Need some 101 Weber help

[Va914]

I have a 2.2T with some weber 40's. (I'll get the weber specs tonight)

The engine idles overall fine (an intake pop every now and again). I am test driving my car for the first time since the topend rebuild and it seems the WOT is not there?

I adjusted the throttle stop and have the carb plates just about 100% vertical, so it;s not that

At about 3000-3500RPM it seems to lose power and sounds/feels like it is losing a cylinder Pulls good until 3000'ish?????????

At idle it reves great.

What jets control fuel at WOT and Mid-throttle?

[Fishcop]

Sounds like they are not transitioning from idle jets to main jets properly. Ensure you have PERFECT float levels in the bowls, the emulsion tubes can also play a part, but it's generally the float levels.

[RoninLB]

vacuum advance and carbs

"What jets control fuel at WOT"
-------- mostly it's the main jets. The devil is in the details.

"and Mid-throttle?"
--------- it depends. The idles may stay dominant till around 3.2k rpm. the mains take more control after that with the idles contributing less.

[Grady Clay]

911s (and 914-6s) with metal air filter assemblies can get rust
in the double wall chamber where the breather hose attaches.
Much of the rust particles are ingested by the engine. Some,
however, land on top of the carburetor and can restrict or plug
the main air correction jets (green). There are three of them
facing up in the carb (red arrows). If this is the problem, you
should see debris on top of the carburetor between the stacks.
IMAGE: WeberDiag01a.jpg “
"
© Dr. Ing. h.c. F. Porsche A.G.


Here is a view looking down on the carburetor. The three air
correction jets (red arrows) are easily contaminated by anything
on the carb top. The auxiliary venturi (green circle) is in the
direction of the air correction jet. Note that one float chamber
feeds only one cylinder (on left) and the other float chamber
feeds the center and right cylinders.
IMAGE: Weber_IMG_1165s.jpg


Best,
Grady

[Va914]

Thanks. I'll check out the main jets and the fuel level.

I used the PMO float gauge before I installed the engine and things seem to be in line. Maybe something got into the carb

FYI:
I have an inline filter before the pump and then again before the Y-Block, but before the pressure gauge. I have the system set at 3.75psi. Could the pressure be dropping at WOT?

I just thought it was funny that I can rev the engine up to 6000 with no issues but once a load is on the engine....no go.

Cheers
Kerry (Back on the road, sort of)

[jpnovak]

I had issues on my ST with the fuel pumps not supplying enough fuel to keep the float bowls full. you could watch the pressure gauge go to zero under no load acceleration (blipping throttle). I assume this engine is in a converted 914 based on your signature. Check the fuel flow and fuel pressure under flow. Just a thought.

[Va914]

Yep it's a conversion

I have been setting the pressure at idle. I'll rev it up and see it it drops any.

[jpnovak]

You also need to check fuel flow volume. I think 100 L/hr was the Bosch rotary pump spec.

[Grady Clay]

Originally posted by Va914
Thanks. I'll check out the main jets and the fuel level.

I used the PMO float gauge before I installed the engine and things seem to be in line. Maybe something got into the carb

FYI:
I have an inline filter before the pump and then again before the Y-Block, but before the pressure gauge. I have the system set at 3.75psi. Could the pressure be dropping at WOT?

I just thought it was funny that I can rev the engine up to 6000 with no issues but once a load is on the engine....no go.

Cheers
Kerry (Back on the road, sort of)

Kerry,

I think you misinterpreted my post. These are the Air Correction Jets in the top of the carb, not the Main Jets in the carriers that go into the float bowls.


There are basically two running circuits in the carburetor – the Idle Circuit and the Main Circuit. Each has a main jet that controls fuel and an air correction jet that monitors air flow. In the Idle Circuit the main gas jet is known as the “idle jet” and is in a carrier at the upper side of the carburetor. The idle air correction jet is not replaceable and is pressed into the carb casting. The main circuit gas jet (Main Jet) is in a carrier near the bottom of the float bowel. The main air correction jet (Air Correction Jet) is the one pictured in my post above.

The engine will free-rev to 6000 on the idle circuit (mostly). As soon as you put a load on the engine, the transition to the main circuit is necessary. This doesn’t happen all at once and is moderated by the emulsion tube below the air correction jet. There are also several other things that affect transition from mostly Idle Circuit to mostly Main Circuit. Just off-idle there are ports that help that transition from idle to the idle circuit. This is an important difference between IDT, IDA and IDS. On the 40IDS3C carburetors there is a third “high speed enrichment” circuit. Then of course all have accelerator pumps that have a large affect when the throttles are moving open.

