Hole in piston

[Hodaka250SL]

I have a 1978 hodaka 250SL and It stopped running yesterday. And I found out that my piston had a hole in it. What would cause that to happen

[matt glascock]

Lean condition, engine air leak, faulty crank case seals, heat range on plug too hot, poor fuel, wrong gas:oil ratio, plug reach too long, preignition/detonation. These (and more) can hole a piston. What did the engine sound like moments before it died? Did you hear a ringing/pinging sound?

[Bullfrog]

. . . and what were you doing when it got noisy . . . and then quiet? Throttle position? Engine speed? Type of riding?

Ed

[Hodaka250SL]

It didn't make any noise when it failed. I was riding back from work and it just lost all power

[Hodaka250SL]

I think I was hearing a pinging sound just a few secs before it died

[viclioce]

From what I've read here about the 250's is they are notorious for this. Some say it's because the compression is too high and have taken to making a thicker, copper head gasket. Others have used 2 head gaskets on top to decrease the compression & increase the volume of the squish area. Though some people recommend against the double head gasket scenario, making a thicker one out of copper can give the needed space. I don't own a TDog so I don't have first hand experience. But this is what I have read here about this problem. I assume you're using a premix ratio of no leaner than 32:1? What spark plug are you using in her? ; D Victor

[Hodaka250SL]

I'm using 4 once said of oil to 1 gallon of gas. And the spark plug in will have to get back to ya that one cuz I have to find it

[---]

Oil/fuel ratios rarely lead to holed pistons. Much more likely is the mentioned detonation, which is a function of compression, octane, and advanced ignition timing. While the hole is obvious, look at the perimeter of the piston crown. It is pitted like it was blasted with rocks. That's detonation.

There are many technical treatices on the net explaining detonation. Just google it. I think there has also been discussion here about the 250 ignition system advancing at higher rpm. I would seriously consider a different ignition system, or check timing on the one you have with a good timing light at higher rpm. If it advances as speed climbs, back off the base setting so that total high speed advance is within usable range. Others here know more about the system than I do and can explain.

[Hodaka250SL]

The spark plug that was in it was a ngk B8ES. The piston has no damage to it except for the melted hole in it

[Bullfrog]

With all due respect, there is considerably more damage to the piston than just the hole. As suggested by Arizona, there is definitive evidence of detonation (the sand blasted appearance at the perimeter of the piston crown). The hole is the final failure due to the ongoing detonation.

As already mentioned - please confirm the ignition timing. Do a pressure check on the engine to assure there is no air leak past a crank seal or at some other location. Air leaks cause lean-ness (and can cause detonation). Check compression - anything over 150-160psi - is asking for trouble in that engine with stock components. Check jetting. You were probably cruising at somewhere between 3/8 and 3/4 throttle, which is the range of throttle where jetting is most definitively controlled by slide cut-away and needle. Since owner tuning of needle clip position is easily done . . . a lean condition could have been introduced in that throttle range.

Ed
PS: ANY time you notice a new and strange sound coming from a two-stroke engine (especially while at cruise) - it is virtually NEVER harmful to RAPIDLY assume that detonation is happening RIGHT NOW . . . and that you should do something different RIGHT NOW. Check mirror - if safe - pull clutch and back off (and/or burp) the throttle and do it FAST! DO NOT cock your head to listen more closely to the new noise while asking yourself, "I wonder what that noise is?" It will get very quiet before you figure out what the noise is. Don't ask me how I know.

[Hodaka250SL]

I took another look at everything and found a small hole in the rubber piece on the back of the carb going to the intake manifold.

[Hodaka250SL]

How do you adjust the timing on these

[ossa95d]

As usual Ed is absolutely right on all counts. However, as Matt mentioned we shouldn't minimize the possibility of old gasoline as a contributing factor. Modern pump gasoline (especially with ethanol) deteriorates very quickly and can drop several octane points in a relatively short period of time. I have seen a 250ED detonate like crazy and we couldn't address it through timing or jetting. Put in fresh gas and were able to go back to the previous timing and jetting. It's a long story but it all happened at the 2010 ISDT RR in Arkansas, right Paul!

The 250 Hodakas seem to be particularly sensitive to these things.

