"it Blowed Up, Sir"

Well, that's racin.....I went to Putnam with high hopes of a good finish in F-500. I was using the cylinders I had ported myself for the first time. The bike was running pretty strong in practice, and everything looked good. I went a little leaner on the jetting for the 2nd practice session and picked up quite a bit on the top end, and my EGT gauges were still below 1200 deg. Dick had some trouble with his bike and didn't make the start, and I got a decent start for once and was running 2nd behind Josh. There were 2 or 3 four strokes running up with us and we were having a good race for about 3 laps. About the 4th lap, I thought I heard a little rattle, and it seemed like I lost a cylinder, the bike started losing power. When I backed off the throttle, it seemed to pick back up, so I kept going. Going into the bus stop, I heard the rattle again, down shifted twice, and dropped it into the corner. I had not lost any places yet, but it seemed the guys in front of me were starting to get away. When I rolled on the gas coming out of the bus stop, the rattle got much worse and I had no power. I got over to the left side of the track and got a hand up, so I would not get run over. About the time I pulled in the clutch, the rear tire broke loose as the motor locked up, and I coasted to the corner worker tent in the grass in Dead Bear. When I was finally allowed to go get my bike, I was shocked at the damage. The right cylinder was ripped open thru the bottom of the exhaust port, thru the flange for the exhaust manifold, and about 2 inches down into the upper case. The rod, minus the eye for the little end bearing was sticking out under the exhaust manifold. The plug on the left side looked good, maybe even a little on the rich side. The plug on the right had not been hit by the piston, but it was burnt white, and covered with bits of aluminum. I'm thinking I had an air leak on the right side, the piston crown melted, and then seized, then the crank rod ripped out. I guess I should have pulled off when I lost power the first time, but I was too busy racing.... I am reloading this week and hope to be ready for the AHRMA round at Putnam next week. Thanks to Tom Bentley and Bill Moeller for helping me with my bike, and for putting up with my dumb questions....

 

that blows toad. ill be at vir or grand bayou this weekend. good luck.

 

Hey Evil, you may have already done this , but check your pilot jet on the right carb. I told you to run the 120's with the 17 tooth front and rear sprockets!! Just kidding.
Oh I didn't finfish last in the mock race just next to last.

 

Sounds like an airleak, but dont rule out other things. Diagnosis is always better in person.
Do you have spares to make vir? I was hoping to check out what you've done.
Just for the record:
In 8 years of racing vintage 2-strokes, the one thing that has saved me more money than anything else is to pull in the clutch and push it home at the FIRST HINT of a problem. If my old rz ever so much as backfired once,(usually a piece of piston/bearing cage flying out the exhaust port) I would push it back to the pits. Not only do you risk a body damaging siezure when you keep on running an ill engine, but think about how many times that piston goes up and down per lap. One time(at band camp... ) I was exiting t7 at road A and the bike hicuped one time, I pulled in the clutch and coasted to the bottom of the 'cavity'.(this was in the old days) It was july, 90 something and very humid, but I pushed it up the hill and back to my pits to find that one of the thrust washers had taken leave of the crank. There was NO damage to the cyls and only 2 dents in the head. (I had a spare bottom end w/ me and rebuilt the whole thing in time to make my race!) The crank was also rebuildable. If I had run that puppy down the back stretch wfo, theres no telling where it would have siezed and how bad the damage would have been. (under the bridge and a totally wasted motor would be a smart guess, not to mention the 90 mph crash that probably would have ensued)
Next time your bike makes a noise that it's not supposed to, stop. Check it out in the pits.

Good luck!!

 

2 strokes, 4 strokes, no matter, it sucks when "They done blowed up"

Any of you jokers going to make the Nelson vintage round?

 

Mark makes an excellent point about pulling in the clutch and pushing the bike back to the pits when something unusual happens. Lots of guys have tryed to retart a seized engine and ended up with damage like yours.

