getting both cylinders to fire at the same location

[poobah22]

I just bought a 1978 off an old friend this past fall and I holed the left piston which caused me to spend the entire summer breaking the bike and the motor all the way down and re-building it. I finally finished and got it all back together and got it running so I'm pretty jacked about that. The problem I am having is when I am trying to set the timing, with a timing light, I am able to get the right cylinder to line up exactly where it is suppose to at low idol, approx. 1200 rpm, right at the F mark however when I try to set the left cylinder I can't rotate the point plate enough to get that cylinder to fire where it is suppose to, at the F mark. I can get it half way between the full advance mark and the F mark. When I first starting working on setting the left cylinder it was firing before the full advance mark. It seems to be running a little rough and backfiring so I am a shade concerned that all my hard work is going to get flushed down the toilet. I am also wondering if this is what contributed to holing the piston. Does anyone have any suggestions of a cause of this and what I need to do to fix it?? Any suggestions would be most appreciated. Thanks.

 

[5twins]

First of all, when you rebuilt, did you install a new cam chain? If not then I would suggest doing so. There are various reasons your timing could be off like it is but there's really no sense pursuing them if you've still got a 30+ year old timing chain still in there.

 

[TwoManyXS1Bs]

Hey, poobah22, welcome to the forum. Sounds like you've got some worn points plate parts.
Things to check:
Screws protruding through backside of points plate, it sets unlevel, point settings can't match.
Excessively worn or mismatched fibre points followers, their contact to the points cam influences timing.
Worn points cam, especially angled (conical) wear on the highside. One point may have matching uneven fibre follower, other may have unworn straight follower.
Worse-case-scenario: sloppy cam bearings deflecting cam during that side's valve lift...

 

[CDNTX650]

+2 on timing chain...

And get a pamco with e advance, no more fucking points and it works. very well.

 

[XSLeo]

On your 78 there are two marks by the F one on each side. when timing the rotor mark should be between these two marks. This is a range of about two degrees.
I might suggest you start over and set the points gap. If one is wider than the other this might be the problem.
Time the upper or right set first. These are mounted on the full size breaker plate. Once set do the lower or left set. Set these by moving the half plate.
If you have a repair manual if explains this, it even has pics.
Leo

 

[poobah22]

I did replace the timing chain and tensioned it properly. I did set the point gaps to the same gap. I was shooting for the .35 mm (range .3 to .4). I was having trouble getting it just right so maybe I will go back and check that. That sounds like it might be the culprit. I have been able to rotate the plate over the main plate to adjust the left cylinder and I was able to adjust it but it was way out. It is so far off it is out of the range of adjustment. So I will give that a shot and report back.

I did buy a manual which is helpful and I did find the manufactures (Yamaha) shop manual on-line which has been light on detailed information but has all the right specs for my bike. The manual has a wide range since it is trying to cover several bikes and the bolt torques and gaps they specify aren't always the same.

Great suggestion on the Pamco. I will look into that.

Thanks everyone. I will check back after I reset the point gaps, again...

 

[CDNTX650]

Pamco with e advance is set it and forget it. i fought with points and wish id bought it sooner. The. I fought with the stock advancer an pamco. till the advancer blew itself up and left me stranded. the new e advancer makes the bike so much smoother on the advance transition. I love it.

 

[poobah22]

That was it. The point gap for my left cylinder was way out. I thought I set them right and never bothered to go back and check them after the timing was way out. I re gapped it and was more careful this time and I was able to get the timing for each cylinder to fire exactly where they were suppose to. Runs a lot smoother now. Thanks a ton for the advice.

 

[peanut]

as TwoManyXS1B's has mentioned wear in the camshaft bearing can cause your points to open up wider than the setting due to centrifugal force . it might not be much but a few thou would throw it right out .
If I suspected the points gap was opening up at fast idle I used to press hard on the cam from the opposite side of the points cam and measure the gap to see if there was any appreciable difference.

If you want this to be spot on you'll need to get seriously anal with the micrometer , needle files and fine wet and dry etc on the points and advance assembly as Twomany has suggested

oops ...seems like I missed your update this morning sorry.

Good to hear that it is sorted anyway

 

[5twins]

You can shim the excess end play out of the advance rod. That can smooth the timing curve out.

 

[Tomterrific]

The dwell is enough that you can juggle the points gap enough to get timing the same for both.

Use the highest pump octane you can get. If it is 91, 92 0r 93 use it. Detonation caused the holed piston and, "Octane is the sure fire easiest way to keep octane at bay." Kind of rhymes. :^)

Tom