PeachParts Mercedes-Benz Forum - View Single Post

Zulfiqar · #15 12-03-2012, 02:23 PM

I have fiddled with carburettors a lot, but my experience is limited to japanese Aisan and KeiHin carburettors, I also had a VW bug that I had to troubleshoot and got it running really sweet.

I was taught by a very oldschool guy and also learnt a lot from experience.

In first adjustment before the carburettor is bolted to the engine a few parameters are checked - so we know that only the carburettor is in question.

1 - overhaul/grease the distributor centrifugal advance, check clearance of rubbing cam to heel of CB points, make sure the points are good, make sure the quench capacitor is good, make sure the ignition coil is correct for application. Make sure the manifold has got some heat supply to it, It helps draw mixture from the carburettor - dont know how or why - but it really improves port vacuum. Also use a brand new air filter. It is also vital to ensure the valve clearances are correct and that the engine can pump good compression and you have no leaks in the intake manifold.

at first install after verifying float level, screw in the idle mixture fully and out about 2 or 3 turns as a base point, fill the bowl and operate the throttle lever, if there is a pump in it you should see it pee or squirt a stream of fuel into the primary throat. If its not flooding over through the emulsification circuits - your needle valve is ok.

attempt to start the engine - use choke or keep it pushed depending on what your engine is doing - it would catch rough but you would need to start adjusting the main idle lever to keep it running. Let it warm up and then start to adjust the idle mixture and idle speed stop screw so that you have the smoothest idle at your recommended idle speed.

Now - stop and let it cool down, after its cool - check the choke operation and cold start idle speed raise systems - what you want to adjust is the cold idle setting and see it transition into hot idle setting.

Now it comes to the big fuelling part, ideally this is checked with a exhaust analyser but you can check the color of your spark plugs (if they are correct heat range and your engine is in good health) to fine tune this.

All the above would be invalid if something like your throttle shaft being worn or having leaks in the system.

The hot intake manifold was put because in my VW - it had heat riser pipes on it that were heated by the exhaust gases, these get blocked over time. For the longest time I would fight with cutting out issues after a long drive or overfuelling on deceleration. Once I decoked the heatriser pipe, the intake manifold would be frosted over at the base from the low pressure zone under the throttle blade. I then dialed down the idle mixture and had to change out the carburettor pump jet to original spec. The car never stalled or cut again, power was restored as well as the fweem sound.