Another Jetting question

It is sometimes worth setting the main jets using a hotter grade of plug until the black band almost isn't there on the porcelain, then fit the colder ones for high speed use. Good idea for two strokes as it keeps you a bit away from disaster. With racing two strokes, they never seem to really perform unless the jetting is dangerously lean.

Perhaps a dyno might not help you get your motor to idle well ? It sounds to me as though you have partially blocked jets , or the breather to the tank is blocked. If your tuning is correct, you should need the chokes on to get the motor to stop spitting back when cold. However when it is warm it should idle perfectly with the chokes off.

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Have now fitted inline filters and waiting on adjustable Amal floats (don't like repeatedly tapping seats to and fro). I think an idle blockage may be the problem but because I have a relatively clean tank and float bowls with no sign of foreign matter I've never bothered with filters. We will see. Tank has clear vent and is fine on road.
On the mixture front yes I know it has to be a little rich to run half well when cold with no choke. Those std Amal settings (which is what they are for a 72 750) would have to be substantially weakened. As it is, below 40F it needs lots of tickling and several restarts to keep it going. When warm there is no sign of richness, excessive petrol smell, black smoke at any throttle opening or load (there was, accelerating in 3rd/4th at 60mph + before dropping the needle to top groove). It has stupidly overworked inlet head (not me), PW3 on std marks, so 5 deg retarded on designed timing (found that when I degreed the cam), Combat compression with 015” base gasket so about 9.7:1.
So I guess my reasoning behind the Dyno thing is to find out not only what happens to the AF ratio when it gets hot and idle/pickup disappears without a fistful of throttle to keep it alive but also to see what it’s like throughout the range.
Trouble is I can see 105 needle jets in the mix and an inability to start when cold, having modified the slides!
Who knows? It would be nice after 15 years and 34,000 miles to be able to ride in town for more than 3 or 4 minutes without it acting up and me having to ride it like a Manx /Goldie on a GP carb with no idle facility.
I do appreciate the help and will get to the bottom of what in reality is a stupid little problem, albeit an annoying one.

How rich do you believe it has t o be when you see black smoke coming out of the exhaust ? Even with the chokes on you should not see that. If you have cut the slides, you have probably lost control of the situation. Perhaps you should buy a new standard set ? I suggest you should strip the carburettors, clean and check that all the jets are clear without pushing anything through them that could scratch. Rebuild the carbs, checking the float levels, and that the float valve is not obstructed. Then you will have a decent starting point. I would be careful of in-line filters , check that you have sufficient fuel flow through them. The starting point in jetting the carbs is to get the motor to idle, even roughly, then set the needles and needle jets by lowering the needles until the motor coughs when you ride the bike around a twisty tight r oad requiring a lot of throttle changes. After you get the motor to cough, raise the needles one notch. The motor shoiuld accelerate cleanly and fast through the midrange of throttle openings. Adjust your idle when the motor is warm leaning off the mixture until the motor starts to spit back through the carbs, then richen the mixture slightly. You should wind the throttle so that the slide cutaways are both at the tops of the chokes and check that the cables pull both slides to the top exactly together, lower the slides then set the stop screws to give a fairly quick and even idle. When the motor is cold you should need the chokes to get it to run without spitting back through the carbs. If it does spit back or cough, that is a sign that you have got it very close to correct. If you grab a handful of throttle when the motor is warm, and all you get is a gasp, you need to reset your needles and needle jets.
It is all easy stuff, but you need to be aware of the sign when you've got the settings too lean. The main jets are the last thing to set, you do that by looking for the black band on the porcelain right down inside the plug after a high speed run on a slightly uphill road. Your standard main jets should be close to correct , and only need a change of one size up or down, unless you change the air cleaner.
When you are tuning you should try to get the mixture as lean as possible without causing missing , except on the mains. If the main jets are too lean, you can burn pistons and valves. If the midrange is too lean, your bike will cough and splutter, and you usually wont persevere without correcting it.

About cutting slides. Years ago we used to do that when jetting for methanol. It was usually a matter introducing an error to fix an error and I haven't done that in the last fifty years.

Hi All
not wishing to join in this discussion but, this site has a very good explanation of reading plugs and its got pictures too (helped me anyway)
Hope it helps.
JohnT

I love the bit about cutting up the plugs with a hacksaw so you can see the ring on the porcelain. Don't you guys have strong electric lights or sunshine and magnifying glasses in your countries ? It is not the colour of the ring which is important . What is important is that you lean off the mains until it is still just there . On two strokes, the heros lean off the mains beyond that point . If you are not prepared to do that your bike will be too slow when the fast guys are around. If you use a hotter grade pf plug when setting the mains, the process is safer. I've just bought a pair of iridium tipped plugs for my 850 - cut them up - you must be joking ?
I suggest the p[oint of the exercise should be to run the motor as lean as possible right through the carb's operating range without doing damage or making the bike difficult to ride, (which can occur if it is too lean at 3/4 throttle ).

Couldn't that be part of the problem ?
I hear lots of praise for a PW3 cam , but for city riding ? .. 5 deg retarded , plus chain stretch and valvetrain flex will retard it even more , this makes for a pretty late closing inlet valve . I can't help but think that this must have a negative effect on a stable idle . Maybe a camshaft expert can give his opinion on this ?

