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ivanrs01

4RT compression test (SOLVED)

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hello guys, I have problems with my 4rt 08 repsol to start it, I have done all the tests to try to see what happens, tell you that I have also mounted the starting lever model 2017 and has improved a bit, the bike comes on but after 7 or 8 kicks

try another gas pump
check valve adjustment
Throttle cable adjustment
electric connections
new spark plug

Today I have done a compression test and wanted to discuss the results with you, with the 125 psi cold engine and the 200 psi hot engine, I do not understand these results because in the manual it says that the figure has to be 65 psi.

any ideas?

thanks in advance
Edited by ivanrs01
problem solved

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4 hours ago, ivanrs01 said:

Today I have done a compression test and wanted to discuss the results with you, with the 125 psi cold engine and the 200 psi hot engine, 
I do not understand these results because in the manual it says that the figure has to be 65 psi.

Sounds like your automatic decompressor (compression release) is not working.   At kicking speeds, the exhaust valves are held open slightly.  This system is very common on modern 4Ts.  It allows a higher rotational speed for starting and easier kicking.

Once the engine fires, the decompressor ceases to operate and the cylinder pressure returns to that of normal operation.  The decompressor is built into the camshaft.  See the service manual for more information.

 

Edited by konrad
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15 hours ago, jimmyl said:

Have you checked tick over  

Quote

thanks for your answer but I do not know what the tick over is


 


 

?

Quote

Sounds like your automatic decompressor (compression release) is not working.   At kicking speeds, the exhaust valves are held open slightly.  This system is very common on modern 4Ts.  It allows a higher rotational speed for starting and easier kicking.

Once the engine fires, the decompressor ceases to operate and the cylinder pressure returns to that of normal operation.  The decompressor is built into the camshaft.  See the service manual for more information.

 
I will disassemble the top cover of the cylinder head and see what the spring works well, is that what you mean?

I think the decompressor works well because the lever is not very hard.

I will also review it, thanks for your reply

 

 
Edited by ivanrs01
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There is more to the decompressor than just a spring.  There is a centrifugal mechanism to inhibit it from releasing compression when the engine is actually running.

Americans call "tickover" the idle speed.  There is an adjustment on the throttle body.  It should be 1700 to 1900 rpm when the engine is up to temperature.  Hard starting can result if set incorrectly.

Also check that the valve clearances are set correctly.  I see that you've done this already.

Edited by konrad
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1 hour ago, ivanrs01 said:
 

I will disassemble the top cover of the cylinder head and see what the spring works well, is that what you mean?

I think the decompressor works well because the lever is not very hard.

I will also review it, thanks for your reply


 


 
 
this afternoon I will check the decompressor,
the tickover is high

 

 
  thanks for the clarification of the tick over, here we call ralenti.

 

thanks

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The photos in the 4RT manual are not clear.  Here is a photo of some of the decompressor parts on a CRF150RB.   It may be similar to the 4RT.

Does your compression gauge read directly in PSI?  Maybe a units conversion error?

CRF150R decompressor.JPG

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the checked the decompressor i think it works well, I leave you link: https://youtu.be/O0T9fJ-2l6o 
 
I have also checked the distribution marks of the camshaft and they are fine, the compression meter that I use is this, I do the test as the manual says, hot engine and the gas completely open

 

 
2qdv37r.jpg
 
 
I leave some more images, I think it's all right

 

 

29zvkhc.jpg

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1hvvrk.jpg


 

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While testing compression are you kicking so rapidly that the decompressor behaves as though the engine is running?  That would explain the the high pressure reading.

I'm just guessing here, but typical kick-starting speed may be 250 rpm. The decompressor probably deactivates at around 1000 rpm.

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I will perform the test again, operating the lever slowly, the problem of high compression can also be due to excess carbon in the combustion chamber according to the manual.  It can also be a problem of bad valve settling ..... I see that maybe it's a good time to open the cylinder and check ... $$$$

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1 hour ago, ivanrs01 said:

...the problem of high compression can also be due to excess carbon in the combustion chamber according to the manual.

True, but that will increase the pressure by a relatively small amount -- not from 65 to 200 psi. 

Did you try jimmyl's suggestion about setting tickover/idle speed?  You need an accurate tachometer to accomplish this.  I tried to set it with my digital timing light, but the reading was erratic.  Ironically, this simple device measures rpm quite consistently: https://www.trailtech.net/tto

 

P.S. I'm curious, what do other 4RT owners use to set their idle speed?

Edited by konrad

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I just tested the compression on my 2016 4RT 260 (which runs flawlessly).  I saw just over 200 psi on a cold engine after 16 kicks with a high-quality Snap-On gauge.  I observed no increase in pressure after 20 kicks.  These were not "fast" kicks, just normal kick-starting speed.  Local atmospheric pressure will have a small effect on the reading, but I would say the decompressor (if it even exists) does not work as Montesa describes!

 

P.S. I'll also add that although I appreciate the quality of the Montesa service manual, it appears Honda just "cut and pasted" some of the information directly from their motocross manuals. Rings every 6 months?  Piston every year?

Edited by konrad

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It seems a very big fault of the manufacturer.  I remember that in the 2017 260 model manual if that speaks of 188 psi.

 

can it be a good idea to clean internally the body of the injection?

 

Edited by ivanrs01
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