Home › Forum › Ask A Member › ’83 15hp, no power at top end
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fleetwin.
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April 13, 2016 at 10:46 pm #4035
The motor does not want to rise to a higher RPM, say much above 2500-3000RPM. Already checked the usual stuff, carburetor is fine, ignition is fine, fuel pump is OK, timing is OK.
Compression is 110-120 T/B cylinders.
Motor IS NOT backfiring or running rich. When I put compressed air into the barrel of the carb, it will run a little bit faster at the top end. Idles down low and fine, but no power at the top end. Smooth power curve up to max timing advancement/throttle, but will not rise higher. It is not stalling out or overheating at any range.
Pulling the powerhead now…anyone have other experiences? Going to check the intake manifold and intake bypass for air leaks.
Is it possible that the powerpack is somehow faulty and limiting RPMs at the top end?
April 13, 2016 at 11:25 pm #34780Did you do a cylinder drop test. These motors run remarkably well on one cylinder except they have very little power at the top end. To do a drop test, fire up the motor, bring it to a fast idle, and with a pair of insulated pliers, pull one spark plug boot at a time. If the motor dies when a plug boot is pulled then you know the other cylinder was never firing or is very weak.
April 13, 2016 at 11:32 pm #34781it’s not that I can tell when a cylinder is lost. I suspect it may be distorted leaf valves at the manifold. 3 out of 4 were not seating completely flat on the manifold surface; going to replace and retest. Will report back once done.
Fortunately this is a fresh water unit so the whole thing came apart in about 15 minutes.
April 14, 2016 at 12:17 am #34784Not smart enough to know about a ’83, but does it have timing marks on the flywheel? With a timing light, they would answer your power pack question.
April 14, 2016 at 12:53 am #34788This one doesn’t. To my knowledge, I don’t think any of the 74-93 run 9.9/15’s have a flywheel index. I usually just stick a pencil into cylinder 1, find a common reference point (i.e. shifter boss), and then use a light to at least see if the corresponding cylinder is firing near TDC.
I am putting my money on worn/distorted leaf valves on this one. I replaced both, and now even when turning over the flywheel with my hand slowly I get the familiar noise owl ‘WHOOO’ as each plate vibrates during induction. Wasn’t getting any of this prior to the repair.
If this doesnt solve the problem I’ll be back to pester folks with more questions. Won’t know until tomorrow AM – just buttoning the motor back up now and then turning in for the night. Back at it at 9am tomorrow!
April 14, 2016 at 1:32 am #34789Might the motor need to be decarbonized ? This a problem with improper oil use.
April 14, 2016 at 10:22 am #34796Mouse nest in the exhaust tuner? Carboned exhaust ports? Sounds like it’s not breathing.
Long live American manufacturing!
April 14, 2016 at 10:23 am #34797I worked on a 20 Johnson that acted like that. I ended up finding that it had the wrong high speed jet…. apparently someone had been into the carb before me and had it all screwed up. I also seem to recall that someone had installed a piece of lead shot into one of the passages [the ones visible on the exterior of the carb] and had it plugged. You may need to trace out each passage using carburetor cleaner with the extension tube, or using alcohol and a syringe. Like my old instructor used to say, ” knowing each passage way in the carburetor and understanding fuel and air flow, is the key to a good repair”. Anyway, I’m sure you know all of this but just an idea…
April 14, 2016 at 12:25 pm #34798Jerry is probably onto it.
If you remember a post I made before the old boards crashed I noted that I am finding more and more jets that have been enlarged internally. What you are describing is indicative of this. Before you go off on a goose chase with your reed theory I would check the HS jet for size and replace.April 14, 2016 at 3:22 pm #34805I switched carbs prior to posting, 1 was #56, the other #58, no significant change on the jets in terms of influence on performance. This one has me stumped. Changed leaf valves and while the motor gets about 400RPM more now, it’s still topping out around 4200-4300RPM. Idles down low to 600 RPM smooth no problem.
Switched powerpacks, switched the float inlet orifice to a larger one, no difference. It’s as if it’s not getting enough fuel at the top end. I went as far as to switch fuel pumps, and the lines are all clear and new.
It’s not bogging down at the top end, smoking excessively, or blubbering, it still runs smooth, just won’t max out in RPMs as it should. It does not stall out at any range. I’ll try a 3rd high speed jet.
One thing of note, it appears this motor >might< have been overheated at some point because the top of the block and cylinder head are a light/medium brown (it’s a white block). So while compression numbers are still 110-120 (OK), I’m wondering if somehow the cylinder head has been affected/distorted. It was necessary for me to change out the head gasket as it was leaking initially in multiple areas.
It seems that these 9.9/15’s run hot on the top of the cylinder even when the cooling system is OK, and it takes awhile for the thermostat to get warm enough to open. So while the top of the cylinder may be 170°, the bottom where the thermostat is may still only be 100-110°, and the actual cylinders themselves (from the port side) are only at 100°.
I did not see any major scoring on the cylinder walls when I removed the head (top cylinder had minor scoring, nothing too major).
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