You may have broken a piston skirt or, more likely, seized the top of the piston and then ripped the wrist pin out of the piston and had the rod flail around doing all that damage. Is there a hole in the center of the piston? Is the edge of the piston eaten away to the ring groove? These are indications of detonation and excessive heat. Causes? Many including decreased fuel flow, an air leak, plugged exhaust. I suggest you examine your other cylinder very carefully to see if you see anything at all that is not normal. Since you are using an EGT and had 1200 degrees your jetting should have been in the ball park but the good cylinder might show some indications of problems.

Broken piston skirts are a problem, especially when you use RZ pistons. Flat bottom intake ports exaserbate the problem and personally have caused me problems. It seems like the porting I learned 20 years ago (using flat bottom intake ports) is not a good thing with today's pistons. (I never broke piston skirts 20 years ago, but now I only get about 400 to 450 miles out of RZ pistons before the intake skirts crack.)

Rick Butler, who monitors this site, has at least one spare RD400 bottom end. You might contact him for replacement cases. You can contact me at [email protected] if you want to talk about your problems some more.
Lyn Garland

 

That would be me. E mail me at [email protected] if interested.

 

hey rick, are you going to race this year or what?

 

No full time employment = very limited racing budget. :-(

I plan to be at TGPR later this month.

 

Yep, It was detonation....

The autopsy revealed the cause of death was a hole blasted through the top of the piston. The entire crown looked sandblasted. The piston seized in the top of the bore and the crank ripped the piston in half. My plugs looked good on both sides all day, and I am running the Pro-Flow Ultra ignition, so I don't think it was a jetting, or timing problem. Maybe the right side had a blockage and starved for fuel? I tore the carbs apart and checked and cleaned them, but I didn't find anything. Anyway, thanks for advice and offers of parts. I have it back together with my "good" cylinders and crank, so I will be racing AHRMA at Putnam. GOOD LUCK TO EVERYONE AT VIR.

 

Evilhorde said:
Maybe the right side had a blockage and starved for fuel? I tore the carbs apart and checked and cleaned them, but I didn't find anything.

While that is possible, is it likely? Detonation usually takes a while to put a hole in a piston and a blocked main jet could cause a seizure but I don't think it would let the bike run long enough to do that damage (since you didn't find any blockage in the jet).

It is my opinion that you have an air leak or that the ignition went bad on that one cylinder. The Hitachi ignitions fitted to the TD/TZs by Yamaha had a failure mode of advancing on one side doing just what you experienced. In my experience when the Moto-Plat ignitions failed, they just quit sparking - no advancing. I haven't had a PVL failure yet, but hope they do the same. The Pro-Flo is a modern unit and I doubt that they have had many fail. You might give them a call and see what they say.
Lyn Garland

 

The Pro flo ignition uses a single double ended 4 stroke style coil, it fires both plugs at the same time. With it if there was a change in timing it would cause a problem on both cylinders.

What about fuel?

Where was your timing set? a little to much advance can cause a motor to slowly burn up if it is close to the edge, ok for a couple laps but as the heat builds up the internal temp overheats and ohhh ohh you have that HUGE new transfer port in the middle of the piston..

Bob

 

Here's my experience on holing pistons. I'd like to hear what others have seen.
The "other" piston has always looked OK when I've had this happen. To me, this does not rule out timing or octane problems. I believe the reason is that the onset of detonation is such that it doesn't take very long from the first few pings to the grey molten mess, especially on an aircooled. The cylinder where it begins is destroyed quickly, before the other one gets a chance to join the party. That's how it looks to me, anyway...

 

I usually set the timing at...hey that's a secret... ...Pretty tricky there Yamaha Fan. I usually have it at 1.5 mm, but I tried running it at 1.65 this time with my usual mix of Turbo Blue high lead and Klotz R50 at 20/1. There is no sign of any problem with the other cylinder. I guess I had an air leak, or a partial blockage in the fuel system on that side? I just hope I get thru this weekend with out doing it again.

