Breaking up and backfiring at 3000 rpm


Active Member
Jun 21, 2016
New Jersey
Hello all, Im not sure if this is the right place to post this because Im not sure what the problem is exactly but the car is running a microsquirt and I just got it running. The setup is a stock explorer short block, e303 cam, ported gt40P heads, holley systemax intake, 75mm throttlebody, on3 turbo kit, 340lph fuel pump, 52lb injectors and Autolite 103 plugs gapped to .025. Timing was set at idle to 15* and confirmed in TunerStudio. As the title says the car will break up at about 3000 rpm under load and start backfiring out of the exhaust. Im at a loss because other than that the car appears to run good. It idles well in closed loop and cruises smooth. Fuel pressure stays within the 38-40 range, stays steady under load and increases under boost. AFR stays around 12.5-13 under load. I tried setting the timing to fixed at the base 15* to see if the advance was messing it up and that didnt make a difference. I even took the charge pipe off the throttle body to see if the problem was boost related and the car still has the same problem. In 3rd gear if I roll into it without the charge pipe the car has almost no power and I cant force it past the 3000 rpm even with the pedal to the floor and it breaking up. Ive attached my .msq file along with youtube links of videos of my fuel pressure, AFR, and boost gauge while the problem is happening. This is my first attempt at trying to tune anything like this so let me know if I left out any critical info. Thanks in advance!




  • 2019-10-21_15.57.04.msq
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Also forgot to add that for some reason the physical boost gauge reads 7 psi different under vacuum than the MAP pressure in TunerStudio. Im not sure if its different under load because its hard to keep an eye on both and drive the car at the same time. I double checked the calibration in TunerStudio and it is correct for a GM 3-Bar MAP
I also need a .msl

if by under load you mean boost then 12.5-13 the car will not be happy by any means.

The distributor needs to be installed with 10* of advance, how exactly are you setting the timing? you should lock the timing in TS to something easy to read and verify on the balancer.
when the car is running is the launch control indicator lit up? if it is that is the issue, your settings for launch control have the hard limiter set at 2950 rpm..

I do tuning classes and am very familiar with the combo you are running.
Also noticed you have the fuel algorithm set to %baro, you cannot run this method with a turbo.. the turbo adds backpressure to the exhaust under load and will throw the fuel off. I suggest running it all straight speed density, you can run %baro with boost but only with superchargers.
I also need a .msl

if by under load you mean boost then 12.5-13 the car will not be happy by any means.

The distributor needs to be installed with 10* of advance, how exactly are you setting the timing? you should lock the timing in TS to something easy to read and verify on the balancer.

I set timing per the microsquirt instructions. Its been a while since I did it but off memory: Set timing to fixed in TS, pulled spout, turned distributor until I had 15* on the timing light off of the balancer, then used the ignition wizard to match the timing in TS using the offset button, reinstalled the spout and reverified that the balancer matched the computer. Ill refamilirize myself and redo the whole procedure again at 10* to make sure its right

A friend of mine that tunes hondas told me that anything less than 13AFR should be fine. Obviously a sbf is alot different than a b-series lol so what would you recommend it be at? Less than 12?

Ive heard of guys running very similar setups on stock ecus without a tune and it being okay so alot of the settings in TS stayed untouched from the factory 5.0 tune that came with it except for the general settings and closed loop idle since Im not sure what most of them are. Thanks for the tip I will switch it to speed density now. I also dont recall touching a launch control setting or seeing a light come up on TS when running. How do I make sure this isnt being turned on without me knowing and would it make the car seem lazy under the 2950 rpm?
I also have not really tried data logging. I experimented with it a few days ago and it saved it as a .csv to open in MegaLog viewer. They only saved the MAP readings for my short cruise, everything else say NaN next to it
Your friend is crazy. 12.5AFR is the standard for N/A power at WOT, anything around 11.3-12 is safe for boost but I like to keep them in the 11.8 range. not all the cylinders get the same fueling and there may be a .4AFR spread between them dyno time has told me there is no noticeable power difference so the safer method is rich.

seems like you have gotten some bad information, boost without a tune is asking for failure. unless they are running low boost [low boost is 6psi] and using an FMU.
The factory timing curve at 10* base is safe for up to 6psi with 93 octane, I dont pull timing until 6psi.

I do not care what the engine is 13 AFR in boost is an invitation for failure.
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how much boost are you running? with the right tune 12-14 should be good and safe on pump gas.

I plan to run somewhere around there. The goal is to have the car in the 10s and I think its doable with that level of boost and the setup based on what others have done. I dont know what spring came with my kit and calling on3 they did not know either. Most likely 5-8lbs. There is not a boost controller on it yet and the gate is venting to atmosphere. So far Ive only seen 3-4lbs before it starts breaking up
I can't look at your file at work, but under the timing settings for spark output if you have it set for going high, try changing it going low. I had problems with mine breaking up and that was the culprit.
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