RZ/RD 350 & Misc. 2-Stroke Tech BBS

The engine had been modified & raced before I got it, melted down also. 66mm bore, head looks shaved. Toomey pipes, stock 26mm Mic US '84RZ carbs, 420 mains (up from the 410's I found in there), Y boot with UNI. Measured squish band at 1.2mm. I , resealed the crank, repaired the bores, added EGT's and observed reasonable temps, around 1200F at WOT short pulls. BUT, the coolant temp was always high. After 350 miles I cranked on it, redlined 3rd, 4th, 5th... resulting in coolant temp pegged, coolant overflowing the reservoir, and a light seizure. EGT max temp logged at 1450, obviously China Syndrome.
The wash on the piston crown looks leaner on the melted (left) side, and that side consistently showed 20-50F more EGT all throughout the rev/throttle range.


Piston crown isn't melted

1200* fahrenheit seems a bit high, what is it supposed to be? Around 1150? I think i remember Ed mentioning a lower temp is desirable. Were you using the stock spark plugs on your WFO run? You even permatexed the head gasket (did you use an OEM head gasket? <---important for good seal). Looks like a typical 4 corner seizure. Possible causes are too quick a warm up, too hot a plug (stock plug, or go 1 step colder) and too lean jetting. Wonder if it could be as simple as too low anti-freeze. Last time I blew anti-freeze out the overflow was a blown head gasket at the track doing a T.O.R. (taste of racing).

YOu mentioned the water temp being high also...probably a good time to inspect / replace all that shi*...impeller/gear /bearing/seal/ pipe o-ring/thermostat/temp/sender. coffee colored anti-freeze is a sure sign that water is in the oil.

That sucks. Looks like you had it all setup. Wonder on RZs how porting affects the jetting. Just wouldn't know. What color is the underside of the piston? if you can find the guy that ported it (hopefully a professional), ask him what the ballpark jetting is. The less work you have to do, the more ride time you have!

When you do get it back together, I remember my tuner saying it is better to get a ballpark jet in the lower gears, initially. The upper gears offer much more stress and possibility of immediate blow-up.

You can seize on the needles too. I remember seizing after a top speed run, JUST as i rolled off the throttle. Also seized on the needles after taking a 3 mile run on the freeway, cruising at higher speeds (70-80). A couple seconds after I took the offramp, the motor had a light seizure. I was testing to see if the lighter clip setting was better. At least you get immediate results and no question you went the WRONG direction...hehe.

So a wild guess. jetting and a step colder spark plug than stock maybe?

Did you get the correct clearances for the Wiseco piston?

Note to self* If motor is always running hot...dont redline the poor thing. Could one expect any other result?

pushstartrearset wrote:1200* fahrenheit seems a bit high, what is it supposed to be? Around 1150? I think i remember Ed mentioning a lower temp is desirable. Were you using the stock spark plugs on your WFO run? You even permatexed the head gasket (did you use an OEM head gasket? <---important for good seal). Looks like a typical 4 corner seizure. Possible causes are too quick a warm up, too hot a plug (stock plug, or go 1 step colder) and too lean jetting. Wonder if it could be as simple as too low anti-freeze. Last time I blew anti-freeze out the overflow was a blown head gasket at the track doing a T.O.R. (taste of racing).

YOu mentioned the water temp being high also...probably a good time to inspect / replace all that shi*...impeller/gear /bearing/seal/ pipe o-ring/thermostat/temp/sender. coffee colored anti-freeze is a sure sign that water is in the oil.

That sucks. Looks like you had it all setup. Wonder on RZs how porting affects the jetting. Just wouldn't know. What color is the underside of the piston? if you can find the guy that ported it (hopefully a professional), ask him what the ballpark jetting is. The less work you have to do, the more ride time you have!

When you do get it back together, I remember my tuner saying it is better to get a ballpark jet in the lower gears, initially. The upper gears offer much more stress and possibility of immediate blow-up.

You can seize on the needles too. I remember seizing after a top speed run, JUST as i rolled off the throttle. Also seized on the needles after taking a 3 mile run on the freeway, cruising at higher speeds (70-80). A couple seconds after I took the offramp, the motor had a light seizure. I was testing to see if the lighter clip setting was better. At least you get immediate results and no question you went the WRONG direction...hehe.

So a wild guess. jetting and a step colder spark plug than stock maybe?

