Sherco Bultaco Graham Jarvis Replica Picton Knock

[cantgetaname]

Hi there i have a Sherco bultaco Graham Jarvis Replica 2000 modle 250 cc when i open the throttle half way and then let it close there seems to be a knock that comes from the piston/cylnder area. The fly wheele has a little play in it i think its my main bearings it only does this knock when it is warm.its driving me insain. What could it be? every opinion counts.

Edited May 11, 2008 by cantgetaname

[dabomb]

Hi there i have a Sherco bultaco Graham Jarvis Replica 2000 modle 250 cc when i open the throttle half way and then let it close there seems to be a knock that comes from the piston/cylnder area. The fly wheele has a little play in it i think its my main bearings it only does this knock when it is warm.its driving me insain. What could it be? every opinion counts.

make sure you have the correct fuel,not normal unleaded,super unleaded is what you should try.Higher octane resists pre-ignition,which is what you may have.(98 octane i think)

Edited May 11, 2008 by dabomb

[copemech]

You could be a bit lean on the jetting as well, as they go lean when you chop the throttle!!!

[cantgetaname]

Hi there again im using normal unleaded fule at 100:1 to 1 gallon exactly of fuel. This is my first trial bike how do i fix the jetting as you suggested? i have the little screw out at 3 notches would main bearings have an affect on this?

Thanks very much for all your help Jorge.

Edited May 13, 2008 by cantgetaname

[sherpa]

I also have an ssdt replica, and had the identical problem. I finally solved it by burning out the head pipe and center section of the exhaust. I would also recommend repacking the muffler while your at it. Your bike will run smooth like a new machine.

[wayne thais]

With that old of a bike, you could have leaking main seals, bad main bearings, some piston slap and worn rings ec. Surely there is someone that you trust that you can have a look at it. It is extremely difficult to diagnose a problem of this magnitude from long distance.

Wayne

[copemech]

Just a couple of thoughts,

Be sure you are using the 98 RON gas, and use a good full or semi-synthic oil at 80:1

Clean your carb and find out what jets you have, I think a 33/120 combo would be a good start, needle in 3rd groove up.

One general way to find out just how gunked up the exhaust is, is to do bout a half dozen "drag" races at WOT through the gears on some old dirt road. This should burn off some gunk and get some heat in the pipe! Usually smoke like hell! You can re pack the muff on that one too if need be!

Mind you, if it is tits up to begin with, you could blow it up, yet I doubt it.

Little to loose by trying above, Cheers,

[sherpa]

I was certain mine needed a piston, yes it was that bad. I cleaned the carb, checked jetting, timing, etc. The only thing I hadnt done was actually disassemble the top end for inspection. I figured I was going to upgrade the top end to 2.9 anyway so I would make due. The exhaust was a fluke. I only it because the center section felt a little heavy. A propane torch is all it took for about 30 minutes. alot of smoke! When I put it back together, I just could not believe the difference. I guess the 2.9 top end will wait now.

I've been around bikes for a long time, and I've done my share of overhauls. This was a surprise to me.

[cantgetaname]

With that old of a bike, you could have leaking main seals, bad main bearings, some piston slap and worn rings ec. Surely there is someone that you trust that you can have a look at it. It is extremely difficult to diagnose a problem of this magnitude from long distance.

Wayne

Thanks wayne speaking of which i had to replace the crank case seal beacuse it was smoking real bad i think i need to replace my main bairings could all this work come to more than

[cantgetaname]

I also have an ssdt replica, and had the identical problem. I finally solved it by burning out the head pipe and center section of the exhaust. I would also recommend repacking the muffler while your at it. Your bike will run smooth like a new machine.

Hi there my front /manifold pipe has some dents in it i repacked my muffer a few days ago and have rode it once or twice i have never touched the center part of the exhaust scince i bought it in november

[dabomb]

Jorge,if you havnt tried the 98 octane yet,this is the easiest option to try.Give it a go,let us know if it helps.From experience,wrong octane does make the engine sound bad,especially if the bore/piston are in good nick.If it doesnt help,you can try all thr other suggestion from the other guys.

Edited May 16, 2008 by dabomb

[cantgetaname]

Jorge,if you havnt tried the 98 octane yet,this is the easiest option to try.Give it a go,let us know if it helps.From experience,wrong octane does make the engine sound bad,especially if the bore/piston are in good nick.If it doesnt help,you can try all thr other suggestion from the other guys.

Hi there again tryed the SUPER UNLEADED petrol but no diffrence at all even tryed the jetting settings but nothing changed still the same old knock took the top of the head off had a look there is no play on piston wrist pin just as it should be the bore is absolutley spotless i think i need a main bearing re-build.

[clav3]

I had a 2000 290 and now have a 2008 290 and they both do exactly the same. The only way to completely get rid of the knock is to run the needle really rich. On my 08 I just dont worry about it. But you do have to make sure that it doesnt do it excessively. Try dropping the needle to its lowest setting, that is clip in highest position. If the engine knocks excessively and it is the same as the sound you are describing, then the bike is not getting quite enough fuel. On my 2000 I ran a D37 needle raised all the way up. The bike never pinged but had so much power slower sections needed alot of clutch control. There is no way my 08 will accept as much fuel as the 2000. Before mucking about with needles and jetting make sure the carb is spotless. You need to get your midrange (needle) right first and then adjust pilot and main jet. I have been where you are but dont worry about a slight knock when the throttle shuts off quickly.

Hope this has helped.

Mark.

[29ty]

One thing you could try is ...............

If the pilot jet is too small i.e. weak, when it is warmed up try it with a little, not all, choke on and see if you can get the knock to disappear. If so your jetting is wrong but so are alot of bikes. Go to a bigger number on the pilot jet for shut throttle mixture and the main jet for wide open mixture.

Those are the basics to get you started.

[copemech]

A bike always tends to go lean on throttle chop, unless the pilot jet is on the rich side! As Clav stated, somewhat normal. Not sure what jets he is running, yet a lean jetting will add to this, yet yield smoother power on roll on.

Finding a balance can be important on a 2.9 to smooth them, I am running a #36 which I can handle on the newer bikes. On the older bikes I ran a #33.

I understand many like the #38 on the newre bikes, yet I would expect it to be more aggressive! The needle comes into play somewhere above 1/8 throttle transition, and has no effect when throttle is shut, as all pulls off the idle and transition circuits! Which must be clean!