Hitchcocks Forum

Good evening one and all,
sorry to put this old chestnut back on here but I'm looking for some advice on tuning me 1995 500cc bullet.
I recently purchased a copy of the motor cyclists workshop by "Torrens" of The Motor Cycle. Got some excellent advice for tuning up a big single '50's style. Polished Head, Barrel, High Comp Piston, etc. To this heady mix I will be adding a 32mm Amal mk1.
Now, to get the arguments going, would it be worth twin plugging the head? I was simply going to grind out the decomp side to take a plug and fit a valve lifter and a twin lead coil. Keep it simple.
Also, timing. How easy is it to set up?
I've been reading old posts on this site and a few others I found on the net and it seems half and half as to worth it or not.
So, as the heads about to come off and the polishing to commence; twin plug or not twin plug, that is the question!
Thanks in advance for any suggestions,
John B

i would say not twin plug the price will out strip the gain also the decompressor will come in far more handy with an high compression piston than a valve lifter,assuming that you are going to fit a valve lifter to your exhaust pushrod in some manner?i.e like a wd/co /model g or indian electra thats mi 2pennorth but as they say its up to you john atb paul.

Not that i'm an expert in head design and combustion efficiency, just a gut feeling that the basic prehistoric dome shape, lacking any form of real squish band would not benifit to any degree by twin sparking... you wouldnt notice the difference.... just sayin. gwilly.

JohnB
I have done the twin plug and am dead happy with it - again many thanks to Tim Busby in NZ. I doubt there was extra power but much smoother, especially the idle. See below.

timbusby Jan 20, 2010
A small mod is involve in Twin Plugging.
Block off the old vent to the exhaust, and ease plug access.
I also refit the decomp between the two plugs, mounted onto a short riser with a 10mm plug thread adaptor dead centre of the combustion chamber, small hole (2mm) down into the combustion chanber.
Minor milling of fins in between the two rocker boxes is required, but the fins being removed are for the most part are useless any way other than radiated heat.

I have done this as Tim says and have kept my 130psi static pressure (Gunson compression tester).

How to fit a plug where the decompressor was? See below.

Posted By john On 2007-03-08 At 23:46:38
Hi wizid,
Procedure involves removing the sealing washer and turning down the round part with the embossed letters of the spark plug {its the part above the threads} to 17.5mm.
Next you need a spacer to fit comfortably over the plug threads its outside diameter is 17.5mm and it is 4mm thick.
I bought a dual output coil from busters on line site. They have 2 types, one 3ohm for points ignition which suited me. I would advise using new sealing washers you need all the washers as used on the decompressor plus one extra small one.
Firstly put the large washer on the plug then the large sealing ring then a small sealing ring on the plug thread then the spacer and finally another small sealing ring.
Screw in the plug with care as the threads may have some carbon in them as the decompressor doesn't quite go fully home.

There, its all quite straight-forward as these two experts showed but you need access to good machining. You can get a twin-coil from our hosts as well as all the washers - this way you don't need 2 condensors (which you will if you uswe 2 separate coils). It works exactly as John and Tim say.
I can copy all details to you - pm me.

Guys - I hadn't realised how much the lack of spaces that ensue when you upload text here affect the sense of my emails! The text is all correct, just remember that where I say "see below" the new section follow immediately starting with the author's name. ChrisD

Having a Bullet EFI, no decompressor of any sort, it would seem adding a second plug would be more difficult to install than with my previous bike. I was never sure if such a mod has any real benefits. Never saw any dyno test results.
However started flying lessons last year on a Cessna 150, which has an engine about as old as a Bullet and from 4 cylinders produces 100 hp at 2,750rpm. Standard preflight check is to prove that both magnetos are functioning. Each cylinder has two plugs and the magnetos are wired so that each cylinder has one plug connedted to A mg and one plug connected to B mag. Engine is set on the ground to about 1,800 rpm, and each mag is switched off in turn. On one mag, the RPM drops, by up to 75 rpm compared with two mags. Ample proof to me that two plugs are better than one both for power and safety. Cheers TrevorCH

Hi there everyone,
Thanks for the posts. Still not the wiser!
Chris, not sure how to pm on this site, my email is [email protected]. Don't mind passing it on this site as fellow bulletiers are of the old school sensible sort.
Refreshing change in day and age!
Back top the point, any info you have is greatly appreciated.
Thank you to every one again
John B

Hi Trevorch
the reason for the twin plug twin mag on the aircraft is just as you say for safety, the reason for the drop in revs is because with twin plugs the burn starts in two places at the same time this in effect advances the ignition as the flame front is faster, to compensate for this the timing on a twin plug engine is set slightly retarded, when switching one mag off the engine runs retarded and the effect is the same as on any old bike with manual advance, retard it and it slows down.
In real terms I don't think that any real power increase would be achieved in this case.

DennisC has described it exactly right.
The reason for twin spark is to reduce the flame travel for complete combustion. As you can imagine, it takes time for combustion to reach the most distant parts of the chamber. The bigger your bore and the more crevices in your chamber (deep valve pockets with high CR) the longer it takes or it might not even burn completely. With a modern 4-valve set-up, there is usually no benefit, as the middle position is really optimum. In theory, only a 3-plug set-up would mean a benefit here and people really thought about this in big big big bore short stroke engines like in Formula1, until it was banned in the rules.
Why is a full and quick burn so important? For optimum efficiency/power and least emissions (that's most important these days) it is important to burn every single molecule at every load/rpm. t full throttle, 50% of the combustion is already over after about ten degrees after TDC if the burn is most efficient. So in order to achieve this, you need to run pretty early ignition timing and might run into knock problems.
In general, you should be able to run at least 5° at high CR up to 10° less ignition advance to get full power with twin plugs compared to single plug thus you can reduce probability for knock. Anyone out there to confirm this?

Whether the power increase is noticable in a road going engine is doubtable. If you gain 5% in power that's about 1.5bhp on a 30bhp engine, measurable, yes but a bit of tailwind gives you more difference in top speed.
But at low rpm/throttle openings the engine should be smoother, even when running leaner mixtures, as the two sparks are "more likely to find something to burn".

Best reason for doing it.....we just love to tinker with our toys, that's why I will try it as well, soon.

@Dennis C and@Nico,

Learning to fly is most interesting; the consequences of not following procedures being most grave. I guess your are right that the apparent power increase with two plugs is small, safety being paramount. I think I leave my EFI with a single plug, but now fit the Power Commander and try a little remapping. Cheers TrevorCH