GX160 power falls of 0.2hp after 10 dyno pulls

K

knighty

Member
OK, we warm the engine up on the dyno, then do 10 dyno pulls one after the other, which is the equivalent of about 1 race lap, and by the 10th pull my motor has lost 0.2hp, whereas my best motor actually gains 0.1hp.......which considering it only makes 6hp, thats 3% power loss, not good.......the piston has about 30 hours on it, oil is changed every 3-4 hours and i use joe-gibbs, the leak-down test at TDC does not imply anything is wrong.......so I'm kinda suspecting either the piston rings or the bore hone condition......any ideas welcome before I go down the wrong avenue!
 
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.
 
many thanks Sir, I have printed off your suggestions and its going off to the dyno-doctor this morning for another health check, failing all else, its going to have a new piston fitted and have the block honed out.....the deck-face was recently skimmed by 0.1mm, and I have a suspicion this has very slightly changed the profile of the cylinder and the rings are not sealing properly once its heated up........they probably also re-used the head gasket, so like you say that could be leaking also.
 
Just been to inspect my disassembled engine with by dyno man, and heres the headlines:

Bad stuff:

1) Exhaust valve sealing land looking a bit rough - needs lapping
2) Two top piston ring gaps both measuring 24 thou - this seems big, whats advisable for a 6hp 6400rpm GX160 select parts build? by dyno man thinks 6 to 15 thou........which sounds more on the money to me
3) The big end of the con-rod had scuffed up a little bit - can be polished out
4) Cam timing was about 3 degrees too far advanced from the regulation limit

Good stuff:

1) valve guides seem fine
2) cylinder bore is measuring up round, both top and bottom, honing marks fully visible around 360degrees, with only minor visual wear, looks good.
3) Piston skirt hardly worn with machining marks still visible around the full 360 degrees, looks good.
4) cam and pockets in great condition
5) followers, pushrods and rockers all look ok, nothing bent or misaligned.
6) crankshaft and bearings all spinning freely with no concerns
7) head gasket looks like its been sealing perfectly
8) inlet valve sealing looks bright and good, but will lap to be sure

ok, so we have decided put a new set of piston-rings in, which I will buy tomorrow, but would much appreciate your guidance on the gaps for the 1st and 2nd rings.......is the oil scraper gap all that critical?.......by the wording of our rules, the piston rings must be totally standard and un-modified, so perhaps gapping the rings is even out of the question, in which case, i just need to know what you guys think they should land at?........dont worry I'm based in the UK, so no chance of me ever turning up at your track!

many thanks again:cool:
 
Ring end gap recommendations from the ring manufacturers is typically .004 per inch of bore. Most put them much smaller than that. I am sure .006 to .015 would be fine. I am not overly exited about polishing the scuffed up connecting rod. IF you can AND keep it round AND not round the edges and give it NO taper then go for it. Most set the oil ring very loose to minimize drag. Timing may be the power issue or possible rod/crank drag but any rod binding that causes power problems in my experience, was well past being able to buff them out. LOL. Good luck.
 
Have just put a new set of rings in and they are measuring 10 thou, which based on the 4 thou per inch of bore rule, is about where I'd expect them to be for a 67.5mm or 2-5/8 inch bore......much better than the 24thou of the old rings!......will be dyno'd tomorrow.
 
Better change that rod before It costs you short block, or worse the whole engine. Stock rods are cheap, change them often
 
Had the engine dynod and its now quite strong, used it on the track, and the bottom end performance is the best its ever been.......which raises the question, how often do you guys recommend changing the piston rings in a 6hp select parts build 6400rpm GX160 motor? I'm guessing around 5 to 10 hours use?......this engine probably had about 50 hours on it :-/
 
Right....Engine not revving past 6000rpm, should be revving with ease to 6300-6400rpm....and I have had it on 2 different tracks with the full range of gearing, engine has now done 2 hours since having the piston rings done, for the first hour I used gibbs break-in oil, am now running gibbs KRT......why on earth would this be happening?.....on the plus side the bottom end performance is the best it's ever been.
 
Check to see if the cam gear on the crank possibly slipped slightly, that could explain the better bottom end and lack of top end power
 
Good idea! the timing could have slipped......last time I checked it the ICL was at 112Deg........will check it tonight.....

Yeah, it revved up to 6300 no problem on the dyno, it was really healthy engine in comparison to my other 2 motors and a same-class engine they had dyno'd the day before, the only things I have changed since then are the exhaust and put some gold heat reflective tape on the air-box cover.

I'm now thinking perhaps the engine has ingested something and its lodged in the intake system, like a small piece of the gold tape I stuck on the airbox cover!......this happened a few years ago on another engine when a piece of the air-filter paper lodged itself down the intake.

Or perhaps the exhaust has broken up internally or is blocked, I have another I will swap it over for the dyno check-up to be sure.

Much appreciated guys :)
 
Bingo.....checked everything and it appears the thick steel gasket between the airborne and the outer carb face was on upside down, so the upper vent hole was blocked off......does anyone think this could be the culprit?......
 
Many thanks!.....am taking it back to be dynod today, fingers crossed......now I look closer at the dyno plot, it made good peak power at 4200rpm.......but from there up to 6300 it tails off quite a bit by about 0.5hp when compared to a previous dyno plot......on the dyno last week I was running Gibbs BR break-in oil in order to help the new rings bed in nicely, which is like really treacle-thick and I thought the power loss simply was due to excessive oil drag.......but obviously not :-/.......hey ho......as the old saying goes over here in the UK "the man that hasnt made a f%&k-up - hasnt made f%&k-all"......my engine builder and dyno man is a retired guy who has built multiple championship willing 4 and 2-stroke engines, he really does know what he is doing, and I absolutely trust him.......I have made a lot worse mistakes myself!

As another question has anyone ever measured sump-oil temperature on a dyno or in the kart? what sort of temperatures should I expect to see for a 7.5hp GX160 engine on an inertia dyno?......I'm now running Gibbs KRT oil if that helps.......I have got a termocouple and we are going to drill & glue it into the plastic filler plug to see how whats going on the dyno.
 
We've never had good luck getting oil temps due to the oil not sitting in the sump like you would imagine on a car, it's misted and everywhere except the bottom of the sump. If I was going to try again, I'd put it in place of the drain plug.
 
Knighty over here most would not know what Treacle is, you should say molasses. I have never used that product, but if it was that thick I would never put it into a splash oil system. Don't waste your time with the oil temp. With your build some fine carb tuning, and you seem to be on an honest dyno you should be able to get your peak torque up to just shy of 8hp.
 
I should have said honey!.....have now got the engine back, the thermocouple was installed in the sump plug and about 50 dyno pulls were performed and the oil temp was in the region of 70degrees Celsius, which I must say I am shocked at how low that really is....I was expecting well over 100DegC.....Peak power has now moved from 4100rpm up to just over 4400rpm.....but top end power at 6000rpm is 0.5hp off from where it was in April......but on the plus side bottom end power is up 0.5hp at 3000rpm.......I'm guessing the engine just needs about 5-10 hours running to bed the rings in......jetting wise it's on the money, but I will keep an eye on the plug colour
 
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