Your description sounds like you aren’t getting transition to the main circuit. This can be from a restricted Main Air Correction Jet causing the mixture to go very rich or by a restricted Main Gas jet causing the mixture to go lean. My bet is the air correction jet circuit.


BTW, the float level should be checked in situ with the engine at smooth idle.
Where is your fuel pump located? What kind? I wouldn’t use a fuel filter prior to the pump, just the in-tank screen filter.

There are aftermarket racing systems for an in-cockpit mechanical fuel pressure gauge. These have an intervening fluid so you are not exposed to a possible fuel leak in the cockpit.

Best,
Grady

[Va914]

DAM

I rechecked the floats and they are fine. Took the mains off and blew air threw them. Took the air correction tubes out seemed fine. Fuel pressure doesn't budge as I rev the engine (I didn't check volume yet) It should have plenty of flow, it is a new carter pump

Reassembled and took it out for a ride and SAME old thing

I have a 2.2T and weber 40's with;
Main's, 125
Idle, 55
Air correction, 180 and F26
Vent, 32


It idles fine and rev's great. Once I drive it and it loads up it has no power after 3000. Kin of hits a wall.

Do the Air correction tubes need to go in a certain way? They have a lot of holes on the side of them.

I guess I'll do more searching

[RoninLB]

JW says,
I've seen a lot of the T shaped primary/secondary venturis in backwards so they don't flow any fuel. the opening on the end goes toward the nearest emulsion tube. they go in either way. just takes one in wrong to create a dead hole as the Rs go up beyond what the idle circuit will run, like over 2500.

[Fishcop]

I'll echo Ron's reference to JWW... Check that the secondary venturis are correctly oriented. They are the green circle in Grady's photo.

[Grady Clay]

Kerry,

The Factory workshop manual shows the auxiliary venturi (red
arrow) installed incorrectly. The retaining spring (green arrow)
goes away from the air correction jet.

Installing them backwards will give the symptoms you describe.

This image is Fig. 42 on page F41 of the early 911 Workshop Manual.
IMAGE: WeberAuxVenturi01.jpg "
"
© Dr. Ing. h.c. F. Porsche A.G.

Best,
Grady

[RetroSC]

Did it run at WOT before the top end rebuild? If so, maybe you're cam timing is way off.

Another thing that happened to me was a bad coil. I replaced the coil during my rebuild and it wouldn't rev past 4500. After fighting the Webers it turned out a very weak spark from the "new" coil caused this.

Probably neither of these, but something to think about.

Good luck, Bill

[Va914]

This is my 1st time with this engine so I don't know about the pre-rebuild operation

The coil is a Crane PS-91.. Does anyone know if I can swap out the PS-91 with an OEM type coil, just to test the possibility of the above problem?

It just seems to run very well with no load.

I will retime the car tonight and HOPE it was off some. I will also set my fuel pressure back down to 3-3.5psi and change my vent's to 30's.....to see if that helps.

Also, someone mentioned I may need to "phase" my crane XR-700 in my dizzy. During the install I set the rotor to #1 (static timing) and timed the car as normal once it was running. No extra work was done.

[dhoward]

Is timing advancing as RPM increases?

[Va914]

IIRC, it (the timing mark) moves CW on the pulley so that means it advances.

[Va914]

Also, once I get near the 3000rpm range and it starts to act funny there are no backfires or popping????
Just the loss of power and a steady funny sound.

[RetroSC]

Mine revved fine with no load as well. It was just under load that the weak spark from the bad coil (actually it was the wrong coil for the ignition I had.. long story) and it would just hit what felt like a rev limiter at 4500. No carb popping or sputtering, just ignition misses like a rev limiter would do. That's what eventually led me to suspect the coil and stop fighting the Weber's.

Another thing that tipped me off to the ignition problem, was that I had to set the idle timing at 10 degrees instead of the normal 5. Just wouldn't run at 5.. another sign of the weak spark.

Just some things to keep in the back of you mind while fighting this.

Bill

[Va914]

Interesting about the coil.

I was looking at the cranecam website and noticed this:

XR3000 only. If a miss occurs
at high RPM, make sure any
ballast resistance has been
bypassed.

Does anyone know if I have ballast resistor?