Misbehaving Weber?

[Ian T]

Hello all, I'm new (again) to carburettors, after learning fuel injections systems for the last two decades and getting really into the test equipment that dealers use to diagnose them... Back to carburettors and it's all new again as the ones I owned 20 odd years ago never really gave any trouble.

The car is a 1968 +2 with Weber 40DCOE31s fitted. The engine was rebuilt in the early 80s by Dave Smith (a Lotus specialist from Ilford) and I have a rolling road result table from that visit, while that was 30 odd years ago, it was also approximately only 7k miles ago, showing a headline of 138bhp at 7k rpm. It's pretty certain that the car ran well at that point and in the rebuild, two CPL2 camshafts were fitted (can anyone explain the meaning?) and Dave recorded all the jet sizes fitted in the carbs, which I have compared to what is fitted now.

Maybe it needs a good set up, but the car has only recently returned from Barry Ely, who last year had it in to fix this issue, but was only able to improve it slightly, It idles pretty well, responds pretty well to a blip of the throttle but then is all lumpy on progression under load, resulting in sooty plugs and a fairly rythmic 'miss'. When I brought it home, I spent a little while at high speed on the M25 and after a couple of miles, the running cleaned up really well and it felt smooth, fast and responsive until I slowed down again, soon after which it was back to hesitation. So while I have no doubt Barry had a good go at sorting it, I'd like to get to the bottom of it and be sure it is right.

When Dave Smith built and tested the engine, the carb had the following:
Main Jets: 120
Air Correctors: 150
Choke: 32mm
Emulsion Tube: F11
Idle Jets: 45F8

Today it is fitted with:
Main Jets: 115
Air Correctors: 150
Choke: 32mm
Emulsion Tube: F11
Idle Jets: 45F8

So the only thing that looks different is the Main Jets (115 now, vs 120 before),

I know I need to measure float levels next (first?), so will read up on that and I've found a guide suggesting that the final position of the idle mixture screws should be in the range 1.5 to 2.5 turns out, so I checked mine:
1: 1 and 1/8 turns
2: 7/8 turn
3: 1 turn
4: 7/8 turn

Here it is: http://www.redlineweber.com/html/Tech/c ... _best_.htm Following that guide, the author suggests that the idle jets could be too large if the best idle settings were as above. I am starting to think I should approach this as though these were new carburettors rather than trying to assume that they are already correctly set up. I'll find the links and post them in this thread.

Somewhere I have a Colortune, so I will have a look at that and see what it suggests is happening.

I don't have a CarBalancer (yet) so I can't confirm the balance of the carbs and from what I have read so far, this is one of the more critical adjustments to get right before I can do much else.

Ignition control is Lumenition and the spark plugs are Bosch Platinum '8' range. I can order some NGK 6 but can't see them fixing this low speed condition.

Some things have changed over the intervening years - namely fuel quality. I know that Brian used to use Shell Nitro in it, to avoid the ethanol of other modern fuels and I haven't added any fuel yet since acquiring it.

Advice and pointers welcome. I've done some searches and this one gave me the most clues: lotus-carbs-f40/weber-jetting-t36196.html

Realising that carbs are specced depending on the driver's wishes and expectations, I'm looking for a smooth, pleasant drive more than a racing car at wide open throttle.

Thanks,

Ian.

[nmauduit]

CPL2 profile is close to a Sprint profile, cf. previous thread :

lotus-twincam-f39/cpl2-cams-true-potential-t22900.html

I believe you have checked ignition prior to looking into carbs (like the saying goes, 95% of carbs problems...), initial/static advance and curve (esp. in the area considered troublesome).

as per the idle screws, coarse version (which you have) is generally considered in range in the literature if between 7/8 and 1.5 turns, so nothing to jump on there really. There is a interaction between idle jets and idle setting via these screws (larger jet likely to need a bit of tightening to get back to same idle conditions), so if you change the idle jets you should redo the idle screws adjustment. I've found the balancer type pictured below convenient for that

carb balancer

balancer.jpg (40.94 KiB) Viewed 2534 times

but the most helpful for me was checking actual richness under various rpm and load regimes, using a wideband lambda sensor (colortune plugs may help for that, checks under load requiring a rolling road then, or just the good old way of checking plug tip color).

But first thing I would do is to start tuning with fresh fuel (if need be from a separate canister).

good luck...

[nigelrbfurness]

Have you checked for minor air leaks around the carb mounting spacers? These can affect the slow running and progression much more noticeably than when the throttles are wide open, in my experience. A colourtune might also give you a some more information about what's happening.

Nigel F.

[mbell]

If am rembering spark plug number right Bosch 8 should be the same heat range as NGK 6?

To me if your problem isn't old fuel or air leak it's either spark plug flowing probably because of the wrong temp range or the idle jet is too low. So after ruling everything else out I'd through a set of NGK 6 plus in and test. It's a pretty cheap and simple test.

[Ian T]

Thank you all.

Lots to go on here and I should go back to the basics and work through from the beginning as you suggest.

Timing looks right (10 degrees) at 900rpm and seems pretty steady. I haven't plotted the advance curve through the progression area - this is a good suggestion, thanks.

