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

[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 . . .

[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, 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?

[bikerphil]

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

[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

[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.

[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.

[mikubuilder]

Everything you say is true, and makes sense. Engine speed(RPM), and actual speed (MPH)are correlated, so depending on your tune you are either efficient or inefficient(hens higher RPM to achieve the same result = 80 MPH)

[BA80]

It does that because when the engine is out of tune it throws a wrinkle into the time space continuum that causes the STEEL crankshaft to turn slower than the STEEL main shaft because flux capacitor that connects them becomes distorted and can't operate at warp speed.

Nanu nanu

Simple

[dbeardslee]

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.

Again, the gearing determines the rpm at a given speed. It doesn't have anything to do with the power the engine is putting out. If the crank makes a revolution there's a mathematical relationship that determines how far the rear wheel will turn. The gears don't change the number of teeth they have based on power. There's only one thing in there that can change, as BA80 alluded to, and that's the clutch. Other than that it's a straight gearing relationship.

XS11 tachs have been known to get flakey, and I think you're putting your faith in the wrong part. If you really want to know what's going on use an auxiliary tachometer with sufficient reliability and resolution and see what that tells you.

[dbeardslee]

And one other thing as it relates to clutch slippage. Higher torque is what makes a clutch slip - not lower. So if the clutch is slipping you would expect it to slip more with higher torque which would yield higher engine rpms.

[Ken Talbot]

Well, now, let's not forget that the crankshaft spinning and working through a series of gears to a spinning rear wheel is pretty much a fixed mechanical link. And remember also that the tach is getting its electrical signal via some sort of electrickery voodoo magic and translating that into a mechanical motion of the tach needle. Lastly, and maybe most critically, the speedometer is getting its input from the front wheel via a springy cable whirling within an outer cable, which spins some magnets inside a metal cup, which mechanically drives the speedo indicator needle.

Oh, and don't get me started on how the circumference of the front wheel changes ever so slightly with changes in pressure and air temperature, and location of the riders's butt in relation to the center of gravity of the bike. As the circumference increases, the wheel spins fewer times for any given distance, which has a direct result on the speedometer indication.

It's a wonder that any of this ever works the same twice!

[motoman]

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 . . .

A cleaner, leaner burn WILL give you those kinda results. An idle mixture re-setting and re-sync is definitely due after and chnge internal in carbs.

[skids]

You two should listen to Ken!

[dbeardslee]

You two should listen to Ken!

Who two you two? I always listen to Ken 'cause it's usually the prudent thing to do.