Vintage Dirt Bike Q & A

my mr175 is nearly in perfect running shape now, my final issues are that my shocks are dead, Can anyone tell me were I can get rear shocks or who makes them? and I guess the front fork seals can be bought off ebay. The other problem being that I think my spark plug is not the correct one I'm using an ngk B8ES. once the engine gets quite hot it stops idling properly and almost cannot be started, once the plug is taken out and cooled down the bike runs fine again. I plan on disassembling it this winter and repainting it. The bike has been a fun little project so far.

I'll probably be getting Works Performance Dirt Tracker DRS/TPL shocks for my MR at some point.

The B8ES plug is the OEM MR175 plug, and works OK. IMO the BP7ES would be better, and I'm using a BP7EVX (platinum version of BP7ES).

Re the heat-related problem; have you checked your compression? It might also be stator coil heat breakdown, plug cap, coil, or something similar.

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'll try different plugs first, because swapping them out immediately solves the problem and the engine still has great power once it's started so I doubt it's the compression. I saw one of your posts saying that you rebuilt your rear shocks. How did you go about doing that?

If stopping a hot motor, immediately replacing the spark plug, then immediately restarting the motor (no cool-down period) cures the problem completely, then the problem may not be low compression. If you've got to let it cool off too, then it's probably the rings, not the plug.

During the short period I was using worn-out rings with very low compression waiting for OEM replacements to appear on eBay, the bike seemed to run quite well after a cold start, then would slowly lose power after it got hot until the motor finally stalled. It was then impossible to restart until it had cooled off. Finally the top ring snapped and chewed up the piston and exhaust port. Knowing what I know now - that Wiseco 2605KD rings work great on the standard bore, and that pistons last 4K+ miles - I would have thrown in new rings in a heartbeat.

Right now my bike's running 175 psi compression. It's a little higher than I'd like, but works fine with 104 octane race gas.

Here's the shock rebuilding info:

viewtopic.php?f=21&t=2575&view=next

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 link, look like it's gonna take quite a bit of tinkering to get theme working again. As for the Rings I'm not losing any power as it heats and it runs fine till I let idle and it stalls but if I start it again immediately it's fine if I let it sit for like a minute it won't start till a cool plug is put in. I'd say it take an hour of ridding it hard till it has that issue.