Fuel Mixture Control in Relation to Throttle Position Diagram

[philward]

Can anyone point me in the direction of the diagram  that shows the 'Fuel Mixture Control in Relation to Throttle Position' - I've seen it in a thread but can't for the life of me find it now! It's the one that shows the overlapping function of the carb jets, cut out, needle in one portrait orientated diagram.
Need it to help with the ongoing fine tuning of my CR750 Replica

[Nurse Julie]

I think it was Nigel, K2-K6 that posted the picture in the past Phil.

[Nurse Julie]

It may have been similar to this Phil
http://www.nightrider.com/bt30/carb_jet_ranges.htm

[philward]

Thanks Julie - have saved it and hopefully Nigel will be along shortly

[K2-K6]

Hi Phil, was out down the pub with a bunch of Landrover-istas last night and didn't get time to respond.

Think Steve (admin) posted a good one some time ago, but julie's site a good resource.

I view it more like this one as believe the idle jet has more influence than generally appreciated in trying to optimise these and especially if you've non std setup to mould into decent response.

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Have you specific characteristics to discuss/dissect to clean up running ?

[philward]

Thanks Nigel – I’m trying to clear up ‘popping’ when de-accelerating at lower revs (below 5000) which I am guessing is a weak mixture. As a reminder, I’m running 836 standard compression Cruzinimage pistons, standard carbs with foam pods and 4 into 4 race exhausts fitted with modded K0 baffles. Boyer ignition/power coils timed as per Boyer instructions.
After much tinkering, the carbs are running 135 mains (initial 120 standard main stopped motor breathing/revving) with needle on middle groove with 45 slow running jets (these are 40’s drilled out to 45 as 45’s not available). Using your guidance on setting the air screws, I get virtually no change of engine tone on any of the 4 cylinders (surely not all circuits are blocked – they all blow through with compressed air). Tried 50 slow running jets (drilled 40’s) and popping cured but plugs then sooty/fouling. Tick over is uneven varying between 1000 – 1100 rpm. Starts on the button.
I think the uneven tick over may relate to intermittent miss on 2 & 3, Boyer ignition has been checked by Boyer as ok but I didn’t send the coils for testing – I’m away this weekend but will test coil/lead values when I get back in case it’s a coil (before moving onto carb tuning).
Haven’t been able to do an accurate plug chop due to getting onto a decent road (M6 roadworks - being changed to smart motorway near me and no suitable A roads).
Wanted to look at the chart to consider whether a move down a notch may be the answer.

[Bryanj]

Is it possible that one of the wasted sparks from the Boyer is igniting gas in the exhaust, boyer is the only one that fires all four plugs together so 3 wasted sparks per cylinder

[philward]

I have run Boyers on previous bikes without this trouble - but not with a free flowing exhaust which I have on the CR Rep.
I  dragged this off tinternet (Dynajet site)
'Decel popping is caused by the detonation of unburned fuel in the exhaust pipe. This happens with high flow exhausts that allow more fresh air to be pulled into the pipe, causing the exhaust temperature to rise and detonate any unburnt fuel'.
Based on the chart showing the influence the needle has across the rev range, I'm thinking of putting a smaller main jet in (130 instead of present 135) and lowering the needle 1 notch to richen the low range and maintain high end mixture. This thought is based on the popping going when I went up a size on the low running jet - although it was too rich

[K2-K6]

I was also, like Bryan, wondering about all the lost sparks floating around too, but as you note it has to have a scource of fuel to allow it to ignite anything and make that popping.

As we know it's a compromise in running significantly away from original fit on these, making sometimes odd setting requirements to get a competent running.

Thinking it's worth discussion of error potential to hopefully help give a more targeted setup to help. The main jet sizing I view as a problem as moving away from std size while still using the std needle to modulate that flow will build in error that's contributing to low speed mixture problem.
When the main jet bore is shifted it will alter the closed position (needle right down into main jet) and let the main jet flow excessively at closed throttle when it should be effectively shutting it off.
This would have the effect of moving that needle modulation patch on the schematic downward into the idle mixture area and "doubling" the idle mixture route. That could explain why you don't get much response to idle mixture screws (there's the other, uncontrolled scource via main ) to see what they are doing.

I don't think the engine specification needs bigger mains to any great degree as the cylinder draws more air volume on intake, to which the std main jet will add a corresponding fuel amount ordinarily. It's something that people historically miss when modification is made to capacity. That doesn't mean you're not experiencing "lean" conditions as you open the throttle though. I feel this is caused by low to mid range mixture that comes with pods that prevents the rpm rising during acceleration so that it just falls flat on it's face as you demand higher torque when riding. In essence you never get to judge ultimately flat out mixture as it'll not transition to that area of throttle.

To give a start point in taking this into account, I'd try the 120 mains with the needle down one notch (clip up one) from central, then put the second largest idle jets in to see if you can find adjustment on them (possibly consider the largest) to encourage flow raised as it comes up to mid throttle a litte like an accelerator pump effect and see if it'll run up to higher range with that.

Also consider running with ignition retarded by 2 degree from book to calm rpm at tickover which would then cause you to lift the throttle slides at tickover to get steady rpm. This will reduce the closed off (twist grip closed) vacuum level and pull less fuel in through those enlarged idle jets. It may reduce the fuel available to pop on overrun too and potentially compensate more fully for running pods too.

Nigel.

[philward]

Thanks Nigel, only just seen your post as away again in the motorhome - I'll try to analyse that info and bombard you with more questions when I get home!
When e say idle jets, is that the slow running jets (40)?

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[philward]

Update on Nigels suggestion, fitted 120 mains, fitted .45 slow running (one size up) and moved needle 1 notch down (richer) - I know opposite to Nigel's suggestion - but only just got round to a test ride today. Popping on over run gone and revs out fine at higher revs. 'Chuffing/popping' on tickover still there with very slight and only slight variance in tickover (probably 50 - 75 rpm).
So nearly there! Probably the Boyer 'lost spark' in conjuction with race exhausts. Will try 2 degree timing retard.
Thanks for all the help