Kawasaki Motorcycle Forums banner

21 - 40 of 62 Posts

·
Registered
Joined
·
661 Posts
Bogging before dying is an interesting clue. What do the plugs look like after it dies? Now you have two reasons to do the spark test:

1) to determine if you have spark right after the engine dies.
2) to determine if the engine is running too rich or too lean just before it dies.

Just a wild theory, but if it is running too rich at highway speeds and with weak spark from ancient coils the plugs could foul to the point of engine dying and in this case it takes about 20 miles before that happens. With enough sitting time, the plugs may dry up enough to give enough spark to start and with some idling and low speed running the plugs clean up enough to get you home. Like I said, just a wild theory at this point.

Let's get that spark test done and report on colour of plugs.
 

·
Registered
Joined
·
203 Posts
Very difficult to diagnose remotely. One of the problems is that we tend to describe a particular problem differently. So please read this and correct any variations in what you actually experience. Looks to me like you ride the motorcycle for approximately 20 minutes and it begins to loose power. Please be very concise here, I am reading that the motorcycle starts to progressively loose power to the point that it eventually quits. Is this accurate? If the engine runs smoothly but with a linear loss of power, no on and off, no jerking, just a steady lose of power until it quits with out you turning off the key or hitting the kill switch, it is highly unlikely to be electrical. My first suspicion if the above is an accurate description is that the engine is running properly in regards to combustion events but that internal friction is consuming the power produced. It was not uncommon in that era of air cooled two strokes for an engine to stick ( minor seizure) cool and restart. Admittedly this was not a characteristic of the S3 as they were very reliable motorcycles. But at this age you have no way of knowing if the oil injection tank was run dry or several other possible causes. Does it totally stop running on it's own, When it quits do you still have full electrical power, horn, signals, neutral light? Immediately after it stops running if you select neutral and slowly kick the engine thru does it turn freely with no additional resistance?
 

·
Registered
Joined
·
203 Posts
One of the problems for any two stroke but especially an old one is air leaks. A properly tuned, healthy two stroke EGT is not far from aluminum melting temp. Factor in incorrect jetting or an air leak and EGT can easily rise to above the temp. required to melt Al. Your S3 pistons are aluminum. At 40 years old you could easily have a leaking crank seal. What really makes this problem a trap is that a two stroke with an air leak is likely to run better and make more power than the same machine in a healthy state properly tuned. Two strokes must be balanced in tune to provide the best possible power but still have enough fuel to aid in cooling. Jetting for, or an air leak that provides max power will lead to a meltdown. Could you take note of the effort to slowly kick the engine thru cold and then also take note of the effort required immediately after it quits.
 

·
Registered
Joined
·
32 Posts
Discussion Starter #24
Bogging before dying is an interesting clue. What do the plugs look like after it dies? Now you have two reasons to do the spark test:

1) to determine if you have spark right after the engine dies.
2) to determine if the engine is running too rich or too lean just before it dies.

Just a wild theory, but if it is running too rich at highway speeds and with weak spark from ancient coils the plugs could foul to the point of engine dying and in this case it takes about 20 miles before that happens. With enough sitting time, the plugs may dry up enough to give enough spark to start and with some idling and low speed running the plugs clean up enough to get you home. Like I said, just a wild theory at this point.

Let's get that spark test done and report on colour of plugs.
👍😊. will do, but again, all 3 at once? will look for wet plug at “quitting time”..... at least it’s another “fork in the road”😊
 

·
Registered
Joined
·
203 Posts
Charging system failure would lead to low battery. S3 is battery ignition. A discharging battery would likely lead to a sequential loss of cylinders as the voltage available would drop the most electrically inefficient cylinder first. Again when the motorcycle quits do you have strong lights and horn? If the motorcycle runs strong for 20 minutes we can to some extent rule out jetting ( with the exception of a lean condition) as the engine has obtained operating temp. within the first few minutes. If it runs strong for twenty minutes, highly unlikely all three cylinders are failing due to over rich condition. Of course this assumes the op is accurately describing a strong running engine. What we need to proceed is a very concise and detailed description of what is happening. The S3 is a very simple and reliable machine but age takes its toll.
 

·
Registered
Joined
·
32 Posts
Discussion Starter #28
Scratch the gen/reg.... going the wrong direction 🤪
Yes....lights & horn..... sounds like it’s getting back to ignition switch (out to coils).... or loop to kill switch....again, a wet/moist plug would signify no spark?
 

·
Registered
Joined
·
32 Posts
Discussion Starter #29
Yes....lights & horn..... sounds like it’s getting back to ignition switch (out to coils).... or loop to kill switch....again, a wet/moist plug would signify no spark?
Yes to the strong engine....VERY responsive for 1st 10, 12, 15 miles... until “bogging) & eventually bog/quit..
 

·
Registered
Joined
·
203 Posts
As WFO-KZ said, your next step is to check for spark when the engine quits. I would suggest taking a known good plug with you and use that for the test.
 

