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

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#1
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???
 
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#4
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.
 
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#5
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
 
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#7
well we installed a wiseco psion kit in te bike that came with its own rings. I thought those were already spec
 
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#8
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.
 
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#9
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.
 
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#10
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?
 
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#11
Obviously, he didn't, but that's not the problem unless the rings have butted and broken the piston.
 
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#15
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.