Motoman's Correlation: Carb Sync. Vs. RPM's

[IanDMacDonald]

I gotta agree with this. The speed the motor is running at and the gear it's in has a mathematical relationship to how fast the back wheel turns regardless of the power it's putting out. The tach, on the other hand, is electronic. My guess is your tach isn't reading exactly on the money.

As far as the idle speed you sync at - The Yamaha XS1100 Service Manual tells you it should be done at approximately 1,100 rpms. It also states that the correct idle for a '79F is 1,050-1,150 rpms.

I think it's also interesting that the manual says to attach a portable tachometer for tuning, so evidently Yamaha didn't trust the accuracy of the installed tach all that much.

Just comparing the op's readings to what I get on Betsy - at 70 mph I'm turning over 3,800 rpms with a 750 FD. It certainly lowers the engine rpms, but not 900 rpms at 70 mph as suggested by the ops 4700 rpm reading.

Actually, I believe stock specs as-mentioned in the other thread for a '78 Standard was 4600 rpm's @ 70 mph. So, when my bike is synced properly, it holds to the 1978 testing. When out of sync., it is off. So, no, I don't think my tach is off. I think that unless someone else on here can confirm, or duplicate my findings, I'll be a lone wolf with Moto and his theory.

[dbeardslee]

The drive train being directly connected to the crankshaft RPMs at any given speed and in any given gear cannot fluctuate regardless of the tuning.

Unless, your clutch is slipping.

I gotta agree with this. The speed the motor is running at and the gear it's in has a mathematical relationship to how fast the back wheel turns regardless of the power it's putting out. The tach, on the other hand, is electronic. My guess is your tach isn't reading exactly on the money.

As far as the idle speed you sync at - The Yamaha XS1100 Service Manual tells you it should be done at approximately 1,100 rpms. It also states that the correct idle for a '79F is 1,050-1,150 rpms.

I think it's also interesting that the manual says to attach a portable tachometer for tuning, so evidently Yamaha didn't trust the accuracy of the installed tach all that much.

Just comparing the op's readings to what I get on Betsy - at 70 mph I'm turning over 3,800 rpms with a 750 FD. It certainly lowers the engine rpms, but not 900 rpms at 70 mph as suggested by the ops 4700 rpm reading.

[jwhughes3]

It takes a certain amount of power to hit and maintain momentum at 70 mph. If the engine power is reduced due to inefficiency, at a certain RPM the speed will be reduced. Therefore it makes sense to reach the original speed the now underpowered engine must do more work to reach 70 mph. To do more work it must turn faster.

I cannot account for why the fixed ratios might allow this. I dropped out of engineering school to be a nurse.

Where is Ivan when you need him?

John

[bikerphil]

Don't forget, head wind or tail wind will also affect your RPM/MPH ratio.

[IanDMacDonald]

So, how do I account for the fact that during a state of a bad sync. on the carb bank, there is a 200 rpm difference @ 80 mph between a good sync., and out-of-synced carbs?

[BA80]

The drive train being directly connected to the crankshaft RPMs at any given speed and in any given gear cannot fluctuate regardless of the tuning.

Unless, your clutch is slipping.

[IanDMacDonald]

So, in another thread, I mentioned that I notice my RPM's flucuate in relation to the tune of my carbs. Moto agreed, throwing-about CV carb terminology. Everyone else said it was strickly gearing. I felt a connection to what Moto said, despite the naysayers.

So, last week, I had a low idle, so I bumped it up to the point of throwing-off my sync. I could tell it was off, as when I let-off the throttle while cruising, the rpm's stayed higher.

Anywho . . . Guess what the rpm difference is up-top??? I'm cruising around 200 rpm's slower with a bad sync., vs. my carb tune sync. Impossible you say? Well, I say you're wrong. 80 mph @ 5k is what my bike was doing with a good sync. 5200 rpm's @ 80 mph is what it's doing right now. 70 mph around 4700 rpm's.

I can guarantee that when I sync. the carbs tomorrow, the rpm's will be back to the lowered numbers . . .