Atlas monobloc carb jets

The new needles are already clipped on the top notch, so they're at the weakest setting possible.

The pilot jets being too rich did cross my mind, so I'll turn em out a bit like you say. Makes sense as it only feels lumpy at low revs. I turned them out two and a half turns when I fitted the carbs but they're a pig to get at and adjust cos they're both on the inner side right next to each other and appear to be touching each other already. Stupid design if you ask me!

The bike runs real nice from cold for a mile or so then starts feeling lumpy at low revs. When I give it some throttle it tears off and doesn't seem to have any flat spots or misfires. It just goes like a rocket. 

Al Osborn reckons if the needles were worn then the main jets will be worn out too so I intend to renew them but not sure what's in there currently (forgot to make a not of thier size when I stripped the carbs) oops. That's why I'm asking what would be a good jet size as an average. I'll then get a few sizes either side of that and change 'em out as needed till it feels right.

Also, how do I check if the slide cutaways are to spec? 

Well, ya learn sumfink new every day! 

Yes I did think the needle jets were the jets. Kinda stupid when you think about it.

I've now ordered a pair of new 106 needle jets 

Old Norton's eh, never a dull moment!

So, I screwed the pilot jet adjusters out one more turn and rode the bike for 20 miles on both fast A roads and also around town. Felt a "little" better than before but still a bit lumpy at low revs. Ticks over relatively smoothly at traffic lights though and doesn't stall. As before, it tears off with no hesitation and keeps on pulling when given a fistful of throttle. Didn't chug and go onto one cylinder like it's done before.

Got back and checked the plugs to find the right cylinder plug has a brown centre with black sooty rim and the left cylinder plug is as black as the ace of spades. And this is with the hotter running BN7ES plugs.

I've now turned the pilot adjuster screws out one more turn on the left cylinder carb (the sooty black plug) and only half a turn on the left one (brown centre with black rim.)

It's quite possible that the main needle jets are so worn none of what I'm doing will make much difference till I change them out but at least there seem to be some changes occurring (on the right cylinder) with just the pilot screws adjustment.

Yep, got that now. I've ordered two 106 NEEDLE JETS

The petrol hasn't got any sand in it either. I checked! (lol) Also, I plopped one of those diamond octane boosters in the tank 

I'll bet it's the needle jets that's causing the excessive richness. Won't know till I fit them I guess. 

This bike has reverse 389's, i.e. the float chambers both face outwards. The fuel runs first to the left side carb then to the right one via a two spigot banjo that's on the left carb.

1/ Float height - no idea if they are equal. I don't actually know how to check that on a Monobloc carb?

2/ Yes, carbs are balanced (well, they were last time I balanced them about two weeks ago)

3/ Ignition advance - It has a Thorspark Electronic ignition which I set to 30 degrees BTDC but only set it on the left cylinder. I can't see how I'd set the right cylinder differently as the Thorspark unit only works one way. I know the Atlas engine timing is supposed to be 32btdc but it has high compression pistons and seemed to run smoother on 30 degrees.

4/ Valve clearances are correct, yes (well, they were when I set them a month or so ago)

A colourtune might be useful, you're right.  

I set the balance first by eye, then use a venturi suction gauge. (This works by placing the conical section over the carb mouth with the engine running then you adjust the opening till the air float reads any number between 1 - 5. Then you do the same with the other carb opening with exactly the same opening in the device and adjust that carb until the air float shows that same number. I've used that to sync Kawasaki triples and its always been pretty accurate.

The manifold does have a pipe connection so I could use that if I had a vacuum gauge, which I don't.

Summary: The engine kicks over on, at worst, second kick but it always needs a bit of throttle to start. Once its started it ticks over smoothly. It feels smoother and less lumpy when just starting off. When it warms up it feels a bit lumpy at lower revs but not terribly. Open her up and she goes like s*** off a hot shovel!

It DOES have siamese pipes running into a single a reverse cone mega. Could that possibly cause it to run rich maybe?

I've already fitted new D needles.

I'm going to renew the jet needles with 106's next and see what that does.

