Lotus Elan

Weber Setup - Need Serious Help

PostPost by: 65 Lotus » Sat Sep 01, 2007 4:13 pm

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.
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Carb Setup.jpg and
26/3795 Carb Setup
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PostPost by: elancoupe » Sat Sep 01, 2007 5:00 pm

Here is the simplest suggestion I could make. Turn your idle screws out to about 1 1/2 turns. If this improves the situation, then your idles are too lean.

I had a very , very similar situation about 25 years ago, and richening up the idle circuit was the fix.

PS, wasn't your car red before?
Mike
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PostPost by: JATLAN4 » Sat Sep 01, 2007 7:02 pm

I had a lot of trouble setting up my idle and assumed all sorts of problems but it was simply me not recognising how sensitive the 4 idle mixture screws are.
I know it might sound stupid but how about just unscrew the screws 1/8 turn from fully in and see if it will start/run. if not gradually increase by a maximum of 1/8 turn until it will. Once it runs then just do the usual fine tuning of each by screwing in and then out by very small steps to get to the fastest speed.
The workshop manual is very misleading as following those instructions will give you a tickover of about 3000 rpm!!
Good luck
John
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PostPost by: 65 Lotus » Sat Sep 01, 2007 7:48 pm

Thanks for the replies guys. The car was indeed red before an altercation with a rock. During the body work, decided on a respray. It's now Viper Race Yellow.

John, if I understand correctly, you're saying to set the throttle plates to the correct position and slowly crack the mixture screws until it starts, then increase until max rpm is achieved and it starts to go rich.

When I do this, the speed will continue to increase until I hit 3000 - 3200 rpm! It just seems like the progression holes aren't close enough to the throttle plate. Too much air seems to be going through for an idle condition at the optimum plate setting for progression hole activation.

Attached is a close-up of the progression holes.
Attachments
Prog Holes.jpg and
Progression Holes - #1 Cylinder
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PostPost by: elancoupe » Sat Sep 01, 2007 8:01 pm

The car was indeed red before an altercation with a rock. During the body work, decided on a respray. It's now Viper Race Yellow.



OK. I met you/saw your car at the last Ohio LOG---really nice original car. :)
Mike
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PostPost by: bill308 » Sat Sep 01, 2007 10:00 pm

Scott,

I believe all the progression holes should be covered when at idle. The most upstream one should be on the verge of opening, but not yet uncovered.

At idle, the mixture should only come from the idling discharge oriface on the engine side of the throttle plate, not from any of the progression holes. Again, all progression holes should be convincingly covered at idle. Once you begin to uncover the progression holes, you will not be able to make the engine idle. Will the engine idle when in this condition? If not, and I suspect it won't as you indicate it is very lean, I think you need to open up the mixture screws a good amount, say 2-3 turns open from the fully closed position. Hopefully, you should be running rich at this point and it should be confirmed with the color tune. Now dial in the mixture for max idle.

What method did you use to balance the carbs?

The other issue is the ignition timing of 17 BTDC, although you say this is at 1500 rpm. Which distributor are you using and do you know what the timing curve is? By the book, 12 BTDC seems to be an appropriate static timing, at least until you sort out the carbs somewhat. Also, when you set the timing, both for the cams and ignitiion, did you first confirm TDC with a dial indicator, positive stop, or other independant means during the build? IMO this is a very important thing to do as all timing is based upon knowing exactly where TDC really is on your engine.

Anyway, those are my thoughts at this point.

Bill
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PostPost by: rgh0 » Sun Sep 02, 2007 3:02 am

Sign up to the sidedraft_central yahoo group and read Keiths white paper in the files section and the discussions around setting up Webers. A lot of this is based on his standard S1 Elan of similar engine spec to yours so should be very relevant . It deals with all of the issues in all the detail you will need. Alternatively I have attached a copy of the paper itself.

Your ignition timing should be set at the 10 to 12 crank degree static up to 1000 crank rpm. The advance curve for a standard Weber car of your specification should be linear and roughly 5 crank degrees per thousand crank rpm above 1000 rpm reaching a maximum around 4000 crank rpm for a maximum total advance including static setting of 25 to 27 crank degrees. This may vary a little depending on fuel used and engine specification but is a good starting point for getting the carb set up right. I have never seen a standard weber twin cam need much more advance than 27 degrees and a full race engine only needs 32 degrees when running on slow burning Avgas at 8000 rpm..

Unfortunately the Lotus manual screws up the specification getting crankshaft and distributor rpm mixed up and Miles Wilkins in his twin cam book does not make it much clearer. Also the orginal Lotus curves for the Weber B cam engine (40953) were far from optimum even in their day having much to much early advance and are even worse now with modern fuels. The later Lotus curves ( eg 41189 and 41225) are closer to the spec I describe above and work better in any twin cam.

cheers
Rohan
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PostPost by: Allison » Sun Sep 02, 2007 5:43 pm

Hi,

you may have got the answer by now but if not..... You should set the idle screws with the revs at about 1000-1100 rpm. Screw them in, then back out to around 3/4 of a turn. Gradually screw them out (or in, if that increases the revs) , as the revs increase outside this range, reduce the idle speed by adjusting the idle speed screw. Turn the idle screws about 1/8 - 1/4 max of a turn at a time and wait until the engine has settled down to a steady rhythm at the new setting.

Each new adjustment may have an effect of as little as 50rpm so you need to watch the rev counter carefully and do not rush it! It goes without saying that the engine needs to be hot before you start playing!


Have fun,
Peter
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