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This is how its always been. Bike came in early '98 with a Fred Barlow version of 2S cam, massive intake lift etc but kind of std timing, knackered carbs. Rubbish in town of course. New carbs added, just the same when hot. I rebuilt with std. cam when the engine blew end '02. No real change in town use.
PW3 cam added early '10. Just the same in town. You can hear the idle in attached video, its perfectly stable until something changes. I've built small block GM and Ford engines with high lift cams (low vacuum - erratic idle) and they were 'rumpy' but the PW3 is pretty good in this respect. The reason I have it on std timing marks is that it was not as tractable set to designed timing which just happened to be 5deg advanced over std marks. I am doubtless missing some top end and no, I would not fit another. It may be very good at 7000+ rpm but the std cam is far nicer and not noticeably slower in my engine. One reason I bought it was the Norton experts claim that it improved the mid range substantially. I do not think so, certainly not on specified timing. Both of those experts are well respected so I will not say any more. The Barlow '2S' was far more exciting as a piece of performance equipment but lousy for tractability.

How rich do you believe it has t o be when you see black smoke coming out of the exhaust ? Even with the chokes on you should not see that. If you have cut the slides, you have probably lost control of the situation. Perhaps you should buy a new standard set ? I suggest you should strip the carburettors, clean and check that all the jets are clear without pushing anything through them that could scratch. Rebuild the carbs, checking the float levels, and that the float valve is not obstructed. Then you will have a decent starting point. I would be careful of in-line filters , check that you have sufficient fuel flow through them.

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Good point AC but I have had chokes on this bike and it has produced a little black (not blue or white) smoke with full choke and throttle opened. Not clouds of the stuff but just enough to know.
Carbs are clean, 100psi of air and thinners through all openings. Did consider the possible reduced flow but at idle? 220 Main jets are only 0.050" dia.
The fact remains that she idles fine until the temperature is elevated or some kind of mysterious blockage occurs and leaves no trace after!
Listen to the vid, the idle is lovely, a bit sharp but that's the cam. If it stayed like that I'd be very happy but it goes from perfect to shit in a matter of seconds.
I think it has to be a blockage and like you I'm thinking what idiot puts up with this for nearly 15 years. I have lots of faults and one is going down the same route time after time (no lateral thinking ). As an example I spent 3 years trying to get the clutch to release cleanly (IN TRAFFIC - WHERE ELSE!!), plate cleaning, adjusting, changing lever pivots etc. I then found I had a Dommie actuating arm installed and when a Commando item didn't completely solve it I pulled the diaphragm and found it distorted. It wasn't lifting evenly. Most of my riding is as expected on the open road so these 'problems' are tolerated because the situations under which they occur are avoided.
Here she is running before I pulled the bowls. Ran for 10 mins with no problem this time:roll:
http://youtu.be/QPu5nqrMpGU

It seems to be idling perfectly, it could be a fuel flow problem. I'd be looking at the float needle and seat, checking the float level, and the rate of flow from the fuel lines. Incidentally, blue smoke from the exhaust is usually due to oil, black smoke due to flooding.

It seems to be idling perfectly, it could be a fuel flow problem. I'd be looking at the float needle and seat, checking the float level, and the rate of flow from the fuel lines. Incidentally, blue smoke from the exhaust is usually due to oil, black smoke due to flooding.

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Stating the obvious is sometimes too simple for me :lol: This is what I found yesterday.....
I think its the main fuel tap! Hard to believe? Maybe not. Long story short, fitted new floats and CLEAR Tygon fuel pipe from a lawnmower repair. Ran for 4-5 mins and as it started to get warm the idle faltered and would not rev cleanly. But, as the revs climbed beyond 2000 it cleared and revved cleanly. But, as soon as I released the throttle it started to die and at the same time I noticed the fuel in the pipe bubbling. When I realised what was happening I opened the reserve and a good idle returned as did the ability to rev well from idle. My taps are BAP's which sometimes get a bad press but they certainly flow ok at speed and don't leak. They have been fitted for 35,000 km.

Now I don't know for sure why it happened but it was obvious that flow on the main side was reduced for some reason (removed tap and all clean, replaced and it did not do it again). Now, repeated closing of the main tap and running the engine until it faltered exactly duplicated the problem. Opening the tap immediately restored idle etc.
A close look at the main tap feed tube (inside tank) shows it is quite long at 40mm from thread. This gives a 35mm fuel depth meaning I need to go on reserve much earlier than always felt necessary. It is possible than low fuel (Roadster) co-incides with riding through towns!! With mixed riding at 100 to 130 kmh I get 65mpg (UK gallon) so I know the cruising mixture is pretty good. I have No3 slides.
I find it strange that with low fuel flow the engine would rev well for 30-40 secs but not idle or pickup well. Surely the idle passage will work to a lower fuel level being slightly below the main jet.
I know on the road I can run the fuel quite low before I turn on the reserve which holds 1litre and 15miles max. The true test will be on the road and to keep the tank filled. I can't believe its this simple but it certainly idles and revs ok for 15 mins. I don't know what was wrong with the tap as it did appear to be immersed in fuel. It has not played up since but I may have put more of the fuel back in the main side of the tank.
Tks for everyones' suggestions. I was going to stop messing with it and put up with it for another year but the response here made me see it thru'. Cheers all.

Glad you've solved your problem. It is not of the type that I usually have myself. On my racer I use a big industrial fuel cock, with no reserve. The tank is aluminium, so there are no flakes floating around. It is all straight forward. I haven't had a road bike for many years, so I've pretty much forgotten the silly little problems which can occur. I seem to remember fuel taps which you used to pull out to turn them on, and they had cork inserts to stop leaks, they were always suspect. ' Another invention from Bastards Incorporated '