Flat spot when blipping throttle to change down

Hi All,

When I slow quickly and change down through the box, I find sometimes the engine does not "blip" is usually around 3-4000 rpm also sometimes a spitback, otherwise carburation etc good. Carbs are Amal concertrics, standard jets etc, serviced and otherwise working fine, cam is a 4S correctly timed. PS also stalls if I come to a quick stop.

Any ideas?

Thanks

1st thing I'd check is ALl carb related fasteners and tube-age cableage that could leak pressure differences. If a boyah and not upgraded its trigger leads you are on your own and hopefully not on side of road. Oh yeah if ya ain't bought plugs in V8 packs to make sure that ain't an issue, might start soon. Prolly just water though.

It sounds as though the needles are one notch too low. If they are in the middle and you raise them one, and it still coughs on gear changes, the needle jet is probably one thou too small in internal diameter. You will probably be using 0.106 or 0.107 inch needle jets. The spit back is a pretty sure sign of leaness. It also depends on the weather. If it is colder than normal the motor can run lean. You need to set up for the coldest average sort of day, on which you would go out riding. Then in hot weather it will run slightly rich. - doesn't matter.
The situation you seem to have, is the very one I look for first when tuning my race bike - setting the needles and needle jets is the hardest bit to get right, the rest is much easier. Get it to cough on gear changes and changing throttle, then lift them one notch. If you want to get really tricky, you can play with different taper needles to get max performance at all throttle openings.

Nigeldtr said:

Hi All,

When I slow quickly and change down through the box, I find sometimes the engine does not "blip" is usually around 3-4000 rpm also sometimes a spitback, otherwise carburation etc good. Carbs are Amal concertrics, standard jets etc, serviced and otherwise working fine, cam is a 4S correctly timed. PS also stalls if I come to a quick stop.

Any ideas?

Thanks

Click to expand...

How do the plugs look. I think this could happen with either too rich or too lean condition.

I don't think plug reading will help set the midrange jetting - usually only the mains after a high speed slightly up-hill run on a straight road. The needle jet setting at 3/4 throttle (the shoulder on the needle) is critical. If you get it wrong (lean) the bike will be a bastard to ride on twisty tight roads. If it is rich the bike can become a bit sluggish.
I know some guys claim that when you do a plug chop, you should cut the engine dead immediately so the plugs don't change and give a false reading. What you are usually looking for is a 2mm black ring on the porcelain right down inside the plug where it meets the metal - I suggest it takes a lot of midrange running to change the appearance of that. If you've got the MAINS jetted to the best, the 2mm black ring on the porcelain just disappears. That is usually the limit before the pistons get eaten, or detonation occurs in a poorly shaped combustion chamber.
Remember we are talking about two things - mains and needles. You can easily get confused about which is doing what. Mains - high speed, full throttle. Needles and needle jets - midrange up to 3/4 throttle.

acotrel said:

I don't think plug reading will help set the midrange jetting - usually only the mains after a high speed slightly up-hill run on a straight road. The needle jet setting at 3/4 throttle (the shoulder on the needle) is critical. If you get it wrong (lean) the bike will be a bastard to ride on twisty tight roads. If it is rich the bike can become a bit sluggish.
I know some guys claim that when you do a plug chop, you should cut the engine dead immediately so the plugs don't change and give a false reading. What you are usually looking for is a 2mm black ring on the porcelain right down inside the plug where it meets the metal - I suggest it takes a lot of midrange running to change the appearance of that. If you've got the MAINS jetted to the best, the 2mm black ring on the porcelain just disappears. That is usually the limit before the pistons get eaten, or detonation occurs in a poorly shaped combustion chamber.
Remember we are talking about two things - mains and needles. You can easily get confused about which is doing what. Mains - high speed, full throttle. Needles and needle jets - midrange up to 3/4 throttle.

Click to expand...

I'll pull the slides and maybe lift the needles, it's sounds just like what you describe and is a pain as It screws up my approach to corners and generally getting speed off when slowing.

PS plugs looked "Normal" but I did not look too close.

So, I tried lifting the needles, were in the middle and it made no real difference if anything, probably not pulling as well, possibly too rich. I have tried snapping the throttle shut in the middle of accelerating hard and every time, the revs die off fast and she stalls. The engine is still a bit tight (done 400 miles now) if I let the revs drop gently she will tick over. Seems there is a problem around the low revs or tick over mixture. Carbs have been throughly cleaned but they are of unknown age. Could too much air be getting past the slides. I sleeved the slides but not the carb bodies?

Regards

Nigeldtr said:

So, I tried lifting the needles, were in the middle and it made no real difference if anything, probably not pulling as well, possibly too rich. I have tried snapping the throttle shut in the middle of accelerating hard and every time, the revs die off fast and she stalls. The engine is still a bit tight (done 400 miles now) if I let the revs drop gently she will tick over. Seems there is a problem around the low revs or tick over mixture. Carbs have been throughly cleaned but they are of unknown age. Could too much air be getting past the slides. I sleeved the slides but not the carb bodies?

Regards

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If lifting the needles (enriching) made it worse, maybe that indicates it is too rich even in the mid position, so why not try lowering?

suspect electrical supply connection, diddle floats so goodest idle gotten close to 1.5 pilot screws out and of course never ever forget to probe open the pilot jet. Too much air past bore/slides is usually undetectable but for too fast an idle on throttle closed, unless idle set too low then stalls out.

hobot said:

suspect electrical supply connection, diddle floats so goodest idle gotten close to 1.5 pilot screws out and of course never ever forget to probe open the pilot jet. Too much air past bore/slides is usually undetectable but for too fast an idle on throttle closed, unless idle set too low then stalls out.

