Hi All

Hope I can pick some brains on here. I've just finished rebuilding an '83 MH349 and am now in the process of "shaking it down" in preparation for my glittering career in twinshock trials (maybe).

I come from a motocross / enduro background and have no experience of trials bikes, but one thing I've noticed is the bike is really noisy. I don't have access to a DB meter, but I'd reckon it's as noisy as a loud 2 stroke motocrosser.

It's got a brand new WES tail pipe from InMotion on it, mated to the original front pipe. It's not leaking anywhere and the WES silencer has got packing in it. Bizarrely, the noise seems to be coming from the front "expansion box" rather than the tail pipe. The noise is a typical "ring-ding" 2 stroke noise, which is audible from the front expansion box even when I put a rag over the tail pipe. When I take the rag off, the noise from the tail pipe is a deeper noise, but the "ring-ding" from the expansion box remains.

I've tried wrapping the entire pipe and expansion box in heat-proof lagging but it hasn't made much difference.

My questions is.....is this normal for these bikes with the WES silencer?

Are there any tricks of the trade to quieten them down?

My bleeding eardrums thanks you in advance for your help!

Gavin

 

NOTE - I've also posted this in the Montesa forum.

You probably need to repack the box.

 

Thanks a lot lineaway.

 

I'd assumed the front box was an expansion chamber, but the link you sent to the other post seems to suggest it's actually a silencer in its own right. Have you got any experience of doing this? Am I best to cut an access hole in it? Or try and grind the seams off and split it into two halves?

 

Thanks again

 Do a search on the Fantic or Bultaco forum for more pics. The Montesa forum does not have as much info. I have not done it, or yet. My old 348 is fairly quiet, but she is very plugged with oil.

Cool. Thanks a lot mate

You can access the inside of the middle box by cutting a plate from one side using a small cutting wheel. Then clean and repack it around the perforated tube, araldite some rivet nuts on the inner surface by the edge of the hole you've cut and make a new plate from some sheet alloy to fix in place with short bolts. Seal with heatproof silicone.

If the box has been cut, then re-packed in the past,as outlined above, it could be that the new cover plate is loose, or not sealed properly.

 

Thanks for the replies guys - really helpful.

cleanorbust - that's an interesting idea about using Araldite. I was initially thinking pop rivets or MIG weld (with a steel cover plate)

Will Araldite withstand the heat?

st pauls - it hasn't been cut before sadly so I'm doing it from scratch.

The front box has a recess on the outside where it fits behind the frame rail, so I'd only be able to cut a smaller section next to the recess. Because of this, I was going to cut out the inside (next to the airbox) so I can remove a larger section to gain access to as much of the internals as possible so I can sand-blast it clean. This presents a further challenge though, because there's only about 1/4 of an inch clearance between the front box and the airbox, so not much space for bolts to hold the cover plate in place....

 

Thanks again

I did this back in the Seventies. I cut a rectangular hole in the back of the box then welded a plate on after packing.  If you can make a removable plate then so much the better as repacking made a big difference to the way the bike ran, being smoother at the bottom end.  

WES type silencers are not very quiet anyway but I feel that these old bikes run better (and certainly sound better) with the original silencer - if you can get a Montesa tail silencer then you can repack as above.

Edited March 1, 20179 yr by 2stroke4stroke

Thanks 2s4s. Do you recall whether you cut the whole side of the box out, or just a small access "window"?

I did try to get the original silencer, but InMotion didn't have any in stock and wouldn't get any until end of April so I went for the WES so I could get the bike running and shaken down. I realise it may be a bit noisier, but I want to get the bike as quiet as possible to protect where I ride (and my eardrums).

I think I maybe went just on to the edge of the curve, so could use a flat plate.  I probably reused what was cut out - I had to stitch drill holes together as Dremels were just science fiction here then.  You can poke the filling in to the ends of the box from that window.  Now I think about it I perhaps only cut three sides of the window so that I could just fold it back.

A pal has a 240 Fantic which had an alloy tail silencer and it really was like a motocrosser, even after repacking the tail silencer. Our complaints eventually got the better of him and he fitted a standard item and now it is fine but the Fantic has an element of baffling in it unlike the straight through Montesa one.

For its day the Montesa was quite quiet so you should be able to obtain a satisfactory effect.

Let us know how it goes.

Edited March 1, 20179 yr by 2stroke4stroke

1 hour ago, montesamadness said:

Thanks for the replies guys - really helpful.

cleanorbust - that's an interesting idea about using Araldite. I was initially thinking pop rivets or MIG weld (with a steel cover plate)

Will Araldite withstand the heat?

st pauls - it hasn't been cut before sadly so I'm doing it from scratch.

The front box has a recess on the outside where it fits behind the frame rail, so I'd only be able to cut a smaller section next to the recess. Because of this, I was going to cut out the inside (next to the airbox) so I can remove a larger section to gain access to as much of the internals as possible so I can sand-blast it clean. This presents a further challenge though, because there's only about 1/4 of an inch clearance between the front box and the airbox, so not much space for bolts to hold the cover plate in place....

 

Thanks again

Yes, you should find that araldite withstands the heat OK, and there should be enough clearance on the inside for the bolt heads (you could always grind the heads a bit thinner). You can use a fairly small widow to leave enough flat surface to fix the rivet nuts and still get access to the innards using some sort of tool to poke the packing into place.

Edited March 1, 20179 yr by cleanorbust

If you are in the UK you could try allensusedtrialsspares.co.uk for an original back box. As has been noted, these old Spanish bikes do seem to run better on their original exhaust systems.

Not done this to bultaco's, but see no reason it wouldn't work. On Rotax first box, cut window, leaving enough meat to fit plate over whole area, fit 4 small rivnuts, repack, seal edges with high temperature silicon. Allow silicon to cure before use.

 

Edited March 1, 20179 yr by b40rt