IDF 40 Questions

[JerryH]

The PO had upgraded to IDF 40s. I guess I got lucky. I've got some questions regarding these carbs. I'm new to carbs but I've leared a lot from other posts on this forum. Here is a photo of my setup:


The car sat for ~5 years before I purchased it. After I purchased the car I disassembled the carbs and cleaned them up. They seem to run well now, except for idling. I've got the throttle screws turned in all the way. The mixture screws don't seem to have a strong effect on idling. Once the engine is warmed up it idles fairly well at around 1100 RPM but when it's cold it stalls unless you keep it going with some pressure on the accelerator. I'm getting over 20MPG and the engine runs well on the road so they must be operating close to okay. I've developed an accelerator pump drip on the front carb (they still have red diaphragms) so now I've ordered a rebuild kit. I'll be taking these apart again. I plan to note the sizes of all the jets. I'll make sure the idle circuits are clear.

Here's my questions:

1. I really don't like the throttle linkage connection (green cable in the photo). The cable is jammed against the side of the engine bay. I plan to cut the bracket and weld it so the cable connection is closer to the carbs. Any comments?

2. Do I absolutely need a synchrometer to tune these and keep them working? It appears that to use the meter you jam it down the air horns. My setup doesn't have air horns--I suppose for clearance issues. How would I get a synchrometer to work with my setup?


3. My engine vacuum advance is not connected. Where would be the best place to connect it? The brakes get their vacuum from a connection on the intake manifold. Should I connect the VA there?

4. The carbs 'start assist' mechanisms are not connected. I was thinking about connecting them to a cable and routing it through the firewall to the throttle knob mounted below the dash. It would help when then engine is cold. Comments?

Thanks for your help.

-Jerry

[So Cal Mark]

to begin with, your idle speed is too high for proper adjustment of the idle circuit. You really do need a synchrometer or have gobs of patience trying to synch them.

[Daniel]

Jerry

You would need to remove the air filter bases to use the sync and to sync them without a meter would be
impossible you might get really close but i fine it hard to believe someone could do this without a meter .

The idle mixture screws should be about 1 turn out more or less start from there .

Vac advance, is there a smaller fitting on the carb around the bottom ? i thought IDFs had theses it's
been a couple years since i messed with any if so connect it to that do not connect it to the manifold
outlet for the power brakes .

The start assist is fine that's what it was designed for you might have to come up with a way
to control 2 carbs .


good luck

Daniel

[ventura ace]

. . . You would need to remove the air filter bases to use the sync and to sync them without a meter would be
impossible you might get really close but i fine it hard to believe someone could do this without a meter . . . .

Agreed, you need some type of instrumentation to compare the flow or vacuum in each carb. I don't know anybody that can do so by ear. I'm not sure that removing the air filter bases will help, Daniel. It's those enrichment tubes sticking up that make it difficult to use the synchrometer, because it bumps into the tubes. You may need to buy a couple of velocity stacks (or air horns as you called them) just for doing the carb sync, then take them off if they are a restriction for your filters.

There may be another option. I've noticed that my carbs each have a threaded port down next to the idle mixture and air corrector screws. I believe you could remove these screws and attach a set of vacuum gauges, and use vacuum for sync adjustments just like we do for motorcycles. I've thought about trying that myself. You'd need a set of calibrated vacuum gauges and the proper fittings, however. You can buy a kit of these for motorcycles.

Cold Starts. Yes, you could hook up the cold starts with a choke pull cable as you described, but I don't think you need to. I don't know anybody that uses the cold starts, even in cold weather. You just plain don't need it! My carbs warm up in just a few seconds, even on the coldest days, which is different than any other type of carburetor I''ve ever had on any car.

Each of the 2 carb housings has a vacuum port on the back side, and I think these were intended for vacuum advance.

You need to set your idle mixture screws at a very low rpm. If it won't idle at low rpm, then perhaps the sync / air corrector adjustments are too far out to get it to idle smoothly. You may need to get the sync dialed in a little better before you can go any further. When you get the idle down low, you can fiddle with the idle mix screws to get the best running for each cylinder. As Dan says, this should be about 1 turn out, and if not, that suggests that the idle jet size needs to be changed up or down.

Feel free to experiment with throttle linkage adjustments. I've seen about a dozen different ways. It's good to look at other cars with IDF's for ideas whenever you get a chance.

Alvon

[124ADDHE]

Have you checked your carb fuel filters and the main fuel filter? If these are good, have a look at the float setting, this, i have found, causes many of the idling and popping noises people have with these things and if your mixture screw is having little effect......worth a look.

You can usually get the carbs to run pretty good without a gauge as the main problem with the balance is usually the adjustment between the two carbs.

Good luck, those IDF's are sure worth it though!

Keith

[Daniel]

you could cut out a hole in the bottom of a paper cup place it over the air filter base
and use the syncmeter that way .

