Change Q-Jet jets on engine or off?
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Change Q-Jet jets on engine or off?
I rebuilt my q-jet and now would like to try 1 jet size smaller. Is it easier to change the jets on the car or to take the carb off?
I rebuilt it for my zz4 in my 1980 vette and I am happy to say that the transition from idle to full throttle is now smooth with no hesitation. I followed the Cliff Ruggles book. I do think it might be a little rich so I want to try the jet change. Compared to the edlebrock 600 (1406?) carb on the zz4 or the q-jet with my original engine, I lost about 10% fuel economy (15.5 mpg to 14 mpg). Not the end of the world but I thought that I might get a little more performance and economy if now I am too rich and I change the jets. If the change is wrong I will put the other jets back in. The "tip in" test indicates that I am not too lean.
I rebuilt it for my zz4 in my 1980 vette and I am happy to say that the transition from idle to full throttle is now smooth with no hesitation. I followed the Cliff Ruggles book. I do think it might be a little rich so I want to try the jet change. Compared to the edlebrock 600 (1406?) carb on the zz4 or the q-jet with my original engine, I lost about 10% fuel economy (15.5 mpg to 14 mpg). Not the end of the world but I thought that I might get a little more performance and economy if now I am too rich and I change the jets. If the change is wrong I will put the other jets back in. The "tip in" test indicates that I am not too lean.
Drifting
Joined: Sep 2004
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From: North Bend Ohio
If the WOT fuel mixture is good you really shouldn't change the jets. The main metering rods have more to do with cruise mixture and fuel mileage. If you want to lean out the cruise mixture go with a larger rod.
BigBlockk
Later.....
BigBlockk
Later.....
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Thanks for the responses
Cliff Ruggles believes in adjusting the APT (adjustable part throttle) to fine tune the part throttle mixture. He set me up with rods that have a greater taper from off idle to WOT. I am currently running with the APT as lean as it will go. It may be the right thing to go with a leaner rod but since I already bought the jets, I will try them first. If I lose something at part throttle, I will adjust the SPT. If I lose something at WOT I will change the jets back and try to get new rods with this taper. If this gets too frustrating I will go back to where I started with the rods and jets from Cliff or stock!
For more info, I always had some hesitation with the stock engine and carb since I bought it in 1984. The stock jets were .071 with .043 rods (.026 at WOT). The new jets are .077 but the rods are .036 at WOT. With these rods I calculate that .075 jets would be the same at WOT as stock. I am going to try .076. He also turned down the Secondary rods from .0565 (CH) to .045. The good news is it runs well. The bad news is we changed a lot all at once.
Thanks for the responses!
For more info, I always had some hesitation with the stock engine and carb since I bought it in 1984. The stock jets were .071 with .043 rods (.026 at WOT). The new jets are .077 but the rods are .036 at WOT. With these rods I calculate that .075 jets would be the same at WOT as stock. I am going to try .076. He also turned down the Secondary rods from .0565 (CH) to .045. The good news is it runs well. The bad news is we changed a lot all at once.
Thanks for the responses!
Drifting
Joined: May 2005
Posts: 1,629
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From: Somers CT
Cliff Ruggles believes in adjusting the APT (adjustable part throttle) to fine tune the part throttle mixture. He set me up with rods that have a greater taper from off idle to WOT. I am currently running with the APT as lean as it will go. It may be the right thing to go with a leaner rod but since I already bought the jets, I will try them first. If I lose something at part throttle, I will adjust the SPT. If I lose something at WOT I will change the jets back and try to get new rods with this taper. If this gets too frustrating I will go back to where I started with the rods and jets from Cliff or stock!
For more info, I always had some hesitation with the stock engine and carb since I bought it in 1984. The stock jets were .071 with .043 rods (.026 at WOT). The new jets are .077 but the rods are .036 at WOT. With these rods I calculate that .075 jets would be the same at WOT as stock. I am going to try .076. He also turned down the Secondary rods from .0565 (CH) to .045. The good news is it runs well. The bad news is we changed a lot all at once.
Thanks for the responses!
