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Author Topic: Big valve upgrade  (Read 2159 times)

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Offline fireman1073

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Big valve upgrade
« on: April 19, 2018, 02:34:20 PM »
Ok so I bought a Thompson big valve kit from a guy on Craigslist
He said it should fit a 75 cb750
They look kinda too big     I don’t know
What kind of machine work do I need to do to the head for this?
I put a piston next to the valve and I don’t know how this could work
The seller claimed Big horsepower gains with these
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Offline DV Red Herring

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Re: Big valve upgrade
« Reply #1 on: April 19, 2018, 02:44:54 PM »
Another great Craigslist find... haha
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Offline Bankerdanny

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Re: Big valve upgrade
« Reply #2 on: April 19, 2018, 02:55:01 PM »
 8)
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Current: '76 CB750F. Previous:  '75 CB550F, 2007 Yamaha Vino 125 Scooter, '75 Harley FXE Superglide, '77 GL1000, '77 CB550k, '68 Suzuki K10 80, '68 Yamaha YR2, '69 BMW R69S, '71 Honda SL175, '02 Royal Enfield Bullet 500, '89 Yamaha FJ1200

Offline Paintedseat

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Re: Big valve upgrade
« Reply #3 on: April 19, 2018, 04:18:04 PM »
You have to change the valve timing. You have to install a much smaller cam gear so the valve will have to open twice and act as a intake and exhaust valve. It's a simple upgrade.
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Offline MRieck

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Re: Big valve upgrade
« Reply #4 on: April 19, 2018, 05:32:58 PM »
Backcut that valve if you want to make some better low lift #'s.
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Offline ekpent

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Re: Big valve upgrade
« Reply #5 on: April 19, 2018, 09:18:53 PM »
 Ha ha - that thing is huge. Any idea what it was used on ?  I bought a big cool valve once still in its original box at a yard sale that was used on B-29 bombers. Not as big as yours though !
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Online Don R

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Re: Big valve upgrade
« Reply #6 on: April 19, 2018, 10:23:21 PM »
 I have a Neway valve seat cutter, I think we may need new seats though.
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Offline PeWe

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Re: Big valve upgrade
« Reply #7 on: April 20, 2018, 03:20:40 AM »
Look heavy.
A good port job is no1. Work fine with stock valves and guides.
Next is Kibblewhite 5mm valve kit, 34mm In and 28mm Ex if an K head. Port job for the bigger valve to use the bigger area. Maybe together with bee-hive springs.
I think this is called JMR Porting stage 4 head ;)
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CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline Flyin900

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Re: Big valve upgrade
« Reply #8 on: April 20, 2018, 04:10:12 AM »
 When I installed one of those the biggest issue was the spring compressor to install the keepers. Needed a backhoe with the compressor attachment.  :o
« Last Edit: April 20, 2018, 04:11:53 AM by Flyin900 »
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Common sense.....isn't so common!

1966 CL77 - 305cc - Gentleman's Scrambler
1967 CL175K0 - Scrambler #802 engine
1972 CB350F - Candy Bacchus Olive - Super Sport
1973 CB350F - Flake Matador Red - Super Sport
1975 CB400F - Parakeet Yellow - Super Sport
1976 CB400F - Varnish Blue - Super Sport
1976 GL1000 - Goldwing Standard
1978 CB550K - Super Sport
1981 GL1100 - Goldwing Standard
1982 CM450A - Hondamatic
1982 CB900C - Custom
1983 CX650E - Eurosport
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1983 CB1100F - Super Sport - Pristine example
1984 GL1200 - Goldwing Standard

Offline fireman1073

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Re: Big valve upgrade
« Reply #9 on: April 20, 2018, 07:01:24 AM »
i think it came from a commercial fishing boat
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Online Jerry Rxman Griffin aka MuthaF'er

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Re: Big valve upgrade
« Reply #10 on: April 20, 2018, 02:20:02 PM »
Rieck's the only guy I know that could make that work  ;)
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As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

Offline gschuld

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Re: Big valve upgrade
« Reply #11 on: April 20, 2018, 04:27:57 PM »
They would made good bases for a table lamp.😉

George
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Offline CarbonK

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Re: Big valve upgrade
« Reply #12 on: April 27, 2018, 08:04:05 AM »
I’ll let yah borrow my micrometer if you need to take some measurements.
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Offline PeWe

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Re: Big valve upgrade
« Reply #13 on: April 27, 2018, 08:28:20 AM »
For viagra guys!
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CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline RJ CB450

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Re: Big valve upgrade
« Reply #14 on: April 27, 2018, 08:46:56 AM »
Think might run into one problem.  That doesnt look like a metric valve...  You gotta bore and stroke to get it to the 45 ci required.  Dont forget you will also need imperial pistons too.
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Offline RAFster122s

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Re: Big valve upgrade
« Reply #15 on: April 27, 2018, 11:21:50 AM »
Quote from: CarbonK on April 27, 2018, 08:04:05 AM
I’ll let yah borrow my micrometer if you need to take some measurements.

Nice set of calipers there CarbonK, I like that storage box. What kind of latches does it have?
I like a nice wooden case or box...even if simple...beats the heck out of plastic every day, every way...

David
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Offline CarbonK

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Re: Big valve upgrade
« Reply #16 on: April 27, 2018, 12:27:45 PM »
Quote from: RAF122S on April 27, 2018, 11:21:50 AM
Quote from: CarbonK on April 27, 2018, 08:04:05 AM
I’ll let yah borrow my micrometer if you need to take some measurements.

Nice set of calipers there CarbonK, I like that storage box. What kind of latches does it have?
I like a nice wooden case or box...even if simple...beats the heck out of plastic every day, every way...

David

Haha, I’d love to tell you but it’s just an image I came across online which reminded me of this thread.
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