Vintage Dirt Bike Q & A

I have a 77 mr175, when I start the bike it idles high (allmost lean) but seems to run good with quick throtle response, then as your standing their after about 20 seconds (which is about the time it takes to burn through a carb of gas) the idle will slowly slows down to barly ticking over and have very slow and boggy response.(allmost to rich) As you pull away their is little power in 1st and it will not rev up to the powerband but 2nd and up once you get the bike rolling it runs good in the high rpm only.
It sounds like the bike gose from lean to very rich
The frustrating thing is I replaced all the gasgets and float bowl shut off valve reset the float to level. So what else shoul I check. Every time it sounded like a dirty jet they were clean.

I live at sea level, my needle is in the middle slot and the mixture screw is set at 1 /2 trurns
just like the book says.
The points are worn but that dosen't explain the flucuation of mixture.

any ideas

Sound like an air leak. The easy way to find out is, while it's idling is to spray the intake manifold area with WD-40 & see if the idle speed changes. Ray Rivers is the MR expert. I'm sure he will give several other causes.

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77 MR175, 89 HD Sportster
74 CR125, 00 Kawasaki ZG1000
79 XR185, 56 Cushman Eagle
03 XR650L, 79 Vespa P200E
72 Vespa Super150,
74 HD Z90

It could be your bike is loading up from a too-rich pilot circuit, which will also mess up your idle big time no matter how you adjust the air or idle speed screws.

My bike's idle has always been an issue, but it's livable now. It varies up and down a bit but never stalls, even with the CR125 ignition and practically zero flywheel weight. With the stock flywheel, it was amazing to me how low it would idle and still run. If you think your points are iffy, replace them; you'll be glad you did.

I'm using a 48 pilot jet, 120 main, and (stock) 200 air jets, with the stock 271005 needle (in the middle #3 position) and stock 3.0 throttle slide. It took me a long time to find these values, as I tried every jet/needle/slide/air-screw combination I could think of. Once you have the 48 pilot jet in, turning the air screw out 3 turns (!) probably helped the idle problem and throttle response more than anything else, but you have to have the 48 pilot jet - the stock 52 pilot is way too rich for good idling and throttle response using modern premix ratios.

I also found part of the problem was the idle screw threads in the carb body, which were worn to some extent on all the carbs I have (six) - not only does the screw rock and change the slide height, I believe that air may leak past the threads as the screw tilts. I finally wrapped the threads with Teflon plumber's tape, this helped out quite a bit - just don't adjust the screw a lot afterwards if you can help it, try to find the idle setting quickly and then leave it. I also matched the angle of the end of the idle screw to the angle of the ramp in the throttle slide just for grins, but I'm not sure if this had much effect on the idle-speed problem; it did reduce the 'divots' that get banged into the throttle slide ramp and screw up your idle speed adjustment, though.

Thanks, Jim! Great suggestion about the WD-40, too.

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