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Discussion Starter · #1 ·
Hi guys, so I’ve got an 06 750 that I’ve been taking to track days for about a year. Long story short original motor blew (sounds like piston slap but not 100% sure) so I bought a used one from a guy on FB. Swap it in and hook everything up and I’ve got a high idle and popping/backfiring above 6k rpm. After some diagnosis I realized if I unplug cylinder 4 main Injector?(bottom rail) then the popping goes away. I swapped cylinder 3 and 4 injectors and problem remains on cylinder 4. Swapped coil packs around and problem remains. Tried with PAIR valve connected and bypassed and same issue. Compression test shows ~150psi cold on all 4 cylinders. Valve clearances show within limit according to service manual specs.

The engine will rev to redline just fine if I blip it but if I slowly increase throttle it will backfire/pop above 6k every time. I also swapped in another wiring harness just to confirm the harness wasn’t shorted. Tried a different ECU as well.

Where do I go from here? What can I try next? Is my used engine bad? It seems to show correct compression but for some reason I have extra fuel in cylinder 4 that isn’t combusting properly. Any suggestions would be much appreciated.

List of things I’ve tried:
new Quantum fuel pump/strainer kit
Checked and confirmed I have spark and also swapped around coil packs
Swapped around main/primary injectors between cylinder 3 and 4
New engine harness
Tried different ECU
Compression 150 on all cylinders
In/Ex valves in spec
Tried a different cam sensor
Brand new NGK plugs
Fresh oil
Tried with quick shifter bypassed and with quick shifter enabled
 

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Welcome.

Good statement of symptoms. Is there a Bazazz or Power Commander on it?

With the good compressions and valve gaps, I doubt it is a bad engine.

It sounds like an injector hanging open. I've had that happen on my K6 750.

It is a bit contradictory that you unplugged the #4 primary injector and the popping went away but swapping injectors did not make the problem travel to cylinder #3.

I'm assuming you unplugged #4 again with the injectors swapped?
 

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Discussion Starter · #4 ·
Yes I tried unplugging them after swapping cylinder 3 and 4 injectors and the response is the same. I know it sounds crazy and I don’t believe it either. I will try again later and confirm again if necessary.

I did pull the cylinder 3 and 4 injectors off the rail and put 12v to them momentarily and the solenoid opens and closes as it should. I was able to blow through each injector with 12v applied. When 12v was removed then both injectors clicked completely shut and I was not able to blow through.
 

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Discussion Starter · #5 ·
My thoughts are to do a leakdown test after work today and see the results.

I was also thinking that I could use the bottom end off the new engine and the head off my old engine and combine them and make one good engine if that makes sense. I was really hoping to get the new engine running right but honestly I might be better off swapping the heads.
 

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I see you swapped in a different ECM also so that should eliminate any thoughts of a corrupted tune.

Did you keep the throttle bodies from the new engine or swap on the ones from the original engine? I ask due to the reference of a "high idle". Did you check for a cracked intake boot on that cylinder?

"The engine will rev to redline just fine if I blip it but if I slowly increase throttle it will backfire/pop above 6k every time."
Was this under load (riding) or just revving it?

For what it was worth, my bike would start up and idle fine but after a higher power run, it would be on 3 cylinders. I finally found that one primary injector would hang open with high duty cycles (power) and flood the cylinder when back to idle.

I can't think of a mechanical problem that would cause your above 6k symptoms except for maybe a sticking exhaust valve. I just have never heard of that on these bikes.
 

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Discussion Starter · #7 ·
The new engine didn’t come with throttle bodies so I am using the same throttle body and fuel rail from my original engine.

I haven’t checked for a cracked intake boot but I did try to run the engine with the air box off just for fun and no difference.

In reference to your rev to redline comment, the engine was in neutral on a rear stand under no load. I honestly haven’t ran it in gear to redline but I’m 99% sure it will be popping/backfiring the entire way.

If you recommend it then I would be willing to get a new injector and try that. Or maybe I can swap 3 and 4 again? Maybe I messed up somehow last time even though I numbered my injectors.
 

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Yes, I would swap the injectors again. How about swapping 1 and 4? You have to pull the rail off anyway.

So if you used your old throttle body, and the previous engine ran fine with it, it should be fine.

Do you still have the high idle?
 

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Discussion Starter · #9 · (Edited)
Ok so I ended up borrowing a throttle body and fuel rail assembly( injectors included) from a buddy. I swapped it over and the engine ran fine.

After some diagnosing I realized I’ve never swapped out my secondary injectors. I swapped his cylinder 4 secondary injector into my throttle body and boom fixed the problem. I assume my secondary was stuck open or somehow allowing too much fuel.

Anyways, problem solved now thanks for your help.
 

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Ok so I ended up borrowing a throttle body and fuel rail assembly( injectors included) from a buddy. I swapped it over and the engine ran fine.

After some diagnosing I realized I’ve never swapped out my secondary injectors. I swapped his cylinder 4 secondary injector into my throttle body and boom fixed the problem. I assume my secondary was stuck open or somehow allowing too much fuel.

Anyways, problem solved now thanks for your help.
Thanks for reporting the fix. I had not considered the secondary injector but that makes sense.
 
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