1973 911 t with triple webers and maranelli dist / popping and backfiring and miss

[kiwicarrera]

Ok, vehicle had carbs recondtioned about a year ago, car ran pretty good occasional pop out the exhaust or carbs, but not that often. Recently car had started to pop back through carbs. carried out complete ignition check over, new plugs,points were ok, checked resistance in cap and all leads and were all similar. set and adjusted dwell and timing.. my front pulley had two marks close together and according to manual my 2000t engine times at either 5 atdc or at TDC, doesnt make much difference as to where its set anyway set at 5 atdc. pulled plugs and noticed were quite sooted/rich. hooked up strobe to coil lead and noticed occasional drop of strobe light. though i mite have a croock coil/permatune cdi. swapped out coil and cdi with known good parts, checked resistance through all cdi loom plug wires and could not fault. decided i may be looking to far into occasional drop in strobe light and decided to strip and cleaned carbs out, had found floats on l/h bank had sqaushed ( must be from big backfire im guessing). replaced floats and measured/adjusted float heights according to repair manual ( 18mm with vernier) re assemlbled and attempted to balance with syncrometer, got them around 2.5 on the gauge at 900 rpm idle and then adjusted mixture screws by listening to engine.. can get car to run average at idle, but still feels like its abit hesitant, after hours apon hours of fiddling with this and that, it no longer pops back through carbs at idle or accelerating but just doesnt go all that great seems to be quite rich at idle and gets slightly better as you get near the redline.. seems to have to be rich to stop it from popping out carbs and rich through all rpm, puffing some black smoke when you rev it up, try and lean it out and i get the popping through the carbs again and i get the occasional pop pop out the exhaust, especially when you give it a big rev.. so any suggestions??? also checked distibutor shaft for play and checked points gap at each lobe, seemed ok.., removed plugs and they were fouled, cleaned them up, checked exhaust for cracks or leaks none found and also checked intake gaskets etc, all ok and so im driving around, thinking of what i may have missed our whats actually wrong!!!

[Mo_Gearhead]

QUOTE:" ...balance with syncrometer, got them around 2.5 on the gauge at 900.."
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That seems like a low reading to me. How many miles on the engine? Worn out rings/valve guides perhaps?

QUOTE: "maranelli dist"
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These also have a bad reputation. I would pull it and take it to a shop that can check it for wear/advance weights properly moving, etc.

[kucharskimb]

popping through carbs is usually lean. popping in exhaust is usually rich. is your distributor still using vacuum hookup? i run all mechanical advance. it is set at 6deg BTDC at 900-1000 and 36deg at 6000. i also agree that 2.5 is low. mine usually read 3.5-4.0 at idle.

don't worry about the 2.5 turns on mixture screws for initial setup. this only applies for new condition and if the springs aren't in the way. just try setting it up the minimum gas/max speed way as a starting point.

did you set the float levels once it was running? 18mm is just the bench starting point. then you have to adjust the washers after you get everything running. could be level is set too high and pouring into the cylinders.

[kiwicarrera]

i did a compression test and it was around 145 to 150 all around, so dont think the engines worn out, plus it had gone ok before. Ive just ordered the float tool for the carbs, so no i didnt measure the levels once installed, distributor has no vacuum hook up(dont think the maranellis did) . Im abit unsure on which mark to set the base timing at... seems my pulley has no z1 mark i have two marks close together and then a mark further to the left (if your looking at the engine) and then one further to the right. Im guessing both marks close together are either z1/5btdc or 5atdc/z1, so im not sure which one . once engine is reved up distributor is advancing. what would be the best procedure for the setting points of the airbleed and mixture screws on an old beast? any thoughts on the strobe light dropping out intermittantly? cheers guys

[kucharskimb]

http://www.pmocarb.com/installation.htm
Weber rebuild

here is some good info. save it. read it.

i have also written some good info after doing some testing with an LM-1 monitor. i will point this out... the mixture screws are much more sensitive than you think. moving all 6 of your screws in by just 1/20th of a turn, will lean out your overall AFR by .4-.5. so if you think you can just adjust all of the screws 1/4 turn at a time, you are drastically changing the AFR.

when i set my mixtures, after warming up engine, i pick one cylinder. i slowly screw in the mixture screw until you can audibly hear that cylinder drop and the engine RPM drop. i then start unscrewing that mixture screw until it comes up to max idle. do it slowly. when you think you have that one at max, go ahead and give it another 1/8 to 1/4 turn open. then move on to the next. this will put you somewhere around a 13:1 AFR. you can always lean out later depending if the exhaust fumes are thick or the plugs are sooty.

go do an italian tuneup and blast some carbon out of the plugs, let it idle for a few minutes. after letting it cool, you can look at the plugs to read their color. if it is sooty, tighten that mixture screw by 1/8-1/4 turn. after reassembly, repeat the italian tuneup...

preliminary check on floats... take off your air cleaners and grab an inspection mirror and flashlight. if you can run your fuel pump without starting the car, do so. look down each throat and see if it is wet or has fuel running down the sides. now start the engine and see if the vibration causes a river of fuel anywhere.

i believe that the marks on your pulley are in this order:

__|____________|__|
__Z1__________30_36

I can't say it's actually 30 and 36, but that should be the advanced side of the pulley.

[kucharskimb]

so, what happened?...

[kiwicarrera]

Well havnt had a chance to try and set them up yet....just been to busy with work commitments. was waiting on the carb float level tool. Im certain my distributor is poked to as every time i try a timing light on any lead and coil lead the light drops out every now and again .. will keep you posted when i attempt to set float levels and getting it running right!!! thanks for your input so far

[911pcars]

Float level setting mostly affects cruising and high speed operation. At idle, fuel source is from elsewhere.

There are several sources of the basic Weber setup. I'd scan the archives and follow that procedure.

However, since the carbs/engine worked fine until recently, it seems like something beyond the basic settings is creating this condition. For example, if one or more idle passages are plugged, that will create a lean condition on the affected cylinder(s) and create some pop-backs. During normal operation, the carbs will gradually deliver more fuel through the main jet/main discharge tube as rpms increase, thus minimize the idle characteristics.

The second symptom of excessive fuel (soot on spark plugs) could be just that. Perhaps the float level is too high. Readjusting the idle mixture screws to compensate (leaner) creates another set of symptoms. The suggestion by kucharskimb is valid. Look into each carb throat at idle. Should be dead dry - only wet at cruising throttle opening. Use the float level tool to confirm.

Hope this helps,

Sherwood