Lotus Elan

SF Bay Area Fuel Quality

PostPost by: type26owner » Wed Aug 17, 2005 2:47 pm

Has anyone noticed the leading brands of preminum gasoline in the Bay Area make the Weber carbureted twinkcam run like total crud recently. Crud would be missing at idle and having the engine die on occasions or slowdown to the point you have to keep blipping the throttle. Trying to stall out everytime on a standing start. Huge hesitation when getting back on the accelerator after taking a corner. Less milage.

Had my suspicions it was the fuel quality causing this effect. Been going around trying different brands of fuel to see there is any improvement by chance. Yesterday filled it up at a Citgo station and had my suspicion confirmed. It drove and acted like I a had just done a complete tuneup! Now I'm really peeved off. I fear we will have the fuel quality of a third world country here soon and enjoying a well tuned Weber outfitted car will be a thing of the past. :x :cry: Gawd, really hoping I'm wrong.

Still have yet to test Mobil, Valero and BP fuel brands but the others are garbage.
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PostPost by: type26owner » Mon Aug 29, 2005 4:36 pm

As I research the gasohol issue I'm beginning to believe that part of the troubles I've been having with tuning the Webers is actually the E10 fuel not vaporing properly due to the summer variety's lowered vapor pressure to prevent vapor-lock. Seems like a really dumb thing to do but I'm going to try and preheat the fuel to about 60C before it goes into the float bowls. When my engine bay gets really heat soaked there is a noticable improvement by way of the all the constant missing at cruise or under acceleration going away. Beginning to suspect I should lean it out to lambda 1 (14.1 to 1 for the E10) for commutting so it runs smoother. The carbies are chilled to the touch but are not frosted now at 12 to 1.

It's becoming more and more apparent to me I still don't have the optimum heat flow through the engine.
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PostPost by: bill308 » Mon Aug 29, 2005 11:40 pm

Keith,

I believe that botique gas blends are oxygenated. If this is the case in your area, I would try richer idle and main jets. Air bleeds modify the A/F ratio slope so these shouldn't be altered unless one has a good rason to do so. If you haven't changed anything else, I would increase the idle and main jet sizes one step from OEM and re-evaluate.

I just ordered a set of Delloto DHLA 40 E's (5 vs 3-progression holes and barrel-to-barrel air flow adjustment via an idle air flow circuit), jetted to Sprint specs with extra (next step richer main and idle) jets for my S2 Elan with Sprint cams. In CT, the local gas is oxygenated with 10% alcohol so I'm anticipating having to go richer.
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PostPost by: bill308 » Mon Aug 29, 2005 11:42 pm

Keith,

I believe that botique gas blends are oxygenated. If this is the case in your area, I would try richer idle and main jets. Air bleeds modify the A/F ratio slope so these shouldn't be altered unless one has a good rason to do so. If you haven't changed anything else, I would increase the idle and main jet sizes one step from OEM and re-evaluate.

I just ordered a set of Delloto DHLA 40 E's (5 vs 3-progression holes and barrel-to-barrel air flow adjustment via an idle air flow circuit), jetted to Sprint specs with extra (next step richer main and idle) jets for my S2 Elan with Sprint cams. In CT, the local gas is oxygenated with 10% alcohol so I'm anticipating having to go richer.
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PostPost by: bill308 » Mon Aug 29, 2005 11:43 pm

Sorry fo the double post.
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PostPost by: type26owner » Tue Aug 30, 2005 2:29 am

Hi Bill,
I had one of those engineering moments when the whole truth is revealed while driving home and not concentrating on the problem at 100%. The kind of moment that sends chills down your back and were all is suddenly apparent and quite obvious. The E10 Reformulated Fuel is EXTREMELY temperature sensitive. If it's too cold the stuff won't vaporize fully and the stuff runs but misses and hesitates every morning. Every afternoon my engine runs noticably better without the constant missing. This has been happening to me for years and I never put two and two together to equal four until today.

