Carb flat spot

Forums

After having the monobloc of my Model 50 sleeved, I tried setting it up correctly and seem to have done it nearly right.

I've been treated with a bike that actually idles when I stop at traffic lights and doesn't even change revs (as much as it did) when I park it on its sidestand. It starts perfectly too and behaves like it should. When pulling away, there's no hick-up when opening the throttle, just a direct response. Weird, isn't it?

So, that's all I wanted to say really...

Of course it's not. Nobody bothers posting here when not in need of advice. And I could definitely use some.

It's all as good as said above, but when I pull away, there's a barely noticeable flat spot, just about where the throttle is 1/8 of the way up. It doesn't stutter, it doesn't fall dead, it just seems to do nothing at that throttle opening. It doesn't take much pulling it through, just a fraction of throttle. In fact, many might not even notice.

Still, I've never dreamt of it running so smoothly and I'd like to get this last bit sorted too to make it perfect. It's definitely rideable and it doesn't really irritate me. I just want to see if I can get it perfect, since I've come so far.

If anyone's wondering; I've left out jet sizes on purpose. That's because I do really want to get some insight in the workings. The how and why, so to say. And as it's old stuff, you can never be sure they've not been reamed. So mentioning numbers could theoretically mess up advice.

I'd really appreciate any thoughts on this matter.

Cheers!

Tim

It will be nothing...

It will be nothing to do with main jet. You might try raising the needle one slot to richen the mixture as you are describing a slight weakness.

You are entering throttle…

You are entering throttle cutaway territory, you could try tweaking the pilot airscrew in a tad to richen or a throttle slide with a smaller cutaway.

Throttle slide

Cheers both!

From what I've read in old manuals, the throttle slide would be the best suspect indeed. However, it has always worked with this one before and now that it is sleeved, it's the one thing that's not easy to change.

I've set up the airscrew by ear first, which was decent enough. But after checking with a colortune plug, it was still quite rich, so I turned the screw out until orange changed to blue. It improvement starting even more (just a slow kick on full advance -it's only a 350- is enough after a tickle when cold). The flat spot remains, but didn't get worse.

I'll have a look at the needle, but I thought the needle only had effect on bigger openings. The plug is on the dark side (black, but not sooted) as it is already. Not really 'coffee with milk' as it should be.

I remembered something though... The 're-sleever' changed the body for another used one (at my request), as I discovered mine was too small for this bike. Regretfully, the quality of that body was less than the one that I had. The float pin (where the float pivots on) was loose in the body, causing massive flooding and extreme richness. The tried and tested splinter-of-a-match trick secured the pin in the desired position, but the brass float was still barely off the carb body. I've folded a strip of brass on the worn out float needle area, which did the trick.

I might have overdone that, causing a too lean overall mixture... But surely, the spark plug (B6ES) then must have been near white and not black and -apart from that tiny flat spot- it does everything perfectly.

Please clarify. If you have…

Please clarify. If you have had the throttle body relined with a sleeve and has this sleeve has been bored to suit the original throttle valve, if so a smaller cutaway valve should also fit.
If this is the case you could then modify the smaller valve by increments to suit your want.
All of your efforts and requirements will be subject to the condition of the rest of the engine and its components such as wear and tare, tolerances and the like. Having had the carb' sleeved are you sure the clearances have been maintained by the machinist.
Without taking the time, effort and money to ensure that all of the above is as it should be perfection may just be a touch from ones grasp and as in most cases a compromise in ones aspirations needs to be taken into account.
Best of luck and ride whilst the sun is still doing its best to cheer us up.

It's common to set idling…

It's common to set idling mixture "correct" by Colortune, then find you have to make it richer to get good running.

It strikes me that, if it…

It strikes me that, if it starts with 'just a slow kick at full advance' when cold, then the ignition is a little retarded. 'Only a 350'? I've had mighty kick-backs from a 350 in the past, forgetting to retard it first before kicking over! Maybe the carburettor settings are not the place to look?

