Synchronizing Throttle Bodies 2004 RC-51 Nikki Hayden - Honda RC51 Forum : RC51 Motorcycle Forums
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post #1 of 3 (permalink) Old 03-22-2015, 10:14 PM Thread Starter
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Synchronizing Throttle Bodies 2004 RC-51 Nikki Hayden

New member been lurking for a long time and can usually search and find the answers, however cannot find answer to this question after many searches. In the Honda manual on page 5-74 it says to set the front cylinder manifold vacuum higher by 20mm +- 5mm. I have the Vacuum Mate four channel synchronizer which I calibrate frequently so holding this spec is not a problem and I have set it to this spec.

My question is why?

I did a search last year for the answer and I found one Ducati mech saying that being the firing order (on page 1-5 of the RC-51 manual), Front - 270 degree - rear- 450 degree - front, that it helps smooth the 90 degre V twin because of the longer interval till the crank comes around for a combustion hit from the front cylinder and some Ducati's (he did not mention a model)vary intake vacuum as well. I am confused because it would seem the rear cylinder would have the higher intake vacuum setting to cause the engine to spin with more energy till the front cylinder hit. All things equal, usually the cylinder with the highest intake vacuum pulls the higher load. In my latest searches all the Ducatis I found and TL1000 were set front and rear vacuums the same. In trying to adapt my thinking, maybe Honda is setting front cylinder to pick up the pace up after the longer crankshaft transistion from after firing the rear cylinder. What aroused my interest is when I was setting my V-Rod, of which there is no spec in the manual, I discovered that the front cylinder was 5mm higher in manifold vacuum and it runs very smooth however it is a 60 degree twin. Any answers for this difference in front to rear bias in the RC-51?
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post #2 of 3 (permalink) Old 03-22-2015, 10:39 PM
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Higher vacuum value means LESS air sucking. Don't forget that you measure vacuum AFTER the throttle's butterfly.

So higher vacuum = less air air flow = less fuel injected.....front cylinder has a slightly weaker bang
That means the rear cylinder has a stronger bang due to more fuel injected.
And that is what compensates for the longer "dead" period until the next bang of the front cylinder......doesn't it?

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post #3 of 3 (permalink) Old 03-22-2015, 11:22 PM Thread Starter
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Thanks Makis, the rear cylinder is filling with more and air and fuel molecules, in laymans terms resulting in more bang. The front cylinder is being more restricted by the butterfly or in this case the starter valve. I was confusing the higher intake vacuum with better efficiency as when tuning most engines with timing and carb settings you try to get the highest intake vaccum. Thanks again, when you replied , I was reading your assistance to the owner with the weak spark. Better than a suspense novel, however the culprit was never mentioned. Again I appreciate the quick response, I can sleep better tonight.
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