I don't need to warm up the fuel. I just need to prewarm the air in the mornings! Bill, pay attention to what I claim and see if it's true for your fuel situation too. My car sits outside on the driveway and is at the ambient temperature every morning. I need to add an airflow restriction to the front grill and divert the warm air exhausting out of the radiator into the airbox feeding the carbies.
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PostPost by: rgh0 » Tue Aug 30, 2005 10:30 am

Keith

A comon design in cars 30 years ago in the early emission control era was to has a dual air intake to the airfilter. One source was a cold air supply and the other was a hot air supply typically by taking air from around the exhaust manifold. The split between the 2 was controlled by bimetallic thermostatic coil flap valve which crudely adjusted the flap position to attempt to maintain a constant inlet air temperature to maintain a constant rate of fuel vaporisation and improve combustion consistency and reduce emissions.

Sounds like with the current fuels you have to live with in California a similar approach is desirable to deal with the low vapor pressure fuels in an old carb engine on cold days versus a fuel injected engine where vaporisation rate is much less critical.

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PostPost by: type26owner » Tue Aug 30, 2005 2:01 pm

Hi Rohan,
Can well imagine the butterflies and main choke area would be coated with ice if only the humidity were higher here. Typical highest value for the humidity around these parts is only around 30%. It runs poorly at 10C but with the same fuel at 30C the missing is almost completely gone. Just so happens I filled it up with ARCO 91 the other day and it's by far the worst behaving stuff I've tested yet so far. Yesterday morning I was expecting the engine to wheeze and just quit. When I ascended the big hill leading up to the lab and built up some heat in the engine bay the transformation was spectacular.

Now that the symptoms are recognized I can apply a remedy. :D
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PostPost by: type26owner » Wed Aug 31, 2005 2:33 am

Hey Rohan,
Here's an idea that's outside the box but I can't find anything fundamentally wrong with it offhand. A totally passive solution to provide the necessary heat to the idle/progressive and mainjet circuits. What if one were to solder a pipe barb onto the inspection covers of the Webers and pipe in the hot air directly from the crankscase. You'd still keep the other crankcase breather as a roadtube or feed it into the airbox as originally done. The air passageway that normally supplies the jet would be left alone. Any pressure differential between the area over the jets a the float bowl would be a disaster. The real unknown is actually how much air do those two circuits consume. :? It's certainly an easy thing to try.
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PostPost by: bill308 » Wed Aug 31, 2005 2:36 am

Keith,

My thoughts:

Intake air heaters are common on carb engines but I have never seen one used on a throat-per-cylinder configured engine. I believe Stromberg equipped Elans and Europas used an intake air heater arrangement with cross over pipes, but in this layout, 4-cylinders shared 2-carbs. I think the purpose of the heated air was to aid the even distribution of the the air/gas mixture to better meet emission standards of the day. Distribution is generally quite good on a carb-throat-per-cylinder arrangement so heate air is not generally used. If you want to try warmed air, why not replace the stock cold air box with a pair of carb mounted air filters. Warm air from the engine compartment will be used and you may gain a little air flow as the stock air cleaner assembly is a known restriction.

Humidity is often well above 70% RH in Connecticut, USA in the summer yet to the best of my knowledge I've never experienced or recognized a carb icing problem. It is my understanding that this phenomenon occurs at high humidity levels in near freezing conditions (during a cold rain?) or at altitude in a carbed aircraft.

I always found the stock jeting on my 40 DCOE-18's to be pretty good. The carbs were carefully balanced and the mixture set to the best lean idle at about 850 rpm when stinking hot. With all else in tune, there was no bog in acceleration from about 1800 rpm and above. I think the low inertia flywheel I have fitted helps as it allows the engine to rev very freely. The sole complaint I had with the Webers was a stumble or partial misfire at very small throttle openings at around 2500 rpm. Under this condition I was likely running on the upper range of the progression circuit, not the DCOE's strong point. This condition was frequently encuntered when approaching a stop sign in a leisurely manner in residential neighborhoods. I believe Dellorto DHLA's will be surperior to the DCOE's under this condition.

