Musings over morning coffee: Dellorto off-idle stumble

[Hedge36]

The ex-RT is on the road again. Damn it's nice to be back on an airhead - however...

Got an off-idle stumble that's pestering me. This is my first go-round with the Dellortos, and while I've got idle and high-end all squared away, puttering through town at 25-30mph with the slide just off the bottom yields a rough run. I noticed last night that half choke smooths it out a bit.

So, refresh my memory: which combination of jet and needle adjustment covers just-off-idle?

[supershaft]

At steady throttle it's mostly a combination of slide angel, needle jet, and jet needle. The idle jet/mixture screw can effect it slightly.

[Hedge36]

Originally Posted by supershaft

At steady throttle it's mostly a combination of slide angel, needle jet, and jet needle. The idle jet/mixture screw can effect it slightly.

Hm... almost finished with Cup #1, caffeine is kicking in...

Up one clip on the needle, then, see if that does it?

[supershaft]

Originally Posted by Hedge36

Hm... almost finished with Cup #1, caffeine is kicking in...

Up one clip on the needle, then, see if that does it?

One way or the other. You never know until you try this, then that, then that, then all the combinations of all of that. With my setup I went to a richer slide angle and smaller needle jets to get ride of exactly what you are talking about but I was already far removed form stock R90S jetting for a lot more power.

[Hedge36]

Ah, yes... experimentation

On it, thanks

[Hedge36]

62 idle jets and the needle on the third (needle in "high" position) clip... not bad.

AFTER I solved the air leak on the right cylinder's intake spigot, of course. I wondered why that exhaust pipe was bluing faster than the other.

Got a bit more dialing-in to do on the accel pumps but damn I think I'm almost there

[Rucksta]

Accelerator pumps can mask a range of evils with your tune.
I thought I had my Dels dialed in until I disabled the pumps and went back to square one.
Couple or a few recursions of the tuning cycle and I still haven't re-enabled the pumps.

It's not like I plan to not re-enable then it's just I haven't found a point where the motor really needs a squirt of fuel to make it respond.

48 idle jet, 260 needle jet, K4 needles middle clip, 138 mains and back to the slides that came with an R90S.
980cc 9.5:1 twin plug big port squish heads.

Slight hesitation on micro throttle change off idle but OK with a twist.
Needs heaps of choke to start so bigger idle jets are indicated but 55 feels a bit much over.
When I get around to buying so 52 or 50 idle jets I'll reevauate.
Maybe it just needs a little less cut away on the slides to keep it rich in the transition.

I don't think this is the perfect setup for my bike but it is better than every other example of the perfect setup I've come up with over the years.

Like the title says slow learner.

[wescnmbkr1]

Pilot Jet----just off idle

Originally Posted by Hedge36

The ex-RT is on the road again. Damn it's nice to be back on an airhead - however...

Got an off-idle stumble that's pestering me. This is my first go-round with the Dellortos, and while I've got idle and high-end all squared away, puttering through town at 25-30mph with the slide just off the bottom yields a rough run. I noticed last night that half choke smooths it out a bit.

So, refresh my memory: which combination of jet and needle adjustment covers just-off-idle?

[supershaft]

Originally Posted by Rucksta

Accelerator pumps can mask a range of evils with your tune.
I thought I had my Dels dialed in until I disabled the pumps and went back to square one.
Couple or a few recursions of the tuning cycle and I still haven't re-enabled the pumps.

It's not like I plan to not re-enable then it's just I haven't found a point where the motor really needs a squirt of fuel to make it respond.

48 idle jet, 260 needle jet, K4 needles middle clip, 138 mains and back to the slides that came with an R90S.
980cc 9.5:1 twin plug big port squish heads.

Slight hesitation on micro throttle change off idle but OK with a twist.
Needs heaps of choke to start so bigger idle jets are indicated but 55 feels a bit much over.
When I get around to buying so 52 or 50 idle jets I'll reevauate.
Maybe it just needs a little less cut away on the slides to keep it rich in the transition.

