Author Topic: Cobra motor disassembly and rebuild  (Read 107472 times)

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Offline Bill/BentON Racing

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Cobra motor disassembly and rebuild
« on: December 16, 2015, 02:38:17 PM »
Yep, as it says. Last week when I tried to crank it, all of a sudden it stopped.  Near TDC on 2&3. Now I see why! Guess it wanted to be rebuilt top and bottom! So it will be! Stay tuned. Bill
« Last Edit: December 16, 2015, 02:50:03 PM by Bill/BentON Racing »
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Offline Bill/BentON Racing

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Re: Cobra motor disassembly and rebuild
« Reply #1 on: December 16, 2015, 02:41:26 PM »
Why you don't force things!! Fixed!  ;D
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Offline Bill/BentON Racing

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Re: Cobra motor disassembly and rebuild
« Reply #2 on: December 16, 2015, 02:53:04 PM »
More!
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Offline 70CB750

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Re: Cobra motor disassembly and rebuild
« Reply #3 on: December 16, 2015, 03:09:37 PM »
The vaseline on the lens makes it kind to hard to see exactly what happened there.

Ain't vaseline, them are tears.
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Offline Bill/BentON Racing

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Re: Cobra motor disassembly and rebuild
« Reply #4 on: December 16, 2015, 04:08:38 PM »
LMAO,  you close to right.... so what made it stop ? Detectives , c'mon.  ;D
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Offline Medyo Bastos

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Re: Cobra motor disassembly and rebuild
« Reply #5 on: December 16, 2015, 04:08:55 PM »
phewww, glad you guys are seeing the same thing.  i thought it was my tears when i read the first post...

Offline NickO

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Re: Cobra motor disassembly and rebuild
« Reply #6 on: December 16, 2015, 04:36:59 PM »
LMAO,  you close to right.... so what made it stop ? Detectives , c'mon.  ;D

Looks like something was jammed in #2 cylinder right at the edge of the piston near the exhaust valve?

There is a witness mark in the chamber on image 174007 which you seem to have polished-out on image 174020, also can see the same mark on the piston crown on image 175206? Seems to me it could have been the spark-plug disintegrating as it is right below the plug-hole?

Offline Bill/BentON Racing

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Re: Cobra motor disassembly and rebuild
« Reply #7 on: December 16, 2015, 04:51:27 PM »
Nick, good observation,  yeh #2 had interference! Not a spark plug PC though. Check these pistons out! K.
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Offline gschuld

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Re: Cobra motor disassembly and rebuild
« Reply #8 on: December 16, 2015, 04:58:28 PM »
12.5:1 or higher.  12.5:1 should have around a .33" dome.  I like the Swiss cheesed skirts.  Reminds me of the early RC modified cb350 pistons they were using in the early 836 kits.

More pictures please 8)..... ;)

George

Offline BPellerine

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Re: Cobra motor disassembly and rebuild
« Reply #9 on: December 16, 2015, 05:07:04 PM »
look like the pistons in your pre cobra article post George.billp
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Offline Bill/BentON Racing

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Re: Cobra motor disassembly and rebuild
« Reply #10 on: December 16, 2015, 05:31:31 PM »
It was past time! Cam chain shot! Man, that crank spins so freely! K.
BentON Racing Website
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Over 35 years of experience working on vintage motorcycles, with a speciality in Honda SOHC/4 with a focus on the CB750 and other models as well from 1966 - 1985.
______________________________________
1993 HRC RS125 | 1984 NS400R | 1974 Honda CB750/836cc (Calendar Girl) | 1972 CB 500/550 Yoshi Kitted 590cc | 1965 Honda CB450 Black Bomber | 1972 Suzuki T350 | 1973 88cc | Z50/Falcons Pit Bike | 1967 CA100| 1974 CB350 (400F motor)...and more.
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Offline Bill/BentON Racing

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Re: Cobra motor disassembly and rebuild
« Reply #11 on: December 16, 2015, 05:33:21 PM »
RC crank, balanced,  lightened and oil holes fluted
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Over 35 years of experience working on vintage motorcycles, with a speciality in Honda SOHC/4 with a focus on the CB750 and other models as well from 1966 - 1985.
______________________________________
1993 HRC RS125 | 1984 NS400R | 1974 Honda CB750/836cc (Calendar Girl) | 1972 CB 500/550 Yoshi Kitted 590cc | 1965 Honda CB450 Black Bomber | 1972 Suzuki T350 | 1973 88cc | Z50/Falcons Pit Bike | 1967 CA100| 1974 CB350 (400F motor)...and more.
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Offline scottly

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Re: Cobra motor disassembly and rebuild
« Reply #12 on: December 16, 2015, 05:47:45 PM »
LMAO,  you close to right.... so what made it stop ? Detectives , c'mon.  ;D
The holes in the head gasket are too small for the bore, and interfered with the outer edge of the pistons, which protruded above the deck. There appears to be a small chunk of loose metal in the #2 cylinder, but with the crappy pics I can't tell where it came from. ;)
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Offline Old Scrambler

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Re: Cobra motor disassembly and rebuild
« Reply #13 on: December 16, 2015, 05:58:39 PM »
I see a mixture of HD and OEM studs ???  Could be gasket material pinched in the cylinder.  I like the flat-faced valves ;) I've seen dimpled skirts.......but not swiss-cheese 8)
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Offline hotdog

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Re: Cobra motor disassembly and rebuild
« Reply #14 on: December 16, 2015, 06:10:31 PM »
Broken peice of guide..? Carb part.?, baby faces favourite chew toy..?, hell I don't know just guessing now.

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Offline MRieck

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Re: Cobra motor disassembly and rebuild
« Reply #15 on: December 16, 2015, 06:49:11 PM »
I see a mixture of HD and OEM studs ???  Could be gasket material pinched in the cylinder.  I like the flat-faced valves ;) I've seen dimpled skirts.......but not swiss-cheese 8)
Those must be RC studs Dennis. The 2 for the oil feeds were tapered. Not a bad idea but you can't torque them to 22lbs.
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Offline Bill/BentON Racing

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Re: Cobra motor disassembly and rebuild
« Reply #16 on: December 16, 2015, 06:56:54 PM »
I never liked that. Going to replace em with better ones, better head nuts also and cylinder stud kit. New cam chain and primary's etc, new mains also. This 1/2 ready for a bath. I cleaned my lens. BF chew toy closest so far!! Closest guess wins a John Stein Drag Racing book!
« Last Edit: December 16, 2015, 07:02:57 PM by Bill/BentON Racing »
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______________________________________
1993 HRC RS125 | 1984 NS400R | 1974 Honda CB750/836cc (Calendar Girl) | 1972 CB 500/550 Yoshi Kitted 590cc | 1965 Honda CB450 Black Bomber | 1972 Suzuki T350 | 1973 88cc | Z50/Falcons Pit Bike | 1967 CA100| 1974 CB350 (400F motor)...and more.
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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: Cobra motor disassembly and rebuild
« Reply #17 on: December 16, 2015, 07:02:05 PM »
Early engine Bill? Bushed and peened RC rods with bigger bolts. Not Goldens.

Post up a couple decent pics of the head and cylinder.
As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

Offline Bill/BentON Racing

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Re: Cobra motor disassembly and rebuild
« Reply #18 on: December 16, 2015, 07:17:29 PM »
After you tell what this is for? ;D
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Over 35 years of experience working on vintage motorcycles, with a speciality in Honda SOHC/4 with a focus on the CB750 and other models as well from 1966 - 1985.
______________________________________
1993 HRC RS125 | 1984 NS400R | 1974 Honda CB750/836cc (Calendar Girl) | 1972 CB 500/550 Yoshi Kitted 590cc | 1965 Honda CB450 Black Bomber | 1972 Suzuki T350 | 1973 88cc | Z50/Falcons Pit Bike | 1967 CA100| 1974 CB350 (400F motor)...and more.
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Offline cbr954

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Re: Cobra motor disassembly and rebuild
« Reply #19 on: December 16, 2015, 07:20:09 PM »
does it have a welded primary?
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Offline scottly

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Re: Cobra motor disassembly and rebuild
« Reply #20 on: December 16, 2015, 07:21:56 PM »
BF chew toy closest so far!! Closest guess wins a John Stein Drag Racing book!
Billy Bling gold tooth cap?? ??? ;D
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Offline Bill/BentON Racing

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Re: Cobra motor disassembly and rebuild
« Reply #21 on: December 16, 2015, 07:32:08 PM »
Close, but Bill closer! Cases just love luxuriating baths!
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Over 35 years of experience working on vintage motorcycles, with a speciality in Honda SOHC/4 with a focus on the CB750 and other models as well from 1966 - 1985.
______________________________________
1993 HRC RS125 | 1984 NS400R | 1974 Honda CB750/836cc (Calendar Girl) | 1972 CB 500/550 Yoshi Kitted 590cc | 1965 Honda CB450 Black Bomber | 1972 Suzuki T350 | 1973 88cc | Z50/Falcons Pit Bike | 1967 CA100| 1974 CB350 (400F motor)...and more.
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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: Cobra motor disassembly and rebuild
« Reply #22 on: December 16, 2015, 07:43:17 PM »
Is the kick starter prawl in the head?
As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

Offline Jerry Rxman Griffin aka MuthaF'er

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Re: Cobra motor disassembly and rebuild
« Reply #23 on: December 16, 2015, 07:43:51 PM »
Close, but Bill closer! Cases just love luxuriating baths!

Wash that damn dawg after you get done with the cases!
As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

Offline Bill/BentON Racing

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Re: Cobra motor disassembly and rebuild
« Reply #24 on: December 16, 2015, 08:28:04 PM »
LMAO, He got a bath last week. ...me too!! Hehe.  ;D
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Over 35 years of experience working on vintage motorcycles, with a speciality in Honda SOHC/4 with a focus on the CB750 and other models as well from 1966 - 1985.
______________________________________
1993 HRC RS125 | 1984 NS400R | 1974 Honda CB750/836cc (Calendar Girl) | 1972 CB 500/550 Yoshi Kitted 590cc | 1965 Honda CB450 Black Bomber | 1972 Suzuki T350 | 1973 88cc | Z50/Falcons Pit Bike | 1967 CA100| 1974 CB350 (400F motor)...and more.
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See our latest build 'Captain Marvel' CLICK HERE