Anybody ever change their weber pump jet size?

I'm going through the same thing...trying to fine tune the jets/tubes. My issue is an off idle hesitation when warm. Curiously it doesn't do it when it's cold. I'm not an expert but the advice I got from a Ferrari mech. was change one thing at a time only to see what change the change makes.

Mine current setup is .55 idles, 135 mains, 220 A/C, F36 ETs, .45 accel pump jets.

As my problem occurs when the idle jets are responsible for fuel supply I'm going to start there. I've ordered .60 idles and will try those and readjust the mixture. Your's may also be an idle jet issue because I think the idle jets are responsible for fuel up to about 3000 rpm. Hopefully some of the carb gurus will chime in here. Anyone..anyone...Birdman...anyone?

It seems to me I read an article by a webber tech that said if you have this hesitation up to about 3-3500 rpm it is either the idle jets (to small/lean) or the A/C to large (assuming everything else is correct like blockage free, carb synch and ignition/timing).

My car is a 1975 series one 308gt4 with Electromotive xdi, and 82mm 10.5 -1 pistons. Stock exhaust.

Good luck and let us know the results.

 

Re-read your post. I'd try the idle jets first. I expect the reason the bog disappears when you "mash" it is because you've activated the accel pumps/jets so the engine is getting enough fuel from them until the mains kick in.

Have you read Birdman's carb sych tutorial? Do a search it's very educational.

 

OK, I think I've got the same symptoms you're talking about...

I've got a '78 US 308 GTS. I've had this for 15 years and same thing, it has done this ever since I've had it. Pretty much in the first 3 gears (maybe in 4th & 5th too, but not as noticeable) under light acceleration (normal driving) it seems to have a "stumble". If I accelerate a little harder (say, half throttle or more) it goes away. My current settings are either 135 (pretty sure) mains, 200 AC, F36, and 55 on the idle jets. With these settings I've always had max rpm at idle with the mixture screws out about 4-1/2 turns. I experimented with 60 idles hoping that I could screw the mixtures in a turn or so, but max rpm was with them out about 4 turns. It did seem to run a little better when cold (makes sense...cold motor likes a richer mixture) but once it was warmed up it still seemed to have the same flat spot. Also, with the 60's my mpg seemed to plummet. With the 55's I used to get about 14 city and 20 hwy, but with the 60's it went to something like 10/15.

I've often thought about accelerator pump. Unfortunately, I don't have a way of checking my A/F mixture.

Let me know what you decide to do.

BTW, did you change to the F24 tubes? Mine came with the F36's and I've been thinking about trying the F24's.

Henry

 

I bet the top end would be even better with 135 mains. That or you could go 185 a/c with your 130 mains. Glad the flat spot is gone !

 

Doug, glad to hear it was an easy fix. This is interesting though because according to your original post the stumble was occuring between 2000 - 3000 rpm. I understood from those who know (presumably) that at that RPM range the mains are not yet contributing fuel. Your experiece seems to indicate that they are at least begining to contribute in this range.

Just got my jet order from Pierce so I'll be tinkering this week.

Do you have any hesitation just off idle? What year is your car?

 

smaller mains do not contribute at lower rpm, but if a larger main is installed it will "reach down" in the rpm band to make the mains kick in a little earlier many times hiding most of the transitional flat spot or removing it all together (essentially by hiding it by a rich mixture). upper rpm flat spots have more to do with emulsion tubes and a/c jets while lower rpm spots are transitional from idle jets to main jets.

 

My car is 1975 series one 308gt4. 55 idles, 135 mains, 220 acs, .45 accel nozzles, f36 ETs. 10.5 - 1 pistons.

Ok, I did some work on the weekend. Thanks to Birdman's carb synch tutorial I synched the carbs up properly. They were slightly out and it made quite a difference. Once synched I drove the car (with the stock .55 idle jets) but it still had the off idle hesitation and while the car ran well it felt like something was holding it back. It felt like that before I synched the carbs also.

So...I pulled the 55s, installed the .60s, screwed in the mixture screws about 1/2 turn each...holy cow. What a difference. It feels like it's a different car. No more "being held back" feeling. I'm pretty convinced by this that .55s were too small for my car. This is the same result I think that Birdman had when he went from 55 to 60s.

Unfortunately I still have a small off idle stutter. It's better but it's still there. I may decrease the ac size next from 220 to 210 or even 200. The real curious thing is that this hesitation is not present when the engine is less than full operating temp. To my way of thinking that should mean that on initial acceleration there is too much fuel. This based on the theory that a cold engine requires a richer mixture. Am I way off base here? If I am can anyone explain why the off idle stutter is non-existant when the block is cooler?

The accel nozzles come into play on mine on virtually any progress of the throttle. Even a small movement will result in a dribble of fuel. Is this normal?

Any comments, theories are appreciated. Blaine

 

With that compression in your engine, you have a solid 10-15bhp at the wheel hiding in there once you find out what the best jetting is for your engine. I would start with 140 mains and 180 air correctors. You are good with .60 in the idle jet department. To give you an idea of what I mean, I have a stock engine and run 140 mains with 200 air correctors but am still a little lean at the top end.