Lotus Elan

Weber Dcoe Throttle Plate Spindle Repair

PostPost by: JACKJABBA » Sun Feb 22, 2004 6:54 pm

Keith,
check out these carbs on ebay, they have the plate with the hole.

<a href='http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=2460932376&category=32094' target='_blank'>http://cgi.ebay.co.uk/ws/eBayISAPI.dll?Vie...&category=32094</a>



Regards Jack.
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PostPost by: type26owner » Sun Feb 22, 2004 7:10 pm

Jack,
No question they are available and the Weber vendors are going to try and sell them to you. But is that the right part for this particular version of carb application? I don't think so. I'm having the same problem as you. Who do you trust? I now only have absolute trust in the Weber Tuning Manual.

BTW, if anyone screwed up the idle air bleeds adjustment on the 151/152 carbs this manual explains how to recover from tampering with the factory preset adjustment. Just like my younger brother did on his Caterham Super7. :D I'm just trying to close the huge performance gap he has on my stock Elan on the race track.
Regards,
Keith
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PostPost by: type26owner » Sun Feb 22, 2004 7:30 pm

The result of changing to 78 degree throttle plates is it now idles with the idle mixture screws closed. I have a small but measurable stumble when just opening the plates. That can likely be compensated with the idle mixture screws after I readjust the idling position of the plates by drilling the air bleed holes. The speed screw is 1/4 from closed to idle at 900 rpm. Going to try drilling the 0.75mm holes like the manual recommends. It responds to changing the idle jets nicely now. Can move around or eliminate any flatspot. Had to down size the size of the idle jets from 50F9 to 40F9s to get the mixture to 11:1 otherwise it was off scale being to fat. My guess about doing this repeatedly is coming true. I'm learning how these things behave now finally. Only first-hand experience does that. Chances are I'll have to reinstall the 79 degree 30 minute plates and chamfer the downstream edges to split the difference now that I've overshot the mark. Still having fun though!:)
Keith
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PostPost by: JACKJABBA » Sun Feb 22, 2004 7:59 pm

Keith, have you got the ISBN number for that manual.

Looks like I will just buy a new set of carbs tomorrow and sell the old ones on ebay. May get a new dizzy as well, as I have no confidence in the 43D4 that I have.

Jack.
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PostPost by: type26owner » Mon Feb 23, 2004 2:23 am

I got it to run like it has fuel injection with an ECU! Finally after something like seven years of driving junk it runs like it's a sewing machine. :D :D :D :D Yippeee! There is allot of things to be aware to do it though. Right now I'm going to celebrate a bit.
Keith
p.s. Was thinking if I were to charge myself for tuning the Webers at $100/hr my bill would be approaching $10K. :rolleyes:
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PostPost by: type26owner » Mon Feb 23, 2004 2:51 pm

Can adjust the idle as low as 500 rpm now. When the throttles are blipped open it returns to an idle and settles everytime within 50 rpm in under 2 seconds. There is no stumble or flatspot and the O2 meter confirms there is a steady mixture through to high rpms. On maximum overrun it does not pop, backfire or snap. It just purrs on down. Accelerates like a scalded dog. Idles for the first time without an intermittant miss.:D

Drilling .031" holes through the 78 degree plates did the trick. When I reinstalled the carbs after drilling the holes I restarted the engine. It spit violently and shook about. Glanced at the O2 meter and it was off the lean side scale. Remembered the idle mixture screws were still closed and opened them up and presto could tell it had worked out very well.

Found out the hard way that balancing the airflow between throats cannot be done by twisting the steel spindles. The bleed holes must be resized accordingly to do that compensation. Also when doing the airflow balancing between the carbs, that it be done at the correct rpm. That being the critical point of 1200 rpm were the first progressive hole is just starting to contribute to the mixture. Don't even try to do this without purchasing a model SK flow meter first.
Hope this helps,
Keith
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PostPost by: JACKJABBA » Mon Feb 23, 2004 7:42 pm

Keith, looks like you need to document all this work, along with photo's and graph's and post it in Technical tips section. :D
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PostPost by: type26owner » Mon Feb 23, 2004 8:14 pm

Jack,
I'm am documenting this stuff. It's just in bits and pieces all over this forum.

The Weber Tuning Manual has a Weber part number only. P/N 95.0000.54PM. Asked the folks at Pierce Manifolds who wrote it. They didn't know. Whoever it is understands it completely and has done a good job at being concise and descriptive.

Bought a Tecalemit Jackson fuel injection kit off of Ebay recently which was made to fit onto a stock Lotus Cortina twincam roadcar (40DCOE). Swore I'd not install it until I got the damn Webers tuned to perfection. Mission nearly accomplished! I've got 35 years of tuning and engineering experience with the TJs. Just a rookie with the Webers though.:)
Regards,
Keith
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PostPost by: type26owner » Mon Feb 23, 2004 8:53 pm

What really peeves me is the realization that this problem was left to the customer (me) to finally fix it. Lotus must have known this was a flaw from the very beginning. Nothing could have changed over the intervening years which could have caused this to malfunction. Maybe the 79 degree 30 minute plates were the correct mean value ones for the bell curve. Maybe with the right stackup tolerance errors the stock plates actually would work correctly on some cars. They really missed the benchmark on my carbs though. :o

The utmost care must be used to align the throttle plates to the carb throats. I stopped fiddling with them when a Be/Cu .001" thick x .125" wide foil tape got firmly caught at six points individually when the plates were closed. On both opposing sides top and bottom and at the +/-45 degree positions. It's all about controlling the airflow with high precision at very low rates around the idling speed. First one took me an hour to adjust. Last one about five minutes. Suggest you buy some Locktite grade 290 to glue the locking screws after they have been cinched up tight. That's about it unless there's some questions. B)
Good luck,
Keith
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PostPost by: type26owner » Tue Feb 24, 2004 1:24 am

For the very first time that I can remember I noticed my Elan does not reek of smelly gasoline fumes when it's been fully heat soaked and shut off. So I popped open the bonnet and wiped my fingers along the bottom of the metal airbox flange and it was perfectly dry!. I'm liking that! ;)

I hope it does not reward me now by producing carb ice. The carbs are cold but not frost covered. The morning ambient temperature is just hovering around 10C though.
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PostPost by: type26owner » Wed Feb 25, 2004 1:36 pm

Gee, discovered that 40F8 idle jets give me about a 15:1 mixture and there is no hesitation and no flatspot. Have decided there's no reason to cruise around at maximum horsepower when doing 50 mph or less and just waste fuel. Lets see here now, looks like I can have it both ways now tuning wise. Smooth running and good fuel efficiency when in the rpm range of the idle jets. Smooth running and maximum power available (12:1) whenever it's desired up on the mains. Weber nirvana? :D

This should also help prevent any tendency for carb icing becoming a problem.

Gotta wonder if the recommended jet sizes are actually the best ones if the carbs are properly setup. I got that engineering wise feeling from having been there, done that from having been pressed into service to do damage control in the past. Sadly, I suspect something is amiss here........ :( :( :(
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PostPost by: type26owner » Thu Feb 26, 2004 8:21 pm

I'm seriously considering not installing the TJ fuel injection at this point. Since I'm absolutely delighted with the results of finally fixing the Weber throttle plate flaw. It runs as smoothly as most modern day performance cars now. Like the difference between night and day.:D

Decided I'm going to tune the main jet stack for maximum fuel mileage for the everyday usage when I can't use maximum power anyway. Going to buy another set of emulsion tubes and the holders so I can jet it for maximum power output for the trackdays. Can't do that with a TJ.
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PostPost by: BillGavin » Fri Feb 27, 2004 12:44 pm

Very impressive results, Keith.

Where on the throttle plates did you drill the holes?

How did you decide on .031"?

I have a heavily tweaked engine in my car, and it shows similar symptoms to your engine. Mine runs OK with standard sprint carb specs, but getting the car rolling is tricky, and the transitions don't feel right. At 30mpg it's not too far off, but driveability could be improved a lot.
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PostPost by: type26owner » Fri Feb 27, 2004 2:55 pm

Bill,
The Weber Tuning Manual will be the best $25 you've ever spent. Along with that purchase the STE #SK air flow meter for I think $50 and waste bin your Unison.

The air bleed hole goes right below the stamped-in degrees designation and centered on the throttle plate. I had a 60-80 number drill set already so the holes are to the nearest inch equivalent size. The manual will explain all the hows and why. Maybe that 'all' should be 'most' cause it's very brief and to the point. That's why I've still got some unanswered questions. Like does my carbs exhibit 'reversal'? The answer is NO not yes (see note below) , I think. I'm still learning how to best read my O2 meter.

The quality of the Weber factory replacement parts is poor. Carefully examine them for defects before proceeding. Consider ordering double the quantity of what you need, keep the good pieces and ship the junk back or you could be stuck working on the weekend.

Thanks for responding. Been feeling like I'm wasting my time even posting here.
Regards,
Keith

Note: Oops, just having a brain fade moment while typing. Suspect on the DCOE there is enough of a pressure gradient because of Bernoulli's Principle that the progressive holes are contributing fuel along with the mains while above 3K rpm. To tune the carbs correctly it's crucial that the low rpm jetting is perfect first before doing any tweaking of the high end.
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PostPost by: type26owner » Fri Feb 27, 2004 4:00 pm

Number two lobe on my intake cam has worn it's tip off. At least 0.5mm to 1mm is gone now and there is a flat which is slowly spalling the bucket. The reason I mention this is cause one of the throttle plates I got clearly has the cutting tool marks that it had caught it and threw it out of the lathe. It had hit something hard and rolled along the floor for quite a distance raising burrs all around the sealing surface. Someone picked it up and threw it into the finished parts bin! I had to reshape the sealing surface with a jewelers file to get it to fit. That one got installed in the #2 throat. Surprisingly it passed my six point shim alignment test.

When balancing the air flow between the #1 & #2 there was enough of a difference to warrant adjusting it because #1 was dominanting that carb. Both holes started at .031". Enlarged #2 to .035" and overshot the mark so #2 had to much air flow. Corrected and got the balance perfect by enlarging #1 to .033". It's a fine line to achieve so don't rush it or get frustrated. It's well worth the effort!

Little wonder Lotus declined to do this adjustment at the factory because it would have really cost a small fortune per car. They just compensated by installing overly fat jets to partially reduce the stumble but not cure it. This means the Lotus Manuals' recommended jet sizes are apparently wrong for a properly tuned carb. I just installed a 105 main and fully expect it to be too fat also. I got a 10:1 mixture with a 110 main.
Regards,
Keith
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