Compression CM400

jcharlet

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Jun 9, 2020
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Location
Greensboro, NC
I've just gotten a compression tester and tested compression on both cylinders on my '80 CM400T. Bike hasn't run in 18 or so years. I have not yet adjusted the valves, either.

Readings are cold.

Lt cylinder 158 psi
Rt cylinder 149 psi

Am I looking ok here? I know these are below spec (185 +-14 acc to the FSM) but again, this is cold and valves aren't yet adjusted. Oil is fresh.

Thanks!
Josh
 
Meaningful compression readings require that the engine be in proper operating condition and at operating temperature.

With your readings under those conditions I would proceed with getting it running. Assuming no major problems like billowing smoke or excessive oil consumption I'd ride it a few hundred miles and then do the compression test.
 
Thanks guys. Waiting on the new programmable CDI and new MOSFET regulator/rectifier before I can proceed. On a side note, given that the timing curve of the CDI will be programmable, is it possible to modify the curve to take full advantage of high-test gasoline?
 
Yes, you can set the advance anywhere you like. I haven't changed my programming since the road trip in 2014. Because of the potential for getting bad quality fuel somewhere I ran the timing a bit late, full advance of 43* at 5K instead of 4500, and also retarded the timing 1* at 9K and another 1* at 9500. Rev limiter set for 10,500. I also ran base timing of 10* instead of 15*.
When I get around to reprogramming I'll be setting it for full advance of 45* at 4500. Spec is full advance of 43* +/-2* at 4500 to 5350 rpm.
 
Awesome. Your post on the electronic upgrades is the only place I have seen any timing curves. Would this be a good place to start with mine, these same settings?
 
When I mention the road trip this was the the follow up from 2011 and finishing riding thru the lower 48 states and into Canada. I didn't know for sure I could get quality fuel everywhere so I went with conservative programming. For yours I would run base timing at 15*, full advance of 43* at 4500, essentially bone stock. I would delay initial advance to 1500 or even 2K. you don't want a lot of initial advance as the engine is put under load leaving a stop.
 
Cool, that's where I'll start, then. At least initially, I will be staying local. There's a local station with ethanol-free 90 octane that I will be using exclusively while I get it running, tuned, and learning how to ride.
 
In truth there is no benefit to running higher octane fuel in these engines. There's actual documentation on the internet of power loss with some engines, not ours, due to the slower burn characteristics of high octane. Now if you cross the 10.5:1 CR threshold higher octane is needed and when you get into the 12.5:1 CR even higher is needed.
 
That sounds reasonable. Honestly I was just kind of curious about the possibilities given that the new CDI will allow for programming of the timing advance curve ... but for now I want to "mimic" bone stock as much as I can. The more important consideration for me was the ethanol-free option I have locally ... however it ONLY comes in 90 octane. I figure the benefits of no ethanol 90 octane will be better than 87 octane with up to 10% ethanol (i.e. the assurance of no ethanol is more important to me than the octane rating). For what I just went through cleaning those carbs, I want to do everything i can to minimize varnish.
 
For example my anecdotal "evidence" has been that I've had a lot less carb trouble in my push mowers since I started using the ethanol free fuel... seems to be much easier to keep them clean, and the mower runs much better long-term. I've also added an in-line paper filter, which I plan to do on my bike as well.
 
Running ethanol free fuel is definitely worthwhile. Ethanol contains extra oxygen molecules which when burned create an inherent lean burn. Our bikes are already jetted for lean burn and making it more so doesn't help any. Your lawnmower is suffering from the lean burn so that's why the free fuel works better.
This is also one of the considerations used in recommending the primary main jet size increase. The actual correct way would be to increase the slow/idle jet 1-2 sizes but the press in jets to do that haven't been available so the next best thing is the primary main.
If your petcock filter screen is intact and installed there's no need for an additional filter on these. I've been all over the country, Baja and Canada an never had a dirt contamination problem with the carbs.
 
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