Model 97 "sherpina" Lurching

[onthegas]

I have an M98 Alpina that I restored to running condition 12 years ago. A couple years after that I turned it into a "Sherpina" for riding vintage trials and rode about a half dozen trials. Seven years ago I was given a basket-case M91 (that had been in a fire). The gears, heavier primary-side flywheel, single-row chain and clutch basket from the M91 made their way into the M98. Shortly after that I moved and only used the bike to ride around in the woods following my kids when they were learning to ride. Last year I entered my first trial, in six years, and noticed the bike was having some issues, but figured I was just rusty at riding trials.

 

This year I picked up a TL125 and a TY80 for my boys and I to ride trials together. Last month we entered a trials and I was having a lot of carburetion issues, so I pulled apart the Amal and found the slide was worn out. I replaced the old Amal with a Mikuni VM26, checked the timing (still in spec.) and cleaned the air filter. All the old problems (hard-starting, flooding, running lean) have disappeared. A couple weeks ago I went to a friends place to practice for a day. One thing I noticed was when crawling through a section at idle the bike lurches to the point where I have to use the clutch and throttle to keep the engine speed up. It was doing this with the Amal carb too, but the other problems sort of overshadowed this one. This makes the bike very difficult to ride in tight sections. I can ride the TL or the TY in the same tight section and not even touch the clutch.

 

Lurching was never an issue when I rode the bike before in competition. After I swapped out the internal parts with those from the M91 I never rode the bike in competition. I'm thinking the heavier primary-side flywheel is the culprit here even though the. engineer in me says a heavier flywheel would have the opposite effect.

 

Has anyone else experienced a similar issue?

 

Edited July 20, 2016 by onthegas

[feetupfun]

_{flywheel weight change is unlikely to be the cause.}

_{Pretty hard to diagnose from your description. It sounds like it might be lean at near-closed throttle position. This could be poor jetting, or a restriction in the pilot circuit, or air ingress downstream of the carby or a combination.}

_{Modern fuel is notorious for blocking the pilot jet if you let the fuel in the bowl evaporate}

_{The magneto side crank seal going hard is a common source of air ingress. If it is leaking, there will probably be a witness of oil near the seal behind the stator. There are also plenty of other possible air ingress sites}

_{Are you sure that there is enough fuel flow to the bowl, to keep the level up? Original Bultaco fuel taps are notorious for poor fuel flow rate}

_{Is it geared low enough in first?}

_{M97 Alpina?. The first Alpina 250 was M85 and the first 325 Alpina was M99}

 

[feetupfun]

I just read again and see that you had the cases apart to swap the gearbox over, which introduces the possibility of air ingress via the centre gasket leaking

[onthegas]

_{flywheel weight change is unlikely to be the cause.}

_{Pretty hard to diagnose from your description. It sounds like it might be lean at near-closed throttle position. This could be poor jetting, or a restriction in the pilot circuit, or air ingress downstream of the carby or a combination.}

_{Modern fuel is notorious for blocking the pilot jet if you let the fuel in the bowl evaporate}

_{The magneto side crank seal going hard is a common source of air ingress. If it is leaking, there will probably be a witness of oil near the seal behind the stator. There are also plenty of other possible air ingress sites}

_{Are you sure that there is enough fuel flow to the bowl, to keep the level up? Original Bultaco fuel taps are notorious for poor fuel flow rate}

_{Is it geared low enough in first?}

_{M97 Alpina?. The first Alpina 250 was M85 and the first 325 Alpina was M99}

 

 

I just read again and see that you had the cases apart to swap the gearbox over, which introduces the possibility of air ingress via the centre gasket leaking

 

Thanks! Actually it's an M98, 175 Alpina. The M97 was a 125. The 175 & 250 (M85) have the same stroke and most M98's, including mine, were fitted with a 250 jug and piston by the dealer.

 

I did split the cases, but reassembled with new gaskets and Yamabond sealant. It's also not burning gear or trans oil, but the ignition side seal could be leaking though I didn't see any signs of a leak when I pulled the ignition cover to check the timing. I know it's not a plugged pilot jet, brand new carb, and I checked for air leaks on the mounting surfaces. I'll try adjusting the air screw first to see if that makes a difference. Also I installed a new fuel tap last year and it's flowing plenty of fuel.

 

Gearing could also be an issue. Before installing the M91 gears I had a 50-tooth sprocket on the back. I went back down to the stock 42-tooth sprocket after swapping out the trans gears.

 

Guess I should try the simple stuff first.

Edited July 20, 2016 by onthegas

[lbhbul]

When you say 50 tooth/42 tooth rear sprockets 428 or 520 chain??

 

I personally have owned Sherpa Ts for many years, m49, m92, m199 & have never had a seal problem, have had very good luck

with all Bultacos.

On the carb issue VM26 jetting, I see you are in the states, for a 250cc what jets are you using?

 

Check your Personal Message

 

Larry