Weber Question 40 IDA v. 40 IDS

[t6dpilot]

Ok guys, I need your help here. I am about to seal a deal on a new car and I just discovered something that may be of concern and need to make sure my thinking is not flawed. The car has a '69 E 2.0L motor that has been rebuilt to 2.2S specs with S cams, pistons, crank, and 67S heads. The car does not have MFI, but instead has Weber 40 IDA carbs. I read on this board that the early SWB cars had 40 IDA carbs and that the later SWB S's used 40 IDS carbs. The difference is that the IDS carbs have a high speed fuel enrichment circuit that minimizes detonation at higher RPM.

Here is my issue... The car cuts out at 6,000 RPM as if it is starving for fuel or has a rev limiter. After reading about the IDA versus IDS differences, I am thinking that this may be the problem. Could that be detonation that I am feeling/hearing? What do you guys think? The seller says he has been told by "experts" that it is just a float issue, but I take that with a grain of salt. The more I am thinking about this, the more I want to solve this problem before I seal the deal on the car. All opinions and suggestions are welcome.

Thanks.

[304065]

Hey there round-engine driver!

First of all, everybody quotes Bruce Anderson's book where it says that the high-speed enrichment circuit was only for "safety" to prevent detonation.

Bruce is a legend but I'm not sure that's enough information. The reason Weber added that circuit is that in order to get the mixture rich enough at high RPMs with an IDA carb, you end up with an overly rich midrange. By adding a high-speed enrichment you can dial back the midrange to the proper ratio, make more power and have better fuel economy.

That feature, and the relative scarcity of anything with an "S" associated with it, are what make the IDS so desirable. Bieker engineering can actually modify a set of IDA's with the additional circuit. It is not an inexpensive mod.

But now back to your situation. First, you need to find out what jets are in the engine, and compare those to the known correct settings for an IDA-carbed "S." "Just a float issue" I don't know what that means. Does that mean that somebody cleaned the carbs with compressed air and crushed the floats? Does it mean that somebody tried to use a high pressure fuel pump and buggered the needle valves? What is the float height? What is the fuel pressure measured before the carbs? Only after answering all those questions can you get a sense of what's happening.

Oh, and one other question, which is also the title of a great jazz song: "How long has this been going on?" Fuel starvation at 6000 rpm under load is NOT a good thing for a high-compression motor, particularly one with the deep 67 2,0 heads, which are universally acknowledged to be worse than the later 2,2 liter heads when it comes to detonation resistance.

Second, suspect the ignition. Most carburetor problems are in fact ignition problems. Does the car have a CDI? What is the timing set at, and what does the timing curve look like over the rev range? Hesitation could be a stuck advance.

A thorough investigation by a paid expert is in order here. Good luck!

[t6dpilot]

Thanks John, I plan on a PPI for sure. Glad to hear it is not associated with the carbs and hopefully it is nothing serious.

Yeah, round engines are great aren't they? The T6 is the Harley of the sky.

[Bill Meyer]

I have IDA's on my 67 with 125 main's and the plugs look great after 5K miles. The car runs fantastic and returns mid 20's on mileage, so it's not overly rich. The high speed enrichment circuit probably is a nice safety margin at high rpm's, but I really feel that properly dialed in IDA's are just fine. PMO's have no enrichment circuit that I'm aware of and many racer's use them as well as IDA's on single plug High HP 2.0 motor's.

That being said, I'd trade my IDA's for some IDS's if I had the chance.