What governs fast-idle (on choke) speed?

[yeadon_m]

Folks,

One of the benefits of having more than one GT is you notice stuff and get to thinking 'why is that?'.

On my 550B, the fast-idle when first started on choke is as high as 3000rpm, rapidly dropping to 2500rpm within a minute. As soon as a minute or two, it will idly stably at 1100rpm off choke.

These numbers are much bigger than with the other two GTs. My 380B won’t idle on choke for very long before bogging, and the falling speed tells me I have to either blip the throttle or take the choke off AND blip the throttle for a couple of mins before it'll idle stably at 1100rpm. Its carbs are of the same design as the 550B, only a size smaller.

My 750A bogs on choke quickly (within 30secs) and also won’t idle immediately when choke goes off. Revs just slowly fall until it stops, unless its warmed up, when it idles as 1000rpm.

Separately from hot idle speed question: what governs fast-idle speed? What changes, if any, would alter fast-idle speed? there are brass tubes which project down into separate;y metered 'choke wells' in the carbs. These tubes have two pairs of lateral holes. They are clear on every bike. Perhaps fast idle is hard-plumbed and I cannot easily play with it even if I wanted to, but I'm wondering whether size, number and location of those lateral holes plays a big role.

Anyway, just musing on 'stuff'. Not only playing in the shed, just done another 40 miles on the 550B.

Cheers,
Mike

[tz375]

Mike,

Cold idle speed is a function of mixture and throttle position. On one bike the throttle slides may be slightly lower or higher than on another of your bikes. Similarly if one is a touch rich once it's warm and another is a shade lean, they will idle at different speeds and will warmup differently.

The bike that needs to come off choke in 30 seconds is slightly rich and the enrichment makes it too rich to run.

We tend to assume that if three bikes all idle at the same speed when warm that they will all behave the same when cold but in reality they may be very different in terms of air: fuel, throttle height, etc. I can get two similar bikes to idle warm at the same speed and one may be one size too rich and the other one size too lean but they can both be made to idle at say 1,100 rpm. When they are cold and we add in the enrichment circuit, that all changes.

If they are all running fine, but behave differently, I'd leave them as they are and explain it as that elusive "character" that other Japanese bikes lack.

[yeadon_m]

Thanks TZ,

That all makes sense. To be sure, I'm not unhappy with the way they run or indeed start and run, cold.

I was just wondering how it was that they fast-idle - Mikuni must have made design choices that govern or at least contribute to what we finally see, and I wondered (if ever one wanted to) what changes to components of the enrichment circuit would lead to changes in cold running behaviour.

I'm less interested in actually doing it (changing the enrichment circuits) than gaining insights (I just can't help but be inquisitive

Again, I'm not proposing to do it, but would not adding air/reducing fuel by enlarging the upper pair of holes in the 'choke tube/straw' make the rich bike (380 / 750 here) cold-idle faster? and reducing air / adding fuel by covering up those upper holes make the lean bike (550 here) cold-idle more slowly? (assuming one didn't want to alter the master idle).

The choke plungers are kinda open or closed, so no adjustment likely there; and the enrichment 'well' has a pressed-in jet which governs how fast fuel from the float bowl transfers to that 'well', again not readily altered.

It may be that no one here has been sufficiently moved to try it....and I'd understand that. I probably won't either!

Cheers,
Mike

[tz375]

Rather than changing hole size (it's basically an air jet) maybe the way to try that would be to drill out the pressed in jet and tap the bowl to take small jets. Standard Mikuni air jets are probably too small in diameter but there are other jets- or have some machined up. That way the carb body isn't destroyed if the drilled holes don't work.

Looking at the design, the air holes connect to the air above the fuel in teh float bowl so it's at atmospheric pressure - more or less. On Homopressure carbs it's not quite the same, but that's another can of worms that does not need to be opened.

Interesting to think about this stuff.

[yeadon_m]

Thanks again TZ, indeed it's an interestng thought experiment. One I will likely leave at that level, I wouldn't be surprised to find that this is a circuit that's quite easy to mess up!
Cheers, Mike