Needle jet still works with a closed throttle !

[John Orchard]

I thought I'd run something past you guys (what again!), trying to get my TS125/VM32 jetting fine tuned, I think (?) umm how do I put this into type .... ok I'll start somewhere.....

In neutral, rev the bike up, chop the throttle, rpm's hang up (probably around 1500 rpms?) like too lean (which it is), after aprox 20 seconds the idle comes down to aprox 800 rpm (nice idle), then finally conks-out.

I think, as the rpm is quite highish, with a closed throttle, just after closing the throttle, the engine is drawing fuel from both the pilot/airscrew AND the needle-jet, once the idle rpm can creep down, fuel stops being supplied by the needle-jet and then the pilot/airscrew has sole control.

So, in all my years I have never had a bike with this kind of issue, probably because I am basically starting from scratch, rather than someone else (the manufacturer) doing the hard work. I never considered that the straight section of the needle/needle-jet can be drawing fuel with a closed throttle! I guess only while the rpm is high enough to have air speed to do so.

While confusing myself, I had the pilot jet too rich and the needle-jet to lean, bike revs its tits off above normal idle rpm (lean), conks-out at normal idle rpm range (too rich) ..... all rpm affected, with a closed throttle !

Ok I'll work on my theory tomorrow, hopefully I am correct :-)

[sleepy]

Remember the slide cutaway has a big effect on the starting point of the main system and if the pilot is to big you have to have the throttle open further to overcome the rich mixture. Have fun. Did the opening out of the main air jet help?

[John Orchard]

Remember the slide cutaway has a big effect on the starting point of the main system and if the pilot is to big you have to have the throttle open further to overcome the rich mixture. Have fun. Did the opening out of the main air jet help?

I see what you are saying about the slide cutaway, I just fitted a WFO slide optimizer, so the increased airspeed at closed throttle is stimulating the needle-jet into action, I think the flow from the needle-jet might be ok, as long as it's not too rich or lean (and lean I think it is).

Yeh I removed the main airbleed, it cleaned up the wide open setting :-)

[sleepy]

Had a 74 CR125 here for an engine rebuild last year and he had a new TM32 on it and the thing was a pig to ride down low. Blubbery at idle even with the air screw out and lean up to about 1/4 throttle. You had to keep it screaming to get away in 1st gear but once moving it would pull down to about 1/4. I put in a smaller pilot and backed the air screw in a heap and then a richer slide( could have been a 2) and the thing was a different bike, he recons it has never gone so well and been so easy to ride.

[John Orchard]

Fixed - With WFO slide optimiser I have dropped the pilot jet from 57.5 to 15, yes that's right, and I've ordered a 12.5 & 10, dropped the needle-jet from an R-6 to an R-2, seems pretty good now. Ticks along at a nice low idle, playing with the air-screw, now I am 2 turns out (that's why I ordered some smaller pilots). Without the WFO slide thingo, the pilot-jet was right on the money, me being greedy, I wanted more off the bottom-end (I shouldn't keep comparing the ported 125 to my modern 450 4-stroke ;-)

I guess when I had a larger pilot-jet, I had to have the slide higher to keep it idling, that extra slide height allowed enough air to go through to get the needle-jet involved. With a leaner pilot keeping the rpm higher, it allows me to drop the slide down further, separating the introduction of the needle-jet circuit, me earlier thinking it was rpm introducing the needle circuit, but no, slide height ....... 1 turn of the idle adjuster!

