 40mph or below, so you're not on the gas pedal much and just maintaining your speed. The car kinda surges then dies randomly, not a steady drive at all.

Any ideas what could be the cause? I think I have the fuel mixture right and don't noticeably hear any vacuum leaks,but I could certainly by wrong.

Thanks or any help.

The Big E
 
sounds like a lack of gas after warm. also check fuel lines for hot spots if the fuel boils it will run like you say.
 
yeah, the choke is opening completely when warm. The car came with no choke at all so I found one and put it on. Works well.

That's an interesting point about the fuel overheating...I think that the fuel line from the pump outlet should run down low at fuel pump height and then come around the front of the engine to the carb from down below in a metal pipe. That setup is gone and the previous owner has a rubber gas line running directly up from the fuel pump to a fuel filter. then the line goes over the valve cover and right to carb in a straight line from the fuel pump.

Should I reroute to keep the fuel cooler and see what happens???

thanks for the info so far.

Big E
 
I think the fuel getting too hot is the issue as advised. I rerouted the fuel line basically like factory and it ran better than ever!!

When we get into the really hot weather though I may try some fuel line insulation to keep it cooler still and maybe some type of float bowl heat shield maybe??? Should I try and locate a real factory metal fuel line to keep things cooler or is the 5/16 inch rubber hose just as good??

the Big E
 
TheBigE said:
I think the fuel getting too hot is the issue as advised. I rerouted the fuel line basically like factory and it ran better than ever!!

When we get into the really hot weather though I may try some fuel line insulation to keep it cooler still and maybe some type of float bowl heat shield maybe??? Should I try and locate a real factory metal fuel line to keep things cooler or is the 5/16 inch rubber hose just as good??

the Big E
Click to expand...
Back in the day we used to use cloths pins on fuel line to prevent vapor lock on stock fuel lines. I still run them today as a joke on my 64. Everyone asks. I have only had one old gentleman ask me if I was vapor locking. LOL!:cheers:

64 vapor lock.jpg
 
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