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Whats wrong with my 1978 RM400 Top End??

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These are pics of the piston in my 78 RM400. This piston is .50 overbore and has only approx 15 Hours on it? Why does it look like there is rust in the bore? The top is nasty too but doesnt look like carbon, looks like rust as it is kind of a brown color? The left side crank bearing also looks this bad?? I clean my air filter after each ride and the bike and the airbox are also very clean. I run Klotz Sythetic and CAM II 110 Race gas 28:1. Why all the blow-by on the Piston?? Thanks, any ideas whats going on in here??

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Did it run good when you took it apart? The discoloration is from heat and burning oil, as you know, 2 strokes have oil in the ga, so they're always burning oil, there is going to be somebuild up. I would say there isn't anything wrong with your 400..... nice looking bike by the way :bonk:

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Did it run good when you took it apart? The discoloration is from heat and burning oil, as you know, 2 strokes have oil in the ga, so they're always burning oil, there is going to be somebuild up. I would say there isn't anything wrong with your 400..... nice looking bike by the way :bonk:

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Looks to me like the ring never sealed up properly if it's that little run time .

What kind of oil did you lube the piston with when you put it back together?

(Assembling with the piston/ring dry is not unrealistic.)

Did you use the Klotz in the first take for break in ?

(some people like to leave synthetic oils out until after break in)

What clearance did the machine shop set it at ?

What grit of stone did they use to do the final hone ?

What kind of hone did they have/use ?

(this is pretty important)

Did they bore it first then hone it , or just use the hone to take it from the last size to this size?

(if it's more than a .010 increase that's not recommended )

If they did bore how much did they leave for the Hone ?

Did they have or use a dial bore gauge ? and if so what was the run out , top to bottom and radially ??

How were the ports chamfered ?

Lastly maybe it's just me but on a bike with a bore that big . I wouldn't use a single ring piston I'd use a double ring piston. Especially since I'm pretty sure your not currently using every last drop of power that thing is making or trying to win any sort of national championship etc.

The twin ring piston will just be a bit more stable in the bore and last a bit longer .

P.S. nice looking bike !

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Too much oil and too much octane.

28:1 is way too thick for Klotz or any other modern oil. Keep in mind that even though when this bike was sold Suzuki did recommend 20:1, HOWEVER, that was back in the day when everyone ran Castrol bean oil. Klotz or any other oil can protect your engine as good or better at 40:1 or even 50:1.

As for your 110 race gas, to put it bluntly your wasting your money. Unless you've spent some serious cash getting it ported and had the compression raised, then there's no valid reason to run a race gas in this bike. Pump premium is all you need.

FWIW: In my 1980 Suzuki RM250 I run PJ1 Silver Smokeless oil mixed at 40:1 with Mobil pump premium and the top end looks fine.

Clean everything up, lightly hone the cylinder, thin out your premix ratio and use a pump premium of your choice. Your problem will go away.

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Did it run good when you took it apart? The discoloration is from heat and burning oil, as you know, 2 strokes have oil in the ga, so they're always burning oil, there is going to be somebuild up. I would say there isn't anything wrong with your 400..... nice looking bike by the way :bonk:

It ran okay in lower RPM. At the higher RPMs it would sound like the detonation was way off? Like the motor was coming apart.

1. I do not know the measurements of what the machine shop bored too back when the top was rebuilt about 1 1/2 years ago. Since then it was ridden a handful of times on perfect track days only.

2. The few times I rode this bike the last 1 1/2 year I did ride it hard. It was NOT a put around the track bike. I ran at the motor hard those few times.

3. CAM II was used for the last 3 years as it starts easier, runs better on the 110 octane fuel. Had the same results with my 1979 YZ250. They just seem to like the higher octane.

4. Extra carbon build up I can expect from too rich but this stuff has small pieces in it. Almost like sand grains?? Stuck to the plug bad. Inside the wrist pin is like a sludgy grainy buildup of the same stuff. Again, if just too rich, why would the crank journals, bearings etc look like a brown color, like rust??

The story behind the last top rebuild was:

Rode the bike in May 2010 to practice for the Casey AHRMA in June. Bike ran awesome, washed it put it away. Friday before the Casey race I went to load it up and it was locked up in the garage???? The piston looked similiar?? Almost like water got in the bore?? Had to beat the piston out with a block of wood and a hammer? Thought I must have got water in the bore back then and then I did not start it after the wash??? Maybe that was never the problem??

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this stuff has small pieces in it. Almost like sand grains?? Stuck to the plug bad. Inside the wrist pin is like a sludgy grainy buildup of the same stuff. Again, if just too rich, why would the crank journals, bearings etc look like a brown color, like rust??

Almost like grains of sugar?

Yep, I know - absolutely impossible! It sure has that shiny brown look of melted, burned sugar to me, though. I was unable to find any pictures of it on the Web, although I read one description that was similar.

Even if a tank/carb check doesn't turn up any sugar, maybe you'll find something else.

Ray

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Listen to zig06...he's right on the mark. there's nothing wrong with that top end except too much oil (which actually LEANS the bike out). Don't use race gas...the only reason to use race gas is to eliminate detonation, and unless you've had the cly. or head decked, the stock comp. ratio does NOT require race gas. Waste of money. And as far as putting top ends on dry, that's entirely feasible...

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