Webers?....

[GotaT]

Ive got a 71 911t with the weber carbs on it. I recently set all the jets the same size and retuned w/ a unisen. everything seemed to be dialed in correctly. ... The only problem is that every once in a while i will hear a small backfire thru the carbs.. (sounds like an A/C compressor turning on) and also when traveling a steady speed i can feel the car draw back, seems like lack of fuel for a second or two. BUT, I am getting horrible gas mileage, i guesstimate 13mpg.

Please give ANY know how on these carbs. THANKS
Mitch

[zachsbug]

I'm not an expert on Weber Carbs but I have played with them a lot though out the years on some performance VW's & now my 68T. I will do what I can & maybe someone will jump in later with other ideas. Does the backfire that you speak of take place at idle or acceleration?

[GotaT]

Thanks for the response.....

Backfires at idle. ... Also at startup sometimes it smokes.. smells like its too rich, but runs like its too lean.. I just dont get it!???

[techweenie]

Originally posted by GotaT
Thanks for the response.....

Backfires at idle. ... Also at startup sometimes it smokes.. smells like its too rich, but runs like its too lean.. I just dont get it!???

Check the intake manifold gaskets. An air leak can cause the symptoms you're describing.

[Joe Bob]

Spray a little ether/quick start/carb cleaner around the base gaskets....any change in idle while spraying will tell you where the leak is.

Don't over do it....that stuff is flamable.

[GotaT]

Sweet.. thanks .... "Capital Idea!"

[jluetjen]

Are the throttle shafts tight in their bushings?

[GotaT]

last time i looked at them they seemed to be fine.

[RoninLB]

all above + any chance the venturi is to large ?

[GotaT]

AHHH... Venturi size!!.. one thing I was wondering about myself. .. what's considered too large.. My main jets are 180's .. if that helps any

[911pcars]

"AHHH... Venturi size!!.. one thing I was wondering about myself. .. what's considered too large.. My main jets are 180's .. if that helps any"

Your Webers should have these approx. specs:
Venturi 30
Main jet 125
Air correction 180
Idle jet 55
Emulsion tube F26

Spitting back usually means the idle mixture is weak (lean), due to a vacuum leak or air leaking past the throttle shafts (worn shaft bushing).

If the engine is using too much fuel (stock?), I'd say the mixture screws are not adjusted correctly, the float level is too high and/or fuel pump pressure is too high.

Idle mixture setting is tricky. a half turn could be the difference between okay and a little too rich/lean. Many folks adjust the idle mixture with a CO meter.

A main jet size of 180 could be the problem, although I think this is a typo.

Did you recently rebuild these? Have you verified the jet sizes?

Sherwood

[GotaT]

Sherwood, Thanks for the post! ok... I'll need to take them apart again and write all the specs down, and while im there check the throttle shafts.. what is the proper procedure?
Thanks
Mitch

[zachsbug]

Since you are going to take everything apart to check the sizes, keep in mind that when you re-balance the carbs take your time. Give the adjustment that you just made a chance to take effect. I have found that the best way to balance carbs is to make the adjustment & wait a bit before making another adjustment. Give the engine a chance before making another adjustment.

[RoninLB]

Some experience with adjusting our carbs makes the deal a whole lot easier. Even a mis adjustment of the carb cross shaft between the two is important.

Sheerwood's info is always great.

[911pcars]

Mitch,
So as not to lead you on a wild goose chase, make sure the engine is in good shape (compression test) and that the ignition timing is set correctly, then you can adjust the carbs (last thing to do).

Before you remove the carbs, spray some propane or carb cleaner around the throttle shafts, wobble the shaft back and forth and see if the engine gets a rise of this procedure. If so, the throttle shafts might be allowing air to bypass the normal route. Unfortunately, this is the typical wear area for well-used Webers.

The exterior float gauge will also provide a real time look at the interior fuel level. This tool can be sourced from PMO; maybe Pelican as well.

Sherwood

[GotaT]

Engine is strong and runs good, recent timing check and valve adj.

I was noticing on the side of my Weber carbs that there are 3 screws w/ springs on them ... one per venturi. After turning these I noticed NO difference. What are these for?

Anyone have a Schematic of the carbs, breaking down all the parts and adjustment areas? This would help i bet.

OH, i checked for leaks at the base of the carbs and the shafts, didnt notice a problem there....

[Zeke]

Originally posted by mikez
Spray a little ether/quick start/carb cleaner around the base gaskets....any change in idle while spraying will tell you where the leak is.

Don't over do it....that stuff is flamable.

Could someone explain to me why Mike and Sherwood tell us to spray flamable liquid on the hot engine? Isn't there something more inert? I have used WD-40, just as flamable. I'm looking to get smarter in my old age. You think?

[zachsbug]

That screw should be the idle mixture screw.

[Joe Bob]

ether/quick start/carb cleaner flashes off real quick.....and your spraying on the intake....if you are stoopid enuff to spray so much that it drips down on the hot heads than you deserve the instant face lift and eyebrow singe it will give ya....

It's like the beer can that says open other end....some things you just KNOW....%^B

Unca Miltie....go have another valium....

[rfuerst911sc]

You can also use a hand held propane tank for leak detection, with the engine running and the propane valve open and NOT burning direct the propane where you want it, if there is a leak the propane will be sucked in and the revs will change. No messy flammable liquids but not stupid proof.