Problem getting back down to idle.

MetalHed · Post by **MetalHed** » Wed Apr 06, 2005 6:35 pm

Hey all,
I just got a 1988 paso 750, and I'm having an interesting problem. This one I think is a little different than most people's carburation problems. This usually only starts happening after a couple minutes, once it gets warmed up. Whenever I bring the RPM up somewhat high for a few seconds and release the throttle, it doesn't drop all the way down to idle (I have it set at about 1400 rpm). It will stick at about 3000 - 3500 RPM for up to about 10 seconds, then it will gradually go down to normal idle. Any ideas?

This site has been very helpful for answering a lot of my questions, thanks!

Post by **Skins** » Wed Apr 06, 2005 8:24 pm

Can you tell us what kind of carburator, air filter(s) and throttle control you have?

Duck01 · Post by **Duck01** » Wed Apr 06, 2005 8:25 pm

Presume you have already checked & lubed the throttle cable......?

fasterdammit · Post by **fasterdammit** » Wed Apr 06, 2005 9:32 pm

What Duck said ...

What's the story with the bike? Garage-stored, mileage, etc? It could very well be some tired old dried out throttle cables, or a very gucky carb linkage.

If you think it's the first throttle cable, take my advice and replace it now - don't even think twice about it. Take it from me: it sucks having a throttle cable snap mid-corner. Especially trying to limp it back home with the choke ...

Post by **jcslocum** » Thu Apr 07, 2005 12:08 am

The engine is running to LEAN. This creates a floaty idle that is hard to get to return. Richen up the mixture a little at a time until it ceases or get the thing professionally tuned and it should be OK.

Post by **Skins** » Thu Apr 07, 2005 12:58 am

That is good advice. Jon is probably right.

Duck01 · Post by **Duck01** » Thu Apr 07, 2005 2:08 am

Ducati running lean at idle??? Seems a contradiction of terms......

Paul · Post by **Paul** » Thu Apr 07, 2005 1:42 pm

I have a similar problem. Mine Paso has twin PHM 40 Dell Orto's. When it is cold it idles OK. When it gets hot the tickover gets faster (up to around 2,000 revs).

I tend to notice it when I am sitting at a traffic light. What I have found is that if I pull in the clutch, engage first gear, then ease out the clutch the revs drop as the bike wants to move. Once the revs are down to where I want them I can pull the clutch in again and take her out of gear. The revs will then stay at this point until I pull away. The next time I stop it may go back down to idle or it could be at any revs up to around 2,000.

This only happens when she is hot. I have tried forcing the throttle closed (on the handle bar) but this makes no difference. I don't think that the cables are sticking or too tight as it wouldn't be rectified using the clutch.

I haven't stripped the fairing off when it is hot so I haven't seen if the butterflies are getting stuck. I've just "sort of got used to it".

Any ideas ?

Thanks

Paul

MetalHed · Post by **MetalHed** » Thu Apr 07, 2005 5:37 pm

It is a weber carb, I'm not sure what air filter I have. I have thoroughly checked the throttle cable and it isn't sticking at all. It has 34,000 miles on it, and was sitting for about a year before I got it, but I have taken the carburetor off and cleaned it all out. I think I probably do need to have the carb proffesionally tuned. I'll try playing with the carb some more, or find someone to tune it.

Thanks for all the help!

Post by **Skins** » Thu Apr 07, 2005 8:15 pm

Paul, this is what I think may be happening:

You mixture is basically good - that's why she idles OK when she's cold. She doesn't increase revs when she's cold because she doesn't want to - no bike does, that's why they have chokes.

But when she's hot, she wants to run - and she will do so given any excuse.

I think you will find, if you take the tops off the DellOrtos, pull out the throttle slides, and look closely at the little flats where the point of the throttle slide stop screw makes contact, that, probably on both slides, a little dimple has been worn into the metal (it's very soft metal).

What might be happening is that when she's idling, her vibration is causing the slides to move, because there is play where the point of the screw sits in the dimple. The slightest movement of the slides (if the mixture is good) will affect the revs. If the movement is up, the revs will increase.

I once measured slide movement vs revs at idle with a micrometer, and it was something like 0.01mm = 100 revs (give or take a couple of hundredths of a millimeter)

If you find these dimples, you need to make the flats absolutely flat again with a file. Adjustment of the throttle slide stop screw (maybe a quarter of a turn it) will compensate for the metal removed.

You may also need to eliminate any similar play in the inner and outer throttle cables from where they leave the twistgrip to where they enter the carbs.

If that doesn't fix the idle, the mixture may be lean - as Jon suggests. However the fact that idles OK when she's cold (without the choke, I assume) would indicate that the mixture is OK.

MetalHed, I'm not familiar with the Weber carbs, but I understand the recommended idle speed for them is quite high anyway. The mixture may be lean, or there may be some play somewhere. The notes above may give you some ideas.

A lean mixture can easilly be caused by a poorly-fitting air filter.

Post by **jcslocum** » Fri Apr 08, 2005 1:18 am

Clean it and adjust it. It may get better. Then again, the weber is crap to begin with.

Paul · Post by **Paul** » Fri Apr 08, 2005 4:54 am

Thanks skins, I'll see if I can look at it this weekend. I have a set of needle files that should be fine enough.

It has always needed the choke to start, even from cold on hot days. But it does idle correctly when the choke is shut after a short while. As soon as I have looked at it I will reply with my findings.

Where do you get all this time from to do things like measuring lift / revs ?

Paul

Problem getting back down to idle.

Problem getting back down to idle.

Similar problem, but slightly different

Thanks