I don't think this is the perfect setup for my bike but it is better than every other example of the perfect setup I've come up with over the years.

Like the title says slow learner.

IMO the main thing the pumps mask is too much throttle input. Riding with CV's for too long will get your right wrist very complacent with over zealous throttle input. The pumps mask that at a the cost of being too rich for anything but covering up too much throttle.

Needing heaps of choke is not a bad thing and very often a sign of getting the mix just right for a hot engine. That mix won't run a cold engine! My Dell's don't have chokes or pumps so I need a ton of tickling to get mine started unless I JUST shut it off.

Wow. I have jetted more than my own setup and so far have needed way bigger than 48 idle jets. I think stock R90S jetting is 60, 260, and a K4 on the third clip? With the stock jetting I have got a huge flat spot with no pumps on bikes with clam shells and flat boxes. I am running my idle and just off idle a little to a lot richer than you and my mains quite a bit leaner yet. Whatever works. I am just putting some other fixes out there to ponder.

[Rucksta]

Originally Posted by supershaft

. I am just putting some other fixes out there to ponder.

And thank you very much for doing so.
Your ideas gave me a new starting point and even though I couldn't get a result with some of your settings it did give me a view to a different approach.

[Kai Ju]

I recently had the pleasure of dialing in a set of VM 38 Mikunis on my 77 R100/7
So obviously the jet sizes and descriptions won't match but the tuning principle remains the same.
Save for the lack of the accelerator pump on the Dellortos which doesn't come in to play at steady state cruising anyway.

I'm surprised that nobody has mentioned fine tuning the pilot jet with the mixture screw.
It's an airscrew on the VM 38's and the original spec called for a 17.5 pilot. The engine would't run unless the airscrew was completely closed. I've slowly worked my way up to 25 pilots and I'm pretty happy with how it responds off idle or at low opening cruising speed with the airscrew 1 turn out from seated.
I'm running the clamshell airbox with a stock filter and the stock tubes. Engine and exhaust are stock.

In short, if you think you're close with the pilot jet size and the throttle slide cutaway, try playing with the low speed mixture screw.
Also, at that speed, the needle position doesn't do all that much. Just sayin'.

[Rucksta]

Probably because the adjustment range of the equivalent part on the Del is so fine and narrow it's usefulness is limited to little more than balancing the carbs.

The mixture is determined by the jet, the amount of mixture delivered is determined by the screw position paradoxicaly known as the idle mix adjustment screw.

Transitional arrangements on the Del are somewhat crude with an orifice immediately upstream of the closing edge of the slide.
Overuse of the slide screw ( Idle speed adjustment screw) of a magnitude indicating a increase in slide cutaway is required can lead to confusion of this primitive transitional mixture arrangement producing rough tranistions or flat spots.

This is one of the evils masked by the pumps.

SS chime in here if you have a different understanding of the idle & transitional circuts .
I've come to terms with it in this way and it has produced results but I'm open to alternate ways of thinking & tuning.

[Kai Ju]

Rucksta, thanks for the clarification on the Dell's.
It sure makes me happy to have gone the Mikuni route.

[Rucksta]

Originally Posted by Kai Ju

Rucksta, thanks for the clarification on the Dell's.
It sure makes me happy to have gone the Mikuni route.

Yes loved the VM38s on a Kawasaki triple but on the beemer don't you think they look a bit too much like, well, umm, err , there is no way of saying this nicely.









They look like Mikunis !!!

[Kai Ju]

Originally Posted by Rucksta

Yes loved the VM38s on a Kawasaki triple but on the beemer don't you think they look a bit too much like, well, umm, err , there is no way of saying this nicely.
They look like Mikunis !!!

What's ironic about your statement is that I was going to admit in my previous post that I liked the look of the Dell's better than the Mikunis.

Here is what they look like:



I have to apologize for the threadjack, sorry.