Fuel Consumption

[DEEBS47Chief1991]

My Chief gets about 35+MPG with a flat head motor. Timing, manifold seal(Cotton has written quite a bit about this subject), and carb setting make all the difference. Setting these old bikes up is not simple. It's an art form that is slowly dying.

A tuned motor also can wear slower than a badly tuned unit mainly because of overheating. One of the easiest ways to tell if you are out of tune is to observe the amount of heat coming off the motor. The motor can actually smell like it's cooking if it's lean. The exhaust smells like un-burnt hydrocarbons when it is too rich. You can also measure cylinder heat with an infra-red thermometer. After-burning or back-fires when you close the throttle fast at speed is another simple lean test.

Valve adjustment is really important with an old motor. You can't really time a motor correctly until the valve lash is completed. If a valve does not fully return to it's seat because there is no gap between the stem and the lifting device, you could be burning up your valve seats and over heat the motor.

Too much advance on the timing is also a very common problem in poor running motors. Don't time a motor "By Ear".

If you been around these bikes long enough you can hear a badly tuned bike as it passes you and unleashes noises that sound like a fork in the garbage disposal. We have all witnessed this.

This does not necessarily mean that your engine is out of tune but it just something to be aware of.

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[T. Cotten]

Pretty sure Otis has manifolds down pat, Deebs!

And plenty of experience listening to lots of machines.

Model M Linkerts have the disadvantage of vertical throttleshafts, so comparing modern carbs is pointless, Chuck.

What robs mileage when everything else seems fine to a trained ear, an experienced wrench, and veteran rider?

....Cotten
PS: If you can't find a big problem, its a bunch of little ones adding up.

[otis71]

I always keep timing and valve lash closely monitored. I usually tune the carburetor a touch on the rich side on the idle needle (10 clicks past lean starvation) but maybe I should try closer to 5. Power needle always seems trickier to get as lean as possible without being too lean since modern gasoline doesn’t let you read plugs. I try to get the sweet spot between black smoke and lean cough under heavy acceleration but that’s a pretty wide range. Any tips will be appreciated.

[T. Cotten]

Well, Otis,...

My brief affair in the '70s with Mikunis was love-hate, but I still take to heart the manual: Tune for performance, nothing else.

That's easiest for the idle circuit, but of course it will take an extended trial to tickle the high speed to its sweet spot, and then repeats of both since the circuits overlap.

Anything arbitrary confuses things, and inevitably sucks gas.

...Cotten

[dukekleman]

Otis,

If measure mpg at cruising speeds of say 50-65 mph, the most influential circuit is the low speed.
You are on the right track leaning the low speed needle 5 clicks. Leaning down the low speed needle should change your idle speed. It should increase your idle speed.
So next lower your idle speed and this will obviously close the throttle disc more.
It will cut off the air flow and expose a different ratio at the transitional circuit. This should help your mpg.
Hopefully no one has modified those transitional holes over the years. If they have, your mileage will always be poor.

Lastly make sure that your those shaft and bushings a "tight" if you will. Air that travels past the shaft unregulated required the tuner to compensate for the added air leak with fuel on the low speed circuit. The ruins for mpg as well.
Remember a properly tuned carbureted engine should always require a warmup enrichment period by the operator. EFI does this electronically...
If your engine feels good to go quickly after start up, I can guarantee you it's way to rich at operating temperature.

Hope this helps,

Duke Kleman

[DEEBS47Chief1991]

I have this Colortune tool and it works great. Takes out the guess work.

https://www.gunson.co.uk/techtorque/...ustion-chamber

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[DEEBS47Chief1991]

I always start with static timing the engine and then use a self powered timing light (on 6 volt system). It's much more accurate than static timing alone. Lots of slack in those timing gears. Paint your timing mark orange and use a clear timing plug.

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[T. Cotten]

I found my Colortune hard to see going down the road, Deebs!

But I sold several.

