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[Adrian]

Hi Norm/Cov Lad.



I took out the three middle fins (if you can find someone with a decent milling machine you can remove a bit less). The hole for the decomp valve is drilled 12.5mm or 1/2" and tapped 14x1.25mm, but the hole is also counter-bored from the top to being the valve down to the correct height. There's nowhere for the valve to vent into the exhaust port so it vents to the atmosphere. Photos re-posted for reference:















A.

[Norm]

Sure is close to the inlet seat isn't it Adrian, but if it works then that is all that matters.Being a lean burn hopefully the head won't get covered with too much black stuff

[Adrian]

The decomp valve is on the centre-line of the combustion chamber, but of course the inlet valve has a bigger head than the exhaust. It also **looks** closer than it should be from the combustion chamber side because I botched the operation slightly and had to use a spark plug thread insert to tidy everything up (successfully). Works a treat in practice. Same technique if you want to go down the twin plugging route, some home market Indian EFI/UCE heads with a similarly-shaped combustion chamber are twin-plugged.



My old Electra had a Dell'Orto carb (later replaced by a Mikuni) fitted when new by the dealer, with one of our hosts unrestricted exhaust downpipes added soon after, so it wasn't lean burn for long! ASBO12 has had a Dell'Orto PHF36 since I first put it on the road.



A.

[Cov Lad]

Thanks for the information Norm and Adrian. Is the head in the photographs for sale by any chance?

[Adrian]

Nice try, but no, it's not a spare! It lives HERE:







Others have carried out this modification and have given good reports. If you can find a friendly neighbourhood machine shop (there must still be a few left around Coventry) it would be an easy enough job for them to undertake.



A.

[Adrian]

Try again:

[Adrian]

OK, forget Postimage, try ImgBB

[Norm]

What is very annoying with this decomp/valve lifter set up, somebody was paid good money to design this rubbish and set the motor back 30 years or more on something they knew was a dud. The original Bullet decomp valve design was perfect, yet somebody thought they were smarter than the average Yogi Bear

[Cov Lad]

Adrian, I think I will be able to find a machine shop in Coventry to do this, by the top hole being counter bored do you mean tapered? Do you have a photograph of the holes from the top? This process is another reason for buying the spare engine - head donor etc. I think I may be able to get it for £500. Can I assume that I will buy the decompressor cable and handlebar fixing from Hitchcoks? In the meantime would you suggest starting the bike with kick rather than electric start? Stopping the engine with kill switch, key or another method? I assuming taking the head off will be simple, no need to get involved with piston position etc. Can it be done on the bike or does the engine need removing?
Sorry for the long list of question but it has been a very long time since I messed around with engines.

[Norm]

Cov Lad, the only really safe way to save the sprag on your motor is to not use it till you fit a decomp to the head, but as you can see from recent posts that you can just be riding along and it just went bang. Fitting the decomp is also to help kick starting it after the sprag has failed.Stalling it at the lights can also cause it to explode. Starting and stopping it using the decomp certainly helps but it only takes one kickback to destroy the sprag, it is not a matter of "if" but "when"