1981 Honda CM400 build log
 

http://ecomodder.com/forum/attachmen...1&d=1404223010

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.

http://ecomodder.com/forum/fe-graphs/graph5250.gif

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.

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?

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?

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?

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.

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.

What gear oil? :confused:

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.

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.

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.

Haha, that would be more helpful if it had a tach. Its a CM400E (E for economy). :) Thus the drums too.

You might want to calculate it then!

Sprocket change should be the highest ranking mod.
Makes for a relaxed cruise too.

I found it very hard to improve my mileage with a bike.
I blame too short a gearing since one can ride in top gear from 30 mph ...

The primaries are probably dirty and the main emulsion tuibes. The hole in the primary jet is so small you have to file the tip of a needle to get in in the hole. Put it in some tarn-X then mechanically clean that hole. Get something that will fit in the emulsion tube holes and mechanically clean them then tarn-x. WD 40 and a blast of compressed air.

Not sure if it has accelerator pumps or not. I think the 450 Rebel did and the 250. You might need to check the floats. Use a clear hose and plug it into the drain and open the drain to see the fuel level in the hose with the top above the float chamber. Make sure both are the same.

I left old gas in the GS500 too long and when I last tried to start it one of the carbs flooded. Haven't messed with it since I pulled the tank, valves need adjusting and it really needs to be stripped cleaned and repainted, frame, swing arm and rear wheel. I think I was pushing 65 MPG in warm weather, 1989 first year of the GS twin air cooled.

Honda twins is a great forum but I sold my 71 CB 350. I prefer CDI ignitions.

regards
Mech

I personally would run a 5W-30 synthetic cycle oil (not standard auto oil) in it if I was looking for mileage, even though most likely 10W-40 is the recommendation. You will get plenty of protection for a 30-grade synthetic, even though it's not as "heavy" as a 40-grade. You will be chugging along at low RPM and load, and will not be putting any amount of stress on the engine. 5W-30 synthetic will flow easier, especially before you hit operating temp. If you do short rides it will help more.

As far as the fuel goes, I would run ethanol-free in a cycle engine of that age. You can get all sorts of carb problems from running ethanol if it's not designed for it. Cenex stations in WI usually have ethanol-free 87 octane.

I would sync the carbs and adjust the idle mixture to get the highest RPM, which is slightly lean for max power and the best acceleration but should be fine for you. I might check into a 1-range hotter plug also. The stock heat range is a compromise, and you will not be operating near peak power levels for an extended amount of time so you should be fine there also.

I would add Techron Concentrate anyway. I feel it's better than Seafoam. The carbs may still need manual cleaning of the pilot jets and all of the tiny orifices.

Gearing up/taller may help, as would some lower handle bars.

What kind of tire pressures are you guys running in your motorcycle tires?

I run stock recommended pressures, but I'm not commuting and looking for MPG.

Run maybe 5 psi over the recommendation, but not a bunch more. There should be a "maximum pressure" for the tire listed somewhere.

I run 35 PSI in my bikes.

10W40 synthetic oil.

Never had a problem with 10% ethanol, even in my (sold) 1971 CB350 with factory carb jet sizes.

My GZ250 came with Metzler tires from the factory and they hold air better at 10 years age than brand new chinese tyres and tubes, many times better.

In winter I just run the bike occasionally and drain the gas and pour it in my car about once every 6 weeks, but there are riding opportunities here year round.

I have began to alter between regular and midrange fuel when it gets hot here, seems to pull better and low speed slight spark knock is eliminated.

regards
Mech

I run stock tire pressure. I know I could increase it a little, but I am too scared of the possible ill-handling effects.
It might be hard to find aftermarket sprockets for your old bike, but you never know. Cafe racer websites might be a source. As already stated, gearing is the first step. Sounds like you are all over it already.
I find that coasting is nearly impossible on a bike compared to a car. Annoyingly so, and there's not much you can do about it.

I run stock tire pressure. I know I could increase it a little, but I am too scared of the possible ill-handling effects.
It might be hard to find aftermarket sprockets for your old bike, but you never know. Cafe racer websites might be a source. As already stated, gearing is the first step. Sounds like you are all over it already.
I find that coasting is nearly impossible on a bike compared to a car. Annoyingly so, and there's not much you can do about it.

