Monobloc carb

[Harry44]

Variety being the spice of life I thought I would play with Edith's carb. Hidden within the myriads of symptoms and defects was the suspicion of running weak.

I had, previous to rebuilding the engine, raised the needle 2 notches which was beneficial if not totally successful. So I ordered a bigger main jet off EBay and shoved that in which again improved things. But I still could not get a decent tickover and it refuses to start except on full advance which is just plain daft.
The carb appears to be brand new but as the previous owner is unfit to put two lego bricks together I thought I should check everything.

The original main jet is correct by the book as was the needle position and slide. So the likely problem was float height. Just to be thoroughly annoying I couldn't find a bit of transparent tube to fit on the pilot jet to check the fuel level against the dot on the float chamber cover and its a struggle to do it with the black pipe I found by holding the end near the dot while the fuel slowly trickles around the thread of the jet and runs all up me arm. So I decided to go all empirical and stick a washer under the seat to raise the fuel level.
Now it just so happens that when rebuilding the engine I bought some fleabay shims to control the camshaft end float and it turns out that they exactly fit the needle seat where it screws into the top of the float chamber. So I added one (I can't remember how thick they are) under the seat and took it for a run. Starting was fun because I now have to learn a new technique. It certainly does kick back now, twice on one kick actually which is quite impressive, and caused one adult neighbour to run away. Spinning it over with the valve lifter produced a backfire through the exhaust that rattled windows and cleared the watching children.

Once I stopped laughing and had it going we went for a ride and it goes beautifully. So I pulled the carb back off and put everything back to standard and added a second shim. petrol went everywhere so I'm back to one. The next job is to go for another run and see how it is. But there is one more problem, how do you do a plug chop on a bike with no ignition or kill switch ?

[Mick D]

Hi

Pull the HT lead off the spark plug

Regards Mick

[leswaller]

Hi

Pull the HT lead off the spark plug

Regards Mick

Preferably wearing a good thick glove!

[Harry44]

Hi

Pull the HT lead off the spark plug

Regards Mick

Preferably wearing a good thick glove!

I had a feeling that would be the answer, but can it damage the magneto ?

[zwarts]

Yes Alan - it can damage the magneto so don't do it! Fitting a larger main jet would have no effect on the tick-over as the main jet only comes into play at larger throttle openings. Plug readings with current fuels are not very reliable or indicative anyway, so save yourself the hassle and forget 'plug-chops'. The fact that the bike ran better with a raised fuel level suggests that you are onto something. It's worth repeating the fuel level check you did earlier with a transparent tube, then you will know for sure if the fuel level is correct or not. Gains can also be made by trying different sized pilot jets and slide cut-aways. The settings in the books were correct for 1950's fuel but in my experience, the current brew requires quite different settings. The only way to find out the best settings for your engine is by trial and error, using the original specs. as a baseline.

[Harry44]

Yes Alan - it can damage the magneto so don't do it! Fitting a larger main jet would have no effect on the tick-over as the main jet only comes into play at larger throttle openings. Plug readings with current fuels are not very reliable or indicative anyway, so save yourself the hassle and forget 'plug-chops'. The fact that the bike ran better with a raised fuel level suggests that you are onto something. It's worth repeating the fuel level check you did earlier with a transparent tube, then you will know for sure if the fuel level is correct or not. Gains can also be made by trying different sized pilot jets and slide cut-aways. The settings in the books were correct for 1950's fuel but in my experience, the current brew requires quite different settings. The only way to find out the best settings for your engine is by trial and error, using the original specs. as a baseline.

That makes sense to me, modern fuels have ethanol to replace the lead. But ethanol has some oxygen in it. So logically it should be run richer.

[Mick D]

Hi Zwarts

How does it damage the mag? Isn't the aim of the exercise to stop the engine rotating so as the plug condition be viewed? Pop the bike into neutral and pull off the plug cap.

In my opinion plug condition is one of the best indicators of correct carburation - even though the fuel composition has changed the correct fuel air ratio is still the aim and will be indicated by the deposits on the plug

Regards Mick

[zwarts]

Hi Mick. To answer your question -'How does it damage the mag?' A magneto will generate an increasing voltage in its H.T. windings up until the spark jumps across the plug gap, ie. it is not a constant voltage output device. Increasing the plug gap allows a higher voltage to build up in the secondary windings until the point where the voltage is sufficient to ionize the gas in between the plug electrodes - then the spark jumps the gap and discharges the sec. windings. If you remove the plug cap the voltage can rise to the point where short circuits occur between the fine wire sec. windings. The safety gap, an internal feature of most mags, prevents the output voltage rising beyond a pre-set level but with old mags and old insulation are you willing to take the chance that the magneto is adequately protected? Having seen first hand how easily the sec. winding insulation can be broken down by excessive demands on the output, I would rather not take that chance!
Re. plug readings. Back in the days of leaded 4 star fuel a plug reading, along with exhaust tailpipe colour, was an excellent indication to mixture strength. Current fuels seem to mask this colour development and I have seen plugs showing all the signs of richness eg. black central electrode, but when the cylinder head is removed the combustion chamber tells a different story. Not sure what the answer is to this one.
Cheers, Bob.

[Mick D]

Thanks Bob

A most informative explanation

Regards Mick

[Rob Harknett]

Hi Mick. To answer your question -'How does it damage the mag?' A magneto will generate an increasing voltage in its H.T. windings up until the spark jumps across the plug gap, ie. it is not a constant voltage output device. Increasing the plug gap allows a higher voltage to build up in the secondary windings until the point where the voltage is sufficient to ionize the gas in between the plug electrodes - then the spark jumps the gap and discharges the sec. windings. If you remove the plug cap the voltage can rise to the point where short circuits occur between the fine wire sec. windings. The safety gap, an internal feature of most mags, prevents the output voltage rising beyond a pre-set level but with old mags and old insulation are you willing to take the chance that the magneto is adequately protected? Having seen first hand how easily the sec. winding insulation can be broken down by excessive demands on the output, I would rather not take that chance!
Re. plug readings. Back in the days of leaded 4 star fuel a plug reading, along with exhaust tailpipe colour, was an excellent indication to mixture strength. Current fuels seem to mask this colour development and I have seen plugs showing all the signs of richness eg. black central electrode, but when the cylinder head is removed the combustion chamber tells a different story. Not sure what the answer is to this one.
Cheers, Bob.

Bob, Logically thinking, we no longer know the correct plug type to match todays fuels, to use observing the plug to check if mixture is correct. If your bike shows the plug sooty, you could swap to a hotter running plug, to burn off more gasses. This may then make the plug look OK, but your mixture may not be correct. I will try and conduct a poll.