Amal Monobloc woes

[lingeringstin...]

This is basically a Honda Rebel 250 clone engine. Some of it's Chinese and some of it is Honda but it's got identical guts to a Honda CMX250 and I'm running a single (slightly modified) Chinese PZ27 carb that actually goes pretty well for the moment but it's cheap shite and prone to niggles like idle jet blockage pretty regularly. Not a big problem but a fecking annoyance.

Occasionally I swap out the PZ Chinky carb on my bike for a 375 Amal and try to run it. So far no luck.

I want to use the Amal because it fits the space perfectly without faffing about and it SHOULD be a better carb than the Chinese crap I usually use, but so far I have had no joy getting the Amal to go.

I built the Amal to basically the specs for old Brit bikes with engines of similar power to mine but even then there's a lot of complication and differences between models. For instance some old Brit bikes used two stroke jet tubes in four stroke carbs, and a Norton Jubilee (250 twin) uses a 130 main jet but a Triumph T21 (350 twin) only uses a 100 according to my information.

My power output is more akin to the Triumph 350 than the Norton 250 but from what I gather the Norton has a bore/stroke fairly similar to what I've got so I kind of lean in that direction for jetting but nope the Amal is just not playing. I know it's a long shot since probably nobody ever puts an Amal carb on a Honda engine but any ideas for jet size, slide cutout, jet tube and all that malarky?

[jeffyjeff]

You probably know this already, but let me state the obvious. Even if you had the best technical data available, your conversion would likely require some measure of trial and error. My humble recommendation would be to start with the smaller 100 main jet and work your way up as needed to get a satisfactory spark plug reading (rather than starting big and working your way down). Also, make one change at a time, rather than making multiple tweaks at once. Honda engines are not very fussy; I swapped out carburetors on a CB450 back in the day, and that worked out well. You will probably achieve a reasonable level of performance on your first attempt. Dial it in and please let us know how it works out.
____________________
History shows again and again how nature points out the folly of men - BOC

[lingeringstin...]

Well it seems to be working pretty well now, but I can't figure it out. Look at the pic of the jets. That's a 100 on the left, a 150 in the middle and whatever the Hell THAT is on the right, which is what works,

I also changed the idle jet to a 30.

Turns out this was a 375/33 carb before I got my hands on it. Still using the two stroke spray tube and a "B" needle which is probably standard for this carb on whatever British two stroke it came off of. But those points don't seem to matter much. It's the massive change in jet size that finally worked and I have to say it works very well (so far).

I just kept running little drill bits through one of my spare jets until it finally seemed to work well. The final drill bit measured 1.91 with my calipers. I have no bloody idea what size jet THAT would equate to.

Anybody know what that thing is I'm using for a choke? Never seen one before but it fits the carb if you hammer it on with a mallet and works a treat.



Hope the pics work. If not somebody might sort them for me:



https://i.postimg.cc/wvF58WFy/a.jpg
[url=https://postimages.org/][img]https://i.postimg.cc/wvF58WFy/a.jpg[/img][/url]



https://i.postimg.cc/HL9XXSqZ/b.jpg
[url=https://postimages.org/][img]https://i.postimg.cc/HL9XXSqZ/b.jpg[/img][/url]



https://i.postimg.cc/CK7nzR89/c.jpg
[url=https://postimages.org/][img]https://i.postimg.cc/CK7nzR89/c.jpg[/img][/url]


.

[lingeringstin...]

seems to work

[Easy-X]

[lingeringstin...]

[quote="Easy-X"][quote="lingeringstink"]seems to work[/quote]

The choke seems to be blowing raspberries when you rev it :lol:[/quote]

Yeah I sort of expect that from a piston port two stroke but I'm not sure it should be happening here. But have you seen the massive jet I'm using? That's not right. That can't possibly be correct jetting but it actually goes very we!!. I can't find any evidence of an air leak so I don't know what's going on.

[jeffyjeff]

[lingeringstin...]

After thinking about it some of this giant jet business makes sense.

This carb has the two stroke jet tube. That design doesn't "suck" as much fuel through it as the flat top four stroke design so a bigger jet would compensate, but THAT much bigger? Mine seems absurdly large now but whatever.

Next thing is the needle. Mine is probably a two stroke needle which is not significantly different at the beginning but does taper to a thinner point which also would allow more fuel to flow past the jet tube at higher throttle use. At the moment there's nothing I can do about that since I don't have a four stroke needle but I AM going to get one for further experimentation.

Even at idle there's a small bit of use from the slide, needle and main jet. The needle at idle does not totally close off the jet tube so you will still get SOMETHING albeit not a lot. However, that would explain why fitting a larger idle jet worked. If there was less fuel getting past the jet tube at idle due to the slanty two stroke design then it makes sense that a bigger idle jet compensates. In this instance it seemed to work pretty well.

