Engine Races Once Bike is Warm

[motomanofgod]

Hello all,

I recently bought a 1980 Honda CB900 Custom from a guy who hasn't ridden it in quite a few years (he estimated that it had been at least 10 years). When I got it home, I replaced the air cutoff valve and then tried to start it. It started pretty hard (which was to be expected after 10 years of not being run), but cleared up after a few minutes of running. Once the engine ran for about five minutes the idle slowly began to rise, and then it reved up to about 5000 rpm. The only way to reduce the engine speed is to turn off the ignition and turn it back on again just before the engine completely stops. When I did this, the engine remained at it's normal idle speed for 15 or 20 seconds, and then began to race again. The bike also seems to idle normally when it's cold, but then revs again when it warms up. I have pulled the carbs and thoroughly cleaned them, but to no avail. I have also sprayed starter fluid around the intake manifold boots while the bike was cold (and not reving) to check for vacuum leaks, but the idle never changed. Does anyone have an idea about what is going wrong here? Thanks in advance!

[Motorcycle Forum]

[rapta619]

your choke might be the source of your problem. maby the cable is stuck.

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[motomanofgod]

Thanks for the reply. I just went out to the garage and played with the choke. The cable seems to be moving freely (though I squirted a few drops of 3-in-1 down the cable sleeve just to be sure), and the choke components in the carburetors look like they are moving smoothly. I also started up the bike and played with the choke with the engine running. When the bike was cool, I closed the choke and it almost killed the engine. Then, when the engine was beginning to race, closing the choke caused the engine to immediately race even higher than usual. Any ideas about why this would happen?

[rapta619]

im not good at explaining things, but when u pulled apart your carbs did you check the needles? im running outa ideas,I wish I could look at it myself. it might be something in the timing or plugs maby. im grabbing straws now. hopefully burn chassis sees this and can help, he's pretty bright.

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[Easy Rider]

I have also sprayed starter fluid around the intake manifold boots while the bike was cold (and not reving) to check for vacuum leaks, but the idle never changed. Does anyone have an idea about what is going wrong here? Thanks in advance!

Do that again when it is hot and exhibiting the problem.

[motomanofgod]

Thanks for your replies! Okay, I warmed the bike up and let it rev itself to 3500 rpm's and sprayed more starter fluid around the intake boots, but the rpm's didn't audibly change. I also checked the timing, but it was pretty much spot on. Next, I synced the carbs (with the engine cool enough to prevent it from reving up) but after warming up it still races. In a last ditch effort, I dumped some Sea Foam into the gas tank and ran it through the engine for a bit. After all of this, the engine still revs itself. Also, the plugs were all replaced a few days ago (though the cables were not).

Another thing that I noticed was that with the gas petcock shut off, the bike will only run for a few minutes (2 or 3 minutes tops). Other bikes seem to run much longer than that on the gas in the bowls so I'm wondering if this is another symptom of the same problem. Any ideas?

[Easy Rider]

After all of this, the engine still revs itself.

OK, back to basics.
When it does this, if you firmly twist the throttle closed, does it make any difference ?
And if you try to adjust the idle stop screw, does THAT make any difference ??
Have you looked down the carb throats to be sure the choke is not stuck part way ON ??
(That is assuming it has an old-fashioned "real" choke......which might not be true.)

[slozomby]

it sounds to me like an old spring is heating up and expanding.


here's the shop manual. not sure if ya had a copy.
Index of /publicdocs/CB900C_Manual

[motomanofgod]

Sorry for my long delay in responding; I had to go out of town for work.

Easy Rider:
Sharply closing the throttle does cause the engine speed to drop slightly, but then it climbs right back up again.
Adjusting the mass idle screw does alter the engine speed when the engine is cool (and not racing), but I haven't tried playing with it when the engine speeds up. I'll have to try that tonight.
The bike does indeed have an old-fashioned choke, but the choke is moving smoothly on all four carbs.

slozomby:
Your spring idea could indeed be plausible. And I didn't have a copy of the shop manual. Thanks for the link!!

Also, I've dumped some Sea Foam into the gas tank and run it through the carbs/engine for a while. Now, the bike seems less prone to racing when the engine is luke-warm, but it still races once it's heated up. This makes me think a jet is clogged?? What do you guys think?

[Easy Rider]

This makes me think a jet is clogged?? What do you guys think?

That would mean less gas and stallling.
Something is causing it to get too MUCH gas (and air).

If the choke plates are still fully open when that happens......AND adjusting the throttle screw will NOT lower the idle speed......then it is likely that you have a vacume leak that slowly opens up as the engine gets hot.

If the idle stop screw WILL allow you to lower the idle speed when it is racing, that might mean a clogged idle jet and the idle speed has been jacked up to compensate for it. If that's the case, the Seafoam might actually help.......or you might need some "real" carb cleaner.

[motomanofgod]

The idle stop screw does adjust the idle speed, even with the engine racing.

Yesterday, the bike would actually idle normally, and would only race if I opened the throttle enough the get the engine above 2500 rpm. Anything below that and the engine would return to its normal idle speed (1500 rpm), but it would decrease slowly. It probably took five seconds to go from 2500 rpm to 1500 rpm once the throttle was closed. This doesn't seem quite like a clogged idle jet, but I don't really know much about diagnosing carbs either. Have you guys ever seen this before?

[Easy Rider]

Have you guys ever seen this before?

Yes. I covered that in the last paragraph of my last post.

You need to give the carb(s) a good cleaning inside and out, using spray on the outside and down the throat.
THEN you need some in the gas. Berrymans B12 Chemtool is good. Drain out the Seafoam first.

Lastly, you need to get it hot enough that it "fast idles" and then turn it DOWN to normal with the adjustment screw.

After that, you might need some extra choke to get it going or to keep it running until the cleaner does it's job......over several days and a few HOT/COLD cycles.

It is still possible that you have a vacume leak. Check the boots ahead of and behind the carbs carefully.

You do have a stock air filter in place, right ??
While all this is happening, what ARE you doing with the choke ??

[intimid8er]

Maybe varnish in the float bowls? Causing the floats to stick open?

[motomanofgod]

OK, I'll try the Berrymans B12 product and adjust the idle stop screw. I've inspected the boots visually, but nothing looks out of line. (It's entirely possible that the leak is actually multiple small leaks that are not visible though.)

And the air filter is stock. I also put a new filter on when I bought the bike. And I am doing nothing with the choke when the problem is happening. It remains fully off (open) the entire time. I've never even had to use it to get the bike started.

As far as a sticking float, I actually cleaned the floats/bowls/needle valves when I got the bike.

Thanks for your help guys!

[Easy Rider]

And I am doing nothing with the choke when the problem is happening. It remains fully off (open) the entire time. I've never even had to use it to get the bike started.

Pretty much as I suspected.
That in itself is not right.
Once you get the mechanical idle turned down to where it should be, don't be surprised if you HAVE to use the choke to start cold.......and that you need to KEEP it partially ON while it warms up.
That is the normal course of things.

With everything set normally, it SHOULD die when the engine is cold and no choke is applied......or at least run pretty rough. As the engine warms, the engine speed should gradually increase and you can make corresponding adjustments of the choke toward OFF until it is fully warmed up and the choke is fully OFF.

When you get close to the point where it is working that way, you may need to adjust the idle air screws......but that can be kind of tricky with multiple carbs. Hope that isn't necessary.