Author Topic: What causes this behavior in a carb bike?  (Read 515 times)

Offline Mayor_of_BBQ

  • Instagram & Twitter: @Mayor_of_BBQ
  • Guzzi Hero
  • *****
  • *
  • *
  • Posts: 2889
  • 'Ever thus to deadbeats, Lebowski'
  • Location: Asheville, NC
What causes this behavior in a carb bike?
« on: July 27, 2021, 11:02:08 AM »
Hi I have a bad case of this on my v65 and a minor case on my Ambo.. I am certain it is a carb tuning issue but not sure what to try first.




When riding very spirited or WOT, with a hot engine and running seemingly well...

Now I come off the throttle, slow down and come to a stop (sign, light, or to turn etc)

The bike seems to drop all the way down thru to rev range, it seemingly 'misses' catching idle speed...  it just sputters all the way down and dies.  So I am hauling ass around the mountain roads, I descend out of the woods or approach the edge of town...  I'm approaching a stop light...  As I come to a stop the bike just putters out and dies (unless I catch it in time and blip the throttle a few times to keep it going).



On the v65 it is quite bad, it will just die as I'm rolling to a stop and shifting into neutral, and then is VERY difficult to re-start.  If I have been running full bore, it is very hesitant to idle at all- must keep a little throttle on it as I sit at the light or it will die..  Once I am puttering around back at 'normal' town speed, it seems to work itself out and then seems fine.


The Ambo is showing a whiff of this behavior, but not as bad, I can usually 'catch it' with a blip of the throttle and it settles into a normal idle as I sit at the light.  But after humping down a long twisty road at high load, when I pop out onto surface streets and come to a stop, it wants to cut out unless I intervene with the throttle.




If you had to guess, what adjustment should I make to improve this condition?  I am guessing too rich but I dont know if that is main jet, pilot jet, or float? needle? at this point I am just naming carb parts because I really dont know.
Chad (Shadrach) in Asheville NC
1970 Ambo (shiny resto-mod)
1984 V65 Cafe Racer (ongoing cafe project)
2007 Breva 1100 (Sport 1200 tribute)

Online fotoguzzi

  • Guzzi Hero
  • *****
  • *
  • *
  • *
  • *
  • *
  • Posts: 18987
  • vee git tooh soon oldt und too late wise -my Dad
Re: What causes this behavior in a carb bike?
« Reply #1 on: July 27, 2021, 11:16:49 AM »
When was the last time you checked valve lash?
MINNEAPOLIS, MN

Offline Mayor_of_BBQ

  • Instagram & Twitter: @Mayor_of_BBQ
  • Guzzi Hero
  • *****
  • *
  • *
  • Posts: 2889
  • 'Ever thus to deadbeats, Lebowski'
  • Location: Asheville, NC
Re: What causes this behavior in a carb bike?
« Reply #2 on: July 27, 2021, 11:25:31 AM »
When was the last time you checked valve lash?

Valves are spot on, that is one of the few mechanical things I can do with confidence
Chad (Shadrach) in Asheville NC
1970 Ambo (shiny resto-mod)
1984 V65 Cafe Racer (ongoing cafe project)
2007 Breva 1100 (Sport 1200 tribute)

Online Sye

  • New Goose
  • *
  • Posts: 67
  • Be nice, feel good
  • Location: Liverpool, England
Re: What causes this behavior in a carb bike?
« Reply #3 on: July 27, 2021, 11:42:28 AM »
Check the spark plug colour as it sounds like the bikes idle jet could be running a bit lean. Try adjusting the idle screw after balancing the carbs.