The only issue I have now is, from a cold start, even with the choke on, I can't get fuel to flow into the venturi (Start ya Bastard or hand choke required), I think that is more my 'expert' porting ;-) I think that combining my 'non reed-valve porting' with the crankcase reed has softened the intake vacuum pulse from the piston up-stroke (because vacuum starts as soon as the reed opens, and the inlet port is still open while the piston is well back down the bore), next mod is to JB Weld the reed passage, take out the TM piston, put the RM-S back in, and make good use of the regained intake port inertia (it kinda works in my mind). :-D

[bazza]

WFO optimiser say halve the pilot jet. Do you think optimiser works?easier to start? Thanks Baz

[sleepy]

Fixed - With WFO slide optimiser I have dropped the pilot jet from 57.5 to 15, yes that's right, and I've ordered a 12.5 & 10, dropped the needle-jet from an R-6 to an R-2, seems pretty good now. Ticks along at a nice low idle, playing with the air-screw, now I am 2 turns out (that's why I ordered some smaller). Without the WFO slide thingo, the pilot-jet was right on the money, me being greedy, I wanted more off the bottom-end (I shouldn't keep comparing the ported 125 to my modern 450 4-stroke ;-)

I guess when I had a larger pilot-jet, I had to have the slide higher to keep it idling, that extra slide height allowed enough air to go through to get the needle-jet involved. With a leaner pilot keeping the rpm higher, it allows me to drop the slide down further, separating the introduction of the needle-jet circuit, me earlier thinking it was rpm introducing the needle circuit, but no, slide height ....... 1 turn of the idle adjuster!

The only issue I have now is, from a cold start, even with the choke on, I can't get fuel to flow into the venturi (Start ya Bastard or hand choke required), I think that is more my 'expert' porting ;-) I think that combining my 'non reed-valve porting' with the crankcase reed has softened the intake vacuum pulse from the piston up-stroke (because vacuum starts as soon as the reed opens, and the inlet port is still open while the piston is well back down the bore), next mod is to JB Weld the reed passage, take out the TM piston, put the RM-S back in, and make good use of the regained intake port inertia (it kinda works in my mind). :-D

With the cold start issue try and drill out the cold start jet in the float bowl. Memory tells me they are around .75mm and from there go up to something like .9mm.

[John Orchard]

With the cold start issue try and drill out the cold start jet in the float bowl. Memory tells me they are around .75mm and from there go up to something like .9mm.

Hmmm hadn't thought of that, thanks.

[John Orchard]

WFO optimiser say halve the pilot jet. Do you think optimiser works? easier to start? Thanks Baz

EDITED

I had a WFO slide plug in the RM125B (VM34) that I sold to Ted (Oz Champs winner), it was awesome, rpm ripped from idle, I had to drop the pilot from 50 to 15, easy to fit, no hassles at all, the one I am fitting now is a right bastard, had to do major mods so it would fit my VM32 carb body (the underside did not match the shape at the point at the bottom of the slide travel), then I had to modify the WFO again to fit another VM32 carb that I had! I thought all VM32's were the same, maybe one was Chinese? I was using two carbs to speed-up tunning options.

I think the WFO's make sense, for a carb venturi to need a depression over the needle-jet, for the life of me why would Mikuni design a slide with a hollowed-out bottom, one of the reasons a flatslide carb rips off the bottom.

[skypig]

If not this forum, where would one find this information?!

Carry on gentleman.

[John Orchard]

With the cold start issue try and drill out the cold start jet in the float bowl. Memory tells me they are around .75mm and from there go up to something like .9mm.

[/quote]

Awesome sleepster! Thankyou very much, found the enrichment-jet was about .40mm, opened it to .80mm, easy start now on this 1 degree morning here in Paradise (I kid you not, that is the name of the town/area that I live in). I think this matter deserves a thread of its own, I've never considered, or even thought about enrichment jetting in all my 63 years :-)

[Momus]

Mikuni has thought about it.
Some of their carbs have replaceable jets.

[John Orchard]

This is great info also ....

https://www.cycleworld.com/story/bikes/carburetor-needle-tuning/

[sleepy]

All good info and it shows getting a good running bike isn't always that simple. I often get scared when I hear of someone say I got a new Mikuni for my restored bike as the original carb was worn out only to find that even with the same Jet spec as factory installed they run like crap and often seize that nice new piston they had fitted. The differences between carb bodies old and new can be quite marked with Air jets fitted or not and different idle passage sizes ect.