View Full Version : hot carb?

tony blanco
05-04-2001, 11:11 PM
my '68 108 has a rebuilt 250, with a rebuilt rochester i-bbl. problem is that after running awhile and then being shut-off, the carb heats up, its hard to restart and blows a lot of black smoke when it finally starts. i tried using a thicker spacer gasket between the carb and manifold and that helped a little but the problem persists. this is the second carb this has happened to. Also, gas mileage seems poorly, although gas gauge and odo are currently on the fix-it list, so hard to get an accurate estimate. any ideas out there? appreciate any help.

05-05-2001, 12:22 AM
Possible cures: Try an adjustable fuel pressure regulator, lower the float level, or rig up a bypass system to bleed off the residual fuel pressure immediately after the engine shuts off. Good Luck! --- TV

05-05-2001, 12:23 AM
Hey Tony,

I've been thinking about this and I keep coming back to the notion that the carb is out of adjustment. My other thought is that the timing could be off.

05-05-2001, 09:49 AM
Ya know, my 65 with the same set up and new carb and fuel pump does the same thing. It has a 1/2 inch aluminum spacer too. I've been struggling with it for months. Carb and timing are set to specs.
When the engine stops the fuel pump push rod may be in the power stroke and will hold pressure on the needle and seat. If the carb and fuel line is too hot the fuel will expand and force fuel past the seat and over fill the bowl and fuel will leak if the air horn is warped or the check ball on the power pump is not seating tight.
This is not the gospel, rather my own long term conclusion.
Try a test, after you have driven it till it's hot, open the fuel line at the carb and catch the gas with a can. Be careful not to spill gas on the hot engine. Let it set as long as you would normally and see if it starts better. Let me know so I can confirm my hunch.
A pressure regulator would be good but I bet there is none that would work in between the pump and the carb and be independent. A relief valve would have to return to the tank and I don't think there is enough pressure or fuel to get back to the tank.
I want to install some sort of cooling system for the dog house that would come on when you park and cool the dog house so you don't get cook your leg. I thought of just a fan on the radiator but you have to cool the engine to cool the doghouse. I would rather just cool the dog house sheet metal. I thought of a fan under the drivers seat blowing from the passenger compartment to the engine compartment. But I fear the effects of the air that would come into the van if there isn't a good back draft damper.
But I have to do something. I have to remove the heat somehow. This is a relatively common problem, I had it with my 67 too.
How about some feedback, fifteen minds are a powerful thing.

05-05-2001, 07:45 PM
I had the same heat-soak / overfueling problem during startup. The bypass doesn't have to go back to the tank, just back to the inlet side of the pump. Tee into the hose or line between the fuel pump and carb, install an orifice (tiny hole) in a line that tees back into the fuel line between the tank and fuel pump. When the engine shuts off, the residual pressure immediately goes away. Some fuel pumps have this feature built-in by putting a tiny hole in the inlet valve disc. --- TV

05-05-2001, 08:11 PM
I'm not sure I fully understand this. Would'nt this cause lower fuel pressure. Please give me more details about the orifice.

tony blanco
05-06-2001, 11:40 AM
i haven't been able to conduct ahighdiver's test due to weather but i'm also curious about turbovan's bypass system. anymore details??

05-06-2001, 11:52 AM
There should be plenty of pressure to supply your carb even with a bypass. Your carb meters the flow of fuel not the fuel pump.
If you have trouble constructing TV's t-valve then use an extreemly cheap fuel filter from a chrysler 325. It has a tiny pressure bleed and is easy to install.

Just run the return line back below the pump like TV suggested.

05-06-2001, 10:45 PM
Yeah, the Chrysler fuel filter sounds a lot more sanitary, probably the way I would have gone, had I known about it. The fuel pump makes more than enough volume to supply both the carb AND a bypass or bleed-off orifice. --- TV

05-07-2001, 03:22 PM
Who knows where and how to install the fuel filter in the 66?

tony blanco
05-09-2001, 10:42 PM
local auto parts store sold me fuel filter with return valve--carquest #86086.i will install this week-end as per TV's set-up and report back. thanks to all for the input.

05-23-2001, 07:48 PM
New to the forum, but not to Vintage chevy Vans. Had a 65 Sportvan for over 15 years. I also had the hot carb problem and for my van, the problem was in the exhaust manifold. The Cold weather preheat valve in the cast iron exhaust manifold was frozen in the up position. This valve blows exhaust up to the botom of the intake manifold to preheat the carb on cold days. They will sometimes stick in this position especially if the engine hasn't been run for some time. Check to see that the valve can be moved. A shot of WD40 may be needed.

05-23-2001, 10:53 PM
I installed the tee with a .0420 orifice and the trouble stopped. I went this way because someday that filter will become clogged and
need replacement. That would only happen to me in the sticks and I couldn't bypass the filter with it plumbed that way. I'll check that valve but I'm installing the headers soon.

[This message has been edited by ahighdiver (edited May 23, 2001).]