JowettTalk

Hoping someone with the knowledge can shed some light on this subject. My vintage has a NOS early thirties distributor fitted instead of the original manual advance type. Both the dynamo and dizzy cogs are new. Yet, there is still quite a lot of freeplay on the rotor arm! I'm questioning how well the cogs are actually meshing. So, does anyone know the relative lengths of the shafts of both types of dizzy if they are different or indeed if they are the same?

Actually, I've just compared the freeplay with the Kingfisher dizzy and it is the same! They are also both new gears on that, so I don't think I need to worry! I think my uneven idle must be due to the fact I've got a compression of 55 lbs on the near side cylinder and 40 on the other! Time for valve lapping I think!!

Hi Ken,
I'm glad you sorted yourself out within 2 1/2 hours, it could have taken me days.
Having read your first post, the little grey cells fired up, and somewhere I've read or heard that a worn shaft bearing, or even a worn shaft itself might just give a sloppy rotor arm as you describe. This is only for posterity as I realise your dizzy is NOS.
There again, I use a bit of thin paper under the rotor arm to keep it 'solid' on the top of the shaft.
I was also told to turn the rotor arm clockwise before I set the dizzy timing when using a tell-tale bulb, but you probably know that.

Do you (or anyone else reading this) know what the compression should be on 7 hp cylinders with valves in good order?

Regards,
Tony.

Good question, Tony, I don't know. Also, I don't know how accurate my Chinese compression tester is. I only bought it, at great expense(£13!!) to tell me any difference between the cylinders, which indeed it has! The better of the two indicates 60lbs which seems fair given the low compression of these engines. The other incidentally shows 45lbs, so a marked difference hopefully cured by grinding the valves. I'm amazed she pulls as well as she does!

Tony, I will do a test on my Kingfisher when it's on the road and run in as it's engine has been completely rebuilt with everything renewed. It'll be an interesting comparison!

An update on the compression drop! Found the reason for the uneven idle; both inlet valve caps were not dead tight and this has resulted in gasket failure. I poured oil over them while the engine was running and could see air bubbling out from the gasket. I have ordered metric copper seals on ebay, however, Geoff Wills contacted me to tell me that the Bradford(?) exhaust manifold gaskets fit perfectly which I somehow happen to have! Just need to find a neat way of cutting them to the required size without damaging the gasket sandwiched between the copper. Any ideas?

Despite a set of valve cap gaskets ordered from club spares, I decided to cut the Bradford exhaust gaskets down with scissors and used them! I now have a much more even idle.

Can anyone tell me when in the 20s, Jowett changed from Delco Remy to Lucas distributors, please?

An update. After further investigation, ie the nearside spark plug was sooted up compared to the offside one. Therefore, still a vacuum leak on the offside! So, off came the inlet manifold and found a hairline crack around the offside flange at the top stud hole. Got it TIG welded at the local engineering shop and he did a great job. Needed to do a fair bit of filing of the flange so that it sat perfectly flat onto the cylinder before making new gaskets.
Now, it idles very well and actually pulls much better, yet has an annoying backfire on deceleration, which was there before I started all this! So, the next thing will be grinding the valves as I cannot think of anything else it could be! The timing is spot on and I can't detect even the slightest exhaust leak which may cause the backfire. Any other suggestions would be welcome.

Valves now ground in and caps refitted with new copper seals from Club spares, annealed first. Now getting 55lbs compression both sides with a very even idle, so I'm happy! Yet to test drive which won't happen until a few days time. Hope not to hear any backfiring!

Still backfiring despite all I've done! Now looking at carb fuel levels, see newer post.

Hi Ken.
Somewhere in the back of my mind is the fact that worn valve guides might cause backfires, but I can't substantiate this.
Tony.

Thanks for your input, Tony! I did know that worn guides could cause a poor idle as well, but mine aren't too bad, in fact they are much better than those on the Weasel and she doesn't backfire! However, my thoughts are still with some sort of vacuum leak, but goodness only knows where!

Hi Ken

Mine is very sensitive to any leaks on the exhaust manifolds and this causes bad back firing. I know it's only one side that seems to be the problem, but I'd be very tempted to change the plugs, HT leads and condenser if they've not been done recently. As time goes on, people forget that condensers are a service part. I always time the older engines with light/buzzer but it's some times it's useful to check them with a strobe just to see the advance is working and there's no issues with 'scatter' timing. Failing coils can cause all sorts of weird carburettor type problems. On 2 occasions, I've had faulty brand new NGK plugs over the years.

I was taught never to start looking at fuel issues before being convinced the ignition was in 100% condition. That said, I was a diesel mechanic.

Barry

Worth going back to basics. A backfire is caused by unignited mixture entering the exhaust system or preignition of mixture in cylinder whilst the exhaust valve is open. The first can be caused by a cylinder not firing and the mixture entering the exhaust. The second can be caused by hot spots in the cylinder or electric spark coming at the wrong time. The latter could be caused by many faults including tracking in the distributor cap, ignition or valve timing, points bounce, capacitor failure, faulty dizzie weights or advance mechanism.