Scott Squirrel – spitting back

[Deleted User]

Sound theory Bob – but isnt the exhaust gas forcibly propelled in the direction of the exhaust port anyway by virtue of the transfer charge being directed upward into the combustion chamber space by the deflector design of the piston?

If my Scott was spitting back (putting thoughts of the nearest canal aside) I think I’d begin with the philosophy of “The Wise Man of Hemel Hempstead” and start by squirting an oily petrol mixture around all the gasket joints whilst the engine was running.

[Deleted User]

All fascinating stuff, I am just to go on holiday and will ponder the sage advice give !

In terms of who did the mag, it was Vintage Magnetos in Bristol, the guy who rebuilds them is an ex Lucas employees, been using him for many many years and have never had a problem with any magneto he has done for me (he is super fast & very reasonable too !), the mags are always thoroughly tested on a ex Lucas test rig for an hour before he passes them on.

[BRIAN MARSHALL]

And piston lengths vary too especially on short-stroke engines, with crowns stamped ‘T’ on early 1927/8 Tourers and ‘R’ on all later engines.
Brian

[Deleted User]

Hi Stan ,
The burnt gases under extreme pressure will get out where they can at the first opportunity and the gas coming up the transfer passage is a much lower pressure and have no effect. They just follow the exhaust gases and the deflector just directs them upwards rather than go straight across the piston and out the exhaust port.
Regards,
Bob Mather.

[Deleted User]

Hi,
More thoughts,
Air leaks around the transfer port covers will cause spitting back.
matchlessg80cs finds retarding the ignition causes the engine to spit back. The flame in the cylinder head spreads rapidly when the plug fires. The area above the piston on the transfer side of the deflector is probably the last bit to burn. Retarded ignition makes the flame later and can cause spit back with a too short exhaust lead. Over advance might stop spitting back but is not good for the engine and can also make it run in reverse. Gauzes will rob the engine of power. If I had this problem, I would be tempted to remove the cylinder block, file 1mm (no more) off the top edge of the exhaust ports making sure they were parallel with the top of the piston and put a slight radius on the vertical edges of the exhaust ports of about 1mm radius to help the gases out. This will give about 2.5 degrees increase to exhaust lead.
Regards,
Bob Mather.

[Deleted User]

Hi Bob,

Exhaust gases are expelled by the Kadenecy effect.

[Deleted User]

Hi,
Afraid not Stan, the Kadency effect is not used to pull out exhaust gases, they need no assistance. It was used on early gas and diesel engine for automatic induction. As the exhaust gases rushed out of the exhaust stub via rapidly opening large valve or ports, it was fed into a large chamber via a stepped pipe. This caused a negative pressure in the cylinder which drew in the next fresh charge.
Regards,
Bob Mather.

[Deleted User]

Hi Bob,

I think you are refering to the specific engine patented by Micheal Kadenacy in 1933, having “discovered” the effect exhaust gasses can have upon engine efficiency etc. – whereas in my earlier post I was referring to the more global application of his findings.

You can read a synopsis of his work if you Google;

https://en.wikipedia.org/wiki/Kadenacy_effect

[Deleted User]

Hi,
I recently posted my thoughts on short stroke Scott engine spitting back and how I would fix it. There have been extreme examples in the past of blowing out a crankcase door.
Today, I was looking though a 1965 copy of The High Speed Two Stroke Petrol Engine by Philip H Smith AMI Mech E, a well known engine development engineer of two and four stroke engines and lo and behold he knew the spitting back problem was caused by too short an exhaust lead over 50 years ago, so I came to the same conclusion rather late in the day. At least I got it right. The book also confirmed all my statements on exhaust gases escaping.
Regards,
Bob Mather.

[BRIAN MARSHALL]

And let’s not forget that Phil Smith was a big Scott enthusiast !

[Deleted User]

😀 😀 😀 Re the above.. Here’s an interesting bit of gen.. I was talking to an SOC/VMCC member from Ulster today ref this problem and he casually mentioned that the gauze’s act like the ones on the old type Davy miners lamps.. IE not letting the flames through (therefore igniting the incoming charge) Now I find this intriguing as in an earlier life I worked the miners lamp room at Ollerton Colliery. We live and Learn….!! 😀 😀 😀 Ted

[Deleted User]

Just checked no gauze’s ever fitted, I don’t suppose there are any members in the Bristol area I could hook up with who could help ? Cheers Al tel: 07712678905

[Deleted User]

Presumably the gauzes work in the same way as the flame-trap “air filters” commonly seen on big singles.

[Deleted User]

😀 😀 I have some gauze which you could cut to try.. it works on my 2 speeder.. Regards Ted 😀 😀

[Deleted User]

Many thanks Ted, what is the specification for the gauze ? Thank you for your offer, I can probably buy some locally which would be easier.
thanks again
Al

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