Vintage Dirt Bike Q & A

You all wanted a progress report on my newly aquired 1975 MR175 with 975 miles on it.

I am please to say that after 37 hours and 11 minutes and a little over $235.00 my 1975 MR175 is back together I am riding it! It started on the first kick and the headlight low and high beems even work! I am one lucky guy to have a bike with all the original parts except a couple small items like an after market clutch lever, a in line fuel filter, a SAE woodruff key, and no tail light!

Ill post pictures in a day or so......Aplty named the "Green Machine" (tank color) it is now days not show room perfect,,,,,,,,but it is beutiful machine with its trail wise wear marks and 17 years of not storage!

Here is what I did:
- Cleaned the carb, jets, installed new carb gasket kit. I did not touch the float settings (carb kit ~$26.00 and Cleaner ~$10.00)
- Cleaned the alternator, set the points to just barely open on the F mark. On T mark the points are at 0.018 (new carnkshaft to rotor Woodruff key $0.17)
-Changed the crankcase oil. I put in 5w-50 and will drain in about 50 miles and then add 10w-40 synthetic. I added the 5w as I just wanted to clean out the internals with a lighter oil first. (2 quarts of official Honda oil ~$10.00)
- Kreem'd the inside of the gas tank. Yes the inside looks like new now (Kreem Tank Cleaner restorer kit ~ $40.00)
- Cleaned the petcock, the rubber "positioning valve is worn and it wont shut off. (gasket maker for Gas and Diesel ~$4.00)
-New Plug and Cap (~$7.00)
-New coil and condensor (~ $90.00)
-I repaired the rear fender the best I could (Fiberglass and Resin/hardner $20.00)
-Replaced the clutch perch and lever.......oiled all cables (Perch,Lever and HW $75.00)
-Replaced Misc HW....Nuts, Bolts, Roll Pin for Side cover latch, ($8.00)

I have three issues I need help with please:
1) I cannot get high RPMs out of it. I can make it run all day as long as I dont try to make the dirt fly.
AKA: It revs up and then bogs down. Its like it has a built in governer like the new cars have on them.
Idle is not perfect but heck its a two stroke with high compression and they never are.

I suspect the carb is the issue as just before it ran out of gas on the first test run the RPMs and power were there. And trust me it was a screamer, this old man almost rode his first wheelie in more then 3 decades.

2) I have moderate leaking on the overflows from the carb. I believe this is the float setting correct me if I am wrong and if I am correct can someone tell me how to set it please.

3) I need a rear fender and tail light

If anyone can offer advise on making this "nice" bike run like new let me know. I am good at rebuilding not good at knowing how to fix carb symtoms

Thanks to all

Larry
-

Problem #2-it does sound like the fuel level in the bowl is too high. If you are seeing gas come out of the overflow,that is a definite indicator. How did the fuel inlet valve look when you rebuilt the carb? -that's the little piece that the float tab moves up and down to let gas into the bowl. If it is worn,it may not be stopping the flow of gas even if the floats are at the correct level. Adjusting the float level is not difficult. Invest in a repair manual for the exact procedure. The "bogging" you are experiencing at high RPM's may be caused by too much gas being fed into the engine because of the elevated level in the bowl..I could be wrong though.

Larry,

Sounds like you're making some excellent progress! I had mine out today, the first time in the woods for maybe 3 months or so; it was lots of fun.

First off, I would advise in the strongest possible terms AGAINST using synthetic oil in the gearbox. I am the absolute biggest fan in the world of synthetic oil, but three slipping clutches later I'm willing to admit that in THIS gearbox with THIS clutch, it's not a good idea. My latest clutch (OEM plates, Barnett springs) has been working great for almost a year now, using Spectro 10W-40 conventional motorcycle oil.

I definitely agree that the float level should be checked, especially since you're getting gas overflow. I would also add that once you get it set properly, do not change it again. I almost had to throw away 3 months of jetting work when I re-set my float level; luckily I was able to set it back again easily. The level of gas in the float bowl makes a real difference in how easily fuel can be pulled though the jets, which in turn makes a real difference when jetting the bike. The correct setting is 20mm from the far end of the float to the float bowl gasket surface (I can scan a diagram if you need it).

By the way, it took me four (eBay) carbs to end up with one I was happy with, so it's possible your carb may have issues in addition to the float level. Re the idle; I'd suggest running your idle air screw out until the motor won't idle any faster, then turn it back in 1/2 turn. My bike idles OK now with a tiny CR125 ignition rotor, it should idle just fine with the stock flywheel. For jetting, I'd recommend a 48 pilot (the stock 52 is way too rich) and 118 main with a stock motor, needle in the #3 (middle) position, stock slide and air jet.

Also, the stock pipe really doesn't permit much in the way of high-rpm power, so I wouldn't expect too much. I was running the stocker today as my other one's being machined, and it really is more of a low-mid pipe. By all means make sure the spark arrestor outlets (around the perimeter of the front of the tailpipe section) are cleaned out, and I've drilled the center of my arrestor 9/16" as well.

I highly recommend the Acerbis Dual Sport taillight unit, which I have on mine - nice big license plate mount and light assembly, light weight, etc. The OEM lights come up pretty regularly on eBay too.

Ray

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

CMA70 and Ray,

I was hoping you would say float, just needed a second opinion before I dive back in. Thank you both so much! Ray if you would post the diagram for setting the floats send it please as I just cant remember how I set them for my 1971 Suzuki 125.

Also what you both imply, it makes perfect sense if the fuel is to high it may be effecting the performance of the jets. That is why when it ran low on fuel and the bowl was getting empty it roared like a lion for couple of seconds. I will also check the rubber connection from the cylinder to carb, it may be sucking a tad bit of air at high RPMs. Some good old WD40 sprayed around the connection should be a quick check on that.

I stopped by the bike shop and I did find a new fender........Honda still makes them for a 1976 MR175 Cost is high at $65.00 so for now Ill use the old one. Its Red but I can paint it to the original color as I am sure the Red is not a plastic color but just paint as I discovered the Silver was just paint and not a plastic color.

Ray, I just may try your after market light. But at this time it will be used for chasing coyotes on the ranch in Nebraska. NO Traffic out there!

As promised here are the unretouched pictures of the bike. It just lwas just a little dirtier than shown when I got it.

Larry,

The bike looks great! It's especially nice to see such a clean-looking tank, so many of them are dented up and rusted out.

The float level diagram is below.

Ray

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