Help ! Why is my Scott running so badly ?

[The Shipley Squirrel]

Dear all,
I was very pleased to finally get my 1949 Scott flying squirrel running the other day after a long renovation. But its running rather badly !

It will not run at all unless on full choke and I am getting a lot of oil leaking out between the exhaust manifold and the gasket to the extent it is running down the front of crankcases, as well as a lot of smoke from the exhaust !

I have increased the main jet size from the standard 170 up to a 200 as was suggested by Brian Marshall to someone in a much earlier post and this has helped a little bit with the running but it still needs the choke on. Should I go up to an even bigger main jet ? I have got a petroil mix of 50:1 in the tank.

Also, have I got too much oil going in through the pilgrim pump ? I have tried to cut this down to approx 8-10 drops a minute but its proving tricky even though I have one of Gerry Howards reduction gears fitted, I am worried about cutting it down too much and it not getting enough oil, but perhaps I am being overly cautious ?
Any advice would be much appreciated.

Mark

[Jeff Reed]

Hello, I don’t know if you engine is a 500 or 600 or what oil you are using but for the record all my 600 engines are run on 200 main jet and I use a low ash 2 stroke oil at 5 to 6 drips per minute plus 50 to 1 in petrol. Suggest you cut down the oil and sort out the leak in the exhaust before trying anything else. Best wishes, Jeff.

[BRIAN MARSHALL]

Hi. I get the feeling that the oil in your petroil mix may not have been properly mixed, and you are getting a VERY oily mix at the moment, too viscous to flow properly through the carb, necessitating excessive use of the choke to get enough petrol in for the engine to run, but this means that even more oil is being drawn in ! Initially I would take off the exhaust pipe and check that the flange on the pipe is dead flat. If a little distorted it can be remedied by holding the pipe by hand onto the belt of a linisher. When you know it is dead flat refit it to the engine using a new gasket with a thin smear of exhaust cement on both sides of the gasket.
Drain the tank and the carburettor, and refill with ethanol-free petrol such as BP Ultimate. Don’t bother with oil in the petrol. If your oil pump is in good condition it should be very easy to adjust when used with the speed reducer.
Try again, and let us know how you get on !
Brian

[The Shipley Squirrel]

Thank you both for the replies.

The engine is a 600 and is late Shipley model with distributor ignition. I’m using a 200 main jet at the moment and the oil i’m using in both the oil tank
and the premix is Millers classic 2-stroke oil which they say is specially formulated for classic 2-srtoke car and motorcycle engines.
The premix in the petrol tank has been thoroughly mixed at 50:1 ratio. I am using BP ultimate petrol Brian as you suggested.

I took it for a short run up the road this afternoon and it did improve a little but it still needed choke to run and was popping and banging and generally being rather erratic.
I have checked all the inlet manifold and the tranfer port covers for air leaks but can’t see any, – all the gaskets are new and have been carefully fitted and all the ‘mating surfaces’ have been checked for flatness. I will check again however when I get a chance. Could the air slide or the needle need changing ? It currently has the standard 6/3 air slide and a No: 6 needle which is on its middle notch.

I am hoping to get this bike sorted in time to bring it to the Scott weekend at Shuttleworth which is not far away !

Thanks for your help.

Mark

[Bob Mather]

Hi Mark,
Your problem is not caused by the main jet, it doesn’t run on the main jet below 3/4 throttle. The problem could well be air leaks in the transfer covers, carburettor gasket and crankcase doors. I don’t know how you checked for them, try squirting WD40 on the joints. If the revs rise while doing this it indicates the engine has sucked in WD40 via a leak. Also try removing the crankcase drain plugs in case there is an excess of oil in the crankcases then refit them.
Regards,
Bob Mather.

[Tman]

Personally I’d do the things as suggested above, and also keep/return all the carb bits to standard (changing things willy-nilly just introduces more unknowns). A new tank of straight petrol, new plugs, charged battery, cleaned and gapped points, recheck timing and try again.
You know the old truism…”75% of carb problems are down to the electrics….”

[Adriangv]

It’s just a thought but have you checked your centrifugal timing control? Are the bob weights piviting on their pins are the springs in good or good enough condition?
Hiding as they do under the points they some times get overlooked; which reminds me, I really should check on mine.
Adrian
Brum Scott

[chris wastell]

What is the condition of the carb bore and slide? I had mine rebored by Bill Harley and the difference was astonishing. Standard jets and needles etc… Good luck anyway

[The Shipley Squirrel]

Thank you all for the replies,

I’ve decided I am going to sort the leak around the exhaust manifold and then check again for any air leaks around
the carb manifold crankcase doors and transfer ports, as I think this is the most likely source of the problem.
I’m confident the electrics are OK, they have all been renewed and checked and checked again, distributor bobweights, points, condenser, timing, everything !
Carburettor seems in good condition too, so I’m going to start by looking for air leaks when I get a chance.

I’ll let you all know how get on !

Many thanks, Mark

[BRIAN MARSHALL]

As the bike is 1949, I assume that it has a separate oil tank ? The reason I ask that is that the symptoms described are entirely typical of a bike that has the oil in the same tank as the petrol, with an internal leak between the two compartments….. Just a thought !
Brian

[The Shipley Squirrel]

Yes, it has a seperate oil tank Brian, I am going to remove the exhaust and check the manifold for flatness tonight and get that
sealing properly.

Mark.

[Ted Robinson]

I always use the good old green Hermetite on my 1949 and never have any problems I think Richard Blackburn stocks it. Ted

[Graham Parker]

Have to agree with Bob Mathern. Main jet seems a re herring as it is only relevant after 3/4 throttle.

Also agree about getting electrics/timing right and looking for leaks.

But after electrics and leaks are confirmed ok I’d forget main jet size and start with slide sizes (6/3 is a good place to start) but I’d also be looking at needle jet size and needle position combinations. Needle jets are pretty cheap so a range of 105 / 106 / 107 is reasonably cheap to get. Also make sure the needle itself is in good nick.

Good luck

[Author]

Posts