Vintage Dirt Bike Q & A

Hey everybody. I am new to the forum. I just picked up a really clean '74 Honda Elsinore MT250 from a friend of mine who was in over his head. Turns out I may be in over my head too.

The bike had recently been treated to new crank seals, and a fresh top end. The previous owner didn't want to re-bore the cylinder, so he ordered a CR piston in the correct size and used that. The cutaway in the CR piston is all that was different from the MT, so it was matched to the MT profile. When the bike was put back together, the acceleration was sluggish, and the bike would not rev past 6,000 rpm. That's when he sold it to me.

Since I have had the bike, I have covered all the bases. I've changed the plugs, cleaned out the carb and the pipe, played with the carburetor mixture settings and needle position. The only thing that has made any change has been dropping the needle, but it has only made the bike "slightly" less sluggish, and it still won't rev past 6,000 rpm. I have attempted to adjust the timing, but I don't have a timing light; I've just played with the point gap, adjusting it a couple thousands at a time. I can make it run worse, but not better. I replaced the coil with a new one, then I replaced it again with a known good one...no change.

What am I missing? It's like the bike has a rev limiter on it. It's not soggy like a rich condition, it just will not rev.

Thanks for any help!

Casey

I'm not sure why an MT piston would require reboring, but a CR piston would not. Did the previous owner give you the impression he knew what he was doing?


A CR250 piston that would work in the MT250 motor should not have required any 'matching'. Hopefully it's not a windowed reed-valve piston or had anything cut off the bottom of the intake-side skirt.

I'd strongly suggest a compression check (meter, not foot :>). I checked my friend's MT recently, it was 160psi (original bore/piston, 3,300 miles).

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

A CR and MT piston have a different cutaway on the side of the skirt. They didn't go to the windows and reed valves until later (75-76?). Most pistons listed as being for MT's are just CR pistons, but if you find an NOS one, you will see the difference in the cutaway I am referring to.

And he couldn't find an MT piston in his current bore size (his old piston had some pretty bad erosion), but he was able to find a CR piston in that size. Everybody told him it would work, but the cutaway on the side of the skirt IS very different.

Hope that helps. If anybody has a decent shape .025 over MT piston, I would be interested in it.

Thanks,
Casey

Casey,

Here's some eBay NOS .25 OS pistons, MT and CR - looks like he's got two of each:

http://www.ebay.com/itm/NOS-HONDA-FL250 ... 0474418365

http://www.ebay.com/itm/NOS-73-74-HONDA ... 91&vxp=mtr

The auction photo angles are weird, but assuming the cutaways are quite different, my point was that in a mildly-tuned motor like the MT250 with tiny carb and restrictive muffler-type exhaust there would be no real benefit in matching piston cutaways to the cylinder liner, so it wouldn't be necessary at all (and could possibly weaken the piston if done wrong). I have CR and MT pistons here with slightly different cutouts (one end is angled on the CR unit, the cutout width is the same) but both are aftermarket from different makers so that may not apply to OEM parts.

There's plenty of on-point MT250 info here at VDB, so be sure to search around if you haven't already.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Hey gang,

I'm still banging my head against the wall on this MT. I've decided to move away from the piston, as the consensus seems to be that it wouldn't cause my problem.

Everything that I have read about the bike failing to rev was pointing to too large of a main jet, so, despite the fact that the jetting was completely stock, I dropped the main down to a 115 (from 122). Instantly the bike was more responsive, and would rev to about 7,000 rpm, so knowing I was on the right track, I dropped further to a 110, and now the bike feels quick, and revs out to 8,000, but it's still not as fast as my buddies bone-stock, tired motored MT.

My question is, why does the bike want to be so lean? To the best of my knowledge, there isn't a power valve or anything that could be leaking and introducing extra fuel. I've boiled out the exhaust pipe, and have a clean air filter, so there isn't any excess restriction in those systems. A 12 jet size drop is pretty extreme, and the bike still isn't 100% right, though it's worlds closer than it was.

Thanks for any help...
Casey

Casey,


It really doesn't - it just doesn't want to be super rich, as apparently it was (a mixture rich enough to significantly reduce power doesn't always create blubbering / smoking / etc.) The question is, why is it so rich that you could drop five main jet sizes (120, 118, 115, 112, 110) and not totally starve the motor at WOT?

My vote is for a worn needle & needle jet - every one of these carbs I've encountered had that problem, to varying degrees - but it could also be other things as well (float height, float valve, leaks, etc.). A good visual inspection of the needle and needle jet orifice (with the main jet out, obviously) would be a good start. New MT250 needle/needle jet sets (16012-358-004 / -014 / -024 etc.) are very lean and fit together closely, with no ovality and little clearance. It's also possible a leaking choke circuit could be the cause. If it were my bike I'd also run it briefly without the air boot to the airbox to eliminate everything upstream of the carb (on the MT that's a lot of stuff) as suspects. Once the cause of the richness has been eliminated, I'd go back up to at least a 120 main jet.

It's great the problem's been isolated to a rich carb mixture, one of the easier problems to deal with.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray,

Thanks for the reply. I checked the needle jet and it turns out that it wasn't completely seated against the carburetor body. The little locking tab slipped, and it backed off a little bit. The needle and jet are sort of sloppy, so I'm of the mindset to replace it, but I just threw the needle jet back in, along with the stock 122 main jet, made sure everything was snug, and took it around the block a few times. It's running better than it was originally, but it's still not as quick as it was with the 110 main. I'm assuming that the wear in the needle jet is accounting for at least part of the problem I'm experiencing. My question is, my stock needle is stamped 271004...which of the above part numbers that you listed should I order? I am assuming that the 004-014-024, etc corresponds to the needle taper. Is "004" stock?

Thanks,
Casey

Casey,

That's the second time I've heard of a leak around the main jet / needle jet area. The last time I replaced the needle jet on my MR175, I used some Gasoila gas-proof sealant just to be safe.

For now, you could try dropping the needle all the way, which should help a bit in the 1/2 - 3/4 throttle range, and maybe put the 115 main back in as a temporary compromise. We haven't mentioned the pilot jet or air screw; if the needle jet is worn neither one will have much effect when changed (if that's the case, I'd run your air screw 2.5 turns out). If your carb uses the shorter pilot jet offset from the carb centerline, you'll be better off modifying your float bowl slightly to use the longer N424-24 pilot jets as the shorter ones are practically nonexistent. I'd also check your air filter etc.; my MR gets slowly richer as the filter gets dirty, which I really don't notice much until I clean the filter, and then wow!

My notes indicate the 004A MT250 carbs came with 271002, 271003, or 271004 needles, which may correspond with the needle/jet kit part numbers as you mentioned. I don't have needle measurements, but in these carbs I've seen variations in needle diameter (bigger = leaner below 1/2 throttle), taper angle (greater = richer towards WOT), and taper length (longer = richer right around 1/2 throttle). I've yet to see any markings on the needle jets, maybe they're all the same and only the needles change. The 125's seem to run richer needles, the 250's are leaner, with the 175's in between.

I'd get the 16012-358-034 set, which is the final version as far as I know (it was used on the FL250 motor well into the 80's, which is basically an MT250 motor without a clutch or gearbox). The -034 set is also pretty much the only one still available, from places like CMSNL, HondaRestoration, etc.

Once you get the carb squared away, you may want to adjust the timing again. You may find that what made little or no difference before now has a noticeable effect.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric