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Author Topic: Montreal Manchild with an '81 Honda CB750K  (Read 37640 times)

Offline Maritime

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #40 on: Nov 30, 2016, 15:13:49 »
If you are blasting it into the intake with motor running, it will get blown out of the exhaust. Seafoam does the same thing. If you are taking the plug out and filling the cylinder and letting it sit, you can then take the plugs out of all cylinders and turn the motor over to blast the cleaner out as well.  Try it running, then the one bad one spray a bunch in the spark hole and leave it overnight, then in the morning leave the plug out, take the plugs out of the rest and spin the motor. put a rag over the one hole that had cleaner so it doesn't fly all over. Once it is all sprayed out, put plugs back and run the motor again till warm then test compression.
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 Maritime

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #41 on: Nov 30, 2016, 15:15:58 »
also if you do this and the rings are bad, the cleaner is going to get into the crankcase and thin out your oil a bit so you might want to check it and likely change it. that depends on how much you fill the cylinder, how bad the rings are or aren't etc.
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 #42 on: Nov 30, 2016, 15:16:37 »
Nice one bud, I'll do that and will let you know how I get on.  I'm going to be spraying into the intake, but will leave some sitting in the cylinder overnight as well as a hail mary =)
« Last Edit: Nov 30, 2016, 15:25:56 by The Jimbonaut »
"I'm telling you Donnie, nuthin' but nuthin' but right"

Offline The Jimbonaut

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #43 on: Dec 01, 2016, 19:01:19 »
Just finished a valve clearance check, replaced a few shims and now have 0.005" clearances across the board. Emptied a can of Honda's carb/combustion chamber cleaner into the carb intakes and compression check now looks better -

#1 - 140 (130 before)
#2 - 150 (same)
#3 - 165 (150 before)
#4 - 165 (150 before)

Question - although the compression has improved, cylinder #4 now has a cold exhaust, whilst the others are hot after running the bike for a while. Ok, not cold, but only mildly warm. Plugs on #3 and 4 look a little oiled.

Weird?
"I'm telling you Donnie, nuthin' but nuthin' but right"

Offline advCo

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #44 on: Dec 01, 2016, 19:41:44 »
Could be your carbs out of sync. Have you done a vacuum sync?
"He broke the mirrors off his Cadillac, 'cause he doesn't like it looking like he looks back."

74 CB360 - Luna - http://www.dotheton.com/forum/index.php?topic=63294.0
82 GS550L - Tracker-ish - http://www.dotheton.com/forum/index.php?topic=67229.0 - Sold
74 XL350 - The Turd - http://www.dotheton.com/forum/index.php?topic=70252.0
Suzuki FA50 "No-Ped" - http://www.dotheton.com/forum/index.php?topic=71189.0
73 Suzuki RV125 -http://www.dotheton.com/forum/index.php?topic=73875.0

Offline The Jimbonaut

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #45 on: Dec 01, 2016, 20:34:03 »
Dude I've been thinking that it could be the carbs.  I gave the bike a once over when I bought it last month (new plugs, battery, bit of a spruce up on the carbs to clean out the gunk but nothing major, clean gas, airbox etc) but that's it.  When I bought the bike it hadn't run for three years, and I just wanted to get it started.

Read in a different thread that someone was having a similar problem (theirs was a SOHC but the issue was the same), and that also boiled down to carbs.  Is there a quick test I can do to see if the carbs are indeed the problem?  How about cranking the engine, with only the plug from cylinder 4 removed, and if I can't smell any gas coming from the hole then that might point to no gas being fed from its carb?

I'm going to be doing a complete rebuild on the carbs next, but first wanted to make sure the valve clearances were as dialled in as possible.
« Last Edit: Dec 01, 2016, 20:35:39 by The Jimbonaut »
"I'm telling you Donnie, nuthin' but nuthin' but right"

Offline farmer92

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Montreal Manchild with an '81 Honda CB750K
« Reply #46 on: Dec 01, 2016, 21:41:05 »
It would really depend on what was wrong with the carbs,

Check the plug, is it rich or lean?
« Last Edit: Dec 01, 2016, 21:42:39 by farmer92 »

Offline The Jimbonaut

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #47 on: Dec 01, 2016, 23:16:55 »
It was a little wet, like it had some oil on it.  Not dripping, just kinda glazed
"I'm telling you Donnie, nuthin' but nuthin' but right"

Offline advCo

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #48 on: Dec 01, 2016, 23:54:58 »
The plug wont tell you much. Assuming these carbs have throttle plates you can bench sync them with a paperclip to get all the butterflies to close/open relatively the same. This will get you "close enough" but the best way to get them dialed in is with a manometer setup for a 4 cylinder bike.

https://www.amazon.com/Vacuum-Carburetor-Synchronizer-Gauge-Yamaha/dp/B004MSJ7E6

Here's an example of the process I'm referring to:

"He broke the mirrors off his Cadillac, 'cause he doesn't like it looking like he looks back."

74 CB360 - Luna - http://www.dotheton.com/forum/index.php?topic=63294.0
82 GS550L - Tracker-ish - http://www.dotheton.com/forum/index.php?topic=67229.0 - Sold
74 XL350 - The Turd - http://www.dotheton.com/forum/index.php?topic=70252.0
Suzuki FA50 "No-Ped" - http://www.dotheton.com/forum/index.php?topic=71189.0
73 Suzuki RV125 -http://www.dotheton.com/forum/index.php?topic=73875.0

Offline The Jimbonaut

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Re: Montreal Manchild with an '81 Honda CB750K
« Reply #49 on: Dec 02, 2016, 12:30:28 »
Bench synch and vacuum synch are firmly on the road map, first I'm going to rebuild the buggers as the gaskets and a few of the jets look rougher than a stucco bathtub

Thanks for the link - do you own one of those things?  I'm thinking either I'll see if I can borrow one from my local shop or maybe build my own - there are a few vids showing how to rig one up with some glass bottles and a bit of know how.  It doesn't show you psi or anything like that, only that the carbs are all pumping out the same pressure (shown by the liquid levelling out in the bottles)

By the way, yesterday I broke the mirrors off my Honda CB, 'cause I didn't like it looking like I looked twee
"I'm telling you Donnie, nuthin' but nuthin' but right"