88 Kdx 200 engine questions

[Trickey]

Hi all. New here and have some questions.
Recently brought my son a grass kart to race here in New Zealand that is powered with a 88 kdx 200 engine.
Engine has been problematic with 3 melted pistons being the biggest problems.
First piston intake boot cracked and engine sized. I gave it a hone and replaced the piston to have the exhaust springs come off and I assume this leaned out the engine and toasted another piston (wiseco)
Took topend to machinest to discover that sleve had previously been replaced by a non stock part and had moved down!
So new sleve and piston and in the second race and it nips up!! Another hone and piston!! I also have had crank bearings and seals replaced trying to eliminate any reason for it leaning out and toasting pistons!!

I have installed a Trail tek Vapour mainly for RPM and head temps so we can see if its getting to hot and cool it down before it melts pistons!

First question: what is the normal idle rpm and max rpm for this engine?

I cant seem to get a good steady idle on this engine and have noticed that the carb slide is worn, and adjusting idle screw has it racing or almost dieing so is there a later model carb that is better than the standard carb as im thinking that the carb is the only thing left that could be a problem. Ive had no luck finding a carb so far here.

Have only put around 30 minutes on the engine so far so still going easy on it and have decided to go up alittle on the main jet to try and keep it alive this time.

Thanks in advance Shane

[Trickey]

Update.
Got hold of another stock carb. Idle and bottom end power were much better. After about 1 hours running in time came to race day.
First race running well. Second race you guessed it locked up again.

Looks like detonation on top of piston, but plug looks ok not lean. Main jet is 320. ( 4 corner seize)

Only possible problem I can see is that the powervalve shaft looks to be leaking, could this cause a lean stuitation?
The class that my son runs in dosent allow the use of powervalves so they are locked open.

I think that the piston and bore will clean up ok and I will try and run with it as don't t to keep putting new pistons in till have found the problem.

I am thinking do I run very rich? and see if it will live.
The races are only around 6 laps of a approx. 700mtr course but unlike trail riding most is near max revs.

So far: crank seals and bearings
new liner and piston
all gaskets and seals new
different carb but same jetting

Any ideas appreciated.

[dfeckel]

I think you are probably running into the very reason all high performance dirtbikes moved to liquid cooling. ONce they get too hot, they run lean and lose power. Too lean and damage occurs. This motor just wasn't designed to handle max rpm for extended periods.

As long as you've eliminated any possibility of air leaks, then your next step would be to try rich, rich, rich jetting to keep it cool. Also, your might want to try a stock-bore, plated aluminum cylinder for better heat transfer, rather than the iron sleeve. But I understand that's a tough move to make with this thing seizing every outing.

ANy way you can use a liquid cooled motor?

[Trickey]

Thanks For the reply.

I pretty sure we have eliminated any air leaks apart from the possaple leak from power port rod. Didn't need crank seals but was trying to cross that of the list of possible causes.

Yes water cooled motor would be great but the rules for junior racers dosent allow water cooling or power port motors.
I had this exact discussion with people on sunday as to why we cant use water cooled motors but that's the rules.

I have found a source for jets over hear so will order a 340 and 360 and try that.

Also forgot to mention that we are running on 100 octain fuel.

Thanks Shane.

[david]

Do the rules allow for a coated piston? You may look into having the piston coated to help it live longer. I used Polydyn 30yrs ago on my '75 CR250R piston skirt for the same reason and it works!

http://www.polydyn.com/coated_pistons.htm

[Trickey]

Thanks for the info david it looks very interesting but I cant see anyone that offers that product on this side of the world.

Cheers Shane.

[Trickey]

Bore cleaned up and honed and fired straight up and all seems fine.

I have bigger main jets on the way and will run rich and see what happens.

One thing that I did notice is that the magnets on the flywheel are a little lose. Not real bad. Is it possible that this could affect
the timing and cause detonation?

Thank Shane.