 

First,I agree with Carl,one side may deto.with one side looking good,from my experience.Until I got my RD tuning act together,I've overheated several(many?)engines,for several different reasons.I'm sorry,but I refuse to use a EGT anymore.A few times,the guage read 1100 degree,and still would hole a piston,what's up with that?I ended up using one tuning method for RD400s:find a mainjet that ran so rich that the bike ran rough from misfires,then lean out one size(10 numbers in Mikunis)at a time until the heavy missfire went away.The plug,after one track session, has a light brown-to brown insulator,tan is too lean.Sometimes the jetting results in a LIGHT misfire,you have to make a judgement call.Run a couple HP down,with assured reliability,or lean one more size and see what happens.Note that a 350 can tolerate leaner jetting,the two bikes are different animals.The piston tops are brown and black,tan too lean.Sure,you can prove me wrong and ride several races with tan pistons,but is your bike as fast as mine?How much compression seems to be the factor,exhaust port height ,pipe,head,all alter compression,you may not have as much as mine.Yes,every race requires the find-the-rich-jet-and-lean-out process.A hassle?not compared to looking at holed pistons at third call of F-500.(been there,done that)The benefits?fast bike.I like that.Another thing,leaned out jetting doesn't seem to go any faster,another myth?or is the extra fuel keeping the engine cool,making power that way?I've tested different ign.timings on a Dynojet. 1.8mm made power,1.5mm made the SAME power,everywhere,1.0mm dropped power about 1.5HP.Here's my advice,tune for the least advance,and the richest jetting,for peak power.By the way,don't jet two strokes on a Dynojet,only at the track,the jetting needs to be WAY leaner for the bike to run on the dyno.So,to sum up,what do you guys call a good-looking piston or plug?Tan,brown,black?

 

Tan looks real nice color-wise, but when I see it I put in a bigger jet. I like brown/dark brown when I get it to that point the bikes run reasonably well and don't seem to seize as often.

I don't know about you folks, but I on some of my bikes I have run different size main jets on cylinders, 10 apart, to get the jetting right. My TD-3 has been running that way for the last two seasons.

 

Additional tidbits

First I believe there is a definite difference between piston port and reeded 2-strokes. Reeded (non crossover tube twins) should slightly stutter at the bottom they actually make more power when setup like that. If you try and jet the stutter out of the motor it will be to lean. Second the fuel helps cool the motor especially in an air cooled motor. I have quite a bit of experience on 500cc 2-stroke singles, and have seen the jetting issue first hand. I agree with Stan and Carl on the detonation causing one side to fail first as it happens quickly. As for EGT a motor that is detonating can have an acceptable EGT reading, a motor with late timing or lean a main jet or an air leak will show a high EGT reading. Detonation causes the protective boundary layer that insulates the surface of the piston and in the squish area to be disrupted, this causes the piston to absorb abnormal amounts of heat very quickly overheating it, this in turn causes it to fail. The absorption of this heat reduces the heat expelled in the exhaust thereby not sounding the alarm that would normally appear on the EGT. Knowing the sound and feel of detonation is critical to saving a motor, a hard thing to sense but if you can you can save the motor…

 

Well said Stan, Boadie and Bob. I couldn't agree more on most points. After years of running rz's w/ very light plugs, (light tan to beige) I quickly found out that rd's like a much darker plug.(richer jetting) I'm sure it's atributed to the air cooled motor needing more fuel to keep it cool. (modern gp guys look for WHITE on their piston crowns)
I'm not so sure that 350's are so much more forgiving than 400's. I've always thought that higher compression made jetting more critical. Gary's motor building philosophy has always centered around lower compression for reliability, strong top end and ease of tuning. My 400 that I'm riding this year is WAY more sensetive to jetting changes than the 350 was, but it's putting out over 40psi more compression. Does a stock 400 have more compression than a stock 350?
I've never owned an EGT and every time I've borrowed someones bike that had one, I got inconsistant readings. I don't trust them. Plug/piston readings will tell you everything you need to know. (IMHO) I do love the Turbine Inlet Tempurature (TIT) gage on my dad's Money 231. (Single engiened, turbocharged, 4place, R/G plane. VERY fast) Lean out the mixture until the engine starts to stutter, (usually around 1350 deg) and then richen until TIT reads -200 deg and you have the ideal mixture. Dynos are somewhat similar, you get you're highest numbers running jets way leaner than you know will survive at a race track.
Timing, I've always run on the conservative side. Why risk holing a piston? Bike runs better up top(rpms) w/ more retarded timing anyway. (TGPR is an exception, there, I'd risk a little more advance)
Detonation is a tough mother! Poor fuel, lean jets, advanced timing and "hot spots" in the combustion chamber can all be causes. The important thing is that the second you recognize detonation, you should pit IMMEDIATELY!!!! The one time I put a hole in the rz's piston, I made one lap of practice and had to pit for a "fouled" plug. (fouled w/ aluminum that is...) I didn't recognize the symptom, put in new plugs and hole the piston on the very next lap. 2 laps total. Brand new pistons. It doesn't take much.
To get to Evilhoards problem, I think the best we can do is guess, but my first bet would be on lean jetting. You were running "new" porting. The general rule for ANYTIME you try something new is to put in the biggest jets you have and work your way dow until it runs right. I know it's a pain in the a$$, but it's better than blowing shit up. I wouldn't rule out an ignition failure, but I think it's highly unlikely w/ such a new unit. We'll see what happens when you put your known, proven, cylinders back on w/ jetting that you know works.
Good luck, I hope to see at least SOMEONE at VIR!