If it's still running rich I'll check the float heights (if someone explains to me how).

If still running rich I'll check the pilot jet adjusters. Maybe they're worn too

If still running rich after all that I'll fit smaller main jets

And if that don't do it I'll bin the Monobloc's and stick some Mikuni's on it! 

What plugs are you running?

Could be airfilters Don, that crossed my mind too. They are quite small its true. I'll try it without them and see what happens.

And David, I'm not worrying, just having a bit of fun tweaking it. I like a bike to run cleanly and be easy to ride in traffic, that's why I want to get the mixture right. I do ride it, and hard most of the time. It's got gobs of power and really goes when I twist the throttle back, it's just a bit rough around the edges at lower revs that's all. I'm definitely going to stay with the siamese pipes and reverse cone mega. The crackle, tweeting and popping on the overrun it makes is addictive.

I'll get it right eventually and I was just kidding about the Mikuni's!

I checked the slides, they're No 3's. I fitted new 106 needle jets and raised the needle notches to the 3rd notch. It now has even more power than before, so much so the clutch is slipping a bit when I accelerate. Never did that before. Goes away when its warmed up though.

The low speed lumpiness has disappeared and it's far more tractable in slow moving traffic. The reverse cone mega is popping a bit more on the overrun which indicates somethings not quite right but it has more get up and go now, and it was pretty lively before!

I removed the air filters but I think I'll refit them now as the mixture seems to have leaned off substantially with the new needles and now new needle jets I've installed. It now needs a bit of fiddling about with the mixture on idle, half throttle and then full throttle. I've ordered a Gunson Colortune to do that and a pair of vacuum gauges to balance the carbs accurately. After that I reckon it'll be running right.

It's improving incrementally with each thing I do. This engine has got some serious grunt that just needs the correct tuning to unleash it's full potential.

Watch this space!

Well, I definitely never knew about the small raised pimple on the side of the float chamber. Thanks for that George, very useful. I'll check the levels before I attempt any mixture tuning.

The floats are black plastic with a brass tang which can probably be bent a tiny bit if needed

The reverse cone mega popping on overrun was always there, it's just increased a bit now that I've fitted the new needle jets. It's obviously running leaner now even with the needles upped to the 3rd notch and I also was running it with no air filters on the last two test rides. I'll replace them now. I don't think the popping will ever go away completely, and to be honest I quite like it!

The clutch was in a bit of a state when I originally pulled it apart. The plates were stuck together with a mixture of black goo and rust which required some serious cleaning off with petrol and a circular wire brush. There were also notches in the splines plus a few raised sections on the bottom of the splines too. I spent some time filing these all down so the plates would move freely. Not much else I could do other than renew the inner splined hub.

Like I said previously, this is a new phenomena that's only occurred since I fitted new needle jets and needles. The power has increased accordingly. If the clutch slip persists when cold I'll reduce the level of the oil a tad by leaning the bike over and letting a little drip out. The oil I have in there is mineral based w20 so it should be ok for a wet clutch. Reducing the levels shouldn't hurt as it only has to skim the bottom of the chain anyway

As far as I can recall I replaced the plates in thier original order. The springs were all the same length and the end caps are adjusted to the top of the threaded bolt. Don't really want to adjust them in any more than that.

Well, I think I've finally sorted it!

I rebalanced the carbs using vacuum gauges, fitted the new needle jets, put the needles back to the third notch, checked the float levels using the clear pipe method (they were ok) and adjusted the primary mixtures which I'd previously leaned out. I also replaced the dual air filters. The Colortune hasn't turned up so I had to do without it for now.

I took the bike out yesterday for a twenty mile run and it was smoother, faster and had really awesome acceleration. The plugs, which are Champion N4C's, didn't foul and when I checked them this morning they both had brown tips and centres with only a blackish ring around the outside. Maybe the right cylinder plug was a tad darker but it's hardly noticable.

The popping from the reverse cone mega on the overrun has become more of a "bark" but that's pretty much how it should sound I reckon. Also, the cold clutch slip has virtually disappeared.

Many thanks to everyone who helped with that.

Result!