Chevy Nova Forum banner

1 - 15 of 15 Posts

·
Registered
Joined
·
116 Posts
Discussion Starter #1
I recently took fi tech efi off and put a quick fuel 750 on. The guy I traded with tuned the carb and had it running great. Only thing he told me to change was the power valve from 6.5 to a 3.5 and primary jets from 71 to 70. I did that and now it surges when cruising and backfires through carb under light acceleration. Engine is a 377 sbc dart pro 1 200 cc single plane intake comp 243/257 @ 50 533” lift 108 lsa. Ignition msd street fire with msd pro billet locked 35 degrees. What should I change
 

·
Registered
Joined
·
1,596 Posts
Those jets seem lean to me. I’d up the jets a few numbers. And go back to the 6.5 power valve. A surge while cruising is usually due to being lean
 

·
Registered
Joined
·
61 Posts
I recently took fi tech efi off and put a quick fuel 750 on. The guy I traded with tuned the carb and had it running great. Only thing he told me to change was the power valve from 6.5 to a 3.5 and primary jets from 71 to 70. I did that and now it surges when cruising and backfires through carb under light acceleration. Engine is a 377 sbc dart pro 1 200 cc single plane intake comp 243/257 @ 50 533” lift 108 lsa. Ignition msd street fire with msd pro billet locked 35 degrees. What should I change
When you say running great do you mean running great on his combo or was it running great on your combo? 3.5 power valve seems about right with all that duration @ .050 you must be pulling very little vacuum. maybe up your squirter, or a 50cc accelerator pump?
 

·
Registered
Joined
·
20,109 Posts
I run a 4.5 power valve with my 255/263* @.050". My buddies cam is about the same as yours and his is a Quickfuel 750 and his has a 4.5 power valve also. Both mine and my buddies show to need a 3.5, but works great and I haven't tried one in it. I just rebuilt his and put a new 4.5 back in his and it works great too.

I'd go with a 4.5 and jet up as mentioned. Here is stock jetting for a Q series 750 : https://www.jegs.com/i/Quick-Fuel/793/Q-750/10002/-1
 

·
Registered
Joined
·
116 Posts
Discussion Starter #5
The initial setup with carb on my motor was perfect. He had his wideband while we drove it and he said it all looked good just a little rich. Motor pulls 7” vacuum so that’s why he told me to switch to 3.5. I’ll try a 4.5 and jet up some and see what happens. Just thought I did something wrong when I went down to 3.5. Thanks everyone
 

·
Registered
Joined
·
1,037 Posts
If it ran fine with the 6.5 in it put it back in and leave it alone. Engines can't read Holley tuning books and don't care that theory is half of idle vacuum for powervalves.
 

·
Registered
Joined
·
20,109 Posts
The initial setup with carb on my motor was perfect. He had his wideband while we drove it and he said it all looked good just a little rich. Motor pulls 7” vacuum so that’s why he told me to switch to 3.5. I’ll try a 4.5 and jet up some and see what happens. Just thought I did something wrong when I went down to 3.5. Thanks everyone
Mine and my buddies both make 7" of vacuum, but still running the 4.5

If it ran fine with the 6.5 in it put it back in and leave it alone. Engines can't read Holley tuning books and don't care that theory is half of idle vacuum for powervalves.
I thought the same thing, put it back like it was. It does show to need the smaller pv though.
 

·
Registered
Joined
·
1,216 Posts
I've found that the "1/2 idle vacuum" theory doesn't work all the time, especially with an auto trans. I like to get the car rolling at light load (maybe 35 MPH) and watch the vacuum.

Like yours, my car idles around 7".. but cruising at light/no load it's around 15". I went up to a 6.5 PV and it helped a lot.


Your best bet is to get a wideband for yourself and keep dialing it in, it took me quite a while to get my old 700DP working right.. and even longer to get my current 750 Vac Secondary running well.
 

·
Registered
Joined
·
116 Posts
Discussion Starter #10
Thanks for all the advice. Going to get a Holley calibration kit & o2 gauge and see if I can’t get this thing dialed in.
 

·
Registered
Joined
·
349 Posts
I recently took fi tech efi off and put a quick fuel 750 on. The guy I traded with
Ok, I’ll ask, why did you trade fuel injection for a carb? Although I do not have a aftermarket FI system yet, it is on my wish list with everything I have read.
 

·
Premium Member
Joined
·
7,732 Posts
You want the power valve to be fully closed at idle and open as the throttle blades open and the vacuum signal drops. The idle vacuum divided by 2 will get you close but not always dead nuts.

As others have mentioned I would go to a 4.5 and I think the engine will run a lot better. My 406 has 7 inches of vacuum at idle and a 4.5 is the perfect value for that engine.
 

·
Registered
Joined
·
1,341 Posts
Primary is lean if popping at cruise. I would invest in an AEM wideband setup. Some of the best money I ever spent on a tuning tool.
 

·
Registered
Joined
·
116 Posts
Discussion Starter #15
Got a buddy to do some tuning on the deuce today and it’s running good again. Changed the power valve from 3.5 to 5.0 (4.5 was a little lean). Also changed air bleeds and metering block gasket. The gasket was smashed and causing most of my issues.
 
1 - 15 of 15 Posts
Top