GX160 power falls of 0.2hp after 10 dyno pulls

am just thinking about why my engine is producing 0.5hp more bottom end at 3000rpm, and 0.5hp less top end at 6000rpm, but the same peak power at 4500rpm......when we put the piston rings in, the bore looked in very good condition, and my builder guy measured the ovality top and bottom with a good bore gauge, so the bore was not re-honed.......I had a look myself and the existing honing was 95% visible with very little wear.....am now thinking we have possibly FUBAR'd and we should have re-honed the bore......but then I'm thinking if the rings really were not sealing the bottom and mid range performance would not be as good as it is , so perhaps the existing honing is ok?.......any thoughts?.......if we strip down again and get the bore honed, do we really need to use a torque plate?
 
Yes on the torque plate and side cover must be installed and torqued properly. Just take the engine and run it the track is a more consistent dyno, you will know if the engine needs attention.
 
Who knows what happens to the bore when the engine is at operating temperature. I can't believe it stays perfectly round or straight. I know I found a huge difference when honing my McCullough's hot or cold. It was such an obvious difference I made it part of my rebuilding routine to heat the block before honing. The first time I did it, while the engine was cold, I noticed no difference in compression when turning the engine over by hand, but after coming off the track, with the engine still hot, you could really feel the difference in the compression. That convinced me that heating the block before honing was a benefit. And I didn't just heat the block in the oven, I used a torch with a rosebud and tried to heat the block in a pattern that might replicate what would happen while the engine was running. I'm sure it was not perfect, really hard to know for sure, but it did help.
 
Many thanks guys....I have ordered a honing torque plate and will install the side cover too.....the guy that will do the honing is a 125cc 2-stroke rotax engine builder on the international stage, he has a stone type old-school honing machine so that will be ideal, I will investigate if they can heat it up also as I also heard hot-honing really helps......all ideas welcome!
 
knighty said:
I will investigate if they can heat it up also as I also heard hot-honing really helps......all ideas welcome!
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I started using the hot honing method in 1970. I wonder if I was the first. If something is beneficial to others, it's always fun to find out you were the first. Of course if it's not beneficial, you don't want anybody to know you did it. lol In any case I've been preaching its benefits on karting web sites for many years. And I understand, it's such a pain to do, few have followed this advice.
 
knighty said:
Yep I have asked if they can hot-hone, but I have a feeling the answer will be a negative - TBC........so as a plan-B I'm looking at investing in one of the Lisle 15000 or 16000 drill hones, it looks good value, are they any good? and which type do I need?.....what stone grade do I need, is 180grit enough?......here's the link

http://vi.raptor.ebaydesc.com/ws/eB...4857&category=82248&pm=1&ds=0&t=1463688678350
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I think that one will work just fine, it pretty much looks like the Sunnun I used back in the 70's. I can't remember the grit stone that I used, so somebody else will have to comment on that. Just off hand, from my memory that is 35 years old, 180 sounds a little corse, but I'm not sure.
 
jsstump#70 said:
It seems it looses power as is builds head. Could be as simple as air fuel ratio. But I suspect that one of the hundred parts is getting "tight" when it warms up. Endplay? The two crank bearings? Timing retarding more with a hot coil? Cam end pockets a little loose letting the cam "walk" or too tight and kinda binding it some? Worn valve guides? Rocker ratio a little of causing it to push "sideways" on the valve instead of straight down? Very small leak in a gasket from heat warping things, like head or intake? Ring ends just a smidge too snug? Piston wall clearance not what you want? Too loose or snug can be a problem. Maybe, just maybe, the clutch being used to test it is slipping enough to show a power decrease when it gets hot? Just a few thoughts. Let us know IF or what you find.
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I'm delving into this motor again, and I think you are onto something with the end-float being too tight / non existent, I have heard stories of people machining out the bearing housing pocket on the side plate to allow the crankshaft to have a bit of axial float, I can have it machined out no problem, but what sort of end float play measurement should I target for a GX160 with 8hp, revving to 6600rpm tops???......I'm guessing like 4 to 8 thou???...
 
Right. I checked the crank float, it's barely noticeable when cold, so I warmed the motor for 10 minutes and the crank was floating a treat.

I keep being told "most problems are in the carb" so I went through everything methodically, and it appears the small o-ring around the end of the plastic idle jet, although not perished, it had lost its compression and is not sealing, so I put a new one in and on the garage floor the motor was much more responsive to acelleration, I will take it out for another track test, then dyno it. Fingers crossed this is the issue. Even in the honda fault finding guide the idle jet can affect both low and high speed operation.
 
I have occasionally heard of gx160 cylinder blocks cracking, if so where abouts does this happen?.....I was thinking perhaps in the steel/iron overcast liner, or perhaps somewhere else which is more externally visible?......is it normally visible to the eye or is a red-eye penetrant crack test advised in the cylinder bore?
 
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