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Author Topic: Montreal Manchild with an '81 Honda CB750K  (Read 37642 times)

Offline The Jimbonaut

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #20 on: Nov 22, 2016, 14:31:41 »
Ok, I'll try and get my own compression tester and will run it again.  Novice question - the engine doesn't need to be firing, just cranking with the starter motor, and then open the throttle?
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Offline advCo

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #21 on: Nov 22, 2016, 14:48:34 »
Well yes. Remove all the plugs so that it turns over easier and doesn't start. The key will have to be on so that you can crank the starter motor over. You will want to hold the throttle wide open while cranking so that there is max airflow into the combustion chamber for proper compression reading.
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Offline The Jimbonaut

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #22 on: Nov 22, 2016, 14:57:32 »
Excellent, thanks a million.  I'll probably pick one up from Canadian Tire but have heard the cheaper tools can be a bit squiffy.  The brand they sell is called Equus, which if nothing else is one cool sounding word
"I'm telling you Donnie, nuthin' but nuthin' but right"

Offline farmer92

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #23 on: Nov 22, 2016, 16:47:14 »
Got one there on sale,
50 bucks, four plug adapters, still working top notch after 10 years.   

Offline Maritime

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #24 on: Nov 22, 2016, 18:28:40 »
I too have the crappy tire compression gauge with the motorcycle adapters it has served me well and is accurate
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Offline Maritime

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #25 on: Nov 22, 2016, 18:33:31 »
And you want all plugs out and gas off, throttle wide open or if you are pulling carbs, take them off. Cold comp should be in the 130-150 on that bike but more importantly it should be within 10 psi up or down across the 4. The #4 number hopefully is just the fact the carb was closed and not letting air build. When you add oil make sure it is only a tsp or less too much will muck up the numbers.
The GL Rebirth: http://www.dotheton.com/forum/index.php?topic=68337.0
CX500 Low budget Bobber : http://www.dotheton.com/forum/index.php?topic=43617.0
"Beer makes you feel the way you ought to feel without beer" -Henry Lawson
"Beer is proof that God loves us and wants us to be happy." - Thomas Jefferson

Offline The Jimbonaut

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #26 on: Nov 22, 2016, 21:28:07 »
Got the tool, ran the test again with the throttle wide open (thanks for the info on that) and got much better results -

150, 145, 150, 125 - cylinder 4's the party pooper

I'll add some oil tomorrow and see what gives =)
"I'm telling you Donnie, nuthin' but nuthin' but right"

Offline canyoncarver

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #27 on: Nov 23, 2016, 00:41:26 »
Got the tool, ran the test again with the throttle wide open (thanks for the info on that) and got much better results -

150, 145, 150, 125 - cylinder 4's the party pooper

I'll add some oil tomorrow and see what gives =)


Have you checked the valves yet?    125 isn't the floor but it's noteably lower than the other three.
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Offline Maritime

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #28 on: Nov 23, 2016, 08:39:02 »

Have you checked the valves yet?    125 isn't the floor but it's noteably lower than the other three.

+ 1 on valve check. a tight valve will cause this as it stays open a little to long to build pressure well. That was what was wrong with my CX500 I had 90 PSI. checked and set valves and went up to 150 in each cylinder.  If Valves are ok, oil in barrel and re-check if up then you are pretty sure it is rings or bore.
The GL Rebirth: http://www.dotheton.com/forum/index.php?topic=68337.0
CX500 Low budget Bobber : http://www.dotheton.com/forum/index.php?topic=43617.0
"Beer makes you feel the way you ought to feel without beer" -Henry Lawson
"Beer is proof that God loves us and wants us to be happy." - Thomas Jefferson

Offline The Jimbonaut

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #29 on: Nov 23, 2016, 11:11:45 »
Checking the valves is next on the list.  Do you have any suggestions on where to buy replacement shims as and when I need them?  I'm in Canada, but of course am happy to buy from anywhere as long as I can get them quick!
"I'm telling you Donnie, nuthin' but nuthin' but right"