Intermittent running fault - LM2

[Dukedesmo]

Towards the end of last year bike developed a little bit of an occasional 'stumble' when opening throttle from a steady 4-5k rpm. Bike would just splutter a little and then go.


First ride out this year bike developed an exhaust leak (poor welding on Agostini pipes causing flange to fall off) and the stumble was more of a flat spot combined with backfiring due to the exhaust leak.


Anyway welded the pipe and that's good now and for the first ten minutes out the and the bike was running perfectly and then the flat spot came back, funny thing is it doesn't always do it but when it does, it feels and sounds like fuel starvation.


Starts around 4.5 - 5.5k rpm, is fine under 4,000 and over 6,000 and when it does it I can usually clear it by chopping the throttle closed/open/closed/open etc.


During the winter service I set the valve clearances and static ignition timing to spot-on, timing checked with a strobe (although not at higher RPM) and was perfect, fuelling seems OK and is unchanged from when it was running OK, carbs were ultrasonically cleaned a year or so ago so I'm thinking they're OK.


Bike will rev OK under no load, problem only occurs under load so it seems I've got either a fuelling or an ignition problem but it doesn't always do it (more not than so) and when it's OK it pulls cleanly through the rev range, any ideas? 

[RinkRat II]

 The intermittent ones are always so much fun, ain't they?   I'd be suspect of your advance mechanism. May just need a clean and lube.
 
  Paul B

[sdcr]

I've been following Chuck's Monza annual maintenance thread. The latest posts includes "draining the float bowls". This is to purge any water that may have settled in the bowl. It can eventually get sucked into the combustion chamber during operation. Might be worth a try, and is probably a good post winter maintenance item anyway.

" and most important is to drain any accumulated water out of the float bowl. It's heavier than fuel, and will sit in the bottom of the bowl. So what, you say? Let me tell you a story. A guy called me one time and said he was at his wit's end. He'd be flying along in his Taylorcraft and that little Continental would be just purring along and then suddenly miss a beat. Back to purring. He said, he'd checked plugs, timing, cleaned the gascolator, checked compression, and nothing helped. It was getting to the point he was nervous about flying it. Did I have any idea what it was? I said, "No.. I know exactly what it is. You didn't drain the carb at the last annual did you?" "Er.. no." There is a reason that by regulation one of the maintenance items at an annual is to drain the carb. Don't ask me *why* it does it, I have a theory, but it does it.
(2) To check that it has the good black float. The white ones will cause you problems sooner or later, just like something with boobs, wheels, or wings.  "

[pehayes]

What ignition?  Electronic or stock dual points?
How old are the secondary spark wires?
What are you using for spark caps?
What are you using for spark plugs?
What coils?  Original or substitute?

I spent a year chasing an intermittent flat spot on my SPII (similar to yours).  Mine stumbled coming off of traffic lights.  Risk of rear ending.  Eventually determined it was a weakened stock coil.  While it may spark under steady running, it gets more rich when you accelerate and it takes more spark amperage to fire.  If the coil isn't up to it you'll get intermittent performance.  Easy enough to tape a substitute coil in place and give that a try.

Patrick Hayes
Fremont CA

[Trogladyte]

Just a thought...

20 odd years ago I had a similar problem on a Le Mans III. Poor running in one spot on the rev range. Took me forever to locate the problem, which turned out to be a worn carburettor needle. It had worn into a tightly waisted shape - like a skinny ballerina. I replaced the needle and main jets all round and all was well.

[Dukedesmo]

Quote

from: pehayes on

Today at 05:57:06 PM

What ignition?  Electronic or stock dual points?
How old are the secondary spark wires?
What are you using for spark caps?
What are you using for spark plugs?
What coils?  Original or substitute?

This is a low mileage (12k) 1980 bike that I bought a couple of years ago, shortly after I bought it there was a piston to exhaust valve interface so I rebuilt with 950cc Gilardoni cylinder/piston kit, new exhaust valves and heads refurbished with new guides seats recut etc. it has a B10 cam

Ignition is original points, I replaced the plug leads last year as the originals were a tad rigid but still worked OK, plug caps are the original metal caps, new NGK BP7ES plugs fitted a few weeks back, coils are original.

[Dukedesmo]

P.S. not sure what's happening to the quoting system on my replies??

[fotoguzzi]

  the original metal caps, ]

I'd start there, the metal caps had a bad reputation back in the day.

[Chuck in Indiana]

It sounds like ignition to me, so it's probably fuel related.  I would suspect plug caps, coils, and *advance mechanism.*

timing checked with a strobe (although not at higher RPM)

[Dukedesmo]

Thanks all, I'll be checking out your suggestions. I'm veering towards the ignition and specifically the advance mechanism as I don't think there's a carb problem but it wouldn't hurt to clean out the float bowls to at least eliminate from the enquiries.


 

[Stevex]

Back in the day those metal plug caps were replaced as soon as the bikes were ridden out of the shop.

