She's hot

[Billington]

Today I took my 1932 BSA 500cc sv, out for the first time this year. After checking it over and adding fresh fuel it started on the second kick. I then went for a ride, after just under 2 miles the engine stuttered and can to a halt. The carburettor was very hot, hence the petrol had become so hot it vapour locked the carburettor. I let it cool down for 10 minutes and it started up again so I rode home.
I had is problem in 2019 when the air temperature was about 20 °C. The kind advice from this forum indicated that I needed to fit a Tufnol flange spacer, which I did. After fitting the 5mm spacer the bike ran okay in temperatures up to 28 °C. However the temperature today was only 16 °C.
I know what the problem is, its vapour lock. My question is why is it now happening at a lower temperature and is the only solution a thicker carburettor flange spacer?

[cardan]

Boiling the fuel at 16C is a bit of a worry - better start figuring why the engine is running so hot.

Worth a quick check on the valve timing (roughly ex closing and inlet opening at TDC on the non-firing stroke), ignition timing (roughly points opening at TDC on the firing stroke on full retard), and compression (if you can't stand on the ks at compression it should be close - don't let anyone tell you old bikes don't need compression).

I guess you're familiar with riding the beast, so giving it as much advance as it wants. After it's warm and before it's hot, will it take full advance and (almost) full air?

When it gives up, is the engine tight? If you lift the exhaust does it swing over freely on the ks?

Is the lubrication system working correctly? Plenty of oil and the tell-tale plunger (does it have one in 1932?) fully out?

The move on to air leaks or jet blockage in the carb, making it run lean. The colour of the plug should give a hint here - hopefully it's not white.

Final suggestion is to check the bike over bike mechanically for anything tightening up - gearbox, chains, brakes or wheel.

Have fun,

Leon

[Billington]

Thank you Leon for all the things to think about, the bike was running okay when I stopped using it last autumn.
I checked the plug be for taking it for a ride and is was a light grey/brown.
I can kick the bike over okay and it has good compression, I need to use the lifter to get it over compression.
I run the bike with the air liver full closed it does not seem to make any difference open or closed, but I will try it open.
I run the bike with almost fully retarded, it seems to like it that way but I will try changing this again. I’ve heard that this can make the exhaust valve hot, is that correct?
No other part of the bike are hot.
I checked the contact breaker point gap before riding it was 0.008 inch however it should be 0.003, I will change this.
I also checked the inlet and exhaust tappet clearance are all fine.
The oil pressure was showing 6 psi which I believe is about right.
I did put a few drops of oil into the cylinder head before starting the bike to raise compression because I had not ridden the bike for 8 months.
The bike was not idling, if I closed the throttle the engine stopped.

[cardan]

I run the bike with the air liver full closed it does not seem to make any difference open or closed...

I run the bike with almost fully retarded, it seems to like it that way...

I checked the contact breaker point gap before riding it was 0.008 inch however it should be 0.003...

OK, so the three things above are a worry - something is not right.

First, 0.003" for the points gap seems wrong - usually something like 0.012", so check for your specific mag before you do anything.

Related to this is the ignition timing, so set the points gap first. The bike should do almost all its running at full advance. There is a reason for this: the magneto is built (new, which yours is not!) to have its maximum strength spark when set at full advance. You might want to retard it a bit when you start the bike, but after that (barring a killer hill with hairpin bends) it should be full advance all the way.

So you need to set the ignition timing correctly - you probably have a handbook that tells you what is required for your bike? Probably about 35-40 degrees BTDC fully advanced.

Lastly the air lever. Again, most running should be done with the air lever full open. If not much happens when you move the lever, the first thing to check is that the cable is connected to the air slide. The air lever should take the air slide from fully closed to fully open - look into the mouth of the carb and make sure it is doing this.

If the slide is working, and the engine wants to run with the air closed, you have a problem with not enough fuel getting through the main jet. First check the number on the main jet (at the bottom under the needle) and compare with the specs. If it seems to be the correct jet, check that it isn't blocked, then check that the level of fuel in the fuel bowl is a couple of mm below the top of the jet.

When the main jet size and the fuel level are correct, the bike should like the air lever fully open for flat road running.

Any of these things - points gap, ignition timing, mixture - can cause the engine to run hot.

Leon

[cardan]

Does the stamping on the carb say 6/112? If so, it is a 1" carb, not from a BSA but instead from a 550cc sv Ariel.

Not to worry, the carb listed for 493 and 557cc side valve BSAs of the period is a 6/014, also 1". Both the 6/112 and the 6/014 use a 150 main jet. Your main jet should have 150 stamped on it.

Even if it's not correct, the 6/112 will be perfectly fine. My guess is it may be a little longer between the slide and the flange than the 6/014.

