Author Topic: 2-stroke montesa Problem  (Read 1486 times)

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runutz698

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2-stroke montesa Problem
« on: September 16, 2008, 09:43:20 PM »
Hi for those who have not read my earlier threads I have a montesa Capra VF250 which I have just done a resto on the motor has had some work done to it before I brought it from a mates dad after his death, It had been sitting in the back room of his house as just a motor and frame but had been rebuilt/refreshed and I never saw this bike on wheels ever in the 10-15year of knowing this guy, So I got it all togeather in one pce(which was a mission in it's self with only 600 odd made as when I got it I was missing lots of things) and got it running, however it ran but didn't stop. It was diseling at 10,000+Rpm but idled fine until you just touched the gas then it was on for young and old.

So I got told air leak so I went on the search for an air leak with no luck. and here is what I have done thus far.

So I replaced the crank seals the fly wheel side was crappy so I was hoping that was going to be the problem.

So thus far I have
a.) taken barrel off honed new base gasket and laped the head in

b.) Replaced crank seals and gaskets.

c.) Change heat rating on the plug to cooler

d.) Covered the motor with water whilst running(hopping it would either suck or blow the water) with no leaks present on any obious parts

The bike starts first kick everytime, I had up against a brick wall before starting it so I could stall it as I was ready for it this time. Whilst sitting there idiling the engine does pick up rpm as it gets warmer but this takes a few min. So I am know thinking

a.) What size jets should be in the carby

b.) What the timing should be set to and how to set it as there seems to be no marks

c.) What is the correct plug for the bike

When starting it feels like it has a bit of kick back like the timming is advanced to far so was going to try retarding it a fraction and see if any improve.

What are your ideas guys as this really starting to give me the sh#$'s,

As it is an unkown thing, I am willing to try anything lol,

But If it is an air leak it is must be so small(as I can't find anything) and for it suck that much air in the short time I have had it running(I am talking minutes max time it has run would be 3min and that was in the first take the hell razor) to lean out that much sort of seems a bit wierd if it is a small leak as I have a had a bike with a leaking crank seal and it didn't do this and was still ridable(but a bitch to start).

Your imput would be great,

Anyone had a simlar experiance as the guy's that are helping me get it running have 70years bike experince and have never come across this before.

So what is worth ago as I don't belive it is leaking now as I have done the majors apart from spliting the case but I have inspected the gasket and it is all there and looks to be good.

TOm

lms6201

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Re: 2-stroke montesa Problem
« Reply #1 on: September 17, 2008, 05:32:56 AM »
hi tom , timing is done by measuring mm before tdc , best way to do it is a dial indicator on top of the piston , make sure the indicator is vertical or the reading will be inacurate ( head off ) or if anybody close to u can loan u the setup that screws into the plug , bultaco made a vernier calibrated tool for this , i don,t have monty numbers ,an estimate would be 2.7mm btdc ,then put some marks on the rotor , anyway retarding it a bit won,t hurt anything but performance , i have a mk 7 that has a similar problem ,    jared would be the man on this one i think ,  but if u need to check the timing i,m sure someone will lend u the tool , i would but am so far away 

runutz698

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Re: 2-stroke montesa Problem
« Reply #2 on: September 17, 2008, 06:44:01 AM »
Cheers,

So did retarding your timing solve your problem?? or is it still ongoing and what have you done to try and fix it??

Tom

Offline JC

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Re: 2-stroke montesa Problem
« Reply #3 on: September 17, 2008, 10:44:49 AM »
Tom,

This happened to a friends IT200 also, & sounds like its not uncommon w guys on this forum. I know from experience that its frighteneing & frustrating.

I would have been checking for airleeks too, & all the other things you've tried.

Perhaps try float level. If its too hi it could be letting too much fuel in so that once you crack the throttle it 'takes off'.

Or is it something as simple as the slide not returning to its seat (once you've cracked it open) for some reason.

W.r.t what to do to kill it once it happens, somebody else on the forum suggested putting on the choke (if it has one)

Once you do find the cause, pls post it here so we can all learn from it.

Offline Speed

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Re: 2-stroke montesa Problem
« Reply #4 on: September 17, 2008, 11:32:34 AM »
Hi Tom, just my two cents worth, if you have looked at all the obvious things already mentioned make sure the carb is sealed gasket under cap and rubber boot on adjuster and so on.
You may also want to see if the crank is true if it is twisted you will also get and air leak this is what my problem was with a bike I had a couple of years ago, It would start easy but after about 15-30sec. the RPM would start to climb rapidly . Good Luck With It....

runutz698

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Re: 2-stroke montesa Problem
« Reply #5 on: September 17, 2008, 01:07:20 PM »
Thanks speed that is one I haven't thought of yet, How did you test if it was running true??

Cheers
Tom

runutz698

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Re: 2-stroke montesa Problem
« Reply #6 on: September 17, 2008, 03:23:39 PM »
Hey Guys it was the big end getting hot and also causing very high comp under revs, It looked ok when inspected before starting the bike and it was tight and felt ok,
however once running it must of been a broken roller in the cage as the conrod looks to of been heated up with an oxy torch now. While the crank and everything around it was all covered in oil and fuel the big end was dry, I think that it was getting hot causing the piston to rise higher in the bore and in turn increasing the comp and making it disiel.

So we checked and o-ringed the whole carby, replaced the carby conector again. But it still did it. so we got creative and thought the comp may be to high so we spaced and the barrel with 2 base gaskets and it ran fine reved idled reved idled,  truned off how it should. how ever after 5min it started to make and noise and threw the bottom end (not completley). So the problem  looks to of turned out to be a compresion problem as it was strectching the conrod under revs through heat hence it idled fine but as soon as you touched the gas it streched the conrod and then ran under it's own steem. The carb was set to run really ritch to stop it leaning out, so all I can say is wow no one could of guessed that.

So that is what we belive happened as it is the only thing that makes sence, A guy who is is an air craft mech, said in the air world they have to de bur every part to a polished surface once they machine a head or black as the bur's can hold enough heat to keep the motor running aswell which I found intresting.

Offline cappra

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Re: 2-stroke montesa Problem
« Reply #7 on: September 17, 2008, 03:44:23 PM »
Do you think the rod could have called it quits during
that initial time when the motor revved to the moon
before you were able to shut it down?

runutz698

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Re: 2-stroke montesa Problem
« Reply #8 on: September 18, 2008, 06:48:36 AM »
Jared that is possiable but the only thing that doesn't make sence is we striped it down yesterday to put the extra base gaskets in at that point we checked the bottom end and it was tight, The next thing is that even with the extra base gaskets it would run under it's own steem but if you gave it a rev it would clear it's self and turn off using the button if not it would just creeping up in revs from idle and the button did nothing. The next thing as of lastnight I split the motor with the mech after work and the crank looks abit suspect to, There are to marks on the bottom off the weights that look as though they should line up to balance the crank and make it run true so I am going to get it put on V blocks and see if my suspesion is correct and that it is out.

We are still kind of in the dark so we are going the rebuild so we know it is all good checking every part as we put it back togeather.

Can anyone measure the head for me and barrel as I am wanting to make sure somone hasn't shaved to much off each to rule out a compression issue which I highly doubt it but need to check.

So I need the full height of the barrel and also the step in the head!!!

Cheers
TOm