So the carb seems to work with an abnormally large jet at idle and into the midrange, but what about wide open? I would almost bet the two stroke needle makes things too rich at full throttle but I haven't had a chance to run it down the road like that yet. If so then a four stroke needle might work better.

The problem is, there's only two Monobloc needles to choose from as far as I know and neither might work in this hybrid 2/4 stroke carb setup.

I loved faffing about with SU carbs some years ago because of the needle tuning. It was a bit of a pain in the arse to find the right needle but once you did (out of the hundreds to choose from) you could really fine tune the carb for the engine. I may have to rummage through my old stock of SU needles to see if there's something that could be adapted to work for this Amal situation if I can't get anywhere with the stock Amal needles.

This is getting interesting and I wish it wasn't. Shit like this always sends me down rabbit holes for years. I'll be looking at alcohol fuel or something before it's all over.

[lingeringstin...]

Further faffing about with the two stroke carb and the massive jet has been interesting. It actually goes very well.

I was certain the massive drilled out jet hole and the two stroke needle would make for wildly rich mixture at anything above middle throttle but it hasn't. In fact it actually just works rather well, and to my mind it really shouldn't.

I only had it out on town streets this afternoon so no long distance but I did give it some hooligan treatment at times. I was on it for a good hour and it was fine. The spark plugs are even the right colour.

I don't get it.

[lingeringstin...]

I could be wrong about the needle being a two stroke needle. I've just been looking at information that would seem to suggest the needles were the same "B" in both types of carbs. If that's the case then the only real difference between a two stroke carb and a four stroke one was the spray tube, and mine's definitely the two stroke version.

Too much information rots the brain.

[that_impulse_guy]

its fine, you've done nothing wrong and it all looks good, my message is purely for someone finding this thread in five years time and trying to apply the same logic.

on a 2stroke engine, start with an oversized jet and work your way down. Do not try small (lean) and work your way up.

(yours isnt and so on, but search engine can find weird terms and match things up later incorrectly)
____________________
Gone: Yamaha DT50lc, Suzuki DR500, Suzuki A100, Kawasaki z250ltd, RD350YPVS, Suzuki DR Big, Kawasaki AR125, Kawasaki KMX200, Suzuki GS1000S, Katana 1100, GS550M, Suzuki RGV250
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[lingeringstin...]

Yeah this has partly been my problem with trying to fit this carb to my engine. There was NO information whatsoever that I could find that specifically addressed fitting a two stroke Monobloc to a four stroke engine.

Apart from wanting to have a better built carb than the cheap Chinese ones I've been using the only reason I even considered it in the first place was because it's pretty easy to get a two stroke Monoblock.

There aren't that many surviving old British two strokes around of a similar engine size that had Monoblocs but there are a lot of people wanting whatever went on their vintage British four strokes and I came up with this two stroke carb for a very reasonable price because nobody else was interested in it.

Not knowing anything about Amal Monoblocs, and only ever having had fairly bad experiences with Concentrics, it was all new to me so I just researced what Brit bikes of similar engine size used what jetting.

The problem there is that all the jetting information I could find was no good for a two stroke carb on a four stroke engine. Everything was for bog standard four stroke carbs and I was way under-jetted to begin with for all the rocket science reasons mentioned previously.

Now that I know you CAN reasonably use a two stroke carb with a bigger jet it would be nice to know what size jet I'm actually using now so anybody else wanting to do this can have some idea of where to start. It's not enough for me to say "I just kept drilling the jet out until it finally worked".

So now I need to find out what my drilled-out jet size equates to in Amal sizes in case I need to make slight alterations myself. The carb is in the ballpark but I doubt it's perfect yet.

I suppose one good thing about having such a huge jet size is it would take some doing to clog it.

Oh and by the way I've discovered the choke thing I'm using is actually apparently an Amal choke from something but I don't know what. I got it years ago (probably at Kempton bike jumble) and have always had it lying around thinking it might come in useful someday.

Although this is for a 250 Honda-ish engine mine's a hybrid of Chinky and Honda parts all mashed together. The heads I use are usually (but not always) from 125's. With these engines (basically Benlys) they all use the same valves and cam from 125 on up, so a 250 is basically just a 125 with bigger pistons and a more robust gearbox.

The only real differences I can find are that most 125 heads have 19mm inlets and the 150, 185, 200 and 250 have 22mm inlets, so the Amal 375 seemed to be about the right size for my general purposes. These Amals are easily interchangeable so I could always get a bigger carb body later if need be but for now the 7/8" seems to function perfectly well.

I still haven't given this new carb a damn good thrashing across the hills to simulate pulling a trailer in Yorkshite on my way to a rally but I might get out today for a lengthy run and we'll see how it goes.

[lingeringstin...]

