Newbie 250C carbs question

[dobrodan]

Have you used a Unisyn to check the draw on both carbs? You can't get these carbs right without proper synching. The front carb cover is different from the rear, so swapping them front to back would mean taking them somewhat apart. It sounds like there is no fuel going into the front carb - no accelerator pump squirts and no firing in the front 3 cylinders. It is common on these carbs for the needle to stick, or for the fuel not to make it there because of a clogged path. Remove the cover of the front carb after you've had it running to see if there is adequate fuel in the bowl. It's a five minute job and will tell you a lot. Check the obvious things first before going for the exotic.

[vwnate1]

I prefer to use a flammable aerosol product to find intake manifold leaks but you can also remove the air cleaner and cup our hands around the air horn, s l o w l y close your hands as the engine is idling so limit the amount of air being sucked in ~ if the idle goes up before it begins to flood, you've found there is in fact a vacuum leak somewhere .

Remember : when installing and tightening a 6 cylinder manifold, you offer it up to the cylinder head then loosely install the fasteners then tighten them from the center outwards working back and forth .

Rarely will there be fasteners on the very ends of the manifold .

[richmike114]

Dobrodan
That’s what I am thinking is that the front carb is not sucking fuel. I pulled the top cover off and I can see fuel in the bowl. Can’t really tell how much is actually in there but there is fuel in the bowl. Looks like about 1/2 up the bowl.

I wonder if there is a nest of some sort stuck in the intake manifold. Car sat for 24 years. There is still crap blowing out of the exhaust. I mean if it were stuck in the exhaust side it would surely blow out.

[dobrodan]

What does the Unisyn tell you? Is the front carb drawing about the same as the back? I think this is just a blocked passage in the front carb. Does it run better or different if you spray a little carb cleaner into the primary side of the front carb when it's at idle? The bowl should be nearly full, so your needle valve could be the problem too.

[aluminum]

Hi
So ?
(1) what is the Compression Now that you made it run ? each cylinder
( This is a Crucial basic test > the car well only run right if you have good ballenced compression . I always do this first before anything elce and yes when they sit that long
its better to run it before testing most often )
(2) With the Motor running Poor or not and the air cleaner top off looking down the Carb
Throats and accelarating the Motor you can see Gas being pulled from the clusters center area of the Carbs . This tells you if Gas is passing through or not .
Accelator Pumps you can check motor off just pumping them but thats not your basic Prob I don't think .
(3) I hope you found and pulled the " emulsion tubes " when you cleaned the carbs / all the jets and such .

Beyond that if the Car is not pouring out huge amounts of smoke and has good compression ( 135 to 150 ballenced ) it should run good unless you have indeed a unfound Vacume Leak .
Easy Test Vacume : Car outside running just run a garden hose slowley about the Motor
advoiding the Spark plugs and Distributor . Intake Manfold / Carbs / Vacume lines Etc.
If you find a leak you shall know it right away as the water flow hits it > You don't need a Huge Water flow for this

Hope this helps sometimes you just have to countinue onward

[richmike114]

Aluminium
Good compression between 140-150 each cylinder.
Looking down the front carb while it’s running it is not pulling fuel and the accelerator pump is not working. The float measures correct. The exhaust pipes on the first three cylinders are not hot like the rear three being fed by the working rear carb.
I pulled the front carb again. Will look at it again tonight. I wonder if there is am out of view nest in the intake manifold runners near the cylinder head. I am not sure why the accellooump and carb are not drawing fuel.

[rumb]

Fuel bowl float? Is there gas in the float resv when you take it apart?

If you tried putting the blue plastic ring on the accel pump during a rebuild, it very likely has failed. I have no idea what they were thinking with that part as it seems impossible to put on so that it will work. stick with the leather one it came with. If the car sat a long time it might have attached itself to the wall.

