Introduction and request for help with '65 B40 please!

highboy_coupe · 20 July, 2023, 20:27:35

Hello, firstly thank you for having me! I have recently been fortunate enough to acquire a 1965 B40 that has been off the road and apart since 1979.
It came to me in bits, with the bottom end and gearbox having been rebuilt in '79 with a +40thou re-bore and a high compression piston.
I have assembled the engine and put it in the bike.

The Amal 376 apparently had a new slide, but the rest of it's internals were caked in various amounts of yellow/grey sludge and aluminium corrosion, I have cleaned it as best I can with carb cleaner and cleaned the jets, I have also put a new float needle in. The mixture was 1.5 turns out.

The ignition timing was completely off and the spark was weak, I did the static timing with a continuity meter and set it so that at on the compression stroke, 33.5 degrees before TDC just breaks connection (with the auto advance locked out), I can demonstrate that bit worked by releasing the auto advance and pushing the cam around with a screwdriver, it breaks connection just as there is no more movement in the cam. The sparks at the plug, however look a bit weak and inconsistent (it is still 6V though).

Hope you are still with me! And that this sounds about right.

I tested the coil, I get 2.4 Ohms across - and +, and about 4.7K Ohms at the output.

I cleaned the terminals and tested the spark plug wire, 3 Ohms.

I have 3 plugs with the bike, an N5, an NC4 and an NC5, all gapped to .20. After about 20 minutes of fruitless kicking, I can get it to start ( ferociously) with a touch of throttle, and what I presume is full choke, pulled all the way towards me.
It seems to fire about half the time, is very slappy sounding and produces a lot of black smoke. It is uneven and has moments of clarity when it seems to pick up and run at double time, then back to a slappy, chuggy mess.

It won't idle, and needs throttle the whole time. The choke seems to do nothing and it uses about 100ml of fuel in about 1 minute of this very bad attempt running. The bad news there is that I think most of the petrol found it's way into the oil, the good news is that there is only one oil leak, and it is from the tank.

I have deduced from this that the mixture may be rich...

Any help would be greatly appreciated, should I scrap the carb? Should I go 12V and electronic ignition?

Thank you,
Grim.

Rog1 · 21 July, 2023, 08:35:15

3 Ohms is a very high reading for such a short length of wire and indicates a problem. Replace your HT lead, and the spark plug cap too if it was included in that reading. The coil readings don't seem to be too far out, though I'm talking in a very general sense and I don't have figures for your B40. I'm presuming you checked the resistance of the meter leads and deducted it from the readings you got. Make sure the coil is wired up the right way round on the low tension side. You'll still get a spark if it isn't, but it will be weak.

It sounds like your carb is flooding, by the way most of the petrol is going into the oil. Perhaps the float is leaking so it sinks instead of floats, and then doesn't cut off the fuel at the right level.

highboy_coupe · 21 July, 2023, 14:18:40

Thanks, I have a new HT lead to go on so I'll check the resistance of that. Now you mention it, I have a feeling the plug might be a resistor plug, as the previous owner supplied a box of bits to convert to 12v, stator, rotor and reg/rec. The only cap I could find was in there and I didn't look. I take it a resistor cap (5k) would be a big no-no?

I know the float is ok, and the float valve is new and holds fuel, I can't vouch for the state of the carb bore or the needle, or jets for that matter.

highboy_coupe · 21 July, 2023, 20:59:26

Ok, I added a new HT lead and N5C plug, spark appears clean and strong enough.

Static timing is set to full advance at 33.5 degrees BTDC. I have new (old stock) points on, gap is near enough but my feeler gauge doesn't really fit in there.

I have noticed the condenser is free to move about a bit, I am not sure if that would alter the tension on the spring much when runnig, I suppose it wouldn't.

Compression is enough that it stops your foot on the kicker and you have to ease it over compression quite hard.

Even when spraying starting fluid into the plug hole or through the carb, I can't even get it to fire once today. I am losing my mind with it now!

Thanks

Stubaker58 · 22 July, 2023, 08:07:50

Hi,

I'm really interested in how you get on as I too am (slowly) rebuilding a B40.

Just one thing that may be contributing to your issues, you said
"what I presume is full choke, pulled all the way towards me."
The choke cable should be slack for choke on and tight for choke off.

Might make a difference?

Regards

rhyatt · 22 July, 2023, 08:11:34

if the condenser is loose then that needs to be sorted immediately.
the condenser is a fundamental part of the ignition circuit

highboy_coupe · 22 July, 2023, 10:04:55

Thanks both, I figured out the choke on closer inspection and realised it was backwards compared to other carbs...
It has not made any difference to starting!

The condenser, I noticed the plate in which it is clamped has some flex in it, it is bolted to the backplate but not rigidly so.

I can't understand why, with a spark and compression, starting fluid will not cause it to run for even a second!?

rhyatt · 22 July, 2023, 11:04:45

As mentioned, you can have all the fuel you like, but the condenser is a vital and fundamental component in the ignition.
This could well have changed the quality and colour of the spark.
Tighten it up and check the points and try again

Beesy · 22 July, 2023, 13:56:01

It sure is frustrating. That's one of the joys of dealing with obsolete vehicles!

The spark could be breaking down under compression.

In this sort of situation it's a case of going over it all again, changing one thing at a time.

A new, properly earthed condenser seems the best place to start.

Good luck, your perseverance will pay off.

highboy_coupe · 22 July, 2023, 17:07:57

Thank you, I guess that might explain it, seeing as there is a spark when not under compression. I have a whole raft of period spares, but then 40 year old spares are probably junk!

highboy_coupe · 27 July, 2023, 23:17:03

Hi, I felt like the Amal was flooding the engine at all times, so I went with gut instinct and threw caution to the wind, I bought a PWK carb and jetted it comparably to the Amal.

One kick from cold and it started! It's now chuffing away at idle like the proverbial sewing machine. I still need to do the 12v conversion but I'm happy it runs so nicely. I thought it was a lost cause!

Thank you for the interest. I'll be needing more help soon. Like how to fit an air filter on this thing... Or the chances of the Amal being fixed by a batch in an ultrasonic cleaner ...

highboy_coupe · 28 July, 2023, 00:42:48

https://youtube.com/shorts/C2VxePZE400?feature=share

Beesy · 28 July, 2023, 07:45:28

Great result. The satisfaction of solving the problem is greatly isn't it?

Just goes to prove that 75% of ignition problems are the carb & vice versa.

highboy_coupe · 28 July, 2023, 08:33:57

Thanks Beesy, I feel slightly guilty putting a modern-ish carb on it, and it makes me mad that I can't get it to run on its original equipment. But I did feel slightly elated when it ran like that! First time in 40 years the engine has been together!!

Beesy · 28 July, 2023, 10:58:27

Don't worry about it not being standard, it's your bike and you can do as you please with it. It's something that can easily be reversed

I've never heard of PWK carbs, a quick Google shows they are cheap, a good idea to get one of those to see if that was your problem. Spending £200 or whatever on a new Amal for it not to be the problem wouldn't have cheered you up.

To me the skill of a good mechanic is quick and accurate diagnosis, which unless gifted, only comes with experience. Rectifying the issue afterwards is, relatively, the easy part.

Glad you got it sorted. You wait for the grin factor when you take it for your first spin!