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Old 07-15-2014, 09:20 AM   #1
MayerMR OP
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Location: Dallas, Texas
Oddometer: 252
Sorting out a recently finished Honda CM91-> CT90-ish restoration

Hey fellas,

I promise I used the search, but I'm still having trouble sorting out my recently finished (are they every really finished?) restoration of a Honda CM91 that I have kinda-sorta-converted into a CT90 clone (basically I took the elements that I liked from each and kept what I wanted!

Here's the finished product!:





But even though I'm able to drive it, it's still giving me some issues. I'll list them below and would love whatever help you all can offer!

First Problem: I can start it very easily and get it idling; however, it seems that the throttle (at the handlebar) has some loose play and it will kind of allow itself to raise and lower the idle speed on its own. Could this be due to a worn-out return spring in the carb or could I have assembled the throttle incorrectly allowing for play?

Second Problem: During acceleration, the the engine kind of stumbles and jerks until I hit a certain RPM range in each gear, after which it will run smoothly. Is this a jet issue or a mixture issue? The idle mixture screw shouldn't affect this at speed, only at idle, correct?

Third Problem: This could just be me getting used to the bike, but the shifting seems clunky and I get a hard jerk now and again when I got into different gears (I think this also has to do with the loose throttle as it comes on quickly after a shift which could lead to the jerkiness post-shift.

Not a problem, but a question: What should the proper shift pattern be for this engine? It seems I have; 1, N, 2, 3, 4, but everything I've read denotes a N, 1, 2, 3, 4 pattern. Not a big deal, but wanted to clarify.

Follow-on nitpicks that I'd like to sort out:

My neutral light had to be switched with a 12v from Autozone as it was missing when I bought the bike - are these available still? The 12v is very dim and hard to see.

The repro heel-to-toe shifter is really crappy and hard to actually use without removing my foot from the peg and kicking it up (heel side). My peg bar is all bent to hell from previous owner tip-overs and I don't know if the pegs should sit lower than they do?

Finally, the repro rear brake sits very low and close to the motor, can you all see from the pics if I have it hooked up incorrectly (I've actually ordered another OEM part just today, so hopefully that'll correct the brake issue). I hope it does because the rear brake is almost useless to use at this point - honestly, I have to remove my foot from the peg and kind of manuever my leg around to even engage it...kind of dangerous at speed.

Thank you all in advance for any assistance!

Best,

-Matt
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1972 Honda CB450 (Rebuilding)
1971 Ducati 450 R/T (utter disrepair)
1971 Honda SL100
1967 Honda CM91 -> CT90 conversion
1986 BMW K75 - Storage until I figure out a way to tell the wife
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Old 07-15-2014, 12:05 PM   #2
MayerMR OP
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Quote:
Originally Posted by Hammerfist7 View Post
Fine looking machine!

I would take the carb off, give it a THOROUGH cleaning*, and expect that surging and hesitation to go away.

Then look for the adjustment to remove the slack from the throttle cable - at one end or the other.

Six volt bulbs are readily available, I'd try ebay for the cheapest price.

And I have to wonder, are your footpegs bent or mounted incorrectly?
That right side sure seems awfully far back!



(no need to respond here, the questions are meant for you to ask of yourself)


* http://www.ebay.com/gds/CHEAP-Easy-R...1800675/g.html
Thanks! I'm going to take it apart and go through it again, it was just rebuilt, but I suppose I could have missed a blocked hole somewhere in there. I think fixing the slack in the cable will help alot as well, it's very annoying.

The footpegs are mounted correctly, but it was laid over a time or 9 and they are pretty bent up. I either need to find replacements or find a machine shop that can bend them back to the right location for me.
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1972 Honda CB450 (Rebuilding)
1971 Ducati 450 R/T (utter disrepair)
1971 Honda SL100
1967 Honda CM91 -> CT90 conversion
1986 BMW K75 - Storage until I figure out a way to tell the wife
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Old 07-15-2014, 07:29 PM   #3
redprimo
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#2 problem sounds like a partially clogged emulsion tube.

