Freshly assembled bike won't hardly run

Hi,

I have a 1973 850 Commando with twin Amals and a Boyer MkIII. I recently got it back together after four years off, after it ate part of a carburetor slide. I've had the bike for twenty years and have put 90,000 miles on it.

I got it to run, but it won't rev much at all. It will pop a few times, the only way I can get it to run is with the choke on. The tachometer isn't working for some reason, but I'd estimate that it wouldn't go much over 2500 rpm. I put a timing light on it and it looked like it was around 25 degrees BTDC. That seems a little advanced, according to the chart I found said it should be more like 20 at 2500. That doesn't seem to be enough to cause this. It never tried to kickback when I started it either.

The carbs were off the bike while it was sitting and I sprayed carburetor cleaner in them and blew them out with compressed air. All the passages seemed clear, but even if the idle jets were totally plugged it would rev more than this.

I'm wondering if I could have gotten the cam timing off. I used a zip tie on the chain to keep the sprockets in time with each other. I was very careful to get the dots lined up, but getting that's the only thing I can think of. Would a bike run this poorly if the pinion on the crank and intermediate gear were one tooth off?

-Eric

ewgoforth said:

Would a bike run this poorly if the pinion on the crank and intermediate gear were one tooth off?

Click to expand...

Yes.

As an alternate to opening the timing chest, If this is your best hunch, start with a degree wheel and note the opening and closing events for the intake and exhaust and compare to some known cam profiles.

If it runs better choked, then maybe you have an air leak? Have you checked the balance tube between the carbs isnt leaking? Or tried spraying WD40 around the manifold joins?

For correct timing the timing marks must be 10 rollers apart on the chain as shown in the illustration.

Dances with Shrapnel said:

As an alternate to opening the timing chest,

Click to expand...

For a fully assembled bike, thats more work than just taking the timing cover off, and checking for sure. ?

Unless a needle has dropped out of the carby, or one carby isn't opening, or something equally drastic.

Rohan said:

Dances with Shrapnel said:

As an alternate to opening the timing chest,

Click to expand...

For a fully assembled bike, thats more work than just taking the timing cover off, and checking for sure. ?

Unless a needle has dropped out of the carby, or one carby isn't opening, or something equally drastic.

Click to expand...

You can get answer with only one valve. Remove one valve cover, remove outer primary cover, attach degree wheel and pointer. What am I missing?

Easy easy.

Do a quick check to see if when you put the carb slides in the needle clip didn't fall of and the needle isn't rising.

I put a timing light on it and it looked like it was around 25 degrees BTDC. That seems a little advanced, according to the chart I found said it should be more like 20 at 2500. That doesn't seem to be enough to cause this. It never tried to kickback when I started it either.

Click to expand...

I suggest putting your strobe light back on and taking it up to about 4000rpm where the boyer should be fully advanced reading around 30 BTDC

is it possible your timing is too retarded?

gortnipper said:

If it runs better choked, then maybe you have an air leak? Have you checked the balance tube between the carbs isnt leaking? Or tried spraying WD40 around the manifold joins?

For correct timing the timing marks must be 10 rollers apart on the chain as shown in the illustration.

Click to expand...

I haven't yet checked for air leaks, but I've had a cracked balance tube before and the bike ran MUCH better than this, I don't hear any backfiring, but then again. I'm holding 3/4 throttle to get the bike to rev to what sounds like 2500 rpm. I need to figure out what's wrong with the tach, so I have a better idea what the revs actually are.

-Eric

So, it wont idle correctly either?

Time to go back to first principles and check cam and ignition timing with a degree disc. Put new plugs in. Then address the carburation problem ?

acotrel said:

Time to go back to first principles and check cam and ignition timing with a degree disc. Put new plugs in. Then address the carburation problem ?

Click to expand...

I've got my degree disk zero'ed and my dial indicator on the left exhaust valve. I'll check my cam timing tomorrow.

-Eric

I need to look up cam timings, but I was getting a peak lift on the left exhaust valve at 260 and 270. The max was .342, if I recall correctly. My base circle measured .002" after I got back over the lift, so that was my error, hysteresis, etc.

-Eric

My peak almost exactly matches Dyno Dave's

http://atlanticgreen.com/images/cam110a.gif

Sounds like you are in the ball park but 260 and 270 is quite a spread. This is where you want to measure two or three times and cut only once; in other words, be certain and then move on to the next thing that may be causing the problem.

I am almost certain that the Norton exhaust lobe is symmetrical so I suggest you read degrees at 0.050" lift at Ex opening and 0.050" lift at Ex closing and then calculate where maximum lift occurs. You can also cross reference 0.050" on the dynodave cam plot. He may also be able to provide you with the data file.

Dances with Shrapnel said:

Sounds like you are in the ball park but 260 and 270 is quite a spread. This is where you want to measure two or three times and cut only once; in other words, be certain and then move on to the next thing that may be causing the problem.

I am almost certain that the Norton exhaust lobe is symmetrical so I suggest you read degrees at 0.050" lift at Ex opening and 0.050" lift at Ex closing and then calculate where maximum lift occurs. You can also cross reference 0.050" on the dynodave cam plot. He may also be able to provide you with the data file.

Click to expand...

The Dyno Dave plot appeared to have the peak at 265 with 260 and 270 being just each side. It jibes with my measurements.

I had 2 things wrong: The points wires were reversed which means I was running 25 degrees fixed advance which made the bike hard to start. I also had emptied most, but apparently not enough, of the old bad gas from the tank. The bad gas left in the tank was what was making it run like crap and requiring me to leave the choke on, etc. I did a more thorough job ef emptying and refilling the rank and fixed the points wires and it started pretty normally.

Good news Eric. So aside from starting normally, how does it run?

Are you making the rally this summer?

It was way too far advanced, about 40 at 3500. I didn't have enough room in the Boyer pickup plate slot to correct it, so I had to take off the rotor and move it. Sounded pretty good, hopefully I'll be able to ride it this Saturday.

I'm planning to attend the rally this summer if the creek don't rise and the Norton keeps running.