200 hot up gone wrong - need help

[leonard1]

Just, done a stack of mods to my 99 kdx 200 and have some issues. Firstly it goes unbelieveable well and the power increase over stock is amazing, but there are two problems:
1. Bubbles coming out through the radiator which I expect means my head gasket is leaking.
2. At hight revs on WOT she makes a repeated snapping wip cracking type noise, not the ball bearing or hammer on metal sound that people refer to when describing pinging and as I have never heard pinging apart from in a car I am not sure what it is.
The mods I have done all in one hit are as follows:
New Wiseco piston and rings
Rejetted to 155 main and needle in middle position.
Boysen Reeds
0.7 mm taken straight off the head with no other changes bringing the squish clearance down from stock of 1.8 to 1.1 mm.
Airbox opened up
FMF rev pipe.
I am running 98 octane pump fuel but it looks like I have overdone the compression. Using a guage I have tested the cranking pressure at 1300 kpa (190 psi). Was about 1000 kpa(145 psi) I am not suprised if it is pinging but the noise just doesn't sound like what is described by others. Could it be the sound of the gases escaping thru the head gasket?

any thoughts?

 

[whenfoxforks-ruled]

Before you even attempt jetting, you have to fix all mechanical issues! You need to figure why the head did not seal. If all else fails, like the head and top of cylinder are not warped, and the dowels are installed, then get the permatex copper high temp spray gasket sealer.

 

[dirt bike dave]

I think you have a form of preignition that is different from the conventional knocking under heavy load and low rpm. Check out the description of 'wild ping' in the research paper linked below.

I would examine the head, piston top, plug and top of the cylinder bore for any pitting or loss of material. As for cures, maybe a thicker base gasket or head gasket would help, as might a colder plug and bigger main jet. Personally, I would pull the cylinder off already to inspect for any damage before riding again. Check for bearing play, too, as more compression combined with a brutal high rpm pre-ignition would be very hard on your big end and wrist pin bearings.

It is possible the reduced cooling efficiency caused by a leaky head gasket is causing the problem. Also, what oil and ratio are you using? You could try adding more oil, to improve cooling and lubrication.

http://209.85.173.104/search?q=cach...+throttle+two+stroke&hl=en&ct=clnk&cd=8&gl=us

 

[leonard1]

Thanks, that article was worth a read. I just can't see how this wild ping could occur since there are no loose deposits as the head is polished and the piston is brand new. Can you shed some light on whether the high compression is over the top and does running high compression require richer jetting.

 

[joebiodiesel]

It might all be related to the cooling problem. If you're getting combustion gasses in the radiator, you might be getting some serious hot spots on the head. That can cause detonation too.
Joe

 

[dirt bike dave]

Thanks, that article was worth a read. I just can't see how this wild ping could occur since there are no loose deposits as the head is polished and the piston is brand new. Can you shed some light on whether the high compression is over the top and does running high compression require richer jetting.

FWIW, in the article, it mentions that polished surfaces are more likely to have pre-ignition than ones that have a thin coat of carbon on them.

You could easily get new loose deposits with any pre-ignition, as the explosions can take little divots off the head, piston top, and anything else in the combustion chamber.

I agree with joebidodiesel that fixing the head gasket and ensuring the cooling system is working up to snuff might cure the problem with no other changes.

It if was my bike, I would pull the cylinder and closely inspect the cylinder, head, and piston for damage. I would make sure the head is flat and the mating surface with the cylinder is good.

If everything checked out and it reassembled properly with no gasket leak, I would test and tune, with the first runs having more oil with the fuel, use fresh fuel, the next colder plug, richen the main jet by one size, and maybe raise the needle one position. If there is a slightly thicker head gasket available, I would try that, too.

You may have gone slightly overboard on the compression for your fuel, but the head gasket leak/cooling problem makes it impossible to tell. Your jetting and fuel might be spot on now, but I would be conservative with your tuning at first as you could easily damage your crank, piston, head and cylinder continuing to run it as is. And if the motor lets go, it could take the crank cases and KIPS valves with it.

 

[leonard1]

Before I redo the head gasket I would like some advice on whether I need to reduce the compression ratio. Is 180 psi too much for 98 octane pump fuel?

 

[leonard1]

Sorry in my last question I said I had 180 PSI I meant 190.

 

[whenfoxforks-ruled]

The head got polished? What other mods were done?

 

[leonard1]

The overheating and leaking head gasket was fixed by refitting the head and investing in a torque wrench. I think I have learnt my lesson. Have been away for the weekend just sorting out the bike and it is so much fun to ride. Yes it is still pre-igniting at WOT but I was almost able to elimninate this by running a colder plug and having the choke out at all times. I believe this is telling me that I need to back off the compression slightly. My theory is that by just shaving the head this has cut into the squish band and left a sharp edge. My next move is to smooth this section out which will also take some meat off that should drop the compression.

 

[leonard1]

Seems my thread has gone a little cold. To change tack what would people say about retarding the timing to help with the detonation problems I have.

 

[whenfoxforks-ruled]

It may stop your piston fron melting. But it will not run good. Get a stock head that is not ruined by polishing. I can only hope you did not polish the ports also. Your squish still seems like a lot to me. How did you measure it? .7 mm is a lot, but it was a kdx. Never seen a kdx that needed 112 octane fuel.

 

[leonard1]

It may stop your piston fron melting. But it will not run good. Get a stock head that is not ruined by polishing. I can only hope you did not polish the ports also. Your squish still seems like a lot to me. How did you measure it? .7 mm is a lot, but it was a kdx. Never seen a kdx that needed 112 octane fuel.

 

[leonard1]

Squish clearance was measured by usisng the solder in the head trick. Stock clearance was 1.8 mm and after milling is now 1.1 mm.

 

[leonard1]

I have done a fair bit of experimenting since my last post and have found some interesting results. Firstly a reminder that the first thing I did was to take .7 mm off the head, which took my squish band down to 1.1 mm. This produced some huge bottom end and mid range gains but at the expense of a little top end and then there was the strange pinging noises at high revs. Even after a fair bit of riding their was no visable damage to piston , head or plug so strange. I took the head back and had 0.9 cc taken out of the dome part of the head. This has almost eliminated the pinging but has now lost some power down low and created a flat spot just before the power valve opens and hits hard into the power band. This really doesn't suit me. It does appear now that the head mods have upset the way the combustion happens and might be time to start with a new head. Next time I think I might take less off the head in the first place, maybe .3 or .4 mm. Any thoughts?

 

[joebiodiesel]

Not to preach doom and gloom, but anytime you start making freestyle changes to a 2-stroke you risk getting in to deep water fast. I've been that route. I've had some pretty fast snowmobiles...that were not so enjoyable to ride anymore. Now. other than rejetting, I don't fool around with them.
You might want to think about cutting your losses...buy a new stock jug/head, or get a performance one from a reputable dealer. It is the easiest path to restoring the bike's performance. A lot of math goes in to making port timing, ign timing, squish, jetting and a pipe all work together. When you make big changes you upset the balance.
Joe

 

[whenfoxforks-ruled]

Personally, I would REALLY just get one from Jeff, or Eric. Just tell them you want to run pump gas. Otherwise, up to .020" may be the area to keep you out of trouble with the stock dome, squish and compression.

 

[leonard1]

On Jeffs site it says he does only 220 heads so I have sent an email to Eric