Misfiring MC5 250

[Lloyd Boland]

I was racing this week in the 50+ intermediate class at a local vintage race in So. Cal. and took first in the first moto.  I felt pretty good about that because there were a couple 450 Maicos and a couple 490 Yamahas of the 1982-1983 years.  I'm no small guy, 230lbs on a little 250, but that MC5 just kept on screaming.  The second moto not a very good start but I was able to get around a few guys when the motor just went south.  It would only run with the motor fully reved, ie. 15000+rpm.  I would have to pull in the clutch rev the heck out of it and slowly let out the clutch to keep it moving, any load on the motor and it would just seem like it was loading up and stalling.  I had to pull off and DNF.  Too bad, it was a good race.
My first thought is something with the carb.
My second thought was something with the timing or the motoplat.
There is compression.  I have not had time to do any work on it yet or to look into it any closer.  
Just currious if anyone else has had a similar condition show up while they were riding.  One minute running perfectly and just hauling, the next it just won't work with any load on the motor. And believe me, I'm a pretty big load for that poor little 250.
As always, thanks in advance for all input.
Lloyd Boland

[tomale]

My first thought was the carb as well, I always tend to start with what ever is the easiest.... A couple of years ago when I first started riding my 250 again I had a lot of trouble just like that... it turned out to be varnish.... I thought I had cleaned up my tank and carb real well but I was wrong... I tried all sort of things and did not have much success... For awhile, I would lean my bike on its side and pull the petcock off and clean out the junk that had built up around the in the hole leading into the valve. I would do this between each moto. I changed fuel filters and clean the carb after each race, there sure seemed to be alot of it, What I realized is that the new gas was attacting the old varnish and loosening it up, it would then pool up in the bottom of the tank. A similar problem that I had early on was I had several wires under the tank that had corroded. one was even loose. So I went to work and redid all the wires and there connections.. I also discovered that the ground wire was looking pretty bad as well and I cleaned it up and wirebrushed everywhere it made contact...
I have been told that coils do not go bad, they either work or do not work, but I am here to tell you that is not true. I had a coil that worked but never did get a good spark. I cleaned everything up and the poor spark just never improved. I replaced it with a different coil and wham, I suddenly had really good spark, nice and blue, the other was kind of a pale yellow... not a good sign. I would start with these, and then work your way toward the really expenive stuff, you would be amazed at what your electric ignition will run with... a couple of years ago I rode a winter Motocross on the 250 of mine and it did not miss a beat... I pulled the igition cover off and it was clear full of mud... I pulled the mag off and the mud was packed all around the coils, I was amazed that it even ran at all but that day, I rode two class on that bike and I won all 4 moto's. What am I saying, the bike will put up with alot, and the stuff under the ignition cover more than likely is fine... If it is a little rusty clean it up but I would be hard pressed to believe that is the problem. I would check your gas tank and the carb for junk, and the wiring under the tank for bad connection....then start looking for bad parts....

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
75' GS400 (project bike)
72'sixday (project bike)

[Jay Jones]

Sounds like ignition or more than likely primary side crank seal gave out?Could be a bad coil or plug,check those first?Jay:)It did'nt eat a bad airfilter did it?[)]

[Jay Jones]

Check for corrosion inside of ignition flywheel and worn woodruff key in crank/flywheel junction with any slop?n it?Check for clean contacts on stator and slator backing plate for tightness on engine cases and cleaness and tightness for a good ground and make sure it did'nt come loose and spin a few mm and altered your timing?No wires are cut or bare and touching and all grounds are clean and tite?If its not a carb float problem,then its probally sucking gear oil?Jay:)

[Jay Jones]

Easy,could just be a plugged exhaust?Try taking out the silencer packing and changing it?

[Merlin]

.............sounds like the Motoplat stator "fell off"...........

[TGTech]

Lloyd:

   What type of carburetor is on the bike? Is it possible that the choke got turned on somehow? If that isn't the case, I'd change the spark plug to make sure it is OK. Then start checking all of the wire connections (stator to coil) to make sure they're tight. It they're OK, then make sure that the wires aren't broken inside of the insulation. Pull on the terminal and see if the insulation stretches. If so, you've got a broken wire insde of the insulation.

   The next step would be to check to see that all of the ignition grounding is sound. And just because the fasteners are tight, doesn't mean that that the grounding will be OK. The coil to frame mounting is the easiest to check. Remove the coil and sand the mount surfaces to be sure that there is no rust between the parts.

   Next, remove the flywheel, the stator, and the stator mounting plate. Take a 4mm Allen wrench, and remove and reinstall the seal plate screws, one at a time, to be sure that there is a good ground between the cases and the seal plate.

   Now, take some fine (220-320 grit) emery paper and lightly sand the back surface of the stator mounting plate and the front surface of the seal plate. When you're done, blow/wipe both surfaces off and reinstall the stator mounting plate.

   Now make sure that the lip of the stator mounting plate is clean and free of any corrosion. If the surface isn't clean silver, then it could be corroded.

   Finally, make sure that the inside diameter of the stator that sits on the stator mounting plate is free of any rust. Once you've got that taken care of, remount the stator, do the timing, and reinstall the flywheel.

