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Wednesday, April 25, 2012

2 steps forward, 1 step back

That seems to be the way with this project. Seamus came over to do his magic with the chain tensioner, and we got the cam timing figured out. (I hadn't accounted for chain stretch when I was lining up the timing marks.) He also helped with a few other pieces of the puzzle that I was scratching my head about, so I finally got the engine buttoned up and exhaust mounted. I installed a fresh spark plug, topped off the oil, and gave it a few kicks. Nothing. I sprayed some ether and tried it again, nothing. I checked for spark at the plug and it was weak. My KTM was in the garage too, and it needed my attention because we had a Thursday Night Trail ride coming up, so I pushed the XR250 back in the garage and gave it a rest.

My next shot at the XR250 yielded no spark at all. I convinced Seamus to stop by and help me out again, since he's much better with a multi-meter than I am, and together we tested the coil and CDI. Nothing wrong with either, but he did find that the kill switch was hooked up incorrectly. Spark was found. A push start later, combustion ensued. In fact it ran really well with a decent amount of torque off idle and smooth power, though I didn't push it too high in rpm. Clutch and tranny felt good too.

The next day I adjusted the idle air screw a bit and kick started the bike and ran it up to temperature. It runs quite well, but still has a hard time at idle so I might have to change the pilot jet.

But before I can get into that I have to replace the clutch cover gasket. It was leaking at a pretty steady drip, more than I could ignore, so now it's back into that job. When I replaced the cover the first time I'd missed a thrust washer hiding in the old cover, so I had to take it apart again and I don't think the sealant bonded very well the second time.



I've got a new seat cover coming, and new fork seals and bushings as well as new steering head bearings. We'll see how it goes after that, but I'm cautiously optimistic that this will have been worth it.

Tuesday, April 17, 2012

Boxshop Speedway

The 2012 racing season has started. This past Sunday I took the '95 YZ125 to Boxshop Speedway in Maine for the first NEMM supermoto event of the year. Aaron Phinney owns and runs the track, which is the home of NEMM, which used to stand for New England Mini Moto, but now it stands for something else since he added big bike classes.

To prepare for the event I put the supermoto wheels back on the bike, replaced the tether kill switch, changed the trans oil, drained the anti-freeze and added Water Wetter, and safety wired a few bolts. I also tried a jetting change, which I promptly reversed after running some practice laps.

I had been a bit concerned about how competitive the YZ would be against modern 250 and open class 4 strokes, and also about how difficult it may be to ride on that track. I needed both practice sessions to get comfortable with the flow of the track and to pick out lines and braking points, and after practice I still had no reference to how I'd stack up against the other bikes.

As luck would have it, I was lined up on pole position for the Open Am heat race, with Rob next to me on a KTM 250 and a few more 4 strokes behind us as well as Chris on a YZ250 2 stroke. The YZ125 is a tough bike to get off the line smoothly and quickly, but I nailed the start and had the holeshot, and therefore the best line into turn 2 which is a 90 degree left. Then comes a short straight with a left/right kink that you can accelerate through to bring you to a sharp left hand 180 onto the dirt section. This was where I figured the big bikes would eat me up, but I got through the dirt section cleanly and back onto the paved track still in the lead. The exit from the dirt is another left hand 180 onto a short straight followed by a very tight 100 degree right downhill turn, another short straight into a left 180 back onto the front straight and start/finish line. The front straight was where I figured the big bikes might have another shot at me, but the YZ wasn't giving up anything on acceleration, so I was able to hang on to the lead for the rest of the lap and most of the race.

Then I ran into transmission trouble. The bike lost the top three gears as I was starting the 7th lap of the 8 lap heat. I could only shift up to 3rd on the front straight, where I had been shifting up to 5th before. I was still able to hold Rob off for a lap and a half, but on the last lap I got into the dirt section a little fast and must've grabbed too much front brake, the front tucked and I went down. By that time I was pretty much out of energy so it took too long for me to restart, all the bikes got around me and I brought it home last.

I let the bike cool down and discovered that the trans oil was low, so I added some and hoped that would restore some gears. No such luck. I wound up racing the features just using second and third gear, to get some track time and help fill out the grid.

So it was a dissapointing day mechanically, but a rewarding day as well, since I discovered that the '95 YZ125 could still hold it's own against the big bikes on a small track.

Bill showed up to help with flags, and was able to snap a few shots of me in practice. Thanks Bill!

This is the short straight after the dirt section, the KTM flying through the background is about to clear a double, right after the berm in the dirt section.

The sharp right hand downhill turn feeding into a short straight before the hairpin left onto the front straight.

This is the hairpin left onto the front straight.

Another line through the downhill right...

Video;

http://www.youtube.com/watch?v=KpQ6udsiisM&feature=relmfu

http://www.youtube.com/watch?v=ZYbsobyqEho

Monday, April 9, 2012

Damn Cam Chain...

I haven't made the progress I'd hoped for on the XR250R. The next step is to install the cam and cam sprocket. Simple enough in most cases but I couldn't get the timing chain loose enough to allow the sprocket to sit on the cam properly. I messed with it until I ran out of ideas and finally called in some help from Seamus, since he is familiar with these engines. The problem I was having was that I couldn't release the pressure of the chain tensioner to gain the slack that I needed. Of course Seamus figured out the problem within about a minute, and together we mounted the sprocket, checked the timing and called it a day.

The next step reiterated that I double check the cam timing, so I did and discovered that we'd mounted the sprocket 1 tooth off of the correct setting. I took it back apart and tried to reproduce what he'd done with the tensioner to allow some slack in the chain, but I couldn't get it to do what he'd been able to. So I'm back to where I was.

Here's the cam in the head.

This is the top half of the head containing the valve rockers and finger followers.

A closer view of the rockers and finger followers. I guess the finger followers reduce the mechanical force required on the rocker shafts because of the steep valve angle.

Any questions?

Wednesday, April 4, 2012

RFVC

Stands for Radial Four Valve Combustion (Chamber) if the internets can be believed. It's a Honda engine designation. In current hip business trend lingo it would have to be RFVC2 or (RFV-C squared), for the extra C. In any case, it's a "Hemi".
I'm making slow progress on the reassembly of the engine. Hope to make big strides tomorrow because I think it will be too cold to get anybody to play golf with me.

The new Wiseco piston bumps the cubes to 263, with a 10.5 to 1 compression ratio.

The cylinder head is a two piece design, here's the bottom half, with the timing chain adangle.

I realize too late that I should have taken a picture of the hemispherical combustion chamber before I torqued the head down. So you'll have to take my word for it that it is in fact a "Hemi".