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High Speed Orifice - Johnson 9R70C

D

dbeamish

New member
I have a 9.5 that will idle correctly - and the low speed mixture seems to be correct - but at full throttle power is significantly low and the low speed adjustment has an effect on the high speed mixture. If I close off the low speed adjustment, the HS mixture seems to come into range and the engine performs well at high speed. I surmise that the HS orifice is either altered - or the incorrect orifice is installed. Would love a HS jet 101 course from someone in the know on the matter. Does my theory hold water?
 
I checked out the orifice - .048 as indicated in service manual - so - I am thinking a defective casting perhaps - a crack and a leak which is in parallel with orifice flow, Is no-one out there using these 9.5s? such a fantastic little engine.
 
Have a bunch for parts / rebuild.-----Post the actual compression values here.-----And does spark jump a gap of at least 1/4" on both leads ?
 
I'm away - will check spark next week. Compression even both cylinders around 70 PSI if I recall. Tried different carb - better but LS still affecting HS. power good when leaned out at full throttle. Thank you for your help!
 
The powerhead was purchased from Sea-Way Marine and was told was remanufsctured by OMC - came in OMC wood crate. Converted '73 style reeds to '69 style which I believe were a better design. Both mag coils, points, condensers, plug wires and plugs replaced. Throttle synchronized. Runs great! - other than this mixture wierdness.
 
faztbullet said:
Revisit carb as a leak,missing gasket or cracked pull over tube
Click to expand...
Thanks for weighing in - all gaskets new - tried 2 carbs - nearly identical. The pullover tube is the small tube from crancase to intake manifold? This powerhead a '73 rebuilt by OMC - I changed the intake arrangement to the earlier manifold and reed arrangement which had me install a nipple into the block for the the crankcase termination for the small rubber hose. The hose I used appeared to have good integrity. Its like the HS orifice is opening is too big - if I undertand that the actual Hi-Speed mix is a combination of low speed + high speed jet output. Definitely puzzling to me.
 
racerone said:
Have a bunch for parts / rebuild.-----Post the actual compression values here.-----And does spark jump a gap of at least 1/4" on both leads ?
Click to expand...
The compression is even at around 72 psi in each cylinder and the spark jumps a 1/4" gap. And as I say - once the HS mixture is corrected, the open throttle performance is as I would expect and the operation is smooth. This engine quite the Frankenstein I have worked myself into. OMC rebuilt '73 powerhead with intake manifold converted to '70 style with the older reed plates - the reeds seal completely. This conversion had me instala a 5/32 nipple for the small hose from crankcase to to manifold which I surmise has something to do with the oil drainage scheme. The hose is healthy and I used an aerobic sealant on the nipple when I tapped it into place. I think this is all tight. This problem quite puzzling to me.
 
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