Q-jet help

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Apr 16, 2010 | 02:46 PM

rham

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From: Murphy Texas

Q-jet help

Working on a friends truck, it dies after idling at stop lights etc. Seams to be rich, checked float level it is ok, leaned out apt and screwed idle mixture screws all the way in and it does not kill engine- still looks like a lot of fuel going into boosters. Any ideas what do I check next?
Thanks
Randy

Apr 16, 2010 | 04:26 PM

redman76

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From: Nashville TN

St. Jude Donor '09

Just thinking about some of the simple stuff . . . have you checked/adj timing and is idle set too low?

Apr 16, 2010 | 05:24 PM

gerry72

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From: San Antonio TX

If the float is not sinking and you are certain it's not the float, you'd probably want to take the carb off, remove the throttle plate and have a look at the fuel well plugs and the main emulsion BBs. They're probably leaking. That or the main body is cracked. There is also a condition with fuel leaking over into the venturi when the main body or top plate are warped or not screwed together tightly (you did put the two screws in that are in the primary under the choke, right?).

Apr 17, 2010 | 01:24 AM

OMF

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From: Salmon Arm, BC

float level....its probably dripping fuel from the primary nozzles, thats why it still runs. have a look down the primarys while its running

Apr 17, 2010 | 11:09 AM

lars

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From: At my Bar drinking and wrenching in Lafayette Colorado

If you have fuel dripping out the main discharge boosters at idle, the float level is too high. This can be caused by several things:

Incorrect float
Fuel-logged float
Incorrect float height (should be .375" for divorced choke carbs and .420" for integral choke)
Needle clip installed backwards
Dirt in seat
Defective seat or needle
Leaking seat gasket
Stripped seat threads in bowl
Cracked bowl
Float pivot pin crushed (not adequate spring)
Excessive fuel pressure (over 7 psi)
Float arms bent and rubbing on power piston/primary rods or bowl walls
Missing bowl filler
Other engine problems forcing an excessive throttle opening at idle, such as incorrect timing, missing/inoperative vacuum advance, incorrect camshaft timing, gross vacuum leaks, etc.

You have to correct the dripping problem before trying any other tuning or remedies - there should never be fuel dripping out the boosters at idle.

Lars

Apr 17, 2010 | 02:33 PM

73, Dark Blue 454

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From: Austin TX

Also, what cam are you running and what vacuum signal do you have at idle?

Apr 17, 2010 | 05:23 PM

rham

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From: Murphy Texas

Thanks for the input all. We checked float level, timing all of the simple fixes. The float level was off we changed that but that did not fix it. We put a new float, needle and seat and fuel filter in just to try that, still no luck. When we took it apart I checked the numbers and the jet and rod combo and I believe they are mismatched from checking Lars papers. The carb is 17059207 which calls for 72 jets and 40 rods but this has 73 jets and 51 rods. I dont know how much that affects the idle. The carb has been rebuilt by one of the major rebuilders.

He has had this truck for a few months doing some body work etc but this is the first time he has really driven it much. It is a 350 as far as we know a stock or very mild cam, put a weiand intake, shorty headers and summit hei in it and that is it.

I thought about fuel pressure, have not checked it but it is a stock fuel pump. Could the fuel pressure be to high?

Thanks
Randy

Apr 17, 2010 | 10:55 PM

lars

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From: At my Bar drinking and wrenching in Lafayette Colorado

Quote:

Originally Posted by rham

The carb is 17059207 which calls for 72 jets and 40 rods but this has 73 jets and 51 rods. I dont know how much that affects the idle.

Jet/rod combo off by a few sizes will not contribute to fuel dripping out the main discharge nozzles at idle. This is not your problem.

Quote:

Originally Posted by rham

The carb has been rebuilt by one of the major rebuilders.

Now there's a big red flag. There could be dozens of critical things wrong with that carb, and there probably are. Drop me an e-mail request for my "Commercial Q-Jet Problems" paper to get an idea of likely problems.

V8FastCars@msn.com

Quote:

Originally Posted by rham

Could the fuel pressure be to high?

Please read my previous post above.

Lars