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Author Topic: Lost in carburation  (Read 2065 times)

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Offline livefast_dieold

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Lost in carburation
« on: June 16, 2015, 10:13:10 AM »
Sooo, last weekend I went trying my CB750 after some changes.
Let's say that the season did not start well: crash on the first race, bike not running properly on the second race (I would have been probably faster on a bicycle). I can't attend the third race as I have a wedding of a friend, but anyhow at least I have some time to get things right for the 4th race in Magione.

The current setup is:

> DP62R pistons (http://www.dynoman.net/images/engine/piston/DP62r/DP62r-2015.jpg)
> DP 315 camshaft (http://www.dynoman.net/images/engine/cams/DP/DP315Cam.jpg) degreed as of its timing card (http://www.dynoman.net/engine/cams/timing-cards/DP315.pdf) even using 0.012" valve lash - which seems very high -
> CR29 carburators (main jet 125 needle on the third slot)
> lightened crankshaft
> racing valve spring (but original valve)
> DYNA S ingnition
> Dyna DC 1-1 coils (http://www.dynoman.net/images/ignition/coils_2012/dyna_dc1-1.jpg)

At the race the bike simply did not run properly, it reaches (badly) 6000 rpm and then it simply stop pulling. Carburation seemed lean, so I tried to go up to 140 main jet but with little improvement.

During practice last week I firstly use the recommended valve lash and the bike seemed to go better. Then I set the needle on the third slot (from below). Again another small improvement.

The thing that really started to get things going was anticipating the ignition, at first by 3° (from the manual recommended position) : finally some pull, but above 9000 rpm, with quite a power hole between 7000 to 9000 rpm.
Anticipating more to 5° increased the pull, but again at very high rpm! The power hole between 7000 to 9000 persist. Unfortunately it's the spot where the bike runs the most during race.

Any suggestion on how to improve the situation? Keep anticipating? Go back to a milder cam? Devote myself to fishing?

Bonus pictures attached: see what I've done to the oil tank!
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Offline PeWe

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Re: Lost in carburation
« Reply #1 on: June 16, 2015, 12:55:11 PM »
I had a bad ignition wire with intermittent failure that interfered with carb jetting made carbs to sound rich or lean depending on when the cable started to fail. Dynatek suppressed graphite cable. (Crap for me) I replaced the cables with std copper wires and NGK 5kohms caps. Much better!

I do not know CR29. My old Mikuni VM29 need correct air jet, very important jet. Too large and it will cause way to lean at ~3600-5500 rpms. Huge pilots to compensate but getting way too rich idle.

Is the ignition advancer OK? Checked that the full advance occure at 2500-2800rpms?  Sloppy springs can cause way too early advance.

Float levels in carbs correct? Verify with clear tube. Too rich at idle can trick you jetting too lean and loos power later on.

Maybe first thing to check, spark plugs look OK, not fouled?
Floading floats foul plugs black, but possible to run way too lean on higher rpm.

edit: Reading plugs..
http://honda-tech.com/forced-induction-16/***-basics-reading-spark-plug***-3063102/
« Last Edit: June 16, 2015, 01:06:35 PM by PeWe »
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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 turboguzzi

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Re: Lost in carburation
« Reply #2 on: June 16, 2015, 03:04:44 PM »
ric, i also would suspect ignition.... specially if its dyna S.....
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Offline livefast_dieold

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Re: Lost in carburation
« Reply #3 on: June 17, 2015, 02:45:44 AM »
It's weird though, the previous one was destroyed during the crash, this one was taken from a perfectly running bike (my CB500 cafè racer)...
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Offline livefast_dieold

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Re: Lost in carburation
« Reply #4 on: June 17, 2015, 03:07:19 AM »
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Offline voxonda

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Re: Lost in carburation
« Reply #5 on: June 17, 2015, 05:10:55 AM »
Use it now for 5th year. Very easy to set up. After weeks of standing still 1, maybe 2 kicks and running. Nothing fancy but a very reliable ignition.

Cheers, Rob
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Better sorry for failing then for the lack of trying.

Offline brandEn

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Re: Lost in carburation
« Reply #6 on: June 17, 2015, 05:30:19 AM »
PAMCO is another ignition option. I have them on both my CB750's and love them. He is a member here as well.
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