Amal Monobloc 389

[cray]

Do the people who still use this carb have an 'angled/chamfered' spray tube?
I am led to believe (with a few exceptions) this is for 2 strokes to aid/prevent fuel starvation.
A few years ago I approached an 'Amal' supplier for advice, needless to say it was if I had just landed from outer space, he had never heard of such a thing and certainly could not supply one.
My question is it really necessary?

Cheers
Colin

[IanCordes]

Spray Tubes

The Amal listing for the 389 as fitted to MDS, MX1/2/3 & ISDT 24 INT. models shows a spray tube 376/136, as opposed to the standard spray tube, so I guess that is it. I can't find it listed in the current parts book though.

The same spray tube is in the spec. for Dot & Cotton scramblers of the period too, out of interest. Must have been a good reason.

Ian

[cray]

Hi Ian, yes it appears to have been lost/forgotten about after the change to the concentric type of carb. As I said the current Amal supplier was nonplussed
about it.
It just so happens I have come across a Jet Block with an angled tube. So now I have both. Just need to get the challenger running and maybe find out if it makes a difference.

Colin

[IanCordes]

Out of interest, the Norton Commando twin Concentrics have angled spray tubes, the only 4-stroke I know of to have them. That doesn't mean there aren't others of course

[ADickerson]

Hi Ian, yes it appears to have been lost/forgotten about after the change to the concentric type of carb. As I said the current Amal supplier was nonplussed
about it.
It just so happens I have come across a Jet Block with an angled tube. So now I have both. Just need to get the challenger running and maybe find out if it makes a difference.

Colin

Hi Colin, the angled spray tube is essential for use on a two stroke whether it be a monobloc or concentric. Hope this helps!

[cray]

Hi Adrian, I think you have just confirmed my thoughts, the limited info I can find certainly points to your assertion.
Do you know what difference an angled spray tube makes?

Cheers
Colin

[ADickerson]

Afraid not! I'm just writing from past experiences. Only I purchased an almost brand new monobloc from a chap who just couldn't get his Hawkstone to run with it. When stripping down I noticed the flat spray tube, unlike the one on my old monobloc, so knocked it out and made an angled tube to replace it. Worked a treat, having subsequently set it up as per the old one.
I've no knowledge at all on air flow dynamics so can't explain it.
A similar situation exists with the needle jet, in that on a four stroke there is a cross drilled hole about half way up, but a two stroke won't run cleanly with it. So I've always made a point of asking for the jet size followed by the letter T. When not available I've got by using the four stroke jet of the same size but just blanking the cross drilled holes with solder! I learnt that from East Anglia's top tuner Ian Bennett!

[Brian Thompson]

Angled spray tubes etc.....

Hi Adrian,

Trust all is well with you and yours old mate!

Chaps....

I had a look for more info on this and found an interesting (and Greeves related) thread here on the 'Brit Bike' Forum; http://www.britbike.com/forums/ubbth...t&Number=50499

One of the posters (John Healy) gives what sounds to me like a reliable explanation of the reason for the use of angled spray tubes in two strokes, at least as far as piston ported engines go. This is the main gist of his post, in response to an earlier post in the thread;

"The cutaway on the suction side of the spray tube would create a pronounced area of depression at the spray tube and so move more fuel during the short intake cycle of the 2 stroke."

Actually it is the opposite. The slanted spray tube is there on a piston port 2 stroke carb to prevent the engine from taking a big gulp of gasoline at low throttle openings just as the piston rises and clears the intake port.

Piston port 2 stroke engines develop a lot of crankcase vacuum as the piston starts up the bore. As the piston travels up, and before the bottom of the skirt clears the intake port, the rising piston increases vacuum in a non-linear fashion. When the port does open vaccum has increased to a point where it creates a "pop" in the intake as the air rushes in to fill the vacuum. If left alone, this rapid incoming charge at low throttle openings, would cause the mixture to be momentarily abnormally rich. .

I made a set-up to demonstrate this. Remove the float bowl of two similar carburetors. Fit a slant spray tube to one and an straight cut one on the. Attach a clear hose to each main jet. Place the end of each hose in a can of water.

With the idle stop screw set the slide equally on each carb so that the slide is about 1/8 to 3/16" off the bottom (I use a small rod to get them even). Place the end of a vacuum hose over each carb manifold and mark how high the water rises in the tubes. You will notice that the water rises higher in the carb with the straight cut spray tube.

If you carefully raise each slide equally you find that the effect of the spray tube evens out as you lift the slide.

Anyway, there's a bit of additional info on the subject, and let's be honest....what more appropriate time is there for a discussion of fluid dynamics and gas flow principles at 8:30 am on a sunny Monday morning.....in my case inspired by the Chinese takeaway we had last night.....

Hope this is of interest,

Brian.

[ADickerson]

Hi Brian, good to hear from you and many thanks for the explanation. Admittedly I had to read it three or four times before I got the gist of it. But I think I've got it now.
Anyway, still here looking forward to this weeks meeting aboard the Hawkstone complete with monobloc carb and points ignition. A rare combination today it seems, but they work ok for me. DinosaursRus.

[Brian Thompson]

Hi Adrian,

Likewise, and good to hear the old warhorse is going strong!

Mind you, with me it's less dinosaursRus and more dinosoreA### thanks to that flippin' takeaway.....

All the best Adrian, and have a great meeting.

Brian.

[ADickerson]

Thanks Brian. I hope your symptons ease as the day progresses, if not, it might be worth trying some of that Emu Oil applied to the necessary area, or taken internally, if thought preferable!
Yes, pleased to report the old warhorse is still going strong. The bike is going quite well too.
But I'm holding my breath on that front as the engine is in need of a complete overhaul so keeping fingers crossed it holds together for one more meeting.

[Soggy Welly]

Brilliant explanation Brian. I was previously content to assume dark wizardry was involved, but it turns out to be common or garden science. thanks for posting

[Brian Thompson]

Thanks Brian. I hope your symptons ease as the day progresses, if not, it might be worth trying some of that Emu Oil applied to the necessary area......

......I would mate, but apparently I'm not allowed to mention emu's anymore on here...... I think it's OK for everyone else though.....

Brian.

[Brian Thompson]

Brilliant explanation Brian. I was previously content to assume dark wizardry was involved, but it turns out to be common or garden science. thanks for posting

My pleasure Paul, glad that I found John Healy's excellent post so we could finally, ahem, get to the bottom of it.....

Brian.