Update in case anybody's interested, and I bet nobody is because this has been an insanity that needn't happen.

The road test today revealed two niggles.

The first niggle is that if you just ease the throttle open slowly the engine starts to falter as if it's not getting enough fuel but it's only minor and doesn't really cause any problem as long as I remember to be a bit heavy handed opening the throttle from traffic lights etc. I don't have to whack it open like a lunatic but I do need to be just a tad more aggressive than I normally would.

To me this seems to imply it's a bit lean at the beginning of the throttle. I'm using a number 3.5 slide so I figure I could just take about a mil off the bottom of the slide for less cutaway and then raise the needle one notch because the needle notches are a mil apart.

Yes, I could probably just get a number 3 slide but where's the fun in that?

The second niggle is that at full whack going down the road it starts to starve out at the very top end. It's not bad, it just means I have to ease back the throttle a bit and then play with it as the engine catches up but eventually it will go balls out pretty nicely so long as I ease into the full throttle and not just pin it and wait for shit to happen.

To me this seems to imply lean at top end. I don't want to faff about with jetting (yet) because the plugs look about right but I think a pointier needle might do the trick for the top end starvation, and I just so happen to have one that looks like it will work. It's from some kind of Russian carb, like for a Dnepr and that sort of thing, and it's near as dammit the same as the Amal needle but the taper at the bottom starts a bit sooner and the whole needle is just a tad shorter but I think it will fit without coming out of the jet tube at wide open.

All in all it's not bad going. At normal road speeds and conditions the carb works fine, it's just not perfect yet and since I'm an idiot I will spend FAR too much time trying to sort it out rather than just getting a four stroke carb body like any normal human being would do and be done with it.

[stinkwheel]

My personal experience has lead me to conclude that all amal carbs are hot garbage. The only thing they have going for them is parts availability. I'd genuinely rather use a Cheap chinese knock-off mikuni.

If you want a cheap mikuni-esque carb, try a mikcarb for an enfield. They are a licenced copy of the mikuni VM24 or VM28. All the mikuni parts fit them too (except the slides for the VM24 which is a mirror image). They would appeal to both your desire for something oddball and your desire for something cheap on account of lots of enfield owners for some reason replacing them with an Amal in the belief it's some sort of performance upgrade then selling the old one on ebay. The VM24 is a flange mount, the VM28 is a stub mount.

I have a couple of oddball Russian carbs here that you'd like but you can't have them because I might want to put them on my Minsk some day. The one I have on it would appeal to you because it's SUPER low profile with an ersatz flatslide made of a bit of bent brass sheet. A K36. They used them on IZH and Voskhods too.
https://lh3.googleusercontent.com/pw/AP1GczNZjFUoD7PhS0hNA3T7vV41UX_2wOZ-_snplGEqpGmmvX-5ZMbJQ2USsqZwlGG1Wc_cSbt1r92HuYBDZzxi5UFdavc8X5njUe1-_gJebDazWVhHSAw46g3RUddpNP1VsOApLF7grw5Iv5xDmUrV96Tb=w1557-h876-s-no
____________________
“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.

[jeffyjeff]

[lingeringstin...]

Air filter? What's an air filter? I think I saw one once but didn't know what it was for.

Nah, joking aside I will just be leaving it open mouth as is. Been running carbs like that for decades and I can't be arsed to go back to air filter complications.

However it occurred to me yesterday that these carbs have a few different jet tube thingy things to choose from so I'll look into that before anything else. Mine is likely a 105 as I think the two strokes used so I should get a 106 and see how it goes. It's almost there but not quite. I think it won't take much to sort it out now.

[lingeringstin...]

[quote="stinkwheel"]My personal experience has lead me to conclude that all amal carbs are hot garbage. The only thing they have going for them is parts availability. I'd genuinely rather use a Cheap chinese knock-off mikuni.

If you want a cheap mikuni-esque carb, try a mikcarb for an enfield. They are a licenced copy of the mikuni VM24 or VM28. All the mikuni parts fit them too (except the slides for the VM24 which is a mirror image). They would appeal to both your desire for something oddball and your desire for something cheap on account of lots of enfield owners for some reason replacing them with an Amal in the belief it's some sort of performance upgrade then selling the old one on ebay. The VM24 is a flange mount, the VM28 is a stub mount.

I have a couple of oddball Russian carbs here that you'd like but you can't have them because I might want to put them on my Minsk some day. The one I have on it would appeal to you because it's SUPER low profile with an ersatz flatslide made of a bit of bent brass sheet. A K36. They used them on IZH and Voskhods too.
[img]https://lh3.googleusercontent.com/pw/AP1GczNZjFUoD7PhS0hNA3T7vV41UX_2wOZ-_snplGEqpGmmvX-5ZMbJQ2USsqZwlGG1Wc_cSbt1r92HuYBDZzxi5UFdavc8X5njUe1-_gJebDazWVhHSAw46g3RUddpNP1VsOApLF7grw5Iv5xDmUrV96Tb=w1557-h876-s-no[/img][/quote]

Yes the Mikcarb does look very interesting. I might go with that if I can't make the Monkbloc work.

