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Two Stroke Trials Exhaust Question


mr neutron
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Hey, All!!!

Have a weird question here. Let me preface it with this: My 2010 Gas Gas 280 Pro was being ridden by a newbie a few afternoons ago (not me; a true "Trials Bike Newbie), and got crashed. It was a loopout from doing a wheelie that went terribly bad. Fortunately, the rider wasn't hurt in the least. He has a strong "self preservation gene" , and just basically threw the bike away.Myself, I'd have sacrificed my body somewhat to minimize damage to the bike, but that's probably because I have dominant stupidity gene......

GasGasCarnage2_zps1d8a990d.png

GasGasCarnage3_zps38800eb6.png

Anyway, my bike's airbox, rear fender, and silencer/midbox portion of the exhaust are all trashed. This is turning out to be an expensive crash to repair. In an effort to possibly save a little bit of money on the exhaust, I've been wondering.....

How badly, or possibly how well, would a trials motorcycle run without the midbox? In other words, if a person simply ran some tubing from the header pipe to a universal type silencer/spark arrestor, does anyone know first hand what it does to performance? How about noise? If it is noticeable performance wise, would that be a terrible thing for someone riding at the Novice level?

Thanks,

Jimmie

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The "mid box " in most two stroke trials bikes is the expansion chamber portion of the exhaust ... and two strokes depend on that for exhaust scavenging I believe , where as a four stroke single cyl. doesn't depend on scavenging as much to control exhaust pressure . I don't think it would run worth a sh?t as you plan to mod it .... But I'm not a exhaust engineer either ... But if a straight pipe would work , it would've been done long ago with success !?!?!

Glenn

(sorry about the crash & damage ! )

Edited by axulsuv
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I would do yourself a favor and just fix it properly as quick as possible to save you and your buddy friendship hassles in the future

Next year you will chuckle about it rather than you feeling burned

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Yeah, i agree with you 100%, Glenn. I'm no "Exhaust specialist', nor am I an engineer of any sort. But everything I've ever read about 2-stroke exhaust theory dictates the need for the "expansion chamber" portion for scavenging, as well as keeping the incoming charge of fresh fuel from flowing out of the exhaust port. There's probably other key points as well......

Sigh. I guess I needed to hear someone tell me this is a stupid idea. I know that the the stock exhaust would be much better than anything I can possibly engineer. I've been told by a dealer here in Oregon that the cost of a new midbox/silencer portion of this Gas Gas model is very pricey. The airbox is around $480, the fender is $168, and there's still some miscellaneous items like a throttle housing (how in world do you break a throttle housing??? I've been racing since 1970, have broken throttle TUBES, but can't recall ever breakng a housing.....). This dealer told me that I would have almost 1/5th the cost of a new bike into these few parts......

Basically, repairing this crash means I'll be off the bike for several months. Worse things could happen to a person, in the Grand Scheme of Things. Right now, I simply cannot afford to just go out & plop down the necessary coin to fix the bike. I was just being hopeful (and stupid) that I could possibly cheapen the cost, in order to get back to riding. I do have a line on a used airbox with a crack already in it, which will possibly help keep my costs down a bit, if it can be repaired. Maybe I can then afford the new exhaust after all..... :wacko:

Does anyone possibly know of any aftermarket exhausts for these bikes, like, say, from Jitsie, S3, or whomever?

Jimmie

Edited by mr neutron
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Wow.... I just re-read my above post, and realized what horriible Whiner I am. :crying: Sorry about that, Fellas! I seriously apologize for that......

Thanks for the replies, 0007 & Lineaway! I wish I was set up for welding aluminum; I'd probably try to do something with the bent up stuff I have ....... I think I'll just Man Up, and save the cubic dollars for a new exhaust. The old one was full of dirt anyway, hee hee...... :hyper:

Jimmie

Edited by mr neutron
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You do not need a fender, you just ripped the top off the air box. The air box should still be functional. I would remove the pieces off the fender and epoxy it parts back together All it is holding is the fender. This would just be a temp fix, but I`ve seen temp fixes last forever! And if you have never broke a throttle housing, you not trying hard enough. Ha! Ha!, sorry I can not tell you how many times this has happened to me through the years!

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Bummer about the crash. I bet the newbie feels bad. I think you can fix the bike for under $100 if you don't care about it not looking "showroom fresh." Here's a couple thoughts.

I have repaired many plastic bike parts with Shoe Goo. That stuff is amazing. Clean both sides of the plastic, let it dry thoroughly then put about a 1 1/2" wide coating of Shoe Goo on both sides. Let it dry for the 24 hours to cure. I have repaired fenders that way and in a subsequent crash, the fender broke along side the Shoe Goo!

A good welder can refit and repair that pipe. Drill out the rivets on the end cap, remove the pipe and packing and get the housing rewelded. Then reassemble, with new rivets (I prefer screws). Probably be real quiet with the dirt in there, but may affect your acceleration. :hyper: You do need the mid box as others have said for proper scavenging. If there are significant changes to the repaired bike it might need jetting corrected.

If the throttle housing is a Domino, look on eBay. I see complete sets with housing, tube, wheel and cover in the $20 range.

Get back on that bike!

Edited by thats_a_five
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The earlier gassers ran separate midboxes & mufflers, look at having a tab welded onto the muffler & a nut on the midbox to tie them together along with some form of mounting brace for further up the muffler if it's possible.

Everything looks fixable without needing to destroy the bank account

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Thanks, Fellas!!!

Seems like reading what you folks have written here, and taking a moment to back away & think about it really helps!

