Carb surge

[kevdog3019]

With the Super Lario I occasionally get a surge after having been at low revs in an upper gear,(for usually an extended time) shift down, and start to accelerate. I'm talking from down in the low 3k RPM range. Problem is it's a quick buck and it's disconcerting. Only happens on rare occasions. I can sense there's a hesitancy before it hits that quick burst. Is it s high compression/low RPM with fueling or what. I'd say it happens maybe 1 out of 10 times in the same situation. Gots me but the bike runs flawlessly and is fueling great otherwise.

[Vasco DG]

The carbs don't have accelerator pumps do they? If not it's probably your slides are too lean.

Low RPM, small throttle opening. Manifold depression isn't very high, fuel will just be dribbling through the atomiser and the slide won't be contributing a lot, you'll mainly be running on pilot and spill from the atomiser and the mix will be pretty 'Gob-a-dollop'. When you crack the throttle the manifold depression plunges and a lot more air is pushed through the Venturi. The problem is without an accelerator pump it takes time for atmospheric pressure working on the surface of the fuel in the float bowl to push the fuel up through the atomiser and into the venturi. The mixture goes critically lean and won't combust so the engine dies until the fuel catches up then it'll fire up again powerfully and suddenly and you get the 'Lurch'!

Best way to address it? Smaller slide cutaway and perhaps go up a tad on pilot jet.

Pete

[canuguzzi]

If I could remember half the stuff you've forgotten I'd be 4 times smarter than I am.

[kevdog3019]

The carbs don't have accelerator pumps do they? If not it's probably your slides are too lean.

Low RPM, small throttle opening. Manifold depression isn't very high, fuel will just be dribbling through the atomiser and the slide won't be contributing a lot, you'll mainly be running on pilot and spill from the atomiser and the mix will be pretty 'Gob-a-dollop'. When you crack the throttle the manifold depression plunges and a lot more air is pushed through the Venturi. The problem is without an accelerator pump it takes time for atmospheric pressure working on the surface of the fuel in the float bowl to push the fuel up through the atomiser and into the venturi. The mixture goes critically lean and won't combust so the engine dies until the fuel catches up then it'll fire up again powerfully and suddenly and you get the 'Lurch'!

Best way to address it? Smaller slide cutaway and perhaps go up a tad on pilot jet.

Pete

Thanks Pete, makes sense.., but I have 36's with accelerator pumps on this guy. Just to let you know, I'm not cracking the throttle open, just easing it up. It's usually when I turn a corner (2nd gear/low rpm) so thought it may have something to do with gas in the bowl,  I'm not so sure that hasn't been 100% the case. It's simply not predictable except at low rpm in lower gear while slowly accelerating. Again... rarely at that. Thanks

[Vasco DG]

36's on a Lario? Could just be too large a choke size. What were they originally? 30's? 32's?

Pete

[Groover]

Sounds to me that is actually rich on the low end loading up. I personally work try leaning the low end a bit, maybe try hotter plugs as a starting point see if that helps.

[Chuck in Indiana]

It also has extremely rich mains, if that helps.

[Vasco DG]

Shouldn't make that much difference at small throttle/low RPM. I'm still thinking that the motor simply can't drive the 36's at low RPM. Small blocks have always been more of a pain to jet than their larger bretheren though. What does the Aero engine run?

Pete

[kevdog3019]

It also has extremely rich mains, if that helps.

Trust me, the bike is none too rich. With my smaller atomizer I have greater mains. Everything is relative. This happens so rarely I just wondered if anyone has experienced it. Its only when in upper gear at very low rpm, shift down to second and begin the acceleration. Never has happened in any gear but second and around 3400 rpm maybe. Why that is I don't know. It's absolutely brilliant otherwise. Not a big deal just surprising at times.
Pete, stock engines are 30's. Mine is far from stock.
Now that I think about it, the bike actually did this more with lower mains and has done it less and less the higher I've gone. I have one size greater mains yet that I could try though I find it strange mains would affect it at such low rpm's. I'm currently on the leaner side of perfect so maybe I throw those in and see what it gets me.  Better late than never to remember my findings.
It makes a bit more sense now: If I'm chugging along in a higher gear at low rpm and not pushing it, coast a bit, shift down, and now I'm easing in on the throttle at low rpm with inadequate fuel delivery and it has to wait a second and boom there it is with a delay, there's the surge. Could it be a vacuum thing?  Funny it's every blue moon and ONLY under the circumstances I mention. Hmmm...

[Vasco DG]

How about a bigger atomiser or raising the needle a notch if you think it's lean?

[kevdog3019]

How about a bigger atomiser or raising the needle a notch if you think it's lean?

Raising the needle makes the most sense. I think it's in the middle so could do that. It's not really lean, just leaner side of perfect. I could see how the needle could supply the needed juice. I had the larger atomizer and things were messy. Too rich down low and good up top. Needle no help. It simply let too much fuel in at low/mid rpm.
Thanks for this.

