IDF Carb Tuning Guide

[sptcoupe]

The pics are posted in the "Performace" thread, the post called "New Cold Air Box for the Coupe, the date is Jan 15, 2012. It shows up about page 2 of the posts.

[maytag]

Hey folks;
I was forwarded the air-horn information discussed on page one of this thread. I will post it up for everyone. But I'm remote right now... So in the meantime:

Does anyone have any experience using the "Unisyn" type synchronizing tool, as compared to a "syncrometer"? Or even carb-sticks?

[sptcoupe]

I've used them all and the Synchrometer is the most accurrate and easiest to use. I still have two Unisyns, but haven't used them since I got my Synchrometer many years ago.

[maytag]

yeah, I just put in new chokes & idle jets, and went to use my unisyncs, and they just aren't cutting it. (could be in part 'cuz I haven't used 'em in a decade and they're all dry and crackly. )

So looks like I'm buying a syncrometer.

[fiat218]

yeah, I just put in new chokes & idle jets, and went to use my unisyncs, and they just aren't cutting it. (could be in part 'cuz I haven't used 'em in a decade and they're all dry and crackly. )

So looks like I'm buying a syncrometer.

what do u have for cabs? and what what numbers are u using as far as jetting. i may want to copy if u have the same carbs

[maytag]

yeah, I just put in new chokes & idle jets, and went to use my unisyncs, and they just aren't cutting it. (could be in part 'cuz I haven't used 'em in a decade and they're all dry and crackly. )

So looks like I'm buying a syncrometer.

what do u have for cabs? and what what numbers are u using as far as jetting. i may want to copy if u have the same carbs

I wouldn't copy me, unless you're at 4k' elevation too.
But at the beginning of this thread there is a great narrative that will help you get some baseline jettimg #'s

[maytag]

here's the air-horn information discussed previously in this thread.

[maytag]

I could use some help from those with experience with these Webers.
Here's what I've got:
1800, standard bore, domed pistons @ mathematical 9.65:1 compression. regrind cams with 10.8mm lift, 300d duration on the intake. Big valves. Ported intake and minor porting on the head. 44 idf carbs. Mariplex dist, advanced to 10d plus the stock vacuum advance, ported from the manifold, not the carb. And I live at ~4000' elevation.

the carbs have 34mm chokes, 4.5 aux vents.
F11 emulsion tubes with 200 air-correctors.
135 main jets
55 bleed-backs (pump exhaust)
and 55 in the idle jets.
I do not know what size Pump jets I have.
Pierce manifolds recommended the 34mm choke, based on my altitude. (??)

So this evening, I set my floats to 10mm open. I screwed the air-bleeds out to 1-1/2 turns, and then set my idle speed screws to 1/2 turn past where they touch. Tried to use my unisyn's then to begin a balance.... but they suck. So I'm ordering a Synchrometer, but I figure as long as I'm placing an order, I might as well get the other stuff too.

Based on what I'm reading at the beginning of this thread, I should think I want to change to the following:
first; add 7m base spacers.
F9 emulsion tubes
192 air correctors
142 mains
00 bleed-backs
Don't know what pump jets I should get?

But I'd like to know what of this will be impacted enough by my altitude that I should rethink it?

As an added piece of the puzzle, I've driven it enough to know what the current base is doing:
Idles well, but rich. It also screams well when all the way into the throttle. (though it does seem to taper-off a little at the top). But the part-throttle is terrible, as is the transition to throttle from idle. It give all the classic signals of flooding: it sounds like it's gargling... but if you keep your foot in it, it finally kicks and coughs and clears its' throat and then screams like a sum-B.

can anyone give any opinions on where you think I should be? Does what I've laid out above seem correct to you?

[sptcoupe]

Maytag - thanks for posting that darn chart. It illustrates visually what I have been trying to say about horns - that not all those horns that are commonly sold actaully improve airflow.

Before jetting comments, it sounds like you may have messed up a bit on the tuning process. The jets are close enough to get it fired up and tuned, so to initially set the carbs up:

- Turn the idle mixture screws in until they are lightly touching the base, then back them out each 1.5 turns.
- Turn in the air by-pass screws until they are lightly seated
- Turn in the idle speed screw until it just touches the linkage tab, then turn it in another 1/4 to 1/2 turn.

Fire it up and use your Unisyn to balance the bores on each of the carbs by identifying the bore on each carb that is drawing the most air. Tighten that air by-pass screw down on each carb and leave those two alone. Use the Unisyn to bring the lower drawing bore up to the other by backing out the air bypass screw until the bores of each carb are flowing equally (or as close as you can determine with the Unisyn). This is balancing the carbs - getting thre flow equal in the two nbores of each carb.

Now you synch the carbs, to make sure the throttle plates of both carbs are open equally at idle, and open at the same rate/time as you accelerate. Then measure the flow of the #2 bore against the #3 bore and using the inter-carb linkage screw, turn it in or out as necessary to get the #2 and #3 bores drawing the same. Now, all four bores should be drawing the same.

Now set the idle speed. If the idle is too high or low, use the idle speed screw to move it up or down until it is +/- 800-900 rpms.

With the idle speed correct, check the #2 and #3 bores to ensure the carbs are still synche'd, and use the inter-carb linkage screw to adjust as necessary to get them synche'd again. Then check all the bores to see if they are drawing the same.

