Vintage Dirt Bike Q & A

I am trying to isolate the source of performance that makes one of my MR175's so much more responsive than the others or my MT250.

My MT250 is a standard MT250 with the base jetting. It runs a little rich, but not much. It can pull me around (200 lbs) but its nothing spectacular.

MY white MR175 has a 1977 CR125 stator, rotor, coil and CDI. It has a nice snap to its throttle, but it runs somewhat like its a little rich. It will readily pull the wheel in first gear.

The cherry colored MR175 has a 1977 stator and rotor and a Mototek brand model CD-125 coil/cdi black box. The carb is jetted the same as the white, but this bike's response is explosive. In first or second, if you aren't paying attention it will stand up on you. If it weren't for the gearing, I would compare its performance to my RM250.

I know every bike and rider combination demands its own jetting, but could the coil/cdi make such a dramatic difference on two similar bikes? Is what I am experiencing atypical for an MR175?

It's the same for me. My MR is way quicker than the MT250 I've been working on, and has better throttle response than my RM (less power, obviously). On Tuesday I rode with a guy on a '97 RM250, and was able to keep up just fine. My bike doesn't make anywhere near that much power, but I think we were both putting similar power to the ground in those particular riding conditions (recent rain, occasional mud but otherwise good traction). The RM guy was really impressed.

I think it's mostly the ignition, myself. Adding all that low-rpm ignition advance and pulling the manhole cover off the crank end really woke the motor up. Porting and pipe work added another huge boost - I have to watch myself in 3rd gear sometimes, when grip is good.

Re the carb richness, replacing the needle jet made all the difference for me, as I've mentioned before. I was running a 115 main / 50 pilot / 3.5 slide with a modified stock pipe, now I'm running a 128 main / 50 pilot / 3.0 slide with a gutted MT125 pipe, and may go to a 48 pilot.

Is one of your MR's a '77 ? The '77 gearbox I'm running now made a big difference in acceleration, and my CR-framed bike will be getting an MT125 box w/MR 5th.

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

Any suggestions where to buy a good selection of jets, cheap (but good quality)? I am playing frankenstein with about 3 old parts carbs, and while they are clean, as you suggested, they may be worn.

I am not sure how to address the year. My buddy and I purchase three MR175 that were posted on the elsinore swap meet (you may have seen them), along with sufficient parts to build two motors. At the same time we purchased another MR175 with a spare motor. I don't know the history on the bikes, so I don't know whether the motors or the parts within them match the years on the bikes, but all bikes are 1976. Although one was made late in 1976.

Two of the bikes we have tried to bring back to stock, while the other two we just tried to clean up. The latter two I refer to White and Cherry in my posts.

Sudco is a pretty good source for Kehin jets.
http://www.sudco.com/

there is also Ebay.
http://www.ebay.com/sch/i.html?_nkw=keh ... &_from=R40

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(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!

Jemco might make pipes for it,might email and inquire about it.http://www.jemcoexhaust.com/

i have a Jemco pipe on my 1974 MT250 which i got it through Krause racing back in 1992 and it really woke up the bike.

it is still no dragster but it added more meat in the mid range and top end and it sacrificed very little down low.

either that or watch on Ebay if a Bassani pipe happens to show up there.

there is also porting,i highly recommend these guys.
http://www.vintageporting.com/

http://home.earthlink.net/~scloughn/index.html

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(2) 1968 DT1 1970 RT1M,1970 DT1C 1971 RT1B Enduro 1974 MT250 1974 DT250A (2) 1975 DT400B 1974 DT100,1978 DT175
1975 CR250M1 1979 CR250R 1979 Kawasaki KZ400H1 Jesus is Lord!!

The place I use now is Jets 'R Us - they've been pretty good:

Main: http://www.jetsrus.com/a_jets_by_carbur ... 25-xxx.htm

Pilot: http://www.jetsrus.com/a_jets_by_carbur ... 29-xxx.htm

NOTE: The above N424-24 pilot jets will only work if your carb's pilot jet is inline with the carb centerline upstream of the main jet. If the pilot jet is off to the side a bit, you should either modify the float bowl to use the 424 jets (that's what I did) or use a carb body with the inline pilot. By the way, the 28mm Keihins are stamped behind the choke to indicate which motor they were used on: 004 = MT250, 005 = CR125M, 060 = MR175, 092 = FL250.

The engine year is easy; a serial number starting with 100 is a '75, 200 = '76, 210 = '77. If the fenders are original, the '75's were silver, the '76's white, and the '77's were red.

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

Thanks for the key for determining which bike the carbs were made for.

I switched the pipes between the white and cherry. The white pipe had the baffle in and the cherry didn't originally. This seemed to reduce the richness in the white and settle the cherry a Little. Next I plan to switch the pipes back, then switch carbs. My theory is that the performance will switch as well. If not, we know its the ignition. If it does then I will first try switching the needles, pilots, etc. ultimately. I want to get to get these two performing equally well, and to a good combination so I can get the stock bike put together getting the best performance I can out of it while remaining stock.

I will keep this updated as I go.

From my five carbs I have 2 @056A, 1@060A, 1@004A and a mikuni 20E. Both of the 056A's are on the running MR175E's.

Of the 6 cases (4 complete motors, and 2 parts) all are 200's. Althugh there is a mixure of black and silver heads and cases. I am no sure how many are their original colors either.

I did switch out the needle the white with a used spare. It was a little snappier, but it no longer idled. Nothing conclusive gained. However, for comparison I rode the cherry. Again, I can only say: wow. Even with the more restrictive pipe with the baffle, it stands straight up in first and second (I didn't try third). I really want to isolate the power gene - I can't wait to switch carbs.

In addition to the carb and pipe, there's also ignition timing (the Mototek CDI almost certainly affects this, especially at lower rpm's), compression, porting mods if any, gearing, etc.

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 checked the compression on the bikes today after a brief warm up. The white bike with the CR125 ignition has a compression of 118-120 PSI. The cherry bike with the Martek CD125 has 138-144 psi. I checked the shop manual, which states that the compression ratio is 6.8:1, but I don't know how to convert that to the standards. Does anyone have the tolerances for a MR175?

The lowest compression I've seen on an MR175 motor was about 148 psi (stock), and the highest was 190 psi (modified). 144 psi is in the ballpark, but 120 is surely low.

For what it's worth, I always measure compression on a cold engine around 70 degrees, kicking with the throttle wide open until the gauge won't read any higher. I also ground the plug to prevent ignition damage.

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 checked both again cold at full throttle using two gauges, tested against an air compressor. I got the same results. About 120 and 144. I check the mt250 as well. It was 160-165.

Give the disparity between the two, I think I will forgo any further carb testing and proceed with doing some top end rebuilds.