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Gents. Bob Mather’s recent posting in the magazine rang some bells with my own feelings when I was just looking at the port heights when my barrels were off. They just didn’t compare to my Yamaha. Anyway I did nothing about it when I was thinking about the spitting and fitting gauzes stopped that anyway. However, if it is spitting because the engine operation is poor and if I can help my engine operate easier, then I am inclined to go with what Bob recommends. Before I do anything though, and without meaning any offence to you Bob, has anybody had any experience of doing this? I am familiar with porting diagrams and the dimension from the skirt or top of barrel are often quoted as a means of knowing how much, if any, work has already been done on changing the port heights. So does anybody know what the standard port dimensions are in the first place I suppose and by raising the exhaust port by 1/16th, does this introduce problems somewhere else for example.
My engine is a 1920 with the plugs at the back of the cylinder.
Thanks very much, Will.
I believe that the basic cure is to fit a central carb and lose the long angled inlet tract, I have seen a bike with the top tube altered to make room. If I were investigating I would run the bike on each cylinder separately to see whether it is one or both affected. I think a lot of thought has gone into this over the years, I expect a more experienced member will know. I would be very interested in any results you get.
To confuse matters further, the spitting on my Super was cured by fitting gauzes to the LH cylinder only. I understood that the root cause was weak mixture to one side due to the curved inlet tract. I am happy to be corrected on this.
Hi Richard. Interesting comments. I did have gauzes top and bottom on both sides and this worked well but restricted power so I took the bottom ones off and it has started again but I also moved the needle one notch down too. I have a 276 carb, which might be too small and all standard size carb components that are new ish and this was clip 4 from the top, so set rich. When the bike is running down the road it does not spit at near main jet speeds and it only seems to occur on the overrun or at lower revs. This suggests to me that it is not a timing thing, nor is it a porting thing but, as you suggest, a fuel air thing. I messed about with the timing lever to see if this made any difference but it doesn’t and just as I was getting home it seemed to like being choked at lower revs.
The snail shell on mine has a big bolt going through the middle and is not restrained from the frame as I have seen on others, so the gases have to negotiate the curved tube and this bolt too which is hardly a smooth path but there again we are only talking a few horsepower. Thank God mine doesn’t go as fast as others seem to because I have yet to master the art of braking in any situation resembling an emergency stop in 100 yards. My seat covering is already the right colour brown as it is.
As far as my brain goes with the spitting, this is either a case of it being too lean or too rich at either the air screw or the needle so I will mess about until I end up back where I started and with no sensible conclusion realised.
Before that I will try the left hand gauze thing too because that makes sense. There is a lot of swirl going on which Yamaha, my second favourite bike company, researched enormously. I wonder if reed valves would do anything?
Fitted a Weldonia fork brace incidentally and this has made the front end a much more confident thing. A work of art to be honest because that was designed by hand and if you fit a big brace along the back, it also works as a steering stop too. It would have definitely needed to be designed by CAD such is the way is passes around everything and it still works really well.
I just browsed through the posts and without really trying to get into your problem I noticed one thing. If you think it’s have something to do with the carburettor, don’t forget the throttle cutout. It’s often overlooked and a too big cutout can lead to lean conditions and a too small can lead to rich. It mostly the region between idle and small throttle lift that’s affected. Every “stage” in a carburettor must perform faultless. Unfortunately I have no idea of what the right settings for your carburettor are.
Best regards, Leif
There are many contributory factors that cause spitting back,it seems to occur in low compresson early engines.However there is one primary cause as Bob Brougham found out.When one cylinder is exhausting flaming gases, some of the flame goes straight into the other cylinder which of course still has its exhaust port open as its piston is coming up on compresson with of course a full charge of unburnt gas in the cylinder.The exhaust ports need to be kept apart from each other.I have read an article written by Rickardo ( yes him of 4 valve fame) before the 1st world war in which he stated that the exhausts from multi cylinder 2 strokes must be kept as far as possible from each other..
Bobs solution was to make a exhaust stub with a divider in it profiled back into the cylinder block so that the gases went their own way into the exhaust pipe and not tracked across into the other cylinder.I had this done to two of my engines and it utterly transformed them.
NOTE Pickardo made the worlds most powerfull aircraft piston engine a 2 stroke !.To late to be be put in production as the Jet age took over.
There was a post giving exact details a while ago but I can’t find it. It also mentioned cutting a small slot in the piston for lubrication of the divider/cooling the underside of the piston crown. It sounds like the definitive cure. Having said that, gauzes are easy and my bike seems ok on poweralthough I don’t thrash it.
Regarding brakes, my back brake is pretty good with new well-fitting shoes. The front does a bit.
