Finally the thing is making it out onto the road —
Bearing in mind that I am supposed to be running in this motor, and horror of horrors I have never ridden a Scot before, what should it sound like!
Now before you all collapse in gales of laughter, the sound I query is what I call a set throttle, in so much that the speed to be attained has sort of been reached, then it sounds like it is misfiring.
Any added load either from gear change or wrist twitch cures it, is this normal ???????
Told you this is a daft post —-
This sounds like the engine is what is called “fourstroking” and is caused when the carburettor mixture is too rich at that particular throttle setting. This is how a two stroke’s mixture is adjusted. If the engine “fourstrokes” at any particular throttle opening it needs to be weakened gradually at this point whether it be cut away or needle etc. until it starts to “twostroke” smoothly. You soon get the feel of it and know when the engine is happy. If you suspect it is weak at a certain throttle opening, you can richen it at that point then gradually weaken it off until it runs smoothly without missing.The fuel level in the float chamber also has an effect here. A two stroke doesn’t need extra fuel to keep cool, it is the correct mixture or nothing.
This was not a silly question!
Thanks Bob, I was wondering about that.
Just to clarify, the sound it makes is heard on this rather nice video of Matt Tschanz’s Power Plus in the Berner Oberland (I know those roads well).
Most clearly from 1.28 to 1.40 minutes in – before he winds it on again.
His mixture is far too rich and he should set the throttle to shut right off and not use the throttle stop.
I would caution about trying to “dial out” the four stroking if the bike is otherwise running well. Its always safer having a two stroke set slightly richer than lean. I seized my Cyclemaster last year after extended running on a part throttle (and yes it was still four stroking). Even “modern” 2 strokes like my RG500 four stroke like a “good un” on a part throttle.
My Scott certainly four strokes quite a lot and it is too lean if anything!! As said, if it bothers you then just lower the tickover adjuster right out 😀
I will bear that in mind, the engine had only been run in the stable prior to the first tentative road trip after a total mechanical rebuild including all chassis components. I had left the paint, I like ’em looking their age.
Anyway, there was a surfeit of oil in the wells I think, and a destroyer could have escaped through the smoke.
Plugs were black and very oily, however after a 9 mile trip, they had burnt back to a dark brown insulator, bright thread face and relatively oil free. Smoke was acceptable.
The problem with a new build however, is that giving it large is not conducive to longevity, so following Amal’s advise to set WOT first is not an option.
I have twin Concentrics, Moss head, pistons and barrel, so normal ballpark settings are out.
What I would say however, that considering this is a rigid frame with girders, it is amazingly stable and comfortable! 😯
If your Scott is four stroking it is not running well, it is too rich or full of oil. My Scott runs clean as a whistle right through the range with no four stroking, does not overheat even in some of the stinking hot weather we get here in Australia. On one exceptionally hot day on a vintage club run, many of the four strokes were calling it a day, but it didn’t bother my Scott, and I don’t run excessive oil with only a bare trace of smoke. It has never seized or tightened up in the 20 years I have been riding it and it doesn’t get an easy life.
There is more to this than meets the eye. If an engine is four stroking and the ignition too retarded it can still overheat and seize.
I see your Scott is not standard with Moss head, pistons, barrel plus twin carbs. All this can well alter the ignition timing required probably it is now too advanced on standard setting. My engine has had many internal alterations plus an effective resonant exhaust system. It is a 1932 blind head 500. After all this I had to completely retune the carburettor and no way would it run on the standard advance of 35 degs any more. I set to work retarding the ignition bit by bit until I could feel it was running correctly and eventually got down to 21 degs full advance. You will not get down to this low figure but will probably feel an improvement as you retard it slightly. If you go too far it will go dead and get hot.You will need to ride the bike for a period of time to get the feel of the engine before trying this.
Thank you for the considered response, your Scott does sound like a good-un!
I have initially set the timing at 1/4″ BTDC, so about 31.5 deg. At this setting I do get some kickback during starting so a couple of degrees less may help.
I will probably leave things as they are for the moment however, this will allow the engine to bed in and I should have a better feel for the beast by then.
The only slight setback is the tank has started leaking at two points, never a dull moment.