I looked up the plug cross-reference to understand the Bosch plugs that are fitted. Mine are Bosch WR8DP (Platinum plus), at the hot end of the range and going by an equivalence chart are indeed equivalent to NGK BP5ES / BPR5ES - here: http://www.sparkplug-crossreference.com ... H_PN/WR8DP

A more digestible form here: http://www.briskusa.com/spark_plug_cros ... ange_chart

Is it still generally accepted that use of the plugs with resistors (i.e. NGK BPR6ES as opposed to BP6ES) is the way to go with electronic ignition? I'll get some ordered, as I agree it's a cheap test.

I've just pulled them out and warmed them gently in an old plug socket (which I also allowed to get moderately hot for a more equal bathe in temperature like the plug would have in the head) using my butane blowlamp to the point where they have self-cleaned. The front pair were cleaner in the recess lower down around the insulator than the rear pair, but all are now free of their deposits for the first couple of mm. Gaps are significantly sub 1mm, but all the same. I have a workshop manual, but that may not be definitive any more as the dissy is running an electronic Lumenition setup. The manual says 0.508mm for non-emission engines and 0.635mm for emission engines. I'd say my gaps are in that range, will need to dig the gauge out and check properly.

I will have to buy an airflow gauge like the one pictured above. On the +2 I don't think there's enough space to use it directly, so I'll need an elbow to make it sit vertically.

Reading the various information online and in the workshop manual about the carbs and their setup has (I confess) led me to this gut feeling, I realise I need to be rigorous about the other potential causes. The gut feeling is that the main circuit is OK as the plugs clean up when belting down the motorway, but there could be too much fuel in the low speed circuit, which bogs progression and fouls the plugs. Could it foul the plugs like this if it was too lean?

While running the engine with the airbox cover removed I couldn't detect any evidence of breathing (It's also all dry and clean in there) at the rocker cover-to-airbox coupling, so I don't think it's that. I wonder if the idle jets have been reamed out at some point - no evidence of it, but from what I have read, it seems generally accepted that larger (fuel) jets sometimes get used to compensate for today's lower octane fuels.

I'll stick some fresh fuel in and see if there's any detectable difference - somewhere I have an adapted can with a fuel tap and pipe on it, so it would be easy enough to connect that instead of the tank. Do you guys run regular 95 RON unleaded or the premium 97 RON?

Thanks for the ideas and encouragement,

Ian.

[mbell]

I will have to buy an airflow gauge like the one pictured above. On the +2 I don't think there's enough space to use it directly, so I'll need an elbow to make it sit vertically.

I think I hacked mine around a little bit so I could just fit the rubber piece direct to the meter on mine. This allows it to fit the front 3 cylinder easily but the rear cylinder is tight and requires a bit of messing to get a good seal.

On the idle yet, missing in the transition phase is normally weak mixture, so is often corrected with a bigger idle jet. I believe that the Big Valve engines (as used in the Sprint) had a 50Fx idle jet, so if your engine is close to Big Valve spec it may need a bigger idle jet. Also remember just because you get a good idle mixture doesn't mean its not too lean under other circumstances.

With US fuels I just moved to a 55Fx equivalent* jet from a 50Fx in my Big Valve engine to get rid of a very noticeable miss through the transition phase. Haven't checked the plugs for mixture yet or fined tuned but it not running and pulling much better.

Good luck.

(* I am running the hyper jets)

[Ian T]

I had a quick look at float levels after finding reference to the correct level of fuel within the carb bowl being 25mm down from the machined face at the top of the main jet bore. My fuel level (measured very soon after stopping the engine running with main jet already removed) was approx 31.5mm below the machined face on both carbs, obviously a lot different to the 25mm seen quoted elsewhere. That article also stated that the reasoning for the 25mm was derived from testing and the fact the drilling for the main jet was just a little above the magic 25mm.

Thinking 'aha!' I took the top off one of the carbs to measure the float level calibration and found it to be within 0.5mm of the prescribed 8.5mm (brass floats) below the gasket.

So now, if I go by the Lotus (and Weber) workshop manuals, the 8.5mm value is the one to go by, but if the generally accepted wisdom is a fuel level of 25mm below the top of the Main jet well is the target, then I have a disagreement in the two measurements to resolve. Reasoning would be that a low fuel level in the carbs would result in weak running. I have observed some spitting noises very occasionally from the carbs when the engine is hot and just reapplying very light throttle after decelerating, if this is relevant.

So, have I got excessively floaty floats? I can't see how as they are brass and look original. Have I got something wrong-sized fitted? I can see the needle valves have been recently replaced and could measure them (I have what I presume to be the old ones loose in holders in the car). If the needle valves had different lengths (slightly longer), I could see how that would upset the maintained level.

Thanks mbell - I am familiar with weak running in injected cars often being misinterpreted as too rich and perhaps I am doing the same here. If I can get some ground truth on float levels (or I may just experiment to get to 25mm) I will look at the jets as a follow-on point. I'll also upload the comparison (Excel spreadsheet) that has all the standard jet sizes for the different engine specs, although that might be broken by modern fuel specifications).

