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Discussion Starter #1
Hi,

My 748 makes some big bangs quite often, not at every rpm but often, and when it does at low RPM it actually halts the engine. I have run it now with the tank of to adjust TPS and sync front and rear throttle bodies, and I then found out that it is the front cylender only that makes the bang, and that it shoots flames out of the front throttle body when it does. What can cause this? I have had the engine taken totally apart and restored (mostly by me) and the valves adjusted by a mechanic, and this problem has started after that. Since it halts the engine at low RPM, I guess it ignites before the piston is at top position and pushes it back, but what should I look for to diagnose this problem, where do I start? My guesses are the following (but they are only guesses):

  1. Intake valve not closing quickly enough
  2. Intake valve not closing completly
  3. Premature egnition (but why? nobody has messed with the ECU)
  4. Air/Fuel mixture wrong (to lean?)

Any help on where to start and what to do to diagnose this is appriciated :)

Thank you
Søren
 

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My guess would be if it is running before and since you made a change. The answer is there? Could you have upset any timing? is there a way you could check the timing is spot on? as you say the basics is always first. when you say you have adjusted the gaps. what was before and how much have you changed? is there a way to check before and after? it sounds very much like the valve gaps are at fault if you have adjusted too much?
 

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Backfiring through the intake is a result of a lean fuel condition— too much air which causes delayed, uneven and incomplete combustion. If the condition is worst at small throttle openings, I'd check for an air leak in the intake tract somewhere. (At larger throttle openings a small air leak has much less an effect on the air/fuel ratio so there's no longer a lean condition).
 

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Backfiring through the intake is a result of a lean fuel condition.
That's the one.

When I had my 916 heads gasflowed it obviously would flow more air so when I rebuilt the engine and started for the first time it was running very lean and spitting flames back through the TBs as it was still injecting the same amount of fuel.

I richened the mixture by using the trimmer until I got around 4-5% CO and it ran smooth again, after a custom remap it made around +15hp.

Check for leaks and then set TPS/sync/CO and you should be be good.
 

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Discussion Starter #5
This makes me remember a thing... I had a faulty ECU chip/EPROM installed at a time, when I switched on power with that EPROM installed the fuel pump did nothing and one of the front injectors (there are two for each cylender, from a 996 I think) made an eerie buzzing sound, I put a finger on it and could feel the buzzing from it so I know it was only one on the front cylender and none on the back. I replaced the EPROM and it was fine again, but with this spitting problem. Can my injector have been damaged you think? Could this be my problem?

P.S. The bike is semi-dissasebled in my basement now, and I can not fire it up down there, so I can not test any theories before spring.

Thank you
Søren
 
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