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Good point Hobot, I was convinced I had a carb related issue on mine last year. When I found an almost blocked pilot jet on the single Mikuni I was convinced that was my issue. But it wasn't. Then when stripping the bike I noticed a very damaged pick up wire on the Boyer back plate. I'm pretty sure that was more likely to be the problem. But haven't run it again since!

Ugh, I guess its pretty obvious I'm really over my head here so most my wisdom and recoveries were found more by accident than checking-testing ordered logic. Two motto's drummed into me on BI list -
If it acts and sounds like carb > check electrics first and when in doubt > NAIL IT! \

Suvee taken down by dogs last fall took me till last month to get going again and got one ride fine but 2nd ride got me 1/3 way to work then started cutting out if iven much throttle - which left me in a common dilemma state >to either try to operate w/o causing the cut out or trying to make it cut out to end up diagnosing or not, dead on side of road missing appointments. That was 3 wks ago and have not worked me back into mood to attempt another probing and dissembling investigation that's as likely to cause another issue as find or fix the 1st. Those that can ride around daily puts me in split brain state to look up to them as what's possible or just throw a stick in their spokes to bring em down to my level.

Fast Eddie said:

hobot said:

suspect electrical supply connection, diddle floats so goodest idle gotten close to 1.5 pilot screws out and of course never ever forget to probe open the pilot jet. Too much air past bore/slides is usually undetectable but for too fast an idle on throttle closed, unless idle set too low then stalls out.

Click to expand...

Good point Hobot, I was convinced I had a carb related issue on mine last year. When I found an almost blocked pilot jet on the single Mikuni I was convinced that was my issue. But it wasn't. Then when stripping the bike I noticed a very damaged pick up wire on the Boyer back plate. I'm pretty sure that was more likely to be the problem. But haven't run it again since!

Click to expand...

Ignition system problems can give the same symptoms as incorrect jetting. However the only time I've ever experienced the spit back, it has been due to too lean and has responded to lifting the needles. When you first start a cold engine and take the chokes off, it is normal for it to spit back until the engine warms up. I would think that if intermittent firing was happening, it would also occur when you are using full throttle and you would feel it. Perhaps you have an air jet or pilot jet problem, however I would think that adjusting the needles and needle jets would over-ride it. Perhaps you have a loose exhaust port leaking air as you shut the throttle - might be giving the flat spot ? The symptoms you described really sound as though the carburation is too lean when the needle is metering the fuel flow on it's shoulder. As I said, when I am tuning my racer that condition is the first thing I try to achieve, and I richen slightly from there. I set the mains afterwards and check that fitting a larger pair actually richens the mixture (might meter off the tip of the needle on full throttle ).

Thanks for the replies.

I have rebuilt several BSAs and have often had similar problems with carburation. I always fully strip, clean and check the pilot jet for blockage however have often experienced hesitation or stalling either when stopping quickly, pulling away quickly or from snapping the throttle shut after accelerating hard. I re-checked the timing, "spot on" and the wires are in fine shape. When I slow for a corner, I normally back off and then blip down through the box so in effect, the carb is blipping from a closed throttle (no fuel or perhaps pilot mix) and then won't blip or hesitates so could it be the pilot mix perhaps?

Could the float level play a role here, I noticed the float bowl gaskets are wet? I can check the fuel height tomorrow with an adaptor I made but not sure where the float should be relative to the top of the float bowl. With all this fuel bouncing, foaming and slopping about the fuel level must be all over the place. Never managed to bottom this hesitation, stalling issue out - wish I could fix this. All the bikes I have, have either one of two concentrics. I am confident I can get the adjustment right but the hesitation and stalling drives me nuts. PS no suggestion to go to single carb, not an option

PS Viton float valves fitted.

Thanks

HIghly reccommend before measuring float level you see what each carb pilot jet is set for best idle above or below ideal AMAL desinged 1.5 turns out which sets base line for everything else. Pilot screwed in less than 1.5 implies fuel level low so requires less air to dilute right and visa versa, assuming of course pilot path is flow free to correct size. I think I solved my SuVee's throttle up stumble cut out by tightening the neg battery terminal and just got back of scary test ride to mail box and back in just shorts and moccasins, about pooping in shorts on return oppostite rut to hit pot holes that slammed trapped mail behind wind screen out for me to grab in time by clutch hand so death grip on throttle side till bouncing skipping past the hazard and no hanging open flesh wounds and no stumble hesitation on throttle up on out going teeth gritter flaunting bare ass flying finger of fate attitude. Got errands to run but not sure I'm up to going it alone again yet.
One thing about cycling, its always a leap of faith on what next.

So quick update. Fitted new "stay up" floats and Viton (I think) tipped valves and set the brass bowl inserts to .575" below bowl top edge (floats now roughly level with top of float bowl when valve seated). Also put in new needles and needle jets and now running really well, So again, thanks for all the tips and advice. I did notice that if I let the throttle "snap" shut above 4 K RPM, and hold in the clutch the engine will stall. Under normal circumstances, I back off and/or close the throttle, in this case, she ticks over fine.

Man don't we have better things to do sheeze. Many reports of pilot jets doing this sort of thing and I forget if already attended to. They should not die letting go of throttle coating with clutch lever held in. Try some grease on slides and see what happens.