Lotus Elan and Lotus Elan Plus 2 Forums

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Finally the DIY tuning aid needed to do wide open throttle tuning at a price I can afford!
Keith

I second that! Its nice!

Greg Z.

Depending on how the sensor works a couple of things to keep in mind with Oxygen sensors are that they dont like leaded petrol and also the sealants we use on our engines gas off and the fumes effect the sensor. You can get special automotive sealants that are OK with O2 sensors as they are used on modern cars.
Clive

First usage and it worked like a charm on my son's '70 RoadRunner. Haven't used all the features on it yet, like the data stream recording. First I'll need to add a throttle position sensor on my Elan's Webers and buy the rpm inductive pickup when it becomes available very soon.

Hoping eventually it will allow us to do on the racetrack realtime TJ tuning of the BRM twincam in our 41.
Keith

Turns out on my 40_DCOE18s, the set of idle jets I had installed by seat of the pants tuning was providing only 14.5:1 fuel/air mixture under a load. 12:1 definitely seems to be better for making the twincam run smooth and to make best power. This Wide Band O2 sensor is well worth the expense IMO. Since you can now can observe the induction system working realtime. Now I need to somehow install a throttle position sensor onto the Webers so the datalogging capability can be used.
Keith

Keith,
So what are the carb specs now?

Greg Z.

Hi Greg,
I gotta ask why? The amount of variables between engines makes that type of information pretty much useless I suspect. Most likely there is some bell curve distribution of Weber settings which this type of engine system obeys. The actual mean value and shape of the bell curve is anyones guess. I've never seen any graph with many data points which I would trust. All I can say is the published values (mean values?) for the jetting are not the best ones for my engine.

I'm still in a tuning mode for at least another week. There's a bit of a learning curve I'm finding. I've yet to make all the necessary mistakes to ascend all the way up it. I can tell you so far it's at least ten times easier for me to do the tuning procedure having mixture insight.
Keith

Keith,
I understand. I'll rephrase the question a bit. Did the readings surprise you as to which way you need to take the jet specs or was it expected based on plugs readings or 'seat of the pants' running?

Greg Z.

Greg.
I thought I was closer to the correct mixture then it actually indicated. So yes, I was surprised. If I had experienced what the correct baseline mixture was supposed to feel like perhaps I could have made it closer to the mark by trial and error tuning. Been hard for me to tell if a hesitation was due to it being a lean or fat condition. Only difference is when it's a really exceedingly fat condition, it hesitates in a surging pattern about once every second or so. Plug cuts have been misleading me at this resolution. Sorry I can't put values to these vague descriptions yet. Haven't had enough experience using the meter yet.
Keith

At the appropriate sized idle jets (50F8) to get the A/F to 12:1. There is another problem to solve which is apparent via the meter. A momentary off-scale lean condition when reopening the throttles after coasting for a few seconds when the rpms have fallen below ~3000. This seems to be mostly independent of the opening rate of the throttles. Can feel a hesitation and along with a spit out the carbs or a moderate backfire or both. Time to examine and repair or tweak the tuning settings of the accelerator pump circuits.

BTW, can easily do away with this problem by installing idle jets which give a 10:1 A/F. One of the tuning books actually recommends doing this. It's a solution alright. Just not a good one!

This has turned a frustrating 'swap and suck' guessing game into a much more positive simplier process. B)
Keith

Thanks Keith. Reaffirms my need (sometime in the future) for the O2 sensor.

Greg,
As I swap the idle jets I can see a corresponding change in the A/F mixture in the upper rpm range (main jets) above 3000 now. Seems to change the main jet A/F at about a delta/2 rate and in the same direction. Always suspected this mixture rpm crossover coupling effect to be the case. Which means the Weber must be tuned from the bottom up or forever chase your tail!
Keith

After doing a full day of swap and suck testing my suspicion towards it being an accelerator pump problem were premature. Actually the 50F8 (F8=1.2mm) idle jets has a flat spot (17:1 A/F) at about ~2000 rpm. A 50F9 (F9=1.0mm) on the other hand causes the idle mixture screws to become insensitive at about 1/2 a turn from closed. The 50F9 quickly fouls the NGK BP5ES plugs but there's no flat spot. I suspect that to sustain an idle, the throttles need to be slightly opened to the point the first progressive hole is contributing fuel too. I can tell this because the idle speed screw must be opened by another 1/4 turn to maintain the same idling rpm.

There is a 40% difference in area between those two jet air bleed hole sizes. Supposedly an F11 & F14 are sized in between but I haven't purchased those sizes yet to know for sure. The only reference chart I can find lists those F-numbers as the same size as a F8. That just doesn't make much sense and I suspect it isn't so. Just in case I ordered some drill bits from McMaster-Carr. Sizes 1.05mm, 1.1mm and 1.15mm just to cover any issues of measurement by using them as pin gages.
<a href='http://w1.401.telia.com/~u40100700/highwood/weber.htm' target='_blank'>http://w1.401.telia.com/~u40100700/highwood/weber.htm</a>

If the other sized jets don't solve this problem. I'll have to consider chamfering the edge of the throttle butterflys and delaying the onset of the progressive holes. The other solution which has been mentioned in some of tuning guides is to drill small air bleed holes through the butterflys.

Greg (opps sorry!), at this point the mains are 115 and the air correctors are 180. Everything else is as bone stock configuration.

Seems the no load mixture no matter what the rpm performs best between 11:1 and 10:1. Unfortunately this does not provide the best fuel mileage. It does give a thrill at wot though.
Keith

Today after putting about 15 miles on it I pulled a few of the plugs which at the conclusion of yesterday's tuning session (left in the 50F8) were totally black with dry soot. I had my doubts they would ever clean up again. They are the picture of heat range perfection as depicted on the NGK website again. I'm prepared to bump up the heat range again if the final mixture I settle on warrants it.

Found out this morning from Doug at Pierce Manifolds there is another variable which Weber uses to tweak the air/fuel mixture in an idle jet. The inner diameter of the jet body (the well) changes via the F-numbering designations too. Not just the air bleed hole size. That complicates things. A 50F14 is in-between the 50F8 and 50F9.
Keith

Doug has phoned down for me the proper brackets to retrofit a Weber brand TPS (throttle position sensor) onto the DCOE_18 which requires no modifications.

Will still need to purchase or devise some sensor mechanism which logs the Elan and Hewland shifter position into recorded datastream. Any ideas?

Oh, forgot to add that Doug confirmed my suspicion that my throttles are opened now too much at idle and it's sucking fuel from the first progressive hole. He called it the 'lazy_idle' symptom. Lazy because it takes 10-20 seconds for the rpms to slow down completely and stabilize at the desired idle. He's assured me there's no way to directly observe this effect happening because the vapors are invisible. Good guy for technical infomation and to do business with.

I'm still on that learning curve. Innovative's second newsletter warns everyone to be careful about getting water into the connector and cables. Ahhh... it's rained here in the last few days and ..... doh! I'm fairly certain I filled up the wire cable all the way to the sensor. :blink:

My son has graciously given me his new sensor to replace the one I've just ruined. I'll take the time to seal up the new one like instructed.
Keith