Tiger Cub Mikuni Carb

[wezzo]

Afternoon All. So my Cub is all together and a runner. Warmed up and tickover set. Starts lovely on the choke from cold and also when warm. However I’ve got a jetting issue. Here are the symptoms

- when I open the throttle gently it runs ok but when I snap the throttle open it muffs badly or stalls.

- I think it’s therefore when it comes onto the main jet?

- after running it for a bit a shiny new pipe is now a gold colour, so getting hot.

Im now about to spend hours researching on the internet, but is someone can point me in the right direction or a good how-to link that would be great 
 

[markparrish]

well, I had a similar issue with my C15 (in recent post) The advice I got was to check no air leak between carb and inlet, which I did. I also stripped and completely cleaned the carb - I suspect there was some dirt or dried out residue in there... there was some visible gunge - especially in the bottom part of the main jet assembly. I seems to have done the trick.

I have a very different carb but I guess these are common places to start from. Good luck

[thai-ty]

I presume you have read the tuning guide for Amals, which is the same as for Mikunis.

Before swapping jets and spending money, i'd have a go at lifting or lowering your needle.

By what you have written it is possibly over rich off idle, hence drop the needle by one groove.

Main jet is 3/4 - wide open throttle only, you have to get there first via slide cutaway, needle position and needle jet.

[wezzo]

Hi. This is a ‘fit and go’ job from Meridan Off-road, however it’s never as easy as that is it. Jets are 

pilots 22.5

mains 100

needle 4D20 middle posn, 3

slide 2.0

I’ll read the Amal manuals tonight, as you suggest, and yes I think it is rich looking at the plug.

This is the first Cub I’ve ridden since I was 9 ( Mum has one), and it’s a project my Dad started 45 years ago and didn’t finish before he died. From the promising start made this week getting it going after an extensive refit, it feels like a lovely balanced bike. 

[thai-ty]

"Fit and go" are always close generalizations and get you in the ball park. It's very rare a carb conversion kit fuels perfectly in all the stages for most bikes.

There's some Mikuni tuning stuff on the net, but they are virtually the same as an Amal.

Needle position and needle jet is the meat and potatoes of throttle position, affecting 1/4 - 3/4, just off idle the slide cutaway affects things too.

I think you'll get there quickly.

[section swept]

As the carb kit comes from a ‘Cub’ specialist, I would have thought that the set-up should to be close for good carburation. A lot may depend upon the state of tune for a particular engine. Certainly as the carb is new a thorough check of the internals would not go amiss....no disrespect to the suppliers. What air filter are you using as the type of filter can have an effect upon fuel mixture. Another consideration, not all engines of the same type are equal; so what suits one may not be the ideal for another. Thai-ty’s suggestions are the way to go, in the first instance. I have to say that I find the Mikuni carb to be longer lasting than Amal....having been brought up on a mixed diet of various Amal types, I found they tend to wear quite quickly. I run a Mikuni on my 348 which had been run by the previous owner with just a wire mesh for a filter and no air box....having made an airbox of sufficient air volume and a dual foam filter element to run with filter oil (the sticky stuff). The jetting had been altered to suit the no air filter and altitude (bike was in USA) with a much larger main jet and different from spec needle and larger pilot jet and in the UK it ran quite rich....as you might expect. Returning to standard jets and needle supplied by In Motion the bike ran much sweeter, very crisp off idle and no hesitation on throttle blip, no staggering or hiccuping and consistent idle. 2T and not 4T I know but I hope  there is some useful info to help. In my experience some Cubs tended to spit back and cough when snapping the throttle open, but as you describe the front pipe discolouring I would say this is normal especially if running the engine in a shed i.e. bike stationary.....its going to discolour unless you have a twin tube exhaust (unlikely). Potentially the float level could have changed due to in transit bumping in the post. Is the engine aTR20 with higher comp. ratio and lower gear box ratios or is it a road orientated engine. The front pipe should end up with a blue band next to the fixing clamp changing to golden partway along the bend and then chrome unless your front pipe is stainless, the colour will be similar and indicates normal running. 