[Chuck in Indiana]

Shouldn't make that much difference at small throttle/low RPM. I'm still thinking that the motor simply can't drive the 36's at low RPM. Small blocks have always been more of a pain to jet than their larger bretheren though. What does the Aero engine run?

Pete

The Aero engine is running 36s, jetted like an LM3. Runs perfect. I'd rather be lucky than good. 
from maintenance log:
 DelOrto 36mm pumper carbs. K18 needle, AR 268 atomizer. 60/3 slide, 130 main jet, 50 idle jet.

[Groover]

Inspect the floats too while you're at it. Make sure you don't have a sticky float, cracked, etc. Could be a sticky float that acts up with a certain rpm/vibration frequency.

[kevdog3019]

The Aero engine is running 36s, jetted like an LM3. Runs perfect. I'd rather be lucky than good. 
from maintenance log:
 DelOrto 36mm pumper carbs. K18 needle, AR 268 atomizer. 60/3 slide, 130 main jet, 50 idle jet.

You sir are getting some major fuel (droplets) with that atomizer. I'm at 264 with 265 being a HUGE difference (18 pts on mains). 

[kevdog3019]

Stuck my 170 mains in (up from 168) and runs great. Just when you think she pulls like a mad dog...
Whoa...she ticks up a lot easier now.  Big difference and big pull from 168's to 170's.  When will the fun stop??  Time will tell on the surge, but I did a couple good test with good results.  That certainly isn't definitive.  My needle has been all the way up all along. It's none too rich for certain behaviorally speaking.  I'll pull the plugs after some more miles but I can't imagine...
Guess I better get the next jet size up and keep going and see where the fun ends.  I'm certain now it wasn't getting fuel down there when it surged. 
 

[kevdog3019]

Yeehaa... Looks like I can keep going! Last change: Bikey reacts to throttle nicey!
None too rich.

[Groover]

Good deal, those plugs look like a perfect burn to me.

[Chuck in Indiana]

I'm guessing and it's only a guess that the super rich mains are masking a too lean atomizer. I would think that it would be pig rich at WOT?   

[Chuck in Indiana]

Ok, I couldn't stand it.. I had to look up some specs.
Whenever I'm doing something "unusual" I kinda wonder why. In Guzziology, there is only one Guzzi that uses 170 mains. (!)

The LM III performance kit uses a 170 main, but it has a K4 needle and 265 atomizer with a 60 slide. That's a lot of carb on a lot of engine, though. That's a 40mm carb. The biggest 36 main jet is a 140 on a LMII.

The 1100 sport with 40s uses a 152, and that's a lot bigger air pump than a 650, for instance.

[Vasco DG]

From memory 170 is as big as they make! I really can't believe a Lario needs 170's, even my stupid little hot-rod only needed 145's and it was 230 cc bigger had similar valve area and revved to close to 10,000. VE dropped off drastically after about 9 though.

[Chuck in Indiana]

From memory 170 is as big as they make! I really can't believe a Lario needs 170's, even my stupid little hot-rod only needed 145's and it was 230 cc bigger had similar valve area and revved to close to 10,000. VE dropped off drastically after about 9 though.

Yeah, it just seems strange to me..

[wrbix]

I would think that it would be pig rich at WOT?   

Runs cool tho, I'd bet!

[kevdog3019]

OK... when I got the bike it had 265 atomizer with 150 mains.  It was a rich pig at low/mid throttle and fine up top.  So... I put 264 atomizers in and started with the 150 mains and it ran swell now in low/mid but needed go juice badly up top.  I crept up one size main at a time over the course of a summer and settled on 168's.  I had 170's to give but it was going up nicely, except for the occasional surge.  I had no idea it would open up like it did with the 170's, so I know I'm damn close.  Plugs look a whitish brown on top.  Pics make it look a little darker.  Why would a 264 atomizer make this thing go up 20 points is a question I can't answer??  According to Chuck's color coated carb chart I did the right thing with the lower atomizer.  This thing is as linear as they come how it runs up and I hardly move the throttle to do it.  What say you with the 264 vs. 265 atomizer?  Forget about the mains as this is the difference maker.

[Groover]

There are some carburetors out there to where the larger number stamped on the atomizer means leaner condition, or at least, leaner condition in certain ranges. Depends on the design of the atomizer and where the misting holes are located. The holes in the atomizer/mixer/emulsifier let air in, not fuel. The fuel is controlled by the main jet, the atomizer adds air and then all that becomes a happy mist being sucked through the Venturi. At least that's my understanding of the atomizer.

[Vasco DG]

Pretty much but of course you also have to take into account needle thickness, height, length and profile.

[Chuck in Indiana]

Someone should contact ATE. He skimmed over the DelOrto manual once.. 

[Groover]

I wonder what ATE had to EAT today..