Topic: Reason for low compression

[upperlake04]

Hello All-
  Except for a few  parts yet to arrive,  the 1978 750F head is ready for reassembly.  But before that I thought it wise to ask the forum techs if I'm mostly on the right track.
  $100 Cdn bike, bought and then shedded for the past 7 years, seems low mileage, mostly original, but cosmetics poor from previously living outside -all fixable with time and money of course. In the short time I rode it the power seemed poor. The  compression tested  low. Last fall I  pulled the engine and disassembled the top end.  All parts have been cleaned and measured and compared to the manuals specs.
  I know dickall about mc mechanics except for what I have read in the manuals this winter. I am going on the assumption that compression can only be lost from the combustion chamber through the rings/cylinder wall gap, valves, or spark plug hole.
                                                                             
COMPRESSION  110-110-102-123  dry test,  13-17 lbs. higher wet,   with cold engine, header off,                                                                                                                throttle open, with starter

  PISTONS/CYLINDERS -  Expected something amiss when the head was first pulled and I could see the top rings in the gap between the side of the piston top and the cylinder walls. But with the reading glasses on, I can make out the factory machining marks nearly everwhere on the sides of the pistons, and with the micrometer it sure looks like they are tapered top-bottom, so I tend to think they are intended to be that way. The skirts measure well within spec, as do the ring sidegaps and endgaps. The cylinder walls are shiny with some scuffing fore and aft, and traces of the last honing are visible over about 80% of the wall surface. The lip at the top where the top ring stops is visible to the eye but not enough to feel with a fingernail. I don't think there is enough wear here to be the cause of the compression numbers.

 VALVE SEATS  no burnt away spots,  blemishes seem to be stains on intakes and very slight corroded patches on the exhaust seats in the head. Pics of worst ones
 
 VALVE STEM-GUIDE CLEARANCE  Intakes   .04 - .04 - .05 - .03 mm  (.10mm max allowable)
                                                Exhaust  .36 - .51 - .44 - .31 mm  (.13mm max allowable)
  The exhaust guides are clearly toast and it is hard to believe that the valves slopping around in the guides at a high rate of speed could seal the combustion chambers properly.
 
 SPARK PLUG HOLES  the threads look good

This is getting long - I have learned on this forum since last fall that poor power can be caused by other things besides low compression ( wheel bearings,poor tune,sticking brakes) but to finally get to my question - is the state of the exhaust guides and the less than perfect seating surfaces of the valves and head consistent with and be the cause of the compression numbers?
 Thanks, David

[Terry in Australia]

G'Day Dave, another F2/F3 "Fragile Swan" (as ooposed to the K7/K8 "Ugly Duckling" ) story, did you measure the piston/cylinder clearance with a feeler gauge? The pistons/cylinders will only wear on the "thrust faces" (front and rear), so "traces of the last honing are visible over about 80% of the wall surface" don't mean squat if the thrust faces are worn away, and the cylinder has "ovalled". Any more than say, 5 "thou" gap between cylinder and piston, and it's rebore time.

Good ol' F2/F3 exhaust valves and guides, you know the worst thing is that the F2/F3 cylinder head design was a "cock-up" and there was bugger-all advantage with Honda running bigger exhaust valves in these engines, and for all the trouble they cause, they should have retro-fitted a K (or F/F1) head with just bigger inlet valves. Anyway, F2/F3 valves are real hard to come by, so send that Head to Mike Reick in Boston, he's our resident head expert, and he can make your head like new (but better) for a reasonable price, he's built a race head for my racer project and he's doing a trick K8 head for my F2, and his work is alloy art! Cheers, Terry. 

[upperlake04]

Terry - thanks for your reply. Yes. the piston/cylinder clearance was measured with a feeler gauge and they seem to show on average a hair  over .002 th on the thrust faces. Master Rieck has already solved the pounded- out valve tip problem on this engine with lash caps  -  he is a good man. I'm not sure if you are saying (gently) that the valve seats need major work.

[Terry in Australia]

I am............