Webers still not right

[bemumford]

In March, I reported that I had installed a pair of new IDR-40's and that the car ran well at high rpms but idled poorly and sooted up the plugs. Another symptom was that at least one cylinder just began to achieve smoother idle with the mixture screw with the mixture screw entirely closed.

The carbs came from CB Performance with 55 idle jets installed. I installed 50 idle jets, and the engine idled better but backfired through the carbs on acceleration. I posed this question to CB, and they said I should lower the float level and keep the 55 jets.

Does a too-high float level sound consistent with the symptoms I've described? How difficult is it to adjust float level on these carbs? Is it better to remove the entire carburetor from the car, or can I simply lift the top off the carb?

It's spring in Iowa, and I'm itchin' to drive!

Ed Mumford

[C. Clark]

Ed,
Adjusting float levels is a common suggestion in trouble shooting the popping
syndrone. Have you adjusted the air by-pass screws?, there the ones with the jamb nuts next to the idle jets.
Through trial and error with information from others I have learned that those jets/screws should be closed completely.
Good luck,
Chas.
69/912

[bemumford]

Chas,

Thanks for your post. Yes, air bypass screws are closed on all four barrels.

Ed

[dtkky]

Ed,
Check the choke enrichment circuit. On 912s, these are usually closed and blanked off. However, during a backfire, the brass cylinder can unseat itself and not return to its seat despite thr presence of a spring. If this happens, the affected cylinder will always run rich, causing the spark plug to foul.
This happened to me once. Took me a while to find out what happened.

David Tan

[bemumford]

David,

Thanks for your post. However, I took the chokes off these carbs and replaced them with metal blanking plates. However, I think the problem is solved. On the advice of CB Performance, I opened the carbs, found the float level was set WAY too high, and reset them. Idle is much smoother, the car accelerates without flat spots, etc. I'll need to do some finer adjustment to get it just right, but it's very close. Not sure this says much for quality control at the Weber factory, though.

Ed

[C. Clark]

David,
Not to defend Weber, but 40 IDF's are not made for Porsche 912 exclusively. In Tomlinson's Weber Manual distributed through CB he suggest float adjustment check as well as all jetting and adjustments before installation.
Glad you got it figured out.
Chas.
69/912

[snowman]

Whatever the carbs the symptom of backfire, ie too lean is the same. Too lean. This means you have the carbs to lean. This may be a symptom of a float level of to low or a jetting of to low. That simple.

[bemumford]

I'm back to the 55 jets. No popping, no flat spots. Car runs great but it still soots up plugs and burns way too much gas. I've just adjusted the float level a little lower and backed out the air bypass screws. Didn't have a chance to drive it with this combination, but I hope to do that today.

Ed

[C. Clark]

Ed,
I believe in fuel regulators, installing one on my 912 improved my MPG considerably. They are usually used in conjunction with a electric fuel pump. CB sold me a rotary/very quiet, pump with a regulator and filter built in, that was over a year and it's worked well. Some of the more traditional 912ers have a problem with elect. pumps but I found it a realtivly easy way improve my situation. I beleive the old Solex's took more fuel pressure than the Webers.
Chas.
69/912

[bemumford]

Chas,

Interesting idea! I'll probably go that way if I don't find success pretty soon with the conventional adjustments. I drove the car briefly today, but I didn't have a chance to look at the plugs.

Thanks,

Ed

[cgarr]

I set my original fuel pump right at 3 psi, experts say (not me) that fuel will push past the needle/seats on Webers at greater than 3 Since I dropped my presure from 4 to 3 the plugs are starting to clean up nice... What engine do you have? p&c's CR Cam etc?

Craig

[bemumford]

Craig,

Engine has big bore kit (1720 cc) with 050 distributor and Pertronix Ignitor but is otherwise stock. You and Chas have given me something to think about. Never guessed that the stock fuel pump would be good for more than 2.5 to 3 PSI. Looks like a regulator is the next step.

Ed

[bemumford]

Well, guys, it looks like I need a regulator. I drove yesterday with my wife following. (This is after I lowered the floats again and opened the air bypass screws a turn each.) She says the car blows considerable black smoke -- even at highway speed.

Chas, you wrote that you're using the CB rotary pump with built-in regulator. Looking at CB's description, it appears that the internal regulator is set to 3.5 psi and that it's not adjustable. Is that the pump you have. Also, what did you do about the stock mechanical pump? Remove it and block off the hole? Block off the fuel inlet/outlet tubes? Something I haven't thought about?

Craig, you said you set your original pump to 3 psi. Do I presume correctly that you installed a pressure regulator in between the stock mechanical pump and the carbs? If so, where did you get your regulator, and how did you measure the pressure when you set it to 3 psi?

Thanks,

Ed

[C. Clark]

Ed,
CB sells two different types of regulators, a stand alone and the rotary I use. I read Tomlinson's Weber manual and got the info on the rotary. I believe Bob Tomlinson works for or is part of the CB Performance family. Give them a call and ask the questions, they'll do there best to help. I believe the manual fuel pump on a 912 runs about 4lbs PSI.
Chas.
69/912

[Duke]

Fuel pressure on the 912 with mechanical fuel pump should be 2.9-3.5 psi.

[cgarr]

I adjusted my pressure by dinking around with the screws holding the pump halves and some shims and it dropped the pressure 1/2 # I am going to order a Purolator fuel pressure regulator $23.99 you can dial any pressure from 1 to 5 psi and it has 1/8" pipe fittings. racerpartswholesale.com has them.

[C. Clark]

Duke
I guess everyone has a ox to gore on this subject but let me be very clear, my 912 after installing webers loaded up and ran like crap. Seldom in life do you find a instant fix for anything.......maybe a sod lawn.... and in the case of "MY 912" a fuel regulator!
Chas.
69/912

[snowman]

Don't get the purlator regulator. It dosen't work properly. Don't knoe why it just dosen't. The Holly one seems to be OK.