Twingle

[davomoto]

Mark, sounds like it could be a jetting issue, though I have no experience with a "twingle" . Got my bike torn down on Monday, I have 16 / 40 gearing on it. Painted frame and swingarm yesterday, will get the small parts painted today, and hopefully shoot some clear coat this afternoon. I'll post some pics of progress under the topic " customs bust it out "

[flathead]

We’re finally back at the twingle project. It’s only 2 weeks until we get to run practice for the upcoming flat track races. We took the bike up to the local drags for test and tune last Friday night with very disappointing results. The bike started up easily, idled great, ran smooth, and just would not rev up past around 5K. (13.3 and 48.5 MPH in the 1/8th, , a real dog!!!!!!) The plugs looked a little lean so I re-jetted from a 130 to a 140. It seemed to run a bit better, but still 13.3 and 48.5. Saturday I was so frustrated that I started to pull the engine to swap over to the 180 crank. About halfway into the tear down, I removed the valve covers and discovered, for some unknown reason, all 4 of the valves were set with about a foot of clearance (exaggerating here but not much). It kinda buffaloed me in that I had set, checked, and rechecked every detail prior to starting the engine the first time. I don’t remember but I must have loosened all 4 up at least 2 turns, for what I don’t know, and simply forgot to reset to spec. There was absolutely no rattling in the top end, no vibration, or any other indication of running too much clearance. Most amazing was this thing would start first kick, idle like a sewing machine, and take throttle up to around 5k,but no more.

So, I set the valves and put everything back together.

I took it out Sunday afternoon to a remote straight section of county road for a quick test run.

I’ve never been so shocked at the change in performance of an engine.
For a 47-year-old 250, this thing is a Torque Monster. Rev’s fast, smooth, and just guessing, as I don’t have a tach mounted, to about 9k with a 230 lb rider. Can’t wait until next Friday night to get to the track just to see what the ET and MPH improvements are.

Anyway, this is something I thought of as I pondered the previous issue.

When using a 360 degree crank, are we not creating a plus and minus pressure situation in the crankcase with every revolution of the crankshaft? As far as I know there is no venting other than the vent at the cover on the head. This would seem to me to be insufficient to relieve both positive and negative pressure created with both pistons traveling in the same direction at the same time. I don’t know if this really makes much difference at all, but I did notice that my crank and transmission seals were weeping a bit which I suspect could be caused by pressure in the cases. Other than running a filtered vent on the oil filler, does anyone have comments or suggestions?

Thanks,
Mark

[G-Man]

Hi flathead

Lokking good and sounds like you have the performance now. As for venting, the motor is the same config as a Dream engine with 360 crank or a Brit twin. The idea with Brit bikes was to use a timed breather so that you end up with a vacuum in the cases. This helps keep the oil inside when it's running.

There's a compromise to be had here. If you pump the air in and out of the cases on each stroke, that just saps power as you have turned the motor into an air pump. Best to let out the positive pressure above a certain limit and then maintain an average pressure in the cases so that the air just acts as a spring which just expands and contracts as the pistons go up and down.

G

[teazer]

The guys are basically right. As the pistons rise and fall they compress air in both the top and bottom halves of the motor. The top half compression you want, the bottom half less so.

An engine is an air pump at teh best of times, but you have an added issue of 360 degree crank.

I would add a big bore breather to the dipstick hole. Remove the dipstick and have someone machine up a barbed fitting to screw in its place. Us teh largest bore tube you can find (within reason) say 1" and route it to the rear - maybe fit a catch tank under the seat or mold one into the seat hump with a drain and outlet.

Fit a PCV valve in line might help too.

[jensen]

Hi Teazer,

Could that be the reason of the strange breather on the early engine ?
I never thought about it, but it makes sense, now find another which verify it,

Jensen

[G-Man]

Flathead

What I meant to say was that you try and average a vacuum in the crankcase and that helps keep the oil inside. Although everyone has one, I think that big breathers use up some of your useful power. I.e. You're trying to push and pull 250ccs of air up and down that tube.

Big diesels often have an auxualliary pump to maintain a vacuum in the crankcases but you can get the same effect by putting a non-return valve in the breather line so that positive pressure escapes up the breather but once created, the vacuum stays inside. You'll have a little more power to drive the wheels then.....

Piston ring blow-by will tend to pressurize the crankcase, but as this builds up it will escape leaving that helpful vacuum in the cases. I used to drive an old tractor which would blow it's dipstick clean out if you let it idle for any length of time. The bores were worn and the gas has more chance to get past the rings at idle.

G
G

[teazer]

G-man,
Great idea but not strictly accurate. Many race motors use a vacuum pump to reduce pressure in the crankcases. Crankcases are always at a higher pressure than atmospheric when the engine is running, unless the vacuum pump can create a stronger negative pressure.

The bigger the volume of a breather set up, the less power is wasted. Small tubes act as a restriction which ensures a high positive pressure in the cases - the opposite of what is needed.

A one way valve (PCV valve) never leaves a vacuum but it does help somewhat get a big one on a large bore breather hose.

Diesels are a whole other matter with extremely high compression, they leak more than gas engines and need more help to reduce crankcase pressure.

Check out what Jerry does with his 180 degree race motors. he uses a big bore breather because they work - he knows.