top end time!

[mechanicmike15]

hey guys got spark to the honda about 4 weeks ago . so i rode my bike all da way 2 autozone today 2 buy fuel starterspray so it would start up shot some in the sparkplug hole and nothin. It has spark for sure, kicked it over with fuel starter and nothing... so i ran ran an engine compression test but the level was below 120. fuel combusted in the cylinder but not quite running.(i could see it combust cause i saw fire from uppere valve wen closing).

i know wat the problem is. the problem is when the engine runs the piston goes back and wen it does the upper valve opens it sucks in the jet from the carb to get the fuel mixture. Once thats done the piston rams the fuel mixture with the air hard enough to combust with spark and the exaust exits the exaust valve. soit doesnt have the air to suck the jet from the carb well and not enough air 2 ram down and combust

would i just need to rebuild the top end and cylinder head or just valve block/head? reason im askin cause i dont wanna do xtra ya know.lol. every engine i do this on i do both i clean the cylinder heads check them for scratches and build the valves with cam of course and lube them And replace the piston rings ... i was thinking of high compression on da engine 2 give it that extra punch but if its easy y not or could ill just keep it stock

BTW: the reason im stressing out is cause i am in my garage in the heat and i hope im not soundin like a pussy, but in like the first 10 minutes im drenched in sweat (hot as hell). jus dont want 2 do somethin uneccesary/xtra.

 

[Epic_username]

Is the intake port all black and crappy looking? Thats a worn intake valve. Its common on these. Did you check cam timing? Sounds like it may be off. Check valve adjustment too. If all that fails, top end rebuild. New valves, guides, maybe rockers and cam (if theyre too bad to re use), new cam chain, piston rings, maybe new piston if its scratched. And of course all the gaskets.

(i could see it combust cause i saw fire from uppere valve wen closing).

Sounds like the intake valve is still open as its igniting, so cam timing is retarded.

Again, check cam timing and valve adjustment, and anywhere else you might be losing compression. Maybe cam chain has stretched/no tension. I had a bike run half decently with only 90PSI compression, with only a throttle bog. Im just shooting ideas out to keep you out the heat, I hate working in the heat too. Hope you figure it out

 

[motodevo]

yeh i agree with Epic, i'd be checking the valve clearances first, the intake valve should be 100% closed when firing, if clearance is too tight(or totally closed up) it can show low comp. Then i'd take off the cam cover and make sure the timing is right and that the timing chain hasn't skipped a tooth.

 

[mechanicmike15]

thanx dave i will check that out.i checked the timing and the timing dot was a little bit off it seemed approx half of the dot off. i will just have to take it off and check it all out. if any1 else has any suggestions that would be gr8 thanx. oh ya iam pretty sure though the bikes valves adjustments are off. 43 years of sitting in a shed are bound 2 cause problems

---------- Post added at 12:05 PM ---------- Previous post was at 11:40 AM ----------

oh ya dave if the valve looks crappy and dirty like you said couldt i just polish it with a wire brush and lube

 

[bry593]

i'd be checking the valve clearances first, the intake valve should be 100% closed when firing, if clearance is too tight(or totally closed up) it can show low comp

Valve seats tend to sink, especially in aluminum heads. When the valves sink, the clearance on the rocker adjustment screw gets smaller and smaller, until eventually, the valve stays open. Once the valve stays open, you lose compression, and if driven that way, you will burn the valve and seat. Seen it many times on Honda cars, not sure if this happens as much on these little bikes that see only 5000 miles in a lifetime and have really tiny valve springs...

EDIT: Forgot to address Mike's question... if after you check your valve clearance (use the procedure in the owner's manual i sent you) you still have problems, then you will have to remove the head. head removal entails: 1. set the motor to top dead center, 2. remove the lash adjusting plugs and then remove the lash adjusting screws completely, 3. loosen the screw on the rights side of the head a few turns then rap it with a hammer, knocking the left cam cover loose, then remove the camsprocket, 4. remove the 3 acorn nuts and one hex nut on the end of the head (note the location of the copper washer), 5. remove the single 10mm screw connecting the head to the cylinder. The head will slide right off the studs (the studs are fastened to the acorn nuts). At this point, you will probably notice a lot of carbon built up around your valves and in your intake port.

You will probably also see some oily residue in the exhaust port. Take a small screwdriver, and gently scrape the carbon deposits from the valve combustion chamber-sphere around the valves. Next, pour some gas into the intake port and check for leakage into the combustion chamber, do the same for the exhaust port. If you have a valve leak, you might be able to lap the valve with 120 followed by 280 grit compound, but if that doesn't work the seat will have to be cut at a machine shop.

Note that because you removed the head, you have also disturbed the cylinder. Because of this, you will need a top end gasket set which includes head and cylinder base gaskets, along with the o-rings that seal the oil passages to the valvetrain.