Vintage Dirt Bike Q & A

i have a few newbie questions regarding restoring my 1975 MR175

one thing i really need is a Left Crankcase (Alternator) Cover... i did some digging at cmsnl.com parts lists and it looks like the Left Cover from an MT125 is identical, and older CR125 covers look pretty close too... hard to say without the exact measurements... I found somebody selling an entire MT125 lower unit with both Left and Right Covers and all the guts - so, question is, if I am correct of the Left Crank Cover inter-exchangeability, are the rest of the lower unit parts also similar to my MR175 ??? Can anyone please confirm exactly what years and models are inter-exchangeable (esp the Left Cover) ???

also, another little mistery - how come do I have 2 spark plug holes ?

Normally use a spark plug in the center hole and put a bolt (special "sealing bolt") in the side hole. You can put a spark plug in both holes and use either one. In the old days we put a compression release (jake brake) in the side hole.

The MT125 and MR175 alternator covers are interchangable for all years. The CR125 alternator covers is too shallow to clear the rotor on an MT or MR.

The bottom end of the MT125 is virtually identical to the MR175. Should be good for parts.

thank you very much for confirming that...

another little question - the alternator has two wires coming out from inside of it... one wire (yellow with black) was connected to the kill switch and then to the coil, the other wire (yellow) was just dangling around... so... is one hot and one ground? would the one with the black stripe be the ground by any chance? and then do i just ground it somewhere on the block?

I scanned the wiring diagram and can email if you give your email address.
There are 2 coils in the alternator; one for lights and one for ignition.
One side of each coil is grounded internally. Black and yellow wire is for ignition and goes to the rectifier routed through the wiring harness. Yellow wire is for lights and goes to the headlight switch routed through the wiring harness.

thanx for the offer! i really appreciate it! i just sent you a PM with my email addy

also, i did some googling, but could not find any good info on jake brakes for motorcycles... where would one go to get one that fits in the spark plug hole? as i understand, it is just a solenoid controlled comp release valve, right?

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2000 XR600R
1979 XR200R
1975 MR175 Elsinore
1987 Husqvarna 510TE

I don't know where to get a compression release but wouldn't recommend it anyway because it will allow dust to be sucked into the cylinder. Suggest you go for the bolt.
(I emailed you the wiring diagram and 2 different clutch adjustment procedures from Honda manual)

kewl, thanx a bunch mang !!! the old bike really had a biscuit and needed some restoring... hoping to giv'n'r in a few days and git'er done... that was real dekey to hook me up with those diagrams, thanx again...

_________________
2000 XR600R
1979 XR200R
1975 MR175 Elsinore
1987 Husqvarna 510TE

eh, so looking at the alternator diagram raised some more questions...

where is the rectifier supposed to mount? it looks like i am missing the rectifier all together, the Y/B wire went straight to the little round deal on the side of the coil, which i am assuming is the condenser... it looks original '75 and hanging by its last breath and only a couple threads of wire... when i first got the bike we got it started, but when we tried to ride it it just died... now i am guessing it was because the needed voltage regulation was not there (no rectifier)...

so... what other models are coil + condenser and rectifier are swappable with???
what is the general rule regarding motorcycle coils and voltage regulation?
i know more about cars, but i am just starting to figure out how bikes work...

and here is that wiring diagram for future use:

_________________
2000 XR600R
1979 XR200R
1975 MR175 Elsinore
1987 Husqvarna 510TE

My MR250 I believe only uses the rectifier for the lights.Yours should be the same.

Lights are AC. Not rectified.
Ignition uses rectifier.
Go to
http://www.mrcycles.com
They have parts lists online with diagrams that you can zoom into to see clearly.
Click on Honda Motorcycle Parts.
Select 1975 MR175.
Scroll to the bottom "WIRE HARNESS @ TAILLIGHT
For the MR175 you can see the rectifier.

if i was to get some new parts...
are rectifiers inter-exchangeable between any models?
also, for coil and condenser, are those inter-exchangeable?
is anything universal aftermarket?

_________________
2000 XR600R
1979 XR200R
1975 MR175 Elsinore
1987 Husqvarna 510TE

i found a rectifier from an old XR175 - would that work?
are all rectifiers similar in how they regulate voltage? so as long as it is three wire, it should work?

also, as far as the coil + condenser... with cars, the more volts the coil produces the better, is that the same with bikes? is the condenser just a capacitor? if so, are they all pretty universal between the models? are there any universal aftermarket ones? maybe coil and condenser in one?

_________________
2000 XR600R
1979 XR200R
1975 MR175 Elsinore
1987 Husqvarna 510TE

ok, so i did some electrical systems research

Rectifier simply converts AC to DC. It is usually used for the battery and lights. Sometimes, in the same unit, it also includes a Voltage regulator to keep steady voltage, to prevent burned out lightbulbs.

It is strange, that according to the diagram, lights run directly on AC from alternator, not regulated or rectified. I do not understand why the original diagram shows that the Ignition Coil is rectified...

although apparantly, some Ignition Coils are AC... someone said that MR coil might be AC, or maybe not care if its AC or DC... also, my coil could possibly have been replaced with an AC one, it looks a little different from the pictures...

Can anyone confirm that I can continue running without a rectifier ???

_________________
2000 XR600R
1979 XR200R
1975 MR175 Elsinore
1987 Husqvarna 510TE

You can only guess if substitute parts will work. I would try to get the right parts. The wiring diagram is correct.
You could try posting on the swap meet for the parts you need or checking ebay for someone selling used MR175 parts and ask if they have the parts you are looking for. Also Western Hills Honda has been a good source of parts for me in the past.
http://www.westernhillshonda.net/
You can check their vintage parts inventory on-line. The rectifier should be relatively inexpensive but the coil may be pricey.
The MR175 can be converted over to CR125 CDI ignition if you don't need lights but this will be a much bigger job than getting what you have now to work.
Good luck