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Discussion Starter · #1 ·
Hi everyone,

I recently got a 1983 CM450A that has been sitting for about 2 years and I'm trying to get it back into operating condition...

I'm planning on going through the stickied "New to you" Bike checklist, but after getting a new key cut (the originals were long gone), new plugs, new fuel lines, new in-line fuel filter, a new battery, new oil, and fresh gas in an aux tank I tried starting it up to see what I'm working with. To my surprise it started more or less right up with almost no effort, however after the first second or two it immediately raced to full throttle and wouldn't come back down to idle speed. I immediately hit the kill switch and checked the throttle controls and cables and found that the throttle control was kind of loose on the bar, turning it with force makes the whole thing kind of roll on the bar, and it doesn't return to fully closed when I let go of the handle.

I started the bike again while holding the throttle fully closed but once again it raced right up to the redline immediately. This happens regardless of what position the choke is in.

My thoughts are that there is either a problem with the cables or the throttle butterfly valve, or there's a significant vacuum leak somewhere. I inspected the carb boots but they look fine. When I pulled the carb I verified that the butterfly valve cycles just fine and fully snaps closed when not engaged.

I'm going to replace both push/pull throttle cables and the control assembly. I'm also going to clean and rebuild both carbs this evening.

In the past I've had bikes that had hovering idle issues where the engine speed would either return to idle very slowly or not at all after being revved, this was caused by vacuum leaks in the fuel system... but I've never had one that goes straight to redline speed as soon as it is started. I'm scared to run the engine for longer than just a second or two since it is running so I'm nervous to do the old "spray carb cleaner by the intakes" leak test since the engine is running so high.

So, my question is what else ought I be looking at for this? The intake boots look fine from what I can see but that doesn't mean there isn't a small crack on there that is causing a leak that I just can't see...but if that's the case would it have this big of a result?
I thought for sure it was an issue with the throttle cables or maybe the throttle spring on the carbs, but once I got them out and saw just how strong that spring is and confirmed that the throttle is closing completely at idle I'm thinking it must be something else. Would this happen with a leaky or blown head gasket? It's almost acting like a runaway diesel engine but as far as I know that can't happen with these motors....right?


Thanks for any advice on where to go from here... I'm really hoping that this essentially free bike doesn't end up turning into a nightmare!
-Joe
 

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Unfortunately the usual way to disconnect the cables at the carbs is after the carbs are disconnected from the insulators. Makes it unhandy to ensure the cables aren't holding the butterflies open but I suspect that is the most likely culprit. The idle speed adjustment screw is also a possibility but seems it would have to be screwed about all the way in to cause what you're getting.

A vacuum leak sufficient to cause your symptoms would likely make it difficult to start, at least without choke.

When the perch halves are installed too far inboard on the handlebar they can cause the grip to stick instead of rotating freely. I'd start there.
 

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Discussion Starter · #3 ·
Awesome, thanks for the reply. I thought after the fact that starting it with the carb still on but the cables removed would be a good test to see if it is a cable/throttle grip issue. I'll give that a shot after cleaning the carbs... Also makes sense that since the throttle grip is already loose on the bar that it may have gotten moved to the point that the cables are holding the valve open, I'll test that as well.

Regarding the vacuum it makes sense that there would be bigger problems if there were that big of a leak. The fact that it starts right up pretty much immediately gives me hope that might not be the issue.

Thanks for the advice!
-Joe
 

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I've had a similar issue on my CB400T.KLIEHN VB31B Carb
I needed to turn the "Idle speed adjustment screw" or the "throttle stop screw" to bring the revs down.
I can't remember which way to turn it but you can figure it out pretty quickly.

Screw.jpg
 

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Discussion Starter · #6 ·
Good news everyone!

Looks like it was an issue with the cables after all. Work got busy so I didn't have a chance to do anything with the bike until today...
I opened the throttle grip assembly and found that the plastic housing that the cables connect to had broken at some point and a previous owner attempted to epoxy the cables to the grip. Somehow it was done in such a way that there was a ton of excess cable jammed inside the thing. That coupled with the fact that the whole grip assembly was very loose on the bar and twisted downward was pulling the cables too tight so they couldn't return to idle.

I replaced the old cables along with the throttle tube and assembly, then for good measure reinstalled the carb without the cables attached. With no cables attached to the carb the bike fired right up from cold and maintained a consistent idle all the way through the warm up.

I didn't have time to attach the new cables to the carb before leaving for work today, so I'm planning on doing that as soon as I get home this evening and hopefully taking the bike for a spin around the block...

Thanks again for the help and suggestions... I'm pretty relieved to see that it doesn't look to be an air leak or mixture problem after all!
-Joe
 
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