Wildgoose Chase Moto Guzzi

Re: What causes this behavior in a carb bike?
« Reply #3 on: July 27, 2021, 11:42:28 AM »

Offline guzzisteve

  • Guzzi Hero
  • *****
  • *
  • Posts: 9760
  • "Just Ride It"
Re: What causes this behavior in a carb bike?
« Reply #4 on: July 27, 2021, 12:43:32 PM »
Raise idle speed to 1000-1100 so that doesn't happen, then adjust mix screws so it's smooth. Sync cables & idle screws.
"Pray through Carlo & your bike shall be healed"
Location: Planet Earth

Offline berniebee

  • 2020 Board Donor
  • Weekend Warrior
  • *
  • *
  • Posts: 383
  • Location: Ottawa, Canada
Re: What causes this behavior in a carb bike?
« Reply #5 on: July 27, 2021, 01:48:18 PM »
Check the spark plug colour as it sounds like the bikes idle jet could be running a bit lean. Try adjusting the idle screw after balancing the carbs.

 I was thinking the opposite. If the bike stalls at idle when hot, (But not when cold) it's a sign of running too rich. But agreed, check the spark plug colour for clues.

Another possibility that has an easy solution: I had an older Japanese bike that had the exact same symptoms as yours. Once the engine was warmed up,  I had to rev it at a traffic light to keep it from stalling, and it was very difficult to restart when it stalled. The engine was flawless when the bike was in motion. I thought at first that it might be an electrical problem, because it was not designed to run with the lights on constantly and I could measure a considerable voltage drop along all the switches, fuses, connectors before voltage got to the aftermarket electronic ignition. And the problem seemed a little less bad when I switched the lights off.

But it turned out that at idle, fuel spends more time in the carburettors and the hot carbs were boiling out the ethanol in the fuel. I caught it only because at one point, having to pull the non starting bike over to the side the of the road on a hot, traffic-at-a-stop-and-go-crawl highway,  I could see a stream of bubbles rising out of the right carb in the clear fuel line.  I switched to a non ethanol fuel and the problem disappeared completely. In retrospect, I think that turning off the lights improved things slightly because there was less load on the engine at idle. In Canada, some companies have no ethanol in their premium fuel, perhaps the same in your parts?

Offline mtiberio

  • Guzzi Hero
  • *****
  • Posts: 4109
    • TiberioRacing
Re: What causes this behavior in a carb bike?
« Reply #6 on: July 27, 2021, 02:12:10 PM »
tight valves as already mentioned. Or tank not venting.

Offline tris

  • Guzzi Hero
  • *****
  • *
  • *
  • Posts: 2361
  • Location: United Kingdom
Re: What causes this behavior in a carb bike?
« Reply #7 on: July 27, 2021, 03:08:07 PM »
WAG .... carb icing?

I used to have an old Ford Fiesta with a go faster Webber bodged on.
After a long blast up the motorway it would die as I came to the first roundabout of the motorway. 
Given a couple of minutes to warm through normal service was resumed
2017 V9 Roamer
2005 Breva 1100 (non ABS) "Bruno" - now sold
1995 Cali 1100 - carby   "Dino" -now SOLD

Offline 1down5up

  • Sunday Rider
  • **
  • *
  • *
  • Posts: 126
  • Location: Sydney NSW Australia
Re: What causes this behavior in a carb bike?
« Reply #8 on: July 27, 2021, 07:43:38 PM »
I had this on a Lemans

I think you will find its your floats level is to low and/or you have a small blockage

In my case it ended up being a small blcokage in the float valve jet on 1 carb - my assumptions was that there was enough fuel getting through when the floa dropped significantly (WOT) but as i backed off and slowed down the fluel inflo slowed as the float came up, and the idle jet was not yet under the fuel and it would starve

If the float is to low and your fuel level is only just sitting above your idle jet (whuch remmeber sits alot higher than the main and choke jets), as ithe fuel refills the bowl the rate it enteres naturally slows down as the float valve closes off - hence why its fine if you just start the bike but if you have been riding in a gentlemans like fassion it may stall

An easy test is to ring its neck for some time then pull in the clutch so it drops back to idle - good chance it will die so be ready to bump start it on the run

