850 fast hot idle

I have a bone stock 850 MK III. No mods except the prior owner has removed the clumsy air filter box and replaced it with a nifty K&N unit that was designed for Commando. It is fitted with Amals which appear to be original units. Timing and points are spot on. Condensers have been changed. It has some oil consumption but it does not affect the spark plugs, and never fouls. It does not smoke. The odometer is at 14,000 miles and is original. The bike is in excellent overall condition.

It runs very well. It starts fine and will idle perfectly with air slides out of the picture within 15 seconds. The idle is "normal" until the bike has ran, perhaps 15 minutes at speed. After this 15 minute "warming", once I stop, the idle will begin to rise to around 2000 RPM and stay there. If I bump the throttle, it will come down and then begin to climb again. It has been doing this for several years.

I have checked for vacuum leaks and do not seem to have them. My next step has me leaning towards evaluating slide wear.... possible heat expansion of the carb body but I figure it would be best to ask 1000 others before I started.

I would imagine I am not the first guy to have this happen. Anyone seen this syndrome?

Yes your not the first. If you dismount the carb leaving it otherwise assembled. Than grab the slide with the thumb and index finger with the slide sitting on the idle screw. Than rock the slide back and fourth in the direction of air flow you will find much more than the .003-.0035 of engineered clearance. The bodies and slides can be re machined with a brass tube on the slide giving back correct spec for clearance about 65.00 US plus shipping Via Bruce see at http://tritonmachining.ca/
When fixed like this they will give much more than 14000 miles service.
Your slides are cocking on their bores. If left like they are you may find pieces of the slide have made their way through your motor sometimes with bad luck for the valves. So it's not just a idling issue.

Ou812. it's a small world.
Are you the same guy that owned a CR500?

0u812 said:

It runs very well. It starts fine and will idle perfectly with air slides out of the picture within 15 seconds. The idle is "normal" until the bike has ran, perhaps 15 minutes at speed. After this 15 minute "warming", once I stop, the idle will begin to rise to around 2000 RPM and stay there. If I bump the throttle, it will come down and then begin to climb again.

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norbsa48503 said:

If you dismount the carb leaving it otherwise assembled. Than grab the slide with the thumb and index finger with the slide sitting on the idle screw. Than rock the slide back and fourth in the direction of air flow you will find much more than the .003-.0035 of engineered clearance.

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My bike (a 1973 850) does something similar occasionally. I do not have air slides fitted. I have a K&N element in the original filter box. I have Amals, but they are new; less than 1000 miles. I bought the carbs from Norvil, and have since received comments that cast doubt on the wisdom of that. I also bought and fitted the chromed slides (from Hitchcocks) because I wanted to avoid premature wear on new carbs. The idle climbs. I blip the throttle and it falls back to normal. Sometimes it will climb again, sometimes not. I figured it was sticky cables (that are also pretty new) that I should lube soon. I didn't think new carbs and new slides would be the problem, but maybe they are.

I know this doesn't answer your question, ou812. More a description of similar symptoms that might, or might not be related.

if you spray wd40 at the slide with no throttle it will seal the slides from air leaks. Your idle will drop. Or push them towards the block with your finger. Either way the slides WILL be worn. The suction of those big cylinders are trying to suck the slides into the engine. That plus vibes and lack of lube just kill them. Norvil say new carbs every 15k miles! I say get chome slides with new carbs and they will last 30k. Then get new bodies.
Best thing I did to my 850 was buy a new set of carbs.

MichaelB said:

Ou812. it's a small world.
Are you the same guy that owned a CR500?

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No, Not the same guy..... just another happy limey owner in NC, USA..... Thanks.

bigstu said:

if you spray wd40 at the slide with no throttle it will seal the slides from air leaks. Your idle will drop. Or push them towards the block with your finger. Either way the slides WILL be worn. The suction of those big cylinders are trying to suck the slides into the engine. That plus vibes and lack of lube just kill them. Norvil say new carbs every 15k miles! I say get chome slides with new carbs and they will last 30k. Then get new bodies.
Best thing I did to my 850 was buy a new set of carbs.

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Thanks, I thought i was headed towards the slides but these forums sure bring a lot of experience to the table! I will check when nice and hot.

Sticking AA unit...

batrider said:

Sticking AA unit...

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Thought about that but could that be if the idle came downwhen blipped, then slowly came up? Also only when hot? I will pull the cover and look though. Thanks.

I'm with you BatRider. It sounds like classic auto advance problem. My two must have mods are sleeved carbs and an electronic ignition. Those two items will transform the starting and idling.

I have a couple Commandos that don't have sleeved carbs - yet. But they all have electronic ignitions and all idle smoothly.