Honda CR250 pinging after new oversized piston

[raksike]

How should the dome cc's be measured.

[mlatour]

Graduated syringe or burette and kerosene.

[raksike]

Seems to hard to do at home. Can you describe more in detail how the measuring process should be?

Edited June 21, 2019 by raksike

[mlatour]

I'm sure there are videos on YouTube but I simply installed a spark plug in the cylinder head,

mounted it 'upside down' (chamber facing up) in a bench wise, clamping on the spark plug's nut faces.

 

Using a 10cc syringe, filling the combustion with kerosene until the level is flush with the mating surface.

[raksike]

I will try to retard for first thing the timing and next month remove some of the cylinder head contact area. By adjusting the ignition timing, should I advance it or?

[mlatour]

Retard the ignition timing.

Despite no audible pinging, keep an eye on the spark plug's insulator for tiny metal specks (signs of detonation)

 

Post some pics of the cylinder head before altering it.

Edited June 24, 2019 by mlatour

[raksike]

Today I have retarded the ignition as much as possible by rotating the stator, the mark on the stator is 1-2 mm off the factory marking. Basically it did not help I would say nothing, concerned the pinging, and also did not notice almost no difference ar all. One thing I did notice was strange idle, I do not know could this was caused by the timing? I did not manage to get it solved by adjusting the air screw, after screwing with the air screw around it starts to idle good, but after some riding it was back. Any ideas on this? The rpms are like jumping.

Done a video of the strange idle: 

 

Edited June 25, 2019 by raksike

[Armbarn sailor]

Today I have retarded the ignition as much as possible by rotating the stator, the mark on the stator is 1-2 mm off the factory marking. Basically it did not help I would say nothing, concerned the pinging, and also did not notice almost no difference ar all. One thing I did notice was strange idle, I do not know could this was caused by the timing? I did not manage to get it solved by adjusting the air screw, after screwing with the air screw around it starts to idle good, but after some riding it was back. Any ideas on this? The rpms are like jumping.
Done a video of the strange idle: 
 

You have an air leak. Or your reeds arent sealing but im pretty sure you are sucking air after or in reeds.

[raksike]

The reeds have been inspected during the last time when it was open, someone has also replaced the OEM ones with carbon reeds. Maybe crank seals?

[raksike]

I have found one picture of the cylinder head from the time when I took it apart for rebuilding. Does this say something to you? 

[mlatour]

I am not familiar with these engines but hopefully by looking at it someone may have an insight 

if certain design cylinder heads are shaped differently depending if meant to run either a flat top or a domed piston.

 

After a bit of searching I found this past thread: 

according to the pic in the 3rd post, your cylinder head's squish band's shape appears to better match one for a flat top piston,

the following posts also mention the risk of a 'negative angle' of squish prone to detonation if these engine components are mis-matched.

 

In the event you disassemble your engine, perform a leakdown test before taking it apart.

Edited July 2, 2019 by mlatour

[raksike]

7 minutes ago, mlatour said:

I am not familiar with these engines but hopefully by looking at it someone may have an insight 

if certain design cylinder heads are shaped differently depending if meant to run either a flat top or a domed piston.

 

After a bit of searching I found this past thread: 

according to the pic in the 3rd post, your cylinder head's squish band radius appears to better match one for a flat top piston

 

In the event you disassemble your engine, perform a leakdown test before taking it apart.

The cylinder head what I have is originally for the domed piston, it has the spark plug on an angle the cylinder head what was in the thread has the spark plug straight. Maybe it has been modified already somehow then to suit the flat piston? But as mentioned earlier it did not work correctly with the flat top piston, it had low compression and was hard to start, with the domed piston these issues are history.

[Dirtbug26]

On ‎6‎/‎15‎/‎2019 at 11:51 PM, raksike said:

Any estimation, will there be any power loss with this?

At this point you should be more concerned with seizing a piston and not with "power loss". Just adjust the timing and don't chase your tail with base gaskets.

[raksike]

I will try to open up the engine in a few days to do one more squish measurement to be sure that the 2mm squish is correct, I have done the squish measurement with tin wire throught the spark plug hole before.

Also just in case I have checked the cylinder head design and the piston what I am using at the moment and the conclusion is that the parts what are installed into the motor now are correct! Domed piston, with cylinder head what is also meant to be used with the domed piston!

[raksike]

I have removed today the cylinder head and done one more squish measurement, result around 1,9-2,0mm.

I did not saw any abnormalities in my opinion. Pictures attached.

 

[raksike]

I have also one question. I have measured the edge on the cylinder head to see how much squish this already gives and the result was that it gave already here 1mm! How can it be that the squish is from OEM already 1mm from the cylinder head + gaskets which give also arou d 1mm and then we get the 2mm what is supposed to be not Ok...

[mlatour]

Squish is measured with the solder and head gasket in place.

 

Just an observation by there are signs on the combustion chamber that the engine has likely suffered a bottom end failure in the past.

Small dings and indentations perhaps from bearing debris, some appear to have been sanded smooth.

[raksike]

1 hour ago, mlatour said:

Squish is measured with the solder and head gasket in place.

 

Just an observation by there are signs on the combustion chamber that the engine has likely suffered a bottom end failure in the past.

Small dings and indentations perhaps from bearing debris, some appear to have been sanded smooth.

Of course, I have done the measurement with the cylinder head and gasket installed. But I am a bit confused, what to do next. I do not know is it wise to reduce the squish, since to me it seems that the squish is around 1,8mm from the factory..

[Grinder74]

You have several options. Turn the cylinder, top or bottom and or the head. Sounds like you should ship it to a shop that knows how to build those engines. The head needs work, squish band looks too wide, which can cause the ping you're hearing. By machining the cylinder, you can change the the power characteristics. If you have the time and money, that's what I would do.

[slotracer577]

Aim for a squish just over 1mm. If it’s pinging like you say I would set the squish band width to 50% of the area, yours looks larger than that. Reducing the width of the squish band width should also reduce compression as well. Squish height should be tight according to Kevin Cameroon, touching to .005” clearance while running. Then adjust the width to control the velocity.