Hard to start with a backfire out the carb. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

evilways · 12-06-2008, 04:55 PM

Should I be using the intake port on the rear of the intake for the vacuum advance?
It is currently plugged.

ChevLoRay · 12-06-2008, 05:23 PM

Regarding the "drain back" issue....I had what I thought was the same thing. I tightened up all of the hose clamps on the fuel lines and solved that issue.
It doesn't take much of a leaking clamp to allow air to enter the line and the fuel to be gone. The time it takes to start it after it sets is spent pumping fuel back to the carb. So, check all of your hose clamps. It could even be a hose that is allowing the air to enter.

The vacuum line to the power brakes connects at either the rear of the carb (large port) or on the intake. You need ported vacuum for the vacuum advance.

68gmsee · 12-06-2008, 06:45 PM

Quote:

Originally Posted by evilways

Should I be using the intake port on the rear of the intake for the vacuum advance? It is currently plugged.

I've used both ported and engine vacuum for vacuum advance. Seems to work ok on either.

Ported vacuum is a vacuum tube on the carburetor that's above the butterflies where engine vacuum is below the butterflies. You should have both on your carb. Just make sure you plug the ones you're not using.

Don't forget to plug all vacuum tubes on the carb when checking your timing, though.

If you want to learn more about pros and cons about ported vs non ported this guy has a good writeup. Gets a little complicated but its a good one to learn about how vacuum advance works.
http://www.hotrodders.com/forum/dist...1-a-59033.html

12-06-2008, 04:55 PM	#1
evilways Registered User Join Date: Mar 2008 Location: Lafayette, IN Posts: 226	Re: Hard to start with a backfire out the carb. Should I be using the intake port on the rear of the intake for the vacuum advance? It is currently plugged. __________________ Mark 1967 C/10 stepside lwb daily driver 1968 C/10 fleetside lwb parts truck 1971 Cheyenne Super swb project

12-06-2008, 05:23 PM	#2
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	Re: Hard to start with a backfire out the carb. Regarding the "drain back" issue....I had what I thought was the same thing. I tightened up all of the hose clamps on the fuel lines and solved that issue. It doesn't take much of a leaking clamp to allow air to enter the line and the fuel to be gone. The time it takes to start it after it sets is spent pumping fuel back to the carb. So, check all of your hose clamps. It could even be a hose that is allowing the air to enter. The vacuum line to the power brakes connects at either the rear of the carb (large port) or on the intake. You need ported vacuum for the vacuum advance. __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was. Last edited by ChevLoRay; 12-06-2008 at 05:24 PM.



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