q-jet problem modified 350
Thread Starter
Intermediate
Joined: Mar 2014
Posts: 26
Likes: 0
From: Vienna
q-jet problem modified 350
hello,my q-Jet on my 74 makes Problems after this mods: dart shp 180 64cc,edelbrock Performer intake,headers,voodoo 262/268.
when i shift fast from first into second or second into third and so on...there is a flat spot for a second and then the power comes back,what can be the problem?
thanks for help and sorry for my bad english
greetings form Austria(Europe)
Franz
when i shift fast from first into second or second into third and so on...there is a flat spot for a second and then the power comes back,what can be the problem?
thanks for help and sorry for my bad english
greetings form Austria(Europe)
Franz
Pro
Joined: Oct 2009
Posts: 664
Likes: 89
From: Marysville Ohio
After my first engine swap to a 355 with a similar cam setup, I had a similar flat spot during acceleration (but was an automatic).
It wasn't the carb - it was a combination of timing and using vacuum advance source (which affects timing).
I needed to advance to somewhere around 13-17 degrees initial advance, 34-36 total advance, and use manifold vacuum for the timing advance, instead of the stock ported setup.
BTW, the QJet is actually a mechanical secondary carb, although it's still quite a bit different that you see on a double pumper, at least visually - but once correctly setup, it will react very quickly. A dead spot could also be due to the spring on the secondary flaps need adjustment - That could also affect how the secondary's react.
Lots of better QJet and timing experts on here then me - but if it ran OK with the previous setup, I would lean towards timing/vacuum advance setup needing tweaked first. I would try the timing setup described in the stickys above, and see what that does - it's easy and free - only takes time.
It wasn't the carb - it was a combination of timing and using vacuum advance source (which affects timing).
I needed to advance to somewhere around 13-17 degrees initial advance, 34-36 total advance, and use manifold vacuum for the timing advance, instead of the stock ported setup.
BTW, the QJet is actually a mechanical secondary carb, although it's still quite a bit different that you see on a double pumper, at least visually - but once correctly setup, it will react very quickly. A dead spot could also be due to the spring on the secondary flaps need adjustment - That could also affect how the secondary's react.
Lots of better QJet and timing experts on here then me - but if it ran OK with the previous setup, I would lean towards timing/vacuum advance setup needing tweaked first. I would try the timing setup described in the stickys above, and see what that does - it's easy and free - only takes time.
Thread Starter
Intermediate
Joined: Mar 2014
Posts: 26
Likes: 0
From: Vienna
After my first engine swap to a 355 with a similar cam setup, I had a similar flat spot during acceleration (but was an automatic).
It wasn't the carb - it was a combination of timing and using vacuum advance source (which affects timing).
I needed to advance to somewhere around 13-17 degrees initial advance, 34-36 total advance, and use manifold vacuum for the timing advance, instead of the stock ported setup.
BTW, the QJet is actually a mechanical secondary carb, although it's still quite a bit different that you see on a double pumper, at least visually - but once correctly setup, it will react very quickly. A dead spot could also be due to the spring on the secondary flaps need adjustment - That could also affect how the secondary's react.
Lots of better QJet and timing experts on here then me - but if it ran OK with the previous setup, I would lean towards timing/vacuum advance setup needing tweaked first. I would try the timing setup described in the stickys above, and see what that does - it's easy and free - only takes time.
It wasn't the carb - it was a combination of timing and using vacuum advance source (which affects timing).
I needed to advance to somewhere around 13-17 degrees initial advance, 34-36 total advance, and use manifold vacuum for the timing advance, instead of the stock ported setup.
BTW, the QJet is actually a mechanical secondary carb, although it's still quite a bit different that you see on a double pumper, at least visually - but once correctly setup, it will react very quickly. A dead spot could also be due to the spring on the secondary flaps need adjustment - That could also affect how the secondary's react.
