New post from Loss of Compression post( viewtopic.php?f=1&t=16807 )- now a carb issue so new thread

First year pilot owner so still learning. A lot.

Extensive rebuild and (almost complete) restoration that ran great for two months. Then it melted the rings out of the blue. Since then, Engine has had leak down test and fuel pump test done, and passed. Twice. And eaten two more pistons for lunch on my tab. Carb has been cleaned and checked(?) and seemed to be jetted well until it leaned out the first time. All rebuilds went through the Hoser recommended break in ( viewtopic.php?t=3114 ) prior to attempting to re-jet. Never able to get it right, Always started and idled well but regardless of jet size, from 160 which was the original jet it ran great on for two months to 182 which ran like crap off the line until you could get rpm's up all came up gray on a plug chop and never got more than 15 minutes of riding at 3/4 throttle after break in before it got hungry - symptom of an air leak.

Picture of piston(s). They are shown in order if use.



Here's the question: Using a 39mm Keihin - will a throttle cable with the plastic covering worn offof the end where it fits into the top of the carb cap leak enough air in to lean out the Engine? Here's what i started with:





There was a pretty big area around the metal of the cable where the plastic had worn away and the housing that it was fit into with nothing to stop airflow.

Here is the after with a little help from some heat shrink tubing (any better ideas for this are welcome also):



Could really use feedback from anyone on this. The Engine and intake do not leak. What is left but the carb?
More time fixing than riding, not cool.

Thanks for any help.
Dave

Yeah I would say that would be an air leak.
Looking at those pistons it looks to me like there is no sign of detonation so fuel would be good.
It seems to me based on what has happened to all my fl350 pistons that your Engine is hot or the clearance is not enough. Opinions from others ?? Lets vote.

I just keep looking at those pistons. They are all damaged in exactly the same spot and look almost identical. If it was detonation of fuel, the top edge of the piston, usually at the exhaust would be chipped away, but that's not the case here so the fuel is good. The pistons are damaged on one side only -- all of them. They appear to be hot in that area only. Can you check to see if all of the cooling ports in the head and more importantly the cylinder, are all clear and not corroded or plugged ?? Is this the exhaust side of the piston or intake ?? (Never mind it's the exhaust. Just figured it out).
You said: Extensive rebuild and (almost complete) restoration that ran great for two months. Then it melted the rings out of the blue.
So what did you change if anything before it blew up ?? That carb cable was probably the same in the two months prior. If you changed nothing then something changed over time. Coolant flow ?? I am really grabbing at straws right now.

Cable on carb won't cause an air leak. When you do a leak down test, you're testing the cylinder and bottom end for air leaks. Looks like your clearance is still too small and that's why the pistons are melting. Any time you have the cylinder bored or honed, you need to check clearances. Measure both the piston and cylinder or have the shop your taking it to show you what the measurements are. Use the clearances recommended by weisco, not the pilot service manual.

A few more things when doing a leak down test.
Take off carb and plug intake side with a rubber plug
Take off exhaust and plug opening on the cylinder again with a rubber plug
Take off hose from pulse and plug the pulse on the cylinder

Pilot Bird is correct, although the cable is not sealed and will let a small amount of air in, the hazard here is more along the lines of wearing out the slide in the carb because of dirt getting into the top of the carb and not leaning out the Engine.

What do the other sides of the pistons look like?

Are you sufficiently letting the Engine warm up before you hit the throttle? The piston itself needs to come up to temperature and expand properly. This is a big issue with a belt drive system like the Pilots and the Ody's have. You can't baby the throttle and gust put around until the Engine warms up. Remember, the clutches don't even engage until the rpm's hit around 3000. You must let it warm up properly!

And again as Pilot Bird mentioned.... what are your piston to cylinder clearances?

Rand

Clearances on this build were at .004 (Wiseco recommendation). Leak down test was done with a leak test kit that had all the correct plugs and was done as you outlined above.

Definitely letting the Engine warm up well before taking off on it and was still babying it around - nothing over 3/4 throttle when it blew and this was after 4 complete heat cycles (up to temp and complete cool down). Clearances were at .004 per wiseco specs.

Here are all four sides of pistons placed in order of their demise, from one to three.

Well seeing all sides of the piston now my money says your clearance is just not enough OR your not getting proper cooling. No damn way can a piston be scared all around like that without clearance issues in my honest opinion. The intake side is the cool side by far, yet it is scared up. That just screams clearance to me, not air leak.

Was there any hone work done on the cylinders between pistons? If so, what type of hone was being used? Were the hone marks uniform all the way around the cylinder?

Normally on jetting issues, the exhaust side of the piston is melted and scored but you are getting scoring on the sides. This is pointing to a cylinder that is out of round or possibly a 4 corner seizure.

