Hi guys I hope someone can help me.
I'm new to old bikes but have been working on cars for years so do have a decent depth of knowledge.
My B31 had a small fuel leak from the bottom of the jet so I took off the carb and stripped it down. It then went back together and I fired the bike up. It's now smoking really badly and it seems like the oil level is going down in the tank.
I rode the bike the other day with no problems at all so this may just be a complete coincidence as I cannot see how that could be related.
Does the bike have a crankcase breather that I'm missing or am I looking at head gasket? Thanks
The engine breather is in the bottom of the RH timing cover. Certainly worth checking that you get puffs of air from this when the engine is running. That said, it's unlikely a blocked breather is going to cause the engine to smoke.
Normally, it is worn valves or rings that cause smoking which would seem to be unlikely in this case as the engine was ok not so long ago. I'm not familiar enough with the pre-unit singles engine but a problem with the oil drain from the cylinder head might be a possibility as I believe it relies on an external oil pipe.
Ok thanks for replying. Yes it's really strange as the bike is running absolutely fine and the engine sounds strong except for smoking me out that is haha. The oil return seems to be working fine as well so it's definitely got me stumped at the moment.
If the smoke is blue and the oil level is dropping and the plug oily then you are burning oil. As Spaceman said that is commonly piston rings/bore wear, valve/valve guide wear. The top end gets its oil from the return line to the tank. Dropping oil level if the bike is left for a few weeks is 'wet sumping' where the oil drains down past the oil pump into the crank case. If you then fire it up after that you will get oil blowing out of any weak joints, out of the breather, into the primary chain case and, if the bores are a bit worn into the cylinder to be burned. Once you clear it and if you use it regularly the oil level will stabilise and the oil burning may diminish.
There are ways to determine if bore is worn (you get more blue smoke on a long over-run ie up to speed and shut the throttle for at least a few seconds. The clincher would be to measure compression with a cold engine, then add a few mls of engine oil and repeat. If there is a significant increase in compression the culprit will be the bores.
If it is grey black smoke when you open the throttle it is fuel and something in the carb is misplaced and you are running very rich.
Any help?
It's strange, I would say it looks grey/black and the fact that I have had the carb apart would point to a fuelling issue but it smells like burning oil and is spitting oil from the exhaust. Could it me overfuelling so much that it's washing the bore? There is masses of smoke coming out to the point it causes a smoke screen. As I said the bike was absolutely fine last week when I took it for a ride and can't see such a catastrophic failure happening that quickly that it's burning that much oil. It sounds strong when running.
Have you looked at the plug - that will tell you what is coming out of the exhaust: oily black -> oil: dry black -> unburnt fuel.
You may have too much oil in the sump. I suggest you remove the sump drain plug (bottom of engine) and tell us how much oil drains out. A normal quantity is about 125ml.
On my 1949 B33, I drain the sump if the bike has not run for a week or more.
Jim
What carb do you have on it?
It's a monobloc but has no make written on it so maybe not an Amal? I've just done a compression test and it's around 150psi so seems good?
Ok I will try the sump oil Jim.
I've taken the carb off again. I'm thinking the original leak was needle valve and now I've disturbed it it's totally failing and massively overfuelling the engine.
Does the needle valve return solely under gravity or is there supposed to be a spring above it where it goes into the inlet?
If it's a Monobloc the needle is held in the slide by a clip and there should be a spring between the slide and the top of the carb. The spring returns the slide (and so the needle) to cut off the fuel/air mixture.
Over fuelling is more likely to be the float needle not seating properly or the float leaking) so the fuel level is too high but not high enough to flood it?
Your mixture on starting and tickover is regulated by the pilot jet and the knurled adjuster on the side. Then it is the slide cutaway then the needle and needle jet and then the main jet.
Assuming that you only stripped and cleaned the carb things that can be wrong might be the needle being set too high or there is debris in the pilot air line making mixture too rich. I presume that you only cleaned the jets and passages with carb cleaner and air and were not tempted to use drill bits or wire to open things out?
To me it sounds more like wet sumping (so empty sump and makes sure the ball bearing and spring under the hex nut on the timing cover are clean) regular use tends to sort this out.
But of course the temporal coincidence with the carb strip cannot be ignored unless it had sat idle for some months.
Success!! Thank you all for your reply's. On advice from Jim I drained the crankcase and about 0.5l came out so it was wet sumping. Bit of smoke while it cleaned I assume and now clean as a whistle 👍🏻👍🏻. Must've been a complete coincidence that it did that when I decided to sort out the carb leak.
Matt A
Happy to hear of your success.
You will be draining the sump again soon. If you don't already have one installed, get yourself a sump cover with a drain plug. This avoids the need to frequently remove the drain plug from the crankcase housing with the potential to damage the threads.
Jim
Good
Make sure your ball bearing and spring are seating properly and run it regularly!