And I have always been interested in those Russian carbs too but I don't think I have ever had a full working one to play with. But whatever fits the space is fine by me as long as it works better than the Chinese PZ I've been using.

[stinkwheel]

[lingeringstin...]

That's mental. So it just clips together or something?

[lingeringstin...]

That's mental. So it just clips together or something? I like the minimalist aspect but does it WORK?

[stinkwheel]

[lingeringstin...]

That's exactly the sort of thing I like.

When I was still running the MZ two stroke engines I dabbled with Amal Concentrics and a few other things and it was all just a terrible ordeal.

My mate had a wee bike workshop in an industrial estate and somebody had thrown an SU carb in the bin at the car workshop next door and my mate pulled it out of the bin and laughingly said "I bet you'll never make THIS work on your bike".

Actually it became the best carb I ever used on that bike. I basically just removed the butterfly and ran a cable down the middle of the slide tube and vented the slide chamber by drilling some holes in the top to let the air escape when I pulled the throttle.

It had no slide cutaway, no idle adjustment or anything and the slide just came to a rest flat down in the carb so I drilled a tiny air hole in the slide towards the bottom to create an air leak at low throttle and screwed the adjustable jet in or out until it idled on it's own. This of course completely "ruined" how the SU is meant to operate but it worked very well once I figured out which needle profile to use, which took literally years of faffing about..

Amazingly that was a really good carb in the end. The fuel bowl had a ticlker so it was easy start and the lack of anything else but a slide, a needle and an adjustable jet meant no complications. Nothing to go wrong or leave me stranded on the road. Not even an idle circuit to become blocked.

The problem is when I went to a four stroke engine the needle profile was all wrong and I was in a hurry and didn't fancy another relentless and time consuming dive into trying out various SU needles to get the right one. Also around that time SU spares started to dry up and they became "vintage"expensive so I just ended up going with whatever was cheap and readily available which turned out to be the fifteen quid Chinese PZ carbs I've been using.

That Russian carb is very much the sort of thing I fancy. I'll spend just a little more effort on the Amal because it's basically almost there and I'd just like to be able to say I finally did it but honestly I like that Russian thing better.

[lingeringstin...]

Ah bloody heck. I have just found THIS information about how Amal jet numbers correspond to bore size:

https://www.coventrysparesltd.com/Amaljetssizes.htm

The problem is, the jet I'm trying to find would have a bore size of roughly .075" (around 1.91mm drill bit size) which isn't on that chart so I'll have to try to figure out how to use the info there to expand the chart until I can get to a jet of my size, and even then I don't know if Amal actually even made one that big. But at least it's a start.

Anybody have a clue what number jets I should be looking for? The rocket science is making my head swim.



I need to guestimate what the one on the right is.

https://i.postimg.cc/SjF6jL6q/c.jpg





After looking at that chart and scribbling on some paper my very rough guess is that I should be looking at somewhere around a 420+ jet. Of course this all goes to pot once I fit a 106 jet tube as opposed to the 105 I currently have, but I appear to be all alone in this wilderness. No sensible person would try to fit an Amal two stroke Monobloc to a slightly modified Chonda engine.

[jeffyjeff]

[lingeringstin...]

Yeah I came up with a ballpark figure for somewhere over a number 400 jet using less specific logic and outright guesswork. I actually thought it might work out to about a 430 or so but your method is going to be closer.

However, I think I have an easy out. Tomorrow morning I'm going to go to the industrial estate where The Old Geezer with the machine shop dwells and ask him if he's got any old Amal jets knocking about that I could measure against my drill bit. He works on vintage Brit bike engines and used to race a bit back in the day so chances are he might be able to help me better pin down the size of the jet compared to my drill bit.

I just want to know what it is in case I end up needing to buy a bigger or smaller jet while I'm faffing about with this carb. Like I said before, it's ALMOST right but not perfect yet and I want to see this little experiment through. If all else fails I'll bite the bullet and order something like a 460 jet and see what happens. Jets don't cost a lot so it's no big deal if I don't get it quite right the first time.

I was out again for a couple of hours this afternoon and it actually went pretty well apart from the slight starvation at just off idle and at top end but I've got a slightly bigger needle jet coming in the post that might help with that.

I'm actually surprised this old two stroke carb works as good as it does on my four stroke engine. As far as I can tell the only difference is the spray tube and jet size, so it makes me wonder why they even bothered making two different carbs in the first place if either one could be made to work on any engine.