Adjustable main jet

[Horsley-Anarak]

Adjustable main jet.

I have been looking at motor cycle tuning.

There are various ways that they are able to adjust the flow in the main jet.

I have a pre war Johnson that has an adjustable main jet, so this is not new.

Have looked at the 400 series Amal carb and have worked out a simple way of having an adjustable main jet.

Before I do the conversion I did think that this must have been done before with Seagulls, does it work?

Perhaps the Kiwis have had a go?

I would think that when going flat out, a little tweak to the carb could get it on song.

I have played with loads of two stroke engines that have a tuneable carb, chainsaws in particular.

When you get the mixture correct they run very well. I can not see that the seagull engine would be much different.

Has anyone had a go at this, or am I breaking new ground.

H-A

[Todd]

H-A asked:

Has anyone had a go at this?

Yup, kind of.

I've had fun tuning my Villiers carb'd Gulls to within the proverbial gnat's ass with an easy modification and a tach.
I got a spare carb cap from SOS John, and drilled a hole in it the same size and location as the recess on the underside of the cap. I simply put this cap on the subject carb and when under way, place a small screwdriver in the hole and turn the jet needle screw while watching the rpms on the "Tiny Tach (http://tinytach.com/tinytach/gasoline.php").
I discovered a couple of interesting things - First, I found my tin ear can't discern speed changes less than 300 rpm, and secondly, with the full throttle rpms tuned to absolute peak, the idle mixture will also be correct. I'll try for a pic or two soon.

The Mikuni VM is an old warhorse which readily lent itself to 2-stroke engine use, and I've wondered if some twisted individual ever successfully mated one to a Seagull ...

Todd

[Rex NZ]

H-A

Many people are running the 400's. Can't say I've ever seen an adjustable main conversion on any.

An adjustable would be good, as conventional jet tuning is a slow & tedious (yet essential) biz.

To this end, one of my (many unfinished) ideas is to set-up an electronic oxygen sensor, complete with digital display

However, Personally I've given up on the 400's. I think 2x Amal twin jets offer more even carburettion across variances in temp & atmospheric pressure changes.

Rex

[Horsley-Anarak]

Adjustable main jet must be the way forward.

I cut my teeth on model aero engines. These were very responsive to the correct needle valve position.

I am sure that a seagull when flat out can be tuned to optimum revs without leaning it out and damaging the piston.

Have just ordered a box of mini o-rings, now to the shed.

I still do not believe that this has not been tried before with the 400 series Amal.

H-A

[Todd]

The promised pic of my easily adjustable main jet, not that a Villiers carb would be used on a serious racing engine. But when combined with the use of a tach, is a convenient way to make the most of what little h.p. you've got. I used a 3/16" bit to drill out the recess in the bottom of a spare cap, as seen in the original one resting on the flywheel.



And if that doesn't piss off the purists, here's a shot of the Atom ignition module nestled in the points can of my CPC. It does work, the purple version being for engines with fixed spark timing. I won't make any claims for superior performance or reliability yet, it's simply an alternative. The cost was comparable to a new set of points. Note: pictured above, the blue wire is grounded, white goes to the coil. Some motors require the opposite polarity.
The only source that has any, as far as I know, is in Canada http://www.bantasaw.com/catalog/viewpro ... 199&p=2206

[Horsley-Anarak]

Looks good, I did think of having a go at a featherweight tune-up.

A standard one pushes my 14' GRP boat with no problem.

I am sure that the 40+ could be improved by simple blueprinting.

I did get some 4mm diameter bar today so I must get back to the adjustable jet for the Amal.

Have had trouble finding the thread sizes of the main jet, they are a non comon size so may need to screw cut them.

Perhaps Amal used non standard threads to stop people making their own jets.

There are two sizes :
0.382 x 20 tpi

0.272 x 24 tpi

First one could be fat 3/8 bsf, not sure about the second one?

H-A

[Rex NZ]

Todd

Thanks for the link to the 'atom' supply

We used to be able to get these in NZ during the 1980's. But then they disappeared. Have been on the lookout ever since.

Rex

[Collector Inspector]

Rex

Australia and New Zealand.

Victa part #MAO-056-17A

BiNorm Part #340-040. (Universal)

Both are fixed timing.

Binorm one is the one I use when needed.

If you have a hot motor with points and want to change, it is important to find where the motor actually fires BTDC first. Then do an offset of the mag plate with a module. Modules just click over when everything is at 100% capacity......fields etc.

Then you do not change your ignition Timing. All mags have different specs when running and the module just chooses when the best time to dump.............that make sense?



Regards

B

[Todd]

Modules just click over when everything is at 100% capacity......fields etc.

Yup, the modules fire at the peak of the mag's voltage rise, but when timing is spot on that's the instant at which conventional points are supposed to open, too, yes?

Todd

[Charles uk]

Not quiet, they come in several colours, the colour seems to denote when they let the spark go in relation to the peak, diffent colour different advance.

[Rex NZ]

Collector Inspector

Victa part #MAO-056-17A

BiNorm Part #340-040. (Universal)

These look interesting.

Questions;
Which Victa model does that relate to? 125 / 160 / 198 / other? Lots of scrapped ones around here for wrecking.
&
Can you expand BiNorm? I don't recognise the name. Who's the supplier for this stuff?

Timing won't be a problem. My project engine has mechanically advancing timing. Basically 2x adjustable postions. TDC & full advance

Rex

[Charles uk]

I'm told they're mounted outside the flywheel & that the Victa ones are metal & screwed to the crankcase to cool them, so unscrew & snip the wires.

They replace the points & the condenser I'm told.

[Collector Inspector]

I made another goof, It is BYNORM

http://www.bynorm.com.au/

http://www.bynorm.com.au/index.php?page=24

The Bynorm part number is for fixed timing, two or four stroke with flywheel magneto that are ORIGINALLY point style.

The victa part number is for replacing said fixed points on any early points Victa. (Could be up to 5 engine makes)

Other victa with from new with a module have different ignition system altogether and you can not swap modules between the two magneto setups.

B

[Rex NZ]

Collector Inspector

Thanks for the tip-off. Module looks like just the ticket.

Learned something new here. All these years & I've never heard of Bynorm.

Rex

[atoyot]

And if that doesn't piss off the purists, here's a shot of the Atom ignition module nestled in the points can of my CPC. It does work, the purple version being for engines with fixed spark timing. I won't make any claims for superior performance or reliability yet, it's simply an alternative. The cost was comparable to a new set of points. Note: pictured above, the blue wire is grounded, white goes to the coil. Some motors require the opposite polarity.
The only source that has any, as far as I know, is in Canada http://www.bantasaw.com/catalog/viewpro ... 199&p=2206

Did you have to adjust the ignition plate timing position or did this work as a simple swap-out for the point set?