Le Mans II idle speed increases when hot

HiI don’t know whether this is common, but my trusty old Le Mans II’s idle speed has started to jump up to over 2k when it heats up after short-ish (10-15 minutes) forays in town traffic, making it difficult to get into neutral, sounding like a boy racer at lights, etc. I’m sure this doesn’t bode well, but it seems to run fine the rest of the time. everything looks fine; any thoughts of things to check?thanks in advancekevink.flanagan2014-07-02 09:20:21

How many miles on it?

oh, about 105,000. but it had a engine rebuild about 30 k ago, and a new (well, a lot less old) gearbox this Christmas


k.flanagan2014-07-02 09:49:24

Have a look at your jets and needles.

Are they unleaded ‘converted’ valve seats…??..if not valve seat reccesing…uneven tickover…common on Beemers…

Check your carb inlet rubbers, reckon you might have splits or loose joint(s). Sounds like a classic air leak downstream of carbs. As a temporary measure / test, try taking the jubilee clips off then wrap in gaffer tape. Put clips back on afterwards obviously.


PS the LM carb rubbers don’t last for ever actually. Develop cracks eventually.

Another way to check for air leaks on the inlet tract is to get some easystart or similar and spray around the suspected area of the leak. If the revs increase, that’s where your leak is.

I had the same problem with mine after I’d rebuilt it. The cause on mine was because I had fitted the “lightest” springs into the carbs, as I’d had a selection that came with the bike to choose from. The cure was to fit some slightly heavier springs, but not gorilla strength that they originally came with.
Next time it happens, with un-gloved hands push the throttle arms on the carbs towards the closed position and see if that stops it revving. If it does, your throttle slides aren’t closing properly.
My Guzzi now ticks over always at about 500 rpm…probably a bit slow. I can only assume that when everything had warmed up that the carbs very slightly altered shape or had expanded with the heat? Is that possible?

Also, have you still got slack on your throttle cables when this is happening?ReggieV2014-07-07 11:45:57

^ Good point. I have put light springs on PHF36’s but didn’t have any sticking slides myself. But something to check.

This problem just reared its ugly on my S3…day before Mallory Park!! Inlets are metal so no leak there, gaskets OK and everything tight. Problem was very sudden so I do not think jets are the issue. The only change is the boosted 99 octane fuel I’ve got in it (don’t ask!) but unsure if this would cause an issue like this? fitted with electronic ignition but I’ve never heard of failures like this! It feels like slides not closing…but they do, needles also seated OK. Did Mike H sort hie issue?

oh yes - carbs recently serviced…

I didn’t have an issue.

Chokes not seating properly…

That would just make it rich, still think it’s an air leak downstream.

Mind you no reply from OP yet.

Same problem on Padgmans sublime S3 this weekend, swapping chokes over and making sure of the freeplay in throttle cables effected a cure Miko…x

OK, something else to check!

Big thanks unto Ricardo Bailey and Guy (with oversight from Kate) who got this fixed at MP. One choke had a worn rubber seal, the weight of the individual plastic choke lever going overcentre with possibly weak spring was enough to leak fuel past. Second track session, with bike which actually slowed down at hairpin, was somewhat less stressful!!!

You can’t half brake late on these bikes - compared with the other 70’s bikes in same session…who obviously then cleared off after hairpin. great fun!! Lafas at 6000 rpm…ho yus!!

Dang, Kate was right…

my money is on worn adv/ret springs and or sticking bob weights .

Can’t take credit for that Miko…twas Ricardo and Guy…