 

Hot issues

The issue of a “hot” cylinder may occur as a result of a poorly centered cylinder head. Most of the really successful high output tuners center the head and pin it to the cylinder for alignment. An off-center head creates a reduced squish and a exaggerated squish condition. There is quite a bit of room for the head to float on a stock cylinder. It is critical that you get the squish right when you try and run higher compression. As you go to assemble your motor next time, get the head bolts down near the point of getting snug, then slide the head around, you will see how much room for misalignment there is. I have not yet built a fixture to align my heads so I can drill/pin them but I am going to do that eventually. The RZ and TZ motors are pinned from the factory eliminating this issue

There is no doubt that high compression motors are much more sensitive to jetting and they have less operating margin. My riding skills do not yet put me in a situation that I need more HP so it is not a current priority. I do try and carefully center my heads when I assemble the motor.

I have purchased a flat piece of marble (tile from Home Depot) I use water based fine lapping compound and lightly surface the top of the cylinder and the head. I think this helps insure a good seal on the head gasket. I am going to try and get a hole water-jetted in a piece so I can do the base gasket surface also.

I have never measured the stock components between the 350 and 400 to see if the static compression ratio is different. My current motor is in the range of 6.25 – 1, and seems low enough to be very forgiving but still puts out good power.

 

Good comments,Mark,common knowledge of these experiences will(hopefully)get rid of the notion that two-strokes are unreliable and blow up all the time.And maybe then we don't have to hear so much trash from Webber! (At least I can jump over to both sides of the fence when the mood suits me)
Tidbits:
I test parts on the HMF Engineering 150 Dyno in Cleveland.Tune final jetting on the four-strokes with the on-board gas analyzer,don't rejet at the track.My fuel,Citgo 110 leaded,doesn't leave any color to read on the plugs anyway,and runs clean even when 3 sizes too rich for max power.My RD400 races with 370-380 mainjets,it doesn't run clean on the dyno until I go down to 280-290 mainjet.I think this is because of the bottom end getting loaded up,not for any real fuel/air reasons.If someone wants to try a long steady-state run on a load cell-equiped dyno,they may get the bike to run in a simulation of the track,but you probably won't replicate the cooling effects of riding it.
Blow-up #329:The bike is running with a little rich misfire at Putnam,go down one size,holed piston.Reason?someone at one time removed the emulsion tube at one time by tapping on the jet with a phillips screwdriver,leaving a closed-in jet from the peen marks.Duh.
Warning:Real Mikuni jets have a square-within-a-square mark on them,there are aftermarket jets with a "RD"mark on them that are about 10#s different in size!Don't interchange the types.
Blow-up #583:don't use non-Daytona stock pistons with the cutaway in the exhaust skirt.Exhaust mixed into your crank makes it real hot!
Scratched my head at the GNF for a whole day#7:A choke plunger was not sealing because the plunger was rotated from the position it was in for the last 20 years and the "nipple"in the choke bore was off-center.