Vintage Dirt Bike Q & A

Hi, I recently picked up an MR250 that was in decent condition for $400. It ran when I took it for a test drive so I figured I couldnt go wrong.
When I brought it home, we were looking at it, and opened up the gas tank to see all sorts of rust and other crud in there, and so we decided we would clean the carbs and get a new gas tank. Right before we were about to shut it off, the RPMs went through the roof (something probably got clogged and caused a lean condition), and it redlined for about 3-4 seconds before we hurried up and stalled it.

Then, we cleaned the carb out perfectly, all jets are clear, and the inside looks like new. Got a new tank in better condition and kremed the inside.
When we started it for the first time after the cleaning, it would start, but was very jerky and wouldnt accelerate past 2-3K RPMs, and it was running very rich, even after just a few seconds of running the spark plug would be drenched in oil/gas.

We went through the carb again, cleaned everything, cleaned the choke, slide, everything, put it back on, and now its even worse. Only starts once every 10 or so kicks, and when it does, there is no throttle response and it just bogs down and dies after 5 seconds or so.

We put a new air filter on it, and we thought that might be the problem, but running without a filter still produced the same results.
I'm going to compressin test it tommorow when I get a gauge, but I have a feeling it is fine, it gets pretty tiring to kick over, and everyonce in awhile the slap back from the compression will whack you in the leg pretty good.

Any ideas?
Thanks

Try doing a search here on 'MR250' - lots of good stuff.

Depending on your budget, you can replace the carb with a Mikuni (the jetting specs are available here) or set the float level and re-jet the existing carb to minimize the intense richness of the stock jetting (you'll need to tell us what jets etc. are in the carb before we can recommend replacements).

Or, you can tweak the air screw, play with the needle clip, clean the carb a few more times, etc. If you go that route, the best of luck to you (you'll need it). :>)

You are not alone:

http://www.youtube.com/watch?v=FocAtnPbo-w

http://www.youtube.com/watch?v=4S5k7BnAOR4

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

Hi Ray thanks for the reply,

I think that I will go with the Mikuni eventually, its just that before I buy it, I want to know for sure that it is in fact a carb issue, not something electrical.
I will be able to tell alot more about the problem once i get my timing light and compression tester tommorow.

Also, what place would you recommend to buy jets? I had looked at Sudco, but they require that you buy a minimum of 4 of each product.

Thanks

Unless your points are set way to0 wide, or your key sheared on your flywheel/crankshaft connection, the MR cannot have the timing set wrong. It sounds like a carb problem. But, while your at it, clean and put some dielectric grease on all your electrical connections. The MR has a very frustrating carb, especially when they are 34 years old. But when you finally get it right, I love this bike. Lots of low end grunt like a four stroke (that massive flywheel does that) and still rips pretty good when you get on it.

I've had good luck recently with Jets R Us:

http://www.jetsrus.com/a_jets_by_carbur ... ection.htm

Your carb uses 99101-357 main and N424-24 pilot jets, and the N424-52 leak jets will work for power jets if 2mm is removed from the end opposite the threads (if the power jet O-ring doesn't look absolutely perfect, replace it with Honda P/N 91308-371-003).

I think it would be a really good idea to throw in an NGK B6ES plug while you're troubleshooting the motor. It won't fix anything, but it will give you a little more resistance to fouling and let you get more tuning in before the plug dies.

Did you set the float level (20mm) when you had the carb apart? There is absolutely nothing more important to do first when the bike shows the symptoms yours does, in my opinion.

It's always a good idea to check timing, compression, etc., but there's only one component I know of that can cause the motor to race as it runs out of gas, and that's the carb.

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

Could you be running the wrong pre mix ? I think the owners manual calls for 20 to 1. I'm using 32 to 1 with no problems

OK, thanks guys, so cleaned some more things on the carb, and then went to kick it over, and it wasnt even trying to start, so we took the plug out and tested for spark, and nothing at all. Checked the kill switch to make sure something wasnt touching there, and then we took off the left cover, and tried doing the test in the manual for the points. We connected a lamp and a 6v battery, and connected one lead of the lamp to the stator, the other lead on the lamp goes to the positive on the battery, and then the negative on the battery goes to ground. The test supposedly checks for timing and making sure that the points are opening in the right places. According to the test, the lamp will darken when the points open, so when the F mark passes the mark on the crank case, the lamp should darken. What we found though was that the lamp would darken at multiple times during a complete revolution, dont know if that means something is up?
Whats weird is that we just had spark the other day when testing it, so maybe the stator is going bad or something.
Thanks for the help, its really nice having other people help out on problems like these.

Edit: also, we have checked the floats multiple times, it is exactly at 20mm now, and we have tried anywhere from 15 to 25mm testing. The carb is spotless, and everything in it is clean.

