'72 250 Zeniths 32/40 backfiring

[rgnprof]

Pulled off the top 2 carb sections - top sections are flat and I replaced the 2 center screws from some older backup zeniths - same length (I have a ton of old Zenith stuff lying around). Tightened a little bit better with the new screws in place.

Floats looked fine but I remeasured float level and replaced one of the needles. While apart I cleaned all the jets, mixing tubes etc. and I'm seeing no obvious problems. I checked the secondary stage diaphragm and replaced the front one - it was leaking down a bit fast for my liking. Mine is a '72 so no venting valves...

I also checked the fuel filter in the tank and the 2 filters in the carburetors - all looked great, even the one in tank was very clean. I have a fuel filter after the mechanical pump and before the carbs but I replaced it not too long ago.

I haven't started it yet because of the fuel in the crankcase - so I need to change the oil.

Bottom line - I still see no obvious reason as to why these carbs are running so rich and dumping fuel.

[rgnprof]

I got everything re-assembled (had to replace the power steering pump as well, so I got that finished).

No fuel dripping down the barrels, carbs are balanced at about 900 rpm idle speed.

But - 1) Very hard COLD start, a lot of cranking necessary as it doesn't seem like it's getting enough fuel. Warm/Hot starts are fine though.
2) Smoke out the tailpipe - weird, as it was not doing this before my recent work (valve adjustment, new plugs, new wires and reworking through the Zeniths.
3) Still have some backfiring and lurching - off-idle/acceleration stumble. It some ways it's actually running worse with new plugs, wires, adjusted valves.

What gives?

[subforry]

For the cold start, make sure the accelerator pumps put fuel down the carb and the choke closes. Should fire right up and run at high idle.

Smoke, could be running too rich. I have not worked with Zeniths in a awhile, but Webers will smoke like crazy if the idle mixture is too rich. Or it could be burning oil. The overly rich running from before could have washed the rings and now they are not sealing well. What color is the smoke? White/blue/black?

Finally use a vacuum gauge on the manifold when tuning the carbs. I had forgotten how good a tool this is when adjusting carbs. Basically adjustments to more correct will increases vacuum.

[rumb]

try adjusting the brass screw on the side like 1/8th turn. Id start in the leaner direction.

[rgnprof]

Think I got things figured out (Knock on wood!). I had forgotten that I had fiddled with the mixture screws when I started having troubles at first. I reset the mixture screws to 1 1/2 turns out - made it leaner- so far no smoke. I checked the accelerator pumps and they were working but I adjusted them a bit to better shoot fuel down the barrels properly. Choke flaps are ok.

Once I made these adjustments, car starts and runs much better. Maybe better than it has for years...Thanks for all the help!

[Sugar Bear]

Great news...your persistence paid off!

[aluminum]

[QUOTE=rgnprof;4263497]Think I got things figured out (Knock on wood!). I had forgotten that I had fiddled with the mixture screws when I started having troubles at first. I reset the mixture screws to 1 1/2 turns out - made it leaner- so far no smoke. I checked the accelerator pumps and they were working but I adjusted them a bit to better shoot fuel down the barrels properly. Choke flaps are ok.

Once I made these adjustments, car starts and runs much better. Maybe better than it has for years...Thanks for all the help![/QUOTE
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One way to adjust those Carbs is to have tools in hand ( screwdriver ) with the air cleaner removed and take the car for a drive to clean out the maladjustment resulting / Motor completely warmed up stop and adjust / Drive some more then stop and adjust > You may have to do this several times sometimes over again until the Adjustment has settled and totally stabilized / Once this is done upon a Longer drive say 25 Miles you may need just a slight adjustment still / Once they are right they well hold adjustment just fine if the Carbs are any good / Note with Motor running and a water hose on low outside you can run water all over the Manifold and carbs Vacuum Lines and find Vacuum leaks very easy. :
Check out the "heat Risers" Closely > see if they are both free with Spring's intact ? or not> most often The Exhaust blows out past the rotating shaft and the Manifold's worn bushings leaking Exhaust making noise
When the case I make a 1/8 plate with drilled holes unbolt the upper and lower Manifolds and Remove the risers completely , you must tap out the Shaft holes and block off W> Bolts / Warm up 3 min extra all good and the Motor Now when warm runs BETTER then " With Risers and now Quite no EX Leaks .
Many of that Year Mercedes like a more ING advance > Retarded you will experience Hard cold start as well as Backfire / Try as experiment 5 or 3 Deg more advance / Mark current Timing mark < Paint Dot > and a second New Dot and try / Cam Timing : are marks on the Cam you Can easily check Valve / Cover off with TDC on Crankshaft you will see Slash Mark " Behind the Cam Gear ** " on the Camshaft " ** ( if Not seen turn Crankshaft 180 degrees Clockwise to TDC again ( top dead Center) to find slash mark / If it not straight up the Cam is retarded ( Very Common ) Offset Key is the fix if needed unless just the Chain is worn out