1985 cr 250 top end rebuild...very little power

acesnkush

Member
Apr 29, 2006
86
0
We just fiised rplacing the top end on a 85 cr 250. After we put it back together it started dit up in like two kicks. However when we rode it for the first time there was like no power. As the engine warmed up it gained some power but not what it had. Once it was hot for a while we pushed more but there was still not as much power as there used to be. What can be the problem???
 

76GMC1500

Uhhh...
Oct 19, 2006
2,142
1
No, the power valve is probably not opening.

To adjust the ring gap, you file the ends of the rings so that the gap is within spec with the ring installed in the cylinder. If they are too tight, the ends of the rings will but together when the ring expands under the heat of combustion. Once the butt, the rings expand outward and breap pistons and destroy cylinders. Ring gaps really have nothing to do with power, until you lose compression because you broke your piston.
 

84cr125

Member
Apr 8, 2007
292
0
76GMC1500 said:
No, the power valve is probably not opening.

To adjust the ring gap, you file the ends of the rings so that the gap is within spec with the ring installed in the cylinder. If they are too tight, the ends of the rings will but together when the ring expands under the heat of combustion. Once the butt, the rings expand outward and breap pistons and destroy cylinders. Ring gaps really have nothing to do with power, until you lose compression because you broke your piston.


well if you knew anything about bikes you would know that a 1985 cr250 DOES NOT have a power valve....... its got a atac chamber
 

76GMC1500

Uhhh...
Oct 19, 2006
2,142
1
What exactly is your definition of a power valve? To me, a power valve is a "valve" on the exhaust side of a 2-stroke engine that varies something (expansion chamber volume (ATAC), port timing/area (HPP), or whatever(KIPS and the like)) in relation to engine speed in order to maximize the useable power range of an engine.

An ATAC valve stuck shut throws off the timing of the expansion chamber so the bike will run fine through the low/mid range, but not have any power on top. It will make the bike feel gutless. Since a top end rebuild was just done and the ATAC valve has to be messed with during the rebuild, it is very likely that it is not operating properly. I would also suggest the reeds be inspected, but if the bike starts fine, I find it unlikely that they are a problem.

Wiseco rings are generally cut to spec, but it's always a good idea to double check because too tight of a gap can destroy an engine.
 

84cr125

Member
Apr 8, 2007
292
0
76GMC1500 said:
An ATAC valve stuck shut throws off the timing of the expansion chamber so the bike will run fine through the low/mid range, but not have any power on top. It will make the bike feel gutless. Since a top end rebuild was just done and the ATAC valve has to be messed with during the rebuild, it is very likely that it is not operating properly. I would also suggest the reeds be inspected, but if the bike starts fine, I find it unlikely that they are a problem.

really, uh huh. well in my manual it says that a defective atac chamber will casue trouble in the mid and bottom end but will not affect the top end. seeing as how the purpose of the atac chamber is to aid the mid and low range to give it more torque.
 

84cr125

Member
Apr 8, 2007
292
0
acesnkush said:
well we installed a wiseco psion kit in te bike that came with its own rings. I thought those were already spec

did you check the rings put putting them square in the cylinder with the piston and measuring the end gaps?
 

76GMC1500

Uhhh...
Oct 19, 2006
2,142
1
On the newer bikes, there is usually a way to check the operation of the power valve, you remove a cover or pull a plug and rev the engine. You should see the valve move from the closed to open position at a certain rpm. I don't know if the ATAC bikes have any such inspection cover. It would be my guess that the linkage that operates the valve is not properly engaged or the governor isn't functioning properly. I once had a power valve that would not operate, there was a fork thingy on the governor (behind the water pump) that was not positioned properly and bound up preventing the power valve from opening. These old Hondas are difficult to set up, especially the HPP models. In fact, on the 87, you cannot set up the power valves properly following the procedure in the manual.
 

84cr125

Member
Apr 8, 2007
292
0
On the ATAC system to check if they are operating in my manual it says that your saposed to take off the muffler but leave the exhaust manifold on. Reach in with two finders and the valve(butterfly valve) should be in the completely open postion.


If you want i could sacn a coupple pages of my manual and send them over to you
 

76GMC1500

Uhhh...
Oct 19, 2006
2,142
1
You could pull the pipe and pull the right side engine cover and try to manually operate the valve using the linkage in the crankcase. I believe it should move to the full open position as you remove the cover and if you push on the cam, it should close. If you lay the bike over on its side before you do this, you don't even need to drain the oil.
 

84cr125

Member
Apr 8, 2007
292
0
Actually as you remove the right side case the valve will close because the water pump govenernor wil now longer be pushing on the linkage system. When the right-hand crankcase cover is installed, the ATAC govenor rocker arm will push on the lower cam which in turn opens the butterfly valve. Adjustment in the butterfly valve is only nessary if the butterfly valve is not completely CLOSED when its not saposed to be (Wide open throttle). If the butterfly valve is more than 45 degrees or more drom the open position, you will have to properly reinstall the exhaust manifold.

You can do a simple test to see if the valve is operating correctly, just take off the muffler but leave the manifold on, leave the right-hand crankcase cover on and stick 2 fingers down the manifold. You will feel the butterfly valve and it should be totally vertical.

If you want, i made an offer before ive got the manual and a scanner. Just give me a PM with your email address and ill do it for ya gladly.
 

decade377

Member
Dec 17, 2006
25
0
acesnkush; My offer still stands,I can come out and take a look at it for you. I haven't sent you an e-mail about my jetting spec's yet as I haven't found my old log book. I'm in the middle of going through my 85 again and will clean out the carb and note my jet sizes so you have some place to start.
I have put several Wiseco piston sets in my CR and the ring end gap was very close,didn't have to adjust at all. It should still be checked before installing the piston though.
 

acesnkush

Member
Apr 29, 2006
86
0
ya decade lol i think you may have to come out. I live in like thousand oaks. email me me so we can set something up. Thanks man thought I can do it without you having to drive from riverside but ya....so much for that lol. My email is acesnkush@hotmail.com
 

lbs4061

Member
Jun 2, 2007
2
0
I just bought a 85 cr250 to play and work on. It runs but not very well. Where should I start on it carb, exhaust engine???????? It sounds like it has the same problem as the bike above. The guy I bought it from did not know the history on it. Anyone have a suggestion. Thanks
 

lbs4061

Member
Jun 2, 2007
2
0
just got a repair manual for the bike. the exhaust manifold is not factory by any means. In fact someone has put an exhaust manifold on it that does not go on it. I am sure that is why it does not run very well. does anyone know where I can get a used exhaust manifold for a 85 honda cr250 it has the atac on it. Thanks
 

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