[Qpermenter]

So i have an 81 cm 400t that i love! But I’m not getting the her i right power out of it. I’m maxing out at about 70mph with 9500 rpm. I thought it was a vacuum leak but bought wd40 and sprayed around the carbs and boots but there wasn’t an rpm change like it should when there is a vacuum leak. I know my carb needs a little tuning for the backfire but it’s close. Is there another’s spot i need to check for vacuum leaks? Or what could be getting me such low power? I’ve heard they are supposed to get up to 90mph. Do i need new jets? Or should i try new boots? Or what’s going on?

 

[Alan F.]

I'd go through the ignition system wiring, make sure every connection is clean and shiny. Then go through the ignition troubleshooting thread. You may also need to clean your ignition pickup coil.
I'm uncertain if the present formulation of WD40 is useful for checking vacuum leaks, it hasn't been flammable for decades.

 

[80cb400t]

It worked for me on a carb boot with a big split and it was only about a 100rpm increase (from 3000 rpm to 3100 rpm lol). For a little vaccuum leak something more flammable would probably be better but wd40 can work

 

[J-T]

70 mph at 9500 rpm? Should be more like 7000 rpm in 5th gear. It would take significant gearing (sprocket) changes to get what you're seeing, or a badly slipping clutch. How is the performance at lower speeds? Good acceleration? Power increasing as rpms increase, particularly passing about 5000 to 6000 rpm?

Welcome to the group.

Got any pictures of the bike?

 

[dtsmjr8dan]

RPM is too high for speed you're going. As above. I suspect a slipping clutch.

 

[Qpermenter]

So how would I go about checking for a slipping clutch or what would the symptoms be for that? I dropped the back gear by 2 teeth and it accelerates fine through the lower gears but I am changing gears at 7000RPM which sounds a little high to me? And its smooth switching between gears as well. I was thinking about rejetting it because it does have pod filters on it but I don't think that would cause that much of a power loss. I bought it with the pods on it so Im not sure.

 

[Qpermenter]

its got fine acceleration, and I just dropped it two teeth in the back because I thought it would help but it didn't do anything for the top speed. Would rejetting the carbs for the pod filters help or would it just be "eh" for power and top speed? how do I detect a slipping clutch?

 

[longdistancerider]

Dropping 2 teeth in the rear is roughly the same a a 1 tooth increase in front which is done to lower the rpm at a given speed. 70mph at 9500 rpm is only possible if the clutch is slipping or the sprockets were completely wrong. 16/35 is the correct ratio.
To test for clutch slippage ride at 25-30 mph, shift to 5th gear, now try to accelerate. If the engine revs w/o much of a speed increase if any then the clutch is slipping.
If your gearing is 16/33 like you suggest then in 1st gear at 30mph you'll be at @7000 rpm and when you shift to 5th the rpm will be @2400, way below the power band which will put a severe load on the clutch as a test. Never do this test more than once.

 

[Qpermenter]

Okay, that would make more sense. I do have to shift through the gears a lot quicker than I was thinking that I would have to. I'll test it out today and get back to you all about what happened when i tested it.

 

[Qpermenter]

So I just went out and did the test that you suggested. i ran it to 25 mph and then put it in fifth. It didn't pick up like it normally would, but it did accelerate slowly. But, when I put it in 5th the RPM still showed around 5500 not 2400. I know that my tach is reading correctly because when I take off its at about 1500-2000. Does this men that the clutch is slipping?

 

[Qpermenter]

So, I tightened up my clutch a little without opening up the whole engine just using the wire and it did just about nothing. I tried the clutch slip test again and it dropped to about 4500in fifth at 20mph and slowly accelerated but didn't rev up through the roof or anything. Im at a loss.

 

[80cb400t]

What kind of oil do you have in it? It doesn't really sound like your clutch is slipping from the test. Also I didn't see anyone pick up on the pod filter thing. If you have pod filters and stock jets you should probably rejet and read the sticky on making them work, then see how it runs.

 

[Qpermenter]

what do you mean read the sticky? and i think I'm running 20-40 on it but planned on doing an oil change just didn't want to change it and then have to change it again to fix the clutch. I ordered a rejetting kit from a different thread on here and will rejet it and then tune them so they are all set and then go from there unless there are other suggestions?

 

[80cb400t]

https://www.hondatwins.net/forums/6...4-cb400-cv-carbs-pods-how-make-them-work.html

This thread

20-40 JASO MA motorcycle oil right

 

[Qpermenter]

perfect! ill give it a shot!

 

[76TWIN]

You cannot just use regular motor oil in these bikes.
Modern motor oil contains friction modifiers that will cause your clutch to slip.

Only use oil rated for JASO MA

 

[longdistancerider]

"stickies" are posts found at the top of each section and are always there. They are specific situational threads like the "How to make Pod Filters work" in the Fuel section or the "Basic Checklist for the new to you" in this section.
Honda spec for oil is 10-40w for under 85 degree ambient riding temperatures, 20-50w for over that. Always use JASO MA rated oil, you may find MA/MA-2 which is ok but MA-2 alone is not ok.

 

[JamesPal]

How do you know the tach is OK? There is something weird going on. On my 450 the tach was close at idle, got way behind between 3,000 and 7,000 RPM and then shot up near redline. I added a digital tack for accuracy.

 

[Qpermenter]

How would I check to see if the Tac is not working properly? or would I just have to buy a digital one and then see if it works like that? I ordered the new jets and everything from a pervious thread and they should be coming in this week or next so I can see if we found the problem.

 

[longdistancerider]

Auto parts stores rent out tachs, timing lights. etc. Use one of those for checking