BSA B50 -500 APG Build

[Peter Jack]

Sorry Tom, it looks like I spoke too soon. It looks like they no longer carry it.

Pete

[fredvance]

Tom it certainly looks like detonation. Maybe everything you mentioned. What gas? I forgot what your compression is??

[Peter Jack]

Tom:

Hold off until you get across that infernal border. Harbor Freight have a dual gauge leak down tester for about $40. At that price it's probably a worthwhile investment.

Pete

[fordboy628]

Tom,

Send me a PM with a phone number where I can talk to you.   Want to call asap.   Or send it to my gmail acct if you like.

Fordboy

[Stan Back]

I bought the HF one a couple of years ago.  I'd been borrowing a buddy's Snap-on.  I couldn't figure out the HF one.  Had to do some math -- which I'm very good at.  Numbers never came out right.  Maybe I'm not that bright.  Went back to borrowing.

[saltwheels262]

maybe too much octane, to rich of a mixture, weak spark , timing retarded too much- any or all or one or the other? -- it seems like a lot of carbon for a race engine Tom.
how does spark plug look?

could there have been salt vapor or (?) sitting on the seat of an open exhaust valve ?

don't know if you could have been doing a hundred forty miles an hour with 70 percent leak down.  seems very high percentage.

good luck on that and at bub.

Bf

[saltwheels262]

 as far as the safety wire tabbed lock washers go--

I don't think the grade of the nut or the bolt is going to matter - the washer
doesn't know ( or need to ) how strong the fastener might be.  or does it ?

franey

[hotrod]

Take a real close look at the plugs that came out of that cylinder, if you're having detonation you should see tell tale signs on the plugs.
Under magnification you will likely see lots of little shinny balls of aluminum (look like microscopic ball bearings) all over the plug electrode and ceramic tip. Every time a detonation event occurs it blows little bits of aluminum off the head or the piston crown and that atomized aluminum tends to collect on the plug.

Also sometimes the electrode looks like it has been chewed on by very tiny rodent with the corners of the electrode eaten away. Not the normal smooth erosion you see from lots of miles.

I would bet that etching near the plugs is from either detonation or very brief pre-ignition. If sustained pre-ignition tends to melt holes in the piston like you held a blow torch on it, but from what I can make out on those photographs looks to me like detonation pinging the metal off the head near the plugs, and probably coating the port with the metal residue.

If that is pitting in the exhaust port rather than metal build up, you might have been sucking salt into the engine and "sand blasting" the surface. Just a guess since the photos are not sharp or large enough to really tell.

[Koncretekid]

Here is comparison of combustion chamber after Bonneville with the most recent one after Loring.  I believe the pits in the combustion chamber became deposits in the exhaust ports.  Also shown are two most recent spark plugs.  The one in the number 1 position is from the dyno runs in New Brunswick before Loring, the one in the number 2 position is about 5 runs at Loring.  I cannot find any evidence of aluminum deposits on the spark plugs.  Although we had a different fuel supplier in Loring than Bonneville, I cannot help but think the reddish color on the exhaust valve shows that the motor was run with the rusted valve seat.  The pits appears much deeper than those from Bonneville, but more concentrated (at least 10 sustained runs at Bonneville compared to five relatively short runs at Loring.)  Of course, effective CR would have been higher at Loring due to lower altitude, higher atmospheric pressure, so maybe more severe pre-ignition or detonation.  I have never been able to hear anything like detonation going on.  I'm thinking pre-igniton between the base of the spark plugs and an overheated exhaust valve, probably made hotter by the fact that it wasn't closing completely.  Does this sound logical?

[Peter Jack]

Tom, a loupe or a proper spark plug reading magnifier will really help you in looking at the plugs. Even the guys who do it for a living use those tools and normally won't make a complete determination without them.

Pete

[wobblywalrus]

Tom, it is not unknown for oxidation in the combustion chamber of an engine that has been sitting to cause a ring to stick in a groove or deposits to form on the valve seat and prevent a good seal.  "Sitting" can be a pretty short time for a motor run on certain types of race gas and stored in a humid climate.  Either can influence your leak down numbers.  Did you spray preservative into the chamber when you put the bike to bed after Loring?
 

[Koncretekid]

No preservative, Bo.   But I will from now on.  I'm really wondering about the new fuel supplier, whose name escapes me, we had at Loring this year.  I asked for something equivalent to VP C-12, but i don't know what I got.  I've never seen rust on my valve seats after tear downs in previous years (maybe I was just lucky).  But the rust along with the blackened oil, which BTW hasn't changed color since I let it sit in the glass measuring cup for the last week or so.  Could the gasoline have had some contributing additive?  Oxygenated, perhaps?

In any case, I made a 5 hour round trip to Kingston, N.S. yesterday to get a proper valve job done.  The exhaust valve has a slight bend in it, so naturally wasn't seating properly.  That along with a slight ding in the intake valve has me worried about the new higher ratio rockers.  I'm considering using the stock rocker for the exhaust along with the new one for the intake.  If I need another mph, I can always put the new one back in in less than an hour.

Tom

[DND]

Hi Tom

I cant see the seat on the ex too good but looks like a wavy pattern either on the seat or above it?

If your seat is outboard of that it look too narrow, for better heat transfer the seat needs to be brought in a touch and have a .020 border with a wider seat on the ex side

Your valve might be getting to hot and helping with the detonation, plus the intake valve pocket on the piston has some sharp edges right at the top flat area and that is not good either

If you are not going to take the piston out I would bring it to the top clean off the top & deck and tape off the area so the chips & alum dust don't get in the bore

Take a cartridge roll and round off those sharp areas then scotchbrite and smooth things up and that should help

Those sharp edges like to glow, and that's bad JUJU in a motor!!!

Hope this helps Tom, Don

[Koncretekid]

Hi Tom

I cant see the seat on the ex too good but looks like a wavy pattern either on the seat or above it?

If your seat is outboard of that it look too narrow, for better heat transfer the seat needs to be brought in a touch and have a .020 border with a wider seat on the ex side

Your valve might be getting to hot and helping with the detonation, plus the intake valve pocket on the piston has some sharp edges right at the top flat area and that is not good either

If you are not going to take the piston out I would bring it to the top clean off the top & deck and tape off the area so the chips & alum dust don't get in the bore

Take a cartridge roll and round off those sharp areas then scotchbrite and smooth things up and that should help

Those sharp edges like to glow, and that's bad JUJU in a motor!!!

Hope this helps Tom, Don

Yes, Don, the seat is too far out on the edge, and the wavy pattern was probably caused by the valve being slightly bent.  I'm re-thinking the new rocker arms, as the valves come within about .30" from hitting each other at TDC.  Actually, I'm having a new larger valve made for next year, as the bowl has been enlarged too far to move the seat.  The piston is ceramic coated, so I can't soften the edges anymore.  I did actually knock of the sharp edges before the coating, but maybe not enough.

I undercut and polished the underside of the exhaust valve to get rid of the nasty machine marks.  I had to go a bit undersize (to about .270" from .312") to ensure I got rid of the stress riser that would have been caused by the machining.  I polished the intake valve while I was at it.  The reason you don't see the new seat on the valve is because the pro that I had do the valve finished it so well that it doesn't leak when checked with a small vacuum pump with a rubber platten over the exhaust port down to -28" Hg.  No lapping required, says the man!  He checked my intake as well with the same result.

[DND]

Back in the day building engines I did my own valve work and always lapped them in, and to double check where the seat and to be on the safe side

When farming out machine work you can not check enough be sure it was done right

After all it is your motor that might break not his!!!

G Don