Are the score marks on exact opposite sides of the piston? If they are, check your bore diameter on the area of the cylinder where the scoring is occurring versus the aide that has no scoring. My bet is there is a difference in the measurement which means your cylinder is out of round.

Rand

I just had another brain fart.
You said it was all running good after a re&re and then suddenly it eat a piston. Then you put in two more and it eat them. But you also found that you had a ground issue on the radiator fan and it was not coming on. So my question is: when did you find out the fan was not coming on ?? Which piston 1-2 or 3 ?? If you didn't find it until #3 Then there is the answer. No cooling = loss of clearance. If you didn't find it until #3 but #3 was still running when you found out, the damage was already done.

I second that and feel silly for not seeing it myself.
The carb cable could have contributed to higher combustion temps ( by leaning it out slightly ) but not enough to melt the piston but just enough to heat up the Engine faster than normal . If the cylinder was out of round I'd think piston scuffing would be on two opposite sides.
So it looks like the scuffing got worse , I'm wondering if you may have extended you're heat cycling each rebuild to be safe and with no cooling fan the damage was done before you ever rode it or was at least starting.
Take everything I say with a grain of salt. I'm no expert I'm just brainstorming here.

Hey guys, thanks for the feedback. Going to try to keep get the answers straight here:

CO - the ground issue on the radiator was a brain fart on my part - missed connecting the ground wire to the radiator when i installed it. Works perfect now and only did heat cycles on it, no riding. Longest cycle was right at around 20 minutes and after correcting, noted the fan coming on at around 15 minutes. When i did get some riding in on it, it was cycling on and off. This was on the most recent piston. As for clearances, was told by the shop the clearances were at .004.

Randman & Fla Edd - Cylinder was bored for the second and only required honing for the third. Couldn't tell you what kind of hone was used but will check and the marks appeared to be uniform.around the cylinder. The marks (piston #1 is scarred on three sides, #2 is on all four and #3 is only on 2 sides) are on almost exact opposite sides of each other with the worst of them next to the exhaust.

My question - If the cylinder was not perfect but ran for a bit, wouldn't I still be able to see some difference in the plugs from different jets? Did numerous plug chops on the most recent piston, 172 seemed to run fairly well but plug was grayish. Went up to a 182 to ensure it would be much richer, ran like crap but i was able to get rpm's up and plug was still gray?

Damn this is a tough one, but my money is still on the clearance.
I run .005 on mine and even have a .006 Engine. They run well, but are FL350 with homemade water cooled heads and cylinders. With pistons that are scared all around that is a clearance issue. What is causing it is the mystery now. I don't know what to say at this point, and that's unusual. I like to blab mechanical.

Take a vernier caliper and measure it out to .004
That's what you got

Just came across this - an excerpt from another thread. Looks familiar. Actually, it looks exactly like all three of my pistons.

FOUR-CORNER SEIZURE
This piston has vertical seizure marks at four equally spaced points around the circumference. A four-corner seizure is caused when the piston expands faster than the cylinder and the clearance between the piston and cylinder is reduced. Another common problem of this type is a single point seizure on the center of the exhaust side of the piston. However this occurs only on cylinders with bridged exhaust ports. The main causes for this problem are too quick warm-up, too lean carb jetting (main jet), or too hot of a spark plug range.



Kind of pisses me off. I don't have the equipment or skills to hone or bore the cylinder myself and check for proper clearances so ultimately at the mercy of the shop that does it.

I can hear the laughter already but can I make a suggestion ??
Have that cylinder bored to .006 clearance. Don't go to the next size, just bore to .006
Reason: I don't like the Wiseco as I feel they "grow" a lot. I can not prove this. It's just an opinion. Wiseco has gone to the maximum profit model of business and they know they are the only game in town for oddy piston, so they don't give a shit. The proof is the pistons today weigh more than they did years ago when Wiseco cared. No longer are the pistons milled in the wrist pin area.
Also verify that you are running good fuel. I run avgas in my machines and they run much much cooler. It does not appear you have this fuel issue here but I am just grabbing at straws.
Just another opinion.
Opinions anyone else on this suggestion ??

Just went through some of my pictures. Check out the writing on that cylinder when I sent it out.
That was the sixth rebuild on my brothers machine and it is still in his machine and running strong.
That was in June 2014.

Yep, we're laughing. You don't need to bore to .006" clearance.

I believe Can Odd is on the right track though with heat and cooling issues. Here is a little blurb about 4 corner seizures. It is from a watercraft post but it pertains here.....

