Backfire through Intake?

[Howie]

It won't, but a leaking intake valve will cause backfire through the intake.  So will an intake valve that does not fully close.

[motoxmann]

the belts look good.
I don't recall the flywheel having a tick mark like that. the only timing marks on the flywheel should be blunt obvious circles. you said there's on of these slightly off to the side out of view not quite lined up?...
it's possible that the ignition timing is out, but there isn't a whole lot of adjustment on ignition timing, probably not enough to cause popping out the intake. but maybe its possible? either way you def need to check/adjust ignition timing. did you make sure to set the gap properly and to spec between the pickup sensors and flywheel trigger "lump"?

my guess due to the above info, and general causes for this, is still cam timing. while your belts are definitely on correctly, are you absolutely sure the camshaft driveshaft gear and it's drive gear on the crank were lined up properly? if that is off 1 tooth it could definitely cause what you're experiencing. and it would explain why the mark on the flywheel in the view window doesnt line up properly even though belt gears all show good.

[brad black]

motoxmann is talking about a carb bike, which is completely different to an injected.  the flywheel should have the single line marks.  i usually set the rotation sensor gap to 0.5 to 0.7mm.  i dno't know if having it at almost nothing will effect the way it runs, but it shouldn't.  ignition timing is not adjustable.

i would confirm that when all the marks line up the engine is at tdc horizontal firing.  if the timing gears are off (how far disassembled was the engine?) than that will only show up comparing timing marks to actual piston position.

if the cam timing is right now and you advance the cams 1 tooth (40 degrees) it should bend the inlet valves.

[motoxmann]

my mistake, I thought it was a carbed M750, should've re-read first post before posting again haha

[Jordan1234]

if the cam timing is right now and you advance the cams 1 tooth (40 degrees) it should bend the inlet valves.

Should as in it will bend them?

[ducpainter]

Should as in it will bend them?

1 tooth can bend valves.

[Jordan1234]

Marks on the timing gear were lined up, but there is some damage on the timing gear that could be the culprit. Its probably from when I installed the alternator - I used an allen key to lock the gears while I tightened the nut. It also could have happened just now when I removed the alternator.

Just ordered a new timing gear set on eBay - hopefully this fixes it. Another week waiting for parts.
And then I'll probably be right back where I was yesterday.

[motoxmann]

ouch, at least you caught the damage this soon before it started wreaking havok on other stuff.

I went through this same type of thing in late spring, did a full rebuild of my engine as well, bike was down for 6 weeks may-june. that was tough

[Jordan1234]

After looking at the wear on the timing gears some more, I've decided that most (all?) of the damage is from just today when I took the alternator nut off.

My theory on why the horizontal cylinder is backfiring is: worn camshaft and/or rockers.

I got the bike for $700 with 80k miles and a broken vertical belt/bent valves. It was the same price to get a whole (lightly used) vertical head on eBay than to buy a new valve, so the vertical head has far fewer miles on it.

The horizontal head appeared fine, albeit in dire need of a valve job.

I had a hard time doing the valve job, however, because the intake lobe was so worn that I couldn't slide a new opener shim on normally; I had take the rocker shaft out, line up the rockers with the shim, and slide the rocker shaft through the assembly.

Thus, my theory is that the bike had horizontal timing problems before I even got it on account of 80k wear on the camshaft/rockers.

From the manual it looks like offset woodruff keys are available to tweak the timing, and it looks like adjustable pulleys may also be available.

EDIT: Turns out the timing is adjustable on this bike after all!

[motoxmann]

The horizontal head appeared fine, albeit in dire need of a valve job.

I had a hard time doing the valve job, however, because the intake lobe was so worn that I couldn't slide a new opener shim on normally; I had take the rocker shaft out, line up the rockers with the shim, and slide the rocker shaft through the assembly.

WTF did I just read?
um, dude? there's your problem right there. sounds to me like you forced the rocker onto a shim that was way too thick, and now your rockers are probably bent, causing the valve lash to be all sorts of out of wack, making the valve hang open slightly almost at all times

[Jordan1234]

I think I stated that wrong -

With everything assembled the clearance on the intake opener shim is in spec.
The problem was that with the rocker arms and rockers installed, the lobe next to the intake opener lobe prevented me from sliding the rocker over on top of the shim.

Using an opener shim thin enough to allow the rocker to slide over into position left me with .2mm of opener clearance.

[motoxmann]

I think I stated that wrong -

*sigh!*

With everything assembled the clearance on the intake opener shim is in spec.
The problem was that with the rocker arms and rockers installed, the lobe next to the intake opener lobe prevented me from sliding the rocker over on top of the shim.

thats normal, mine is the same way, and has nearly zero wear on all valvetrain parts. you usually need to remove the belt and rotate the cam a bit to get the rocker to slide over

[brad black]

should as in i don't know because i've never actually tried it because i don't want to bend my valves, but from my measurements of lift and clearance it should.  if i say it will and it doesn't then i'll get some petty prick telling me i was wrong.  and i'm somewhat over that.

so to sum up i'll say, imo, it's a really bad idea to advance the cams one tooth.

[ducpainter]

I've personally seen the results of one tooth off.

I'd listen to Brad.

[Ddan]

I was a tooth off after a valve adjust and it's immediately obvious after starting that something is wrong.  I never took it apart to see if there was any damage and it still runs strong so I'm 'assuming' nothing is bent.  YMMV, but there is no way I'd set it up a tooth off intentionally.