Backfire through Intake?

[Jordan1234]

Hello all!

I just finished getting everything put together on a 2002 M750 (injected) after a rebuild.

Currently I'm trying to get the crank position sensor shimmed correctly. I've been told I should get it as close as possible without it actually touching.

When I get the crank shim as close as possible I can start it fine and it idles nicely, but when I give it some throttle it backfires up out of the intake.

I can tell its the intake, because I have the airfilter/box completely off and the intakes exposed... I can see the spark from behind the throttle plate.

I was very careful about the timing belts so I'm confident its not that - anyone have any ideas on how to stop this from happening?

Thanks
-Jordan

[Howie]

You are probably either running lean or have a vacuum leak causing a lean condition.

[motoxmann]

do the injected bikes fire at both TDC'ish positions like the carby's? or only on the compression stroke? (the answer to this question may be related to this thread, hence me asking here)

[Jordan1234]

You are probably either running lean or have a vacuum leak causing a lean condition.

Would running with no air cleaning make me run lean?

do the injected bikes fire at both TDC'ish positions like the carby's? or only on the compression stroke? (the answer to this question may be related to this thread, hence me asking here)

Theres only 1 crank position sensor so I assume it sparks both cylinders, and since I would assume they fire out of phase I think the answer to you question is yes.

[motoxmann]

thats not what I mean.
car engines only spark at TDC on the compression stroke, meaning every other revolution. but the carb monsters fire at tdc on the compression stroke and at the end of the exhaust stroke, meaning every revolution. I was asking if the injected monsters spark every revolution like the carb monsters, or if they spark every other revolution like a standard car engine.

[Jordan1234]

thats not what I mean.
car engines only spark at TDC on the compression stroke, meaning every other revolution. but the carb monsters fire at tdc on the compression stroke and at the end of the exhaust stroke, meaning every revolution. I was asking if the injected monsters spark every revolution like the carb monsters, or if they spark every other revolution like a standard car engine.

I'd have to take the alternator cover off (again ) to figure that out... why do you ask?

Also: how would a lean cause backfiring through the intake? It seems like, if the intake valve is closed, then exhaust/combustion shouldn't be able to make it up into the intake manifold.

For vacuums leak: the net effect of this is that the engine would run lean, right? another way of asking the same thing: vacuum leak is a cause of lean mixture?

I can't see any reason for there to be a vacuum leak - I plugged up the emissions ports on the intakes and the o-rings are brand new. Everything is torqued to spec. I don't see any other places for a vacuum leak to occur.

I appreciate the advice!!!

[wiggy_nz]

To the best of my knowledge they have a wasted spark..  ie they fire on every 360 degree cycle NOT 720 degrees like a car.

[Dirty Duc]

To the best of my knowledge they have a wasted spark..  ie they fire on every 360 degree cycle NOT 720 degrees like a car.

Many cars are wasted spark.  injected motors can also be bank injected: where both injectors fire at the same time at a nearly unrelated to piston/cam position... it isn't as efficient as say the super cool direct injection currently creeping in, but it is easier to do.

[krista]

Injected ducatis get timing from the jack shaft which drives the belts, so it's possible they don't have wasted spark. I believe they only fire once per cycle but I do not recall actually testing for that.

[ducpainter]

Injected ducatis get timing from the jack shaft which drives the belts, so it's possible they don't have wasted spark. I believe they only fire once per cycle but I do not recall actually testing for that.

You are correct.

The injected models do not use a wasted spark.

[motoxmann]

huh, ok, good to know.

my only guess then is that one or both timing belts is off a tooth or two. I know you said you're pretty certain they are on correctly, but in every engine I encountered backfiring through the intake it was due to either the spark plug wires being on incorrectly or the timing belt(s) being on incorrectly.
and if you said it idles decently, then I'm sure the spark plug wires are correct, which only leaves cam timing (timing belts). it typically occurs when the intake cam is advanced 1 or two teeth, which allows still-burning exhaust to go up the intake at a point when there is load and retarded ignition.
I would triple check the timing belts, and their tension. a loose belt can even cause the valve to flutter around that point. if the valve adjustment is too far out of whack, that could also cause it I'm sure.

I'm not too experienced specifically with monster engines yet, so I'm learning some things here too, but the above is my common experience with other various engines in general

[Howie]

Running with no air cleaner will make the bike run lean if it is tuned to run an air cleaner.  A vacuum leak will create a lean condition at low throttle, such as cruise.  You can have a vacuum leak where the carbs mount to the manifolds and where the manifolds mount to the heads.  Easy, cheap stuff first.  And, yes, recheck valve timing.  Did you do anything with the valves and valve seats during the rebuild?  If so, think about a cylinder leakage, or, at least, a compression test.

[Jordan1234]

As far as I can tell the belts are exactly right. When I installed them I put bolts through the holes in the back of the case to lock the timing gears at TDC.
I put the engine at TDC and took some pictures...

Here is the main timing gear - There is a smaller dimple at the top of the pulley... don't know what thats for but the larger one lines up perfect.


Here is the Mark on the alternator - the window was dirty so I highlighted where the tick mark and indicator arrow are. There is also a "dot" just outside the view through the little window, but I think its the timing mark for use with a timing light.


Here is the horizontal cylinder... there was no "mark" for me to work off, but the screw hole in the back still lines up with the mark


Vertical cylinder:


Here is me pinching the belt - I used a 5mm allen key when I snugged it up. I used a 6mm on the vertical cylinder.


And here is the engine running. After it started I gave it some gas then held the throttle steady. Backfires start at ~15s.


Could lack of air cleaner really cause this much backfiring?

I really appreciate the advice!!!

-Jordan

[J5]

timing is out , believe me i have been down this road before on jap bikes

either the belts are out a tooth or the spark is at the wrong time

2 things you have to do

check the belts are right meaning with a degree wheel

2nd is using a timing light to check the actual spark timing

i would go with the timing light first since its easier and you are playing with the sensor

but i would also like to degree the cams to make sure they are right since you its a fresh build

rechecking valve clearances

its the only way you are going to fix it

[Jordan1234]

If I advance the belt by 1 tooth do I risk bending a valve?

The valves were re-seated during the rebuild, but I don't know why that would effect timing as long as the valve clearances are correct (which they are - I rechecked them today and they're spot-on).