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Slow running Whippet

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AJW16/03/2019 20:12:51
288 forum posts
117 photos

I'm about to start machining castings I bought a while ago for Westbury's Whippet Engine.

Like my last project it will not be required to work for its living but to be run up occasionally for personal enjoyment!

I like my engines to be able to run slow and was wondering is this purely dictated by flywheel rotational mass?

If this is the case then I shall be considering some more mass for the Whippet which in my case shall not be powering a boat where gyroscopic effects may need to be considered.


JasonB16/03/2019 20:25:34
17856 forum posts
1954 photos
1 articles

If you make the ignition contacts bracket so that it is easier to adjust while the engine is running you can also advance and retard the timing which will also affect the speed at which the engine runs. This can often be done by replacing the short clamping screw with a longer one which will act as a lever for adjusting the position of the bracket.

Although not a 4-stroke take a look at this video from about 1min in where Nick is using the long lever to alter the timing and only making small adjustments to the carb but it has a marked effect on speed and if taken to extream allows the engine to run the opposite way.

Mick Henshall16/03/2019 20:44:30
531 forum posts
31 photos

There's got to be a joke here somewhere 🐕

Mick 🇬🇧

AJW16/03/2019 21:17:06
288 forum posts
117 photos

Thanks for the video.

That's some slow running! Huge variation in timing although I didn't sense any pre ignition and knocking.

I found with my Centaur a wide variation in revs could be achieved by timing variation although I also noted that it wasn't as happy with a large advance.

I would definitely make the ignition adjustable while running.


Hopper17/03/2019 05:18:46
4411 forum posts
94 photos

Not just flyhweel mass, but flywheel design. The larger diameter flywheel with the weight mostly around the outer rim as found on "mill engines" etc will tend to run smoother than the small diameter marine flywheels of the same weight. Remember too that in a boat, the prop shaft and prop add to the flywheel effect. So good idea to fit heavier/larger flywheel for stationary engine application.

Lower compression also helps slow speed running but I think with the Whippet's side valve design it would already be pretty low compression, probably about 5 to 1. Most fullsize sidevalve engines are quite happy at 5:1 compression, will run at quite a bit less, and suffer breathing problems over about 6.5:1 due to restricted combustion chamber space.

My old 1942 Harley has a sidevalve engine and manual spark advance/retard control. Idles down real slow with fully retarded spark. (Makes good backfires too when running downhill on a closed throttle. devil )

Howard Lewis17/03/2019 09:33:44
3149 forum posts
2 photos

The ability to slow run will also be affected by carburation. If the carb does not supply the correct mixture, you will never get a good stable low idle.

Mixture strength and ignition timing are inter related.

Carburetor designers have gone to a lot of trouble, on full size engines to ensure that mixture strength is maintained pretty closely, over the operating range. (Emulsion tubes, air bleeds, capacity wells etc and many hours of test bed time to obtain the correct jet and ignition settings ) This is a lot harder to do with a shop made device, which is going be fairly simple in comparison.

Also, induction tract temperature will have an effect. A good stable idle, after a long heavy load run, may change as the tract cools, and vapourisation deteriorates. The carburetor meters the fuel, much of it vapourises en route to the cylinder. If the carburetor is not supplied with enough heat, ice will form. (The latent heat of vapourisation will come from the carburetor rather than a hot spot or water jacket, and reduce the temperature, to below the dew point and then below freezing).

Sorry to complicate matters!


Hopper17/03/2019 09:42:48
4411 forum posts
94 photos

Yes there is always that. Luckily on a sidevalve engine the intake tract is handily warmed up by the adjacent exhaust port to some extent. But a well tuned carbie would certainly be of the essence for slow running.

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