Reasons why 500's ping, Now 2-stroke porting idea's ;-)

[John Orchard]

** EDITED **

Well couldn't sleep, spent the night with my mind being 'a fly on the venturi wall' trying to understand my carb.

So, even with a closed twist-grip (there is still a tiny air-gap under the slide), the contributing circuits to fueling the engine are; 'pilot circuit' (pilot jet & air screw) and needle straight section in the needle jet.  Yes I know all the charts say that it is only the pilot circuit but in this case I feel it is both circuits.

At Idle speeds with a closed throttle grip, the major contributing circuit is the pilot circuit, due to minimal 'concentrated' airflow over the needle jet to pull the same amounts as the pilot circuit can supply.  At a certain point the vacuum pulse over the pilot outlet becomes less and air-speed picks-up over the needle-jet then making the needle the major contributing fuel supplier.

With the two circuits supplying fuel at idle, this is like having a choke passage open, now realizing why I don't need choke to start the bike cold, even with dropping from a 68 to a 50 pilot jet.  Having dropped that far in pilot jet makes me think that the little less fuel supply at further throttle openings might require stepping-up the main just a touch, might jump up from the 160 to a 165 (168 is stock).

With a closed twist-grip, the needle is on the straight section at the needle-jet, and stays on the straight section for the first 10mm of throttle slide movement (to about 1/4 throttle in a 38mm venturi).  There is no denying that with a fresh 500cc cylinder, the intake vacuum at a closed throttle IS still enough to pull fuel from the needle-jet.  Whether this is just a design fault with the Keihin PE38 I don't know, but I will say that I swear the same thing is happening with the VM38 Mikuni on the KX400 (carb is off a KX250A5).

For a bit I thought that because I have the bike set so it would idle, that the slightly lifted slide might bring the needle-jet into play at a closed twist-grip (and I guess it does) but then I realized that with any amount of slide lift, the depression over the needle jet will draw fuel fuel past the needle, so with the same given intake vacuum from the 500cc piston, and, the fact that the straight section of the needle will still 'allow' fuel to flow out, fuel HAS to flow out if the slide lets any air past.

I guess the next step is to try (even though I don't want to disturb something's that now working ;-) ) letting the slide drop to completely to the bottom to see if I can stop fuel supply from the needle-jet with a closed twist-grip?

To keep the status-quo in the fuel curve, it makes sense now that running a smaller (richer) straight section needle required a drop in pilot jet size, it just took me a while to juggle the correct ratio to balance closed throttle & 1/8th throttle, as both circuits are contributing, varied amounts at various openings.

I now wonder how many other open-class two-strokes have this issue, with the them having such strong intake vacuum at closed throttle?

** EDIT ..... I just realized since I made this post that I have been confusing 'closed throttle' (slide all the way to the bottom of the carb) with 'closed twist-grip', because the high idle rpm setting meaning the slide is open (yes on the needle!).

[John Orchard]

In my experience (spent 10 years driving a dyno) the idle - pilot circuit responds a differently to air correction compared to main. The air screw is called the idle air screw and has it's greatest effect at idle.

Thanks, I now tend to agree with you.

[OverTheHill]

just another probably [probably not relevant] thought, have seen lots of needle jets simply worn out & causing loading up on the line [more than normal], constant throat clearing plus ritchening on a closed throttle heading into a corner [rich when opening up after that]. Worst one of mine was a 125k back when it was only a few years old. Could [i have] block the needle jet off & start on the pilot with closed throttle just to check the pilot lean/rich etc but any throttle above a fraction will snuff it of course. Or is the needle designed to bleed a bit anyway on closed throttle to help avoid seizing after a long full song then shutting off for a long slow down. Someone hasn't drilled the air bleed to the needle jet &/or pilot jet [if they have brass air bleeds in the holes]. Seen that done but of course the customer doesn't tell you he's done it--just i've tried everything & of course you fit a new needle & emulsion tube & then go chasing the next problem. Anyway--off track again as i see the heading is 'pinging', run her on av gas!!

[Lozza]

But why ping when only on the pilot circuit? Surely the combustion chamber is under more load to make it ping with more throttle on.

I'll down-size the pilot and screw in the air-screw and see  :-)

With the throttle not open the reeds are not opening fully which leads to irregular combustion. The needle won't be dominant until around a 1/4 throttle due to the annulus being smaller than the pilot jet with the throttle closed, or when the annulus between the needle and emulsion tube becomes bigger than the pilot jet area. You should be able to pick the transition(pilot to needle) that will be where the air screw no longer has any effect on rpm. I don't believe the needle jet will bleed fuel at a closed throttle if it does it would not be as much as the the pilot circuit. The needle shroud is still inside the slide. I've never had idling change by raising or lowering the needle clip or changing the needle. To check shine a timing light down the carby bore when running

[Tahitian_Red]

** EDITED **

Well couldn't sleep, spent the night with my mind being 'a fly on the venturi wall' trying to understand my carb.

So, even with a closed twist-grip (there is still a tiny air-gap under the slide), the contributing circuits to fueling the engine are; 'pilot circuit' (pilot jet & air screw) and needle straight section in the needle jet.  Yes I know all the charts say that it is only the pilot circuit but in this case I feel it is both circuits.

