Carb flat spot

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.

@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.

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.

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.

Right, I surprisingly had some time today, so I took a bit of clear tube to attach to the pilot jet.

A bit of wiggling saw the fuel level rise up to... not where it's supposed to be. I took an old float chamber cover with 'level dot' and held it next to the one that I've put on. Seems the level should be flush with the bottom of the 'ma' letters (the photograph is angled, the carb is not).

As the bit of extra copper strip on the float isn't really easy to make thinner, I took the file to the Viton tipped brass float needle instead. I've always suspected it was longer than the original plastic Monobloc one anyway. Before filing, I did check if I had a spare though!

The fuel level is now pretty much near (but definitely not over) where it should be. Starting is harder now, but the flat spot has gone. It feels less fast in acceleration now, but I know that a flat spot will let the picking up after that feel like massive power. Evening things out will make it feel less spectacular.

I guess next start, I'll try tickling a bit les and don't let it overflow. See if that makes starting better. Now time for a cooldown so I can get the plug out and set the points and timing.

A 200 km test ride today went well. Yesterday, I did the timing, which revealed it didn't advance properly either. With that sorted and the fuel level as it should, the bike runs very nice.

It was 18 degrees Celcius outside and I decided not to tickle. It started first kick with a steady tickover. No tickle perhaps indicates the pilot mixture being a tad rich, but making it weaker made picking up less brisk.

When pulling away, it picks up well now and the flat spot has definitely gone. After a couple of hundred kilometres and being soaked to the bone because of torrential downpours, it was a real blessing to have a bike that ticks over relaxed when stopping for a crossing, while picking up without the risk of it stalling when pulling away. Such a luxury!

All the advice above definitely gave me a better understanding about the goings on inside the carb. Valuable knowledge, so thanks for that. But as it turned out, it was the float bowl fuel level that made the difference. 

Laying up for winter with modern fuels asks for some preparation indeed Matthew. I drain the petrol and fill with Aspen (synthetic fuel). That stuff is amazing. One kick and it'll start. I discovered it when using the 2-stroke variant for the chainsaw.