Main jet

[Jan Buchwald]

What size main jet is recomended in a Birmingham scott with 6/151 carburator. At the moment it is size 180.
The engine has sized a couple of times after running a couple of miles at 70 mph.
Too weak a mixture could be the problem as it backfired.
best wishes
Jan B

[Stan Thomas]

The standard main jet is a 170 – but that is probably not your problem!!!!

The main jet only affects mixtrute stength above 3/4 throttle, and at 70mph, you may not be running on the main jet. I suggest you raise the needle first.

Also, check the timing, and make sure the advance/retard is working. Time the engine with a strobe to about 30 degrees advance at 3000 rev/min.

Does the machine seem “overheated” when it seizes?

However, check the oil delivery first, as that may be where your problem lies.

Let me know how you get on.

Stan.

[Jan Buchwald]

The timing is set to 35 degrees at 3000 rpm
It does not seem to be running hot.
It frees instantly.
It seem to be able to run at 50 mph for hours
I have been running on modern 2-stroke oil for outboard engines, as normal 2-sroke oil resulted in oil flowing out of the exhaust. (Have changed to normal 2-stroke oil now)
What made me think of the jet, was the backfiring.
Yours
Jan B

[Derek Swetnam]

Dear Jan….Sounds exactly like mine (1960 Special). This gives me no pleasure at all as I haven’t a clue what to do. Stan’s advice is always valuable. Please let me know if even the faintest whiff of a solution can be tried. Did I understand you to say that you put 2 stroke oil in the oil tank?
Derek Swetnam “The last chance saloon.”

[Jan Buchwald]

Yes, but I am in no position to recommend at the moment.
The former experiment (with two-stroke oil for outboard engines) resulted in broken rings and a damaged piston (that could be the reason)
I have now changed to ordenary 2-stroke oil, but only for about 10 miles.
The plugs was a little “greasy” when I looked.
Jan N

[Roger Moss]

Stans advice is sound but if you are still in trouble, go back to basics.
take the jet access nut off the bottom of the carb and measure how much fuel you are getting through per minute.
I would hope that you are getting at least 350ml / min
If not check what you are getting from the tap
I would hope that you are getting at least 700ml / min
If not, drain tank and take out tap.
Check if it has a filter that it is clean and check that there are no pieces of debris obstructing the tap.
For my engines I need two taps delivering 1000ml / min each and if the float chamber is OK I should get about half that to the carb bottom banjo.
ie 1000ml / min
Is is not uncommon for the carb to be starved by poor feed from rusty tanks etc.
Do check that the pipes and float chamber are clean and unobstructed.
If the bike has been “tuned” then you might have to knock the timing back between 5 and 7 degrees.
I do sympathise as after a good few years, you cannot always see immediatly if something has been altered.
My philosophy if I have problems, is always to start at the beginning, hence fuel flow. If that is given a clean bill of health, we can look further, such as is it always one side that seizes, are both plugs identical in colour, if not, their could be an air leak.
All good character building stuff!
Kind Regards
Roger Moss

[Roger Moss]

PS
Just for safety’s sake, I suggest you check that the air hole in the tank filler cap has not become blocked — it has been known!

[Rod Norris]

Hi Jan,
I’m no expert on the Scott engine unlike previous contributors. I have a number of old bikes and a 49 Flyer is just one. I have followed your posts over a period of time and you seem pretty knowledgable on Scott’s. I would however just add a couple of points that can easily be over looked.
Is water flowing through the radiator satisfactorily, ie is it sufficient at higher speeds. The general mistake with 2 strokes is to add more oil to the petrol when seizures take place. This can be counter productive as it weakens the petrol / air mixture as a proportion of the petrol is taken up by the oil. For this reason Roger’s suggestion of altering the needle height seems a good starting point. Best of luck & let us know of your progress.

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