Trying to solve final piece (I hope) of new carb

2strokerfun

Member
May 19, 2006
1,500
1
Have a 1976 Honda MR250 currently running a CR cylinder, which effectively makes it a CR engine with wider gearing and one heck of a heavier flywheel (CR cylinder different in that exhaust port 1mm lower). Gave up on on worn-out 34mm Keihin about a year ago and bought 36 Mikuni. Had to start sorting jets and needles from scratch, but I finally thought I got it jetted perfectly.
However, the last few times I rode I noticed a problem: Once the bike is good and warmed up, if I really really romp on it at high rpms under a heavy load (and about the only time I ever get in that situation is on a long straight section of track) I'm getting detonation-like clatter. Back off the throttle it disappears. If I'm already going really fast with throttle open, it doesn't exist. Only does it under heavy load at high rpm. Only does it after the bike has been running 10 minutes or so--really warmed up. Throttle has to be close to 3/4 open or more for this to happen. In other words, I'm really running the heck out of it when it does this.
I did plug chops forever when trying to jet this carb from scratch. Finally got what I thought was the correct main, needle, needle jet and pilot jet. Plug chop still looks correct. Took the head off to inspect the piston. No signs of damage from detonation or bearing coming apart yet, but I have no doubt I will soon see some soon if I don't address this.
Bike is still running great when I experience this. No signs of anything wrong other than the noise. Now I am extremely sensitive to the sounds of my engines. It isn't a loud clatter at all, but it is definitely there. I think that most riders wouldn't even notice it. (for instance I don't think even I would hear it if I were riding in flat desert. The weeds and trees along the track seem to echo the sound back to me).
I'm thinking I'm completely or almost completely off the needle when this happens, but I'm not sure. My bike is extremely sensitive to the needle size/settings with this carb. In other words, if I raise the needle, I'm definitely too rich at points.
Even though my plug chop looked perfect, I increased the main one size. It might have helped a pinch, but it might also be my wishful thinking.
Probably going to try going up another main size this weekend and test it, but I'm wondering if anyone has any other ideas. Have new main bearings, but crank rod bearing is getting long in the tooth. Still no up and down play.
I'm probably overly paranoid but I hate exploding engines.
Thanks
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
Is the bore true? Skirt and ring gap with in spec? Rod and mains good? Leak down test? Is the bottom end the same volume as the cr? Pipe and silencer? CR or MR ignition? How are you reading the plug chops for the main?
 

2strokerfun

Member
May 19, 2006
1,500
1
Bore .40 over has about 15 hours on it, still true last time I checked. Piston has about 5 hours on it, ring gap correct. Mains new. Rod, I need to check. No up and down play but side play near limits. No leak down test ever done. Bottom end same volume as CR. MR points ignition. About 25 hours on points, probably need to check and at least re-gap them (didn't even consider that). Pipe is MR, but so minutely different from CR I don't think it would cause this. FMF muffler with 4" cut off. Plug chop done going up a grade, then cutting base of plug off using drill press and hacksaw blade (yes it can be done if your careful!). Nice mocha color.
The only way you can adjust the timing with this flywheel, points set up is by gapping the points. Glad you asked about ignition as I'm going to explore that.
 

2strokerfun

Member
May 19, 2006
1,500
1
Fox: I actually have a CR setup, but in order to install it, I'd have to ditch the super heavy flywheel of the MR. It's like riding a 4 stroke down low with that 4 pound piece of iron they call a flywheel. It still screams on top, but much more forgiving when trying to creep out of a creekbed they call a trail.
I'd go with the PVL cdi, but it simply costs too much.
I narrowed the point gap and went riding yesterday and the problem is almost solved. I think if I purchase a real set of Honda (Nippo Denso) points instead of what is probably a Chinese set I bought last time, I'll have the problem solved completely. Read somewhere the Honda points have stronger spring, thicker backplate and are superior in every way to crappy no-name points.
Thanks for your checklist. I had never even considered point float.
 
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