... and I thought this was just an MR175 thing... apparently it's an equal-opportunity Honda MR embarrassment... At one point my MR started igniting the crankcase mixture, which was real interesting in an OMG! kind of way. I never found out the cause, although I suspected a bad points-cam design, perhaps combined with crank wobble and/or points-assembly wear (contacts/pivot/scraper). I vaguely recall someone having a sheared Woodruff key as mitchie mentioned too, this would certainly be worth checking when the flywheel's off.

I immediately replaced the ignition with a CR125 unit before it blew out my case gasket, although it did manage to pop the left crank seal out halfway; the performance improvement was about as subtle as a supercharger.

The 30200-358-004 points are still available (Sudco P/N 616-407). I got a set for a friend's MT250 from the place below, for about $12 shipped:

http://www.olypen.com/retro/30200-358-004

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

New set of Points !! They are about $13.95 or so. Try Honda Dealer because the original Nippo Denso points have a thicker backplate and siffer spring than aftermarket and don't float at high rpm. I think you can also get them at Re-Mx.

Alright thanks guys! New points are on the way. Do you think that the points could have caused my issues? I figured the mixture was just insanely rich since fuel would always be on the plugs and it bogged down, but it kind of make sense that if the spark was weak or irratic due to bad points, it may have just not been firing correctly or weakly, causing the fuel on the plug.

Also, I tested compression today, and it was 135 after 4-5 kicks, is that on par? I had once read somewhere else on this forum that anywhere above 130 was strong for the MR, but just wanted to confirm it.

Last question (i feel bad asking all these questions im sure are easy answers, I just dont have much experience working on 2 strokes, even simple old ones like these), is there anyway to get that giant flywheel off without a puller? Im kinda of on a budget, and am trying to make due with what I have, but maybe its not possible without the tool. Theres not anyway to pry it since the case covers the whole left hand portion of it, but maybe some other sort of device?

Thanks for the help I really do appreciate it

I think it's quite likely that your ignition needs some work and it's a great idea to pull the flywheel, replace the points, etc., but I'm about 99% certain your carb is a huge part of the problem, based on your symptoms and the track record of these bikes. If it were me, I would go down two sizes on both main and pilot, drop the needle all the way, and turn the air screw out two turns; I'd much rather tune richer from a too-lean condition than battle plug fouling, poor starting, and sluggish performance coming at it from the other direction. Leaner jetting should wake this bike right up.

135 psi seems a bit low, but nothing to worry about for now. How many miles are on the bike?

You should be able to use a bearing puller to get the flywheel off, if the jaws will fit inside the flywheel cutouts or around the back edge of the flywheel. Prying or hammering is not recommended; the flywheel's just on there too tight.

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

I'm with Ray. I think you need new points to shore up the spark part of the equation, but I do not think that is your major problem. I wish I had your carb here to look at. Something is fishy. I'm sure this has been asked and answered, but once I start typing these, I can't go back and look or I lose what I've typed. Exactly what jets do you have in there now? And get the numbers off the needle and needle jet while you're at it. Let us know.
Now, on the puller. I've trieda bearing puller it and it is too easy to mess up your stator when doing this. I advise against it. Any bike shop should have a flywheel puller for about $15 or less that will easily take your flywheel off. It is a tool you will use many times and is well worth the hassle and cost of obtaining.
When you cleaned your carb, did you use spray? did you disassemble and soak it for a day or so and then blow all passages with compressed air? Still wonder if a choke circuit passage is clogged up.

+1 on getting a flywheel puller - easy as pie, zero chance of damage - but they may not be that easy to find (the MR175, MR250, and MT250 pullers all have different Honda part numbers). This is the only one I found:

http://www.cmsnl.com/products/puller-fl ... 333950000/

If you can search around a bit more and find out what threads you need (my MR175 unit uses 27mm left-hand threads for an internally-threaded flywheel), perhaps there's a generic puller that will work:

http://media.denniskirk.com/catalogs/of ... road21.pdf

http://www.motionpro.com/motorcycle/too ... l_pullers/

I've used mine in 'extension mode' with a couple of wood blocks to pop cylinders up off the base studs, too - it worked great. It's a K&N 82-0150, same as the 28-1953 in the .pdf link above, and also the Motion Pro 08-0026. NOTE: I've never seen this puller listed as working on an MR175 flywheel, but it does. The MR175 and MR250 cranks both use 12mm hex nuts and the thread openings both look similar in relation to the nut, so perhaps the MR175 puller will fit the MR250 flywheel; I really don't know.

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

It's a 20mm x 1.5 RH puller. Just google MR250 flywheel puller and you'll get a dozen options from $9.49 up to about $16.

OK thanks guys, I ordered some points, and hopefully this will fix the problem. I will update on what happens with the new points.