At Idle speeds with a closed throttle grip, the major contributing circuit is the pilot circuit, due to minimal 'concentrated' airflow over the needle jet to pull the same amounts as the pilot circuit can supply.  At a certain point the vacuum pulse over the pilot outlet becomes less and air-speed picks-up over the needle-jet then making the needle the major contributing fuel supplier.

With the two circuits supplying fuel at idle, this is like having a choke passage open, now realizing why I don't need choke to start the bike cold, even with dropping from a 68 to a 50 pilot jet.  Having dropped that far in pilot jet makes me think that the little less fuel supply at further throttle openings might require stepping-up the main just a touch, might jump up from the 160 to a 165 (168 is stock).

With a closed twist-grip, the needle is on the straight section at the needle-jet, and stays on the straight section for the first 10mm of throttle slide movement (to about 1/4 throttle in a 38mm venturi).  There is no denying that with a fresh 500cc cylinder, the intake vacuum at a closed throttle IS still enough to pull fuel from the needle-jet.  Whether this is just a design fault with the Keihin PE38 I don't know, but I will say that I swear the same thing is happening with the VM38 Mikuni on the KX400 (carb is off a KX250A5).

For a bit I thought that because I have the bike set so it would idle, that the slightly lifted slide might bring the needle-jet into play at a closed twist-grip (and I guess it does) but then I realized that with any amount of slide lift, the depression over the needle jet will draw fuel fuel past the needle, so with the same given intake vacuum from the 500cc piston, and, the fact that the straight section of the needle will still 'allow' fuel to flow out, fuel HAS to flow out if the slide lets any air past.

I guess the next step is to try (even though I don't want to disturb something's that now working ;-) ) letting the slide drop to completely to the bottom to see if I can stop fuel supply from the needle-jet with a closed twist-grip?

To keep the status-quo in the fuel curve, it makes sense now that running a smaller (richer) straight section needle required a drop in pilot jet size, it just took me a while to juggle the correct ratio to balance closed throttle & 1/8th throttle, as both circuits are contributing, varied amounts at various openings.

I now wonder how many other open-class two-strokes have this issue, with the them having such strong intake vacuum at closed throttle?

** EDIT ..... I just realized since I made this post that I have been confusing 'closed throttle' (slide all the way to the bottom of the carb) with 'closed twist-grip', because the high idle rpm setting meaning the slide is open (yes on the needle!).

Hi John,

The 38mmVM Mikuni on the CR480 you raced ran the UFO slide mod and had the pilot jet completely blocked off.

[John Orchard]

Yes I remember Jay, at kickstart speeds we had to have the twist-grip at 1/4 throttle for it get fuel from the needle jet before it would start.  Proving an open-class machine has the suction to pull it out.

[John Orchard]

OKAY FOUND ANOTHER ISSUE.

I swapped the crank seals around so that crankcase vacuum pulls the seal lips into the crank rather than away (seal recess away from the crank flywheels).

Gearbox oil was causing inconsistent readings as it partially sealed the crank seal.

The resulting vacuum in the gearbox was bringing in dust, making the gear oil dirty quicker. I didn't follow my suggestion of a fuel filter on the breather ( I was getting around to it).

I feel crankcase vacuum is much stronger than compression, especially at high rpm/low throttle, I'd much rather have the chance of losing some crankcase compression, rather than sucking in air and causing a lean mixture.

[Lozza]

If the throttle isn't completely open nor are the reeds 

[John Orchard]

If the throttle isn't completely open nor are the reeds 

A bike set to idle will have a partially open throttle valve that will let 'some' air past, with a piston rising in the bore and creating a vacuum, the reeds will only open enough to let available air through it.  So as you say "completely closed throttle valve means completely closed reeds".

[yamaico]

Great thread John. A logical progression of analytical thinking, informed decisions and seat of the pants testing - well written and well executed. You're braver than me though, diving in the deep end and changing half a dozen things at the same time, but that makes it all the more impressive.
This thread is what this forum is all about,
Cheers,
Pete.

[Lozza]

What would happen with a one way valve only letting pressure out of the crankcase solve this? Tried different seal brands or some teflon seals?

[John Orchard]

What would happen with a one way valve only letting pressure out of the crankcase solve this? Tried different seal brands or some teflon seals?

Why would we want to let 'pressure' out of the crankcase, it is not getting as extreme as the vacuum (lower pressure) in the crankcase?  The negative pulse from the expansion chamber and the opening transfers into the combustion chamber would be stopping any excessive 'positive pressure' build-up?

I think keeping a very low pressure in the crankcase would give an instant throttle response once the throttle is opened, yeh?  Now I've rotated the seals it seems to be holding the 'low pressure' (vacuum).

[Lozza]

I meant the gearbox part of the crank case As explained before the highest pressure in the crankcase is lower than residual cylinder pressure.  Throttle response more carburation than anything else

Pressure in the crank case(red),transfers(purple green blue) inlet (black) and exhaust(brown)

                                                                                                Pressure in cylinder(red) exhaust(black) crankcase(green)

[Daryl Jones]

I see crankcase pressure exceeding exhaust port pressure between TPO & IPO.
Cylinder pressure = exhaust port pressure, after blow-down.

Second graph shows nothing useful, because the scales of the measurements are so different.
Resolution of a combustion pressure transducer is not suitable for very low & negative pressures.
(Explained by the manufacturer, previously).

Cheers, Daryl

[John Orchard]

Thanks Lozza, very thought provoking, I'm sure I'll come back with questions & idea's after studying them  :-)