The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: RichardL on 15.09. 2014 21:18
-
Gents,
After about 200 miles following a rebuild where work on the bores was only a honing to increase piston clearance and work on the pistons was a light filing to cleanup scoring from the tight bore, I took the bike up to 80 MPH, briefly. Then, there was some unusual noise, but not the klonking I had heard two rebuilds ago when the left-side big-end shell let go from lack of lubrication. A few moments after the noise began, and after I'd pulled to the shoulder, the engine flat died. Well, I was not particularly close to home, and my wife had no way to help me. I kicked it over with the plugs disconnected and things were not horrible, but there was an unusual sound, kind of a clicking (which I am yet to figure out.) I know it's almost a sin to think of riding your bike when you think something may have gone seriously wrong, but after a couple minutes of thinking about it, I decided to start it back up, if only to get off the interstate, which I did.
From the gas station near the offramp, I called a motorcycle recovery service for a quote. $200! for about 13 miles. Against all good reason, I decided to take the chance and ride it home very slowly, which I did at less than 30 MPH all the way. By the time I got home I was happy that nothing seemed any the worse. I knew that at least the top end must come apart, which it did. You could say I was ecstatic to find that the rods were stable on their journals. In fact, I was hard-pressed to find anything blatantly wrong until I noticed the black streaks on the right piston. (The other streaks are from before the recent rebuild.) When I wiggled that piston I found that it was a bit stiff rotating on the pin versus the free-moving left piston. Since I have billet rods, there is no small-end bush. I decided to slightly hone the small end to free things up, and that seems to be good (not running yet, of course).
Now, on to the photos. When I first looked at the top of the piston I thought it had a web-like pattern of hairline cracks only in the surface. Looking under high magnification I realized that the cracks were actually very tiny ridges. In the last photo of the piston top, you can see where I have taken a fine file to reveal that the ridges are aluminum. The whitish spots are actually semi-loose flecks of aluminum, and there are some near-micro beads of aluminum stuck to the top and spread about. I haven't seen any other photos like this, nor read anything about it, but I am guessing that this is the very first phase of reaching the melting point. I suppose Muskrat has seen this before, tagging it as maybe caused by overheating from pre-ignition. (Others may recognize it, as well.) Since the bike was running really well, I don't know that pinging would be the cause. I am more thinking that the heat would be due to the slightly tight bore running at a speed that the bike probably hasn't seen since 1970, but I am not going to ignore the possibility of pinging. I will he re-timing to be sure it's not too advanced. I don't think there has been any significant damage at this point, so I just plan to file down the ridges, scrutinize structural integrity and reuse the piston.
OK, this has been a long story, so I'll leave it here. Thanks for reading this far (if anyone has). Really appreciate thoughts of any kind. (Sorry, but "Don't you have anything better to do than write longwindedly on the forum?" is already taken.)
Richard L.
E, I think the photos might be just good enough to show what I'm intending, but the ability to zoom with the iPad might be cheating.
-
What's this then Richard? Have you been exploring an unknown part of the Milky Way? Your intergalactic camera seems to work, anyway, look forward to reading the story. Cheers, Bill
-
Looks like a slight case of pinging.
Cheers
-
In case you were looking, I have now added way too much text to my opening post.
-
Using Muskrat's knowledge to guide my searching, I found other pistons (mostly in even worse shape) damaged due to pre-ignition. Thanks, a ton, Muskrat. (Here, "a ton" does not mean that I will next be trying 100 MPH.")
Richard L.
-
Have a good look at the combustion chamber as well Richard. Usually find a few craters in there as well.
Retard the timing a tad of richen the mixture or both, and don't labor up hills.
Cheers
ps the cafe has done a ton & 1/4.
-
...what Groily said... *ex*
and how does it work if you do pics at BDC....??
Yup I read it- after I recovered from initial inter-galactic flash- back
-
Having more than your fair share of gremlins at the moment Richard ! What comp ratio are those pistons by the way ? and finally you haven't changed the grade of gas you been buying for her have you ,you know one brand at such and such an octane rating whilst going inadvertently for another gives another a bit lower , just a thought and please don't think I'm teaching you to suck eggs but it's easy just to miss something like that , best of luck with it , Bob.
-
Muskrat,
Didn't see any pits or ridges in the chamber, but there were some of the aluminum flecks. I am wondering a few things:
- maybe pinging on one side is sign of unequal gap between lobes
- maybe heat from pinging is reason for excess piston expansion and tightening at small end
- maybe lead substitute is effectively reducing octane
- maybe my feel for lugging is off the mark
Bob,
First of all, for me, there is always something basic to someone else that is news to me. So, no worries that something too simple might be suggested. Recent example: the eccentric adjustment for the cam-ring locking pin.
Pistons are 9:1 and I always use premium with a dash of lead substitute. The last time I filled up it did occur to me that premium octane isn't what it used to be. Here, it's 92 or 93 and we are only around 200' above sea level.
Further, regarding sucking eggs, don't forget to put a pin hole in the opposite end.
Richard L.
-
Hi Richard,
Comiserations on the further woes of your A10
It could be something as simple as too small a main jet ??
You more or less said that you had not had the bike up to 80 before
So maybe it leaned out at the higher speed at it would have been running on the main jet
Just a thought
John
-
John,
Thanks for your thoughts. I've previously confessed my ignorance regarding jetting. Would your comment be applicable to my 930 as well as to the carbs that originally came on A10s?
Richard L.
-
ah ah this will be of interest to me as well cos i'm on a 930 and although don't go to mad when I get the chance of by pass speeeds mine just seems to fade slightly afer about 1 and 1/2 miles ( trying to keep ahead of the traffic to my roundabout ) regards bobh
-
It could be that John, but I'd expect to see the piston crown a lighter colour.
Bob.The easiest way to check main jet if you have a choke fitted. Get on a straight road with a slight incline. In 3rd gear bring her up to full throttle and hold her there for a few seconds. Now drop the choke 1/4. If she increases speed or feels better you need a bigger jet, if she stutters your jet is right or a little rich.
Without a choke get her up to full throttle and close the throttle 1/8. If she slows your close to spot on if she increases or feels better your lean. Or do the old plug chop, get her up to full throttle and cut the engine as you pull in the clutch. Check colour of plug.
Cheers
-
HI Richard
It would apply to all carbs
John