warped cylinder head

[MK]

FYI:
Yamaha OEM 48H specs say BR9 for high speed riding
https://www.rd350lc.net/specRZUSchass1.jpg

European past 1986 models say BR9 for all models, except the restricted 27HP 1WX
https://www.rd350lc.net/spec-86chass1.JPG

The earlyer 31K actually had a BR8 recommendation, but that's for stock bikes with stock compression and obeying the traffic limits. After lots of meltdowns they changed that for the later models.

The only application where the BR8's make sense in RZ/RD engines is when you never put much heat in your stock'ish engine. Like riding in winter, around town or in countries with harsh speeding fines.

As soon as you put a tad more heat into your engine, you'd need a 9 heat range. Like the case we're talking about in this thread (remember, he's using an EFI which is like "38mm carbs")
Extremely high tuned engines in race applications even use 10.

For those brave enough to test values "in between" : ND has different steps like 27, 29 and 31. The 27 is a bit in between the 8 and 9 NGK.

[RZtuner]

Spark plugs actually have quite a wide operating range 500-950 degrees C. Yamaha spec a colder BR9 to err on the safe side for a two stroke they release to the masses, they do the same with jetting, always a bit rich, just to be safe. BR9 runs on the cold side of the spectrum.

So how hot should it be? A spark plug should be hot enough to keep its' insulator nose completely clean and burn off all deposits away, but not so hot that the electrodes show serious overheating. That’s it! And all this information is there for you…. if you learn to read your plugs properly.

When I first got in to land speed racing 30 years ago, I was very curious about plug heat ranges and their effect upon maximum power, so I experimented a lot with a couple of RZ350's. A BR8 showed no signs of overheating and ran nice and clean. Nice mixture ring at the base of the insulator. I even tried a BR7 for one 3 mile run at WOT. The electrodes just started to show signs of overheating, so definitely too hot, but no, it did not detonate and explode. Not the sticks of dynamite that the naysayers claim.

Lesson learned: plugs with their wide heat range are actually quite forgiving. To say a BR8 is too hot for a long period at WOT is simply incorrect.

I stepped up to an RG500 production bike which spec a BR9. After porting the motor, I put the bike on the dyno to dial it in. Plugs were not super clean so changing to BR8’s increased RWHP by 6 horsepower. That is the cheapest 6 horsepower I ever found! The bike set the 500cc production record which remains unbeaten to this day. Topped out at 162mph on my best run.

A few years back an old friend of mine who has about 30 or so world land speed records came up to me in the pits at Speedweek. He was running a 125 GP race bike which was running like shite. Seeking advise from a fellow two stroke racer he showed me the plug, a BR10 covered in oily crap. He told me the manual spec is a BR10 for the bike. So, I gave him my plug heat range lesson: “You have to give the engine the plug it actually needs not what someone in a book tells you to run”. I sent him away with a new BR8 which fixed the bike immediately and he set a new record with it.

So, what happens if the plugs too cold? One year I signed a factory contract with Buell to race a Buell at Speed week for the 1350 prod record which was at 144 mph. I got a full race team and a factory mechanic none of whom had race experience. First run the bike was staggering at high speed. Too much fuel. Mechanic says, it’s too lean, I said no, it’s too rich, the fuel is building up and extinguishing the flame front and making it stagger. It took me 3 runs to dial down the jetting to the sweet spot and bingo, the next run was at 150 and we were in impound. Repeated this for the return run and we put the rookie team in the record books, first Buell over 150 mph. Then the mechanic says can we do another run tomorrow. The bike ran like a bag of nails at 130 mph. Without me knowing he’d changed the plug to a colder plug because he’d read somewhere you must fit a colder plug for extended periods at WOT. When he told me what he had done he apologized profusely, lesson learned: Fit the plug heat range the engine needs, not what someone in a book tells you.

Gordon Jennings wrote the bible on how to read plugs and select them. If you are interested in this, I recommend you read it through a couple of times. It’s quite long but very informative.

https://www.strappe.com/plugs.html

[MK]

No need to get triggered and especially no need to insist having the only truth especially when it's pretty outside everyone else's experience.

