Vintage Dirt Bike Q & A

75 MT250, new Wiseco piston, already set the timing properly, now it just runs pig rich.
Is there a different setting for the plastic float than for a brass float? The manual shows a round float that I'm assuming is brass, mine has a chunky looking plastic/foam float. I can't set anything parallel to the carb body, it would make the fuel level even higher.
I am at sea level so even the rich stock settings should be close to useable, but it blows fuel out the carb and won't run hardly at all. I just put a kit in the carb and that made the problem worse.

Update 1- Jets originally in it were a 115 and 58, I pulled those and put in a 122 and 60 from the kit, the smaller ones are back in it now. It still needs a smaller pilot, but I also noticed a hole in the carb flange. I don't know if that is new from today, or has been there a while, but it explains some of the rich/lean combination I've been chasing.
There is a guy local to me who has quite a few parts bikes, I'll see if he has a carb flange, I may even get a whole carb from him if he has one as well, just to try.
This bike was someone's parts bike, and I keep finding little things that remind me of that.

Ok, I'm going out of my mind on this thing. I've changed out the float, still had to set the level at 22-23mm to keep fuel from pouring out the overflow, dropped the main jet to a 105, and it runs ok at cruising speeds. It surges trying to maintain 35-40mph, plus it goes really flat until the throttle is opened almost all the way. It requires almost full throttle to go 55, and I cannot get the low speed mixture lean enough to get it to idle. I have tried a 60 and 58 jet, but I can't find these old-style pilot jets anywhere around here. I'd like to try a 55, but can't find one and the two other carbs I've torn apart here both had 58s. As it approaches idle, it'll bog foul the plug and die. I have done a long pull up a hill, shut it down at WOT at the top, and pulled the plug to check. It was a nice tan color, so I guess all the richness is coming from the pilot circuit.
Does 55/105 sound lean for sea level?
What sort of mileage should this be getting? It's currently getting about 25mpg.

It sounds like you might have the same problem I did - a worn needle jet and needle. One test would be to remove the pilot jet and plug the hole with a small screw or whatever; if the bike is still rich, it may be the needle jet. Something else might be wrong too, like a leaking choke, sticking float pivot, leaking float needle or seat, cracked carb body, dirty fuel, plugged air filter, etc. etc.

The MR175 and CR125M needle/jet sets are slightly richer than the MT250, but at least they're available:

http://www.ebay.com/dsc/i.html?_sadis=2 ... +16012-373

It may also be possible to build up the needle's straight section with solder or epoxy paint - I've never tried either one.

The stock jetting (122 main, 60 pilot, depending on year) is actually a bit lean, believe it or not. Forget about finding the 28mm long pilot jets, if your carb uses those; I modified my carb to use the 32mm N424-24 jets available from jetsrus.com.

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

Ok, it has a new needle and a new main jet (I put a Keyster kit in it), and the old needle was quite worn where it passes through the slide. The new one still has some slop, and I noticed that none of the carbs I tore into had any washers in the pocket with the needle. Most of the Mikunis I have worked on on sleds had a nylon washer that you put above or below the snap ring to either bring the needle up half a notch or just fill the void. Is there supposed to be one in this carb? Also, where can I get a needle jet, or is that the main jet?
I had previously passed over the main circuit as it isn't rich at higher rpms and working hard, like running up the hill. Also I checked the air filter (I did a lot of my testing with the airbox boot off to make jetting/float changes easier) and the spark arrestor (drilled out the cone) with no change at all. The bike pulls decently starting at about 3900rpm, and once cleared out will pull right to 7k without issue.
I did investigate the choke circuit, and it is sealing. I used carb cleaner to verify fuel was not getting through the enricher with the valve closed.
This is probably the cleanest carb ever seen on one of these bikes now, too.

Unfortunately today while I was changing the front rim I found this:

So now I need to wait for new shoes to get back to testing.

I've used washers under the clip a couple of times, but the MT250 carb didn't come with one.

The needle jet is the large brass tube the main jet screws into. It works with the needle's straight section (directly under the slide) to meter fuel below 1/2 throttle. When new, the needle-to-jet clearance is tight and very little fuel gets by, but In the worn ones I've seen, the normally circular needle jet I.D. becomes egg-shaped, allowing more fuel (sometimes a lot more) to leak past and richen the mixture. It can get pretty bad; my MR175 (basically the same carb as the MT250) started easily without the choke, wouldn't idle, and blubbered uselessly unless the throttle was nearly wide open. Replacing the needle jet made a huge difference.

This place has the needle jet set, at least for now:

http://www.cmsnl.com/products/needle-se ... 012358034/

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

That's a big price for a little part- I may just ditch the Keihin altogether. I am making a new intake to replace the stock one, and it'll fit a 34mm Mikuni, which I just happen to have as well while keeping everything in the stock location.
I may have to tear the engine apart again as well, as I think the crank seals may be leaking. It smokes really badly when idling, and first thing in the morning, but uses very little 2stroke oil. I have found the gearbox oil low, and assumed it was a leak (the shift shaft seal may be leaking) but it may also be a leaking crank seal. It has nasty goop around the silencer seam.

Well, I have an update. I got new brakes front and rear, traded the 4.10 rear tire out for the proper 4.60, and put the needle jet out of a parts carb into the carb on the bike. It actually is managable in town now, and even idles. I'm still going to go to a Mikuni, but should be able to finish out the season as-is.
It does seem to be rather thirsty still, running at 50mph it seems to turn in less than 30mpg. I thought these things were rated at 75-ish, I was hoping for 50-ish, I know a 2stroke is going to use more fuel than a 4stroke, but still it should be getting better mileage.