[Chuck in Indiana]

Back in the day those metal plug caps were replaced as soon as the bikes were ridden out of the shop.

My experience is they are ok.. unless the humidity gets over 70%.    If it starts raining, you'll *probably* get home..

[Arizona Wayne]

Does your bike have brass fuel filters?   Since the problem happens at higher rpm (more fuel needed), I've had a brass fuel filter get partially plugged(outwardly you couldn't tell) and that would cause 1 cylinder to starve for fuel intermittently until I found the problem and replaced the inline fuel filter. I run see thru fuel lines so I can see if fuel is running thru the lines.  This issue happened on my `81 CX100(LM II).

[flynguzzi]

Replace the condensers would be my take on the intermittent issue. Stock Guzzi condensers have been known to create problems, sounds like your approach is fairly solid so far. I had a problem that was similar to yours and one of the condensers was weak, replacement of both solved the problem.  jb

 

[rodekyll]

Air filters clean?

One way to rule out lean running is to pull the chokes when it stumbles.  If it clears up, it's lean.

[flangeman_70]

Have you pulled the plugs for a looksee? What do they tell you?

I have just 'fixed' my SPIII for the umpteenth time and it was displaying the same symptoms you describe, acceleration was ok but was a bit lazy and at about 5k it just flattened out. I opened her up more but it just made more noise and not much else. Pulling the plugs told the story, the right was perfect, left was black and sooty.

I immediately super heated the iridium plug with a torch and burnt the soot off and disconnected the left start enrichment plunger (often confused as being a choke) put her back together and went for a ride, needless to say, normality returned.

I went home lifted the tank and found the cable had pulled out of the splitter. Refitted cable measured extended length of plunger was longer than depth of carburettor chamber and installed and tested.

The left was a gnats hair open occasionally, the contamination of the plug just helped mask the issue.

As someone mentioned Chuck's thread, he also covers the start enrichment plungers.

Good luck with the troubleshooting

Adam

[Dukedesmo]

Thanks, interesting suggestions.


It was dry yesterday when I was out testing so not a rain/humidity problem on the plug caps, IIRC there are plastic mesh filters in the carbs under the fuel feed pipe I suppose there could be some dirt/rust in them, there are no air filters so I can rule that one out, condensers are probably the originals and certainly look old so wouldn't hurt to replace, I've not checked the plugs but since they are new (maybe 80 miles?) then any over-richness etc. should be visible.


Plenty of things to check.     

[pete roper]

Does it do it if you leave the fuel filler cap ajar? Sounds like a blocked vent to me.....

[Dukedesmo]

Quote from: pete roper on Today at 12:29:59 PM

Does it do it if you leave the fuel filler cap ajar? Sounds like a blocked vent to me.....

Now, funny you should mention this. I replaced the fuel cap gasket with a 'home made' one from Nitrile rubber, left the bike a few weeks before testing as the weather was shite and then it was running badly as above.

I checked for fuel and the gasket had swollen up, filling the entire area and had a similar consistency to snot. I bought some 1mm thick Viton sheet to make another which has held up fine and it was an exact copy size-wise of the original which I only changed because fuel would dribble out and leave a film around the front of the tank - it still does this BTW with the new gasket.

How does the vent work? I assume it vents through the filler cap by going through the centre/around the gasket but is it possible that it got blocked, perhaps even with some of the 'snot'?   

[Kiwi_Roy]

I haven't seen anyone suggest a break in the HT leads yet.
Measure from the cap to chassis on each side, should be the same both sides


Sent from my shoe phone!

[Dukedesmo]

To update on this, I've been in the workshop checking over a few things today; plugs are fine, just the right colour but, as per one of the first replies above I found water and other related bits in the float bowls.


Took the float bowls off & cleaned them out, also cleaned the screens in the carb/fuel line fitting as there was a small amount of debris in them, ran some carb cleaner through them and took it for an Italian tune-up and it's running fine again.


I guess the problem shows at 4-5k rpm as that's the point that the engine starts to pull harder and so, presumably sucks up more fuel, along with whatever crap is mixed in with it? Anyway running great now, glad it was a simple one but a tad disappointed I hadn't thought of checking it myself.


Thanks all.   

[Chuck in Indiana]

It's one of those annual maintenance things that I emphasized in the Monza thread. Doesn't take long, but as Pete says.. it makes your hands smell bad. 
Once water gets into the float bowl, it pretty much stays there.

[sdcr]

Chuck,

 Monza thread is excellent. had I not seen the float bowl drain tip, I would have skipped it on my LM III spring maintenance. Thanks!

It's one of those annual maintenance things that I emphasized in the Monza thread. Doesn't take long, but as Pete says.. it makes your hands smell bad. 
Once water gets into the float bowl, it pretty much stays there.

[Dukedesmo]

Quote from: sdcr on Today at 06:13:43 PM

Chuck,

 Monza thread is excellent. had I not seen the float bowl drain tip, I would have skipped it on my LM III spring maintenance. Thanks!

Just as I did. Lesson learnt for the future.