Backfire through Intake?

[Jordan1234]

Day before yesterday I misread someones response and tried it one tooth off -

The valve was obviously hitting the piston. I took the head off (the horizontal head is easy) and the valve is not bent, amazingly. i used a sharpie and some lapping compound to check the valve seating and it looks good.

I'm pretty sure my problem is that the factory timing is no longer sufficient for whatever reason - lobe wear I guess.

My manual is for a carbureted M750 - it says intakes should open 11deg before TDC and close at 70deg after BDC, exhaust opens 50deg before TDC and closes 30deg after TDC.

Are those numbers the same for the injected M750s?

[motoxmann]

My manual is for a carbureted M750 - it says intakes should open 11deg before TDC and close at 70deg after BDC, exhaust opens 50deg before TDC and closes 30deg after TDC.

something about that sounds horribly wrong, even assuming you meant ex opens 50deg before BDC.
is this true? the carbed 750's have 261deg duration intakes and 260deg duration exhausts? I would think they'd barely idle with overlaps and durations like that

[Howie]

From the 750 carby manual

Data measured with 1 mm clearance
and timing belts tensioned with
service tool no. 051.2.001. 1A at
11.5.
Intake valve opens at:
11⁰ before TDC
Intake valve closes at:
70° after BDC
Exhaust valve opens at:
50^o before BDC  TDC is probably a typo on Jordan's part.
Exhaust valve closes at:
30° after TDC
Valve lift
Data measured with 0 mm
clearance.
Intake:
9.3S mm/0.368 in.
Exhaust:
8.SO mm/0.334 in.

750IE uses the same cams so lift will be the same and so should the timing.  You need to be anal about clearance and belt tension for degreeing.  In your case, not so much.  Yes, a worn intake lobe can cause your symptom.  The wear would be obvious to the eye.

[motoxmann]

interesting. I guess that does explain why they only pull around 8-9in/hg vacuum at idle

[Jordan1234]

Ok! It took me just 7 months to get back to this project.

I have a degree wheel on the crank, and a micrometer on the intake valve.

I've got the crank at TDC...
The main timing gear indicates TDC: http://i28.photobucket.com/albums/c235/FurrorNos/vlcsnap-2013-06-08-16h12m43s133_zpsaea713d2.jpg
The windows on the other side shows TDC: http://s28.photobucket.com/user/FurrorNos/media/vlcsnap-2013-06-08-16h12m57s227_zps02f3167b.jpg.html

But it seems like TDC for the horizontal cylinder is like Crankshaft TDC+360deg. Is that right?
In other words:
- The intake valve is 1mm open at Crankshaft TDC + 350deg
- The intake is 1mm from being closed at 70deg after BDC, which is like crankshaft TDC + 610deg (360+180+70)

Also, when I set it such that the intake is 1mm from closed at 70deg after BDC, the intake is 1mm open at 4deg before TDC. Its more important that the intake is closed at the right time than open at the right time, right? I assume the deviation from spec (10deg before TDC open, 70deg after BDC closed) is due to lobe wear.

Thanks!!
-Jordan

[Jordan1234]

Wow I just talked this through with my dad and realized its a very dumb question. Of course the intake valve opens 360degrees after spark.