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8hp Timing adjustment procedure

D

davenaus

Member
Hello All, Just finished rebuilding my 2003 Tohatsu 8hp, 2 stroke. Replaced piston rings and reassembled. Motor running pretty well but seems to have a random misfire at all speeds. Using 25:1 fuel in accordance with break-in procedure. Engine has one hour of operation so far. New spark plugs. Misfire could be fuel as it is a few months old. Have read the procedures from the repair manual for adjusting timing and find them confusing. Anyone have the procedures in understandable English? Would incorrect timing create the miss? Thanks, Dave
 
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Figured out the timing adjustment procedure but I have bigger problems than timing. Did a compression test to find upper cylinder at 100psi throttle closed and 90psi throttle open, Lower cylinder 70psi throttle closed and 60psi throttle open. Spec compression is 57psi with both cylinders within 15psi. I replaced the rings as the compressions were basically the same as above and the motor was running quite rough. It is running smoother but still with a misfire. The cylinder wall on the lower cylinder has been partially eaten away by corrosion. Could that be causing my compression problem? Guessing that the gasket can't create a good seal with so little support available. Thinking about trying a gasket sealer as an easy fix. Any thoughts?
 
Always check compression warm with throttle open. Try a new head gasket. Make sure you torque it properly. A little sealer won't hurt, as long as it is hi temp stuff.
 
pvanv said:
Always check compression warm with throttle open. Try a new head gasket. Make sure you torque it properly. A little sealer won't hurt, as long as it is hi temp stuff.
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Thank you. Loctite 515 states is good to 150 Deg C. Is that hi temp enough? I'm not completely clear on the correct torque value for the cylinder head bolts as the repair manual I'm using doesn't cite a specific torque for those bolts.
 
GENERAL SERVICE INFORMATION
2-5
1-/ 2-Cylinder
SERVICE SPECIFICATIONS Standard Torque Values
These torque values apply only when a special torque specification is not listed in the Special Torque Values chart at the beginning of each section.


in-lb ft-lb N-m kg-m
M4 9 - 18 1 1 - 2 0.1 - 0.2
M5 18 - 27 2 - 3 3 - 4 0.3 - 0.4
M6 44 - 53 3 - 5 5 - 6 0.5 - 0.6
M8 97 - 133 8 - 11 11 - 15 1.1 - 1.5
M10 204 - 274 17 - 22 23 - 31 2.3 - 3.1
 
I see the chart with those torque values. Seems that the cylinder head bolts are the M4 category. Would you agree? Getting 90psi compression out of the top cylinder would suggest the torque is ok. How about the Loctite 515? I'm not using a thermostat as in salt water so I'd say the engine will run cooler with constant water flow. Not sure about temp at full power for extended periods.
 
Yes, 6mm. I was doing pretty well and was just about done when a bolt snapped. So now I've got bigger problems. I read a post that new bolts should be used rather than reusing old bolts. Once I get the snapped one out should I use new bolts? Thanks.
 
Snapped going together? Some crud in the threads apparently. I always use never seize on the threads. Yes, new bolts is prudent.
 
pvanv said:
Snapped going together? Some crud in the threads apparently. I always use never seize on the threads. Yes, new bolts is prudent.
Click to expand...

Snapped going together. Think I over-torqued it. Got lucky as bolt snapped above the core so I was able to screw it out. Have purchased new bolts. Discussed corrosion of wall with used outboard mechanic. Have built up wall with metal putty and sanded smooth to give more surface area for gasket to seal. Going to try Loctite 515 again to see if I can get above 60psi compression on lower cylinder.
 
Have put it back together and ran it for 5 minutes. Seems to be running better. Compression lower 80psi and holding better, top 110psi. Need to do break in time to see if lower compression will come up. Cheers, Dave
 
Lower compression still at 85psi lower, 112psi upper. Engine till miss firing and won't rev smoothly due to missing. Need to take head off again and try to get that 85psi within 10% of 112. Not sure where I'm losing the 15psi but still learning.
 
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