Author Topic: 1972 CB750 Sluggish Throttle Response - Oversize slides possible?  (Read 978 times)

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Offline denward17

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Re: 1972 CB750 Sluggish Throttle Response - Oversize slides possible?
« Reply #25 on: August 06, 2023, 07:51:28 AM »
From the honda service manual I found this, which appears to indicate 2500RPM you should see full advance.

Next, raise the engine speed above 2,500
rpm : if the timing index mark is between
the two timing marks located at 23.5~26.5°
ahead of the " F" mark, the timing for both
idling and full advance is satisfactory.

Offline PeWe

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Re: 1972 CB750 Sluggish Throttle Response - Oversize slides possible?
« Reply #26 on: August 06, 2023, 09:33:31 AM »
Both my bikes with cut springs get full advance at around 2600-2700 rpm.
Just cut them...
This is a Honda design flaw as lack of frame kit.

Pull engine for open valve cover is silly!!
The oil weaping CB750 need to lift head without pulling engine too.

I had to pull my engine the first year I owned it due to ugly stock oil weap after 20.000km total since new.
« Last Edit: August 06, 2023, 09:55:55 AM by PeWe »
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 Tracksnblades1

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Re: 1972 CB750 Sluggish Throttle Response - Oversize slides possible?
« Reply #27 on: August 06, 2023, 11:59:50 AM »
You could put a timing light on it and see if the timing is scattering about below 2000 rpm or especially when it dies.

Back in the late 70’s a lot of 836’s locally ran 125 main jets with bread box air filters. They were plenty fat with stock carbs and cams…

I would slowly use the choke at the point of idle instability to see if richer is better or worse..
The same can be done at 5,000 rpm but it’s not very safe if it’s waiting to climb on the pipe…
Age Quod Agis

Offline TheWiredNinja

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Re: 1972 CB750 Sluggish Throttle Response - Oversize slides possible?
« Reply #28 on: August 07, 2023, 02:36:14 PM »
The advance is pretty solid at full advance marks.  That's all I've been really able to witness since it won't dip further, but considering the shaft has been trued, I feel it won't be an issue.

New update: I cut the springs by one ring.  Installed on the bike and it runs like crap now.  I assume this is because the bike is so rich, it needs the advance to fire properly.  This also indicates that full advance has now been raised closer towards +2100 RPM ( I will have to confirm this but that's all the bike could really rev at now)

Just waiting on those jets!

Offline PeWe

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Re: 1972 CB750 Sluggish Throttle Response - Oversize slides possible?
« Reply #29 on: August 08, 2023, 01:14:13 AM »
CB750 specified full advance is 2500rpm.
A few hundreds later - no problem.

Earlier - problem

This is what I have noticed, stock and with various cams.
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 HondaMan

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Re: 1972 CB750 Sluggish Throttle Response - Oversize slides possible?
« Reply #30 on: August 08, 2023, 08:05:00 PM »
CB750 specified full advance is 2500rpm.
A few hundreds later - no problem.

Earlier - problem

This is what I have noticed, stock and with various cams.
+1 on this.
Today (in the USA) the gasoline burns very slowly, by design. Slower spark advance goes with this.
See SOHC4shop@gmail.com for info about the gadgets I make for these bikes.

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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.
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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

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