So check your float height or blockages in your float valve

Online chuck peterson

  • Guzzi Hero
  • *****
  • *
  • Posts: 4462
  • Location: New Haven CT
Re: What causes this behavior in a carb bike?
« Reply #9 on: July 28, 2021, 05:44:57 AM »
Sounds like, from 10,000 miles away, fuel flow

Look for a slightly kinked hose..
Gas cap venting?..
Filter..
Dellorto carb filters..
Petcock filter clean..
Tank bag blocking the gas cap..
Etc

Float bowls arenít staying full is my wag
"I'd like to thank all my friends who have kept my Guzzi's going, but mostly...TOMB."
150k on Verts
750 Nevada
400f
R5 Yammie
BV250
4x 1976 Moto Demm Smily,, now 5, oops now 6, oops now 7
1980 SP1000 in little bits and pieces

Online SIR REAL ED

  • Guzzi Hero
  • *****
  • *
  • *
  • Posts: 1178
  • uh.... it's personal....
  • Location: Forest, VA
Re: What causes this behavior in a carb bike?
« Reply #10 on: July 28, 2021, 05:47:39 AM »
Sounds like, from 10,000 miles away, fuel flow

Look for a slightly kinked hose..
Gas cap venting?..
Filter..
Dellorto carb filters..
Petcock filter clean..
Tank bag blocking the gas cap..
Etc

Float bowls arenít staying full is my wag

Yep!  Any or all of the above.
« Last Edit: July 28, 2021, 08:04:02 AM by SIR REAL ED »
"There's two kinds of people in the world; those for whom no explanation is necessary, and those for whom no explanation is possible."

2016 KTM Duke 690
2019 Beta EVO 250
1982 Yamaha Seca 650
1999 Suzuki DR 650 w/790cc kit
1986 Honda XL600R

Offline Cam3512

  • Guzzi Hero
  • *****
  • *
  • Posts: 6207
Re: What causes this behavior in a carb bike?
« Reply #11 on: July 28, 2021, 07:42:19 AM »
Are you adjusting the idle screws when hot?  Take a screwdriver with you, and make even adjustments on each side DURING the ride.  Having someone sitting on the bike in gear is even better.  Mine drops on the Ambo when riding in hot weather too.
Cam - NJ Rep MGNOC
'67 Stornello Scrambler
'71 Ambo Police
'74 V7 Sport
Ď20 V85TT

http://mgnocnj.forumotion.com

Online chuck peterson

  • Guzzi Hero
  • *****
  • *
  • Posts: 4462
  • Location: New Haven CT
Re: What causes this behavior in a carb bike?
« Reply #12 on: July 28, 2021, 09:39:54 AM »
Bad white float..not..floatin g..

Leaking
"I'd like to thank all my friends who have kept my Guzzi's going, but mostly...TOMB."
150k on Verts
750 Nevada
400f
R5 Yammie
BV250
4x 1976 Moto Demm Smily,, now 5, oops now 6, oops now 7
1980 SP1000 in little bits and pieces

Online Old Jock

  • Guzzi Hero
  • *****
  • *
  • Posts: 2482
Re: What causes this behavior in a carb bike?
« Reply #13 on: July 28, 2021, 10:36:58 AM »
Have I got this wrong but if the float was punctured, wouldn't that tend to raise the fuel level in the bowl and flood the carb?

When my float stuck down on the LM1000 down fuel started to leak out of various places and there was a strong smell of gas.

Is it going onto one cylinder, which isn't sufficient to keep the engine turning at idle?

I'd pull both carbs and check the float levels and jets as already has been suggested.

Then I'd check the idle balance and idle enrichment, does the bike have a low idle anyway?

With the 1000 again, when I had a lean pilot what tended to happen was the opposite, the bike would rev up and be very slow to come back to idle.

Just wondering if it's a heat issue as it only happens after hard running and not any other time. I tend to find that when the bike is really hot (as in too hot) one of the first signs is the bike stalls.