Running WOT is not the most critical state as the fuel cools quite a lot when vaporizing.
You can imagine that road bikes run over a wide range of throttle position, load and rpm. Monitoring EGT may surprise you when you see that for example at some 6-7k and only 15% throttle you have more than 600 Deg C.
And in these bikes security for all kind of operating circumstances beats performance.
And that's why I had to put your BR8 recommendation in the right context - otherwise you'd talk people into their next expensive engine rebuild.

Talking of security: Yamaha TZ350 ran a 10 heat range. Seems they disagreed with your findings.

[RZtuner]

Martin, I am simply responding to JumJums question: "I did a top speed run the other day and destroyed a leftside piston at 160 km/h and it locked the motor up. Can this be caused by running BR8ES plug at high load?"

Based on probably over 100 runs at WOT for 3 miles at Bonneville I can quite confidently, from my experience, say no. I have run a BR7 in an RZ350 WOT for 3 miles and all it did was slightly round off the sharp edge of the center electrode. First sign of overheating, but the 8's were fine for me.

There are lots of folks who just go by the book and never question why their bike is running shitty. All I am saying is do your research, learn how to read plugs correctly, and give your engine what it needs if you plan on doing high speed runs.

Knowledge is a valuable thing, but a closed mind is like a closed book.

[RZtuner]

Yeh, I raced a TZ350 at one time. Sure, like always, Yamaha spec a BR10 to be on the safe side, knowing full well when the tuners get their hands on them everything gets changed. They are not disagreeing with my findings, they are just playing it safe to avoid responsibility.

My friends Red Bull Cup KTM had the same spec, a BR10 plug, but it ran way better at WOT on a BR8.

[brrrappp]

Since the RZ is a twin cylinder and the problem is on ONE side....
The mostly stock RZ350 doesn't have a cooling issue that skews toward one side.
In stock trim, the RZ350 doesn't have an imbalance of intake air-flow unless the carbs are WAY out of sync.
This leads me to ask;
If the problem was the spark-plug's heat range and the guy is running the same plug on both sides, the same jetting on both sides, doesn't that make anyone wonder why the opposite piston did not burn-down too?

Some of you have a wealth of racing experience with twins and might be able to explain this phenomenon of only one side melting on a twin when the air/oil/ignition is identical for both cylinders.

[RZtuner]

Good question. They always seem to squeak on just one cylinder when the bike is running too lean like jumjums. Remember one cylinder is on the way up compressing fuel/air mixture which gets ignited, and the other is on the way down drawing in a fresh cold charge of mixture and cooling the piston somewhat, so the one that reaches the critical melting point first is the one on the way up and is first to reach the ignition flame.

[brrrappp]

Good question. They always seem to squeak on just one cylinder when the bike is running too lean like jumjums. Remember one cylinder is on the way up compressing fuel/air mixture which gets ignited, and the other is on the way down drawing in a fresh cold charge of mixture and cooling the piston somewhat, so the one that reaches the critical melting point first is the one on the way up and is first to reach the ignition flame.

I get that idea and you could be absolutely correct.
- wouldn't you still see signs that your plug on the other side was too hot? - like a white plug insulator or other initial sign of pre-det. For instance, a hot spot on the bottom of the opposite piston.

These are my trouble shooting thoughts:
If one side looks perfect and the other side melted.....
You can probably rule out ignition timing since both sides fire at the same time on every stroke.
Bad gas should also affect both sides.

Something caused pre-detonation on one side only at high engine speed/load.
If there is no sign of this from the other side and the plug also looks normal, logic seems to exclude the plug heat range.

Things that cause pre-detonation to start only on one side can be:
excessive carbon on the piston crown or in the combustion dome.
A burr in the surface of the combustion dome that gets too hot.
A burr or excessive carbon on the plug.
A leaky base gasket or crank seal.
Leaky carb boot/manifold
A clogged jet on one side.
Low float height for the carb on one side or fuel obstruction to needle/seat of one side.
mismatched porting or squish area
Uneven oil delivery from the oil injector on one side (seize is more likely result).
If you have individual air filters, it's possible one is flowing more air than the other or is obstructed externally making the jetting rich on one side lean on the other. If you did set of the pants jetting, you might go lean to compensate for poor throttle response and burn-down one side.
I'm sure tuners here can come up with other ideas too.

