<-=- Club Bevel Heaven -=->

I had posted before, several months ago, how my 75 860 had a rattle noise from the engine under slight load. Turns out that I got pissed off one saturday afternoon (at my bike) because I had been through most everything, save for a rebuild, chasing that damned noise. Anyway, back to being pissed off on a Saturday-- I figured I'd ride the bike and just wring it out. If something was going to break, well, it was going to happen then. It did. I was tucked in, going about 110 MPH straight down a road where nobody goes when the rear cylinder lost compression. Long story short, I limped it home on the front cylinder. This brings us to present day. I have replaced the pistons, rings, clips and pins. Valves are new as are the springs, guides and seals. I bored the cylinders out just about 20 thou. and fit some 86.6 mm slugs from the Loud Bike Store. The bike idles and runs great. I've still got break-in oil in it, so haven't taken it out too far yet. My question is this... When I spray carb cleaner or brake spray around the head/barrel joint and base gasket area, the bike bogs down. I've torqued the nuts to exactly what is specified (31-34 ft lbs). Can I go further without causing any clearence issues? Will this help close the gap? This was the cylinder that failed me---should I remove it and deck barrel and head? Is there a joint compound Duc gurus use? Thanks in advance. Cheers.
JD

Of course its always the rear cylinder, which means you have to drop the motor to disassemble... never is the front! I wouldnt think that a bit more torque would create any squish band clearance issues. That being said, I also dont think that will fix your problem. How well does it run without spraying the mating areas?

The bike runs great. Blipping the throttle or rolling it on pretty hard, the bike returns to a very steady 1k RPM almost immediately. The pipes are not bluing, nor are they different temperatures indicating a leaner run. Thanks.

If it runs great, and the plugs look good then I wouldnt worry about it. Seems you have an air leak at thje base gasket . But if the plugs look good then i would think no unwanted air is getting into the combustion area.

mizike77 wrote:Of course its always the rear cylinder, which means you have to drop the motor to disassemble... never is the front!

On my 860GT it wasn't necessary to drop the motor to remove the rear cylinder head. There was just enough room to pivot the motor using the rear motor mount.

Bruce

Nip `em down a little more!! I had an issue on my 860 where it was leaking around the head joint after I took it off. Rear pot too. I just nipped the head nuts down a little tighter and it`s been fine since. Do it evenly and in a cross-over pattern.

So, a question...

When you lost compression, do you know what actually broke/changed?

And a few thoughts...

It's odd that squirting any accellerant around the base gasket area makes any difference at all. For a start, there's no ignition source below the compression surface of the piston. Also, properly torqued down, no ambient pressure gas will get through there at all. Bevels basically don't leak through there if the gasket is at all intact and tightened down. Standard base gaskets I've seen are .3mm and .5mm, but this only affects your valve to piston clearance anyway.

Something to look at...are you sure you've used the correct oil way o-ring sizes (2 different sizes) on the head/barrel interface and they are positioned correctly? If they are sufficiently large, or out of place, you may not be able to torque down enough regardless of pressure. Whilst they are made of rubber, both viton and nitrile rubber have limited "squash". After that point they compress no more than any solid substance.

You should never need any additional sealant between the head and the barrel.

Something else to look at...are you sure the accelerant entry is not through the intake manifold rubber gaskets. Over torquing, or using old gaskets is a common cause of air leakage (although, generally this will cause lean running - but you may have tuned it out.

Kev

Kev-
When I took the bike apart, the pistons and rings still looked somewhat new. Some slight vertical markings on the skirts, but nothing too deep or obvious. The rings were especially clean. They were free of any carbon build up or anything else. They were also spaced properly, I thought maybe they had lined up somehow, but, no. They only variable that got lost in my little investigation was the base gasket- it came apart while I was removing the barrel, so I couldnt tell if it had been compromised.
For the record, I am not spraying around the intake, as a matter of fact, I am spraying the front side of the head and barrel. Strange that it would have an affect down low. It's only a few bucks and some labor to get that head and barrel off to replace gaskets and o-rings if need be. I may just pop it off and swap some parts and try again. The investigation continues...
Thanks
JD