Now we set the idle mixture.

Turn each idle mixture screw in until the engine stumbles, then reverse until the engine smooths out and go about 1/8 turn further. If you turn the idle mixture screw in and nothing happens, then turn it out from the starting point. If the engine smooths out and idles higher, then continue turning it out another 1/8 turn or so. Do that starting with the #1 bore and then move to #@, #3 and #4, for all four bores. The idea is that you set the idle mixture just slightly richer than the point at which it becomes too lean, but be sure you are not too lean. The idea is not to have all the idle mixture screws turned in or out the same number of turns (as some tuning guides suggest), but rather to get the optimal mixture for each cylinder. This is a slow process, just keep repeating and re-setting idle speed as necessary until the engine is running smoothly. Then re-check that the carbs are still synche'd, etc.

The syptoms you decribe are likely tuning errors as much as jetting. Those same symptoms that we automatically assume is a rich condition are also caused by bad idle mixture settings and poorly balanced and poorly synche'd carbs. So try that again and see what happens.

As far as jetting goes, you are probably very close.

Do you have a header? Also, your effective CR will not be as high as calculated at 4000', and you'll have to experiment some to deal with the cams (different from one side to the other). The 34mm chokes are OK, but they will restrict the top end somewhat.

I would try the following jetting using the F11 emulsion tubes:

140 mains (the 135s may be fine with the cams and no header),
195 0r 200 air correctors
52 idles
35 bleed backs
200 needles
50 pump jets

However, I wouldn't change anything before I re-did the tuning.

Also, I don't quite understand the float adjustment you did. The float should be 10 mm from the carb top when it is closed (11mm with the gasket in place), and 32.5 mm when open. If you only have 10 mm when open, it will hardly run.

Again, the part throttle symptoms you describe are precisely what poorly tuned carbs will produce. It finally quits gargling and then kicks in and screams if you keep your foot in it is because it finally gets on the main jet circuit and gets past all the bad adjustments. And the fact that it screams means you are probably pretty close on jets now, so get it tuned and synche'd up before you change anything!

Keep us posted.

[sptcoupe]

Whoops - forgot to mention the spacers. Get the crans tuned and jetted first - then worry about the spacers. You probably don't need 7mm where you are.

[maytag]

Jeff..... You are awesome! Thanks for all the help!

Yes; I'm running Allison's header, and 2-1/4" pipe the rest of the way.

I will work on the running again in a couple days, and will follow your recommended procedure and report back.
Thanks again!

[maytag]

- Turn the idle mixture screws in until they are lightly touching the base, then back them out each 1.5 turns.
- Turn in the air by-pass screws until they are lightly seated
- Turn in the idle speed screw until it just touches the linkage tab, then turn it in another 1/4 to 1/2 turn.

Fire it up and use your Unisyn to balance the bores on each of the carbs by identifying the bore on each carb that is drawing the most air.

Last night, I went back out in the garage very late. I did as above, and the car would barely idle. 300-400 rpm, and wouldn't idle for long. And certainly wouldn't sustain that idle once I placed a restrictive Unisyn over one throat. And that Unisysn is in such bad condition it wasn't reading anything 'til 1500rpm anyway. I'm junking 'em.

I'm thinking the mixture may still be fat, and that's why she's idling so low. I'll try leaning-out with the mixture screw and see if the idle comes-up enough to perform the procedure you've outlined? Or would you suggest I bring it up with the idle speed screw instead?

I found a VW shop that has a Syncrometer on the shelf, so I'll stop by and pick it up this evening.

[maytag]

update:
I picked-up syncrometer tonight, and then went through the steps outlined above. Man the thing sure runs alot better! WOW!
but there's still a stumble on tip-in. I feel like it's too rich from the accelerator pump / pump-jet. that's a guess.
Could this be caused by too large pump-jet? and can I change the point at which that acc-pump comes-on by adjusting those threaded-rods on the bottom?

[maytag]

alright!
Today for my birthday, I installed Mark's distributor-less ignition. I had to pull the head off too, to replace a head gasket, so I took the opportunity to refine some other issues as well.

(by the way, the ignition system went together very easily. It's very-well made, and came with clear, idiot-proof instructions.)

then I went back to the carbs, to try to fine-tune them a bit more.

I think that now they are all tuned and balanced about as good as a redneck with a syncrometer can get them. And I've still got some issues.
They sound great, and idle well. they also pull well once you get into it. But the problem I'm having is at throttle-tip-in; it bucks, spits, pukes, gargles, whatever you want to call it, and then finally (if you keep your foot all the way in it) it comes back on and screams like a banshee... or whatever.
If I cruise down the road at 40mph in 4th, it'll load-up and stumble all over itself for five seconds if I try to accelerate.

I think it's just plain fat all through the entire lower range.

So I think it's time to change out some hardware.
I'm going to do as Jeff suggested and go to a 35 bleed-back, but I'll buy some 00's as well, just in case.
I'm also going to verify I've got 55's in the pump-jets so we know where to go from there.

Anything I'm overlooking?

I wish I didn't have to wait days-n-days to get jets.

[sptcoupe]

I have a lot of jets you can use to get it right. And maybe a call would be helpful when I can talk to you while you are working on the carbs so I can walk you through the process. Just back from Vietnam tonight and bushed, but I'm at 713 594 9963.