Leon

[Billington]

Hi Leon, thank you so much for the advice. You have given me a starting point for things to try. I will be working over the next two days, so on Wednesday I will try starting the bike by setting the throttle at about on eighth open and the air lever shut. Then move the ignition control to about half full advance. I will flood the carburettor by depressing the tickler for a short time only. As soon as start I will move the ignition control to fully advanced position, when the engine is warm I will open the air lever about three quarters of its travel. I will let you know how I get on after a test ride.
The carburettor details are as follows:
BSA W32-6 Carburettor
Type = Amal 6/112
No = 45A
Float Chamber 64/079
Jet =140 (Main Jet) B140)
Needle = 1
I set the Magneto timing by the BSA instruction book, however I’m not sure if these instruction work for both 1932 quality fuel and 2022 quality fuel.
I use regular petrol in my bike. If I’m honest I did not look at the E value when I purchased the petrol but I will have a look next time I’m filling up. I did however put about 25ml of Redex Lead replacement fuel additive. I’m not shore if this could have an effect.
To set the magneto timing I followed these BSA instruction.
Release the magneto pinion and set the piston at top dead centre of the compression stroke. The ignition is timed in the fully retarded position. With the piston at T.D.C and the ignition fully retarded I set the contact breaker points open .003 inch, then re-tighten the magneto pinion.
Thank you for any helpful contributions.

[Oggers]

All useful advice - to which I would add mix. If it is a weak mix then any carburated engine can overheat quite easily. Personally I would clean the carb out thoroughly, check main jet (and size), needle position, clean out pilot air passages etc. Too close a tappet clearance on the exhaust valve can lead to overheating. I run mine slightly more than the recommended exhaust clearances. Rather obviously perhaps do check the lube oil feed and return. 

[R]

To set the magneto timing I followed these BSA instruction.
Release the magneto pinion and set the piston at top dead centre of the compression stroke. The ignition is timed in the fully retarded position. With the piston at T.D.C and the ignition fully retarded I set the contact breaker points open .003 inch, then re-tighten the magneto pinion.

That all sounds a bit peculiar ?
And very likely the source of these troubles...

Is there a fully advanced ignition timing number (degrees/inch fraction) you can time the ignition to. ?
This is how most ignitions are timed.
And avoids all the vagueness of worn parts / things not within spec.

And, why open the air lever only three quarters open ??
If its running correctly, should be fully open ?

[cardan]

Ah, fuel additives... I once tuned a four-cylinder FN for a guy, got it running nicely, rode it about 30km around the place, then gave it back to him for an upcoming rally with strict instructions on how to ride it. He was on the side of the road within 10km, and arrived at the regrouping stop with the thing running on about 2 cylinders. Of course I had to forego my cup of tea to look at it. The plugs were indescribable - it turned out he had filled the bike with 98-ocatane fuel and an additive of some sort. I just use standard fuel, with an eggcup of oil added in if really necessary.

A comment on the process of setting the timing with the points open 0.003": This was OK when parts were new, but with 90 years of wear it no longer guarantees that the points open 35 degrees before TDC at full advance, which is what is required. If the best position for the mag lever at 30 mph is not fully advanced (wire fully tight) the timing ignition is incorrect.

Leon

[Billington]

I don't have a full advance timing number, sorry.

The bike was running fine before I removed all the fuel from both tank and carb, for storage. The only new variable is the new fuel and the fuel and Redex. On Wednesday I will try running the bike on full advance, if that does not work I may drain the fuel and try fuel from a different petrol station without adding Redex.

[R]

In the meantime, I think you should try exploring where exactly that 'full advance is taking you.

I don't know this bike, but can you stick a pencil down the plughole, and get a feeling for when the piston is at TDC ?
If you put the traditional cigarette paper in the points, and the pencil down the plughole.
If you mark on the pencil when the paper is released, this gives an exceeding accurate indication of ignition timing.

I see that Leon mentions 35 degrees - is this for this exact model ?
Doesn't sound too far out of kilter.
Somewhere round the 1/4" to 5/16" perhaps.

It was also mentioned about giving the carb an exceptionally thorough clean.
It only takes a flake of rust blocking off a jet to upset the apple cart, mixture wise.

[cardan]

I see that Leon mentions 35 degrees - is this for this exact model ?

No, but better than what it is now!

[Oggers]

I mentioned cleaning out the carb as I encountered similar after a couple of my bikes had been laid up. The first instance was when I filled up the VB600 for the first time after winter lay up. Almost as soon as I left the filling station, the bike died and would not re-start. Fortunately I was not far from the house, so after an hour or so pushing the thing back into the garage, I stripped and cleaned the carb, replaced it, and it started first kick. It has been fine ever since. The second instance was again post lay up when the MSS just would not idle at all well. I blew out the pilot air passage and that seemed to cure it.

In both instances, I think a bit of crud from the tank had become dislodged over Winter, found its way into the carb, and messed things up. For sure it does not take much.       

[Billington]

I went for a ride today to try the ideas suggested in this link (air temperature 10 C). My first ride I tried the air lever full open and the ignition half advance, however the results were poor, the bike did not get so hot that it vapour locked but is ran poorly an cut out on the one mile run.
In the afternoon I changed the oil and played with the throttle stop; I could set it a tick over but if I stopped the bike and started it again it would then stall on closed throttle which seems inconsistent. However I went for a 3 mile ride (air temperature now 17 C), with the air lever closed (because I had forgot to open it!) and trying different ignition positions as the road dictated (I live in a very hilly area of England). The bike did not stall because I was managing this with the throttle. At the end I could touch the carburettor but it was still warm.
Thank you for all your advice and questions.

[Rex]

If you ever get to the stage where you can't touch the carb then there's big problems somewhere. It passes X cubic feet of air a minute which keeps   it cool, and if it's hot to the touch then it needs further investigation.