[kawagumby]

The loose magnets are unlikely to change the timing - as long as they're held in place. However, they will eventually break completely loose and create havoc (a known issue with these engines)...the trick is to break them loose now and re-epoxy them back in place before that happens.

[Slick_Nick]

The power valve is blocked OPEN (high RPM) correct?

[Trickey]

Thanks I will look into sorting the magnets. ive had to do the same thing on my outboard motor.

Yes powervalves locked open.

[kdx87]

Have you checked out changing the plug to a cooler one? That may help the wide open runs. Also check that the motor has all the airflow it can get.

[KDXGarage]

What spark plug are you running?

[Trickey]

Running a B9EGV plug
I got a selection of main jets and we ran it with a 380 in last weekend and survived the day. 380 seems way to big but seemed to preform ok
Another race meeting this Sunday so hope it continues to run. 6 good races last weekend on a track that was a small speedway track. Used half track and twisty stuff on the infield so it had a good workout.
See how this weekend goes.
Cheers Shane.

[Trickey]

Well yesterday started off good.
Two
races on a 1 km track doing 6 laps and so 12 good kms and going strong.
Third race and looses power and stops!
No compession so I pull the head and find that the exhaust side top of the piston is gone.
I pulled the plug after each race and looks rich as it should be with a 380 main.
So why would it lean out??
Only possible reason I can see is one of the head studs is sitting proud and poss making the head gasket leak?
Would a small head gasket leak cause this?? I was thinking that it would only cause it to loose compression, or am I wrong?

Any Ideas???

Thanks Shane.

[dfeckel]

Sorry to hear your bad luck continues.

I would have the head checked for warping and get it machined flat if it is.

Do you use any kind of coating on your head gasket when installing? I have had very good results with Copper Coat, and copper-colored, super-sticky aerosol spray application onto both sides of the metal gasket before installation. This stuff has allowed me to reuse metal head gaskets on several occasions, and it salvaged a new but leaky head gasket when I rebuilt my motor only to find coolant bubbling out. Give it a try on your next go around. Also you might want to check head torque between races.

[Trickey]

Thanks for your reply .
I was also thinking of getting the head checked to see if that was a problem.

Do you think that a leak in the gasket would cause it to lean out as I was thinking that it would only be leaking compression out?

[Trickey]

What is everyones thoughts on a head gasket leak causing this engine to lean out enough to cause the exhaust side of the piston to melt?
Small possible leak on intake side due to stud becoming loose.
Cheers Shane

[kdx87]

Quite possibly yes, would also affect low end performance aswell.

[adam728]

One big issue is that you are asking the engine to do something it wasn't designed for, running heavy loads at high rpm for extended periods. Heat kills, but you know that!




Has the engine been blueprinted?

The sleeve
Does it measure square with the cylinder top and bottom?
Is the porting exactly the same as stock (height, width, open area, timing)

Head
Has the squish band been measured? What is the clearance?
How much squish area are you running?
What RPM does the engine run at?

What's the measured compression ratio? Not what the book says it should be, but what it actually is measuring head cc's, gasket thickness, deck clearance, etc.

What exhaust are you using?




A few thoughts -
These engines were designed as grunty enduro engines. Low specific output, long life. Running near peak power for extended periods is not their cup of tea.
Getting heat out is a big factor. Things like running a smaller squish area, larger diameter stinger/silencer, and really fat jetting can help. Retarding spark timing can play a factor as well.


A story -
My old 220 was fairly heavily modded (porting, head work, big carb, exhaust). Setup for trails and enduro, worked great. I ran 152-155 main jets for most places, even when getting into deep sand washes that had some high speed runs.
But in the dunes it was another story. A few minutes would have it detonating and boiling, even with 100LL fuel and a bigger main. I ended up needing to go to a 175 main to keep it alive for a weekend. So rich it would stumble and blubber until it finally cleared it's throat, then ran well. Running near WOT and at 8000-9000 rpm was just not what everythign was tuned for, from cylinder head dome shape to exhaust tuning.