Make sure the advance wieghts are free. Also Check for worn (wobbley or loose) breaker plate. Reset the dwell
Q-Jet Problems
Thread Starter
Instructor
Joined: May 2001
Posts: 136
Likes: 0
From: Hillsborough North Carolina
Q-Jet Problems
Hello Everyone,
I am having some problems with the Q-jet on my '72 Vette. The carb has been recently rebuilt(by a pro, not me). It is on a base model 350. It idles great and and will rev freely until about 4000 rpm. It then sounds like just as the secondaries try to come in that, the engine stumbles and will not rev any higher. This happens whether you ease into the throttle or if you go straight to full throttle although it is worse when going strainght to full throttle.
Anyone have any ideas your suggestions on how to correct this?
Thanks,
Mike
I am having some problems with the Q-jet on my '72 Vette. The carb has been recently rebuilt(by a pro, not me). It is on a base model 350. It idles great and and will rev freely until about 4000 rpm. It then sounds like just as the secondaries try to come in that, the engine stumbles and will not rev any higher. This happens whether you ease into the throttle or if you go straight to full throttle although it is worse when going strainght to full throttle.
Anyone have any ideas your suggestions on how to correct this?
Thanks,
Mike
Safety Car
Joined: Oct 2004
Posts: 3,725
Likes: 5
From: Minnesota
I don't mean to sound smartypants or anything but in my opinion the best fix on these old carbs is to put a new Edelbrock Performer on. I've put them on several different vehicles. They seem to hold up the best with this new kind of gas we have to use now. I just put one on my 75 on Saturday and it runs great now. I've never had much luck with carb rebuilds either. My $.02.
Race Director
Joined: Sep 2002
Posts: 10,362
Likes: 5
From: San Diego - Deep Within The State of CONFUSION!
Originally Posted by piratemike
Hello Everyone,
I am having some problems with the Q-jet on my '72 Vette. The carb has been recently rebuilt(by a pro, not me). It is on a base model 350. It idles great and and will rev freely until about 4000 rpm. It then sounds like just as the secondaries try to come in that, the engine stumbles and will not rev any higher. This happens whether you ease into the throttle or if you go straight to full throttle although it is worse when going strainght to full throttle.
Anyone have any ideas your suggestions on how to correct this?
Thanks,
Mike
I am having some problems with the Q-jet on my '72 Vette. The carb has been recently rebuilt(by a pro, not me). It is on a base model 350. It idles great and and will rev freely until about 4000 rpm. It then sounds like just as the secondaries try to come in that, the engine stumbles and will not rev any higher. This happens whether you ease into the throttle or if you go straight to full throttle although it is worse when going strainght to full throttle.
Anyone have any ideas your suggestions on how to correct this?
Thanks,
Mike
Le Mans Master
Joined: Mar 2002
Posts: 7,456
Likes: 2
From: Coming home from Luckenbach Texas
My secondaries were malfunctioning for 2 years due to a gasket interference issue. Still got to 4500+ rpm. It sounds like an ignition problem to be. (But check the secondary lockout lever.)
QJets are great carbs...just misunderstood.
QJets are great carbs...just misunderstood.
Race Director
Joined: Jun 2000
Posts: 10,039
Likes: 670
From: FL
Get Lar's paper on Q-jet tuning from www.corvettefaq.com and Doug Roe's Rochester Carb book and you will be good to go.
Thread Starter
Instructor
Joined: May 2001
Posts: 136
Likes: 0
From: Hillsborough North Carolina
Thanks for all the replies. In response to a few of the questions...The engine is brand new. It has been broken in but, at this point the car is not driveable so there is no more than about 2 hours of run time on the engine. The distributor has been"refurbished"...new points,weights and light weight springs.rotor,cap,wires and plugs. I do not have a dwell meter so, could you tell me a way to set the dwell without a meter?
Thanks or all the info.
Mike
Thanks or all the info.
Mike
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Intermediate
Joined: Oct 2004
Posts: 36
Likes: 0
From: NEW MEXICO
Mike,
To Set The Dwell Without A Meter - Turn The Adjustment Clockwise Until The Engine Stalls And Then Turn Counter Clockwise 1/2 Turn.
I Think The Secondaries Sould Be In Before 4,000. I Agree With The Others - You Should Check That Out.
To Set The Dwell Without A Meter - Turn The Adjustment Clockwise Until The Engine Stalls And Then Turn Counter Clockwise 1/2 Turn.
I Think The Secondaries Sould Be In Before 4,000. I Agree With The Others - You Should Check That Out.
Tech Contributor
Joined: Aug 1999
Posts: 14,373
Likes: 6,371
From: At my Bar drinking and wrenching in Lafayette Colorado
If you don't have a dwell meter, set the point gap with a feeler gage: With the point rubbing block positioned on the highest part of the lobe (points widest open), the gap should be .016". You should have a snug slip fit with a feeler gauge. This will produce a dwell of about 28 degrees, which is close enough to not cause any performance problems (optimum is 30-31 degrees).
Once you have set dwell and verified that you are getting 36 degrees total timing, check your secondary airvalve windup spring tension: these are often set too loose on rebuilt carbs, and this will make the car fall on its face when you go into the secondary side. Set the spring tension to 3/4 turn windup. Also, make sure your choke pulloff is hooked up and functioning correctly since this will affect secondary opening rate.
Once you have set dwell and verified that you are getting 36 degrees total timing, check your secondary airvalve windup spring tension: these are often set too loose on rebuilt carbs, and this will make the car fall on its face when you go into the secondary side. Set the spring tension to 3/4 turn windup. Also, make sure your choke pulloff is hooked up and functioning correctly since this will affect secondary opening rate.
