First what is intermittent? Does it stumble at speed or under acceleration? I've never in over 35 years had a stator go bad but a few rotors although they never made the bike run bad.

Good day and thanks for the reply. At nearly the same location on the hwy I experience a significant loss of power, there is a loud grrrrrrrrrrrr (sounds like downshifted and engine braking down a hill), which continues until I pull to the side of the road and milk the throttle until a normal idle returns - about 30 seconds. This occurs after accelerating up an on-ramp onto the hwy then progressing at 70-75 for a 1/2 mile. Prior to hwy entry, I had ridden a couple sedate miles from cold start using mild choke. I remove the choke during entry onto the on ramp.
I can limp home back through city streets but the condition persists even with mild acceleration at city speeds. The next morning, all appears fine then the cycle repeats.
There is a healthy list of alternator tests I can perform from Snowbum's article though I would do so more out of curiosity and a health-check rather than expecting an issue here.

► Anytime the weather is above 50F, you shouldn't need the choke for any longer than 2 minutes after starting OR once on the road, which ever comes first.

By lengthening the time the choke is used, you are telling us that the machine runs very lean. Has this machine sat with fuel on-board for a significant time (more than 2 months) without being ridden ? The fact that the tank needed cleaning tells me the answer is "yes". The chugging sound is most likely the effects of lean running called "spark knock", possibly accompanied by mini engine seizures. (See the follow-on post.)

► Since the simple preventative steps weren't taken (removal of the float bowls and emptying the fuel tank), now you get to do the far more time consuming and expensive repairs to correct the long term presence of ethanol laden fuels. Due to the bike's age, we also can't rule out ethanol induced rubber rot, so while you're in there you'll want to renew all the "usual suspects", for added insurance.

• The first step is, as always, to discard all the fuel in the tank and carbs. You can pour this into your car, which really won't care. Maximum useful age for ethanol fuel in a vehicle without fuel injection is about 8-10 weeks.

Then, replace all these parts on both sides...
• Intake rubber tube
• Float needle
• Carb diaphragm
• Carb needle (in the slide)
• Carb needle jet (above the main jet)
• Carb main jet
• Carb pilot jet
• Float bowl gasket
• All o-rings on jets and adjustment screws

• Disassemble one carb at a time to avoid mixing parts. Make sure the new needles are installed to the same exposed length as the old ones... and the same exposed length on both carbs. Set the pilot screws at 1-1/2 turns. Removal of the carb top will necessitate re-balancing the idle speed screws and cable settings. This last step is mandatory in your case due to the present fuel mixture issues, and will require you to buy or make a manometer.

• Set the ignition timing at high RPM using a strobe lamp and the "F" mark. This last step is mandatory in your case due to the present fuel mixture issues.

• Adjust your valves clearances.

• Also buy a bottle of StarTron fuel treatment and run one ounce with every fill up your normal high-octane fuel until the bottle is finished. That will clean the remainder of the fuel system out.

Good luck.

Incidentally, that knocking sound (spark knock) is more appropriately called "pre-ignition".

The sound is actually small, ill-timed explosions inside your combustion chamber(s). These explosions are literally blasting away the crown of your piston(s). If allowed to continue, the condition will eventually eat a hole right through the crown of the piston. Not only will you loose all compression on that cylinder, but molten aluminum particles will be showered into the interior of your engine and engine oil supply.

Needless to say, whether you decide to clean and renew the old engine block, or replace the engine block, you are in for a very expensive repair. At least 15 hours at $60 to $100 per hour labor, plus parts.

Pre-ignition has many causes, mostly based on...
• Lean fuel mixture
• Inappropriate ignition timing (typically too advanced)
• Operating the engine at very low RPM under extreme load (aka "lugging")
• Wrong heat range spark plug(s)
• Too low of an fuel octane rating

These conditions are exacerbated by heavy acceleration (getting on the expressway), and high wind loads (riding on the expressway).

There are certainly other causes, but what they have in common is that they all contribute to hotter than normal combustion temperatures, which air-cooled engines simply cannot tolerate.

