How are decompressors used?

[lotty]

The top of my DT 175 cylinder head has a port for the spark plug going in vertically, but it also has another port going in at 45 degrees (blanked off with a bolt). The only thing I can think of is for a decompressor which if I remember rightly was to help kick over large engines, and I seem to remember as a young lad that some of the trials bikes had decompressors.

So, what is that 45 degree 2nd port for, and how are decompressors used when riding trials bikes?  The correct answer wins a good feeling in the pit of their stomach, haha. Doug

[NE0]

Oddly enough I answered a similiar post recently, when another member asked about the two spark plug holes on the top of his DT head.

He posted a photo of a cylinder head with two plugs screwed into the head. Here's the text from my post...

I know the early 70s AT2's (125 &175) had dual spark plug holes. As far as i know they weren't designed to run with twin plugs as one thread hole was often plugged. However, my mate had one and had both holes with spark plugs,(like your photo) when one became fouled up he'd pull the HT lead and put it on the 'spare'.

It doesn't look like an AT2 though as the plug holes were either side of a central fin. Your photo shows one directly in the centre and one offset which means its from a dual shock DT175.

Here's a brand new one on ebay.

https://www.ebay.co.uk/itm/334146920758?hash=item4dccb56136:g:3E0AAOSw6fBhPx2o

 

I appreciate that my "answer" doesn't answer your decompressor question but it may be of some help.

 

[lotty]

Thanks for that NEO. I myself can only think there must have been a good reason why the designers would have put in an extra spark plug hole, so it's still a bit of a mystery.

For some reason i never ever have ridden a trials bike. I watched them a lot, but that riding never appealed to me, though that tv programme 'Junior Kickstart' presented by Peter Purves was very watchable. It seemed a very different skill, and they had a little lever on the handle bar to decompress, and i suppose the use of it would only be known to a trials bike rider. But maybe even a DT in certain situations could benefit from a decompressor. But I don't know. Doug

[blackhat250]

Decompressors are for big bore singles, [ or diesels ],  XT500 had one, the DT400 ha an Automatic decomp, works of the kickstart via a cable,, i blanked it off .made little difference...  

[NE0]

I'm sure you've all been there but I had a sleepless night last night, mainly with my brain popping up random thoughts that i couldn't get rid of....and it kept me awake!

One of them though was "Why DID early Yams have twin spark plug holes!!?"

I came to the conclusion that it was either for 'Performance' or 'Diagnostic'......

"Performance" to make it better or worse!

or "Diagnostic" to see what was making it better or worse!

yup not the best discussion to have with yourself at 4 am....but....

It also got me thinking about what was available back in the 70's to do either of them...As regards to 'Diagnostic', there was Colortune sparkplugs, which were a popular 'accessory' that found their way into many a christmas stocking for dad who had everything. It was a plug with a glass inspection construction so you could 'see' the colour of your spark!.....oh joy!....come on... it was 4am!!

but would Yamaha construct cylinder heads just for this purpose?....unlikely I thought!...unless it was a high performance engine......not your run of the mill Trial bike...    it was probably 04.15 by now!

Which made me think of 'Performance'. Was there really a need to equip their cylinder heads with dual spark plug holes to run twin plugs? Presumably that would mean twin HT Coils and maybe twin contact breakers?  makes you think doesn't it?

it was probably at least 04.30 by now!

I don't recall my mate who had twin plugs on his AT ever having twin coils, true he had both spark plugs inserted but only one was ever connected at a time.

He would change the HT lead over to the other plug if it got too fouled up...but thinking about that today (04.40) the chances are the carbon fouling was probably all over THAT spark plug too!!....but heh we were just 16!...it seemed to be the 'right thing' to do, swapping HT leads.....wow...the performance change...Not!!

Anyway, (04.45) I came to the conclusion that dual plug holes was probably related to performance more than anything else......mass produced?...domestic market vs sports market?.......then just blank off the hole that's not in use and make use of the twin holes for the sports related performance. Seemed logical, and probably not untypical of 70's thinking.

 

No I  didn't drop off ...I still had Quantum and String theory to discuss with myself along with what am i going to do today?, what I'm going to be ABLE to do today with so little sleep!!.... what's that ringing?...it's the alarm......time to get up!

Then I thought "Why don't I google it? "

Twin spark plugs on Cylinder heads....

I was right after all....its performance related...

https://www.rediff.com/getahead/report/slide-show-1-biking-and-motoring-why-twin-spark-is-superior-to-single-spark-tech/20140104.htm

 

So I just have to answer one final question.... Why didn't i get up at 4am and look it up? ...would have saved myself all that 'deep thinking"!

Gee ...i'm tired!!!

 

 

 

Edited November 30, 2021 by NE0

[finnerz89]

I know a lot of small capacity bikes for the Asian Market run twin plugs due to poor quality fuel. Firing them at slightly different times ensures a complete burn I think it's the theory. 

My vtwin rotax engined aprilia has 2 plus per cylinder. But both fire at the same time and you can run with just one without any noticeable performance loss so no idea why they're fitted

[lotty]

I was also awake, pottering, turning, thinking and wondering if a decompressor helps navigate a tricky bit of terrain like a rocky drop where much control and finness is desired and a decompressor could help by being able to keep the bike in low gear without having any power on or stalling.  So it seems none of us win a warm feeling in the pit of our stomach....haha

[Cynic]

Decompressors can control engine braking on a decent to stop wheels locking, or a stall. As well as making an engine very docile.

The twin plugs on small bikes comes from having 2 plugs in the head, they typically have different heat ratings so the engine can be used without switching plugs all the time.

Some competitive vehicles have 2 plugs for reliability, my land rover i used to compete with had external condensers, (2 off) and 2 ignition coils (coils would fail often in off road overheating covered in shite)and associated wiring. Quick swap of the king lead and i would be running again.

[lotty]

Dear Cynic. Well, after all, I reckon the truth is we are all stumped. It's a come down for us all, perhaps. I mean, we just don't know. It's a leveler. Tough deep down. But....well.....er......at least we are 16 again haha x

[Cynic]

Yep. Just one of those things that disappeared in history. 

I used to maintain a generator when I was younger. Old 3 cyl raf thing used for shows and carnivals on an estate. 

That had de compressors on each cyl. You closed them in sequence as the cylinders warmed up. 

Do it wrong and she would stall, then you had to try and start it half warm. Not good.

The clockwork starter was also an art. Got a good sweat on winding it up even in the winter. 

[slice]

Decompressors are used so that the engine does not have to start with the piston at TDC it allows you to pass the TDC point then get a swing on the kickstart so you don't break your instep with a kickback. Most BSA's  had one plus the odd high compression engine that would fire you over the bars if you tried to start it witout taking it past TDC. That's why my old biker boots had a metal strip on the instep! cos it hurt like fuck if you got it wrong.