On my BSA M21 600 SV I had…

On my BSA M21 600 SV I had an ignition control and I would set that to retarded when starting then advance it when running and the engine note would change and speed up. Could there be a confusion between tight / slack advance?

In order of appearance...

@Paul
I messed up there in my explanation. The body has been bored out, but not fully, to keep some meat for structural integrity. The slide has been turned down and fitted with a sleeve. The man really did a nice job. It's just a shame he didn't check the fit of the float pin.

The bike is very nice as it is now, I just want to see if I can get even closer to perfection. It doesn't use oil and has healthy compression.

@Michael
That's something I thought about too. As all I hear Monoblocs are all about compromises. But the flat sport before and after colortune is the same. But you have me doubting now. Might try richening the mixture again (it was only a quarter turn) to be sure.

@Ian
My bad (again). When I want to prevent it kicking back on full advance (because I need to tighten the lever and I keep forgetting that), I really go for it with all by weight and speed. Normally it's second or third kick.

But yesterday, it started first kick, again and again, cold or hot. Wich gave me confidence to just give it a normal kick. But real point is that it started very easily.

@Mark
There is a big difference in advance and retard. It does slow down when I retard (tighten) the lever.

@All of you
It's really near imperceptible. But if I can do better, I'm always keen to learn. I'll try the pilot screw first and if that doesn't work, will try to figure out the actual fuel level inside the float chamber. Won't be in the next couple of days regretfully, but will report back.

Hi Tim, I have the same…

Hi Tim, I have the same carb restoration on a 99 and as you say it has transformed starting and the idle . The flat spot could also be richness as the sleeving tends to produce that .On the 99 i had the same issue of a flat spot just off idle .It has pretty much gone once i overhauled the advance and retard ,you could check your timing .If it is weakness you could consider richenning the pilot by tuning the colortune from blue towards yellow a bit. The non adjustable Bipass drilling supplies a fuel/air mixture for the transition off the pilot but can be richened by increasing the pilot jet size .Not so easy to change the cutaway on the slide . Definately do a fuel level check with some small bore tubing on the pilot jet . A plug chop at 1/8 th throttle may give some clues as to rich or weak.A long steep hill run is good for this .

No hills in the Netherlands

No hills in the Netherlands Robert, but I know what you mean. Run the bike for a while at 1/8 throttle, then cut it off immediately and take the plug out.

Nice trick with the tube on the pilot jet! I'll definitely do that. And the timing will be done too, though I have to look up how far -fully advanced- before TDC it should be. I think I once set it on 3/8", but different books and manuals say different things.

Right. So, first a fuel level check, then a timing check and then see what else is needed.

Easy starting and good…

Easy starting and good tickover means pilot must be OK. So the standard Amal checks on slide are described by Amal. EM Franks writes: Run it on small throttle at the flat spot. Close air slide slightly and note if running improves or worsens. If it improves, that tells you it is on the weak side, and you need a smaller cutaway. If it gets even worse, you need larger cutaway.

If your slide has been sleeved to a non standard size, that leaves you with a problem if it is weak and you want smaller cutaway. If it is rich, a larger cutaway can be fixed with a file. I don't know if a smaller cutaway can be achieved by filing off the flat bottom by 1/16"? Adjusting the needle might achieve nearly what you need, at zero cost.

No air slide

Thanks David. No air slide on this one, but still a good idea. I can slide my hand in front of the bellmouth instead.

The cutaway has not been messed with in shape or height, but if it IS the area that needs altering, your idea of taking off some of the bottom might be interesting, though I have to see if it's as easy as it sounds.

But the slide will be a last resort and I might leave it well alone. First float height and timing. Then we'll see, but I do appreciate al the insights given. It definitely gives me a better idea of the workings of the Monobloc, while also providing me with confidence to try some things and know what to expect.

Norton Owners Club