That said if you're giving it welly in clear air, the bike should be cooler than when running in city traffic.

It could get really hot I suppose if giving it lots of revs in low gears on tight twisty roads. It's also indictative of summit else going on when both bikes exhibit the same behaviour, although the V65 is worse.

Offline SED

  • Guzzi Hero
  • *****
  • *
  • *
  • *
  • *
  • Posts: 1549
Re: What causes this behavior in a carb bike?
« Reply #14 on: July 28, 2021, 12:59:41 PM »
Mayor, My V65 Monza does this a little, but I've not fixed it yet.  The LMIII did this and the solution was to richen the idle jet.  (Apologies for the length and probably tone - hope it is not too teacherly.  :undecided:   I've been struggling to make the slicer run well on a PHBH28 from the Monza so I've given this a lot of thought and trying to be clear.)

It sounds counter intuitive but what seems to be happening is that when run hard and fast a lot of air is pulled through the carbs and they are actually cooler than when you are just puttering around.  I think carbs are more sensitive to it with 10% ethanol fuel because it runs leaner and has greater evaporative cooling than non-ethanol gas.  Add to this the tendency to set idle mix with the bike sitting and hot.  The old Amal recommendation was to set idle mix warm, not hot. 

There are 2 ways to richen the idle. The first, of course, is backing out the idle mix screws, but this did not help the LM (or the slicer).  It needed a larger idle jets. The idle jet controls both the mix at idle (fine tuned by the mixture screw) and the transition mixture from idle circuit to throttle slide (the fuel comes out the little hole between the needle and the downstream edge of the slide).  The LM was cured by larger idle jets and resetting the mixture screws - they are now 1 1/8 turns out.  V65 screws are 7/8 turns out, but it also has some idle slowdown, just not as bad as you describe.

To test if the idle jet is too lean, get the bike idling well and turn in the idle STOP screw (Not the mixture screw) on 1 side. Engine rpm should increase all the way until the screw bottoms.  If the engine begins to stumble as you screw in the stop the jet is too lean.  Return to the original setting and check the other side.

Just to be clear, it is easy to make the bike idle nicely with the mixture screws and still have it too lean on the transition.  This was the cause of the LM falling idle when stopping after a hot run. 

Let us know what works!

1983 LeMans III
1981 Monza
1947 Ariel Red Hunter
1939 Ariel Red Hunter
1937 Guzzi GTV

Offline Mayor_of_BBQ

  • Instagram & Twitter: @Mayor_of_BBQ
  • Guzzi Hero
  • *****
  • *
  • *
  • Posts: 2889
  • 'Ever thus to deadbeats, Lebowski'
  • Location: Asheville, NC
Re: What causes this behavior in a carb bike?
« Reply #15 on: July 28, 2021, 01:27:59 PM »
Lots of good stuff folks, thanks for the suggestions.

couple things:

Valves are correct .15 In and .20 out

Tank venting is not an issue

I dont have the white floats, I have the good ones

The main reason I thought it might be too rich was I also have HEAVY popping and barking out the (wide open) exhaust during decel/engine braking....  I will be double checking for exhaust leaks as I work on these other issues.



I am going to check out all the other advice here and I will report back.
Chad (Shadrach) in Asheville NC
1970 Ambo (shiny resto-mod)
1984 V65 Cafe Racer (ongoing cafe project)
2007 Breva 1100 (Sport 1200 tribute)

Offline SED

  • Guzzi Hero
  • *****
  • *
  • *
  • *
  • *
  • Posts: 1549
Re: What causes this behavior in a carb bike?
« Reply #16 on: July 28, 2021, 02:01:21 PM »
Popping on decel is also a sign of lean (or exhaust air leak).  Are you still running 40 idle jets?
1983 LeMans III
1981 Monza
1947 Ariel Red Hunter
1939 Ariel Red Hunter
1937 Guzzi GTV