I would suggest that at 100 (this is the standard across the Cub range and 90 also speccd) the main jet is not flowing enough fuel and the weak mixture is causing the discolouration you describe. Try a 105 or even 110 but raise the needle a notch first to see if any improvement can be observed. What bore is your Mikuni carb VM 20 or VM 22 ?  Either way I would be very interested in how you get on...maybe your father is looking down wishing you success, whatever enjoy every minute with the Cub ?

[farmer58za]

21 hours ago, wezzo said:

Hi. This is a ‘fit and go’ job from Meridan Off-road, however it’s never as easy as that is it. Jets are 

pilots 22.5

mains 100

needle 4D20 middle posn, 3

slide 2.0

I’ll read the Amal manuals tonight, as you suggest, and yes I think it is rich looking at the plug.

This is the first Cub I’ve ridden since I was 9 ( Mum has one), and it’s a project my Dad started 45 years ago and didn’t finish before he died. From the promising start made this week getting it going after an extensive refit, it feels like a lovely balanced bike. 

Hi Wezzo
There is a needle jet that is part of the main jet holder, into which the jet needle protrudes. You haven't stated that in the jet sizes above. If the needle jet orifice is too small, you could get leanness and a flat spot at 1/4 throttle, especially where the throttle is snapped open. 

To get to the needle jet, remove the carb slide, main jet and washer, then tap the main jet holder upwards so that it exits through the carb throat at the centre. The main jet holder is also the needle jet and will have numbers on the side, such as P-6, P-8, Q2,  or similar. The number range goes from  around N-0 to R-8. Each number has a specific diameter and diameters increase with higher numbers in the range. Higher number - bigger diameter- richer

Before you take your carb apart, try the following to see if you can improve the response from idle to midrange: Turn in your idle mixture screw 1/2 turn. That will richen your idle and possibly affect the way the bike picks up.

Regards

 

[farmer58za]

On 4/26/2020 at 7:41 PM, wezzo said:

Hi. This is a ‘fit and go’ job from Meridan Off-road, however it’s never as easy as that is it. Jets are 

pilots 22.5

mains 100

needle 4D20 middle posn, 3

slide 2.0

I’ll read the Amal manuals tonight, as you suggest, and yes I think it is rich looking at the plug.

This is the first Cub I’ve ridden since I was 9 ( Mum has one), and it’s a project my Dad started 45 years ago and didn’t finish before he died. From the promising start made this week getting it going after an extensive refit, it feels like a lovely balanced bike. 

Just as a comparison: I was directed to this site today for Mikuni settings for all types of 4 strokes  http://www.braigasen.com/Mikuni_jetting_chart_four_stroke.htm

There are Mikuni settings for a Trials Cub as well.
Regards 

[wezzo]

Thanks for your advice folks. I had a play with the needle and have got close to a goo set up. I raised the needle a notch and it ran much better, raised again and it was worse so I reckon somewhere in between any be perfect. I’ll give it a go for now and tinker once it’s been competed, whenever that may be. I built this bike for the Somerton 2 day trial in June but alas looks like that may be off.

[thai-ty]

On 4/27/2020 at 4:26 PM, section swept said:

. I have to say that I find the Mikuni carb to be longer lasting than Amal....having been brought up on a mixed diet of various Amal types, I found they tend to wear quite quickly.

Concur.

Until.... a few years ago when Amal brought out the Premier range for MK I Concentrics, these are superbly made, different materials, hard anodized slide, removable pilot jet (hooray!) and are just top notch kit, certainly better than the stock versions you can still buy. Night & Day.

I have to say, Amal have really upped their game in the last decade, all the older models like the 376/389 Monobloc and even older stuff is all being remade and IMO, better quality than ever before.