In my profession experience, by far the most common cause over the last 15 years has been partially restricted or completely clogged carb jets caused by allowing ethanol fuels to sit for extended periods. This is not a problem in pressure-fed fuel injected engines... only gravity-fed carburetor engines are plagued by this issue. And the smaller the jets, the faster the symptoms manifest themselves. (Think... moped, lawn mower, weed whacker, string trimmers, etc.)

Hope this helps.

Hello Wobbly, when you say "Maximum useful age for ethanol fuel in a vehicle without fuel injection is about 8-10 weeks."
What exactly are you saying, e.g. in 8-10 weeks too much water will be absorbed and start corrosion on the brass parts or the rubber in the presence of ethanol will be destroyed? In the first case if I'm constantly cycling the fuel I'll have no problem, but in the second new ethanol proof rubber is needed.
Unfortunately my state, Maine, seems to have bought hook, line and sinker the "green" nonsense and it seems I can't get away from some ethanol. Many stations just say, "May contain up to 10% Ethanol". 🙁 But fortunately my bike seems to only be susceptible to the green crud corrosion! SO RIDE MORE!!!
Thanks
Edward

Wobbly - I appreciate the methodology and the time you put into sharing - thank you.

Since purchase in '92, I had no issues like this until this winter when the symptom appeared - at first only for a few seconds under strong power situations: like you have noted, a high wind load and/or rolling the throttle to maintain speed up a rise in the road (at hwy speeds). In March the exacerbated situation reared its ugly head which puts me to the side of the road. Maybe it was 'self-correcting' before? Though the bike did not sit for extended periods I understand your points and believe going forward I need to start putting careful consideration into what I pump into my tank - my moto. Presently the tank and carbs are clean, clear, and appear to be mechanically healthy - all ports clear, etc.

My friendly source for a loaner alternator, rotor removal special tool (to allow camshaft oil seal change) didn't materialize. Any recommendation on a source of purchase for one (88-88-6-123-600)?

Hello Wobbly, when you say "Maximum useful age for ethanol fuel in a vehicle without fuel injection is about 8-10 weeks." What exactly are you saying...

There are multiple bad things going on with ethanol fuel when it is allowed to sit. What I'm saying is that after 8-10 weeks of not riding, all these things seem to team up against you.

• Ethanol is NOT absorbed by the gasoline. The 2 liquids are NOT compatible and without the agitation and vibration of vehicle use, will begin to come out of solution and separate back into their component parts. Keeping the liquid in solution is one of the main jobs of the ethanol fuel additives.

• After separation is when the ethanol really stars to attract water. So when you go to start your vehicle, you might get a slug of water or a higher concentration of ethanol. Either of these will foul the spark plugs, and so you kick and/or grind on the starter to no avail. Some cars have needed entire fuel systems rebuilt when those vehicles received a dose of high percentage ethanol. Although their systems could accept 10%, buying fuel at a station with very few customers could easily get you concentrations of 20% ethanol or higher.

And don't get too smug if you are able to buy "ethanol free" fuel in your state....

• The EPA has also mandated that all fuels have fewer volitiles. That is, there are far fewer free vapors ("fumes") given off by modern US gasolines. In carburetor systems, it's these free floating vapors that actually start the engine. The liquid is not sucked up through the jets until the engine starts to run. So the age of your fuel is also directly proportional to its ability to crank an engine with carbs. (This is not an issue for fuel injection engines, which have liquid fuel literally injected under pressure.)

Things to watch....
• In the old days your dad may have told you the best way to preserve your fuel system over the winter was to fill the tank to the top. That was true in 1965, but it has been the death of many classics since the introduction of ethanol. These days the best way is to completely drain all the fuel during long term storage. Then leave the gas cap open during storage to promote drying.

• To that end, and since no one knows exactly when the next ride might be, it's always best to shut OFF your fuel valves about 1/2 mile from the garage when returning home. This won't completely empty the float bowls, but it will help reduce the amount of fuel in the bowls.

• If you live in a rural area where gas stations rarely sell high octane fuel, then it's best not to buy fuel there. While the fuel in the regular grade storage tank might get daily use and stirring, that will not be the case for the high octane tank.

