Is there a trick to setting the carb?

[Andymann]

My 3YF was generally running well but had developed a hard starting condition when cold and also an irritating backfire on the overrun. I hadn't done a proper once over since I bought it, so decided the time was right to do a bit of a refresh.

I ordered new inlet rubbers and an air cutoff switch from Yambits which solved the backfire and I found the inlet valves were tight which was the cause of the hard starting.

When I checked the carb I found that the manual said the secondary carb should open at 6mm slide height on the primary - mine was set closer to 12mm and the butterfly on the secondary carb wasn't opening all the way either at fully open. So I ultrasonically cleaned the carbs and carefully set everything back to what the manual said. It was terrible - massive flat spot at 1/4-1/2 throttle. I eventually set the airscrew out 3 1/2 turns, and set the secondary carb to only open at 10mm slide gap on the primary carb. Now the bike runs beautifully around town but at 110-120kph on the highway at 4000rpm surges like its running out of fuel. Never used to have that issue, although once through the surging will pull up to 5500rpm in 5th which it never did before. So pretty much running better than it did, apart now from that surging and even momentarily die-ing 3/4 throttle.

I never changed any of the jets and left the secondary carb needle clip on the lowest groove - so the needle in the richest position.

Anything else I could have done wrong?

[motorang]

The mm setting is only a starting point, I would fiddle with the setting until you have a sweet ride :-)

Float chamber level is quite critical in that regard.

Andy

[Andymann]

Thought I would document what I have found so just so that others have something to work off!

I ordered a jet-kit from 6Sigma in the US - they have been super helpful so far.

My bike is mostly ridden at 1350m above sea level and has a free-flow tailpiece.

Replaced the standard 130 on the primary carb with a 140 supplied
Replaced the standard pilot 48 pilot jet on the primary carb with the 50 supplied.
Set Float height at 27mm.
Air screw 2 1/2 turns out


Set the needle on the secondary carb to the middle position.
Secondary carb main jet is the standard 165 for now.

Bike starts on the button now whereas before it took a few goes - especially if standing for a while
Much nicer around town - pulls really well.
Most importantly, the backfiring on the overrun is much much better - still there, but more of a pop now than a bang.

However, that irritating flat spot at 4000rpm on the Freeway still exists - you can ride through it but if you cruise at 110kph which is about 4000rpm it feels like its running out of petrol.

I'll play around with the float height this coming weekend

[donnybach]

I’d check the fuel height in the bowl, but the motorway speed at 4000 shouldn’t be affected. More main jet. For what it’s worth my 1vj runs a 150 main. But the secondary is only 125.
Throttle open fully?
All the electrics fine? No air restrictions?
I found half the sigma useless as the keys were smaller than standard. But modified the secondary as suggested. Have you drilled the holes in the secondary slide yet? Mine picked up better after these mods. Def suggest a larger main though.

[Andymann]

So I will add that the bike was running really nicely apart from hard starting when cold, coupled with lots of black smoke when it did start, and the backfiring which was really irritating - someone had also fitted a pipe from a Raptor 660 so it was really unpleasant.

All I did was set the valve clearances (which were tight) and replaced the Air cut-off valve and the inlet manifold rubbers - and reset the secondary to open at 6mm - just that caused a whole world of issues. But to answer your question - no electrical gremlins and apart from a now proper tailpiece, everything else is standard.

As for the 6Sigma kit - it came with two primary carb jets - a 140 and a 145 - standard was 130 so I have the 140 in now - I think I will try the 145 over the weekend.
As for the secondary - 6Sigma recommends a few things - drill a hole in the CV piston, weaken the spring by cutting a coil out and (and this is where I am a bit worried) drill out the secondary main with the drills provided in the kit. Now my Secondary jet is already a 165 so drilling it out will make that circuit richer.

Interestingly, I mailed Motoritz in Germany and they say I should go down a size on the Secondary, not up - which kind of ties up with what you say - your secondary is pretty small. The bike also was running with the secondary needle clip on the lowest position - to counter rich running so I really am hesitant to drill out that jet.

[donnybach]

I’m with you on the secondary. I’d lower the needle to middle or just one up with a smaller jet. Sounds way too rich.

[donnybach]

Just looked at my old post. I did drill the jet to first size, raised the needle 1 clip, and drilled the slide holes. But with the 125 jet. Again this is a 1vj, so there’ll be differences. Can’t see there being a massive one though. Keep us informed.
Just reread a lot of the posts. I answered stuff already answered etc. beer involved.

[Andymann]

No stress! its at least great to have someone to chat to - even Moritz says the 3yf is quite a rare bike!

So the plan for the weekend will be to set the float height again, but I'll use the fuel level measurement - not the height of the floats like I did last time.

I'm not going to drill out the secondary jet, but I am going to drop the needle to the lowest position - to counter the bigger jet. and try and lean it out on the secondary carb as much as possible.

I'll drill the extra hole in the piston too.

Finally, I'll set the secondary to open at 12mm not six like the book says.

Hold thumbs!