Forum

[Durham Tim]

Standard Bullet 350 Classic with our hosts Indian-made freer-flowing exhaust and pipe.

I recently bought the Mark 1 Concentric Amaml carb from our hosts and had it fitted to the above, by an experienced motorcycle mechanic. (I didn't fancy doing the job myself!!). Hitchcocks actually supplied me with the 28mm version as they didn't have the 26mm version in stock at the time. Although my bike has the standard head, I was assured it would be ok.

I noticed a definfite improvement in how the bike was pulling on the first few runs out, but now it doesn't seem any better than when it had the standard mikcarb. (It is actually harder to start and I think it's slower than before, to pick up when I initially roll on the throttle).

Once or twice it has spat back in the carb.

After doing a bit of research, I did a "plug chop" earlier today, i.e. ran the bike for a good mile or so at full throttle before pulling in the clutch and killing the engine. The plug tip and firing end is very white as opposed to "coffee" coloured. (Certainly lighter in colour than any of the reccomended colours on our hosts notes).

Am I right to assume the bike is running lean?
Is the best first approach to "raise the needle" and "lower the clip" on the carb?

I'm led to believe I might have to remove the tank to get access to remove the top of the carb, but other than that, I can't find any diagrams to show me how to do this. As my title suggests I'm a novice and don't want to make the problem worse.

I imagine there is also a potential problem of burning the valves by running too lean?

Any advice would be very gratefullt received!! Please don't tell me I should have left the original carb on!!

[Dennis C]

Hi Tim, it may be a good idea to read our hosts technical notes regarding the Amal carb, in short doing a plug chop at full throttle only gives info on main jet size, as you say it appears lean then you need to fit a bigger main jet, then do another plug chop after running a couple of miles at about half throttle this will give you info on the needle position, often spitting back when opening up from idle indicates the mixture screw is set too lean or the slide cutaway is incorrect.

[Madoc]

If it spits back on blipping the throttle then you _usually_ want to go for a richer slide.

If it's an amal, then you now have the "air lever" (or choke) on the handlebars, right ?
What this handy device allows you to do is to reduce the amount of air (i.e. richen the mixture) as you are going along by lowering a slide into the carb

With this mind, while stationary, observe where the air slide starts to drop into the carbr and mark that on your lever (or remember it). Go for a ride.

When flat out, start to lower the air slide (using the lever).
If your main jet is about right, then the bike will immediatly start to bog down as the mixture becomes richer. If the bike starts pulling better then it's running too lean in the first place and a bigger main jet is needed. Of course, if it makes no difference then you are way too lean

This works at all throttle openings but unfortunately different parts of the carb do different things and a variety of speeds. It's hard to tell whether idle jet, slide, needle or main jet are at fault.

I would usually mess with main jets on the wide open throttle first and get that about right.

Keep an eye on the colour of the plug as well.
Modern plugs don't like running too rich for any length of time ..

This is an art, but it will get you close.
The scientific way is to pay for dyno.

[Les H]

The carb should have been set up with the correct size main jet which is obviously too small as it is. Moving up to a larger carb can sometimes mean a larger main jet is required. In your case the 28mm carb normally fitted to the 500cc motor has only 350 cc of air flowing through the venturi and sucking at the main jet when the carb throttle valve is fully lifted whereas if the same carb and jet is sucked at with a the 500cc then a larger depression is created which lifts more fuel even in proportional terms (mixture strength richer)) than the same carb fully open with just 350cc flowing through the venturi. You also have the open exhaust that weakens the mixture too. It seems that another 20 or 30 on top of what you have already in the main jet size might be a good starting point. Possibly the good news is that you can fit the main jet by removing the float bowl (posidrive screwdriver from below) be careful the gasket breaks easily, and check the main jet that the main jet screw in is not loose as that can suck in air at the joint. Mucking about with the needle is fiddly so get the main jet right to start with and you might find you can leave the needle as it is as the whole fueling range will become richer apart from tickover.

[Les H]

Correction: "and check the MAIN JET HOLDER that the main jet screws into is not loose as that can suck in air at the joint".

[Alan R]

Hello there ------------ on the Mk1 Concentric you should be able to change a main jet by removing the drain screw at the bottom of the bowl and employ one of our hosts jet spanners. --------------PART No. DESCRIPTION PRICE
AM/622/104 MAIN JET SPANNER £8.00
------------------------------------ I'm sure that on the race track or the roadside etc you don't have the luxury of time to fiddle with the bowl screws.

[Alan R]

Hello DURHAM TIM ----Your "plug-chop" readings are quite correct-----that's a classic lean (or weak) plug reading and you are right to be concerned about burning the exhaust valve, this being the hottest part of the whole engine. You don't mention what air filter you have on as this also makes a big difference to Fuel/Air mixture ratios etc. I'm assuming that you have the correct plug fitted ??---OK, to start with find out what size main jet is fitted----add 10% to that No.---there's your new main jet size to re-fit and re-test. You now know that you won't be getting any weaker so that's a good thing. The GOLDEN RULE with carburation test and adjust is }-----ONE THING at a time !!! and if it helps, keep a log of each change that you do. I would suggest that you check to see just where the needle is set at the moment. Normally you can expect to find 5 grooves in the head of the needle and convention is that they are numbered from top to bottom. Start with the clip in the No.3 position--- this will allow you to adjust either way as needs require later on. OK---I know I said one thing at a time but just now this should get us at a reasonable start position. Go and do your MAIN JET plug chop and see what this new size gives you. It does require a bit of patience and self-discipline but hey!! --- you get to go for a nice ride or 3 and that's a GOOD thing---right ?? PS---If it helps I went through all this shenanigans last year on a 1975 Honda GL1000 Goldwing. And that also included calculating, making and fitting shims to raise each of the 4 needles to help compensate for mid-range weakness after up-sizing the 4-carb assembly. GREAT FUN !!!! ( not ).

[Durham Tim]

Thanks for all your replies!!



Alan - You mentioned the air filter? It's our hosts standard conical, which is actually supplied as part of the Amal carb kit. You also mention the jet tool. If I use this, is it simply a case then, of just screwing the old jet out and screwing the larger one in as a starting point?? I'm anxious not to make the situation worse.

Many thanks.

[Alan R]

Hello there DURHAM TIM, ----- Have a look at our hosts catalogue. It gives suggested jets for various different scenarios. If you still have the box the carb-kit came in from Hitchcocks you should find the Blue & White AMAL box that held the carb. On this MIGHT be details of the carb set-up. Compare this with the suggestions in Mr.H's catalogue. Now you will know what bits to order----and you haven't even got your hands dirty---yet !! Yes, the jet spanner should remove the main jet ( you might have to round-off the outside edges of the spanner---depends how close a fit it is into the drain hole.). As LES H also mentioned the main jet holder may come out/ or be loose. I normally make use of my 1/4"sq. drive BA socket set to replace this item first with some THREADLOC for extra security. When replacing the main jet use just a firm "nip"---as it's brass it'll grip securly. Just as an after thought---- you have our hosts free-flowing exhaust system so you might try increasing the silencer back-pressure a bit by altering the "tangs" within the mute, but do that as the next step after fitting the larger main jet.

[Alan R]

--------------just re-read my last post. Hitchcocks carb settings are for the MIKCARB I believe but you have the AMAL which I think I'm right in saying use different Nos for their jets?? Give our hosts a phone call---they're "quite nice" really and don't bite---much. Tell them your probs. and see what they come up with. You can order your new items at the same time----now that's very "green" of you. Sorry about the mix-up there.