Vintage Dirt Bike Q & A

Hah, the piston had under 50 miles on it and it melted the top and made a hole the size of a nickel next to the left transfer ports.

I was riding about 40mph on a dirtroad under steady throttle. it just lost power and died.

Sorry to hear that. My bet is air leak, followed by ignition timing followed by jetting. Did you replace your crankshaft seals and re-build the carb back to stock spec.? (see previous discussion).

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

yup i sure did. i put in brand new gaskets all around. brand new carb rebuild kit with new jets. i set the timing to 6btdc, used a new b6es sparkplug.

idles it for 5 mins to get the gaskets nice and settled in and to warm the engine up, and i checked my leakage which was basically none. retorqued the head too.

i jetted the carb to be about 10 sizes too rich on the main jet. normal on the needle and idle. the reason i did such a rich main is for a margin of safety incase i did somthing wrong during the rebuild.

using 4 oz/gallon for breaking in, i think it was the oil. i went out and checked my oil and i think i used outboard oil instead of aircooled oil. :\

I doubt oil had much if anything to do with it. You would have scoured the cyclinder and maybe seized first if so.

That plug seems rather hot to me. I run a B9 in my race bike (CR250) and a B8 in my trials bike (Sherpa T).

When you say "i set the timing to 6btdc" do you mean six degrees or six millimeters?

Loren

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Those who can make you believe absurdities can make you commit atrocities.

Loren Davis
51y

According to the Champion spark plugs web site you need an N2x plug which is equivalent to a NGK B9xx. Perhaps the NGK B6xx plug was too hot for this application?

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

damn, so it must of been the b6es sparky that did her in. probably preignition made my piston holy then.

when i got the bike it had a b6es plug in it, so i had assumed thats what it called for. it figures the perviuous owner had the wrong plug. lots of other stuff was wrong too.

man thats annoying to have this happen. it was a perfectly good wiseco piston too.

ah well live and learn perhaps.


do you guys think i should rebuild the bottom end now? i mean what if theres chunks of piston in the lower end bearings?

The book calls for a B9 heat range. It also calls for the ignition timing to be at 1.5mm btdc. I run mine at about 2.0mm I've run between 1.7mm and 2.2mm.

_________________
Those who can make you believe absurdities can make you commit atrocities.

Loren Davis
51y

mine was at 6 degrees btdc. i dont know what that is in mm's but i know on the flywheel there are 2 timing marks. theres a T and a F, the F also says 6 btdc so i time it to the F,which i thing means Fire.?

I don't have my calculator handy but 6 degrees sounds about right.

Also, just for further info if you didn't have it, the breaker gap is 0.3mm-0.5mm (0.012-0.020 in.) I run a PVL so I never worry about those things.


Just as somewhat funny aside I had to go out to my bike shop to get the owners manual to look these specs up and discovered that last time in there about two weeks ago I left the fuel valve on my Sherpa T in the reserve position instead of off. You know what happens when you leave the fuel on to a Bultaco? DOH!!!

_________________
Those who can make you believe absurdities can make you commit atrocities.

Loren Davis
51y

You said you replaced the gaskets but did you do the main seals? I think alot of carb jetting problems and seized top ends are caused by old dried out main seals. Guys pick up an old two stokes for cheap or nothin, do a top end that fails soon after. I don't bother doin a top end on a 25 to 30 year old two cycle bike wothout doin the bottom too and replacing both main seals. A lot of jetting and other problems will be solved with new seals. Most bikes I do bottom ends on have main seals that are hard as a rock. MT250s are real hard on the main seals. The MT250 motor I did recently the right main seal came out in two pieces. One half stuck to the crank the rest in the case. The bike ran but smoked somethin bad.

The expense, time and tools involved make doing a bottom-end rebuild a tough call, however having suffered two dramatic failures - one that cost $500 for a replacement crankshaft - I highly recommend that you have this checked out sooner rather than later. Definitely if you have jetting problems or smoke in the exhaust

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF