Lotus Elan

Weber Tuning

PostPost by: Matt7c » Wed May 11, 2005 7:01 pm

Hi all,

Having now solved my fuel starvation problem, I've been able to begin the process of tuning the carbs, but have reached a point where I need a spot of tuition to move forward.

I have a synchrometer and have verfied that the air flow into each choke is the same. Then I have been following the instructions in my Haynes Weber Techbook which are to set the idle to the desired rpm, then turn the idle mixture screws in or out until the highest revs are attained, then repeat the loop. Did all of that, and boy was I running lean beforehand!

Then, the final test was to ensure that each cylinder gets the same amount of mixture by removing each HT lead in turn and observing the drop in revs, which should be identical. I did that, and in true Haynes style, it gives no clue what to do next if the drop in revs is not identical :( . The observed drop in revs from a base of 1000rpm when each HT lead was removed and replaced in turn was approx:

Cyl 1: 300
Cyl 2: 150
Cyl 3: 300
Cyl 4: 150

Trouble is, I don't know which is the correct drop...or indeed what to do about if if I did know which was the better drop. So, do I need to work on 1 and 3, or 2 and 4. And what work is required anyhow? :blink:

Thanks,

Matt
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PostPost by: type26owner » Wed May 11, 2005 7:43 pm

Just because you've balanced the airflow into the trumpets does not mean you've balanced the airflow entering the cylinders. There two major sources of airleaks. Both leak by the throttle shaft from either side. The dust cover leaks air into the cavity between the bores. The leather seals leak air by on the outboard ends. What is the lowest idle speed obtainable via the idle speed screw? Hey, the idle mixture screws are probably sucking in a ton of air down the threads too.

Which brand synchrometer are you using? Which version of Webers are yours?

Throw the Haynes manual away it's useless. Read up on what I've posted here in the past. I've already gone through the painful part for you and you'll be miles ahead if do some homework now.
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PostPost by: type26owner » Wed May 11, 2005 8:10 pm

Don't bother trying to find the air leak by squirting butane or carb cleaner or water or any other cockamamie way cause they don't work at the detection level required. Suggest you try squirting these things directly into trumpet while the engine is running and try to tell if there is any rpm change that you can detect before wasting your time relying on this voodoo stuff. That will at least give you a baseline of what to expect. The only plausible way to do this is use a smoke machine but they are extremely expensive for a weekend warrior to afford. You'll have to teardown the carbies to really find the leaks.

One visual clue there are airleaks is in the carbon deposits around the throttle plates look for a vee pattern leading away from the throttle shaft openings in the carb body. The vee shape will be seen as a cleaner area in the carbon deposit. If they are there then tuning the idle very well is impossible. This also forces the throttles plate to rotate more closed to maintain the same idling rpms which also moves it away from the first progressive hole so you get a bog just off of idle. The airleaks must be fixed first before doing anymore tuning.

I balanced the idling airflow on my brother's 151 DCOEs just last weekend for the first time. Having the air bleed bypass screws makes this process a snap. He said it has never idled that good ever.

It can get expensive from here. In fact you're probably money ahead if you just find new replacements. Too bad Weber has gone bye bye.

Oh, Weber does not tell you this but the ideal rpm to set the idle at appears to be 1200 or higher for the 18 with the stock 79 degree 30 minute butterlies. With no airleaks this positions the throttle plate to come onto the first progressive as soon as they move the slightest amount. I'm doing this crap on my son's Holley right now. They don't bother to mention the optimum idle speed either to get onto the transfer slot. Seems to be common oversight with carbie manufacturers. Wonder if it's intentional. :unsure:
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PostPost by: Matt7c » Wed May 11, 2005 9:50 pm

Thanks Keith, I can't answer the questions about the brand of synchrometer, I'll check it out tomorrow, but its probably a relatively inexpensive one. You'll probably tell me to junk it when I tell you! :D

The webers are 40 DCOE 18s with the jet and tubes all fitted as per the original standard for the TwinC.

