1936 Model 18 Compression Test

I'd say that was fine for a 6(ish): 1 compression ratio.I would expect to see a multiple of atmospheric (say 15 psi) and the compression ratio so I wouldn't expect it to even be that high.

I must say I've never tried a compression test on a kickstart bike (or actually, anything else) as I don't think I could kick it hard enough for long enough.....

According to Bacon the CR for that year is 6.2 so at a simplistic level (ignoring losses and the fact that it was a cold engine - presumably) you would get 14.7 x 6.2 = 91.14. Makes 110 psi (gauge - I assume) very reasonable. You will have to look elsewhere for the lack of performance!

George

The compression test is 'adiabatic'. That is - effectively no heat loss owing to the short test duration. So the hot gas inside tries to expand, so you get significantly more than 6 to 1 pressure increase. So that's why yours is over 90 (6x15) psi. Anyway it looks OK.

If it used to run OK, I'd suspect something simple- like partly blocked main jet or slide not fully opening. If it's never run properly, then ignition timing might be too retarded. With manual lever you can always set it a bit more advanced than the book, and retard it a little if you need to. I have a paint mark on the book figure. Although it's only a 16H, it goes a lot faster than 45mph. Different books have different values, so I pick the most advanced. Our fuel octane rating is much higher than in 1938.

If you are getting 45mph with the throttle wide open it could be a problem with the main jet or possibly the needle position, or a fuel blockage?

George

Out of interest,Steve, I found the following: Carb settings for 16H 1933/37: Type, 76; Size, 1" ; main jet, 160; Slide, 6/4; Needle position, 3. Hope this helps a bit.

George

For top speed, most of the carb settings are irrelevant. Just the main jet (and the needle correctly clipped). Is there a blocked gauze filter in the tank? Or has the jet come unscrewed? Is the carb slide going all the way up? The air slide must be clear of the throttle passage or your plug would be black.

It's not likely to be the type of plug. The wrong one might foul up or burn, but that doesn't seem to be your problem. The plug itself might just be faulty but seems unlikely if it starts easily and does not misfire. Have you checked the Green Spark Plug site?

If it starts easily, it's not likely to be the magneto either, unless it does not re-start when hot. But the ignition timing might have slipped. That happens...

I find earplugs make my bikes go much faster.

Dare I mention the fuel cap vent. Give it a poke.

Hi Steve,

The easiest test of the engine compression is to stand on the kick-start lever, this should hold your body weight for a couple of seconds before the gas escapes. Are you using your advance/retard lever correctly? This should be a tight wire advance on your bike, there was a thread on this last month. The other options for the poor performance could be slipped timing as David pointed out or valve timing slightly out. You don't have to strip the engine to check this, just take off the primary chain cover and main-shaft nut and fit a degree disc, get TDC through plug hole and check ignition and valve timing. Your bike should do at least 75mph in top (4th) gear with a 20 or 21 tooth engine sprocket.

Regards, Richard.

Previously steve_burden wrote:

Hi my bike has a slack wire for advance although it is the correct magneto and I have read it should be tight slack. The advance retard is working as it should beacuse it speeds up in the correct direction and was timed with this in mind. I will check the timing again. Should I advance slightly more then what was given when new to take into account modern fuel?

Hi again Steve,

The ignition timing will be a bit trial and error depending on which petrol you use as ethanol and benzene were used pre-war before tetraethyl lead became the norm. I would try the standard 45 degrees and see how it goes, theengine should run at a fast tick-over fully advanced or fully retarded, so if it cuts out either way should give you an idea which way to move it.

Regards, Richard.

Previously richard_cornish wrote:

Hi Steve,

The easiest test of the engine compression is to stand on the kick-start lever, this should hold your body weight for a couple of seconds before the gas escapes. Are you using your advance/retard lever correctly? This should be a tight wire advance on your bike, there was a thread on this last month. The other options for the poor performance could be slipped timing as David pointed out or valve timing slightly out. You don't have to strip the engine to check this, just take off the primary chain cover and main-shaft nut and fit a degree disc, get TDC through plug hole and check ignition and valve timing. Your bike should do at least 75mph in top (4th) gear with a 20 or 21 tooth engine sprocket.

Regards, Richard.

Steve - you might already know - a quick check of timing is simply to see if the mag contacts open on full retard close to TDC. But if it 'pulls like a train from the off', it doesn't sound like timing can be far out. Didn't you just say you found the tank cap vent hole blocked? And is the main jet clear and correct size?

As Richard says - slack wire advance is non-standard - I wonder why, and are you sure someone changed it? The racers liked it so they didn't necessarily lose if the wire snapped...

With these old long stroke engines, piston drop is as accurate as you need to be to measure advance angle, and it's a lot quicker and simpler to do.

I think it wasn't only racers who liked it. Tight wire advance can easily slowly retard as you ride along unless you have the friction screw very tight......