Amal Carb Jetting

[Mike]

Hi folks - sorry for the slow response - I think you're confirming my surprise at having a rich condition here. The carb was new when I installed it in 2016, and to be honest - the bike has done about 20 miles since then, so there shouldn't be any issues with component wear on the carb

Have you put some fresh fuel in? And presumably you are using E5?

I did put fresh fuel in - initially Super (E5), but then noticed that this caused it to run richer still, so drained the tank, and replaced with standard 95 (E10). This appeared to have a ~2-3% reduction in CO measured at the exhaust.

And the basics of timing, valve clearances are OK?

Not checked recently, but was previously ok - it's running sasche ignition, aligned to TDC, I'll re-check the timing and clearances at some point soon, I think.

Where is it running rich though? The main jet only affects the engine when the throttle is 3/4 open, below that it is the needle and needle jet. If you have not done so, try dropping the needle down a notch and see if that makes any difference,

That makes sense, thanks. I've been measuring purely at idle - however even at idle, there is definitely a significant difference by changing the main jet size (160 blows soot out of the exhaust, 140 does not). The needle is currently at the mid position, I'll move it to the top position and see what happens - off the top of my head I can't recall what the needle jet size is (I have it written down at home)

plus (just checking) you do have the choke slide open all the way (the lever should be fully tightened).

I have no choke slide fitted.


Thanks,
Mike

[themoudie]

Aye Mike, noticed you bide in "Central Scotland", I'm up near Perth.

As for the carb, having a 450 MKIII, I've only ever run a Dellorto 29mm that was soon replaced by an AMAL MKI 32mm for reasons of spares/cost back in 1976. Now running, with a AMAL MKII 32mm.

I run E5, or alternatively E10, with additive if E5 unavailable. If either E5 or E10 are lying for more than 8 weeks, without additive in other machines that I run, their efficacy appears suspect! Some of the additives in E10 also appear to be biased toward "lean burn" engines and their combustion deposits quickly accumulate on plugs, valves, etc. I have not personaly had problems, but have seen cylinder heads/plugs on '70's engines that didn't appear too healthy!

I assume that you do not have an air filter fitted? But note that you do not run a choke slide, the same as I used to on the MKI.

Is your carb a RH 26mm 4 Stroke MK I Carburettor Part Number: 626/300, or a Premier RH 26mm 4 Stroke MK I Carburettor Part Number: 626/300PREM?

The former's spec. is:- 3 Cutaway Slide, 160 Main Jet, 106 Needle Jet and a Pilot Bush

Whilst the latter is:- 3 Cutaway Slide Hard Anodised, 160 Main Jet, 106 Needle Jet, and a Premier Pilot Jet

With the latter Premier carb you can change the pilot jet size and adjust the pilot needle. Whereas with the Pilot bush carb you'll just have to rely on adjusting the pilot needle.

I found with a No.3 slide that I couldn't get the engine to pick up cleanly, with E fuels! I realise mine is a different beastie, but found that a No.3.5 slide, needle clip in the 2nd from bottom notch, needle jet: 106, pilot jet: 25, pilot screw: 1¼ off the seat. Main jet is a 230, work for me!

Is your float level correctly set and is the float needle sealing against it's seat?

Do you have a fuel filter between the tank and carb, to prevent any paint/rust flakes finding their way into the carb?

The AMAL "Carburetter Settings and Spare Parts list" 11th Edition, has a table of settings used on MotoTrans machines to which AMAL 'pattern' carbs were fitted and the following are the settings used in the 24 Horas carb:

Carb: R267/424*, main jet: 376/100-230, pilot jet: 124/026-20, needle jet: 622/122-105, needle position: 2 (middle notch), Throttle valve (slide) 622/0603.

There are other carb models and settings for alternative 250cc models that MotoTrans produced.

AMAL's current catalogue is available from this link: AMAL_carbs_link

I hope this helps, good health, Bill

[Mike]

Hi Bill - I'm down in Linlithgow

I assume that you do not have an air filter fitted? But note that you do not run a choke slide, the same as I used to on the MKI.

I have an air filter that screws into the carb intake. It doesn't seem to significantly change the running behaviour, but I feel comforted that it reduces the risk of small animals and children being pulled through the engine from the raw power of the induction stroke

Is your carb a RH 26mm 4 Stroke MK I Carburettor Part Number: 626/300, or a Premier RH 26mm 4 Stroke MK I Carburettor Part Number: 626/300PREM?

It is a 626/300 - I do believe that I read a recommendation for this model in a conversation on this forum, many years ago. I also spotted that they are used on Triumph Tiger 500 twins, which seemed to suggest it to be about right for my wee 250.

The former's spec. is:- 3 Cutaway Slide, 160 Main Jet, 106 Needle Jet and a Pilot Bush

Yes, that's how it was delivered. It still has the 106 Needle Jet, and I previously downrated the main jet to 140 (and still have the 160 somewhere).

Is your float level correctly set and is the float needle sealing against it's seat?

Good stuff, thanks - more things to check!

Do you have a fuel filter between the tank and carb, to prevent any paint/rust flakes finding their way into the carb?

I don't - from memory, I think the petcocks have strainers on them, and have been relying on that.

Thanks again,
Mike

[ranton_rambler]

Although I said I have limited experience with Amals, I do have one on my pre-war Rudge.
It was a terrible starter and would regularly foul its spark plug and stop when I first got it last year.

The biggest single improvement was from lowering the float level. In my ancient carb, the float needle has 3 notches to clip the float to and I have moved it to the bottom notch i.e. lowest fuel level.

