SOHC.co.uk Forums > CB350/400
The elusive surge
400 Cafe Racer:
--- Quote from: Alexrayz on January 10, 2022, 09:46:47 PM ---Hey Dennis, thanks for all that sweet sweet info, I think I will do the vacuum gauge sync next time working on the bike. Seems like it will clear up more questions and hopefully solve my issues.
I was actually wondering if there is a proper methodology for the mixture screw settings. I know that the look of the plugs is a telltale sign but I was wondering how precise it gets without requiring an IR exhaust gas probe.
Cheers!
--- End quote ---
Hi thanks Alex, you are welcome,
I would say that the if your bike is standard spec regarding air intake, air filter design and exhaust system, that the standard procedures and guides documented for carb settings are the best to use. The plug colour can be a useful guide for normal settings.
In any event the matching of equal slide heights from idle to full throttle is desireable to ensure equal fuel/air volume to all cylinders and in theory deliver equal power from each cylinder. There inevitably could be small variations on position of the idle mixture screw position between individual cylinders for final setting, due to small variations in manufacture. This is fundamental to all tuning.
On my 400F the air filters and exhaust are non standard and I have an Innovate Air / Fuel Ratio guage already fitted which was very useful during re-jetting and setting up under actual running conditions on the road. The settings I have used are to deliver best power, torque and engine temperature, with no specific regard to economy or plug colour.
I have assumed that all is OK with the Ignition advance unit and that if you have standard ignition, that all your contact breaker points, condensers and other factors are correct.
As Ted has hinted, its worth checking that all is OK with the static timing and centrifugal advance unit. Look for anything that could cause variation. Seized or stiff action on the spindle or weights, preload on the springs with no looseness, correct full advance as Bryan says at 2500 RPM
I am sure with patience and perseverence you will nail the problem - and keep us up to date with how you get on. ;) :)
Dennis
daz-j:
Thanks Alex, yes I have checked the float heights previously but I'm thinking maybe under-estimated how accurate they need to be, so will be revisiting that when I reassemble the carbs with the new O rings that arrived today (thanks Julie).
Really interesting to read the viewpoints on carb balancing thank you. I've bench sync'd previously before refitting, pretty confident that I did it correctly but will go through it carefully again and report back.
philward:
On my 400F the air filters and exhaust are non standard and I have an Innovate Air / Fuel Ratio guage already fitted which was very useful during re-jetting and setting up under actual running conditions on the road. The settings I have used are to deliver best power, torque and engine temperature, with no specific regard to economy or plug colour.
[/quote]
Dennis, I'd be interested to hear how the Air/Fuel physically fits on a 4 cylinder 4 carb bike?
Lobo:
… the easiest way to rule out an issue wrt fluctuations in the timing would be to ‘jam’ the advance mechanism to fully regarded and run the bike at idle.
400 Cafe Racer:
--- Quote from: philward on January 11, 2022, 09:17:38 PM ---
On my 400F the air filters and exhaust are non standard and I have an Innovate Air / Fuel Ratio guage already fitted which was very useful during re-jetting and setting up under actual running conditions on the road. The settings I have used are to deliver best power, torque and engine temperature, with no specific regard to economy or plug colour.
--- End quote ---
Dennis, I'd be interested to hear how the Air/Fuel physically fits on a 4 cylinder 4 carb bike?
[/quote]
Hi Phil,
To avoid distraction from Alexs' post I have put a link to my post showng recent guage additions. I have just added a final one to show where the sensor is fitted in the intermediate pipe.
Dennis
http://www.sohc.co.uk/index.php/topic,26987.0.html
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