Carb conundrum

[muskrat]

 G'day GF,
              might sound silly at this late stage but does it have a filter in the hose banjo at the float bowl?
 A low float height will affect all but idle. Only need 1 tap to  run. The left one should be reserve.
Cheers

[GoldenFailure]

Hi Muskrat

It does have a filter in the hose banjo, but I have cleaned it with carb cleaner and blasted it with an air compressor, is that what you meant, that it might be blocked up?

Would a low float height make such a huge difference? Ill strip it tomorrow (for what feels like the millionth time) and measure it.

[muskrat]

 Might have to do a bit of plug chopping. Get some whiteout and mark the twist grip and throttle housing, now twist the grip to full and mark it. now mark 1/2 way between two marks and do the same again. You now have 4 marks. !st sector is slide 2nd the needle jet 3rd is clip and 4th is main.
 Get it up to temp (at least 5 min running) and find a gentle hill. Hold it in 3rd or 4th at the desired mark for at least 200 yards then pull clutch and kill(stop) motor. Check the colour of the plugs, deep down the porcelain. Tan good, black rich, white lean.
 Another way is to hold on a mark and close the throttle a tad, it should slow. If not it's too rich.
 Then I saw you have a choke. At each mark put the choke on to see if it gets better or worse. Better your lean, worse your rich.
 I don't enjoy setting up new carbs but once done right it's worth it.
Cheers

[Triton Thrasher]

The mixture is governed by the parallel portion of the needle in the needle jet at around 1/4 throttle, maybe less throttle with a small carb on a big engine. Needle jets wear quickly and used ones are not advisable for re-use.

Setting float level certainly is not done in relation to the tickler. I expect this forum has lots of info on float levels. There are various recommendations.

Another thing that can cause intransigent richness at low rpm is use of a sporty camshaft, perhaps with big valves. I don't know if you've got that in your bike.

Has anyone mentioned the danger of piston seizure or holing with a weak mixture at large throttle openings?  That's why we make sure it's too rich (so rich that it runs badly), then work our way down from there. I expect you live far away from me, up here on the north coast of Scotland, but I have a pretty big collection of main jets after nearly 40 years of arsing about with old bikes. If you tried my spare 350 jet and it was still weak at full throttle, then we could be pretty sure it's not the jet.  There must be similar losers to me,  with piles of jets somewhere in your area.

[Triton Thrasher]

Are you happy that the ignition timing is correct at full advance?

[GoldenFailure]

Ok. So I finally got a nearly correct carb, its a 376 monobloc, 1 1/16th bore, which is right for the swinging arm model, mines a plunger, so should have the 276, but I'm guessing the engines are so similar that it doesn't matter.

Its got a 3.5 slide, 106 needle, 25 pilot, needle on the richest groove and my drilled out to maybe 230 main jet, but its still frighteningly lean (burnt white plugs) at every throttle opening.

I haven't checked the timing yet but apart from the lean ness it starts and runs fine. Maybe it has scary cams, I have no idea.

My only thought so far is that there is no air filter (missing rubber knee thingy) but its jetted as per the Haynes which probably presumes there is a filter. Would this make such a huge difference? and if it is a lack of filter, is anyone running a similar set up who could recommend jet sizes for no filter, monobloc, and (I guess) standard silencers?

Your continued advice much appreciated.

[beezermacc]

Hi, The lack of an air filter shouldn't make an enormous difference, however going up a few sizes on the main jet is a sensible precaution against holing your pistons. Maybe you have a peaky cam and lumpy pistons in which case the settings should be more like those for a Super Rocket. I have a 1958 A10 Golden Flash in standard trim which I run without a filter and standard jetting and the plugs look OK. The latest iron heads were dsigned to run with a 389 - you can tell which head you've got by measuring the inlet tract. On the other hand the alloy head models have specifications with/without air filter showing a big difference in main jet size, 270 with filter, 410 without. I have a RGS 'lookalike' without air filter and I run the 410 main jet so there is considerable scope for variation and experimentation. I'm assuming you have the correct plugs. I would be inclined to keep going up the main jet range until the plugs are an acceptable colour after a short,fast run then deal with any fine tuning at a later stage, which is broadly what Amal used to recommend.

[GoldenFailure]

Hi Beezermac. Cheers for the advice.

I'm getting closer I think, the RH side plug is tan up to a 1/4 throttle, but then lean all the way after, even with my comedy over sized drilled out main jet, so considering going from a 106 to a 108 needle jet, does anyone have any advice on how big the jump is between needle jets? should I go even bigger?

Also starting to wonder about mismatched timing, as LH plug always looks to be burning leaner, its white throughout the throttle range, which is why I'm considering electronic ignition. And the LH header is blue.

[beezermacc]

I'd try a 107 which is quite a common fitment on monobloc carbs. Re mismatched timing, check for variation in points gap. Quite a few K2F's going through my workshop have imbalanced points, a £30 fix. Much cheaper than electronic! Also, let me know what the numbers are on your carb flange.

[muskrat]

 I'll ditto what beezermacc said. If the timing is the same both pots then you need an anti-bias gasket to even up mixture Sounds lean and or retarded.
Cheers