Weber 48IDA Basic Education & Help Needed!
 

Hi all,

GT40 guy new to this forum, but previously built and owned FFR Cobra!

Just completed a 3 year build of an RCR GT40 MkI. Power is a 363ci small black Ford (Dart block), 10:1 compression, with 4 x 48IDA Webers and MSD 6AL ignition. Carbs, intake manifold, and cam were all spec'ed by Jim Inglese and shipped directly to Craft Performance Engines who put the motor together last year. According to Craft's dyno, it is producing 467HP and it starts, idles, and runs smooth and cool following initial issues with fuel percolation preventing re-start attempts that were solved with phenolic spacers.

Carb specs are:

Idle Jet: 65
Idle Jet Holder: 100 (F10)
Main Jet: 155
Air Corrector: 200
Emulsion Tube: F11

The question I have is: Why is this car getting 4.1mpg?? The jetting was all specified by Jim Inglese and re-confirmed with him to be correct for this motor. Running 2.5psi fuel pressure and shows no visible smoke Produces great power. I have been driving it fairly aggressively (revving to 5,000+ rpm consistently for each shift) but only because I notice the engine is smoothest and happiest there. Very twitch/jerky when rolling along in 2nd or 3rd gear at 2,000 or so RPM.

Here are some specific questions and concerns I am hoping those with Weber IDA experience can help me address:

1. Is 4mpg fuel economy insane or about right for a 6L motor like this? Wouldn't I notice if it were crazy rich? I can run my finger inside the tail pipe and it comes out black and somewhat greasy -- is that normal?

2. I have a crazy amount of backfiring through the bundle-of-snakes exhaust on deceleration following hard acceleration. In other words, when I gun it, then let off the gas, there is a (glorious) cacophony of snapping and crackling through the pipes complete with flames shooting out. No amount of fiddling with the idle mixture screws changes this.

3. The carbs sat for a year after the engine was built and there was varnish that caused the needles to not seat and fuel to flood through the accelerator pump jets. The carb has been disassembled and cleaned since then and all pump jets function as they should.

4. The idle speed is steady around 900-1000 rpm but occasionally after a spirited run, the idle speed will creep up to 1300-1400 for a while then work it's way back to 1,000 a few minutes later. What could be causing this?

My biggest concern is if the carb is running crazy rich and washing the lubrication off the cylinder walls and hence the 4 mpg. I also suspect a slightly dragging parking brake which I am going to address this weekend but can't imagine that is having a massive effect on fuel economy. What is a reasonable MPG I should expect from a powerplant like this?

Thanks very much in advance!

Sounds like you are running very rich, especially with the mileage and backfiring. 1) how many turns out from fully seated are your mixture screws? 2) Have you checked the float levels?

Thanks, CompClassics! I am exactly 1 turn out on the mixture screws. I tried 7/8 and it seems to run better at 1 turn. Opening wider than that doesn't seem to make any difference to my untrained ear.

Haven't looked at float levels. Assumed they would have been set by Jim Inglese and looking around on the internet it seems you need a special tool to measure it. I've seen a method of measuring with the emulsion tube hole and another with the tops off. Any guidance is appreciated.

Does the jetting profile look reasonable?

What size venturi? 48 IDA stock is 37mm.

What is the camshaft specs or grind part number?

Starting with the basics first, initial settings, you need to double check the float level settings, especially given your previous issue of fuel percolation. Never assume somebody else did it right.

I am not a weber expert but would suggest...

* confirm the balance of the carbs
* confirm floats are floating (no sinkers)
* install fresh plugs
* start with fresh gas - nonethanol if available
* confirm the state of existing plugs
* confirm the number of turns out for the idle mixture screws.

Write down everything!

BTW, Mr. Inglese and others have way more experience than I do.

Hi Gaz64,

I believe the venturi on the 48IDA is stock.

As for the cam, it is a Comp Cams custom grind, with the following specs:

224°-232°, .556-.566 114°os

Not sure what any of that means, but that's what's noted in the build sheet!

Does that tell you anything?

With that mileage, your plugs should be soaking wet with fuel when you pull them out. If that’s the case, don’t run the motor until you figure out why so much fuel is being dumped into the cylinders. Keep it up and you will wash the oil off of the cylinders and you’ll never be able to get the rings to seat.

You have given excellent advice. I moved to another state and have not yet found a permanent mechanic for my car. There is a workshop near the house, so they didn’t name even half of these people there.

First thing to do is CHECK/SET THE FLOAT LEVEL! Ask me how I know. :/

This seems to me like a simple 3 step problem to fix:

1) Call Jim Inglese and describe the problem.
2) Do exactly what he says to do.
3) Enjoy what is surely a spectacular car!

A big V8 is not a 4cyl. When you're driving like you describe it's the most fun but it's like being on a Race Track which sucks gas.....it sure is fun though!!

Your idle jet 65 and holder orifice 100 (idle air jet) are what you tune for light throttle economy.
Your choice depends on where your mixture screws are now. 3/4 turn out is a good starting point.
Try a 60 with the same 100 first. If you need more 1.25 turns out, then stay with 65, and step the air up to 110, then 120 if necessary. You should use a wideband 02 sensor and data logger if you are able to.
Your cam is a nice mild lopey grind for a street car.
I think the engine could go better up the top end with a 38 venturi, possibly even up to 40mm.

Definitely check the float levels, Jim sent mine to me and they did not set them. The fuel was so low in the bowl it could hardly suck it over, causing horrible jumping and drag between the primary and secondary jets.
I made a jig to check and set the levels according to his web page to accomplish this.

After the bowl level was correct I was able to get the air bleeds set correctly and it was all downhill from there.

If you are running high rpm you will be running on your larger jet size and definitely eat some fuel instead of running on the idle jets.

My 2 cents.

Jim will provide a setup that is close to what you need, but then it is up to the engine builder to dyno tune it to get it exact. With Webers you are never going to get good gas mileage; you should be getting better than what you are getting unless it is a full race engine. When my 289 race engine was properly dyno tuned it was using fuel at the rate of 32 gallons per hour at WOT. Remember, unlike a four barrel or dual fours, Webers are not progressive, it's all eight in all of the time. I am running 40mm chokes on the race engine, you may need to increase the chokes to get better airflow for that engine.

Ragtime, Don’t assume your engine builder tuned for anything more than a full throttle dyno pull, which leaves a lot more tuning to be done. I had an engine built with a Holley four barrel carburetor by the same company you used and that’s all they did for me. Your carburetors are much more complex and time consuming to properly tune, I doubt they invested the time to do more than what was required for their needs. Running the engine in an excessively rich fuel state will cause increased wear! If you have someone in your area with experience in properly dyno tuning your Weber carburetors for the full RPM range, that’s your best solution. If you can’t afford to pay an experienced professional to do the job, it’s time to educate yourself. If you’re going it on your own, Kirk V and Gaz64 have given you a starting point. Good luck sorting this out.

It might take a little time to find a shop that understands Webers, but keep looking until you find one. The dyno shop that did mine had O2 sensors on each header tube so that we could see how each individual carb was running and adjust the idle mixture so that they all were running similar. Not all shops know how to adjust Webers. Good luck.