Well the title says it. My story starts with an MC28 with a 300 kit. So I've put about 1500mi on the crank and 250mi on the top end this year. Was running great and I went to switch domes from rocket fuel to pump fuel because its putting me in the poor house. Found a dent in the lower piston ( the one that always blows up?) TYGA sent a new one right out. Put it in went up one main. Making the front 2 sizes richer than the back. Did one heat cycle and headed to the track. First 2 laps nice and easy under 10 grand. 3rd lap full throttle 4th gear (pop) on the straight.

So I checked the compression it was 0. I knew it wasn't good. Later inspection revealed a hole almost the size of a dime dead center and tons of detonation marks or marks from pieces of piston around the squish band.

I don't belive it jetting because it went 250mi on 117oct and loved it. The rear looks great.

I am going to do a leak down test when my $188 piston gets here.( yeah the US dollar sucks and its comming from thia moon)

The only thing I can come up with is an air leak or ignition problem?

The pickup coils are tight and in the right spot. All connection are good. The top cylinder is good so that should cover shared ignition components.

I unfortunatle didn't do a leak down test before I tore it down.

I know this is a loaded question and only a few will be able to help but, if I spend another $600-$1000 for 2 more laps I may sell the NSR. I said I was going to last time but I talked some sence into myself.

Thanks,
Jeremy
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1995 A kit TZ 250
1994 Rothmans NSR 300sp SOLD
1998 RM 250 freestyle/motorcross
1998 Meathanol burning CR 500 AF super retard/motorcross/sand dunes/play bike

My 50p is on compression ratio. What card are you running too?
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Andy.
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Think about it, you ran rocket fuel for 1500miles, changed fuel and have now wreaked two pistons, so the underlying issue is likely to be one or more of;

High compression.
Weak mixture.
Too much ignition advance.

You can run whacky fuel and get away with a lot with all of the above, run the junk pedaled as petrol on the typical forecourt in the states and all three are critical.
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Hi,
As you say you were fine on "rocket" fuel, change the fuel and you have problems, in an attempt to overcome problem you have increased the MJ size , do not forget that this only accounts for flat out throttle position, as mentioned by another post you most probebly have a problem with over advance/mixture/ Octane problem but much further down the throttle range, many peaople forget this. I suspect the damage has been done whilst pulling between 1/4 1nd 1/2 throttle where yor may have too much advance to support your octain rating/mixture and thus the piston is already glowing with pre ignition when you start your flat out run causing piston failure.
Always set up your jeting/ignition map to suit the LOWEST octain fuel you are going to use , this way you will only be loosing a very small less than 1-2% of potential power when you go onto high octain fuel BUT IF you can not get this then the engine is safe to use.
When racing MX we used different maps/jetting to suit different octain rating when ever we changed fuel.
Make sure that you have the correct card to siut the octain rating of the fuel you are using as these control the ignition advance map to suit the fuel octain. Change these cards around to suit fuel in the tank. Alternativly you can use a detanation sensor placed into the cylinder ( much machining involves) and use this to control the retad of the ignition timing when it senses detonation. This is an expensive route to go, maybe you should just keep running high octain fuels.

BR.
Dave

I am running VHM heads. I ran the 117 on domes cut for 117. I found the dent when I switched to domes cut for 92. So I just caught it in time because it got dented with 117. The one with the dent even had a crack on the inside just behind the lower ring half way around the piston. I understand lead lets you get away with more agressive set up but the rear still looks great which tells me its not and advance problem or they would both be cooked. the rear has 0 detanation marks on it.

I am running an 030 card with an HRC harness.
_________________
1995 A kit TZ 250
1994 Rothmans NSR 300sp SOLD
1998 RM 250 freestyle/motorcross
1998 Meathanol burning CR 500 AF super retard/motorcross/sand dunes/play bike

The front is nearly always the first to detonate, hence the reason for many tuning companies advising on 0.6mm base gaskets when doing the HRC head conversion.

I know the HRC conversion isn't applicable in your case, but I am just pointing out that the front cylinder tends to be the weaker of the two.

The jetting for the front cylinder would appear to be just over the edge with the race fuel, and well over the edge on pump fuel, regardless of the increase in one MJ size when you fitted the UL inserts. One MJ size obviously wasn't enough. My MC21 runs 2 sizes difference front to rear (it also runs the needles in different clip positions), but there is nothing cast in stone that says it needs to be this way. There is no reason why there can't be a 3 or even greater size difference, or that the top can't be richer than the the bottom, as I have seen in a couple of 300's.

