Suzuki gs low compression.n eed help

[mr72]

Didn't know low compression could cause hanging idle as well. Interesting.

It did on my bike!

I have theories as to how and why, but I can't test or verify any of them and frankly I have ceased to care. It is a very durable opinion that motorcycle carburetors are enigmatic and pointless to try to understand, and the internet is so loaded with wrong information about motorcycle tuning that it's useless as a resource.

Who can explain why a lean mixture causes hanging idle? There's good info here: http://alt.motorcycle.sportbike.narkive.com/oHTaYnXK/blip-throttle-and-revs-hang-rich-lean#post4 but this suggests that the hanging idle is caused by a RICH mixture.

So I am not a true believer that in all cases hanging idle means a lean mixture. And it's a red herring anyway since in a 20+ year old motorcycle the likely cause of a lean condition is vacuum leaks which really add a difficult to diagnose wrinkle in this, enticing the home mechanic to enrichen the mixture which will only make things worse.

[qcbaker]

Didn't know low compression could cause hanging idle as well. Interesting.

It did on my bike!

I have theories as to how and why, but I can't test or verify any of them and frankly I have ceased to care. It is a very durable opinion that motorcycle carburetors are enigmatic and pointless to try to understand, and the internet is so loaded with wrong information about motorcycle tuning that it's useless as a resource.

Who can explain why a lean mixture causes hanging idle? There's good info here: http://alt.motorcycle.sportbike.narkive.com/oHTaYnXK/blip-throttle-and-revs-hang-rich-lean#post4 but this suggests that the hanging idle is caused by a RICH mixture.

So I am not a true believer that in all cases hanging idle means a lean mixture. And it's a red herring anyway since in a 20+ year old motorcycle the likely cause of a lean condition is vacuum leaks which really add a difficult to diagnose wrinkle in this, enticing the home mechanic to enrichen the mixture which will only make things worse.

I would agree that carbs are enigmatic, but I don't think its pointless to try and understand them. As for why different f/a mixture conditions cause certain symptoms, I haven't a clue. All I know is that I've read all the time that a hanging idle is usually due to a lean f/a mix; no clue as to why that's the case. For all I know the carbs just contain a tiny sorcerer who casts a spell to get the fuel and air to mix, and if they didn't pour enough goats blood into the goblet or they didn't sprinkle enough unicorn hairs into the fire or whatever, then you get a hanging idle.

[gsJack]

Interesting qc and mr, I bought my 97 GS new in the spring of 99 and that summer on a very hot day I had hanging idle.  I adjusted the problem out by turning the idle speed down very slowly until it dropped off while sitting on the bike at a stop and did that another time or 2 until it was gone never to return for the 4 years and 80k miles I rode it.  I've always attributed it to the lean jetting which I never changed.

[mr72]

Interesting qc and mr, I bought my 97 GS new in the spring of 99 and that summer on a very hot day I had hanging idle.  I adjusted the problem out by turning the idle speed down very slowly until it dropped off while sitting on the bike at a stop and did that another time or 2 until it was gone never to return for the 4 years and 80k miles I rode it.  I've always attributed it to the lean jetting which I never changed.

OK this is a really good point. You had the idle speed set too high and you discovered (as I have) that there is a range of setting of the idle speed adjuster where it doesn't actually affect the idle speed directly, but it does affect the "hanging idle" or "drooping idle" conditions. If the air passageways in the pilot circuit are clear then the bike will probably idle with the throttle nearly closed, and closing it too much makes it idle rich and you get an "idle droop" while running it too high makes it idle lean and you get "hanging idle".

I have observed this phenomenon.

Here's hoping the shop sets all this correctly when my bike goes back together. I am guessing "top end rebuild" will include setting idle speed, mixture, and balancing the carbs, since flow and vacuum should change a lot when going from 80/120psi compression to 160+psi on both cylinders.

That, by the way, brings it back to the original problem. Vacuum is a direct result of compression. Low compression will mean low vacuum. Low vacuum will mean less fuel being sucked through the pilot jet, and thus a lower idle speed, since you have less fuel and must match it with less air. Adding too much air via the idle speed knob would make it lean and you'd get hanging idle. Making it richer is hardly the answer since you're going to go to 3-4 turns out and still not have nearly enough vacuum to pull enough fuel, and once you rev it off idle vacuum goes WAY up and it'll run mega rich at part throttle and low (under say 7K) revs. Ask me how I know all of this...

This is precisely the same thing that happens when you have a massive vacuum leak... low vacuum means poor fueling from the jets blah blah you wind up with a lean-rich-lean kind of condition that's undiagnosable without fixing the root problem first... in this case the root problem is low compression but it may as well have been (and may still also be) vacuum leaks.

[mr72]

FYI, gold nuggets in here too.

http://rec.motorcycles.tech.narkive.com/X7yWLgwN/yamaha-xs400-mikuni-bs34-what-s-this-hole-in-my-carb#post5

I'm going to wind up reading that entire archive at this rate.

Anyway, the point here is about setting the idle mixture too RICH, then upping the idle SPEED to try and compensate (more air), then when the engine is warm, idle hangs at 4K rpm.

And if you do this for very long, you wind up with carbon on the valves, bent valves, and an engine rebuild bill (again, ask me how I know).

[qcbaker]

Psshhhh, everyone knows drooping idle is caused by the carb sorcerer forgetting to add the second newt eye to his potion.