Concentric Mk1's on a twin - problems!

[b.l.hayward@tesco.net]

Background: Rebuilt twin engine using new parts only where necessary; currently completed just over 100 miles; fitted with 30mm Mk1 Concentrics 930 – settings: pilot 25; needle jet 106; main jet 200; needle no ? (no markings); needle clip position = middle and bottom groove (no difference with problem – see later); slide 3. Am using 2 left-hand concentrics. Refurbished magneto ignition. Also noted that the inlet port dia is 34mm (carb choke is 30mm). The carbs, top end and bottom end are not related and were purchased separately!
Symptoms: Engine starts easily and at and just above tick over seems to run on one cylinder (the front – no 2). When I try to pick up the motor with a slight throttle opening, No 1 cylinder (the rear) spits back thru carb and often ‘bangs’ in the exhaust. If I close the air lever it stops and both cylinders seem to fire as normal – open the choke and the spitting/banging in No1 returns. No1 cylinder gets much hotter and quicker than No2 and the exhaust pipe (chrome) is blueing. If I open the throttle more than 1/8 the spitting/banging stops and all seems well (but is it?).
My Original Conclusion: Weak mixture in cylinder 1 in the pilot phase.
Actions So Far:

1. check ignition timing – ok
2. check for air leaks carb/stub and stub/head – none
3. replace concentric ‘O’ ring – no change
4. screw in pilot air screw all the way (richest position) – no change
5. remove pilot jet – no change
6. move needle clip to lowest (richest) position – no change
7. check pilot air/petrol passages are clear – done but no change
8. petrol flow to carb – ok (tickler floods).

Thoughts:

1. verify that I have the right needle – what no. should it be and would this really make a difference early in the slide opening range?
2. try a ‘richer’ slide – 2 ½?

Help!: Any other thoughts on problem and likely solution gratefully received. I am baffled!

 

[Bill Thomas]

Hello, You didn't say you checked float level ??,You could change bowls front to rear or put a bit of pipe to the rear bowl after you take it off and hold it in your hand, You can get" Stay Up floats" that you can easy bend the floats to adjust. Cheers Bill.

 

[Howard]

Hi

1. Forget the needle for now, put the clip in the middle notch.
2. 3 slide is about right - you may find 3 1/2 is better (later).
3. All jets sound about right, main may need to go up if you have a very open silencer.

Weak pilot mixture sounds like a good diagnosis, and Bill's idea of swapping float bowls is definitely the next step, If that doesn't point out the problem, I would be inclined to swap the complete carbs - minus the bits hanging from the cables - they shouldn't be the problem.

If it's not the float height, my guess is it's a blockage in the pilot passages that you've missed, or the pilot drilling behind the adjusting screw is too small - you don't say if you've used the carbs before.

http://ebookbrowse.com/bushmans-carb-tuning-secrets-pdf-d19778631

Have a look at this site, Bill and I differ on the effectiveness of some of the tips, but the float height setting is about right.

H

 

[b.l.hayward@tesco.net]

Many thanks for thoughts...The carb in question (No1 cylinder) has been vapour cleaned and flushed (by me) with a petrol squirter and has not been used before by me. But of course there could still be a solid blockage in the pilot somewhere but I did blow through it and it seemed clear. As to float level, this is fixed in relation to the level of the main jet orifice by virtue of the design, isn't it - or are you saying that it could still be wrong because of crud manufacturing tolerances? I guess swapping the carbs (cylinder 1 to 2 & 2 to 1) would probably be the best test at the moment? Does anybody have the needle number - just in case!

 

[Bill Thomas]

Hello,I read that the float level is never right !! I set my Stay Up floats to what amal say and the float touched the bottom of the body !!. I think the needles should have 2 lines above the grooves, Cheers Bill.

 

[van drenth]

most probally you have an airleak. slide 3,5 is better. yet 220. on which compression rating you are running, 9:1 ?
Regards, van Drenth JC

 

[christulin]

Float height can be changed by warming the bowl and tapping the brass seat with a 1/8" drift to either raise the float level up or down. Move the seat slightly and check height of float (after cooling) by gently pushing down on the tab. You may need to lower the seat a little to raise the float height to get a fatter mixture. Once I learned this, every repop Amal needed the float level changed. This was in a Triumph factory bulletin for MkII carbs I have somewhere. I run the same carbs but needed to also compensate for high altitude. Agree air leak is possible. Idle mixture and slide size usually are more important at low throttle settings than needle or main jet.

 

[Howard]

http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html


Sorry, I don't think my last link was right.
Something I haven't seen mentioned, the hole from the pilot mixing chamber (see picture on this link) into the choke can get blocked, it's worth a prod with some thin fuse wire.

H

 

[b.l.hayward@tesco.net]

Many thanks for all the inputs – especially Howard with the link to the Bushman Concentric problems – fascinating reading! Anyway, have had the rear cylinder carb to pieces again and took the trouble to poke the idle fuel way with thin wire followed by a blast of WD40 and then compressed air. Nothing I could see came out - and I then checked the float level based on info in the link. Result was that it looked a bit below where it should be (ie weak) so I heated the float bowl in boiling water and tried to tap the brass bush with the needle seat a bit lower. No joy, unfortunately, because the brass bush was already in as far as it would go – it didn’t budge! Put the carb back together and started – joy! Almost no spitting back and ran much better – perhaps some of the tank sealer had been dissolved by the ethanol in the petrol and a sticky blob had caused a blockage? Anyway, it’s 90% better than it was and I am thinking that maybe the fact that I have a 30mm carb/stub bolted to 34mm inlet tract could be part of the problem? Each time I have removed the carb there has been a little puddle of petrol in the inlet sitting in the step between the stub and the inlet tract. Might try to open the rear of the stub to 34mm ie it will then go from 30mm (carb) to 34mm (inlet tract). Any thoughts very welcome! PS if you are wondering why the head seems non-std I bought the entire top-end from Ted Croft after somebody had blown up a Norvin - looks as though it may have been breathed on and I didn't realise it!

 

[christulin]

Good to hear your problems are better. I have thought about modifying my inlets as well, but I have plenty of power as is. I'll continue to follow the posts on this thread.