3mb expansion chamber, cone restrictor removal and re-jetting?..
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I've been digging through the info here and elsewhere on the web but I can't find out - I have a French import 92 DTR with the original 3mb expansion chamber. Bike is currently around 14-15 bhp.
I've read that these chambers of this era often have a cone shaped restrictor on the engine end of the pipe on the sub 15bhp models, and once removed they run up to around 18 bhp.
I take it there's a jetting change needed to compensate, and I'm wondering if there's a ball-park figure to start at for an otherwise standard French 92 model?
Cheers me dears!
Shaun/FloWolF
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@Calum I can't comment on the best thing to do with the DTRe exhaust as I've never owned one.
@FloWolF I think the person you spoke to about this is referring to the washer that is spot welded into the 3MB exhaust header pipe. Some have this, some don't but it will be pretty obvious if it's there as it practically halves the header pipe ID. Besides this I wouldn't go full-on gutting the stock 3MB exhaust as these are quite well-known for not having any restrictions. The DT125R/DT200R styling was based on the 1988 YZ250 and the expansion chamber is about the same physical size. Within that there was enough space for an appropriately-sized, functioning expansion chamber for a 125cc road/trail bike and enough noise suppression material for the bike to pass a noise test (whereas the YZ is for closed-course competition which is why it has a single-skin racing exhaust). If you chop it open with an angle grinder and rip all that stuff out, it will not only sound like a wasp in a tin can but also you’ll probably lose power as you no longer have an expansion chamber which matches the porting/carb/crankcase volume etc. By all means take the washer out of the header pipe but the stock pipe and silencer aren’t restrictive besides that; I’ve owned several full power French import DT125Rs which have all revved to 10k+ rpm with the stock pipe. In fact in one case I bolted on a DEP silencer and the bike immediately refused to rev higher than 9k; this was cured by refitting the stock silencer. I reasoned that because the OEM pipe and silencer are designed to work together, changing the silencer to a less restrictive type reduced back pressure and compromised the efficiency of the expansion chamber.
Stock jetting for that era should be either a 210 or 240 main, don't have the others' info to hand ATM but it's in the Haynes manual or you could try Yamaha France parts lookup which sometimes allows you to enter your VIN number and look up parts for your exact bike. Sometimes a bit long-winded as you then have to search for the part number elsewhere like Fowlers etc. for the jet sizes but the information is there:
https://www.yamaha-motor.eu/gb/en/service-support/parts-catalogue/
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My 2005 DTRE had the honeycombe internals in the expansion chamber, can't see that being any good for performance and it was gutted without my permission.
Either way, fitted a DEP later on and that was night and day, can't remember how I felt about a gutted stock pipe, can't have been that noticeable. For me, the biggest factor in these things is weight. DEP is significantly lighter than stock.
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@Calum I wonder if this honeycomb could have been a catalytic convertor? As I say I hardly know anything about the DTRe but when Yamaha UK officially released the TDR125 in about '93 they said it had one, and underneath all that TDM850-style bodywork the pipe and silencer were pretty much the same shape as a DTR.
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