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Old 07-01-2014, 09:59 AM   #1 (permalink)
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1981 Honda CM400 build log



I've been riding my 1981 Honda CM400 (CM400E) a LOT more this year (and having tons of fun while doing so). I have a few tanks under my belt now, and I am thinking about what I can do to improve mileage.

My daily commute is 7 miles one way. About 1 mile of 25/35 mph followed by 5 miles of 45 mph, and then another 1 mile of 40/25 mph.

The carbs on the bike aren't running 100%. I get some hesitation on mid range power on cold starts (low or high throttle input gets me going). I'll be taking them off this fall for a good through cleaning. It was running nearly flawlessly last year, but I didn't drain the carbs during winter... dumb move I know. When the bike is warm it runs very well with the occasional back fire at low throttle inputs. It also starts very well all the time.



Thus far, I've been getting just below 60 mpg which seems average when looking at our garage of motorcycles of similar engine size.

Obviously, I know the basics, but I'm looking for more bike specific things. Lately, I've been engine off coasting to stop lights/signs at city speeds to avoid idling. Obviously keeping the speed down helps. Additional technique tips would be greatly appreciated.

At this point I'm not looking to do any real involved mods as I have enough other projects. If its quick and easy I'd be interested though.

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Old 07-01-2014, 10:03 AM   #2 (permalink)
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I haven't ridden since I sold my one and only bike - a Ninja 250 - about 10 years ago.

But I'd be interested in hearing about tire pressure. What are you running and how did you decide to set it there?
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Old 07-01-2014, 10:07 AM   #3 (permalink)
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Right now I'm running factory recommended tire pressure. I'd be interested in bumping it up, but am a bit hesitant due to how it will effect traction. What are others running?
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Old 07-01-2014, 10:08 AM   #4 (permalink)
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Not a technique tip, but I would add some Techron Concentrate fuel system cleaner (1 oz per gal) for the next tank or two. If you have some moderate deposits it may clear them out. How do the plugs look (before running Techron)? Have you made adjustments to the idle mixture to optimize idle and low speed running?

Are you running ethanol-free regular?
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Old 07-01-2014, 10:19 AM   #5 (permalink)
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I've run a generous amount of seafoam through it over the past couple of tanks. I think its going to require manual cleaning.

No, I haven't messed with the carbs too much. In fact, they're only tuned per my highly calibrated ear. The plugs do look great though. They're always a nice light tan.

I run 87 octane ethanol gas.

I also checked and adjusted the valve clearance earlier this year.
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Old 07-01-2014, 11:25 AM   #6 (permalink)
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The next thing to check is the lubrication systems for wheels, drive train, and engine. The bearing grease could be upgraded to include moly disulfide as in CV joint grease. The gear oil could be multi grade. The engine oil could be a 0w-30 synthetic. Some folks have claim a non O-ring is more efficient.
The bolt on mods are usually a gear change and aero improvements. A touring fairing and a good size cargo bag on the seat help. Anything to lower the shoulder height by angling the back forward or backwards from vertical will reduce frontal area.
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Old 07-01-2014, 12:45 PM   #7 (permalink)
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What gear oil?
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Old 07-01-2014, 12:51 PM   #8 (permalink)
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Frank is right. The gear and clutch oil is the engine oil. There is only one oil in it. I used a cheap non-synthetic non-energy saving oil as per some reading on hondatwins.net. Anything too thin isn't a good thing for the clutch because it wasn't designed for it. Thats what they say at least. I believe the weight was 10W-40 or 15W-40 which is what is recommended for the engine.
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Old 07-01-2014, 01:23 PM   #9 (permalink)
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You are darn lucky the lil guy has drums! Bike disks are sooo draggy.

At those speeds I wouldn't go out of my way to work on aero. Pump up the tires 5psi at a time and see what you think. Chain tension is critical; better to be a bit loose than a bit tight. Be sure to check tension while you are sitting on the bike, and roll fore and aft, checking at several points. Sometimes things aren't perfectly concentric and there may be a tight spot. Get each wheel up in the air and give it a whirl to make sure nothing is dragging.

Fix the carbs. You may not even have to disassemble; drop the bowls and take the air filters out and shoot carb cleaner in there. I like to blast the carbs with compressed air after doing that.
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Old 07-01-2014, 01:52 PM   #10 (permalink)
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RPM sweet spot should be 3000-3600 (that is 1000-1200 f/m piston speed).

You might be able to skip-shift ie. 1-3-5.

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