If you are positive that the compression ratio is where it should be, and you are using an 030 UL card, and the motor is in otherwise A1 condition, then the jetting will almost certainly be wrong.

However, if there is an air leak, or an O-ring leak (allowing coolant into the combustion chamber and consequently increasing the compression ratio) for example, then that will have a similar effect on the piston.

Running lean, low octane fuel, high compression, and too much ignition advance will all produce similar failures. Even the wrong choice of spark plug can cause the piston to melt in extreme circumstances! Combinations of these issues will just cause it to happen more quickly.
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Andy.
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I did start running those fancy 105 plugs recently. Can a plug cause a piston to hole? The hole is dead center under the plug? I don't know if this is common, I have only seized from a lean condition never holed a piston.

It only made it 3 laps after I changed fuel and compression accordingly but, there was already a dent forming in the first piston which would have most certinly turned into a hole very soon. could the change in compression speed up the process from 250mi to 3 laps?

I also checked the trans oil and it was not over or under full. I did not check the water for over or under full.

I guess I will install new piston. Do a leak down test. Jet SUPER RICH and I may even install some E.G.T.'s and give it another go????????????

If I only make it three laps there will be an NSR in the trash bin at Grattan Race Way
_________________
1995 A kit TZ 250
1994 Rothmans NSR 300sp SOLD
1998 RM 250 freestyle/motorcross
1998 Meathanol burning CR 500 AF super retard/motorcross/sand dunes/play bike

In my experience, jetting a 300 "super rich" isn't the answer; they are extremely sensitive to compression ratio. It sounds to me like the compression ratio was already incorrect with the LG inserts, and the race fuel (at some point) only just saved you from piston failure.

Fitting the UL inserts (even if they were correctly machined) has just shown up the compression ratio problem previously masked by the race fuel, and the lower octane/lead free fuel was unable to compensate for the incorrect setup any longer.

I would personally be inclined to pull both barrels and measure them.
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Andy.
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Wasn't it a 230 main that one died on Andy?
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Yes, 230 mains at one point, and the plug came out white!

It was actually considered at one point leaving the main jets out all together, just to see what would happen!
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Andy.
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I'm at 188 in that cylinder. I didn't think it was jetting. I think were getting somewhere with compression and crappy gas. I am going to measure the cylinders. Do you know where to get diferent thickness base gaskets? Can I just run two? I've done it on motocross bikes but once you get on the asphalt everything is different. So I'v learned. If so should I adjust both cylinders or tune individually?

I am waiting for 110 oct domes also which I hope helps. Its not $11 a gallon just $6 which isn't to far from pump gas anymore.

Thanks for all the help guys!!!!!!!!!
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1995 A kit TZ 250
1994 Rothmans NSR 300sp SOLD
1998 RM 250 freestyle/motorcross
1998 Meathanol burning CR 500 AF super retard/motorcross/sand dunes/play bike

jkolewski wrote:
I am waiting for 110 oct domes also which I hope helps. Its not $11 a gallon just $6 which isn't to far from pump gas anymore.

I'm sure you guys just quote things like that just to make us who live in the real world laugh!

Your race gas is cheaper than our pump unleaded!
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Andy.
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Don't forget their gallons are weeny!
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Have you physically checked your squish on both cylinders? Many 300's like Andy pointed out also run richer on the top so bear that in mind when jetting.

What carbs are you running? I'd have a good look at fuel range the needle is covering when re-jetting the rebuilt motor also.
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NSR300R - Why did i ever have a 250...

Maxim I'm running 39mm PWK's with 1269 RS needles.

Just measured the cylinders and one is .010" taller than the other. I don't have to tell you which one is the short one.

I don't have my piston yet (Where is Matt by the way?) so I can't check squish.

Is .010 a lot? I really don't know. I'm assuming its enough since my TZ gaskets come in .010 incruments. Can someone confirm?

If this is my problem how can I corect it? The stock base gasket is .020" Does anyone make .010 or .030 base gaskets? Are all model NSR's the same thickness?

Thanks again guy I think we're going in the right direction.

I'm not having a good season I've killed 4 pistons and 2 cylinders in 3 bikes
_________________
1995 A kit TZ 250
1994 Rothmans NSR 300sp SOLD
1998 RM 250 freestyle/motorcross
1998 Meathanol burning CR 500 AF super retard/motorcross/sand dunes/play bike