Amal Monobloc woes

[lingeringstin...]

UPDATE on using the Amal 375/33 two stroke carb on my Chonda four stroke engine (CMX250 clone).

3.5 slide
"B" needle
30 idle jet
105 jet tube

I believe these are all bog standard fittings for this carb.


The 106 jet tube came in the post and I tried it. I could not tell any difference between that and the 105 I have already been using. I suspect that since I'm running an absurdly large jet it doesn't make a whole lot of difference which jet tube I have at this point.

I went to see The Geezer on the industrial estate to see if he could shed any light on what size jet I might be looking at to match the drilled out 1.91mm.

The biggest Amal jet he had laying around was a 420 so he poked a few drill bits at it and came up with a size of 1.7??. I can't remember what he said but it was basically 1.7-something for a 420 jet.

So the previous rocket science estimate about it being about a 490-ish Amal size for my 1.91mm hole appears to be spot on (cheers for that!) but even THAT seemed a bit small in road tests so to complicate matters I've now found a 2mm drill bit and drilled the jet out even more to see what would happen.

Basically it helped. The carb is still a bit iffy at just off idle (almost dies out like it's not getting enough fuel) but then once you're on the go it works fine clear up until the very top of my useable revs for this engine where it still starves a little bit but you can play with the throttle control and eventually creep the revs up without the engine bogging, but I suspect it's running lean at that stage so best not to push it. But at most routine throttle openings it now seems to work pretty good. Not perfect yet but getting there.

So my current drilled-out jet size (if anybody's interested, and I doubt anybody would be) currently stands at 2mm which would put it somewhere over a 500 jet in Amal terms.

If I can get my hands on a slightly-bigger-than-2mm-drill-bit then I'll have another go to try and clean up the top end starvation, but in a pinch the carb would work OK as is with just a couple of small niggles.

This is of course all pointless and unnecessary. I COULD just fit a four stroke jet blocK in the carb body and do things in a civilised manner with appropriate jets and readily available information but I'm not quite done beating this dead horse yet.

[lingeringstin...]

slight update

I found another needle that was exactly the same diameter as the Amal one and the taper was pretty much identical but a it was overall just a bit shorter.

I only just barely got the bike started and even then it was obviously WAY too rich and only ran for a few moments with me frantically blipping the throttle before it died altogether.

Of course that opens up a whole new can of worms where it might be possible to use a smaller jet than what I currently have but I'll save that sort of fun an games for later if I need to experiment that way. But this got me thinking.

Obviously the needle and the needle jet do play a role in carb operation even at the very bottom. I know there's a lot of information out there basically saying only the idle jet works at up to 1/4 throttle but that's not correct. The needle never fully clogs off the main jet hole so there will always be some fuel draw there. It ain't much but it's enough to make a difference.

Likewise the needle never fully exits out of the needle jet so even at full throttle it does make some difference and you're not just running on 100% jet alone.

I have noticed that unlike many other carbs I've messed about with this Amal design is REALLY picky about the precise length of the needle, and that gives me ideas.

Since I drilled the jet out to 2mm it's beginning to become just a little rich in the midrange. It's not bad, the spark plugs are still a reasonable colour but there's been some definite darkening. I figure that could be sorted out with a bit more slide cutaway but that's not so important right now. It's actually going pretty well at everything apart from absolute top end.

That makes me think I don't need a bigger jet but just more fuel flow at the very top end from the jet I have. Since the Amal is so picky about needle length I'm going to leave the current jet at 2mm (somewhere over a 500 jet size in Amal speak) and play about with needle length. I have a spare needle to faff about with so I won't be ruining anything in case my ideas don't work and there are needles from other Amal carbs that are the same diameter as what mine is but only vary slightly in length and taper so there's scope for experimenting.

I know I really shouldn't be wasting my time with all this but I feel like I'm really close to making a defunct vintage two stroke Amal carb work on an unpopular four stroke engine, not that anybody anywhere would care about such shenanigans. But it's too close for me to give up now.

[jeffyjeff]

For what it is worth, plugging in a 2mm diameter bore into the spreadsheet results in a jet size of 543! I think the R&D you are doing on your Amal is really cool, even if you are the only one who ever uses it. How do you plan to go about re-shaping the carb needle? Sandpaper or emery cloth? Cheers and good luck.
____________________
History shows again and again how nature points out the folly of men - BOC

[lingeringstin...]

I'm going to see if I can get things working better without altering the needle for now. I just got a 560 jet cheap because nobody has much use for that size. That might be fairly close to correct by now. I know I'm almost there jet-wise.

I was lucky enough to get my hands on a #3 brass slide so that might sort out the lean feeling at just off idle. Probably won't do much at midrange and top end. I can always drill an air hole in it if it turns out to be too restrictive. Done that before to liven up a slide cutaway that was too rich and it works well.

It would be interesting to get this two stroke carb working well because I'm beginning to quite like certain aspects of it's design. The tickler makes it the easiest starting carb I've ever had on this engine. And a jet that big will be hard to clog. It's still not a "great" carb but it's an interesting experiment.

[lingeringstin...]

Update for the Amal 375/33 two stroke carb.

Cold start with new 560 jet, needle lowered by a notch, #3 slide, 30 idle jet.

Removed the choke as it's not needed with the tickler.

Bike starts better now than with any carb I've had on it so far.

I haven't actually road tested it yet because I'm fitting a sprung saddle and waiting for some springs to come in the post. I'll review progress after I get my seat sorted out but it certainly seems to be working pretty good so far.

[jeffyjeff]

nice video! you'll be popping wheelies in no time.
____________________
History shows again and again how nature points out the folly of men - BOC

[Easy-X]

[lingeringstin...]

OK so here's the final word (for now) on running an Amal 7/8" two stroke carb on a basically CMX250 Rebel engine.



It's a 375 Amal with a two stroke jet bloc in it.

105 needle jet
560 main jet
30 idle jet
#3 slide with needle dropped one notch
no air filter



It actually works pretty well now. I haven't done a lot of testing yet, only around town with a couple of moments of hooligan shenanigans, but as far as I can tell it's working just fine. It will be interesting to see how it goes over the winter and on long outings but so so far it seems fine.

So the big deal is that you have to run a huge main jet if it's a two stroke Amal carb on a four stroke engine. The very same carb with a four stroke jet bloc in it that is meant for some average British bike of similar engine specs runs a main jet of somewhere around 100 to 150 depending on what it's for.

I suppose as long as spares are readily available this is a viable carb option but these carbs are like three hundred quid brand new so if I was after a new carb I'd definitely be looking at one of those oddball Russian ones, but that's because I'm a cheap bastard that likes weird things.