   It is possible that the frame to engine connection can cause a break in the ground, if there is rust on the frame mounts and corrosion on the engine cases. I would take the pipe off, remove the engine mount bolts from the front and bottom mounts, lift the engine to where you can sand the engine and frame surfaces. This insures that you don't have problems there. I've personally experienced problems with that, so I make sure they're clean.

   When you're done doing all of this, check to see what the spark looks like. If you're not getting a nice, bright blue spark, you could have problems with the ignition components. At that point, I'd suggest sending them to us for testing to see that things are working OK.

   If there is a problem, it would most likely be the capacitor charging circuit of the stator.

   Obviously, going through the carburetor to make sure it's clean, would be a good idea.

   Hope this is of some help.

Dane

[Lloyd Boland]

Thanks for the suggestions.  This weekend I hope to have time to sort threw all the suggestions.  I will get back to all of you so we all will have a little more knowledge/experience.  Again, this is the greatest bunch of guys.  Thanks for all your help.
Lloyd Boland

[Lloyd Boland]

Thanks for everyones input.  Cleaned the bike real well.  Pulled the pug - great spark, bright and strong with each revolution. OK.  

Checked the timing - perfect.  No evidence of any corrosion, leaking, all wires appear good and no rubbing or shorting out. (again great spark)

Pulled the carb (Bing) off.  The needle still attached properly (had a thought that maybe somehow the needle slipped or something).

Nothing in the float bowl.

Floats set properly.  

Pulled all jets - no evidence of a clog or any malfunction.

Then, there it was.  The little nylon tab that keeps the slide in the proper position was missing.  The little tab had slipped sideways and was no longer protruding into the carb to hold the slide in the proper position.  The nylon piece is still there, it just appears that it loosened up and slipped a little and was loose.  I could push it back in from the outside of the carb and I could push it back out from inside the throat of the carb.  I can actually push it completely off the carb.

It looks like from the ouside of the carb it was previously held in place by crimping or flattening out the edge of the carb where the piece slides in - like taking a screwdriver and hammer and denting the body of the carb putting a small dent in it to keep the piece from sliding back out.

Has anyone else dealt with this? In 1972 I had a 400 cz with a BING on it and the same thing happened but the piece came completely out and was lost.  I had no time to order the piece back then so I took a nickle, ground it down and stuck it in and glued it into place with something, problem solved.  This time I thought I would ask for help.  Should it be glued in with something and them tapped to prevent it from coming out or just put abck in and the aluminum tapped to prevent it from coming out again?

Once I get this back together, I will see if this solves the problem.  The solution isn't always the obvious one.

Thanks again to everyone for suggestions, it always helps to make sure your not missing something.

Lloyd Boland

[Lloyd Boland]

Thanks for everyones input.  Cleaned the bike real well.  Pulled the pug - great spark, bright and strong with each revolution. OK.  

Checked the timing - perfect.  No evidence of any corrosion, leaking, all wires appear good and no rubbing or shorting out. (again great spark)

Pulled the carb (Bing) off.  The needle still attached properly (had a thought that maybe somehow the needle slipped or something).

Nothing in the float bowl.

Floats set properly.  

Pulled all jets - no evidence of a clog or any malfunction.

Then, there it was.  The little nylon tab that keeps the slide in the proper position was missing.  The little tab had slipped sideways and was no longer protruding into the carb to hold the slide in the proper position.  The nylon piece is still there, it just appears that it loosened up and slipped a little and was loose.  I could push it back in from the outside of the carb and I could push it back out from inside the throat of the carb.  I can actually push it completely off the carb.

It looks like from the ouside of the carb it was previously held in place by crimping or flattening out the edge of the carb where the piece slides in - like taking a screwdriver and hammer and denting the body of the carb putting a small dent in it to keep the piece from sliding back out.

Has anyone else dealt with this? In 1972 I had a 400 cz with a BING on it and the same thing happened but the piece came completely out and was lost.  I had no time to order the piece back then so I took a nickle, ground it down and stuck it in and glued it into place with something, problem solved.  This time I thought I would ask for help.  Should it be glued in with something and them tapped to prevent it from coming out or just put abck in and the aluminum tapped to prevent it from coming out again?

Once I get this back together, I will see if this solves the problem.  The solution isn't always the obvious one.

Thanks again to everyone for suggestions, it always helps to make sure your not missing something.

Lloyd Boland

[wildman]

I,ve sealed mine in with Penton orange silicone seal but now I use a little dab of JB weld. Keeps the dirt out too.

1975 250 Cross Country, 1974 175 Jackpiner, 1975 125

[Lloyd Boland]

Update:

Tried a couple of different sealers, but the nylon tab kept sliding out if I put any pressure on it.  So I took Wildman's advice and used a little JB Weld, but I did it a little different.  When I pushed the tab in, there was a little lip of the carb that extended out past the nylon piece on the outside of the carb.  I drilled 2 very small safety wire holes and ran safety wire behind the nylon tab so it won't slide out, then put the JB Weld so the wire was also inbedded in the JB Weld.  Now that should hold it.

I gave it 72 hours to dry.  Put the carb back on.  Fired it up.  Runs like a champ.

Next week, Glen Helen.

Thanks for everyone's help and suggestions.

Lloyd Boland
SVRG #1 on a 250 MC5 (and CZ 400, and YZ 250 and KX
 500)