You're right; I believe the airbox is repairable. I actually already have a repair on it that has held up very well, with some "Black Gorilla Snot" (black 3M Weatherstripping Adhesive), and then stitched together with zip-ties, Frankenstein style.... I should be able to do that again. And thats_a_five, I've used that Shoo Goo Marine sealant stuff for a variety of things. It does indeed work very well. Should work well in this situation., That, and some more zip-ties....... :hyper:

I do have a line on a used 2009 Raga airbox. It's got a crack in it, and the owner already has a new airbox for it. He doesn't want to sell the new airbox, but has said I can have the old one, after he gets around to removing it and installing the new one. Until he got his new Racing model last year, he's been riding with that old box with the crack in it......

The fender, well, I'll just deal with & live with it..... One of the holes where the factory push fastener went through is ripped, but I think I can still figure out a way around that....

On the exhaust, I'll try to get a photo of only that & post it. But it's bent & squashed more than I feel I can take care of. I'm not certain the average welder could work a miracle at straightening this mid box/silencer combo. Where the midbox ends and the silencer begins is where it's wrinkled up pretty badly. The mount tab (for the midbox) that attaches to the upper shock mount is broken, and the lower one at the rear master cylinder is stretched pretty good. There is a pipe repair place in the very state I live in (Pacific Pipe Repair) that has done some miracles with big off road bike pipes that have been squashed. ( www.piperepair.com ) I'll call them Monday (tomorrow) and see if they possibly are up for a challenge, hee hee.... I'm fairly certain they don't work on this type of exhaust, however, but it's worth a try......

Anyway, I guess I pushed the panic button first, thought the worst, and then took the time to actually think about it later. My apologies ot all again for that. Thanks to each of you for your help, solutions, and words of encouragement. You guys truly do Rock, and I appreciate you all taking the time to reply here. I think it'll work out where I might be riding again shortly.... :thumbup::banana::bouncy:

Thats_a_five, I'm surprised you're not at the 2 day event at Lolo Pass? I'ts a PNTA/COTA event, I believe. That's a really great place to ride, but I do understand it's a bit of a drive for you. It's the one trial on our schedule that's practically "in my back yard", and I badly wanted to ride it this weekend, as well as the 1 day event next weekend.....

Oh yeah, on the throttle..... Stu at Jack's Cycles informed me about a throttle housing that is a direct replacement for the Domino throttle I have on my bike. But it has a built in screw adjuster that keeps the throttle from opening up, or to allow it to open to whatever degree you wish to set it at. Kind of like those throttle slide stops on kid's bikes that don't allow them to open up to full throttle. Except on this deal, the "limiter" keeps the throttle tube from opening up to whatever you have it set for. I've got one of those ordered now, but it's a few days too late, I think...... :wall: I would still let anyone interested in riding a trials bike ride my bike; I just think I'll have this as a possible insurance policy......

Jimmie

Edited by mr neutron
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Over here when someone wants to borrow your bike we say "you break it you bought it!" think I would have slapped them with a bill to put it back to how it was or just have them but the bike. Moral of the story "never a borrower or a lender be". Good luck with the repair / bodge :thumbup:

Edited by old trials fanatic
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Hey mr N, sorry about the prang.

I cant offer much help here but I had to repair a crack in my fender and I used a hot glue gun. Provided its not too close to the pipe or any hot bits, hot glue sticks pretty good.

As for sharing my bike or lending it out, I just cant do it....I would rather share a tooth brush with a stranger. It might be selfish of me but its just something I cant share. If I break it myself, its only me to blame.

Hope you come right and get back on the pegs soon, I feel your pain :(

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Thanks, Guys!

Well, I guess I have a "different" attitude about loaning a bike out. I've been the beneficiary of having a bike loaned to me, and truly appreciated it. It helped convince me that I wanted to stop off road riding & mx, and go with Trials. I'm thinking if someone else rides my bike, and decides they'd like to go Trials riding, buy a bike, and all the gear, then I've helped the sport somewhat..... Plus, it's only material stuff; not that important in the Grand Scheme of Things...... But I do definitely understand what y'all are saying here......

Here's the pics of the bent mid box. It's got some wrinkles in it that, to me, look fatal. It looks like it would be hard to bend it back to anything close to the original shape that clears the master cylinder, frame, airbox, tire, & etc..

GasGasCarnage5_zps433f59f0.png

GasGasCarnage6_zpsbe3ac775.png

I've got the airbox cleaned up, and pretty much ready for glue/sealer & zip-ties. May possibly get that done tomorrow, I hope. Maybe I'll take some pics of that after it's done, but it won't have any graphics on it, however.....

Jimmie

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Mr Neutron,

If you feel comfortable loaning out your bike, keep doing it. There's a couple guys that I ride with that are pretty free with their bikes, I have loaned a bike and parts of my bike in the past, but I am a little "picky" on who I loan to.

But it would have been nice if the guy that borrowed yours would have at least offered to toss a bit of $$$ towards the repair.

on the exhaust. The mid box does look a bit crinkled, but maybe (big maybe) it could be bodged out a bit to at least be useable until you save up/decide to buy a new one.

The top mounting bracket being broke I am sure could be wiggled into a good shape and welded up. The mounting tab at the rear brake master could probably have something done to make it usable.

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Thanks, Zippy!!!

Yeah, it's the first time I've loaned it out that it came back in worse shape than it was before a newbie ride..... I realize that I take a risk when loaning it out, assuming that a crash is always a possibility. I'd just assumed it would likely be a broken rear fender, or bent lever to worry about, or something along those lines. These bikes are amazingly tough, and I'm still kinda surprised at how bent & broke stuff got......

I guess I'll get my chance to help the sport out by buying some high-dollar parts, huh? :hyper:

Jimmie

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