I was afraid of that. I was hoping to get more driving technique tips here versus mod ideas. I already have my hands full with my small fleet of vehicles. I'm really not looking for another project right now.

One technique that comes to mind is not spending much time accelerating. Accelerate briskly, shifting near the torque peak, and get into top gear ASAP - then minimal-throttle cruising.

I know you don't want to do a lot of mods, but lower bars and a front fairing/windshield might give some improvements. Maybe not as significant as if you were on the hwy, but something.

What RPM are you cruising at most of the time in top gear?

You do not want to use motor oil that has friction modifiers in it. This engine runs the engine/transmission and clutch in the same oil. Honda makes an oil to their specs. I usually run 15w40 Rotella in my Honda and Yamaha 4 strokes.

60 mpg is not bad for commuting. Have you had a chance to take it out on the road?

One area that I didn't see mentioned is a valve adjustment. Also maintaining lubrication on the chain is also important. When you have to change chain and sprockets add a tooth to the countershaft sprocket.

For sprockets check with

http://www.shop.pbisprockets.com/med...Catalog(2).pdf

They list sprockets for the CM400E. I've had luck with calling them with something that wasn't shown and getting them to check their cross reference to see if there is something that will fit but isn't cataloged that way.

If they don't have a countershaft sprocket look in the online parts fiche and find your model and take the sprocket part number and put it into the "where does this fit" search and you may find a different model that PBI does catalog.

cheers,
Michael

As for technique, I found it quite easy to EOC to lights but it requires to coast down with the clutch in and find your neutral when stopped or about to stop.

Rotella T is an excellent choice. It is diesel motor oil that has a cult following with motorcyclists. So much so that Shell went out of their way to get it JASO MA certified for motorcycles. Bobistheoilguy has very good things to say about it, and it is dirt freaking cheap. I run it in all my bikes.

Never met Bob but he must me some kind of smart. Rotella does work. I use Diesel grade oil in all of my gas engine too. Last winter Mobil1 was $13 dollars a gallon at Auto Zone. I use it in my diesels.

Here's Bob: Bob Is The Oil Guy | The Internet's Number One Motor Oil Site
An amazing resource for all things oil.

I took the bike up to my brother and sister in laws this weekend. That quickly put on enough miles to finish off the tank of gas. I hit the freeway, but kept it to 60 mph. That netted me a 63 mpg tank, best yet!

Well done! I once had a CB400T, which was the "sport" version of the CM, and 58 mpg was about the best I could muster. Admittedly, I never really tried hard for an entire tank. I find that I get bored hypermiling motorcycles and have never made it through an entire tank.:(
Maybe I should fill the V-Strom tomorrow and use some inspiration from you and "persevere" for at least a 100 miles or so.:snail:

My 87 Rebel 450 only had 3 tanks in my garage @68.7, but it had 6 gears. I think yours has 5. Probably be the same if I had stayed in 5th.

regards
Mech

CM 400 have the 6 spd gearbox, don't they ?

63 mpg is good.

Too bad you don't have a tach to assess where you are from peak torque ...

82 CM 450 had 6. Earlier was 5.

regards
Mech

Have you dropped the carb bowls for cleaning and tried the +5 psi and the skip-shifting?

My 2 cylinder 4 strokes give better economy with a single carb and a y manifold. I have used 1.5" pvc glued it to shape and wrapped it in carbonfiber/epoxy. So far my 700 virago has been good for 6 years. of 50+ mpg. I used a VM Mikuni carb. They have every jet available for them.

Nope, not quite yet. I haven't done anything lately with the bike but put gas in it and keep the chain lubed up.

The Civic is currently occupying all of my excess time as it is immobile at the moment with no head on the block.

Erm... skip shifting is one of those requested driving tips...

I've never understood why skip shifting would be considered for fuel efficiency. You claim best BSFC at a specific piston speed. Skip shifting would bring you out of that sweet spot more than normal shifting, no?

That being said, most of the time I do skip from 3rd or so to 5th as I'm accelerating.

I think the shifting process itself is very inefficient and the less of it the better. I do it in the car for good results too.

That would be a benefit only if you are done accelerating.