My ‘25 two speeder has a “splitter” fitted in the inlet manifold of the crankcase to prevent fuel mixture bias to one cylinder after it has traversed the bend in the side mounted carb manifold.
Each bike has its own character, e.g. mine likes the air screw fully (but gently) home otherwise it spits. Before doing anything radical make sure that your carb settings and ignition timing are correct, no air leaks anywhere. I believe some owners have the ignition as advanced as possible to get round spitting. Trial and error. Much better minds than mine have grappled with this problem. Some bikes just don’t spit anyway…
Thanks for all those comments and thanks Glyn but I head already read and actioned your advice about the exhaust divider as this was one of the things you wrote about in one of your Potty mods I believe.
So I took Chitty Chitty Bang Bang out for another 30 mile ride around Cheshire and caused a lot of consternation to riderless horses and small children. before I set out i moved the needle to clip 2 with the air screw just over 1 turn out and she ran well to be honest but when it got hot things started to go wrong which I think is a lean thing, especially in that just on the needle bit. Once onto main jet, it’s fine.
So I stopped after 10 miles and turned the air screw in to half a turn out and moved the clip to 3 and this probably did not do anything except when going down hill and you back the throttle off whilst at the same time going fast, produced the most noise yet. Nothing I did would stop it and it can’t be good for the engine. The difference was though that it was coming out of the exhaust and not the carb so that’s backfiring which is rich. Next time I will turn the air screw to 1 again and if I would have had my wits about me I would have done it there and then but the noise deterred me. Having re read this posting, it seems obvious that I should choose to have the air screw out 3/4 of a turn.
Interesting to hear Leif on about the slide. I think it is a air/ fuel mismatch that affects some engines differently. I am inclined to agree with Richard that each bike has become slightly unique over time and likely also in manufacture.
I was going to stick the gauzes back and be done with it but I have a number 6 carb that is bigger than the 276 I have fitted so I am interested to see if what this does. As the internals are the same and hence the petrol flow, the increased bigness of the bore should mean more air and more spitting which I am growing to appreciate.
Going back to my Yamaha carb fiddling, either a less wide needle is to be tried, or a throttle cut out with less of a cut out. Or gauzes.
The Yamaha needles are all shapes, some with very skinny bottom bits and fat top bits and clearly Yamaha were imparting all their knowledge to get the mixture dead right to go to the trouble of having such widely differing profiles. You wonder if the bottom part of the needle on the Scott were skinny that it would help fuel flow in the very first parts of when the needle comes into being…………… Or gauzes.
Enjoying your thread.
Both my 2-speeders were former carburettor spitters…
Neither have an inlet splitter or gauzes fitted, one is running on a 1” Amal 276 with a 190 main jet and a 3 1/2 slide and the air screw out 1 turn, the other is on a 1 1/16 Amal 276 with a No. 3 slide, the air screw 1/2 turn out and a 210 main jet…
In my case I found several conditions to increase the likelihood of spitting back through the carb:
1) Retarded ignition
2) Air screw too far out (too lean)
3) Cutaway too lean or slide / carb body worn
4) Air leaks
5) Running out of fuel
I would be tempted to try a richer cutaway on the slide, turn the air screw in a touch, double-check for any air leaks with soap suds on a running engine and check the timing…
Exhaust spitting I have no experience of, so forgive me if this is all superfluous.
A study on port sizes appears in technicalities as I recall either Potty or Tim Sharp measured a multitude of barrels and shared their findings
Just to get my head round this, spitting back the through the carb is due to flashover from the exhaust port through the transfer port and crankcase? So separate exhaust ports would cure it which is what they did on the 3-speed motors?
Richard , Yes that is the main (but not only) the cause,thats why it does not happen with a Flyer type.
Thanks Glyn, hope to see you when this is all over.
Thanks Peter. That gives me hope then. I have fitted a number 3 slide with 6 needle and a larger than usual but unstamped 106 nozzle on clip 3 with 3/4 turns out of the air screw with a 170 main. I did have a number 4 slide and a 190 main and a genuine 106 nozzle so I have made the whole thing have less air and more petrol at the crucial point hopefully. Getting rid of the gauzes is my aim but maybe this should be accompanied by work on the brakes too.
I have been busy stripping the threads on the cylinder head bolts on my YR5 Yamaha so test ride number 76 on the Scott has had to wait. Incidentally I have had a drive chain break and a mag chain roll off the sprocket and plugs go all wierd on me during my sojourns and this has prepared me well because my spares and tools have taken on a new exacting life and I have yet to be unable to get home. That will doubtless change though. In fact I was able to catch up accidentally with friends who I was unable to see whilst at the roadside only for them to reassure me that they thought it was going to be me anyway.