Gnasher328
Senior Member
- Joined
- Aug 9, 2012
- Location
- Chiang Mai / New Forest, England
- Bikes
- Ducati 851 & Mille S2, HD FXR, Trumpet R3, BMW R100RS, CRF 250
Last year my CRF250 developed an annoying rattle under certain conditions. Lots on t'interweb about it, blaming everything from tappets and cam chains to loose fairings but nothing conclusive and lots of scorn from some t**ts who obviously didn't have it, as it seems not to affect all bikes.
This year I started pimping the bike, one of the mods being a Yoshi copy exhaust. Hmm - odd, no rattle, and I was sure it wasn't the extra noise covering it up, especially as the rattle occurs on nearly closed throttle.
I wasn't that impressed with the exhaust so I refitted the now 'gutted' original. Incredible result, nearly as fast as the straight through but overall quicker with heaps more torque than before, now pulling strongly across the range a gear lower for any given speed.
BUT - rattle back.
Due to the exhaust affecting it and the fact it was so throttle sensitive I figured it might be something like induction resonance. While installing a CBR throttle body I got working out what the mass of tubing and valves was up to, and noticed one going from the air box to the cylinder head via a solenoid valve. Turns out it's purge air to the exhaust. Aha -there's the link. Thinking that on a neutral throttle the valve might be 'dithering', I decided to block it off. Couldn't find any suitable blanks in CM so I found a dried up marker pen whose top fitted fairly well for the cylinder head hose, the airbox hose had to make do with a bend and a cable tie.
Went for a ride and I could swear it was better, but still noticeable. Hmm, perhaps it was the valve itself. Disconnected and removed it, no change. I was certain it was better though, almost tolerable so I went back to Google, trying CBR instead of CRF. Out of the dozens of wild guesses I found a couple that made sense on CBR250.net. It turns out there is also a reed valve on the head. Aha. As soon as the bike had cooled I went to have a look. It was fairly easy to get the top off and when I prodded it there was a clicking sound. Right, that's it then, so I pulled the reed asembly and a mesh screen out and decided to bend the reed closed. There is a tiny crosshead screw holding it together, but it was so tight it simply sheared off, they might as well have rivetted it together.
Bugger, now what? I had a pot of superglue handy so I have just glued it all back together closed, and will try it again tomorrow after the glue has had plenty of time to set. All the parts are too big to go through any holes so the worst it will do is rattle again if the glue doesn't hold. It's supposed to be good for @100degC but I will be on the lookout for some JB-Weld or similar. I had been meaning to get some for the emergency toolkit anyway since my 'off' North of Pai bent the rad.
This year I started pimping the bike, one of the mods being a Yoshi copy exhaust. Hmm - odd, no rattle, and I was sure it wasn't the extra noise covering it up, especially as the rattle occurs on nearly closed throttle.
I wasn't that impressed with the exhaust so I refitted the now 'gutted' original. Incredible result, nearly as fast as the straight through but overall quicker with heaps more torque than before, now pulling strongly across the range a gear lower for any given speed.
BUT - rattle back.
Due to the exhaust affecting it and the fact it was so throttle sensitive I figured it might be something like induction resonance. While installing a CBR throttle body I got working out what the mass of tubing and valves was up to, and noticed one going from the air box to the cylinder head via a solenoid valve. Turns out it's purge air to the exhaust. Aha -there's the link. Thinking that on a neutral throttle the valve might be 'dithering', I decided to block it off. Couldn't find any suitable blanks in CM so I found a dried up marker pen whose top fitted fairly well for the cylinder head hose, the airbox hose had to make do with a bend and a cable tie.
Went for a ride and I could swear it was better, but still noticeable. Hmm, perhaps it was the valve itself. Disconnected and removed it, no change. I was certain it was better though, almost tolerable so I went back to Google, trying CBR instead of CRF. Out of the dozens of wild guesses I found a couple that made sense on CBR250.net. It turns out there is also a reed valve on the head. Aha. As soon as the bike had cooled I went to have a look. It was fairly easy to get the top off and when I prodded it there was a clicking sound. Right, that's it then, so I pulled the reed asembly and a mesh screen out and decided to bend the reed closed. There is a tiny crosshead screw holding it together, but it was so tight it simply sheared off, they might as well have rivetted it together.
Bugger, now what? I had a pot of superglue handy so I have just glued it all back together closed, and will try it again tomorrow after the glue has had plenty of time to set. All the parts are too big to go through any holes so the worst it will do is rattle again if the glue doesn't hold. It's supposed to be good for @100degC but I will be on the lookout for some JB-Weld or similar. I had been meaning to get some for the emergency toolkit anyway since my 'off' North of Pai bent the rad.