Definitely good, rich piston crown wash on the RH side, leaner on the LH (melted) side. That anomaly mystery needs to be solved. Obviously, staggered jetting will help, I'd rather find the source.
BR9ES plugs right from the go, old sled experience here. I can always change a fouled plug easily.
Warm up was complete, 5 miles in 90F weather, temp needle cycled a few times as thermostat opened, warmed up the rad. The rad cap is (WAS now) a .9 bar pressure, going up to reduce the spitup into the bottle. Water pump impeller inspects good, plastic impeller firmly attached to shaft (a common failure as well in the VW car world). I removed the rad, verified flow, takes a full 3/4" garden hose flow with 60 PSI.... no worries. Looks clean as a whistle.
So... a few thoughts for discussion...
A) the head has a couple pock marks from previous failure, the DPO blended them acceptably well with a dremel (visible in the image). Possible detonation beginning point, those marks?
B) The LH carb idle mixture screw will only back out 1.5 turns. EVERYTHING else was removed, cleaned, verified during intial build. I was going to drill this out, but Sudco was out of stock, (still waiting). So, I left it, it idles, low speed runs great. Any chance that can impact mid-high speed mixture?
C) squish measured at 1.2, may be too big?
D) NOT using an OEM head gasket because, at 66MM bore it would hang into the combustion chamber significantly. Wiseco is a multi leaf, copy of the Yam, but bigger bore. No sign of leakage, no oil residue in the coolant. Copper coat used to help with slight corrosion on head surface.
E) Piston clearance set at .0025", RH side lived just fine, despite a pegged temperature reading.

Questo vecchio rz wrote:Note to self* If motor is always running hot...dont redline the poor thing. Could one expect any other result?

After reading dozens of posts here, and seeing many warm/hot/overheat post, I was learning more about this particular machine, the gage calibration (Fluke digital IR temp gun was in my tailpack at the time, been logging ACTUAL temperatures on cyls, head, rad), and so forth. I had verified the thermostat, inspected for oil residue in coolant, verified flow, checked coolant level multiple times. Pressure cap was inspected. This was the first real pull, and I had hoped to shut down before a seizure. Perhaps I was expecting that coolant temps would rise to 3/4 scale and level off? Yup, I failed. But I sure tried to do it right.

concours wrote: E) Piston clearance set at .0025", RH side lived just fine, despite a pegged temperature reading.

I think Wiseco call for .003-.004" for a 66mm bore. How did you measure it? RHS might have not been far behind, it had to start somewhere. Are you sure you had no air leaks?

RuZty wrote:

concours wrote: E) Piston clearance set at .0025", RH side lived just fine, despite a pegged temperature reading.

I think Wiseco call for .003-.004" for a 66mm bore. How did you measure it? RHS might have not been far behind, it had to start somewhere. Are you sure you had no air leaks?

Yes, Wiseco piston spec is .003". I had sworn off forged Seizeco's years ago on sleds, rattle cold and seize hot. Only 66mm piston I had found, so I used them. Contacted them, they assured me the newest generation is so much more thermally/dimensionally stable.... But, yes, I think more clearance would have delayed meltdown a few seconds.

As sure as I can be no air leak. I meticulously assembled, new crank seals, Yamabond cases, reed blocks cleaned, inspected, gasketed (included Loctite 518)

Mitaka piston kits go right up to 66.50 mm

i had wisecos in one bike i got . someone sized it and it rattled like crazy and all these stories put me off forged

my thought is that if it needed to be forged then it would have been , even right up to the last of the banshees were cast

nzminis wrote:Mitaka piston kits go right up to 66.50 mm

i had wisecos in one bike i got . someone sized it and it rattled like crazy and all these stories put me off forged

my thought is that if it needed to be forged then it would have been , even right up to the last of the banshees were cast

Mitaka US distributor?

nzminis wrote: my thought is that if it needed to be forged then it would have been , even right up to the last of the banshees were cast

casted is a lot cheaper to produce in larger quantities, so you'll hardly find any forged piston's in standard bikes.

concours wrote:

nzminis wrote:Mitaka piston kits go right up to 66.50 mm

i had wisecos in one bike i got . someone sized it and it rattled like crazy and all these stories put me off forged

my thought is that if it needed to be forged then it would have been , even right up to the last of the banshees were cast

Mitaka US distributor?

Who knows ?. get them direct from yambits

This looks like a intake leak to me with such rich piston wash it shouldn't have melted unless it's all idle then sustained wfo.
Leak down test ?

Update....
I had a standard bore set of jugs standing by, bought the Mitaka cast pistons and put it together to ride while repairing the scored cyl. Time marches on, very doggy by comparison, but, it was running.
This winter I committed myself (in addition to a full Norton engine rebuild project) to put the 66mm ported jugs back on. Mitaka cast pistons. Head squish and chambered by Scott Clough (based on measurements I supplied) Bigger radiator. 430 mains. Feels good on a short test ride.