[JerryH]

I passed emissions this morning! I can't believe how well the car is idling now. Idles well at 700 RPM. It's very quiet.

Here's what I did:

I purchased rebuild kits from Webercarbsdirect. Also purchased a synchrometer. Checked the timing (10° BTDC). With the kits installed, it still wouldn't idle well below 1100RPM. So with more investigation I found that the PO's linkage setup didn't allow the front carb to fully close at idle. The back carb butterfly valves closed fully, but not the front ones. The way it was set up, I don't think it ever operated correctly. The engine wasn't idling with the carb's idle ciruit--it was idling with the b-valves cracked open. And you had to open it a bunch more with the throttle screw just to keep it running. So, I took apart the linkage and made some adjustments, put it all back together and viola--the car can now idle reliably below 500rpm. Now tweaking the mixture screws causes the engine to behave just as it should. I was able to peak up the idle speed with the mixture screws.

I checked the intake levels with the synchrometer. I purchased one velocity stack so that I could connect it to the carb without interfering with the enrichment tubes. I stuck some foam weatherstripping to the bottom of the stack to eliminate leaks. It tured out that the bypass screws on my carbs are well adjusted--all four intakes read about the same on the synchrometer.

Keith, I checked the fuel filters and due to your suggestion I paid special attention to the float settings--one of them was at 15mm. I tweaked it back to 10mm.

Thanks to everyone for your suggestions. I'm very happy I got this car through emissions.

-J

[JerryH]

So now I've got another issue. The car idles sweetly at 700RPM, but I can't adjust the idle to go above this level. As I tweak in the throttle screw, the engine goes from idling at 500 RPM to 700 RPM and then starts to decrease in RPMs and starts to run eratically and eventually stall. I expect that I am entering the 'transition region'--where the carbs go from running on the idle circuit to the main jets. Is this correct?

Is it possible that the PO chose the wrong jets because the carb wasn't set up to truly operate the idle circuit?

Here are my settings:

Main Venturi: 32mm
Main jet: 125mm
Air corrector jet: 210
Emulsion Tube: F11
Pump Exhaust Valve: 80
Idle jet: 55
Pump Jet: 40
Needle valve: 1.75

The car is a 1.8L. I honestly don't know if the engine and head are stock or modified. I do have a 4:2:1 header and the IDF 40s so I wouldn't be surprised if a PO had done other mods to the engine.

Let me know what you think...

-J

[124ADDHE]

Hi Jerry. Glad to hear that!

Regarding the idle, you are correct, that is strange. How many turns from seated are your idle-mixture screws? I think that they should be 1.5-4 turns out, probably close to 2. I think you should be on the idle circuit for quite a ways past the throttle position that would give you a reasonable idle. I would start with the idle mix screws, crank them out and see if that helps.

Keith

[ventura ace]

Ideally, the idle jet setting would be smoothest operating (highest speed) at about 1 turn out. Much higher or lower than that would mean that cleaning is needed or a different range idle jet.

My guess is that the main jet circuits may be plugged or restricted. It should run fine with the jetting that you have selected, which is very similar to what I have in mine. A little improvement may be realized with 130 or 135 mains. Needle valves of 2.0 might be better for wide open flow. But what you have in there should run fine if all the passageways are open and no vacuum leaks.

Alvon

[JerryH]

There still seems to be a range where the engine sputters with the throttle just above 700RPM. I've checked for vacuum leaks.

Just curious if altitude would affect carb jet selection? I'm running my Spider in Colorado at a mile high.

-J

[maytag]

There still seems to be a range where the engine sputters with the throttle just above 700RPM. I've checked for vacuum leaks.

Just curious if altitude would affect carb jet selection? I'm running my Spider in Colorado at a mile high.

-J

yes... absotively!

open it up and let it breathe a little more. don;t go down in fuel-jet size at first. try to feed it more air. eventually, you'll reach a point where you've given it all ya can, and still fat. at that point, go down a size on the jets.

[tbrunke]

There still seems to be a range where the engine sputters with the throttle just above 700RPM. I've checked for vacuum leaks.

Just curious if altitude would affect carb jet selection? I'm running my Spider in Colorado at a mile high.

-J

Hey Jerry, did you ever figure out what jets work best for you here in Colorado? I need to also know what size would work best. Thanks

[JerryH]

Sorry to have left this thread hanging. I ended up disassembling the carbs again and cleaning them thoroughly. Then I spent a little time tweaking the timing and now the IDF 40s are running great! For the past two years I've been running without carb troubles. Vacuum advance works well. Smooth operation at all RPMs. I didn't change the jets from what I listed earlier in this thread. I suspect that whoever installed these carbs had the jets sized for Colorado operation. I'm certainly no expert--the car seems to be running well, but maybe there would be some additional performance to be had if I jetted the carbs differently?

tbrunke: I assume you're located in Colorado somewhere? I'm near Erie.

-J