Lots of better QJet and timing experts on here then me - but if it ran OK with the previous setup, I would lean towards timing/vacuum advance setup needing tweaked first. I would try the timing setup described in the stickys above, and see what that does - it's easy and free - only takes time.
with the previous setup it ran perfect...so i will try another Timing Setup.thank you!
Le Mans Master
Joined: Oct 2010
Posts: 6,936
Likes: 848
From: melbourne florida
2023 C3 of the Year Finalist - Modified
i think at least one thing is that secondary door. the quadrajet has mechanical secondary BUT that air door is vacuum operated. so you have to get both doors open so to speak before any air flows. you need to richen the carb up a bit, you have much better flowing heads and a bigger cam to boot. yours isn't far off what i have but i have a quick fuel 750 double pumper , to get the flat spot out of mine i have gone up 4 jet sizes and 2 squirter sizes and now its about right. if you want to keep that quadrajet id email Lars and get his thoughts on jetting etc.
Melting Slicks
Joined: Jul 2010
Posts: 2,503
Likes: 33
From: McGrady NC
St. Jude Donor '15-'16
My engine is similar to yours and I had Lars set up my carb recently. A good rebuild with good parts, jet size changes and applying the settings he knows from experience that work with my engine made a big difference. Once I got the timing set up correctly it was like I had a different engine. It was so much better in every way. Now it starts easily, idles MUCH better and at a lower RPM, runs MUCH smoother around town, and accelerates like it's rear end was on fire! SO much better all around!
Le Mans Master
Joined: Aug 2010
Posts: 5,339
Likes: 546
From: Central Texas
If you want to stay with your current Q-jet I would get in touch with Lars @ v8fastcars@msn.com. He is the resident Q-jet guru so to speak.
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Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
It takes two seconds to do this and it's free.
Racer
Joined: Jul 2010
Posts: 283
Likes: 1
From: The Netherlands EUR
I know that experienced guys probably can feel immediately if the spring is too loose because they had felt it dozens/hundreds of times before but if you have little experience with carbs and no good carb to compare, what is "too loose" and what is "good"?
Thanks for explaining.
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
There's a simple adjustment procedure in every Qjet book. Going from memory, I think it's with the spring loosened, push the door open. Tighten the spring slowly until the door just closes, then add 1/4 turn more.
Not rocket surgery.
Not rocket surgery.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
OK, i read the advise to check things a lot, but how do you know what is the correct adjustment if you have nothing to compare to?
I know that experienced guys probably can feel immediately if the spring is too loose because they had felt it dozens/hundreds of times before but if you have little experience with carbs and no good carb to compare, what is "too loose" and what is "good"?
Thanks for explaining.
I know that experienced guys probably can feel immediately if the spring is too loose because they had felt it dozens/hundreds of times before but if you have little experience with carbs and no good carb to compare, what is "too loose" and what is "good"?
Thanks for explaining.
Scott
Last edited by scottyp99; Mar 23, 2014 at 12:43 PM.
Drifting
Joined: Mar 2011
Posts: 1,886
Likes: 9
I would start the air valve preload at 1/2 turn. Then try 5/8. Most Qjet bogs are caused by a too loose setting of the air valve spring.
Or it could be a failed choke pull off canister, also allowing the air valve to flop open too quickly.
Or it could be a failed choke pull off canister, also allowing the air valve to flop open too quickly.
Thread Starter
Intermediate
Joined: Mar 2014
Posts: 26
Likes: 0
From: Vienna
today i tried to tightening the spring,but there was no difference,then to loose it,also no difference...there is always this flat spot when i shift from first to second,second to third and so on....when i shift very slowly then there is no flat spot...
sorry again for my bad english
sorry again for my bad english
Thread Starter
Intermediate
Joined: Mar 2014
Posts: 26
Likes: 0
From: Vienna
today i tried to tightening the spring,but there was no difference,then to loose it,also no difference...there is always this flat spot when i shift from first to second,second to third and so on....when i shift very slowly then there is no flat spot...
sorry again for my bad english
sorry again for my bad english