Author Topic: CB750FO won't start  (Read 497 times)

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Offline Dave B

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CB750FO won't start
« on: February 19, 2021, 11:55:50 AM »
The bike has been unused for a number of years
I have cleaned carbs and have fuel to the plugs
I have fitted new battery,plugs,points and condensers and checked timing.
I have a continuous blue spark at all four plugs
All four engine intakes are sucking
All four exhaust outlets are blowing
BUT the engine will not fire at all!
Should the plug spark be continuous all the time i press the starter button.
Cheers,
Dave

Offline 70CB750

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Re: CB750FO won't start
« Reply #1 on: February 19, 2021, 12:08:12 PM »
It has two coils, one for cylinders 1,4 and the other one for 2,3.  Common problem is swapped wires between coils, try switching them.
Prokop
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I love it when parts come together.

Dorothy - my CB750
CB750K3F - The Red
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2006 KLR650

Offline PeWe

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Re: CB750FO won't start
« Reply #2 on: February 19, 2021, 12:37:39 PM »
Ignition or fuel.
Kill switch might be off.
Starter spray in carbs if you have a spray can on hand. 

If ignition is OK it will start on the spray.
I use an extra plug to test on each cylinder. Disconnect plug cap, connect extra plug that you can place on valve cover.
Test with starter and see any arc.
No arc at all, check if coils get voltage.
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 Dave B

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Re: CB750FO won't start
« Reply #3 on: February 19, 2021, 01:19:28 PM »
Just to be clear
when i press the starter button all four plugs spark continuously
is that correct
Cheers,
Dave

Offline newday777

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Re: CB750FO won't start
« Reply #4 on: February 19, 2021, 02:32:15 PM »
Just to be clear
when i press the starter button all four plugs spark continuously
is that correct
Cheers,
Dave
2 at a time, then the other 2. Not all 4 at the same time
Stu
Honda Parts manager in the mid 1970s Nashua Honda
My current rides
1975 K5 Planet Blue my summer ride, it was a friend's bike I worked with at the Honda shop in 76, lots of fun to be on it again
1976 K6 Anteres Red rebuilding project, was originally my brother's that I set up from the crate, it'll breath again soon!
Project 750s, 2 K4, 2 K6, 1 K8
2008 GL1800 my daily ride and cross country runner

Prior bikes....
1972 Suzuki GT380 I had charge of it for a year in 1973 while my friend was deployed and learned to love street riding....
New CB450 K7 after my friend returned...
New CB750 K5 Planet Blue, demise by ex cousin in law at 9,000 miles...
New CB750 K6 Anteres Red, to replace the totaled K5, I sold this K6 at 45k in 1983, I had heavily modified it, many great memories on it and have missed it greatly.....
1983 GL1100A, 1999 GL1500 SE, 1999 GL1500A

Offline Dave B

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Re: CB750FO won't start
« Reply #5 on: February 19, 2021, 02:41:25 PM »
Thank you
looks like iv got a busy day tomorrow

Offline evinrude7

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Re: CB750FO won't start
« Reply #6 on: February 20, 2021, 12:34:03 PM »
Thank you
looks like iv got a busy day tomorrow

what brand points, condensers and plugs (+type)? 
cb750 k6 - ugly