·
Registered
Joined
·
203 Posts
When it quits do all three plugs look wet? Can you verify if you lose power from all three cylinders at once or are you loosing power because you are dropping cylinders. Important difference as you will not foul three plugs at exactly the same time. If you have a multi tester could you also report battery voltage at the time the engine stops. The only way you will get to the bottom of this is to methodically test the systems.Many on this site capable of helping you but we will need tests performed and results posted. Once repaired I think you will be very happy with that motorcycle. Some of my best motorcycling memories after almost fifty years of riding are the memories of my 1976 KH400 which was just a slightly upgraded renamed S3.
 

·
Registered
Joined
·
32 Posts
Discussion Starter #32
When it quits do all three plugs look wet? Can you verify if you lose power from all three cylinders at once or are you loosing power because you are dropping cylinders. Important difference as you will not foul three plugs at exactly the same time. If you have a multi tester could you also report battery voltage at the time the engine stops. The only way you will get to the bottom of this is to methodically test the systems.Many on this site capable of helping you but we will need tests performed and results posted. Once repaired I think you will be very happy with that motorcycle. Some of my best motorcycling memories after almost fifty years of riding are the memories of my 1976 KH400 which was just a slightly upgraded renamed S3.
Obviously you guys know your way around....😀. Day off today for yard & home maintenance 🤪. Will do the spark plugs/testing manana😊
Looking forward to solving this mystery so i enjoy my rides 😎
 

·
Registered
Joined
·
3 Posts
Having trouble w/ ? Runs like a bat outta hell for 20 miles, then dies...like fuel restriction.... carbs rebuilt 90 miles ago & adjusted for this issue but still exists... affects ALL carbs so gotta be fuel? Throttle cable? Gas tank venting? HELP please🙄
5
 

·
Registered
Joined
·
3 Posts
When it quits do all three plugs look wet? Can you verify if you lose power from all three cylinders at once or are you loosing power because you are dropping cylinders. Important difference as you will not foul three plugs at exactly the same time. If you have a multi tester could you also report battery voltage at the time the engine stops. The only way you will get to the bottom of this is to methodically test the systems.Many on this site capable of helping you but we will need tests performed and results posted. Once repaired I think you will be very happy with that motorcycle. Some of my best motorcycling memories after almost fifty years of riding are the memories of my 1976 KH400 which was just a slightly upgraded renamed S3.
the pep ****....
 

·
Registered
Joined
·
1 Posts
Having trouble w/ ? Runs like a bat outta hell for 20 miles, then dies...like fuel restriction.... carbs rebuilt 90 miles ago & adjusted for this issue but still exists... affects ALL carbs so gotta be fuel? Throttle cable? Gas tank venting? HELP please🙄
Mine did same crap for years always had to plan out where I was going and where stopping. Clean tank clean carbs everthing like it rolled off showroom. Then I found out after replacing all three fuel lines they where the problem even though they flowed great when tested them they and the gas where cold when the lines got a little warm there would swell internally and restrict gas doesn’t sound right but did to mine and then a buddy’s years later
 

·
Registered
Joined
·
661 Posts
This is not the case here. The OP pulled the fuel lines after it quit and plenty of flow but thanks for your input.

However, the OP may wish to consider replacing the lines anyway due to age. The rubber begins to deteriorate internally and flakes of rubber will plug up the carbs. But again this is not the case here, at least not yet because after a cool-down the bike runs fine. It's just a good preventative maintenance item to be done on vintage bikes, unless you love cleaning carburetors...
 

·
Registered
Joined
·
4 Posts
Having trouble w/ ? Runs like a bat outta hell for 20 miles, then dies...like fuel restriction.... carbs rebuilt 90 miles ago & adjusted for this issue but still exists... affects ALL carbs so gotta be fuel? Throttle cable? Gas tank venting? HELP please🙄
Does your bike have a CDI ignition system? (capacitive discharge ignition) Electronic components have been known to break down when they get hot and fail. When the engine shuts down, pull a plug wire and crank the engine and see if you have spark. Capacitors do get hot. They recharge and discharge large amounts of current at a very fast pace. Check for spark. Vapor lock can be eliminated by wrapping the fuel line in tin foil. it reflects the heat away from the line. just a thought
 

·
Registered
Joined
·
661 Posts
We are still waiting for spark plug test results and colour/condition of spark plugs right after it quits.
If you have spark and if you have fuel then I am leaning towards 1981GPZ550's theory of "internal friction".
 

·
Registered
Joined
·
32 Posts
Discussion Starter #40
Does your bike have a CDI ignition system? (capacitive discharge ignition) Electronic components have been known to break down when they get hot and fail. When the engine shuts down, pull a plug wire and crank the engine and see if you have spark. Capacitors do get hot. They recharge and discharge large amounts of current at a very fast pace. Check for spark. Vapor lock can be eliminated by wrapping the fuel line in tin foil. it reflects the heat away from the line. just a thought
3 carbs, 3 fuel lines, slide diaphragms (no vapor lock?), compression start....switch only.....actually do not know about cdi....
 
21 - 40 of 62 Posts
Top