Bike won't idle, poor compression.

[Krauser4valve]

It's been sitting in the garage, occasionally messed with for a few days, then left unmolested for about 15 years. I popped the heads off last year to take a look at the bores and didn't see any corrosion, just alot of oil that reeked of gasoline. Plenty of carbon on the valves and piston top too.
The guy that built the engine knows what he is doing, ex B&S employee, might even be on this board for all I know, so I think that something just went horribly wrong during my attempt to break it in.
If I do need to re-hone it, do I need new rings? I know that place in the Netherlands has them, no idea how long they would take to get here though.

[Major Softie]

IF you need to hone, something that we don't know, that does not necessitate new rings. However, if they didn't seat because they're the wrong rings, then, obviously, you would need to replace them.

[Boertjie]

I would certainly have a look at new rings, and you can get it here http://www.motobins.co.uk got me 2 sets of rings, one for the newer nykasil barrels for a colleague and one set for my older R100S 77, delivery was within a week. UK to South Africa.

Regards
Johan

[vanzen]

The guy that built the engine knows what he is doing

So, we might ASSUME that "the guy" used components and techniques that resulted in a proper build.
We might as well also ASSUME the possible consequences of a period of running time
that allowed carbon to accumulate on the valves and piston dome
and the consequences of 15 years of sitting ...
Carbon indicates prior use. That, plus 15 years of sitting,
would be reason enough for me to pull both heads and cylinders
for measuring and inspection.

VERIFY the engine's current condition.

If I do need to re-hone it, do I need new rings?

I cannot presume to answer this question
without seeing the rings, measuring their fit to the piston,
and the fit of the piston assembly into a straight bore.
Same with honing –
Cylinders must be measured for taper and out of round
to understand if honing, boring and honing, or replacement is necessary.

[Krauser4valve]

I would certainly have a look at new rings, and you can get it here http://www.motobins.co.uk got me 2 sets of rings, one for the newer nykasil barrels for a colleague and one set for my older R100S 77, delivery was within a week. UK to South Africa.

Regards
Johan

Thanks for the tip on the rings, the guys in the Netherlands never got back to me.

[Krauser4valve]

]

I cannot presume to answer this question
without seeing the rings, measuring their fit to the piston,
and the fit of the piston assembly into a straight bore.
Same with honing –
Cylinders must be measured for taper and out of round
to understand if honing, boring and honing, or replacement is necessary.[/quote]

Assuming things were correct that probably aren't has wasted
a lot of time over the years. Time to pull it apart and take a look.

[Duane Ausherman]

Was the engine sitting all of those years with the pistons at TDC? If not, then it is most likely rusted cylinders.

[Major Softie]

Was the engine sitting all of those years with the pistons at TDC? If not, then it is most likely rusted cylinders.

He popped the heads last year and saw no such problems.

[mattcfish]

I would certainly have a look at new rings, and you can get it here http://www.motobins.co.uk got me 2 sets of rings, one for the newer nykasil barrels for a colleague and one set for my older R100S 77, delivery was within a week. UK to South Africa.

Regards
Johan

Thanks for the tip on the rings, the guys in the Netherlands never got back to me.

I don't think you need to go to the Netherlands for rings. Rings are available from custom piston builders (Venolia, Wiseco, etc.) as well as direct from ring manufactures for any size and width you specify. I still can't see how only 10 miles of riding could ruin your rings, unless they were used to begin with or the wrong ones. You also stated that it had the same issue 20 years ago when you stopped riding it.
Was this a new set of heads and pistons or used? What shape were the barrels in when you had the kit installed?
I still think your leak down test isn't giving you the whole story. A cold test with rings that aren't even broken in yet, is not conclusive. Also, bikes will idle just fine with bad compression, where you notice it is lost power under load and oil burning.
Are the plugs oil fouled or just carbon coated? Are they the right ones? Sorry about all the questions, but I'd hate to have you tare this all down to find out that your real problem is a bad coil or plug wire.

[Krauser4valve]

I don't think you need to go to the Netherlands for rings. Rings are available from custom piston builders (Venolia, Wiseco, etc.) as well as direct from ring manufactures for any size and width you specify. I still can't see how only 10 miles of riding could ruin your rings, unless they were used to begin with or the wrong ones. You also stated that it had the same issue 20 years ago when you stopped riding it.
Was this a new set of heads and pistons or used? What shape were the barrels in when you had the kit installed?
I still think your leak down test isn't giving you the whole story. A cold test with rings that aren't even broken in yet, is not conclusive. Also, bikes will idle just fine with bad compression, where you notice it is lost power under load and oil burning.
Are the plugs oil fouled or just carbon coated? Are they the right ones? Sorry about all the questions, but I'd hate to have you tare this all down to find out that your real problem is a bad coil or plug wire.[/quote]


The heads were used, I got them from CC products, who did a valve job on them before they were sent. The pistons were new, though I have no idea if the rings included were the correct ones or a substitution, apparently only the top ring differs from a stock BMW set (hmm...).

There was some pitting on one of the barrels, the builder fitted a good replacement.

The plugs are carbon-fouled, not oily at all. As for the type, I've tried the stock Champion G63's and nearly any other Japanese substitution. I think there are CR7E's in it now as they are a warmer plug on the cross-reference chart. The wires are new, the coils are green Dyna's and there is a fat spark when I crank it over.