[Mike]

I've found this useful document for the Amal, which I'll work through:

https://accessories.hitchcocksmotorcycl ... 4&_xt=.pdf

On float level, some google-obtained internet wisdom appears to suggest that float level is less of an issue for the Amals, but none-the-less, I think worth checking, particularly with an explicit eyeball of the float needle.

I expect I'll carve out some time this weekend to have a look.

[Mike]

Float level is absolutely fine, as was the float needle. Sasche ignition timing reset from first principles using degree wheel and piston stop. Will check the clearances tomorrow, but I'm not expecting a problem there.

It starts easily enough and idles with a little trace of white smoke, but as soon as you start turning the throttle, the smoke from the exhaust increases and it starts to struggle, like it's far too rich (which was confirmed with the Gunson CO meter).

The carb is spotlessly clean, has fresh (E10) fuel, and is running a 140 main at the moment. Everything I can find seems to suggest that it should be running a larger main, though the behaviour seems to suggest that it wants a smaller one.

This happens with or without the air filter fitted, so there is no problem with air starvation.

I'm stumped here. Does anyone want to buy a 250 before I torch the damn thing?

[blethermaskite]

Mike, what colour is the increasing smoke from the exhaust?
Cheers,
George

[George]

Float level is absolutely fine, as was the float needle. Sasche ignition timing reset from first principles using degree wheel and piston stop. Will check the clearances tomorrow, but I'm not expecting a problem there.

It starts easily enough and idles with a little trace of white smoke, but as soon as you start turning the throttle, the smoke from the exhaust increases and it starts to struggle, like it's far too rich (which was confirmed with the Gunson CO meter).

The carb is spotlessly clean, has fresh (E10) fuel, and is running a 140 main at the moment. Everything I can find seems to suggest that it should be running a larger main, though the behaviour seems to suggest that it wants a smaller one.

This happens with or without the air filter fitted, so there is no problem with air starvation.

I'm stumped here. Does anyone want to buy a 250 before I torch the damn thing?

The main jet has no effect until throttle is 3/4 open unless it is exceptionally small, in fact if you removed main jet it would make no difference from tick over to 3/4 throttle opening. I mark throttle at closed, 1/4, 1/2, 3/4 and wide open to give some idea of which jets are supplying mixture, Up to 1/4 pilot jet, 1/4 to 3/4 needle jet, 3/4 to wide open main jet. With some overlaps. If your CO meter is showing to rich at 1/4 + throttle as you say try previous suggestion of lowering needle one notch. Good luck

[themoudie]

Aye Mike,

Following on from George's response to your last post I re-read your post dated 10th, June in which you say:

The carb was new when I installed it in 2016, and to be honest - the bike has done about 20 miles since then, so there shouldn't be any issues with component wear on the carb

In your intial query you posted:

It starts and idles just fine, but it is clearly running quite rich. I have a Gunson gastester device, and it's showing about 8% at idle.

Then in your post of 15th, June you say:

It starts easily enough and idles with a little trace of white smoke, but as soon as you start turning the throttle, the smoke from the exhaust increases and it starts to struggle, like it's far too rich (which was confirmed with the Gunson CO meter).

I suspect George (blethermaskite) is onto this and I too, having referred to your earlier posts think that the reluctance to rev cleanly has nothing to do with your carburettor, or the engine running rich. The symptoms point to either the piston rings are not sealing correctly in the bore, due to having had less than 20 miles use since 2016. Or, alternatively, the valve stem rubber seals have perished and oil is being sucked down between the guides and valve stems, or both faults exist!

Have you taken the plug out and checked it's colour, I suspect that it will be wet and oily! The Gunson won't pick that up, just the increase in CO, as more oil gets sucked into the combustion chamber. I should add that the only Gunson device that I own and use is their Colourtune dating back to the 1970's!

This may not be welcome news and I would park the Gunson for the time being and check the spark plug heat range and colour.

The recommended spark plug by NGK is a NGK B6HS. However, I found this too "hot" a plug, with blistering of the insulator. Nigel Lacey, recommended using a NGK B7HS, or NGK B8HS as these are "cooler" running and having switched to a B7HS, I found that the 450 ran much better, the blistering of the insulator stopped and had a good salmon pink colour ring at its base that I believe indicates a good mixture, at ¼, ½, ¾ and full throttle, when held steady, with a load on the engine at those throttle openings, as George suggested in his post.

If your plug is wet, black, oily, or any combination of these symptoms, I would suspect oil coming into the combustion chamber.

As for:

I'm stumped here. Does anyone want to buy a 250 before I torch the damn thing?

I'm just up the road! I hope that you are able to sort the wee dodger and enjoy it. I spent 2 years battling my 450 to get it to run and in the end replacing the points and AAU, with Hall effect EI proved to be the answer. That saga is documented on another thread on this site, if you are interested?!

Good health, Bill

[ranton_rambler]

If the needle is shaped like those in Dellorto carbs, there is a parallel section for the first part of throttle opening.
Raising/lowering the needle may affect the point at which the taper starts to take effect, but the other thing which affects small throttle openings is the needle jet itself i.e. the annular space around the parallel part of the needle.
I don't know about Amal, but Dellorto publish the needle profiles so I was able to make a spreadsheet showing the relationship between annular area and throttle opening for the needles that I had, so I could work through them in a sensible order. I bought a "knock-off" pack of 10 various needles for roughly the price of 1 genuine one. They measure up very accurately!