July 21, 2013 at 9:34 pm #4404
Did this today, but I’m not sure how. It was not quite right off idle, so I played with the mixture screw and made it a little better but a quick opening of the throttle off idle would cause a bog but ran well on the high side.
I autocross and it just locked up on my second run. I am running the same jetting and everything else that I have run for the past 2 years. I am just trying to figure out what happened so I can make sure to correct the issue.July 22, 2013 at 7:52 am #4451
IMO it needed a new piston and ring about a year and a half ago.July 22, 2013 at 9:09 am #4454
need more info Pete. Was this the first time you ran it? I’ve stuck two pistons like this (melted down at the exhaust port) and they both were due to severe lean condition due to a leak in the carb boot. I kept running larger and larger jets, but just couldn’t keep up with the leak.
Probably not due to the low end circuit. Bogging when quick off the throttle is usually a rich condition. What the the exhaust pipe (exit) and spark plug look like?
"Karting Expert Since 2015"July 24, 2013 at 1:52 pm #4779
So if he were in fact running “RICH”, what would he expect to see at the exhaust exit and spark plug? Contrast that please with what he would expect to see at both locations if he were running “LEAN.”
Thanks in advance for the education.
~AndyJuly 24, 2013 at 2:09 pm #4783
Andy – to be more complete:
Running rich would cause the motor to bog and have a strong hesitation when you hit the throttle. Couple of things to look for:
1. Excessive smoke out of the exhaust. Note that you will always have some smoke, but you shouldn’t be creating a cloud.
2. Oil/fuel dripping out of the end of the silencer. Oil/fuel leaking between the manifold of the engine and the pipe (causing a messy block!). Oil/fuel leaking between the silencer and pipe.
3. Oily gummed up plug. The plug will go black over a time, but it shouldn’t have a build up of goo.
Running lean or correct, you’d not see any of the above. But running lean you’d see the plug burning itself up/whitish, you’d see pitting in the piston top near the edges, you feel like the engine was zingy (basically it’d feel ‘too’ responsive).
In that photo you can see pits all around the edges of the piston. BUT, note that this same phenomenom can come from poor cooling (which is the same as running too lean, the oil cools the system).
"Karting Expert Since 2015"July 24, 2013 at 2:14 pm #4785
Ray, I owe you a beer. Start me up a tab.
~AndyJuly 25, 2013 at 9:44 pm #4992
The plug looked nice and gray. Slight leak of oil between exhaust and cylinder but not much. I had pulled apart the carb and all passages were clean. I am suspecting either a vacuum leak at one of the seals or a coolant leak at the head dripping onto the piston. I should have replaced the piston and ring earlier and I am learning the false economy of stretching the time on replacing them. If anything the engine smoked a little less than usual. I did drop the needle half a groove but the outside temperature was hotter than usual. I don’t think it was a lean stick due to the carb settings…. maybe elsewhere but not the carb. Motor temp was up to about 170, radiator temp 150. Above ideal but I didn’t think dangerously high, and that was in grid before actually starting my run. Could an old piston fail in this way or did I kill it?
The one thing I did notice is that when I would spin the wheel to start the motor it felt… rougher…. Like not enough oil. I was running the same ratio as always, a different brand, but I don’t really believe that the brand would make that significant of a difference. At the previous event there was just the slightest hint of a stumble when snapping open the throttle. So I think it was a problem that got worse quickly but progressively.July 26, 2013 at 12:29 am #4999
Really the worst is to have an old piston ring break apart. Does a lot of damage to the cylinder walls. But an old piston, nah. You lose the original concentric grooves on the surface of the piston that helps with distributing an even coat of oil on the piston. And you might have some wear on the piston body that causes an increase in blow-by and a subsequent reduction in compression.
Only reason I suggest looking for an air leak in the inlet path is that I’ve got two pistons that look identical to what you have (no clear indication of detonation, melted on exhaust side).
Look for excessive oil around the seals of your carb boot.
Also, what oil to fuel ratio are you running? What engine is that?
"Karting Expert Since 2015"July 26, 2013 at 2:30 pm #5042
Engine is TM K9B, I run 22:1 which is more oil than some recommend. The ring is as old as the piston I think. If you look at the picture, part of the ring is gone, but i don’t know which went first. I haven’t changed it in 2 seasons but autocross isn’t like open track. An autocross run is about 1 minute and we get 4 runs per event. So I may put 1 hour of actual run time on a year.July 26, 2013 at 4:25 pm #5064
What fuel are you using?
Double check your silencer and exhaust system for excessive back pressure (broken silencer core, bent exhaust, etc)
Remove fuel filter inside the Dell”Orto carburetor and look for debris.
Look for pinched fuel lines.
If the bog is a distinct “shut off”, then quickly accelerates as you increase throttle, you have a lean condition due to small pilot, to open on the air screw, or clogged jets. If the engine bucks or hops, or is very sluggish (but trying to clear out), the low end jetting is to rich.July 26, 2013 at 5:25 pm #5068
The carb fuel filter was the first place I checked, I pulled the carb apart and all is clean and open passages. I guess you could call the bog a “shut off”. If snapped open at idle the motor would just stall, but opened more smoothly would rev fine. If I snapped open the throttle after revving it out, response was crisp. The silencer is clean, I can see straight through it. What would be the symptoms of a leaking seal at the timing shaft, or intake, maybe even cylinder. The setup I am running is what Tom Barth had recommended, and is also run by my competitor. If anything I am setup a little on the RICH side.August 13, 2013 at 7:01 am #6887
So, what did you find?August 15, 2013 at 2:33 pm #7144
The plug was ‘nice and gray’? Not sure I ever want to see a ‘gray’ plug when I come in. I prefer a very light (REALLY LIGHT) brown to tell me there’s plenty of lubrication going on.
Where physically are you racing? What temperature was it outside? What was the barometer?
As you can see from the piston, the ring actually snagged the port and broke off part of the dome, but apparently not before enough heat was generated to burn that hole in the backside.
Looks to me like there’s multiple scars on the piston as well, as if it got too lean before, but didn’t get so lean that it burned up.
Gray plugs…not good. That’s usually ‘gray’ aluminum coating the electrode.November 2, 2013 at 9:30 am #14526
Soo, I can’t answer the question of what happened to the motor. As much as I would love to tell you all what happened, I haven’t seen my motor in 2 months. It got lost on the way back from the builder and wasn’t insured for replacement value. So now I wait to see how the builder will handle it.November 11, 2013 at 8:38 am #15248
If anything I am setup a little on the RICH side.
I think you need to set up a little more on the rich side.
FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience
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