Leakdown For KX85?

[razorboy]

Hi Folks,

Looking for a leakdown tester for a KX85 motor.
Who makes those?

I got a KX85 that won't start after full rebuild, bottom and top. Its got spark but it won't fire at all. Wets the plug down almost immediately too so I am ordering a valve for it.

In the meantime, my compression number is around 120psi which is pretty low according to the manual of 119-185? Thats a pretty broad range however.

What does a Honda CR85 manual state for compression on its motor? Just want to be sure these numbers are right cause if they are, I have a leak somewhere?

Cheers
Bernie

 

[whenfoxforks-ruled]

http://www.motionpro.com/motorcycle/tools/category/2-stroke_leak_down_tester/ . The best. Vintage Bob

 

[Patman]

Did you happen to measure the bore prior to the rebuild? Was it tapered or out of round? What size piston did you install? Taking it apart and putting new pieces in doesn't always fix wear issues.

 

[razorboy]

http://www.motionpro.com/motorcycle/tools/category/2-stroke_leak_down_tester/ . The best. Vintage Bob

Thanx For That.

The motor was rebuilt due to a burned up crank bearing. I decided to put a new piston and rings in it as part of the rebuild. It was the same make, size piston and rings that were in it previously on the stock bore and sizing. The topend never suffered any damage from the bearing failure that I could see?

Gotta start with the leakdown first to understand where the problem may be?

 

[Patman]

So the answer to the question about measurements & inspection would be.... no? Like I said just replacing parts with the same thing may not cure a problem if the problem is a dimensionally out of spec part that is not being replaced (replated, bored, etc...). What about the crank, was it checked of just new bearings put in? I also wonder if you may have a seal that was either damaged or installed backwards?

I seem to recall it not being too difficult to fabricate your own leak down "kit" with some freeze plugs, clamps, etc... back in the day.

 

[whenfoxforks-ruled]

Back in the day? That still can get fabricated for under 30 dollars, but the instructions may cost 270? Vintage Bob

 

[Patman]

Yes but it doesn't have the cool box.

 

[razorboy]

Crank is new and all seals were installed correctly.

The entire bottom end is new and with the exception of the cylinder and head, so is the top end.

So I had a leakdown tester for a Honda RS125 and my machinest whipped me up a new insert to fit the KX85 motor. I pressurized the intake to 4 psi and it dropped about 1 psi per hour. So a little under 2psi after 2 hours. Tried to go higher but the line won't hold more than 4-5 psi without popping off.

From my research, seems like 1 psi loss per hour is pretty healthy. Sound good to you guys?

It may simply be a problem with the carb but won't know for sure until the new float valve arrives. Strange that it only gets up to about 120psi of compression and according to the manual, thats the low end of things for this motor. And also strange that it simply won't fire. Seems like it gets flooded before it can even get a good kick.

If the float valve doesn't help and I can't find anything else outwardly wrong, I guess I will pull the cylinder as suggested to get the runout checked on the bore.

Cheers
Bernie

 

[whenfoxforks-ruled]

Send the top end to Forward Motion, and get the psi up a tad. If its been a while, years, replace the float valve for peace of mind if nothing else.

 

[Bonehead]

what is the end gap on the rings?

 

[razorboy]

Send the top end to Forward Motion, and get the psi up a tad. If its been a while, years, replace the float valve for peace of mind if nothing else.

It dawned on me last night that the rings are new so they haven't had a chance to seat at all since the bike won't start. The compression is based on new rings and certainly will go up once the motor runs a bit.

 

[whenfoxforks-ruled]

That sounds better. What was the ring end gap set at?

 

[Bonehead]

is there a rag in the air box???

 

[razorboy]

So I went through the process again yesterday of checking leakdown and compression. I wanted to do this before tearing the topend off again to readdress the internals.

Leakdown came up again perfect.

But as far as compression it was weird.
I had the exhaust off for the leakdown so I left it off. As I kicked the bike for compression, I heard it hisss out the exhaust port and the compression would go away. Tried to figure on why it was doing this? I gave it a couple of more good kicks and then it stopped hissing and started building compression. Got it up to 150psi this time before it levelled off.

Threw the carb back on and fed it some fuel. One kick and it started? Did this over and over again for about 5-6 times and it started first kick everytime.

Trying to understand what would cause an exhaust leak only to stop after a few good kicks and wondering why it didn't do that before?

Anyway, bike is now running and sounds good. Thanx to the board for the help.

B

 

[helio lucas]

Trying to understand what would cause an exhaust leak only to stop after a few good kicks and wondering why it didn't do that before?

if the engine is dry and doesn´t have any oil to seal the rings or if the oil isn´t properly distribuited it will allow leakage and take some time for the engine get full compression.
of course the leakage noise you hear from the exhaust port may be well due a incompleted piston closing :cool: