Random Intake Backfires

Currently I am on an FSAE team running an Yamaha '08 R6 engine with a custom intake and 20mm restrictor. We have been experiencing some very rough intake backfires lately and I am lost as to how to fix this problem.

They come periodically and randomly from the past 6 months only during cranking. It has backfired while the engine is warmed up and while cold. The car runs great and cranks great, but every now and then it gets a hard backfire, rough enough to split the bond on our carbon fiber intake. Here is what we have gone through to troubleshoot (with no luck so far).

The engine is running fully sequential fuel with COP wasted spark. Timing has been confirmed with a timing light while running. Cranking advance is set to around 11 degrees, and has had no problems with kickback. Injector timing has not been modified or touched. We used to crank with batch fire, changed that to cranking fully sequential and still no change. The cranking pulse has been moved around some and without a backfire, the car cranks good with the added fuel.

A couple other things to note; we machine our own injector flanges, currently the injectors are directly pointed at the back of the intake valve (not sure if this could add to the problem). We are using the primary set of stock injectors and our ecu is a MS3 Pro Evo. Fuel psi has been checked, spark plugs look fine, compression has been checked, all of the basics have been taken out of the question.

We have been through many other forums and searched all over the web. Any help is greatly appreciated as we are lost as to what to check next in order to fix this random and unpredictable problem.

Hmmm, seems like you've covered about all the bases.

[my thoughts/ramblings]

Sometimes the fuel charge in the cylinder doesn't ignite from the ignition and can be ignited by the previous cylinder's burning exhaust gas as it's exhaust valve opens - is it possible the mixture during cranking is too rich or lean to ignite properly?

Following on that, but you seem to have addressed it in your trials at one point, is with batch fired ignition - if the charge isn't ignited by the spark on compression, it can be ignited by the 'wasted' spark on overlap.

If the intake valve isn't perfectly sealed, or isn't closing every time, it may be possible for the burning gases to pass back into the intake port - this may be due to slight warpage/poor seating, or weak valve spring seat pressure, insufficient valve clearances, and/or a slightly too tight stem clearance causing enough drag to hold it slightly open.

Something else we haven't thought of yet.

With the manifold(s) off, can you see it there's any evidence of a particular cylinder being different from the others, as that may help point to the problem?

In the meantime, if you can't solve it, perhaps designing in an easily reached, and changed, burst panel might save the air-box? As it's a N/A engine, with little pressure differential across it, you can probably use something like the aluminium foil used for cooking, as it should be strong enough in normal use but easily burst if needed?

Waste spark on high overlap cam will always have problems unless your cranking injector phasing and pulsewidth is perfect to fire on the first viable cycle. I'd run sequential ignition. You can probably improve the fueling too.

The old haltechs used to pulse power on power up, our old car used to pop a bit.

I have a couple of suggestion

check valve clearance tight intake clearances Can cause this type of issue,

the other would be a faulty cam or crank sensor or trigger up, Hard to imagine that it would be random tho but worth checking.

the other thought I had is possibly a ecu issue

Ross's second suggestion brings up something else - is it possible the voltage drop, when cranking, across the ECU gets low enough to cause it to false-trigger the ignition from unstable voltage levels? Might be worth checking the voltages across it?

So we have been testing it some more to troubleshoot. The voltage does sag but the ECU does not loose sync. The trigger logs look good when cranking. We have tried to run the car in fully sequential COP spark. We used to run Wasted COP and it cranks up fine with that (with exception of random backfires). With the only change being sequential COP spark, the engine will hit just for a split second then die. Every time when cranking. Any ideas to what could be causing this? I have logs of it cranking and running with Wasted COP and trying to crank with COP. Unfortunately I cant post those here on the form. Wrong file type. Any help is appreciated.

Note, we checked that sync loss is all good at this point, fuel and spark are both identical to when it will crank with wasted cop. Just wont run more than two or three engine cycles on fully sequential COP.

Thanks!

Are you 100% sure they are on the right cylinders? They will fire with 2&3 back to front, I know that much.

Yes, we know the firing order for the 08' R6 and have confirmed both the injectors and coils correspond with the firing order by commanding fuel and spark to each cylinder specifically.