[RZtuner]

All those are true, there are just so many variables, but on an engine that has been properly assembled, has been pressure tested for leaks, has new pistons and plugs, it's rules out a lot of those and boils down to just too lean and the first to the finish post is the first to fail.

I rebuilt my production bike's carbs with all new parts and Yamaha sent me two new needle jets in the correctly marked bags and I put them in without checking the number on them. One was the wrong needle jet. The bike burnt the piston, just like JumJums on the first run. I found the problem when I got home and dismantled the carbs. $5 part cost me a record attempt. Now I check everything!

[MK]

Some of you have a wealth of racing experience with twins and might be able to explain this phenomenon of only one side melting on a twin when the air/oil/ignition is identical for both cylinders.

IMHO a good explanation would be crank twist.
Each cylinder ist putting torque on your crank which slightly twists the crank. Starting from left to right the torsion is the highest on the outer right side, but the ignition "reference" is the angular position of the outer left (at the rotor).
High tune state RZ engines were measured having about 1 - 1.5 deg difference in ignition advance at peak performance.
A sophisticated tuning compensates that with an offset between left/right cylinder (Ignitech and Zeeltronic offer this feature).

As it was mentioned: "Identical" jets do not necessarily flow the identical amount of fuel.
Check this measurement that can easily be repeated with your own stock of jets:
https://www.rd350lc.de/downloads/MK-Dok ... nglish.pdf

Another theory would be fuel distributing unevenly through the fuel lines (i.e. depending on how and where you put T joints, you happen to supply one side more than the other)

Even Yamaha was aware of identical cylinders running differently.
Check the carb specs of the post 1986 RD350 models. They feature a slightly different power jet for the right/left side though everything else is nominally identical.
https://www.rd350lc.net/spec-86eng1.JPG

And even though it's always one side melting, I often saw "the other side" having deformed piston domes or seizure marks, too.

[jumjum01]

Here is a picture of both pistons and spark plugs. RH piston shows small signs of starting meltdown on top and edge. LH sparkplug has been cleaned with brake cleaner so it has no color, but lots of fragments from melted piston.
Cylinders has been bored today by yours truely (my 1st time boring a cylinder ) to 64,72mm and next step is honing to 64,75mm. Bike is torn down to bits and frame is going for sandblasting and powdercoat.

[RZtuner]

Complete lack of any carbon build up or flow patterns on both piston crowns indicates its running really lean on both cylinders and they were both getting ready for meltdown. Richen up your fuel map on the top end quite a bit, then keep doing plug chops and reading the plug for the correct mixture ring, jetting down as you go.

It's a shame you cleaned the plug. After that sort of event its best to cut the threads off the good plug and see exactly where your jetting is. Don't wash away all the evidence. The plug should have a 2 - 3mm wide mixture ring at the base of the insulator to be safe. 1mm is the danger zone, you are on the knife edge, and no ring you are in deep trouble.

Hope this helps.

[(F5)]

I'd still be keen to know what the compression ratio is if I was you. As part of the complete study.

[brrrappp]

And even though it's always one side melting, I often saw "the other side" having deformed piston domes or seizure marks, too.

It was nice that jumjum went ahead and pictured the other piston since it clearly shows that both sides were experiencing trauma. I made a prior assumption that his problem was isolated to one side only.

Your observation captioned above has been consistent with my past experience from 2 stroke twins. At the snowmobile shop I worked for in the 80's I would estimate that 1 of 5 meltdowns were totally isolated to a single side. The majority of L/C failures being cold-seize of both sides and overall A/C + L/C from overly lean on both sides. It was almost always the case that one side was more serious than the other. As you can imagine, Sleds have a number of contributing factors you don't see on an RZ350's

[jumjum01]

RH spark plug chop. Ring is 1mm. It seems i am on the lean side. My original fuel map was based on autotune with O2 sensor. When I had the bike dynoed the bike was rich according to the dynos O2 sensor, so I leaned the map out. Guess my own O2 sensor was more realistic. Seens the best way to check is to do the spark plug chop test with new plug at WOT. CR will be calculated when I recieve the pistons and dome cc can be measured