1978 CB400T High Speed Power Loss

[cleeuw]

I bought this bike about 3 months ago and until recently haven't spent much time on the highways. Yesterday I took it out for a ride and after being on the highway for less than 10 minutes at around 110km/h I noticed sudden power loss in 6th gear (the engine is a 81 CB400T 6 spd). At first I just had no power, but eventually the bike stalled out and I had to coast to the side of the road. I shut it down, let it rest a few minutes while I checked my fuses and spark plug wires, and it started right up. Ran great, then again once I hit 110km/h it died again. Same thing, started back up and I took it home at around 80km/h with absolutely no problems.

Runs great in the city - strong acceleration, slight backfiring on decel but never any stalls. Never need to choke it either.

When I got back I took pics of the plugs and it looks like the left cylinder is rich and right is lean (correct me if I'm wrong - my knowledge of this is minimal). I did some searching on here and hondatwins.net and someone had the same problem with their float levels being off (so, at high speeds, they were draining the float bowl faster than it was filling, they said). Does this make any sense? I checked the spark and put in new plugs already.

My plan of attack right now is:

1. Inspect petcock and fuel lines to make sure there's no obstruction.
2. Remove the carbs, check float level.
3. Clean carbs.
4. Sync carbs (never done this before - any links to a how-to?)

Specs: 81 CB400T engine, 6 speed, no kickstart, 2-1 exhaust (installed by PO and he drilled out one baffle to avoid re-jetting).

Any information you guys can give me would be much appreciated.

Also, I should note that the tank was clean and rust-free and that I haven't had the carbs apart since I bought it.

 

[haplo_84]

Sounds like you have the right plan of attack there, but you still may need to rejet the carbs if the po put a 2-1 exhaust on it regardless of drilling the baffle out. I would actually think that would make it worse.

 

[Swapmeet Louie]

It got hot from running lean... Thats my guess.

 

[cleeuw]

Thanks for the advice, fellas. I'm going to start with the cleaning/syncing and then do a plug chop to see if the rejetting is needed and go from there.

Louie - I know you built one of these old twins. Did you ever get a max speed on it?

 

[Swapmeet Louie]

Thanks for the advice, fellas. I'm going to start with the cleaning/syncing and then do a plug chop to see if the rejetting is needed and go from there.

Louie - I know you built one of these old twins. Did you ever get a max speed on it?

ya, it did about 110. But I installed a cb450rear wheel and sprocket.

 

[Maritime]

I lost the right cylinder to lean running 120 KPH on the highway. Stock bike, 5 Speed (81) before I knew much about bikes etc. The oil light was coming on every once and while so I would pull over, check oil, it was fine, start bike back up and ride and it was ok?? What was happening is the right cylinder was way lean due to a clogged jet and when it got too hot the oil light would come on. I was riding to work on the highway and the oil light came on, but I just checked the oil before I left, I was 2 exits from work so I kept riding figuring I would check the oil when I got there. Then bog (power loss), then crunch, crash, bike stalled and I coasted to a stop. I let it sit, then tried the starter and nasty sounds ensued. Long story short, right piston detonated and bits went all through the motor. Check your carbs but there are at least 3-4 members who have lost right piston to lean detonation on these 400s. Double check everything is clean and floats are correct etc. The power loss is the piston over heating and if you push it, you will blow it and wreck the motor, I know, I did it. I now have a 450 re-jetted with pods etc. and it runs slightly rich, on purpose. Never fouls plugs but I like it slightly rich versus the lean, they are designed to run slightly lean from factory and if you have any crud in the main jet etc. or your floats are off, it can be deadly. You are lucky you stopped and let it cool down.

Cheers,

Maritime

 

[Big R]

Could also be a clogged tank vent or a dirty fuel filter too.

 

[cleeuw]

I lost the right cylinder to lean running 120 KPH on the highway. Stock bike, 5 Speed (81) before I knew much about bikes etc. The oil light was coming on every once and while so I would pull over, check oil, it was fine, start bike back up and ride and it was ok?? What was happening is the right cylinder was way lean due to a clogged jet and when it got too hot the oil light would come on. I was riding to work on the highway and the oil light came on, but I just checked the oil before I left, I was 2 exits from work so I kept riding figuring I would check the oil when I got there. Then bog (power loss), then crunch, crash, bike stalled and I coasted to a stop. I let it sit, then tried the starter and nasty sounds ensued. Long story short, right piston detonated and bits went all through the motor. Check your carbs but there are at least 3-4 members who have lost right piston to lean detonation on these 400s. Double check everything is clean and floats are correct etc. The power loss is the piston over heating and if you push it, you will blow it and wreck the motor, I know, I did it. I now have a 450 re-jetted with pods etc. and it runs slightly rich, on purpose. Never fouls plugs but I like it slightly rich versus the lean, they are designed to run slightly lean from factory and if you have any crud in the main jet etc. or your floats are off, it can be deadly. You are lucky you stopped and let it cool down.

Cheers,

Maritime

I haven't ever seen the oil light, but it's good to know that there's a pre-existing problem to be aware of. I'm waiting on a carb rebuild kit at the moment and I'm not going to ride it until I can get everything working properly (at all speeds). Sorry to hear about your motor - at least you've got a 450 now though.

- Chris

 

[Maritime]

I haven't ever seen the oil light, but it's good to know that there's a pre-existing problem to be aware of. I'm waiting on a carb rebuild kit at the moment and I'm not going to ride it until I can get everything working properly (at all speeds). Sorry to hear about your motor - at least you've got a 450 now though.

- Chris

Yep, live and learn. I think sXe had the same issue and caught it before he wrecked the motor, just ended up with fresh bore and pistons. Not sure why these motors end up loosing the right piston but float hieght really sounds like the culprit. starving that side for fuel.

Good luck. I scored my 450 motor for $150 and to rebuild the 400 would have cost me close to $400.