I?ve been slowly sorting out the issues on the twin cam with Webers in my S1. Need some serious help guys. I’ve probably got 100 hours into trying to get it to run properly. Here goes, please hang in there with me.
Engine was initially a mess, rebuilt with Dave Bean head work. Cam timing is correct, valve clearances correct with stock cams. 10:1 compression ratio. Compression values are 187 psi +/- 7 psi. Ignition working properly, new everything. Dwell is 60 degrees. Timing set at 17 BTDC @ 1500 rpm. NGK plugs set at .025 gap.
Corrected all of the many vacuum leaks in the carbs (including new throttle shaft seals, lapping and clocking the choke valves, sealing the rear spring cover plates, Viton o-rings in the mounts, and more). Carbs are properly synched. The 78.5 degree throttle plates are centered beautifully. Setup on the car always returns to same idle position. Carbs have stock specification jets, tubes, venturis, however…
I now know the rear carb is a 40DCOE2 body but it has a -18 cover on it. The front carb has a -18 cover, but the casting externally is different than the one in the rear, so I’m not sure if it’s a -18 or maybe just a later -2 as the progression holes are drilled the same on both bodies. I don’t have a known -18 to compare to and have been unable do find out if the -18 and -2 have the same progression hole drillings. I read on this forum that someone has been running -2’s without a problem.
Here’s the situation. At idle, and into the progression range, it was running very, very lean with lots of popping and sneezing. It idle down as low as 900 rpm and I can get it to idle “ok” but not great, rough with random lean popping on all cylinders, verified using a colortune. This is the case with all cylinders. Idle mixture screws are between 5/8ths to 7/8ths turn out on all carbs.
I pulled the progression hole covers to have a look where the throttle plates were living at idle. All were in the same position, where the plate was only covering half the hole. The half of the hole that is showing open to the bore (not covered by the plate) is on the upstream side. Ok, so this makes sense about the extremely lean condition during progression. The plate is trying to cover the hole more, not open it up as the throttle is opened.
I then realized the idle arm stop tang was resting on the body casting itself, not even touching the screw. The engine wants to idle with the plates in that closed of a position! Looking down through the progression holes, I turned the idle speed adjust screw to see where the throttle plates would properly cover the first progression hole as it should be positioned at idle per all the manuals. This was exactly one full turn from where the screw touches the arm. I backed it off, replaced the hole covers, and fired it up. I then screwed in the idle screw to the “proper” idle position. The tach is reading 3000 rpm in this position.
I left it there and got it to idle down to 1000 rpm by massively retarding the ignition to something like 5 degrees BTDC, but of course it isn’t going to run there, just burn things up. It did run smoother in that condition than I’ve ever seen it run though. I only ran it there for about 15 seconds or so as I could smell bad things happening in the exhaust. I double checked cam timing; at TDC, the two #1 lobes are dead horizontal pointing away from each other. If I run the car and get it up on the main circuits under load, or stomp on it in any condition, it runs great, so it’s got to be the idle circuit. I verified that no one drilled out the idle air holes.
I know you can file the plates to get the progression hole in action as soon as the plate starts to move, but I would have to file away half the thickness of the plate. Plus, when the throttle would be open to about the 3000 rpm position, only then would the plate be in the “proper” idle position whereas by then the second hole should certainly be in play. The “filed” setup would only have the first hole opened up at that point. It would be a mess.
It acts as though either the throttle plate angles are too big or the progression holes are way out of position. Or maybe I?ve got something else dicked up. The only other place I can think of for a air leak would be around the idle jet holders, and it would have to be massive. And on all four cylinders?
The engine?s talking to me guys, but I just don?t know the language. Can someone please recommend an approach or at least a starting point to help get me straightened out? I?ve run out of talent.