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Discussion Starter · #1 ·
Hello all. Fixing up the new/old bike we picked up a few days ago. (1981 CM400C) The specs say spark plug gap should be 0.026 to 0.028 so that's where I set it. But in searching around on this forum I've seen mention of a 0.035 gapping. She fired up well and throttled up smoothly when we picked her up Sunday. The plugs that came out are gapped between 0.03 and 0.035. Put in the new plugs and it was a difficult start up, tried it w choke and w/ out. Once it finally fired up the engine would stumble and bog down when rolling on the throttle. Didn't smoke or back fire. And once it got over 2500 RPMS it seemed to run better. Would you say that sounds like the result of too narrow of a gap? I'll pull the plugs tomorrow and see if there's any sign of unburned fuel.

Thanks!
 

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Discussion Starter · #6 ·
Those are the plugs I'm using. Thanks for the link I'll check it out! So I just cleaned the old plugs swapped them out and fired up the bike which I put on a battery tender last night. Fired up, stuttered for a few seconds and died. Fired up and stayed on the second attempt. Same rough bogging down until I got it up around 4500 rpms then it seemed to smooth out, rode it up and down the street a few times. I then swapped out for the new plugs again and it fired right up. It was running smoothly right away. Which leads me to one possible conclusion. The bike needed to be warmed up, and had a tired battery the other day that added to the issue. I've just taken her around the neighborhood and down a long dead end road going through all the gears and taking it up to 55pmh. Seems like the engine wakes up and likes to cruise around 5500 and 6000 rpms. Where do you normally keep it at when riding? It's been a decade since I've been on something with less than 100hp so it's hard to judge right away where the sweet spot is on the power curve.
 

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First, your bike has a CDI ignition with it's own power source. You can remove the battery completely and not affect the ignition.

Coming into the power band around 5000rpm is pretty normal, but it shouldn't be really bogging below that.

Needing to warm up before it will run (along with your initial symptoms) sounds like a carb issue, just seems like quite a coincidence that it would show up right after you changed the plugs.

When you initially pulled the wires off the plugs you may have disturbed the connection from the wires to the caps. Worth checking out.
 

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Discussion Starter · #8 ·
First, your bike has a CDI ignition with it's own power source. You can remove the battery completely and not affect the ignition.

Coming into the power band around 5000rpm is pretty normal, but it shouldn't be really bogging below that.

Needing to warm up before it will run (along with your initial symptoms) sounds like a carb issue, just seems like quite a coincidence that it would show up right after you changed the plugs.

When you initially pulled the wires off the plugs you may have disturbed the connection from the wires to the caps. Worth checking out.
Just so I'm not misunderstanding this regarding the CDI. I can start my bike without a battery? I swear a bike can tell when it has a new owner, all kinds of little things crop up that hadn't been an issue before. Thanks for the suggestions, carbs are next one list once we get the front brake sorted. Seized pistons being pulled at the shop. Hopefully they'll be okay once we get them out and cleaned. No sign of pitting from the exposed sides that we can see. On another side note, I can't imagine why this would matter but I did just change from (what I assume was) 10-40 to 20-50 weight oil. That should not effect anything we're discussing right? Again many thanks for all the shared wisdom and info.
 

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20-50W is called for when riding in 85+ degree weather, below that is 10-40W. Oil weight isn't the problem.
I'd suggest adding Sea Foam or Berryman's B12 to the fuel following directions and run it for a couple of tanks. The VB22 carbs on your model have an accel pump that when inop can cause a flat spot.
Yes, the CDI system works w/o a battery. It shares components like the stator for power and timing as well as the ignition switch.
While you're doing the brakes invest in a SS braided hose, makes a big difference
 

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Discussion Starter · #10 ·
20-50W is called for when riding in 85+ degree weather, below that is 10-40W. Oil weight isn't the problem.
I'd suggest adding Sea Foam or Berryman's B12 to the fuel following directions and run it for a couple of tanks. The VB22 carbs on your model have an accel pump that when inop can cause a flat spot.
Yes, the CDI system works w/o a battery. It shares components like the stator for power and timing as well as the ignition switch.
While you're doing the brakes invest in a SS braided hose, makes a big difference
You sir are a treasure of information. In fact braided lines are being fitted to the new banjo bolts presently. It was a balmy 80 degrees here today in fact, a bit unseasonable but showing no signs of stopping. As the bike was acquired for warm/hot weather riding I opted for the 20-50 instead. And knowing my wife she's never going to take it out on a cold or rainy day. Any preference between Sea Foam or Berryman B12?.

Cheers and many thanks.
 

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If its been standing for some time particularly in your heat, fuel system can be problematic, but, check out the ignition timing and advance, as well as a tune up, you know about the balance chain system ?

Carbs and fuel system are the last items to check/clean/service etc..

Check ign advance to rule that one out., cold ( none ?? ) and when hot ( now works and bike "takes off" ).
 

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Discussion Starter · #12 ·
If its been standing for some time particularly in your heat, fuel system can be problematic, but, check out the ignition timing and advance, as well as a tune up, you know about the balance chain system ?

Carbs and fuel system are the last items to check/clean/service etc..

Check ign advance to rule that one out., cold ( none ?? ) and when hot ( now works and bike "takes off" ).
A lot of this is new to me, it's been 27 years since my last CM400 and all I had to do to that one was oil and chain maintenance. With only 15k miles in the last 37 years I shall assume there have been a few points of the bike just sitting there.
 

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Bike looks good after a clean up. LDR is the site expert on the SOHC. A good carb and fuel tank cleaning is important for bringing these old bikes back to life. I always ultrasonically clean the carbs, remove the throttle shafts to clean the bushings and lube the shafts and felt seals, then blow out all of the air passageways. I just competed bringing an '85 GL1200I back to road worthy. It had been parked for 11 years. This bike has very similar carburetors. I got it running last summer, but it had a flat spot below 4K RPM and would come on after that. It turned out the rubber on the carb boots had hardened and would not seal. I also replaced the carb diaphragms. Now it runs great.
 

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Good day. You got the part number of that GM coil module. i had heard they work fine. to date i haven't a problem but certainley wouldn't mind seeing it and seeing if of course its a direct fit or larger or smaller. Kinda like a plug and play??
 

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Discussion Starter · #16 ·
No real preference on B12 or SF, I've used both. Grew up with B12 so I tend to grab that first.
Picking some B12 up tomorrow, thanks. Side note question. What front cowl do have on your "road bike"? Looking for something to make the upcoming road trips a little more comfortable.
 

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Picking some B12 up tomorrow, thanks. Side note question. What front cowl do have on your "road bike"? Looking for something to make the upcoming road trips a little more comfortable.
It's from a 1996 Ducati Monster, same for the headlight.
 
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