Since purchase in '92, I had no issues like this until this winter when the symptom appeared - at first only for a few seconds under strong power situations: like you have noted, a high wind load and/or rolling the throttle to maintain speed up a rise in the road (at hwy speeds). In March the exacerbated situation reared its ugly head which puts me to the side of the road. Maybe it was 'self-correcting' before? Though the bike did not sit for extended periods I understand your points and believe going forward I need to start putting careful consideration into what I pump into my tank - my moto. Presently the tank and carbs are clean, clear, and appear to be mechanically healthy - all ports clear, etc.

I don't want to insult your intelligence because you seem like a highly capable person, but we are not looking for dirt, scum, sediment, or anything you might have looked for in 1990. Pull the slides and if there is ANY green or brown colored coating on the brass needles, or the needles have ANY white crusty areas on them, then the needles AND the needle jets all need replacing. The needles should be bright, brassy gold in color with NO vertical scratches, and absolutely NO pitting.

Most people simply cannot conceive how fine the metering systems are on modern carburetors. The slightest irregularity in the needle surface (up to and including even a varnish coating) will upset the correct fuel flow. Generally, needles and needle jets need replacing every 10 years anyway !!

My friendly source for a loaner alternator, rotor removal special tool (to allow camshaft oil seal change) didn't materialize. Any recommendation on a source of purchase for one (88-88-6-123-600)?

The battery will have no issues filling in any voids in electrical service that might cause a sputter in the ignition. Any issues with the alternator will likely be reflected in the health of the battery. Take your battery out and take it to any qualified battery store and have it "load tested" by a professional. They'll have a proper battery load tester (get a Google image). I bet your battery is fine. If your battery is fine, then by inference your alternator is most likely not the problem.

Bad alternators simply don't require more choke.

All the best.

Hi, After reading your symptoms it seems like it may be losing one cylinder. What I would try is to ride until the problem comes up hopefully close to home. Then with the throttle holding about 2,000 rpm pull off one of the spark plug wires see what happens then the other. If there is no change then you just pulled off the dead cylinders wire. If the bike dies it's the good cylinder wire. As long as you have the stock points set up this is a safe thing to do if you have replaced them with electronic then not safe.
I have seen coils go bad and break down with heat then work again after cooling off.

What I would try is to ride until the problem comes up hopefully close to home. Then with the throttle holding about 2,000 rpm pull off one of the spark plug wires see what happens then the other. If there is no change then you just pulled off the dead cylinders wire. If the bike dies it's the good cylinder wire. As long as you have the stock points set up this is a safe thing to do if you have replaced them with electronic then not safe.

The energy in the coil must find a way home. If you don't supply a path through a spark plug, then it will burn one through the insulation layer of the coil.

Now what you are saying is safe if you install a spare spark plug into the cap, lay it on the cylinder head, then start the engine. That way the energy has a safe way out.

pull off one of the spark plug wires see what happens then the other.

not a good idea..... as wobbly says stick a spare them restart the bike after you lay it on a cylinder so it will ground itself to the bikes system.

Take your battery out and take it to any qualified battery store

qualified means just about any FLAPS . . . . any auto parts store can help you with this.
f riendly l ocal a uto p arts s tore

My friendly source for a loaner alternator, rotor removal special tool (to allow camshaft oil seal change) didn't materialize. Any recommendation on a source of purchase for one (88-88-6-123-600)?

EME has them
keep it on the bike protected

http://www.euromotoelectrics.com/product-p/boalt-rotor642tool.htm

@all -

thank you for your ideas. I have some great leads to follow up on!

The thought of the energy in the coils pent up without a way home will keep me from pulling plug wires. Snowbum has some great reading on sync'n carbs which extol the travails of a coil with no path to ground. There are some good examples of tools to be made that can help play games with shorting plugs without risking coil damage when I am ready to go that route.

I'll report back with some progress or - hopefully - a smoking gun root cause.

Cheers.

silver soldered from brass tubing, threaded one end for plug , detent on other for cap
and a 16p nail