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Discussion Starter · #1 ·
I am trying to set up the six

I am trying to set up the six 38DCOE's on my 400 Auto and having a problem or two.

I have rebuilt the carbs a couple of years ago with new gaskets, needle valves, fuel filters etc. (using Weber rebuild kits). The car runs OK but not great, a bit lumpy and pops from the exhaust on the overun. So it is time to set up the carbs properly.

I have disconnected all six linkages from the throttle system so that each carb is acting individually. I have set each of the six throttle stop screws (the ones that set the tickover) to 0.002" clearance with the linkage and then screwed them in 1 complete turn to give the engine some tickover speed at around 800rpm.

I have screwed in the 12 idle mixture screws until they just nip the body and the unscrewed them ONE complete turn (should this be 1 or 2 turns? - I will set the tickover mixture with a Colourtune later on).

I have screwed in the 12 air bypass scews until they nip the body and this is where my questions start. Should I leave these air bypass screws screwed in and then try and balance the two chokes on each carb by trying to get the weaker choke sucking the same amount as the stronger choke using my 'scynchronometer'?

This procedures seems to make sense to me but I have five specialist books here on Webers that either don't mention these air bypass screws (I think they may be a 38 & 40 DCOE part only) or say that they both should be unscrewed either no turns, three-quarters of a turn each, one turn each or two turns each!

Final question is this. What can be the cause if adjusting one of these air bypass screws has no effect on the synchronometer reading? Clearly something must be wrong but where do I begin to look? I have sprayed 'cold start' (which is basically an ether based product in an aerosol to aid poor starting that is available in the UK) around all the manifold gaskets and air filter gaskets and this had no effect on the tickover so there are no leaks there.

Any advice welcome please.

Simon in London

Discussion Starter · #2 ·
Simon, the process is really v

Simon, the process is really very simple. I have tuned the Webers on my Dino and C4. I am with you with respect to idle speed adjustment, and mixture, but am not sure what you are referring to with the air bypass screws. I suspect that are not to be adjusted, merely screwed in until they stop (gently).

But I ramble. First, turn all of the low speed (idle) MIXTURE screws in until the come upon their stop, and then back out 1 and 1/2 turn. This is a starting point. Disconnect linkage, and turn all idle speed screws in until they stop (compress their springs until they stop), and then out 3 turns or so.

Start the car and warm to operating temperature. Using your synchometer, adjust the idle speed screws to get all carburetors sucking approximately the same amount of air (flow equal across all carbs). Now adjust each of the idle speed screws one-by-one to lower the idle speed to as low as it can go - 500 RPM if you can do it. You may be closer to 1,000 RPM, but tune the idle as low as you can get it and still have the engine run. The reason for this will become clear in a minute. You will be moving the synchrometer from carb to carb as you lower the idle speed to maintain equal (or close to equal) air flow as you adjust the idle speed screws.

Once you have it idling at low RPM, then you will attack the low speed (idle) mixture screws. Put aside the synchometer for a moment. Slowly turn in one mixture screw, listening carefully for the engine RPM to rise. If the mixture is too lean, the cylinder will not fire as "vigorously" and the RPM will drop as you lose that cylinder. If the mixture is too rich, the same will occur. You want to turn the mixture screw slowly in and out to find the maximum RPM for that adjustment. The reason for the low RPM idle is that it is easier to notice a change as you knock off one cylinder, and it is easy to notice when you achieve optimum tune as the RPM increases. If idling too fast, it is more difficult to hear a change.

Once you are satisfied that you have found the max RPM for that idle mixture screw, move to the next. Repeat this process, of finding max RPM for a particular mixture screw, for each of the carbs. Once you have the mixture optimized for each carb, go back and check the airflow, which will now be off. Adjust the idle speed screws to bring airflow equal amongst all of the carbs.

One more pass on the mixture screws with this new airflow, and you should be there. Then adjust each of the idle speed screws to bring the idle to where you want it.

Hope this helps.

Jim S.

Discussion Starter · #3 ·
The air bypass screws of which

The air bypass screws of which you speak are to balance each choke of an individual carb. Start with them turned in all way, then unscrew the slowest reading to equalise the chokes, this is independent of the idle mixture and throttle plate screw. If you get no change, suspect some crap in passageways.
the basic setting on idle mixture will be about 1.5 turns out. When you are satisfied with the balance, the mixture screws can be adjusted. Some people turn the screw in one at a time until the cylinder drops, then out a half turn. Takes some practise.

regards, brian
a limey in Indy
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