Tuning questions

genolan · #1 (**permalink**) 03-15-2014, 01:05 PM

I have a Backdraft 408 making 520Hp so the engine spec sheet says. The engine has vic jr heads and intake along with a holly 770 street avenger carb.
Other engine spec's are unknown. The car pulls and runs much better with the changes I made to eliminate the Bog.

I had some pretty severe bog and changed the squinter from .25 to .35 along with switching the cam from a green to brown. This took care of 95% of the bog with only hard acceleration from 2500 rpm giving a slight stumble(no black smoke at all when nailing it). Exhaust pipes inside look almost black.
Carb is stock with the following
pri jet 72
sec jet 75
pump nozzle increased from .25 to .35
pwr valve 65
secondary diap spring silver (vacumn secondary)
idle set to 1000 rpm

Issues?

Popping out of exhaust when at steady state 2500rpm and also when deceleration.

Should I increase squinter from .35 to .37 i'm trying not to go over this since i'm not comfortable changing the pump to 50cc.

I'm really enjoying playing with my new toy even though it's freezing out.

I haven't had a chance to check vacuum or timing yet.

Any ideas or is popping normal

Z-linkCobra · #2 (**permalink**) 03-15-2014, 01:31 PM

Need to know the timing. I timing issue could cause a backfire. Also un burnt fuel in the exhaust will cause as well. An exhaust leak will also cause a back fire under de acceleration. Sucks clean oxygen into the exhaust and cause micro explosions in the exhaust. Make sure you don't have any slop in the linkage on your accelerator pump. A little slop goes a long ways to causing a bog. Since you went to a larger squirter and still have a slight bog, that might be your problem or at least part of it.

genolan · #3 (**permalink**) 03-15-2014, 02:31 PM

I checked the linkage and it's tight the Bog occurs when hitting the gas in third gear around 2500 rpm's so the linkage has to be engaged. Also using the brown CAM helped and it has more lift than the green CAM I removed. What's a good timing at idle and then advanced. I have an MSD mechanical advance distributed.

Gaz64 · #4 (**permalink**) 03-15-2014, 03:26 PM

Gary,

A few things:

I would try another carb on that engine, like a QFT 750-850.

A Brown cam is for a 50cc pump, you can't run a brown cam with a 30cc pump.

Sounds like your carb is lean everywhere and you are trying to make up for fuel curve inadequacies with excess pump squirter volume.

Look at some of the "highly tuned" double pumpers that can still run white cams (218) or long throttle duration pink cams (330).

Accelerator pump tuning is the last circuit to adjust, the others must be addressed first, preferably on a dyno.

Jerry Clayton · #5 (**permalink**) 03-15-2014, 05:10 PM

change to a double pumper carb--you won't get the amount of squirt you need from a single 30 cc pump on an engine like that in a light car like a cobra---

Texasdoc · #6 (**permalink**) 03-15-2014, 10:50 PM

I second the lean issue. My $0.02: Stumbling at cruise is a lean condition and has nothing to do with the squirters. +1 for tuning on a dyno, or buying a Holley book and figuring out how to tune it. You can *try* to turn the idle mix screws 1/4 turn out. This *may* help, but it should just be a temporary patch until you can get it tuned correctly.

genolan · #7 (**permalink**) 03-16-2014, 06:07 PM

Thanks Gary
You said I can't run a brown cam on the 30cc pump will I damage the pump or just empty it early? I'm new at this and wanted to experiment before sending it to the experts. The brown cam makes it run nearly stumble free so what one should I switch to to give similar performance. Also does the black header color mean i'm running rich or lean. Many thanks since I am really grateful to all and I am excited about attempting to tune it myself.

Clois Harlan · #8 (**permalink**) 03-16-2014, 07:31 PM

I agree with the others on your crb choice. Double pumper would be a better choice, I experienced a similar situation w my Fe. I went w a quick fuel dp which solved all my problems. I would also check my header bolts to make sure everything is buttoned up pretty good all the way to your side pipes.

Gaz64 · #9 (**permalink**) 03-17-2014, 04:19 AM

Quote:

Originally Posted by genolan

Thanks Gary
You said I can't run a brown cam on the 30cc pump will I damage the pump or just empty it early? I'm new at this and wanted to experiment before sending it to the experts. The brown cam makes it run nearly stumble free so what one should I switch to to give similar performance. Also does the black header color mean i'm running rich or lean. Many thanks since I am really grateful to all and I am excited about attempting to tune it myself.

Gary,

I wouldn't muck around with that carb, a street avenger is way too mild for your application.
You really need a more race style double pumper, with more tuning options, emulsion bleeds, idle feed restrictions etc.

DesertMK4 · #10 (**permalink**) 03-17-2014, 03:04 PM

I am not an expert, but I have a Street Avenger (670cfm) so that makes me one. Holley has a lot of video information which can help (HolleyTV). I agree with the others that the pump is one of the last things to tune. I set my mains by reading the plugs. Altitude effects jet size. I then set pump cam. Mine is the stock red. Never heard that you can't use brown cam with 30cc pump. I got the JEG's set of cams that has a useful chart of all the cams. I tried several more for fun than to cure a problem and found very little difference. They help some at this amount of throttle opening and hurt at another point, or RPM. Never heard that idle screws will tune anything other than idle and just off idle. Off throttle pop sound like too rich not too lean. As one of the Holley videos says "More isn't always better." The last thing I learned was to make one change at a time and to keep a record of what I did each time. Good luck and remember it's not a Weber.

Quote:

Originally Posted by genolan

I have a Backdraft 408 making 520Hp so the engine spec sheet says. The engine has vic jr heads and intake along with a holly 770 street avenger carb.
Other engine spec's are unknown. The car pulls and runs much better with the changes I made to eliminate the Bog.

I had some pretty severe bog and changed the squinter from .25 to .35 along with switching the cam from a green to brown. This took care of 95% of the bog with only hard acceleration from 2500 rpm giving a slight stumble(no black smoke at all when nailing it). Exhaust pipes inside look almost black.
Carb is stock with the following
pri jet 72
sec jet 75
pump nozzle increased from .25 to .35
pwr valve 65
secondary diap spring silver (vacumn secondary)
idle set to 1000 rpm

Issues?

Popping out of exhaust when at steady state 2500rpm and also when deceleration.

Should I increase squinter from .35 to .37 i'm trying not to go over this since i'm not comfortable changing the pump to 50cc.

I'm really enjoying playing with my new toy even though it's freezing out.

I haven't had a chance to check vacuum or timing yet.

Any ideas or is popping normal

Gaz64 · #11 (**permalink**) 03-20-2014, 01:46 AM

Brown and Yellow are for 50cc pumps.

http://www.yenko.net/attachments/357...mpcamchart.jpg

ng8264723 · #12 (**permalink**) 03-24-2014, 03:25 AM

The best way to tune a carb is with a wideband. They are so cheap now it is crazy not to use them. I have one on my twin turbo Silverado and i'm getting 13.8 MPG! It idles with a AFR of 19!. With the twin turboes cruizing it is around 14. There is NO WAY I could tune to this level without one. As TOM KIRKHAM says your just guessing otherwise. This summer I'm going to add one to my Kirkham and get the dang carb tuned. I bought PLX kit for around $200