Saving Old Seagulls

1942 102: The spark plug is not getting wet. Checked the usual ie no blockages clean fuel etc etc. The problem I think is more fundamental. Using the drill on the top nut mothod of starting the following occurs............Nothing, then smoke from partial ignition, then the odd flame from the exhaust outlet (Underwater and rather spectacular) at the gearbox with backfire and water shot above head hight. The timing is correct and it sparks well! Water from the outlet is either very good or non existent and smoke also comes out on occasion. While tinkering I squirted oil down the fuel inlet hole while the motor was turning and eventualy the oil spurted out of the plug hole. I put the carb back on inserted plug and BINGO!! it ran beautifully for about 10 seconds untill all the excess oil had gone.I kept trying with the drill but nothig, Took the plug out and it was dry again. Put the carb on another 102 and it's fine, put the carb off the working 102 same symptons! The question I am pondering is what on earth is going on, why will it suck no fuel in?

How do you know the timing is correct? where did you set it?

It doesn't sound like it's in the right place,
it should be around 20 degrees before BTDC, not 50 degrees after.

What have you set the points at?

Remove carb, just by loosening the pinch bolt, & put your thumb over the inlet stub, can you feel any suction?

How much cylinder compression do you have?

Hello Charles, the baseplate locking screw is secure in the dimple, points 15 thou plug 20, compression feels higher than my other 102's it is very good. The inlet suction feels poor BUT, was greatly improved when I filled the engine with straight 40's engine oil. The mix is a good 10 to 1 and have tried it with an even higher ratio of oil to petrol than that. Could air be sucked into the crankcase from anywhere else other than through the carb thus reducing the inlet vacuum pressure?.........Thanks!

Now go & check to see if your spark plug lead on the baseplate is pointing in the same direction as your other 102's in relation to the crankcase.

Hello Charles, yes they are all the same at about 5 o' clock' ish if viewed directly from above with spark plug facing away from you. Also checked the timing physically and the points are just opening at 20 degrees B.T.D.C. Good thinking though and well worth asking the question! Thankyou!

What condition are your main (crank) bearings in , sounds like loss of crankcase compression , they are a VITAL part of a healthy twostroke.

Andy.

Twostrokenut: The answer is I don't know! I fully concur with your reasoning though, putting engine oil down the inlet manifold improved things greatly, which me think about the crankcase pressure getting lost and the thick oil sealing it and getting it to run for a short time. At the moment I am trying to make a silk purse out of two sow's ears I got from guess where? This means a strip down of this 102 will have to wait a few weeks. If the bearings are shot will John have any I wonder? Thanks for taking the time to reply.

Not sure if John would have any , it's a fairly painstaking job fitting new main bushes , I've never done it on a Seagull , but have on other twostrokes.Obviously a quality in-line reamer is required , coupled with infinite patience , but it can be done , and the end result (provided everthing else is OK) is astonishing.

Andy.

If you look at your Seagull logically you will see how unlikely it's loss of crankcase compression that's causing your problem.

The gas pressure starts to rise somewhere around the point that the inlet port is half closed, about 30 degrees after TDC, at this point there should be, if everything is working properly, 1 atmosphere inside the crankcase exactly the same as the preassure outside the crankcase, there might be a little more when the inlet port closes 60 degrees after TDC, but only at full revs, the crankcase then builds up pressure until the transfer port is about 5 degrees open, 125 degrees after TDC, at this point the crankcase gas is being pumped into the cylinder through the transfer ports, so no preassure rise after this point.

The maximum theoretical crankcase compression ratio of a 102 is around 1.2 : 1, so .2 of an atmosphere is the maximum pressure rise, 2.8 lbs, the pressure will only be of this area of high from the time the transfer port opens, until it is about half open, somewhere around 30 degrees of rotation or 1/12 of a revolution.
At 3500 revs this pressure peak would last 0.001428 of a second, not a lot of time to lose a lot of gas, so a hole big enough to lose enough pressure to cause these problems would have to be almost big enough to slide an 1/8" drill through.

I think you might notice that, none of the above figures are accurate to your engine, they are only guesstimates of average Seagull port timings.

To calculate the exact figures for your motor I would need all the port timing figures accurate to a degree, the exact crankcase volume at TDC & the conrod length to within .01 of a mm & an hour of spare time, hence the guesstimate.

SD main bearing bushes are no longer available, new later ones have to be modified to suit.

If it's not the ignition timing, then your going to have to strip this motor to see what's happening.

Thankyou Twostrokenut and thankyou once again Charles! Well, both suggestions include a strip down so that is what I will do. Hopefully my bits and pieces from John will arrive tomorrow to put another 102 engine back together, so when that is done I will start again. At least it is an easy job as it comes apart as though it was put together last week, I had the leg and exhaust off a couple of days ago and I don't think it has ever ran or been in water. The square driveshaft is still shiny and shows no swelling at the ends and no sign whatsoever of carbon in the exhaust tube or in the gearbox chamber! A pal said it must have ran once upon a time because it would have been tested in the factory, but would they have done that in wartime? Anyway, thanks again for your help!!

Before you strip this Seagull, are you sure it's an SD? post a picture if you can,
The base plate position on an SD, with the spark plug at 12 should be between 3.30 & 4.
Give it a try at 3.30.
We need a pic to ensure we're singing from the same hymm sheet.

Hello Charles
It is SD 1581. It is the British G.D.P. motor we discussed on the phone. I will adjust the timing as you suggest before a strip down, Thanks.

What's a G.D.P. motor?

I have to say I share the other Charles' thoughts about timing...

CharlesP: Thankyou for your thoughts. It is a war (1942) engine in khaki with a tank decal that says British G.D.P. Nobody seems to know what it means. I think John wants to post pictures on his site. Allow me to get this timing issue correct then. With the flywheel rotating clockwise the points should just be opening at 20 degrees B.T.D.C.? What whould that equate to about` three eighths if I used a drop guage in the plug hole? Thanks again folks!