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[ChrisD]

Hi guys. My crankcases are still apart whilst I await the lightened and rebalanced flywheels; so I can do work on the bottom end easily now (1996 535cc Bullet). But since I don’t plan to do any more such work on this bike because I have 2 others waiting (for time and money), I want to get this one right.

I’ve had issues with breathing above ~4000rpm – so every time I go through the gears, oil weeps out around the head. I have a steel conrod, roller bigend, new mains and crankshafts, 32mm Amal Mk2, high lift cams, ported head and plenty more including metal-metal head joint with viton oil rings around the pushrod tubes. The breather at the side of the crankcase has been opened to 8mm diameter and ends in the prescribed duckbill oiling the chain quite well, which is fine up to ~4000 rpm. Yes, I know not to ride at 4000rpm plus, but to accelerate through the gears past that revs is OK.

I recollect seeing an additional breather suggestion entailing drilling into the crankcase from behind the tappet cover stud to release excess pressure from rocker boxes and timing chest. Can anyone remember where this advice was (maybe not even on this bulletin board) and if there was any resolution on its effectiveness?

Cheers, ChrisD

[stinkwheel]

I had thought about fitting a breather stub on the exhaust rocker cover to ensure it didn't overpressurise the timing side. I've seen this done in a couple of poked bonneville motors before today. This has the plus that it's 100% reversible because you can try it with a cheapo Indian rocker cover (I think they can be had for a tenner or so). I'd thought about modifying one of the breather valves (part number: 140167) by sanding a curve on the bottom of it to facilitate fitting a hose then either using a duckbill or other non-return valve (I have a diesel fuel line one-way valve on my 350 bullet breather setup).

[Bullet Whisperer]

Hi Chris, the breather modification you refer to is one I came up with a couple of years ago and it will work for some, but not all crankcase types, unless internal modifications are made. Basically, I drill part way along the tappet cover stud from the inside end to about half way along the hexagon body and then cross drill the hexagon, to provide an exit. Some crankcases have a hole right through to the inside where this stud screws in and are relieved from the outside surface of the cylinder liner, which permits extra breathing from this point, while others seem to need drilling right through and a channel forming downwards along the crankcase mouth to permit breathing. With this extra breather operative, I also put a small breather in one of the rocker covers. The ones I have done to date are obviously not one way, but with a bit of effort and imagination they could be, although the bore is so small, nothing untoward seems to happen and they are just a little bit of extra help for the main breather and can't make any mess externally. If you should decide at some point you don't want the extra breather anymore, just fit a new tappet cover stud and you are back to normal again. Regards, Paul.

[Bullet Whisperer]

Incidentally, 4000 rpm is nothing for even a standard Bullet and with your engine and conrod setup, even 6000 rpm shouldn't worry it.

[ChrisD]

Stinkwheel. That’s a good thought, maybe I need to acquire one of those breather stubs from a jumble sale here (Cape Town) and try that.
Your diesel one-way valve is, I guess, like the one I had also planned to add to assist the duckbill. Does that work OK or just stay fully open at speed?

BW. Thanks for your comments, I shall certainly consider this option since everything is still apart.

Please also put my name down for a set of the “S” cams should you have other sets made.

Cheers, ChrisD

[another Allan]

My Tollgate 625 (1999 engine in a 2003 bike) has always had an oil weep from the head fins (porosity?) When I first bought it, it had the Hitchcock extended breather/oil filler, the exit tube being connected to one of those nasty catch-can things, with a pipe from that to the back of the bike.

I removed the catch can and still had oil weeps. I fitted a duckbill after the Hitchcock breather and still had oil weeps.

Then I fitted a reed valve with the inlet connected to the Hitchcock breather outlet, and the reed valve outlet pipe running to the back of the bike. NO more oil weeps! The reed valve creates a partial vacuum inside the engine when it is running.

In fact, after stopping the engine and pulling off the breather pipe (before the reed valve) there is a 'pop' as air is sucked back into the engine. The reed valve is actually an EGR valve from a Yamaha 125, readily available as a used item on Ebay (here's one 182463562742.)

The same reed valves have been used on Commandos and have the same beneficial effects.
btw, have you tried blowing and sucking on a (clean!) duckbill pipe? You can suck in almost as easily as blowing out. They may work ok on a standard 350 or 500 but my experience with a larger displacement engine is that something more sophisticated is needed.

[another Allan]

Sorry, I've forgotten the code for paragraphs on here.......

[stinkwheel]

The diesel fuel breather certainly works fine at a fast idle. Difficult to tell what it's doing at high speed/load but it definitely flows a lot of gas away from the crankcases. Mine had the odd, very late model breather system with no crankcase stub, a stub in the oil tank and (originally) a return stub from the catch can into the timing chest. I landed up drilling the crankcase and fitting a breather directly to it with a taper thread nipple. Trouble is, I haven't blocked the hole between the oil tank and crankcase so that's still open. I wanted to see what effect leaving the oil tank stub open or closed had so I measured it and videoed it. You will be able to see just how much gas it flows



Long story short, with the diesel non return valve at a fast idle, it certainly and measurably only breathes out and at a fair volume. More subjectively (by holding a hand in front of the hose), there is a steady flow of gas out of the breather with the engine under load and revving harder. Not sure exactly how I'd check that dynamically. Possibly with a small flow meter or by rigging it up to blow bubbles in a jamjar with a video camera on it. I suppose there is absolutely nothing stopping me fitting a duckbill after the non return valve in case it does stay open under load.

[Alan R]

Hi there another Allen}---

OK, so holding down the upper case arrow on the L/H side of the keyboard-----------you then press < followed by B then R then > ......If you want a bigger space then repeat it again without a break.....
It'll show on the screen as those characters but will change to the spacing required when you press}--- "Post Reply" key....

[Adrian]

Alan R, lower case b and r works fine, thus:







You lett your keyboard's hyphen key have a rest and wear out the chevron keys instead, now! )



It's a bit of a pain having to remember to type it after every paragraph, though.



A.