Funky Ida jetting

[Peter]

hello,
My Ida's are jetted like this:
IDLE :65, holders: 120
main :135 air correctors: 200
f11 tubes, 37 vents
They sit on a 2276 fk8 9.5cr

When i use the rule i get 148 for the mains, so i bought 155's and used them in the beginning, but it was way too fat (afr is 10)
So I put back the 135 thinking it would be too lean, but at wot, now i have about 12 for afr, so still a bit too fat.
I think its strange that its still fat if i look at other motors with ida's and their jetting, because i

[Peter]

I think its strange that its still fat if i look at other similar motors with ida's and their jetting, because i am using 200 air correctors (they should lean it out more i thought?) I thought main and aircorrector values should be like 160 mains and 200 airs or 140 mains and 180 airs...

I try to find more tuning tips on IDA's but there is not much to find (like a manual or something)

And then there's a lean hole at 2700 -2900 at very low loads...like driving down hill at 3000 it spits sometimes?

I am thinking to get 100 or 110 idle jet holders and F7 tubes ... any suggestions?

THANKS!

[richie]

Were the carbs new? if not its quite likely the 135 mains have been drilled out bigger,the 110 idle jet holders may help with the lean spot around 2700,you may have the carbs slightly out of synch causing the spit driving downhill etc,the airs only really come into affect at high rpm in my experience

cheers richie

[Jesse/DVK]

Float height and pressure is correct?

[Peter]

thanks !
No they are used italians
float height set at 24.2 with your manual
pressure i am not sure... i use a Hardi pump without reg.
I think it was about 0.2- 0.3 bar...
about the lean spot.... this is only when cruising or going down hill... when i stab it at 2000 lets say, the afr stays at about 12 through the range.
1 thing i dont understand: if you stab it at 1500 or so, are you still on the idle circuit? because what i read is that the switch from idle to mains is only rpm related? so mains start from 3000 independant of throttle angle? common sense tell me that its both, but i dont know anything about carbs, so i am not sure...

cheers, Peter

[richie]

thanks !
No they are used italians
float height set at 24.2 with your manual
pressure i am not sure... i use a Hardi pump without reg.
I think it was about 0.2- 0.3 bar...
about the lean spot.... this is only when cruising or going down hill... when i stab it at 2000 lets say, the afr stays at about 12 through the range.
1 thing i dont understand: if you stab it at 1500 or so, are you still on the idle circuit? because what i read is that the switch from idle to mains is only rpm related? so mains start from 3000 independant of throttle angle? common sense tell me that its both, but i dont know anything about carbs, so i am not sure...

cheers, Peter

When you are cruising it will stasy on the idle circuit to around 3000rpm,then start to affect the main circuit,but the carb doesnt know what rpm your engine is doing ,only the vacuum signal it gets from the air travelling through it[that may not be quite the correct term? ] when you hit the pedal hard it creates a different vacuum signal to cruising and affects both circuits differently.

What is the spec of the rest of the engine,that might give some clues to jetting and venturi sizing

cheers richie

[Peter]

hello Richie,
engine is 82 * 94
deck is 1.6 mm
cb wedgeports (44 * 37)
+- 9.5 CR
1 5/8 header
37 vents

I ll re-synchronize them and try again tonight 

[speedwell]

 

[Peter]

thanks Fabs!
still got to get rid of the ignition system and install the scintilla 

[nicolas]

thanks Fabs!
still got to get rid of the ignition system and install the scintilla 

send the ignition over to me then!

the 37 vents can explain part of the 'problem', but like Richie said if you cruise at a steady speed (higher rpms or not) you will be on the idle circuit, the mains affect it a bit, but they play a bigger role in accelerating and higher rpms when the air sucks creates a lower pressure and sucks in fuel.

the lean spot going down hill is this because you let go of the trottle? if so you have indeed a leaner situation, just floor it when going down hill as well 

[Peter]

Guys,
I recently changed to a magneto for ignition.
since changing, the motor ran fatter (black plugs) (same jetting as above)
and it didnt run very well anymore...
(also had to change needle valves because the old ones were leaking, but now they are 3.0)
so changed the idle jets to 060 and the mains to 150.
I know the 150 mains probably are too small but i bought the reamer to enlarge them.
I thought by reading the plugs i can find the good mains..
So did a testrun, and the car felt crisper in the idle circuit, but when the rpms went up, and i think the mains take over, the engine holds back,
but not really backfiring or anything...
tomorrow i ll get new and hotter (NGK D7EA) plugs and try again.
What size mains do you guys suggest? I ve got 40 mm vents now, so i thought i should aim at about 170?
Air correctors are 200.

Thanks, Peter

[Erlend / bug66]

Do you have a wideband?

My IDAs like big fuel and air jets.

[Peter]

yeah,
but its in pieces,
i have to get it working again

[spanners]

I run 185x200 on 40 vents, but on DRLA, 60 idle and pumps, I would guess you should be there or thereabouts, I only use plug chop tuning for track racing, I think rolling roads have limited use on variable throttle racing, I aim to be over stoich  anyway and my plug colours are were I want them, track cars always lean off towards the end of a race, I often see motors getting slower and slower with increasing temp, so some fatness can help with a passive inter cooling affect in the chambers.
Another factor may be your manifold porting, a 40 vent may not be pulling the juices if the manifolds are under ported, let alone even smaller ones going the same way. Good luck with it all anyway.

[Peter]

thanks,
all changed when i removed the distributorless ignition.
the plugs before had a nice tan...
i ve had the magneto recoiled and checked over but the spark must be less then before i think...
I also free revved it and kept it in higher rpms to see if the points were floating or something,
but without load, it didnt misfire or stumble, so i guess the mag works..
 

[Peter]

Guys,
I fixed it! 
i received the jet reamers today and a jet gauge...
so first i wanted to know what mains i had been running in the past...
turns out they really were 135's! i still dont understand how it could run that good with those jets..
any way, i removed the new jets and reamed them out a bit..
hey, it runs better, but still hesitation!
So out with the mains again and ream a bit further..
I didnt really expect this, but damn, she really runs good again!!!

So I am a happy man today

[nicolas]

Guys,
I fixed it! 
i received the jet reamers today and a jet gauge...
so first i wanted to know what mains i had been running in the past...
turns out they really were 135's! i still dont understand how it could run that good with those jets..
any way, i removed the new jets and reamed them out a bit..
hey, it runs better, but still hesitation!
So out with the mains again and ream a bit further..
I didnt really expect this, but damn, she really runs good again!!!

So I am a happy man today

what size did you end up with? just very curious.
i am calculating the jetting for my 2007cc engine, but it has very small 34 vents (DCNFs though) and seems to me the 135 mains should be OK, even if this seems also really small for what it is. probably the jets are correct, but in the end i will need to step up to 36 vents. but for now we'll see how this will work out. i have a size or two bigger if needed.

how does jetting change with a change in CR? it must affect the 'sucking signal' (i am sure this is the wrong term  , but you DO get the picture)

[Peter]

Hey nicolas,
I dont know what i ended up with to be honest,
the gauges i got only went to 150 
i thought they were up to 200 when i ordered them.
i just went bigger by feel, reinstalled jets, testdrive,  again, until it ran better,
i must say in higher rpms it runs better than ever! theres an extra kick in the ass when it revs up
i will order the bigger gauges and will let you know!
next now is buy a signal converter so i can install the tach

[alex d]

oops