We haven't mentioned manifold flange warpage or throttledisc borewear.

....Cotten

[T. Cotten]

Otis,

If measure mpg at cruising speeds of say 50-65 mph, the most influential circuit is the low speed.
You are on the right track leaning the low speed needle 5 clicks. Leaning down the low speed needle should change your idle speed. It should increase your idle speed.
So next lower your idle speed and this will obviously close the throttle disc more.
It will cut off the air flow and expose a different ratio at the transitional circuit. This should help your mpg.
Hopefully no one has modified those transitional holes over the years. If they have, your mileage will always be poor.

Lastly make sure that your those shaft and bushings a "tight" if you will. Air that travels past the shaft unregulated required the tuner to compensate for the added air leak with fuel on the low speed circuit. The ruins for mpg as well.
Remember a properly tuned carbureted engine should always require a warmup enrichment period by the operator. EFI does this electronically...
If your engine feels good to go quickly after start up, I can guarantee you it's way to rich at operating temperature.

Hope this helps,

Duke Kleman

You realize the idle and power circuits overlap, Duke?

An arbitrary lean setting on the low speed starves air to the high speed circuit.

....Cotten
PS: We could do a lengthy thread about throttleshaft bushing replacement.

[otis71]

Throttle shaft bushings are within spec

[dukekleman]

You realize the idle and power circuits overlap, Duke?

An arbitrary lean setting on the low speed starves air to the high speed circuit.

....Cotten
PS: We could do a lengthy thread about throttleshaft bushing replacement.

Cotton,

Yes, of course the idle (low speed) , transitional and power (high speed ) circuits overlap.
What some people forget or don't understand is the moving Venturi.

[T. Cotten]

Cotton,

Yes, of course the idle (low speed) , transitional and power (high speed ) circuits overlap.
What some people forget or don't understand is the moving Venturi.

If the venturi is moving, Duke,..

The main nozzle must be missing!

Please explain this again, as it ignores the overlap entirely:

You are on the right track leaning the low speed needle 5 clicks. Leaning down the low speed needle should change your idle speed. It should increase your idle speed.
So next lower your idle speed and this will obviously close the throttle disc more.
It will cut off the air flow and expose a different ratio at the transitional circuit. This should help your mpg.

Let us not forget that the idle circuit meters fuel at slow motor speeds, but reverses to meter air to the power circuit once sufficient velocity is achieved.

Arbitrarily changing one changes the other.

.....Cotten

[dukekleman]

Cotton,

With proper engine vacuum on the engine side of the throttle disc and close to atmosphere pressure on the Air box side the throttle disc, the throttle disc's position dictates the Venturi affect.
It moves the Venturi, if you will, from the idle passage hole, to the transitional holes/slots and onto the fixed Venturi you are referring to.
Picture the Venturi affect moving along the side wall of the carburetor back and fourth with the throttle shaft position turning on fuel circuits based on it angle.
And of course they overlap...any circuit exposed to the engine side is on, they don't magically shot off once exposed to the engine side.
Probably not the greatest explanation at the moment but hopefully gives a picture and helps.

Duke

[T. Cotten]

I think you mean Bournoulli effect, Duke!

The throttledisc directs airflow into the front of the bleeds for mixing, and motor vacuum draws it out the rear bleed hole, but only at speeds below 35 mph or so.

When the disc is opened past the bleeds, the true venturi kicks in, and only air comes out of the bleeds;
A lean setting on the lowspeed produces a richer mixture on the highspeed, and vice-versa.

One must optimize the lowspeed, then optimize the highspeed, which will then require the lowspeed setting to be refined again, etc. When both circuits are optimized for performance, the fuel is used most economically.

....Cotten

[dukekleman]

Cotton,

The Venturi effect is a direct relationship to Bernoulli's principal.

It's not necessary a speed (mph) , mph is derived from blade angle, load and gear selection.
The Venturi effect is directly connected to a Throttle position (throttle blade angle)

Duke