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Old 04-07-2006, 11:23 PM
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Q-jet problems

Hey

I just got my 454 running, but I have been having a problem with the Q-jet carb. The carb is off of a 1978 350 3/4 ton truck. I believe it to be a 750 cfm which should be plenty for my mild 454. I rebuilt it because fuel was coming out of every hole possible.

There are a couple of problems that I have been running into.

First off the carb is running extremely rich at idle. So rich in fact the exhaust burns my eyes. LOL my eyes started watering just thinking about it. I have had no luck adjusting this at all. In fact I have had a mechanic look at it and he says that the carb could be wore out because nothing adjusts properly on it.

Second the electric choke is not set right. The choke comes off too fast. And when warm (not operating temp, but run into a store for 20 min and come back out cooled down type temp) it starts horrible! But if I pop the hood and prop a screwdriver holding the choke partially closed it starts right up. How can i fix this? I think the problems are related.

Third question is about my fuel pressure/fuel pump. The pump I'm using is a stock ac delco unit off the 454. I couldn't tell you what the pressure is at. I will have to check it. On the fuel pump there is 3 fittings. One for a inlet, one for outlet, and one last fitting that is a barbed fitting that would connect to a rubber hose. My question is what does this connect to? It was plugged up with a bolt by the previous owner. Could this be a factor why my carb is not working?

I only though of the fuel pressure of being a problem while I was looking at a new edelbrock carb and it says the pressure should be between 4-6 psi.

I would like to keep my Q jet for now because when they are working properly they are as close to fuel injection as possible. And right now all I have.

Thanks for the help

John

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Old 04-08-2006, 01:47 AM
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The extra outlet on the pump is for a fuel return, and yes its a requirement. You have a couple options: You could run a return line to the tank, or you could buy a different BBC pump that doesn't have the return. I wish I could help on which ones don't have it but I just can't. All of my experience is with ones that DO have the return, but I know the non-return pumps exist.

The problem is when you plug up that return line it delivers more than normal pressure to the carb. That could be causing your problem. The pressure from the pump may be overpowering the floats and overfilling the bowls causing a massively rich condition.

The other thing I suggest checking is the primary metering rods. Since they are spring loaded and tend to splay themselves outward, its possible that you got one or both of them not in the jets. I've done it a couple times. You put the air horn down on the body of the carb and everything seems right, but sometimes those rods pop up out of the jets, spring to the outside, which lets them open to fuel. Its not too much of a problem on a properly tuned idle, but most Qjets are partially exposed to the primary circuit at idle and it may be affecting your mix.
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Old 04-08-2006, 01:54 AM
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Oh, and the choke question... fix the other problems first. Its quite possible that they are related things. Fixing the idle mixture problem might help solve the choke issue. The reason I say that is because going into the store for 20 minutes isn't long enough for the choke to cool off. If the engine is requiring choke to start after sitting for 20 minutes, its not a choke issue, its something else.

Heat soak and gasoline vapor lock might be the cause of that problem. Fix the idle mix issues first. Then set your choke so that it's loosely closed at cold set. After startup the vacuum brake should pull it open about 1/8". Custom engines obviously require custom settings, but you can use that as a baseline and adjust yours tighter or looser as needed. If you still have the same 20-minute restart problem it's probably vapor lock.
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Old 04-08-2006, 10:49 AM
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Curtis is onto something to check. Fuel pressure can kill a qjet. For the pump problem, just go buy a SBC pump with 2 lines. Its no biggie. Most all '70 version trucks had 2 line pumps. That very well may cure the flooding problem. Try that and let us know how it goes. I've only had one qjet in my life that I just gave up on finally and tossed in the trash.

Personally, I like the newer electric choke qjet like you are using. I'll agree too that the electric choke comes off very fast since its not dependent on engine temp. Mine is VERY cold blooded once the choke comes off but before the engine and carb warm up. I'm thinking of reducing the voltage to the choke in hopes of slowing down the time it takes to un-wind.

Mark
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Old 04-09-2006, 11:06 AM
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Thanks guys you just what i was thinking. Just wanted to confirm that first. Still have not got around to test my fuel pressure.

That is what happens when you buy a old truck from a farmer for the motor. It's kinda funny that he was having problems that he had a bad fuel pump. And when he went to replace it he didn't have the fuel pump push rod all the way up so he bent that. Just a comedy of errors.

Well I think that I will go get a different pump. I have all ready had the air horn off and dropped the float level to try and fix some problems. I'm also thinking that I need to use the thick insulating gasket. Instead of the thin one that I have now.

Thank you for your help,

John
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Old 04-09-2006, 11:44 AM
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Quote:
Originally Posted by Jmark
Personally, I like the newer electric choke qjet like you are using. I'll agree too that the electric choke comes off very fast since its not dependent on engine temp. Mine is VERY cold blooded once the choke comes off but before the engine and carb warm up. I'm thinking of reducing the voltage to the choke in hopes of slowing down the time it takes to un-wind.

Mark

You may want to look for the thermostatic type choke coils. Some were built into the coil cover, and others mounted on the intake. They had a bi-metallic strip inside that would not close until under hood temperatures reached a certain point, then they would close and allow the choke coil to heat and let the choke off. This combined with the initial delay, put the engine into a good operating temperature, where the choke was no longer needed.
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