Four corner seizure - This is by far the most common type of seizure found in personal watercraft engines. Both sides of the piston will show heavy scoring and seizure marks on each side of the wrist pin hole. The pattern of these four seizure points often appears to be a perfect square, hence the slang term "four corner". The scoring takes place in this pattern because those areas of the piston casting are the thickest. When the piston is seriously overheated, the thick areas will expand and distort the most. High output motorcycle engines usually experience this type of seizure pattern when a piston has been fitted with too little clearance. Most experienced , and well meaning, motorcycle mechanics would take one look and immediately say that insufficient piston clearance is the cause. However that diagnosis, on watercraft engines, would be wrong about 99% of the time.

Four corner seizures in watercraft engines are almost always a result of the Engine creating more heat than the cooling system can exchange away. That is not to say that most cooling systems are under built, but rather that it's easy to make a modification that creates too much internal heat for even the most beefed up cooling systems. Even though a constant feed of cool water is being moved through the cooling system, the cooling system must be capable of exchanging the Engine heat away at a rate quicker than the Engine is creating it.

The Engine factors that have the greatest seizure related effect on operating temperature or excessive compression ratio or ignition advance, high rpm, insufficient fuel octane level, insufficient cooling, or any combination of these. Properly adjusting these same factors will have the greatest effect on total power output. The job of the professional Engine builder is to find the right combination, or "blend", of these factors that will result in strong overall power output at a pace that your cooling system can keep up with. There are many mechanics and Engine builders who have mastered their own combination "blend" that can get you all the power your after without risking a seizure.

A group of mismatched modifications is a first class ticket to "seizure-land". Any inexperienced individual who sets up your Engine with over 200 psi of compression and advanced timing, is also guaranteeing your arrival. If your big mouthed motorcycle buddy down the street tells you that he can make any Engine "roost"...you should think twice. You could be in for a very expensive lesson.

When I bought my Drakart it had a burnt valve. After many hours of testing and checking things I traced it down to a tiny hole in my vacuume advance diaphram.
You think .006 clearance is crazy just listen to me a minute.
Take the carb off but leave the cable attached. Block the air filter side. Take a plastic soda bottle and cut the bottom off. Tape the cut end to to the intake side of the carb.
Now see just how much air is passing you're cable with and without you're shrink tubing fix buy pulling air through the soda bottle top. ( I avoided telling u to suck on it )
You could go one step further and take a empty soda bottle and make a hole in it and see how big u need to make the hole to match how the carb feels.
I'm not saying this is the only issue but may be contributing to it. If hardly any air passes I would think it not an issue. However a bunch of air I would consider it a contributing factor.

You can start laughing now.

check your cooling system, make sure you are pumping coolant back into the radiator, you should feel heat going through the hose that goes to the top of the radiator, they are not crossed up are they your pump will not work very well if sucking air from the top.

Plug chops are difficult to analyze if you are not familiar, Hoser has some posts here about it, you need a 10X magnifier and a good light to see way down inside the plug to do it correctly.

another clue here is you say it always happens at 3/4 throttle? at this point you are mostly on the needle and could be running lean, I don't recall seeing what the top of your pistons? from the sides they do not look like your burning them but???

when you measure the clearance, piston to cylinder, you know you must measure the bottom of the skirt front to back? they are not the same size top to bottom, your shop should know this but then again you should not be going through pistons like this either.

probably most everyone has pulled a dumb move before, I know I have on many occasions and just tossing out some stuff for you to look at which could all cause similar results.

Don't be discouraged, these machines are a ton of fun and well worth a bit of head ache.

Keep the rubber side down

CO - thanks for keeping the ideas coming but I already have the 82mm on the way. I agree these bastards think they can do what they want but unfortunately and as you said, they are the only game in town.

Randman - thanks, I have already read that and so much more and I still have not confirmed the root of my problem. I do have a much more thorough understanding of 2strokes than I ever thought I'd need though. That's progress(?).

Fla Edd - Agreed, more than likely not the primary cause but definitely a suspect for a contributing factor.

Kuma- I'd like to say with confidence I'm pretty sure the cooling system is functioning as I should but at this point, I will confirm again because, well just because. And - I agree, wheels down is the way to be.

happy Friday eve

For jets, the carb is normal. I looked up the needle you on Kehin scamatics and a DGJ needle is in the norm of not being to lean or rich. A 52 pilot jet is within a normal range. Did you check how many turns out your air screw is?

Also, did the radiator fan ever kick on when you did your Engine break in?

He said it came on at about the 15min mark.

I would like to make another suggestion here -- for the break in period and the first hard ride I would run avgas. You can run 50/50 avgas premium fuel mix along with your regular premix ratio if you want.
I know your heat will be a lot less. When we run the FL350's with avgas we don't even let them warm up, we just stomp on it. The reason I suggest this is to eliminate the bad fuel thing for sure.
Any votes on this anyone ??

Cruz - air screw was only out 1/2 turn which I though was strange but wouldn't that allow less air creating a richer situation?

CO - definitely gonna try the change up on the gas for break in. good idea for helping run cooler.