Vintage Dirt Bike Q & A

I have a 1976 MR175 that has been under maintenance for a couple months. I will list all that I have done below:

-Replaced the carburetor with a brand new Mikuni VM28-49 Round Slide (28mm)
-Replaced the old throttle cable with a new CR125 cable
-Replaced the handlebars
-Replaced levers, mounts, and added new mirrors
-Replaced the pawl spring on kick starter
-Changed oil
-Replaced old fuel with fresh fuel with 32:1 ratio
-Replaced the fuel line
-Replaced spark plugs

A week ago, my bike would run and idle just fine. Now, it doesn't even act like it wants to start. The gas tank is a little rusty on the inside. I have adjusted the carburetor and tried starting with the choke opened and closed. I do not know exactly how much compression I have, but when I kick, I get some pretty breathtaking compression strokes.

My spark plugs have spark. My airbox does not have a filter in it, so it is getting plenty of air to the carburetor. I cleaned my carburetor and the hoses going to the head and the airbox.

I do not know where to start with this bike. Where should I look? I would appreciate any advice or information!

-Chris

Chris,

From your list, it seems you've done all the easy stuff - which often works - but having been there many times before, here are several more items which should at least give you a better idea of what's wrong by process of elimination:

1) check compression (with a gauge, not your foot - see below)
2) set ignition timing (try setting points gap to .012")
3) clean exhaust spark arrestor (just ahead of tailpipe section; you'll need to remove tailpipe insert first, then I used long ultra-coarse pipe cleaners from a pipe/tobacco shop)

Cheap MC-friendly compression gauge:

https://www.walmart.com/ip/Equus-3612-I ... 3=&veh=sem

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray,

Sorry for the late response. I have finally gotten around to looking for all possible issues. I have spark going to my brand new plugs, so I know it isn't a spark issue. I have checked my exhaust pipe and there is no spark arrestor in it. After removing my exhaust pipe, I checked inside of my cylinder and the piston, rings, and walls are in good shape. My compression is low, so I can only assume a head gasket.

I guess I am unfamiliar with setting points gap on these MR's. I found the point in the left crankcase, but I do not know what screw to slightly loosen to adjust the point. I measured the gap and it is near .012" but not exact. I have a few things to do, but with that information, I wasn't sure if you would have a clue as to what could be the reasoning for my MR's failure to start back up.

-Chris

Really! - because unless you have plainly-visible black spooge around the head gasket area, I wouldn't assume that at all. My cylinder / piston / rings looked great through the exhaust port too, then 2 rides later the top ring snapped & nearly destroyed the exhaust port.

You can rebuild your top end, or continue to chase your tail - your call.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric