Carbureted LSX forum - ultra hp tuning

Just put a new 850 on my lq9 ms4 swapped fox and can't get the thing tuned in. I've tried adjusting everything on the Carb plus timing and it still falls on its face out of the hole and back firing flames through the Carb really bad. Does anyone have any tricks or tips to get it working?

Flames through the carb usually means timing is way off. Maybe your cam sensor is wired backwards or not working.

Im using a msd box and it worked up until last week so it must be the sensor out. Is that a very common thing to have happen?

what have you got for a timing curve?
Get that close first, then go into the carb.

12 deg at 0 rpms then ramping to 34 at 800 across. I had a 750 with some wore out slicks but it still trapped 122, then i put new slicks and a 850 on it and something must have happened because now with both carbs its acting as if the timing is really off when I bring up the rpm quickly. It seems as if its not speeding up the firing of the coils in relation to the speed of the motor, but im not sure the cause.

that strait 34* timing curve will be just fine.
Sorry obviously cant really help with the new problem, but I think it is electrical if it happens with the original carb that worked well before???
Cam sensor should not cause your problem while running, only during startup.
I would double check battery voltage at the MSD first, then check all electrical connections, grounds.
Good luck, those kinds of issues suck
Barrett

yea its got me confused. It has two new batteries and everything was working fine before. Its showing good voltage at the msd too.

When I was having trouble with backfiring out of the carb, this was the suggested steps given to me by tech (Edgar)support at MSD:

Sounds like the box is not seeing the cam sensor follow these steps:

Cam

DVOM probe the brn/wht wire closest to the box

You will be looking for a fluctuation on your meter

*No signal then test on opposite side of harness

*If you do not see an activity then you need to reverse the two outside wires and test again

No signal then test on opposite side of harness

Still no signal, then you have a faulty cam sensor

Edgar Lopez (915) 857-5200 ext 2234.

When I was having trouble with backfiring out of the carb, this was the suggested steps given to me by tech (Edgar)support at MSD:

Sounds like the box is not seeing the cam sensor follow these steps:

Cam

DVOM probe the brn/wht wire closest to the box

You will be looking for a fluctuation on your meter

*No signal then test on opposite side of harness

*If you do not see an activity then you need to reverse the two outside wires and test again

No signal then test on opposite side of harness

Still no signal, then you have a faulty cam sensor

Edgar Lopez (915) 857-5200 ext 2234.
This is true, and good advice,
BUT once the engine is running the cam sensor is redundant, so I believe the problem here is something else.

I have the same problem I have a 950 hp Holley doesn't matter what I adjust still have backfire so I am putting 50cc acc. Pumps on mine and putting 35 pump squirters according to Holley the pumps will correct the stumble and the nozzles will help the transition

Did you say you are going straight to 34° at 800 RPM ? Seems massive to me. What is your idle RPM? I think the box ramps the timing up between setpoints. So unless your idle is something like 500 RPM you are at 34 degrees all the time.

Going to a 50 CC accel pump is a last resort. You need to get the AFR right with small throttle changes first then use the pump shot to correct for fast throttle transitions.

Do you have a wideband? Expensive carb, seems like a wideband would be a worthwhile investment.

I plan on getting a wideband pretty quick I'm just confused on why it just started back firing like that with two different carbs when it was working good

Wideband was the best $250 I have spent on my car

I have a wideband on mine and it just goes lean for a second not right off the line but before 60 ft and I mean a full second according to video. It goes from 12.8 to 15.3 on wb then back to 12.3-12.8 the rest of the run.

I haven't had any problems like this when I switched to my 750 Ultra HP