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The large knob is indeed the idle adjustment, air/fuel setting is factory set by the pilot screw covered up with a limiter cap. Unless you are replacing the pilot screw, don't touch it. It is not the same as adjusting a lawnmower...

FIRST... get a compression test, or you may do it all for nothing. I purchased a motorcycle compression test kit on Ebay, very easy to do, but beware of the fact your spark plug are inserted at a slight angle toward front (practice this with a plug you insert rolling with your fingertips first). Otherwise, you or a mechanic may cross thread your spark plug, which will require a major repair or replacement of the engine head (I've been through it). You also need to adjust the valves every 8000 miles, or they may burn and cause major damages to your engine (especially if you are running with an overrich mixture).

I suggest you get your muffler pinhole soldered, replace your jets and perform a carburator synchronization.
Your carb is a Constant Velocity type, 30mm, from Keihin, model number VB22A
Primary jet is a 72, secondary is a 118. Initial pilot screw adjustment is 2 turns out (if you must replace it). It would be smart to get new carb bowl gaskets and replace both float needles too. This can be done without much risk for damaging your carb. If you notice the rubber plug next to the jets is degraded, you may replace it too as a precaution.

After 28 years, It would be smart to replace the original rubber gas line with a new one to make sure you don't get rust or rubber debris into your carb after you cleaned it. 1982 and up versions already have fuel strainer (rough screen filter) into the petcock valve. Get the same benefit by adding one now !Personnaly, I also added a small transparent conical fuel filter between the petcock and the carb fuel intake. This is cheap insurance and it allows you to monitor fuel flow problems instantly.

You may want to have a look at this site to get an idea on what you are dealing with: http://www.gadgetjq.com/keihin_carb.htm Yes, Keihin carbs are used on Harley Davidson bikes too, so it is easy to find someone who can help with a complete overhaul.

I strongly suggest you get a copy of "Clymer Honda 400-450cc Twins 1978-1987". It contains basically all the procedures you want to do. Honda's OEM shop manual is useful for some specifications, but information is scattered and explanations are very limited.

I don't recommend you completely dissassemble your carb as on
http://dirtbike.off-road.com/dirtbike/a ... ?id=413750 because yours appears to simply be off because the jets are damaged.

One last critical detail: Make sure you use carb cleaner that is SAFE FOR RUBBER!. Stay away from GUNK unless you are replacing all rubber parts, they will swell and desintegrate.

Have fun !

Martin

system9 wrote:The bike is a 1981 CM400T
The one thing i'm not familiar with, are the 2 screws that sit on the bottom of the carbs. They are adjustment screws, but i'm not sure where they should be at. There is also a rather large knob type screw, that directly adjusts the throttle, i guess its for setting the idle? or is it some kind of air/fuel setting? see, between the 3 different screws, there is plenty of head scratching.

[quote="lamimartin"]The large knob is indeed the idle adjustment, air/fuel setting is factory set by the pilot screw covered up with a limiter cap. Unless you are replacing the pilot screw, don't touch it. It is not the same as adjusting a lawnmower...

I did get myself a clear fuel filter earlier, as i was worried that the inside of the gas tank may be a little corroded. Fuel flow has been good though.
Where would be a good place to but the carburetor pieces? ebay? or is there a good online store you know of thats reliable?

I beleive you can get it from Honda Dealers.
You can also find some on Ebay but not 1981. Essentially, all CM400 carb kits are the same except for jet sizes and float needle type.
See http://crc2onlinecatalog.com/cmcb400-450info.htm for details

www.Siriusconinc.com use to carry them all, but they only have 79 kits lately which does not have the right float needle for you. You could order float needle and jets separately from Honda Dealers.

You've got the detailed specs, check on with Google and good luck.

I picked up a 1980 CM400T and the carb is really bad.

What kind of cleaner do you recommend I use?

Also, what other models of bikes are compatible for replacement carbs?

Thanks

Crash

Crash007 wrote:I picked up a 1980 CM400T and the carb is really bad.
What kind of cleaner do you recommend I use?
Also, what other models of bikes are compatible for replacement carbs?
Thanks
Crash

Hi,
If the carb is really bad, it needs to be cleaned with Gunk which will destroy all rubber seals and parts. Next, you have to find replacement parts to refurbished it to new. This operation is rather risky because some spare parts are hard to find and that reassembling and readjusting a carb from scratch is not easy, to say the least. You may also consider a professional refurbishing to reduce the risk to ruin this critical component.

Unless you are a pro mechanic, it is failry safe to limit your level of carb maintenance to a new carb float, main and secondary jets and new float bowl gasket. This is only possible if the internal rubber parts are in an acceptable condition and if nothing is jammed solid.

My best bet would be to check for one in better condition on Ebay, enough to be salvaged by cleaning it with any good quality Carb Cleaner SAFE FOR RUBBER PARTS). Any carb for CM400 will do as long as you change the jet sizes which are sometimes different. The major other difference is the float needle type (you must know the year of the original bike to order the right replacement part).

CM400 will accept some parts from the CM450 (including the whole engine) and a few from other bikes like CX400.

Major incompatible differences between CM400 versions are brake systems (single or dual piston, or even drum for the front wheel), wheel types and automatic transmission (CM400A) vs manual engine.

I hope it helps.


Martin

It does ty. I am going to try to clean up the current ones, not having much luck finding complete replacements.

Whats the brand name or link to the carb cleaner that is safe for rubber parts?


Thanks

Crash

Crash007 wrote:It does ty. I am going to try to clean up the current ones, not having much luck finding complete replacements.

Whats the brand name or link to the carb cleaner that is safe for rubber parts?

Thanks

Crash

What I have is the OEM Honda brand Carb Cleaner. I'm not very hot about OEM brand name oil and stuff, which is usually overpriced, but so far I found nothing else that says "safe for rubber" on the label. This is a product that has been strongly recommended to me by a Motorcycle mechanic for that reason.

The 1980 version of Kehin carb rebuild kit is currently hard to find. You may order the 1979 version and get the float needle and jets separately.

Will a 1985 Honda CB450SC carb be a valid replacement for a 1980 CM400T?

Crash

Crash007 wrote:Will a 1985 Honda CB450SC carb be a valid replacement for a 1980 CM400T?
Crash

Unfortunately, this carb is the only one that has a completely different range of primary and secondary jets. I doubt it can be made compatible. Main differences are jet sizes (you will have to readjust), and the presence or not of a fuel pump (marginal performance improvement, not at all essential).

The entire list of specs can be found at page 230-233 of Clymer's Service rapair and mantenance manual. This is a MUST to have one with such project.

I have the bowls off both carbs, floats needs jets out for cleaning now.

I do see a problem, not sure how to resolve.

When at idle, the 2nd carb is not fully closed, on the side connected to the engine. I adjusted the middle screw and when i open throttle both are about equal. When I go back to idle, the 2nd one still remains open some. When I throttle the lever pulls them open the same, but as it goes to idle the 2nd one has nothing to pull it back that little bit I had to adjust to get them even in the first place.

Gummed up rod, spring tension or something?

Thanks

Crash