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raksike

Honda CR250 pinging after new oversized piston

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Posted (edited)

There is no shop around where I am located and have to manage somehow myself. So basically what I was thinkin to do is to machine the cylinder head down 0,70mm. To achieve the squish around 1,2mm. I think that I would not be capable to measure the cc to optimize the dome shape.

 

But still I have the question, how can it be from the factory already designed so that the squish is already almost 2mm?? I dont get it.

Edited by raksike

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Maybe before shaving off anything I should try to replace the base gasket with some type of silicone? With this I could reduce the squish anround 0,3mm I guess.

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Posted (edited)

Any sealant you use on cases, reed gasket etc. must be gasoline resistant (Threebond, Hondabond, Yamabond, Motoseal)

Typical automotive RTV sealants are not and will eventually get softened allowing air leaks.

 

As I mentioned in my last post, by the latest pictures you posted 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.

While it doesn't look too bad, imperfections that have sharp edges can create localized hot spots and be a source of pre-ignition.

 

If the cylinder head / combustion chamber has been machined by a previous owner to remove damage,

it's quite possible the squish band now sits deeper than stock

 

Pending the one you are using is of the correct thickness, I would not mess with any thinner base gasket or using sealant on the cases.

If there is a machine shop local to you, I'd start by having the cylinder head milled/shaved by 0.5mm 

It can also be done by carefully lapping the head in figure-8 patterns on sandpaper/super flat surface but, will take a lot of time.

Edited by mlatour

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Ok, I will search a machinist who could help me with removing 0,5mm from thw cylinder head.

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Came just from one person, who is working on racing car engines and has worked pretty much on 2 stroke bikes also.

He rejected my work and told that this squish topic is nonsense basically in my case :D

He called over the phone three other persons, who are working with race engines and two of them are known locally as experts in this section. All of them told that it can`t be the case and they suggested the following things:

1) Go more richer on the main jet.

2) Go from 0,4mm -> 1mm base gasket.

3) Try different gas station fuel, since the station from where I have taken the fuel has the worst quality fuel according to them.

So now I don`t know what to do next :D Maybe order next size main jet? Next size is 185. How much is the pilot affecting this thing? Right now it is way rich on the pilot..  

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Here is what I did to my head. Common mod that's done to YZ490 heads is to narrow the squish band to fix the knock problem. The stock head has a really high MSV. This head was machined so that it had NO squish band...
To fix this I machined .050 off the face and another .030, at 12.5° to match the piston surface, to get a squish band 8mm wide. It really should be 10-12mm wide but I didnt want to machine more off. I also added an o-ring to reduce squish and not machine more off. I should be at .050 clearance now. This engine has an 88.5mm bore.20190708_202348.jpg

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Thank you everyone for finding time to answer and support on this case.

First of all I think I will do a compression test to see how much I will get with the current setup ant then decide will I try to mess with the dome myself or try to set only the squish.

But I have one question. Have I understood correctly, that when I would change from the base gasket from 0,4mm to 1mm gasket (or two 0,4mm gaskets) then it would stop pinging ang I will lose some bottom end? If it is so, then why it will stop pinging? The squish would increase then from the 1.9mm to 2.8mm!! I dont get this point at the moment.  

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One question. Is it Ok, when the piston does not travel the whole way to the end of the cylinder top side? I noticed, that when the piston is in TDC, then there is around 1mm room to the top of the cylinder, is it Ok? I have done a drawing to explain what I mean, drawing attached.

piston.png

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Posted (edited)

Depends on engine design but most 2-stroke engines will have the piston sitting almost zero deck height meaning flush with the top of the cylinder.

 

The point on reducing the squish band height is to make it as minimal as possible, just enough to avoid physically contact between the piston and cyl. head.

That way there is no air/fuel mixture that can remain 'stuck' in that area during the compression stroke, making it prone to detonation

but rather forced out into the compression dome area.

 

Adding base gasket thickness may reduce overall compression but it makes the squish band worse.

Edited by mlatour

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25 minutes ago, mlatour said:

Depends on engine design but most 2-stroke engines will have the piston sitting almost zero deck height meaning flush with the top of the cylinder.

 

The point on reducing the squish band height is to make it as minimal as possible, just enough to avoid physically contact between the piston and cyl. head.

That way there is no air/fuel mixture that can remain 'stuck' in that area during the compression stroke, making it prone to detonation

but rather forced out into the compression dome area.

 

Adding base gasket thickness may reduce overall compression but it makes the squish band worse.

Adding base gasket makes squish worse, but it will stop pinging as I have understand, why it will then stop pinging, would like to understand this logic.

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Posted (edited)

The engine can ping for a few reasons, bad fuel and incorrect timing would be first on the list.

 

If the compression is too high (pending the squish is correct),

adding base gasket thickness can help lower compression without going into cylinder head machining (a band-aid fix)

 

But it can also ping if the squish band height is too big, leaving air/fuel mixture trapped in there during the compression stroke.

By your measurements, the squish band height appears excessive.

IMO your piston is also sitting too much below deck height but more experienced Honda members will have to confirm.

 

Fix the squish first, then if compression is still too high, correct the cylinder dome cc's.

 

You can keep adding more and more base gaskets to the point compression will be much lower and it won't ping

but also loosing performance and combustion efficiency.  Base gasket height also changes cylinder port timing relative to the crank.

Edited by mlatour

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Piston on my engine is below deck also. Gives you options of machining head and or the cylinder. Just machinevthe head together the squish right. If you then have pinging, reduce the squish band width from the chamber.

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have you tried retarding your ignition timing a skoch?

how can you be sure that this is in fact detonation you are hearing?  

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Posted (edited)

I have tryed once to retard the timing as much as it can be done with the current ignition plate and I did not notice any difference. Without modifying the ignition plate holes it is not possible to adjust the timing very much.

At the moment I have put one size larger jet into the carb 180->185 and changed the fuel. Have not had time to check if it changed anything.

Regarding the pinging. I think it is pinging :) When the engine gets hot, then under very high load, sandy tracks or in a condition when I am going up a hill with a high gear and suddenly accelerate, then I get the noise from the engine, it remembers me to the noise when a 4 stroke valves are clapping. 

When I first assembled the engine with the new parts then it done the same, but already on a low engine load, then I went with the pilot and the main jet one size richer and it was making noise only on a very high engine load.

When I ride on an easier mx track then I get basically never the engine sound out.

And also as mentioned above, it is occurring only when the engine is hot, or it has been run for about 20 minutes or so before I cant get any noise out of that engine.

Edited by raksike

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Posted (edited)

One question, is the oil amoubt also affecting the pinging? I use 32:1 premix.

Edited by raksike

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One question, is the oil amoubt also affecting the pinging? I use 32:1 premix.
Only if the bike was jetted for, like 50:1, then you would be running a little lean with 32:1.

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Posted (edited)

Still with my pinging issue :D

One question. I have checked the VHM cylinder head and there is written that with this head it is around 1.6mm. Could someone explain why it is 1.6, not 0,8 to 1,25 what has been suggested earlier?

https://www.motoracingshop.com/en/vhm-cylinder-headset-with-perpendicolar-sprarkplug-and-inserts-for-honda-cr-250-92-01.html

 

One more thing what I have noticed, is that I will not get a single pinging from the engine until it is totally hot, the pinging comes only when the bike has been having lots of revving and right after the hard riding it will ping when on high gear and opening it up. When you continue to ride rather not so agressive then it will also ping, even under high load with high gear when opening up the throttle.

Could it not be caused by I wrong heat range spark plug?

Edited by raksike

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