Gregory Bender

2012 October 07: Convert compression

Updated:

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This entry is extracted from a series of email exchanges between myself, Charlie Mullendore of Antietam Classic Cycle and Gordon Kline of MG Cycle. Both Charlie and Gordon were very supportive and helpful.

Hi Gordon,

I got back home from the monthly MGNOC breakfast this morning on the Convert. With the engine fully warmed up (and both throttles wide open), I checked compression: 145 left and 140 right. Seems okay to be for an 850 engine. What do you think?

Regards,

Gregory Bender

Gordon's reply to Gregory:

Well, no smoking gun there, since they're both about the same. I wonder what's going on.

Gordon

Gregory' reply to Gordon:

Hi Gordon,

Maybe I just need to wait. I've only put 2,829 miles on it since the rebuild.

Regards,

Gregory Bender

Gordon's reply to Gregory:

I don't know, Greg, 2800 is getting to be some real mileage now, it's not normal to be blowing oil like that. Something's not right. I have less than that on a couple of mine, and none of them are huffin' oil. I should see what my Eldo is pumping now, it's only got about 200 miles on fresh cylinders.

Gordon

Gregory' reply to Gordon:

Ug. I'll fit a catch can and see how much I am losing.

Gordon's reply to Gregory:

You're using a standard breather box with an intact, original type metal-disk-on-the-inside check valve, right? Do you have one with a disabled disk and a ball type check valve to put in it's place? That might be a worthwhile experiment. Maybe it's as simple as your box isn't working properly. That would be nice, and not hard to imagine. Or maybe you've already been down that road. 850 cc - what pist/cylinder are you using?

The riding season is coming to an end up here. Not every day is a good day for a motorcycle ride anymore.

Gordon

Gregory' reply to Gordon:

Hi Gordon,

I'm using a standard loop frame breather with the disc check valve. I bought it brand new, so it should be good.

All the same, I could snatch the new V7 Sport breather off my G5 and see how it works.

Cylinders are nikisil for 850 T3 pistons. I had one re-coated by Millennium because the PO broke an oil ring during install and then ruined the coating (rest of cylinder okay).

I'm going to fit a catch can first and see how much I'm blowing off. Then I can try another breather and compare (trying to get somewhat scientific about this).

I wrote to Charlie Mullendore the other day about the stock breather configuration:

The Guzzi belch tube is 14 mm ID; while the inlet to the breather is 18 mm ID. Personally, it makes zero sense to me that the stock breather exit hole is *smaller* than the inlet hole. Seems to be that would act as a restriction to evacuating all of the potential inlet air. I would think the exit hole would need to be at least as large as the inlet hole.

All that said, my Ambassador is great with the same breather and no sump spacer. The only time I had trouble with blowing oil on it is when the sleeved cylinders were ready for the dumpster...that was years ago.

Regards,

Gregory Bender

Gordon's reply to Gregory:

I never heard of a breather box being bad, but who knows, maybe the manufacturer screwed one up somehow. All of your leakage does come from the breather's discharge tube, right? Nothing pushing past the front or rear crankshaft seals? I never thought about Charlie's comments regarding size of the tubes, but I'd think it's not an issue considering they normally do the job. Maybe they're designed that way, maybe something to encourage the separated oil to run back into the sump.

I can think of two real significant oil huffing instances that I've had, one was the broken rings with Venolia pistons story that I told you about, the other on a brand new set of Gilardonis when using both a check ball valve and the box disk flapper valve. If one is good, two must be better, right? :) In both instances they huffed serious oil. In the first case, I installed Gilardonis and the problem was gone (probably in 1993 and I'm still running them today in the blue/silver T3), and in the second case, I pulled the ball check valve out and the huffing immediately stopped.

Gordon

Gregory' reply to Gordon:

Hi Gordon,

I agree, it is hard to see how the breather box could be bad. Even when they rust horribly inside, they still function properly because there is pretty much nothing to go wrong with them. I suppose they might have not made the exit pipe inside the breather box tall enough???

Indeed, all of the oil is coming from the breather tube. The seals are in great shape there is no leakage evident from the bellhousing, transmission, etc.

I thought I might have installed both a check ball and the flapper valve...checked that months ago now already and it was not the case.

That statement about exit hole sizes is what I sent to Charlie (not sure if Charlie agrees or not). I did think about the case where perhaps restricting the atmosphere size actually helps push oil back down the return tube. I suppose that could be the thinking, but I still don't like it. It would be pushing oil against the same pressure that is already in the crankcase. Maybe that would be a good idea if it were pushing oil to another container, but not to the same container that is constantly trying to rid itself of pressure. When the breather is at its maximum capacity and cannot evacuate any more, then pushing oil back into the sump is not going to help matters at all because the pressure will be equalized between the breather and the sump.

I've read other stories about guys putting in two check valves (the ball and the flapper) and that causing all sorts of breather problems. But why? The check ball/flapper are only there to prevent dirty air from getting inside the crankcase (forget the return line is open just now). They do not regulate how much air is evacuated in any meaningful way as they are supposed to easily open with only the slightest pressure. I see the problem with two check valves as placing too much back pressure inside the crankcase - restricting the volume of air that can enter the breather, making the oil return line the easier path for excess pressure to escape.

Still waiting for my catch can to arrive...want to get scientific about how much I'm blowing before making changes. We'll see...

Regards,

Gregory Bender

Gordon's reply to Gregory:

Hey Greg,

That's what I was thinking about the breather box, maybe something inside isn't right.

And, you're probably correct about why 2 check valves are a problem, or at least it seems to make sense. I wonder how much pressure differential vs gravity is responsible for returning the oil to the sump. Wouldn't a pressure gauge on the crankcase be interesting?

Gordon


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