Runaway revs

[ron_in_conn]

Hello.  I'm the long-time owner of an '81 CB750 custom, and I'm having an issue with the engine getting 'stuck' at high revs (around 5K), AFTER the bike is warmed up.  Sometimes it'll slowly go back down to 'normal' idle of 1200-ish, but then it'll stick again each time I hit the throttle.  I can SOMETIMES get it to drop back down by really hitting the throttle hard and then letting go, but often it just hangs out around 4500-5000.  The  bike starts easily on choke, and behaves pretty well until it gets warm, then gets 'stuck'.  

Any thoughts on what the issue might be?  

Some extra info:  I sealed up the gas tank (rusted through a few years ago), but I was very careful to do it 'right' (relative, I know), and I also added an inline filter, which is still nice and clean.

Thanks, anyone and everyone, for any thoughts on what the issue might be, any tests I can run, etc.  I love this bike, and would like to get it running nicely again.

[Re-run]

So if you manually close the throttle, does the idle still stay up? If so, most of the time, it is an air leak in the boots between the carb and engine. Make sure the clamps are tight and then spray wd-40 around the boots when the engine is running and see if the idle drops.

Search around here, a number of people have had the issue and you might come across other ideas.

[ron_in_conn]

Thanks hugely for quick reply, and advice.  I'll search through the forums, and see what I find.  The main clues seem to be that it only happens after the engine is pretty hot, and that SOMETIMES it goes back down when I hit the throttle hard, then drop it quickly.  Manually returning the throttle doesn't seem to affect it.  Somewhere, someone had mentioned that it could also be a frayed piece of wire on one of the throttle cables, so I'll also look into that.  I've neglected the poor thing for almost a year (actually went out and bought a 2004 Shadow Aero to ride), so I'm going to take a few days off from work and give the bike the attention it deserves.  Thanks again!

[mitchberry]

could be a list of things.

1. air fuel mixture
2. idle is set too high
3. floats need to be set.
4. cables on your handlebars need to be re-routed.. too much tension on the cables.. this happens if you route the wires incorrectly or put lower bars on a bike sometimes.
5. carbs need to be synced
6. a slide is sticking in one of the carbs (needs to be cleaned thoroughly)
7. one or more rubber manifolds are cracked
8. leak / crack in a carb..  it's not spilling gas on the motor is it?

there's just so much it can be...  but those are the most likely..  i'd check in this order..

7, 6, 1, 2, 5, 8, 3, 4


that's my 3 cents..

[ron_in_conn]

Damn, you must be an Engineer, Mitchberry; that's a great checklist.  

It's definitely not leaking gas; my landlady's dog, who lives in the garage during the day, is still alive and unexploded.  I haven't touched the cables, but I certainly have messed around with the idle stop.  However, when the bike isn't 'racing', it idles OK, so I think the stop setting is generally OK.   However, you make me realize that the problem could be generally LOW idle, and maybe I'm compensating by riaising it, so that it's 'normal' idle is now 4500, but it would die otherwise.  It doesn't seem like that's happening, but anything is possible.

I'll work my way through your checklist, esp. checking manifold boots.  As I think back, this problem DID come on somewhat gradually, which could fit manifold boots, sticky slide, or generally crapped-up carbs (causing low idle, which I'm overriding with idle stop).  I DID reseal the gas tank a few years ago, but the inline filter is still clean, so don't think crud is blocking anything.  

Half of me loves the thrill of the chase; the other half would rather be riding it, or, now that I've got a Shadow, maybe selling to clear up space.

Thanks again for your advice!

[Re-run]

Did you spray the boots yet?
Also, dirty idle jets can cause idle issues for sure.
A simple thing you can try depending how bad it is, get a can of seafoam and dump in half a can and run through the tank. If it is jets, this may clear it up some.

[ron_in_conn]

Got my head into the bike this weekend, and, although I feel like a freakin' idiot, think I've got the diagnosis.  It runs great with the air cleaner out; nice, controllable idle, drops right back down after you gun it, etc.  When I put the air cleaner back in and seat it,  the engine races off and idles at 4500.

So, at the risk of sounding even more foolish - is it just the air cleaner being clogged and not allowing enough air?  Or is the air cleaner business an indicator of a different problem?

Many thanks in advance for your expertise!

[Re-run]

Well I suppose it may be possible the the filter is so plugged that the engine is pulling fuel through the mains and that could be making your idle high.
What brand of filter is it? I have had poor luck with emgo filters. Well made but much too restrictive.

[ron_in_conn]

Don't know the brand of filter; wouldn't be shocked if it was the original.  I just ordered an Emgo as a replacement (before I saw your reply).  Guess if it still sucks, I'll try something else.  Funny - I've used the "clog up the air intake" trick to get other engines started, esp. if carbs have been sitting for a while; gets even better draw than full choke.

Thanks again for assistance!

[ferroussphinx]

ron_in_conn wrote

Don't know the brand of filter; wouldn't be shocked if it was the original.  I just ordered an Emgo as a replacement (before I saw your reply).  Guess if it still sucks, I'll try something else.  Funny - I've used the "clog up the air intake" trick to get other engines started, esp. if carbs have been sitting for a while; gets even better draw than full choke.

Thanks again for assistance!

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