Amal Monobloc shenanigans revisited

lingeringstin... · Spanner Monkey Joined: 01 May 2014 Karma :

I'll try to post some relatively pointless pics but if I screw it up I'm sure somebody will come along and fix it. Anyway...

I'm using a two stroke Amal Monobloc on a four stroke engine.

I never got on with 1970's era Amal Concentrics as found on so many classic Brit bikes. For whatever reasons they just never worked very well on whatever I was trying to fit them to. However, this older model Amal Monobloc is a different thing altogether.

It's fairly primitive and probably inefficient and just to add complication (because that's what I do) this is a two stroke carb on a four stroke engine.

The one I'm using is a 7/8" model 375, the sort of thing that went on a Velocette LE or something in like 1954, cobbled onto a basically Honda CMX250 clone engine.

As far as I can ascertain the only difference between a two stroke and a four stroke Monoblocs is the needle jet tube. On the four stroke it's cut straight across and on the two stroke it's slash cut.

Presumably this changes the behaviour of the fuel delivery to suit whichever type of engine it's on, with the slash cut (two stroke) spray tube effectively making it a bigger, oval shaped needle jet hole without making it a bigger component.

There is of course faffing that can be done with jet sizes and slide cutaways plus or minus a wee bit either way but that's about it. As far as I know they even use the same needle for both carbs, which I find a bit odd but that's just how it is.

The ins and outs of how a four stroke engine and a two stroke engine deal with carburation is best left to somebody that actually knows what they're talking about because it's all gibberish to me. I'm pretty much just a trial and error guy.

A fairly standard main jet size for a four stroke Monobloc carb on an engine of similar spec to mine is around the 140 size with a 25 idle jet.

HOWEVER...

There was NO WAY this two stroke carb would work with anything like standard jetting. After a lot of jiggery pokery, and far too many main and idle jets purchased, I've finally got it going pretty well on a 550 main jet and a 40 idle jet with a standard 3.5 slide.

WHAT, I hear you cry, HOW BIG of a MAIN JET? Surely that can't work!

Well actually it works pretty well now but it was a bastard figuring it out. I don't know all the rocket science behind how carbs work but all I can say is if you're going to fit a two stroke Monobloc to a four stroke engine (and nobody anywhere is ever going to do such a daft thing) then you'll need WAY bigger jets than you thought.

I'm guessing the slash cut two stroke spray tube is a bit like having a built in air leak and you have to increase the jet size to compensate. At least that's my theory. All I know is it runs pretty well now and still gets about a hundred miles a gallon just like the PZ carbs I used to use, only the Amal Monobloc starts much easier thanks to the tickler. Idle is still a bit lumpy but it'll do.

Of course none of this aggro and expense would have been necessary if I would have just used a four stroke needle jet tube in the first place but as I've said before, where's the fun in that?

Pointless pics:

https://i.postimg.cc/VkWJhrxn/a.jpg

https://i.postimg.cc/85jssy7G/b.jpg

https://i.postimg.cc/J4b0yHz4/c.jpg

https://i.postimg.cc/G28hbHfd/d.jpg

https://i.postimg.cc/HsCsCW4v/e.jpg

stinkwheel · Bovine Proctologist Joined: 12 Jul 2004 Karma :

I wonder if you're having fuel height issues and your big jet is compensating for that?

It's canted a fair way forwards there and the thing about monoblocks compared to concentrics is that they aren't concentric.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

lingeringstin... · Spanner Monkey Joined: 01 May 2014 Karma :

Yes that's always been a bit of a worry. And there isn't much scope to mess about with the float and needle so you just have what you have, but it seems to work fso far. I know the fuel bowl level is a bit askew but it doesn't seem to matter much. If it wasn't for the tickler I would have stopped messing about with this carb ages ago, but that tickler is the best thing I've ever had on this engine for reliable starting.