Author Topic: CB750 K1 spark plug read - oh and damage, bent insulator WTF?  (Read 1236 times)

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Offline Ace

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CB750 K1 spark plug read - oh and damage, bent insulator WTF?
« on: December 20, 2010, 07:55:35 PM »
G'day all, after a tune up on the last weekend, I got some dodgy fuel and all the carbies started leaking fuel.  Whatever was mixed in with the fuel attacked the rubber orings etc.  Not happy about that.  I synced the carbs and thought, idiot, should have done valve clearances and tension cam chain.  Not too worry, started that today with no problems until I decided to pull the plugs as it has been about 18 months and 1500 miles since I put new ones in.  I also decided to check as I've just put a set of HM300s on as well so I want everything running right.

Pulled the plugs out, #4 oily #3 rich #1 rich and finally pulled #2.  The picture says it all.  The insulator tip has moved to the side but it's not loose.  The worrying part about this is if you take a close look at the pictures of the plug by itself, it looks as if something has hit it as there are little score marks around the edge and it looks like the tip has been pushed in closing the gap.

Bit of background info: Went for 300 mile ride, dropped down to 2 cylinders working 1/4.  Intermittent problem.  When cold would run on all 4, warm up back down on 1/4 and consequently when it did fire up on all 4 it managed to put a hole in #3 header due to rust weak spot.  Replaced the Dyna S ignition with points and condensers and ran good.  Put the HM300s on and gee does it sound awesome.  After carb sync reved it a little up to 6k and hear a disturbing sound coming from top left side 1-2 cylinders.  Pulled plugs today and there you go.  Not sure what the f**k has happened, merry christmas to all, and to all a happy rebuild.  I reckon that's what I'll be doing and not putting more plugs in and see how it goes.  Anyone disagree with pulling the engine down and taking a look?  I need to put a set of rings in it as it's blowing smoke out of #4 and the plug confirms this.
1971 CB750 K1 - Sold
1978 CB750 F2 Supersport - Sold
1981 CB900 Bol d'or - Sold
2006 CBR1100 XX Super Blackbird - Sold

Offline HondaMan

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Re: CB750 K1 spark plug read - oh and damage, bent insulator WTF?
« Reply #1 on: December 20, 2010, 09:33:51 PM »
I saw a big chunk of carbon blow out of a sooty K1 once and chip off the side of the ceramic on a plug. It didn't bend it like that, though! The K1 generally got real sooty because they were jetted too rich (up around 115-120 mainjets from the factory) and the emulsifier tube's holes too small (at .031", should be more like .035" to .039"). Then the carbon could break off in bits, even blow out the HM341 pipes in chunks (even my own did that!). The pipes often plugged their drain holes (at the upswept points) from the loose carbon bits, rusting the pipes out right there.

That same carbon, if left to harden for [fill in this blank with a 2-digit number] years, can get real hard, break off when the engine is restarted, and damage ring lands and breaking rings.

The K1 is much happier if you either drop the needles a notch, or lower the mains to 110.  ;)
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

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Offline Ace

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Re: CB750 K1 spark plug read - oh and damage, bent insulator WTF?
« Reply #2 on: December 20, 2010, 11:57:53 PM »
25,000 miles since complete rebuild, 9 years ago.  I know what you mean about the carbon pieces, blocked the HM341 pipes up.

Tear down then I take it to find out HM?
1971 CB750 K1 - Sold
1978 CB750 F2 Supersport - Sold
1981 CB900 Bol d'or - Sold
2006 CBR1100 XX Super Blackbird - Sold

Offline HondaMan

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Re: CB750 K1 spark plug read - oh and damage, bent insulator WTF?
« Reply #3 on: December 22, 2010, 08:27:08 PM »
25,000 miles since complete rebuild, 9 years ago.  I know what you mean about the carbon pieces, blocked the HM341 pipes up.

Tear down then I take it to find out HM?
Either that, or see if someone might loan you a [lighted] borescope for a peek inside? It's not hard to tear down, just hard to get out of the frame.  ::)
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com