Your shifting issues might be either a worn/bent shift drum stopper or even something as simple as a loose screw on the shift drum star. Without hijacking this into an oil thread these bikes do exceptionally well running shell rotella t6. The wet clutch seems to really like this oil.

Shifting can best be described as a deliberate stomp just shy of abusive. And down shifting its easiest done by removing your foot from the peg to stomp on the heel pad.

the correct instrument lite can be can be ordered from dratv..com
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Old 07-15-2014, 08:08 PM   #4
MayerMR OP
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Quote:
Originally Posted by redprimo View Post
#2 problem sounds like a partially clogged emulsion tube.

Your shifting issues might be either a worn/bent shift drum stopper or even something as simple as a loose screw on the shift drum star. Without hijacking this into an oil thread these bikes do exceptionally well running shell rotella t6. The wet clutch seems to really like this oil.

Shifting can best be described as a deliberate stomp just shy of abusive. And down shifting its easiest done by removing your foot from the peg to stomp on the heel pad.

the correct instrument lite can be can be ordered from dratv..com
Ok great, I think the shifting really was just a me-getting-used-to-it kind of thing as it's working better and I'm starting to understand what it needs.

The main problem being the the loose shifter...gotta get the dang thing figured out.

So the Shell T6 you're talking about; the diesel oil? I used to use that stuff in my cummins - loved it!
__________________
1972 Honda CB450 (Rebuilding)
1971 Ducati 450 R/T (utter disrepair)
1971 Honda SL100
1967 Honda CM91 -> CT90 conversion
1986 BMW K75 - Storage until I figure out a way to tell the wife
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Old 07-16-2014, 11:04 PM   #5
redprimo
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Quote:
Originally Posted by MayerMR View Post
Ok great, I think the shifting really was just a me-getting-used-to-it kind of thing as it's working better and I'm starting to understand what it needs.

The main problem being the the loose shifter...gotta get the dang thing figured out.

So the Shell T6 you're talking about; the diesel oil? I used to use that stuff in my cummins - loved it!
Yep that's the stuff. not sure why but the wet clutch in these really likes that oil.
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Old 07-17-2014, 08:11 AM   #6
Rizingson
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Location: Parker, CO USA
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Quote:
Originally Posted by MayerMR View Post
Not a problem, but a question: What should the proper shift pattern be for this engine? It seems I have; 1, N, 2, 3, 4, but everything I've read denotes a N, 1, 2, 3, 4 pattern. Not a big deal, but wanted to clarify.
When you say it's a CM91, are you referring to the frame only, or is the motor # also CM91E-xxxxxxx. If the motor is a CM91 it will be a three speed with a 1-N-2-3 pattern (toe up for 1st) also neutral is a positive stop, meaning you have to push down the toe pedal twice to go from 1st to 2nd. This is a bad design IMO for any trail riding. The CT-90 engine will have the N-1-2-3-4 pattern up to 1975. Smooth shifting will come with practice as you learn to anticipate the rpm change with the throttle. I really dislike the Heel Toe shifter as well because I can't make my heel move the lever far enough, so must also resort to moving my toe back to shift, giving my leg a big cramp in the process
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Old 07-22-2014, 08:15 AM   #7
MayerMR OP
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Quote:
Originally Posted by redprimo View Post
Yep that's the stuff. not sure why but the wet clutch in these really likes that oil.
Wow, redprimo, you were right. These little horizontal Hondas just plain love that Shell T-6. I changed the oil last night and took it on a little 5 mile jaunt around the neighborhood. The difference in shifting was almost immediate

Thanks for the advice!

-Matt
__________________
1972 Honda CB450 (Rebuilding)
1971 Ducati 450 R/T (utter disrepair)
1971 Honda SL100
1967 Honda CM91 -> CT90 conversion
1986 BMW K75 - Storage until I figure out a way to tell the wife
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