Interesting to note you think its an air leak - seems likely. I'll have a scan around for leaks tomorrow, but, while I am looking, what is the more usual drop in rpm for a well tuned engine when one HT lead is removed. I may be barking up the wrong tree, but if my amateur tweeks can easily solve the issue, I'd like to know what solved might look like so I recognise it when I get there. Or is this particular test unimportant? Is there anything to be deduced from the fact that, based on the observed drop in rpm above, each left choke and each right choke appear to produce the same drop?

Please bare in mind that I'm a long way off from seeking perfection here - I'd be very happy with bog standard performance at this time. Which, for me, from where I am now, means eliminating back fire in the exhaust and spit back through the chokes...which might point back to ignition? So much to do, so little time...
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PostPost by: type26owner » Thu May 12, 2005 1:41 am

I can virtually guarantee you your leather seals on the throttle shafts have shriveled up completely due to the time it sat. You can and should make your own new ones. There are press-in sheetmetal cups that will have to be replaced because upon removal they get destroyed. All the parts are still available so far. The quality is poor though and the cost is starting to skyrocket. I learned to purchased more parts than I needed and gleaned the ones which were okay and returned any that were junk. That way I was fairly certain to have the parts to complete the job over the weekend and avoid discovering the bad parts only then.

Testing the dizzy for the correct timing and any scatter takes all of 5 minutes tops. You'll need someone to help raise the rpms slowly and read off the rpms as you measure the advance. Best to graph the timing curve by 500 rpm increments. Do this second.

You really should use a long reach dial indicator through the spark plug hole and directly measure the piston going over TDC. That takes all of 5 minutes to do if you have the tools handy. Do this first.

The rpm drop test is vaild in theory. They should all cause the same amount of rpm drop. It's not good practice to pull plug wires off while they're firing. They'll arc to ground somewhere and start the beginnings of a carbon track short circuit. That's a good way of spanking the crap out of the dielectric insulators in the dizzy and bring on the short sooner. Better to just short it out at the plug rather than pull the wire off. I think the testlight will do this okay but I could be mistaken. :blink:

Just take your time and it'll become all right before long. There's a learning curve associated with all this stuff.
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PostPost by: type26owner » Thu May 12, 2005 5:09 am

It's coming back to me again so you'll notice the values are changing in preceding posts. To get the idle down to 900 and no bog off idle the butterflies will need to changed to 78 degree ones. It's just a matter of adjusting the cubic feet per minute of ingested air past the butterflies to match the positioning of the upstream edge of the butterfly to the first progressive hole. You're locked into their solution unless the butterflies are changed. It sucks that Lotus installed stock butterfiles which must be set to a really high idle speed to eliminate the bog.
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PostPost by: Matt7c » Sat May 14, 2005 6:53 am

I tried to improve upon the "remove an HT lead to check the rpm drop" test and have now attained an equal drop across all 4 cylinders. I decided to use the highly scientific "spark plug deposit colour" method! I noted that Cyl 2 plug was black as a black thing, while Cyl 4 plug was white tipped. So I left 1 and 3 alone, while adjusting the mixture for 2 down and 4 up. Now the idle seems much steadier and it runs smoothly at 850 rpm when its warm.

Regarding further enhancements, I am in awe of Keith's knowledge of the Webers and TwinC. I'm still driving relatively slowly on private roads, so I'm sure that more imperfections will reveal themselves once I have her out on the open road. But for now, my main effort has to be passing the MOT before the end of May in order to acheive my original goal of having her on the road by summer. I thank you for the advice and will come back to it in the future, but I'm content with the engine for now and must concentrate on a few other bits to get the MOT. New butterflies can wait until the winter maintenance period!
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PostPost by: DrEntropy » Sun May 29, 2005 2:13 pm

Matt7c wrote:
Regarding further enhancements, I am in awe of Keith's knowledge of the Webers and TwinC.


Find some of his other posts and you'll really be blown away. The man has no concept of "half way!" Pro'lly FORGOT more about Webers than most of us could ever learn.

Best thing is: He's willing to SHARE that knowledge/experience.
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