40IDA Hesitation/Stutter Between 3-4000RPM

Hey Gang,

During Parade last month they had quite a bit of time to get acquainted with my carburetors. After some around town drives and a few short trips I plunged headlong into the road trip from Salt Lake to Monterey.

As with any new motor you're always anxious and listening for anything that may be wrong. As I got out on the open road immediately began to notice a hesitation between 3000 and 4000 RPM. Originally I thought CDI boxes but I quickly realized it was a carburetor issue.

The first leg of the trip was Lake Tahoe so the high-altitude settings were pretty safe and standard. Once I got down to sea level the following day I noticed my hesitation and stutter was still there between 3000 and 4000 RPM. It was especially annoying because highway speeds were right in that RPM range.

I did a little bit of reading (on Paul's wonderful site) and the only thing I can think of is the progression air correction jets. Another knowledgeable friend suggested that I might need the taller secondaries.

Does anyone have any similar experience with this type of an issue with the 40IDA3C?

The motor is a 2.5 L twin plug. It has smaller 38 mm intake ports and I live at 4500 feet so the 40s actually work well with this motor. It has the Web 102/140 mod 906 cams.

Thanks in advance. Looking for suggestions from anyone who's BTDT. ;)

Eric:

You might try calling Kurt Donohoe at Partsklassik. He is very generous about talking through problems with Webers and knows them well. He is also close to you in Flagstaff, AZ.

http://www.partsklassik.com/Default.aspx? He rebuilt my 40s and they are wonderful with no stumbles or hesitations.

Thanks Gib, I know Kurt well.

Did you change main jets to a larger size when you came down to sea level? It would be good to know actual jetting (venturi size, main, E-tube and main air correction) to get a better idea of what might be the culprit.

Thanks Paul,

I was hoping you'd weigh in.

No, I'm afraid I left the mains alone. I just adjusted the mixture. I didn't feel like going to the hotel parking lot and smelling like Eau de petroleum.

Here is the happy jet chart for 4500 feet. And again, maybe this isn't so happy because I have the issue here between 3000 and 4000 RPM.

Venturi's - 34
Mains - 160
Air correction – 180
Idle – 6
E-Tubes - F3 which are supposed to be the ticket for these 906 type cams.

I've been told to keep the 34s versus the 36 for drivability. I kept the 160 mains for altitude and I have 170s for sea level that I didn't install. Same thing with the air correction 180s for altitude 190s for sea level. The car seems to do better with 60s in the idle circuit vs. the 55's that calculate out with the 34 venturis.

Perhaps it's hungry with those cams and needs 36v's, 170 mains and 70 idles. Aaron B mentioned the mains should go up then lean them out some more with the air correction jets.

Thoughts?

F3s...well...I've been pondering E-tubes quite a bit lately and have been having some good success with F26s in S-cammed motors. Actually, the F3s were developed for use with the IDS carbs which used High Speed Enrichment tubes but the original S engines when supplied with IDA carbs used F26 E-TUBES. I just had a customer in Florida swap F3s for F26s in a 2.5S and the midrange stumble like you are experiencing went away.

F26s have a set of holes located just above the fuel level in the float bowl which is designed to help blow air across the surface of the fuel in the Emulsion well and initiate fuel flow earlier in the transition. The F3s do not have such an arrangement. Curiously the F3 equipped S-cars used 125 mains with 185 air corrections and the F26 cars used 135 mains and 170 air correctors...larger mains and smaller air corrections to help supply fuel earlier and later; the HS enrichment tubes helped provide the fuel higher in the rev range so they could use a smaller main jet...theoretically and somewhat verified.

However, a 906 cam for a street engine is a little different from most of what I have seen so I don't have much to offer. I currently am using 35mm venturis in a set of highly modified Zeniths (http://forums.pelicanparts.com/porsc...ith-weber.html) on my S-cammed, 2.3 twin plug and am very OK with driveability. I'd think 36s would be fine for a light car like yours and actually, my venturi selection chart predicts peak HP developing at 6500 RPM while 34mm venturis would develop peak HP at 6000 RPM.

I have 150s in my trick Zeniths with 35mm venturis but haven't finished the project yet, I am needing to hook up my AFR for a better diagnosis but pretty good right now, a little hesitant during midrange roll-ons which implies lean. Main jets are sized to the venturi and the altitude in your case. So, I expect the 160s should be good down here but a bit rich in Sandy, Utah. I'd think 170 air corrections would be needed, also tall auxiliary venturis will help initiate the main circuit a little earlier. A 2.5 should flow enough through the 36s to get a pretty good signal. Another item to consider, you ARE using IDAs and not IDTPs, right? IDTPs can be manipulated to work well but the progression circuit on IDAs is superior.

F24s were OEM E-tubes for 906 engines and are actually VERY similar to F26 tubes. Get a set of F26s and give them a try is my recommendation.

A couple of comments regarding 906 cams:

• they like open exhausts
• they have issues with reversion pulses diluting intake charge which is exacerbated by street style exhaust mufflers; this occurs at your operational engine speed of 3-4K

Yes. IDA's.

I'll mess around and report back.

The cam is basically a "mod 906" (longer duration like a mod S). The longer duration and the 38mm intake ports give the car more torque and drive-ability.

Eric ..... If you have not solved your problem, try the smaller idle jets. You are transitioning from idle to main circuits and it may be too rich.

Thanks Jim,

Paul directed me toward some F26's which should help for the remainder of the year.

I spoke with Kurt in depth yesterday. We did a little cam analysis and, it seems this is the main issue with the engine. With Aaron's twin plug dizzy, I should be at 28 degree advance and I am currently running at 32 degrees. This is because the cam has the intake and exhaust overlap and I need to advance to give it compression for idle. I've tried 60 idles and it won't stay running and dips into the progression circuit causing high RPM swings at idle.

Bottom line with all of this is; E-Cams will probably be installed over the winter break here in Salt Lake (only 17 weeks away) :O and, this combo should be good with the F26's as well. With the E-Cams, I should be able to set the dizzy where it needs to be and we'll start over with some more normal jets. Kurt recommends I=60, M=145, AC=140 and again, the F26 E-Tubes.

Big thanks to Aaron, Paul and Kurt. Now to get some E-Cams on order so I can make the motor much more funnerer. ;)

Also!!! Jim, I don't think I ever had the opportunity to thank you for the CD. I had initially lost your info but wants to say BIG THANKS!!

Eric:

My Webers have those Kurt Donohoe recommended jets for my single plug 2.7L with 9.5 CR...but with Solex cams, which have more lift and duration than E cams. Take a look at using Solex cams for your 2.5L. They will give you more early torque and better overall performance I think....just my opinion. My engine runs like a scalded dog all the way through the rpm range with Kurt's rebuilt Webers and no stumbles or hesitations.