For more info, I always had some hesitation with the stock engine and carb since I bought it in 1984. The stock jets were .071 with .043 rods (.026 at WOT). The new jets are .077 but the rods are .036 at WOT. With these rods I calculate that .075 jets would be the same at WOT as stock. I am going to try .076. He also turned down the Secondary rods from .0565 (CH) to .045. The good news is it runs well. The bad news is we changed a lot all at once.
Thanks for the responses!
When you said hesitation, are you referring to light throttle response or when stomping on the gas going to WOT??
Bill
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I've reworked my carb according to Cliff's book also and have seen a dramatic improvement in performance. I'm still not satisfied with it but it's getting there.
When you said hesitation, are you referring to light throttle response or when stomping on the gas going to WOT??
Bill
When you said hesitation, are you referring to light throttle response or when stomping on the gas going to WOT??
Bill
With the new motor, I first installed a Barry Grant that was supposed to be set up for the motor. It smelled of gas and blew out black smoke it was so rich and had a very rough idle. I have to admit it accelerated well. I then installed the Edlebrock 600 but that didn't have as much power and hesitated a little. Since I rebuilt the Q-jet there is no hesitation and better performance than the Edelbrock. I don't think it is quite as fast as the Barry Grant. In following Cliff's book, I did "recipe 1". I sent him an email question and he told me to call him. He spent quite a bit of time on the phone. I did a lot of the work but sent it to him to bush the primaries and reduce the size of the main air bleeds. Everything is "recipe one" except the rods and jets that he recommended on the phone.
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Drifting
Joined: May 2005
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From: Somers CT
Very interesting, I did the same only used recipe #2. Reason I asked about the hesitation was, mine had a hesitation/bog when going to WOT before so I notched out the secondary air flaps just slightly as he shows in the book. It's much more responsive now.
I had the air horn off yesterday and by accident I discovered the secondary butterflies were only opening about 30% of the way due to the throttle linkage setup. Might be worth checking on yours also, I bet I gained 50 hp by getting them all the way open!
No adjustment on these cars that I could find, I had to bend the gas pedal linkage which is crazy but it does seem to cure the problem.
Good luck with yours.
Bill
I had the air horn off yesterday and by accident I discovered the secondary butterflies were only opening about 30% of the way due to the throttle linkage setup. Might be worth checking on yours also, I bet I gained 50 hp by getting them all the way open!
No adjustment on these cars that I could find, I had to bend the gas pedal linkage which is crazy but it does seem to cure the problem.
Good luck with yours.
Bill
Drifting
Joined: May 2005
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From: Somers CT
Boy you got that right Noonie! I need to do that..mine doesn't have a plug there though, there's a circle in the casting right where it should be but for some reason there's no hole. It's a real pain taking thar airhorn off all the time, I gotta get out the drill for sure!
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Noonie, I see that you reduced the size of your main air bleeds also. Did you follow the Ruggles book? Glad to hear that adjusting the APT was a big help. That is why I want to try the smaller jets. My APT is now as lean as it can go allowing the rods to go in as far as they can.
Top Gun, I think that mine looked like a circle over the APT. I think it was a plug. Anyway, Cliff removed that and taped it for me.
Top Gun, I think that mine looked like a circle over the APT. I think it was a plug. Anyway, Cliff removed that and taped it for me.
Drifting
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From: Somers CT
Bill
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From: Florida
Noonie, I see that you reduced the size of your main air bleeds also. Did you follow the Ruggles book? Glad to hear that adjusting the APT was a big help. That is why I want to try the smaller jets. My APT is now as lean as it can go allowing the rods to go in as far as they can.
Top Gun, I think that mine looked like a circle over the APT. I think it was a plug. Anyway, Cliff removed that and taped it for me.
Top Gun, I think that mine looked like a circle over the APT. I think it was a plug. Anyway, Cliff removed that and taped it for me.
Also drilled out the idle screw holes to get better idle adjustment.
The post 75 smog carbs are great, but set up way too lean for tuning purposes on modified engines.
Works perfectly at all rpms.
Holleys are set up way too rich and I drill them to make the idle circuit a lot leaner.