Bill
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PostPost by: type26owner » Wed Aug 31, 2005 2:52 am

Hey Bill,
I can help you cure that stumble. Don't know the Dellortos all that well but the Webers I got covered like no one has before. Try removing the emulsion tubes from the carbies and go drive the car. You'll get firsthand experience on the actual rpms the progressive holes actually cover. The progressive holes only cover an rpm range from about 1000 to 1800 tops in the higher gears. Could barely pull 3000 rpms in first gear only going on level roadway. Just another bullshit myth proven to be wrong!

My mainjets kick in at 1300 rpms. I know this because I've devised a simple test. In top gear slow the engine down to a target rpm like 2000. At that point give it WOT and hold it there. For an instant the engine will pull on the accelerator pump shot. After a second or so if the engine continues to run than the only possible source of fuel is from the mainjets. Continue doing this at lower and lower rpms until it won't pull at all. In fact it will behave like you reached over and switched off the ignition when the fuel is not flowing. That's the rpm threshhold that the mainjets are set too. You want the mainjets to come on before 1800 rpms or you'll have the famous flatspot or stumble.

I'd like the best of both worlds. Supplying the engine with warmed up less dense air just goes contrary to the first principles of Lotusing. I don't want to detune the engine and reduce the horsepower by feeding it hot air down the air trumpets. That just goes against the grain. :cry:
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PostPost by: type26owner » Wed Aug 31, 2005 3:05 am

BTW, I confirmed with Mike Ostrov today I will be at his tech day on Oct 1 demonstrating how to tune the DCOEs with an Air Fuel Meter. Had to buy my daughter another laptop so I could have her old one to run the analysis software on. I'm totally mobile now. This stuff is the trick way to tune!

Sarto does that still work for you?
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PostPost by: type26owner » Wed Aug 31, 2005 2:56 pm

I wonder if Weber already designed in the carb body a flame quenching air passageway that supplies the jets to protect against a backfire igniting a potential combustible mixture in that area. Suppose I could spotweld a fine mesh screen across the openings of the airbox steel backplate as an extra precaution.
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PostPost by: type26owner » Thu Sep 01, 2005 2:01 pm

The other main reason for the flatspot on the older flavors of DCOEs is the biasing spring on the auxiliary venturi has gone soft. That spring is there to push the AV hard up against the bore of the carb body to minimize the leakage of the emulsion tube passageway. If it's loose there it does two really bad things. First, there is a vacuum leak so the time for the mainjets to kick in and flow the emulsion is delayed so it happens at a higher rpm than it should. Secondly, the rate of fuel leaking out past the outside of the air trumpets at the bottom goes way up. This is easy to semi-permanently fix.

That design of a P-shaped flat wire steel spring is a disaster. The auxiliary venturis are too loose if you can reach in with your fingers and wiggle them about. A replacement spring can be made which is much stronger and will not fatigue in a short time. The spring is a tension type coil spring with an OD of .325" x .030" wire diameter. Cut it so there is just two full coils and stuff the side with the cutoff ends of the wire into the female pocket of the AV. This prevents the sharp ends from scratching the groove in the carb body. It will be take quite a bit of force to insert and remove the AV with these springs.

If you have not done this upgrade than there's about a 100% chance you need to and this is the reason for the nasty flatspot you've been living with all along...... :wink:

Only on the latest version (151 & 152) of the DCOEs did they finally eliminate that damn spring and use a grubscrew there instead.
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PostPost by: mac5777 » Thu Sep 01, 2005 11:53 pm

Keith, sorry for the delay in responding. I just fixed the tach again as it had a wire that stopped the tach from going over 5400 rpms.. It's OK now and yes I will be at Mike's.
See you then, Sarto

Keith, you will appreciate this, knowing your inventiveness, I may have a new designed knock on/off spinner socket that locks on to a three eared spinner using a 1/2 inch drive. With torque it only tightens it's grip and won't slip off. A two eared will be next. I should have it done by Oct 1. I'll bring it to Mike's.

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