Offline Huzo

  • Guzzi Hero
  • *****
  • *
  • *
  • *
  • Posts: 9947
  • Location: Creswick Australia
Re: What causes this behavior in a carb bike?
« Reply #17 on: July 28, 2021, 03:03:05 PM »
I donít think itís a venting or flow thing, because you say you can ďcatch itĒ, by manually blipping the throttle.
If it was starving, thereíd be no fuel when you lifted the slides.
I think itíll be an idle mixture setting and looking at too lean. Iíd experiment by reducing the air at idle and raising the rpm by a hundred or so on the throttle stops.
It seems to like the mixture setting up the range but not right on the bottom. Starvation due to blockages or poor venting, will allow the engine to run at low throttle settings, but not at higher demand.

When it goes to cut out, does it drop onto me pot ?

Offline mtiberio

  • Guzzi Hero
  • *****
  • Posts: 4109
    • TiberioRacing
Re: What causes this behavior in a carb bike?
« Reply #18 on: July 28, 2021, 03:25:10 PM »
I have seen enough weird sh*t with fuel starvation that I was never able to solve and it drove me crazy (street and race). I would try a nuclear approach to rule some stuff out.

Remove the petcock tank screens
Remove any fuel crossover
Remove any inline filters
Buy some Tygon F-4040-A clear yellow fuel line

When you hook up the new fuel lines (one per carb/petcock). Open the petcock and watch the gas trickle down (have a full tank, gives a better pressure head). Does the entire fuel line fill up or is there a bubble (air gap at top)? If so, squeeze the fuel line like an old siphon bulb until the gas fills the line all the way up into the tank. (you will hear gurgling during this process which will diminish when the line and petcock are full.

Run your experiment again. Starvation gone? Now you can try backing out the mods one at a time. Me, on my Eldo, I simply run one line per petcock, no reserve or filters, and clear tygon all the time.
« Last Edit: July 28, 2021, 03:26:03 PM by mtiberio »

Offline Huzo

  • Guzzi Hero
  • *****
  • *
  • *
  • *
  • Posts: 9947
  • Location: Creswick Australia
Re: What causes this behavior in a carb bike?
« Reply #19 on: July 28, 2021, 04:15:31 PM »
Itís significant that the bike will run well at high demand.
Fuel flow/starvation ?
Empirically, seems not.
 But I would double down on my attempts to get it to fail at mid/ high workloads, if it does not (and this seems to be the case), forget about poor flow issues. I do like Tiberioís approach to open up the lines of communication though, so as to eliminate ANY possibility, then reverse engineer your way back to the cause.

I think the direction to look, is that it cuts out immediately after high demand work, consequent high cylinder head temps and consequently marginally lean.
Take the air bleeds in a touch (if it has them), and raise the idle slightly.
When you hold the throttle on to keep it from stalling, thatís really all youíre doing...(raising the slides a bit..).
At least thereís no stinking Guzzidiag to antagonise you.

Incidentally.
If you go to kickstart a CT110, (which I tend to do a bit lately...), sometimes when hot, it will spit back and takes two or three tries before it fires. I think it dumps wet fuel onto the plug and needs to evaporate off before the spark will ignite the mixture.
Your suggestion that your V65 is hard to start after sputtering back, is symptomatic of this same event.
« Last Edit: July 28, 2021, 04:23:12 PM by Huzo »


Harper's Moto Guzzi
Harper's Moto Guzzi. Where we still answer the phone, use the highest quality parts, and also do Transmission, rear drive, and carb rebuilds fast. Call us at 816.697.3411 and get your problems resolved.
http://www.harpermoto.com
Advertise Here
 

20 Ounce Stainless Steel Double Insulated Tumbler with donation credit
Buy a quality tumbler and support the forum at the same time!
Better than a YETI! BPA and Lead free.
Advertise Here