Opening Out the Pilot Outlet on Pre Monoblocs?

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Iron Head
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Opening Out the Pilot Outlet on Pre Monoblocs?

Post by Iron Head »

After continued cold and warm starting and difficult tickover control issues (not magneto) on my '48 G80 I have decided to bite the bullet and rebuild the carburetter. I had contemplated a complete new 289 from Burlen but they won't have stock in until March/April and they are north of £340 with VAT and delivery. I have fortunately found and bought a NOS 89/403 mixing body, choke/jet barrel and throttle valve from eBay for a great price and have new 180 main and 106 needle jets coming from Burlen. Will transfer everything else over from my 89B/1AK. The main thing I want to improve is the inevitable mixing body bore wear that my 70 year old carb has seen.

My question concerns a reference in the F.W. Neill book to a modification to all models of pre-54 carburetters to "obtain full control over the pilot jet adjustment".....Neill suggests opening out the pilot outlet with a pin vice and a 0.033" drill. Apologies if this has been asked before but has anyone encountered/tried this and, if so, did it give "full control". Did Amal get something wrong here on (likely) hundreds of thousand of carbs or is it perhaps plating or something build-up?

Thanks, Iron Head

Incidentally, very useful link here on Amal carbs.

http://www.southwalessectionvmcc.co.uk/ ... 0carbs.pdf
Dixter
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Re: Opening Out the Pilot Outlet on Pre Monoblocs?

Post by Dixter »

Andy, drilling the pilot outlet is an interesting item. I'm having some tuning difficulties with a freshly sleeved 89B/1K. I'd not run across this idea from Mr. Neill. Would you kindly post a link to this article, or give me the reference info?

Thank you,
Dick
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bjork
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Re: Opening Out the Pilot Outlet on Pre Monoblocs?

Post by bjork »

Yes, I did this on my 49 G80 and it did make a difference to the effectiveness of the air screw control. Obviously it is just the body that needs opening out, the jet built into the jet block is left alone. (don't be like someone I know :o )


I've only done it the once and found it a bit nerve wracking but it all went well with a little patience. I miss that bike now...
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Iron Head
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Re: Opening Out the Pilot Outlet on Pre Monoblocs?

Post by Iron Head »

Hello Dick

Page 56 of the attached link has the relevant F.W. Neill info on the pilot outlet modification concept. I've got a set of numbered jewellers drills on order and am going to use the plain shank end of the #66 drill as a gauge to see how the land lies before/if I decide to 'mod'!

Iron Head


https://onebaddaddy.files.wordpress.com ... _neill.pdf
Dixter
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Re: Opening Out the Pilot Outlet on Pre Monoblocs?

Post by Dixter »

Thank you Andy.

Safe journeys,
DC
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Re: Opening Out the Pilot Outlet on Pre Monoblocs?

Post by Dave T_LAPSED »

Just a thought, councelling caution, try passing a drill of the original size first, if the jet is corroded at all internally the corrosion wouldn't blow out but would look open even though restricted if you see what I mean? Once you've opened the jet up you can't go back!
As I say, just a thought.
Good luck with it, Dave.
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Iron Head
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Re: Opening Out the Pilot Outlet on Pre Monoblocs?

Post by Iron Head »

Good point Dave! The jewellers set I have ordered has a range of smaller sizes so as I can work my way up to the #66. Got the 70 year old NOS mixer body today and I don't know where it has resided but it reeks of and is covered in nicotine?!
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Re: Opening Out the Pilot Outlet on Pre Monoblocs?

Post by Plugsnpoints »

I suggest you use a 3/16" drill and you'll be rich beyond your wildest dreams! ;)
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Re: Opening Out the Pilot Outlet on Pre Monoblocs?

Post by bjork »

Dave T wrote:Just a thought, councelling caution, try passing a drill of the original size first, if the jet is corroded at all internally the corrosion wouldn't blow out but would look open even though restricted if you see what I mean? Once you've opened the jet up you can't go back!
As I say, just a thought.
Good luck with it, Dave.
The job is not to drill out the jet, it is opening out a passage after the jet. The pilot jet on the standard carb is part of the brass jet block. The passage that needs modifying is in the alloy carb body. It was found by the factory to be needed to avoid blockages in such a small drilling. It works too! So drilling the correct passage won't alter the mixture strength at all, but it will allow the air screw to be more useful.

Just make sure you know 100% which bit you are threatening with your pin vice and you can't go wrong. (apart from going wrong by breaking the drill bit or going off centre ;) )
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Iron Head
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Re: Opening Out the Pilot Outlet on Pre Monoblocs?

Post by Iron Head »

Haven't decided on doing the pilot ‘mod' or not yet but I have rebuilt the carb tonight with the NOS mixing body, slide, jet block and new jets and what a transformation at start up! Bike sounded quite different and was ticking over evenly within a minute off choke and no blue smoke. I was running so rich previously I suspect I had been washing oil from the bore as I had blue smoke haze all the time on the road in spite of new rings and mega compression. The old brass mixing body and slide had probably 0.8mm of side-to-side play in the worst wear area?!

Test ride at the weekend and then some all important hot starting which has so much been my nemesis I daren't even stop for fuel previously......

Iron Head
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