IDF Tuning

[Hatracks]

Engine build:
1679
rotating assembly w/ lightened flywheel and kennedy stage 1 balanced as unit
ACN L3 heads
1.1 rockers
Webcam 218 grind type 1 cam with 108 LC
1 3/8 merged header/J pipes
Dual 40 IDFs w/ 28 venturis, 115 main 180 air 50 idle
034 SVDA timed at about 7.5 btdc
Fuel pressure right about 3 psi at 3000 rpm
Synchro tool is right about 3.5 on both carbs at idle.
I get fuel squirting as soon as i touch the throttle.

The car idles rock steady at about 850RPM and when doing best lean adjustment I am turning in until miss, back out to best idle and it is very definitive and the engine sounds great.

When driving though the car is very responsive through the gears when you are in throttle, when you are cruising though if you let the RPM fall into that 2200-2400 range and get back in the throttle sometimes there is a little bit of hesitation before it really picks up again.

I dont feel like i am more than 1.5 turns out on the idle mixture but maybe the idle jets are to small? Or am I just not used to driving an aircooled car and need to down shift and keep the RPMs above 2500?
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73 Super Build

[67rustavenger]

Up your main jets to 120. I have the same sized engine in my 67 but with a different webcam. I could never get the hesitation to go away. But could lessen it.
Your idle jets are in the right range for that engine.

I changed to 36 Dellorto carbs and they help it drive Sooooo much better.


Good Luck.
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I have learned over the years.
Cheap parts are gonna disappoint you.
Buy Once, Cry Once!

There's never enough time to do it right the first time. But there's always enough time to do it thrice.
GFY's Xevin and VW_Jimbo!

[Frank Bassman]

Great advice from 67.

I worked on a sandrail with dual IDF's and mystery displacement. Don't you love it when you can't get a positive assertion about engine size?

Anyway owner said it was "a 1770 or 19-O-something"

Guess he didn't really care, just wanted it to run good.

Tuned it and drove it, hesitation where you describe.

Upped mains to 120 and idles to 55.

Has a baja extractor header, 1 - 1/2 ". Mystery cam.

Anyway plugs look great, has a TON more power, and no hesitation with 009.

try that.

-Frank

[Coyotemutt]

I haven't played with IDFs much, but if it is anything like most carbs I HAVE played with, it sounds like you actually might need a slightly larger air jet to bring the mains in sooner and clean up the transition.

The 50 idle is probably fine. If you aren't getting any bucking at a steady throttle position under 3000rpm or so, the idle is sized right. Since you are having issues transitioning between part throttle and heavy throttle I would expect that the mains could be too small, mains coming in late, or the accelerator squirt is not what it needs to be.

Take that with a grain of salt though. I'm coming at this from a background of different carb setups.

[67rustavenger]

[modok]

Might need to go up to 120 main and 200 air. Very common. Usually don't say to change two things at once but in this case I would.

OR, could be something to do with the float level and emulsion tubes.

The emulsion tubes have the 8 holes around, and you want the fuel level to be just BELOW that. So, if the holes are drilled a little too high.....you need the float level higher, or, if they are drilled real low right IN the step, it will want a 11mm-11.5mm float level. Depending on fuel pressure

[67rustavenger]

If your not familiar with the term "Step" with regard to emulsion tubes.
I believe this is what modok is referring to.
Disclaimer- This a F16 emulsion tube for demonstration purposes.

Good Luck.
_________________
I have learned over the years.
Cheap parts are gonna disappoint you.
Buy Once, Cry Once!

There's never enough time to do it right the first time. But there's always enough time to do it thrice.
GFY's Xevin and VW_Jimbo!

[modok]

Yes, exactly.
That step is almost always in the right spot, but the row of holes varies a bit. sometimes they are IN the step, other times they are 2mm above it.

[Hatracks]

John at AC.N had me reset the timing to 10 BTDC and reset the idle mixtures before we started swapping jets. I tuned it real quick for best lean and took it out for a drive. She woke up quite a bit and the hesitation seems to be mostly gone. I will drive it more this week and report back.

Cant say enough for the support at aircooled.net even answering emails about carbs on a Sunday
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73 Super Build

[Hatracks]

So the car is still running well and driving well once warmed up. It wants to backfire a tiny bit right after started when cold (40-50 degrees out).

And I am back to #1 seemingly not firing at idle, was fine last week but this week when adjusting for best lean idle it seems to run best with the mixture valve all the way closed (idle picks up) and if I pull the plug wire off the distributor it does nothing again.

Car runs really well though
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73 Super Build

[Vanillagurilla]

[[email protected]]

if it's misfiring when cold, but not when warmed up, the mixture is very close if not perfect. If it doesn't pop when cold, it's too rich when warmed up.
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[Hatracks]

[Hatracks]

I ended up throwing in some 200 air and 120 mains and the car does make more power(seat of the pants dyno). Timing does still need to be at about 10 BTDC though if I retard it to 7.5 the small hesitation does come back but its only really noticeable if I am in first and kick the clutch hard into second and jam the accelerator to the floor.

Ill drive it a bit like this (50 idle, 200 air, 120 main, 10 BTDC) and then pull the plugs and see. Idle mixture screws are about 1 turn out on all of them, maybe a little less.
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73 Super Build

[modok]

Sounds about right.

I ran a set of 40idf-70 for a few years and they did have a odd hesitation after a fast shift that I never did tune it out. It would only do it when temp was 45-55 degrees, and would go away if above or below that. 3-4 degrees more timing would cover it over. I don't want to send you chasing fictional baloney but........... that fact that every OTHER model of carburetor did not do this, makes me think it was related to the idle system feeding back into the main well. It's just process of elimination. If changing to 36 idfs or 40idf-68/69 eliminates the spot......then I'm just looking at what the difference is.

Maybe it is unrelated, OR maybe your particular combination is making this problem more pronounced.

[Hatracks]

[[email protected]]

Thermostat?
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[Hatracks]

[[email protected]]

really with IR carburetion, if the carbs are sized properly for 4-5k RPMs, they are "too big" for off idle. A little driving experience will stop you from going WOT at 2k and lower anyways, because that just bogs.

If it hesitates slightly when cold but not hot, you are probably fine. If you tune to get rid of the cold hesitation, then you are too rich when warmed up. The carbs have no chokes, so it's a compromise.
_________________
It's just advice, do whatever you want with it!

Please do NOT send me Private Messages through the Samba PM System (I will not see them). Send me an e-mail to john at aircooled dot net

"Like" our Facebook page at
http://www.facebook.com/vwpartsaircoolednet
and get a 5% off code for use on one order for VW Parts ON OUR PARTS STORE WEBSITE, vwparts.aircooled.net

[Hatracks]