Rochester carb troubleshooting
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From: North Alabama
Rochester carb troubleshooting
question for the experts on the rochester Q jet. I just took off my holley and the performer manifold and put the stock manifold back on with the stock q jet...but I am noticing that there is fuel dripping into the veturies at idle causing a too rich condition...I know this isn't right...but I dont know how to fix it because I dont know what would casue this condition...help?
Tech Contributor
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From: At my Bar drinking and wrenching in Lafayette Colorado
There are 2 things that will cause fuel to drip out of the main discharge nozzles at idle:
1. Float level too high
2. Excessive throttle blade opening at idle, allowing fuel to be pulled from the main metering circuit.
Item 1 is caused by faulty float, incorrect float setting, incorrect pre-load on float hinge pin, leaky needle/seat.
Item 2 can be caused by a wide variety of issues, including incorrect ignition timing, big cam, vacuum leaks, incorrect power piston stop height, lean jetting, incorrect power piston spring, plugged idle circuit, warped airhorn (leaking fuel transfer passage area at airhorn gasket), or any other issue that causes the carb to engage into the main metering circuit at idle.
Lars
1. Float level too high
2. Excessive throttle blade opening at idle, allowing fuel to be pulled from the main metering circuit.
Item 1 is caused by faulty float, incorrect float setting, incorrect pre-load on float hinge pin, leaky needle/seat.
Item 2 can be caused by a wide variety of issues, including incorrect ignition timing, big cam, vacuum leaks, incorrect power piston stop height, lean jetting, incorrect power piston spring, plugged idle circuit, warped airhorn (leaking fuel transfer passage area at airhorn gasket), or any other issue that causes the carb to engage into the main metering circuit at idle.
Lars
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From: North Alabama
I set the float initially and verified it again last night...set at 9/32...
Regarding the second point frrom Lars...I printed off your tips to help me out. more details of my setup...initial timing 10 degrees; cam is a little larger than stock...I thought this would be the issue, but I was able to bring the idle back down far enough that it would idle with the throttle blades about where they should be on the transition slot. No major vacuum leaks, if any. The power piston spring may be too stiff for the cam...but when I ran without it, I still had the same effect.
A couple of other possible clues...It runs fine when initially started and the engine is cold (I thought this might be because it could used more gas when cold) but after it starts to warm up, I get the hesitation and running rough...thought this might be associated with too much fuel because of the leaking nozzles. Also, not sure if this has anything to do with it, but I only have a paper thin gasket between the carb and the manifold...and yes, the carb gets pretty HOT...thought maybe somehow I had fuel boiling over, but it still leaks when cold. But what about the fact that the carb is that close to the maifold? would this create a vacuum situation that would pull gas through the nozzles?
Regarding the second point frrom Lars...I printed off your tips to help me out. more details of my setup...initial timing 10 degrees; cam is a little larger than stock...I thought this would be the issue, but I was able to bring the idle back down far enough that it would idle with the throttle blades about where they should be on the transition slot. No major vacuum leaks, if any. The power piston spring may be too stiff for the cam...but when I ran without it, I still had the same effect.
A couple of other possible clues...It runs fine when initially started and the engine is cold (I thought this might be because it could used more gas when cold) but after it starts to warm up, I get the hesitation and running rough...thought this might be associated with too much fuel because of the leaking nozzles. Also, not sure if this has anything to do with it, but I only have a paper thin gasket between the carb and the manifold...and yes, the carb gets pretty HOT...thought maybe somehow I had fuel boiling over, but it still leaks when cold. But what about the fact that the carb is that close to the maifold? would this create a vacuum situation that would pull gas through the nozzles?
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From: North Alabama
For those continuing to follow this saga...tonight a friend and I messed with the float level, needle and seat...no luck. We have ruled out a vacuum leak...starting to think maybe in the idle circuit...tomorrow we will pull the carb apart and go through it. 
Le Mans Master
Joined: Aug 2001
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From: Standing still with all my might Texas
Cruise-In X Veteran
St. Jude Donor '09-'11
I had the same problem and it was because I had the idle set a little too high and the throttle blades were too far open for the idle circuit to work.
Try bumping your initial timing up to around 14-16 degrees. This should allow you to close the throttle blades a little more and still obtain the same idle rpm.
Try bumping your initial timing up to around 14-16 degrees. This should allow you to close the throttle blades a little more and still obtain the same idle rpm.
7th Gear
Joined: Dec 2004
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From: Austin Texas
Originally Posted by vettepilot68
question for the experts on the rochester Q jet. I just took off my holley and the performer manifold and put the stock manifold back on with the stock q jet...but I am noticing that there is fuel dripping into the veturies at idle causing a too rich condition...I know this isn't right...but I dont know how to fix it because I dont know what would casue this condition...help?
cheers
Jordan
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From: North Alabama
Originally Posted by Mr. JZR
What have you decided to do with your old Holley? Fortunetly, I'm going in the reverse direction, away from the "quadra-bog". I have purchased the Edelbrock performer and in need of a carburetor. Are you selling your Holley? How did the performer/Holley setup work?
cheers
Jordan
cheers
Jordan
Le Mans Master
Joined: Nov 2004
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From: North Alabama
Originally Posted by vettepilot68
The Holley ran real well...the only reason I took it off is to bring the car back to stock...yes, I will be selling it, but not if I cant get this qjet to work...so I am not ready to get rid of it yet.
Team Owner
Joined: Sep 2003
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From: eville in
Originally Posted by Mr. JZR
What have you decided to do with your old Holley? Fortunetly, I'm going in the reverse direction, away from the "quadra-bog". I have purchased the Edelbrock performer and in need of a carburetor. Are you selling your Holley? How did the performer/Holley setup work?
cheers
Jordan
cheers
Jordan
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From: North Alabama
Update...still same problem...fuel leaking out of main discharge nozzles at idle...one more thought...I took the filter out of the carb because I am running a glass inline filter...with no filter at the carb, could this cause some strange pressure or flow issue for me? just a thought... 
Team Owner
Joined: Feb 2005
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From: central America
St. Jude Donor '08-'09
Just a suggestion...
I'd get rid of the inline filter and keep the filter that is in the carb...there are alot of qjets out there that run like the dickens set up the way from the factory, and if you need some background on why its wise to keep the in carb filter in, check out the Lars tuning for beer thread in SF and all those pretty white crystals that an inline filter can't keep out (at least the washable, reusable ones).
My car runs well with that setup, and unless you are making some serious fuel demand (norval comes to mind), that in carb filter won't cause you flow problems.
just my opinion,
trw
I'd get rid of the inline filter and keep the filter that is in the carb...there are alot of qjets out there that run like the dickens set up the way from the factory, and if you need some background on why its wise to keep the in carb filter in, check out the Lars tuning for beer thread in SF and all those pretty white crystals that an inline filter can't keep out (at least the washable, reusable ones).
My car runs well with that setup, and unless you are making some serious fuel demand (norval comes to mind), that in carb filter won't cause you flow problems.
just my opinion,
trw