[aluminum]

Great ! if you have 140 / 150 LBS Compression your in good shape then and the motor can be made to run just fine !!!
However its easy to say that but may require a lot of Carb and tuning work
Just adjusting the Carbs > (1) so they are balanced opening and closing at the same time /Linkage adjustment is a good basic start
(2) To get the carb's operating right can sometimes require a few tear downs
I concur with RUMB and in fact Most often I Use my Old Leather accelerator Pumps rather then any I have found so far in any Kits . You can soak them and revive them by use I have found .
To discuses Carburetor rebuilding may shake the tree of opinion's I feel humm
So ? I will simply say that these are very simple Carbs to work on .
(1) Go Slow (2) don't lose parts (3) Take into account that air leaks between Carb body parts will cause very poor running . Point being inspect / replace any and all Bad Gaskets / O rings Etc.
You can clean the Carbs Out in many way's . Most likely few will agree with me But I often blast them out with a pressure washer / then blow them out compressed air ( 125 Lbs +or - not some can of compressed air from some Auto parts store )
Idle Mixture Screws : Do Not turn them all the way in and damage them
You can turn them in slowly until you feel resistance if your seeking a reference point ( try turning them out about 2 complete turns as a starting point then begin setting them running
Once You have a basic Good Idle set you should drive the car and readjust
the carbs Perhaps several times with motor completely warmed up .
Most Likely you will need to adjust several times to get it right .
If you have Vacuum Leaks it simply will not run right
( you can run water over the motor when running to look for such leaks just
low pressure stream will find Leaks fast outside not in the garage )
Ignition Timing : Mercedes like advance depending on your elevation
If its retarded you will have a hard time setting the carbs .
Once you get it right the Carbs will Hold adjustment just fine with your compression .The Point Gap is Important take your time setting the Points . You can buy a Tube of point lube online for the rubbing block of the points
without these things it is hard to set the carbs
Make sure Your Plug's and coil wires are in Good condition . I check them 1 by 1 motor running for a nice blue spark ( coil wire to ground cranking motor test )Make sure your Vacuum advance is in order as well and has no leaks

[richmike114]

So I opened the top of the front carb. Fuel in the bowl. The accelerator pump works and I rebuilt it but the kit was junk. The other carb - rear- I re used the original leather type plunger and the squirt has much more volume

Anyone know where to get the original leather type plunger?

The fuel return valve leaks too. I can see fuel running out of the front carbs emulsion tubes into the throat of the carb so o know it is pulling fuel into the engine.

But all that being said I am still ruining on the rear carb only. Here is what I found.
First three cylinders are NOT firing. Pulled the plug wires one at a time. Of course they are fed by the first carb. No spark at the first three.
Why is that ?
So I guess I am going to order new plugs, cap, wires, rotor and points. I mean why would the first three NOT fire?

If I cover the rear car throats the engine boys and stalls. If I cover the front carb if sick my hand down but doesn’t affect anything.

Happy 4th everyone!!

[mercifiknow]

Very nice car!

[rumb]

Sounds like you have 2 separate problems. Ignition and carb.

original plungers are usually unobtainium, however here's one on fleabay

https://www.ebay.com/itm/174793205856?chn=ps&norover=1&mkevt=1&mkrid=711-213727-13078-0&mkcid=2&itemid=174793205856&targetid=4580496732614416&device=c&mktype=&googleloc=&poi=&campaignid=418233788&mkgroupid=1230353745471221&rlsatarget=pla-4580496732614416&abcId=9300542&merchantid=51291&msclkid=482b69ff0e01107be5e4091b89565b25

[1971 250/8]

Sounds like my story. Bought a 1971… tore the engine down to brass tacks… except yours has far fewer miles so the wear should be light all around. Does appear that rats took up residence, no spark to cylinders 1-3 sounds like they chewed through wiring.

[dobrodan]

If the fuel return valve is leaking you probably need a new disphragm. You can get them here:
https://classiccarbs.co.uk/product/zenith-inat-carburettor-fuel-return-needle-valve