[ossa95d]

While I was writing my reply it looks like you may have found the smoking gun if in fact there was an air leak between the carb and the engine.

[Hodaka250SL]

I just filled the take up with fresh gas that morning when it happened

[Hodaka250SL]

As I get the top end rebuilt I'll keep everybody update on how it goes

[DGardner]

I know that the hole in the stinger on the 250 is very small and it makes me wounder if this could be part of the problem. Kels has talked about this before and this is was Bell has to say about it from his 2 stroke tuners book. (good find on the air leak)
TABLE 4.6 indicates the stinger dimensions which I have found to be most
successful. You may find that a minor reduction in pipe diameter will raise the power
output, but do be careful. A stinger pipe smaller in diameter or longer than specified
could easily result in engine overheating and seizure. Therefore, I would suggest you
make only small changes and then thoroughly test the pipe before going any smaller.
Engine overheating, in its early stages, is indicated by the presence of oil burnt dark
brown under the piston crown. On the next stage the burnt oil turns black, until finally
'death ash' appears. After this, the piston can be holed at any time.
TABLE 4.6 Stinger dimensions
Cylinder size (cc)
50-80
100
125
175
250
350-500
Stinger length (mm)
205-230
230-250
265-290
270-295
280-305
285-310
Inside
17-19
19-21
22-24
25-27
26-28
27-29
dia. (mm)

[Hodaka250SL]

What is the stinger

[Bill2001]

AGree with Bullfrog, Victor, Ossa, DGardner, et al. Wish we had a "Like" Button.

Big problem is Detonation. Caused by: Timing, Fuel quality/Octane, Mixture (slide# needle# needle jet#, fuel level)(check for air leaks), just for starts.

--Bill

[Hodaka250SL]

I believe I found it and will do more testing when I get the top end rebuilt. To see if I can find anyone air leaks

[---]

Once rebuilt, do a compression test. If you have never done one, ask when the time is right. It takes more than just a compression gauge.

Also do a leak down test. This will help establish if there is a seal leak, a cylinder head leak, a base gasket leak, an intake manifold leak, or a whatever leak. Cylinders, even when fresh, do leak down, but must not do so faster than an acceptable rate, so again, once you get to that point, ask if this is a new process to you.

An air leak in the intake manifold between the carb and the cylinder will play heck with jetting and can cause failure. A leak between the air filter and the carb simply introduces dirt but doesn't cause leanness or failure per se.

I would consider modifying the cylinder head on the 250. Rich Gagnon can post photos of what the mods look like, and they do help prevent detonation. Or look at his website for photos. Rich's Taylor'd Porting if I remember right.

[---]

What is the stinger

"Stinger" is a term referring to the tailpipe of a tuned two stroke exhaust system. It is commonly the smallish diameter, straight pipe between the belly or final cone of the pipe and the silencer. Sometimes, the silencer is technically part of the stinger, so simple charts showing lengths and diameters of the "stinger" is of little use by itself, even when the data on such a chart is correct in theory. Stinger charts generally assume there will be no muffler or silencer after the stinger, i.e., an open pipe. No one allows such things anymore except, perhaps, Bonneville. Not to mention that for any given combination of cones and head pipes, there is a fluctuating range of acceptable stinger, and the combination of cones and head pipes and stingers is predicated on displacement, porting, carb size, and just where in the power band you want peak torque and hp. It is a very fluid formula.

Bottom line here is if you don't expect to build a tuned exhaust from scratch, don't worry about your stinger. Just keep the silencer and pipe clean.

[Hodaka250SL]

How do you preform both of those tests. I would like to learn how to do them my self so I keep my cost down. Since I drive a school bus. And I am off for the summer

[DGardner]

look up part number 73 that's your stinger......see if it is plugged up with carbon. I think Danny has found out the stock Combat stinger or spark arrester was on the small side so the 250 could be also.

[MWL]

This is my leak down tester. I have adapters to fit both my 125 and 250. Clamps into the intake manifold in place of the carb. The rubber gasket blocks off the exhaust port. I carefully pump 6 psi with a bicycle pump. You can look for leaks with soapy water. I'm sure there is some debate on what is acceptable pressure drop, but any more than 1 psi per minute then you have a problem.