Monobloc carb slide

[MalcW]

Hi All

Quick question - is the number on the top of a Monobloc carb slide the cutaway size? Just wanted to check because a mate's Model 31 isn't running quite right. It has a 389 carb with clean jets which are the correct size, and the needle is in the correct slot. The spec. says it should be a #4 slide, but the top of the slide is marked '2'.

I realise of course that this concentration on carburation almost certainly means it's an ignition issue...

Thanks,

Malc

[dave16mct]

Yes it's the cutaway size in 1/16" stages. So a No 3 will be 3/16" and No 4 will have 1/4" cutaway. So your mates bike will be running very rich I would think.
Dave.

[MalcW]

Yes indeed. On a test ride today on a sustained 60 - 65 for a mile or so his bike started to misfire and then backfired a couple of times, very loud and shooting black smoke back at me, following on my model 31. Certainly focussed my attention.

Malc

[MikeM.]

What do the plugs look like, are they very sooty?

[MalcW]

Yes, brand new (genuine) plugs that were fitted immediately prior to the ride, and are black after 50 miles.

Malc

[MikeM.]

According to my works manual the 59 Model 31 had number 3 cutaway and the 60,61 and 62 had a number 4 cutaway, of course it could also be to do with the needle position, the 59 cental and the 60 to 62 number 4 position. I’ve lowered the needle on my 59 Model 31 also put in smaller main and pilot jet as it was running very rich, and that was on a new carb.

[MalcW]

According to my works manual the 59 Model 31 had number 3 cutaway and the 60,61 and 62 had a number 4 cutaway, of course it could also be to do with the needle position, the 59 cental and the 60 to 62 number 4 position. I’ve lowered the needle on my 59 Model 31 also put in smaller main and pilot jet as it was running very rich, and that was on a new carb.

Hi Mike

It's a mate's1960 bike, exactly the same as mine except he has coil ignition and I have a magneto. Mine runs fine with the standard settings so I've ordered a #4 slide to try first, and then make other adjustments if required, as you did. What replacement jets did you fit?

Malc

[MikeM.]

Hi Malc,
I reduced the main jet from 400 to 390 and the pilot from 30 to 20, I have increased the slide cutaway by filing the cutaway away by about 30 thou, nothing scientific in the way I work. The plugs are still sooty but at least the motor isn’t running as rich as it used to.

[Joker_Bones]

Like the idea of filing out the cutaway, never heard that before

[g5wqian]

hi malc ;
my ariel single was running badly and i was heading in the wrong direction at first changing things .
the bike had a flat spot from just off of idle if you were coasting and then applied the throttle and it didnt seem to pull to high revs freely ..

the plug wasnt bad but got black quite quickly and i didnt get crisp throttle openings when bike sat idling .

initially i thought it was lean so put a number 4 slide in and raised the needle , but that made it worse .

i lowered needle as much as it would go , better but still too rich .

what i ended up needing to do was to put a smaller needle jet in and a slide with lower number , and a smaller main jet .

although i have a 376 carb , i changed the needle jet from 106 to 105 and then changed cutaway to a 3 , i also changed the main jet from 210 to a 190 , it now runs nice and has more go and will pull it with one more tooth on the engine sprocket ..

i dont have an air filter and just use a trumpet with bit of gauze in to stop the bugs getting in .

i also have an AJS model 30 , it uses a 376 carb and runs fantastic but one thing i have found is that if i have any backfires especially when bike is not moving , i need to check around back of carb quickly in case its got any flames there , because i was out one day and when i was trying to start it up it backfired and i didnt at first notice a small flame coming from back of carb , luckily i spotted it and blew it out but if i had let it sit there it may have set the thing alight .
the model 30 carb is standard settings .

hope you get it sorted .
ian