Ian.

[mbell]

As I understand it a low fuel level will cause a delay to the main circuit operation but shouldn't effect the idle circuit. For the main circuit to kick in you need enough airflow through the throat to create a vacuum strong enough to lift the fuel further. So this take more revs/open throttle to get the increased airflow to lift the fuel further, potentially causing lean running in the transition phase

I have my fuel level set to ~25mm from the top of the jet holder (measured with carb lid & floats fitted). Personally I don't like the set fuel level by the float to lid method as it so easy to get it wrong.

If I was you once I ruled out everything but the carbs, the fuel level is the first thing I'd set. I'd check the fuel level again, possibly with engine running and main jets removed to be sure thou.

Mark

[Ian T]

Hi Mark, thank you, that makes a lot of sense.

I drove it down to the North Kent Lotus Group meeting this evening to meet Richard Cox and a number of other members there and while I found the drive there to be pretty moody, the trip home was much better, almost acceptable. The difference? Either the car likes the cooler air, or it likes the dark.

Cooler air is one thing, but dark just might be interesting. The headlamps were on, meaning there was some load on the electrical system. I wonder, just a little bit, whether there's something in that and will have to go out for a run with the headlamps on in the daytime and see if there's anything in that. I have an oscilloscope and can have a look at the supply to the ignition system and improve regulation and smoothing if necessary.

The float level really has me interested though - you have confirmed the 25mm measurement I got from sidedraftcentral and that means that the workshop setting procedure (on my carbs at least) results in a level setting that is some 6mm lower than the 25mm level that has been found to be 'good' more generally. Are today's fuels slightly heavier?

I doubt I will have much chance to play for a couple of days, but I have something to go on and will come back and update the thread.

I appreciate all the advice - it keeps me looking at the right things.

Ian.

[seniorchristo]

I also have a flat spot which stretches from 1000 to almost 2500 rpm. I currently run a 50F8 idle jet but am going to try a 50F9 based on advice from a friend.. Mark (Mbell), I see you are active on Keith Franck's forum. I am interested in his Weber parts but I would like to dial the in carbs first with standard Weber jets. How useful was his theory in setting up your carbs versus trial and error? It's a fascinating subject but seems as much black art as science.
Later
Chris Herr

[nmauduit]

Being only an amateur in the art of tuning, I enjoy the discussion and thoughts exchange, so here is what comes to mind :
- I consider the float level impact primarily on main jet effect, as nebulization directly depends on the height the fuel foam has to travel up to get to the engine, so I rather measure it in situ (base setting 25mm from the machined face below the round jet cover). One can even look at it at idle speed with one main jet off. I try to be as accurate as possible for this, and do the measurement several times for a final tuning, using a caliper depth gauge in the center of the main bore, and increasing by half mm till the tip touches the center of the meniscus, to be observed under strong light. Pre tuning the float on the bench is fine, but I now always fine tune in the car so that the whole chain (pump, float, valve) is taken in account to set the function (fuel level for the jet). Then fuel level may be used to alter operating conditions (e.g. raise the level to get the transition idle to main at lower rpm), to a moderate extent (depart from stock settings at your own risks...).
- on the jet size issue, esp. idle since flow would mostly result from carb operation rather than head/valve for the corresponding rpm range, I would think that engine displacement would be the driving parameter here, since what one wants is to have a mix of a given stoichiometric ratio in each cylinder when the spark fires it (I try to keep it at 12.5:1 for power). For a given low rpm range, one cylinder will pull a certain amount of air that will need a certain amount of fuel for the mix to be just right (too rich or too lean will give a combustion of lesser power) - the difficulty being to maintain this ratio across the variety of flow condition in the carb across the whole rpm range. So for any LTC engine idling around 1000, regardless of its head porting condition/valve size/camshaft diagram, on webers choked at 30 I would start with 45 idle, possibly 50 idle if choked at 33 or above, and make sure everything regarding idling is right (including ignition timing : if not optimally ignited it is not of great use to fine tune richness, baring in mind that flame progression will depend on richness; closed butterfly flow and position respectively to the transition holes) before doing lots of experiments. When trying to tune my engine (which had severe combustion issues but coming from a specific carb version rather than jetting), I did a lot of experiments with idle jets (each time with a lengthy idle setting), to get back to stock values in the end when I checked actual richness with a wideband lambda sensor : that insight of actual combustion was what helped me most in the process.

[mbell]

Mark (Mbell), I see you are active on Keith Franck's forum. I am interested in his Weber parts but I would like to dial the in carbs first with standard Weber jets. How useful was his theory in setting up your carbs versus trial and error?

Hi Chris,

I haven't really finely tuned them yet. I got it running well but need to do some more tuning to make sure I am at optimal setting. So far the behavior I am seeing seem to line up with what Keith says.

I never had a set of correct standard jets for my car (bits missing when I got it). My tuning has consisted of setting the fuel level to 25mm, careful balancing of the carbs. I experimented with the H20 hyper jets to try get rid of the transition misses I had, but couldn't get rid of it fully. I experimented with different air hole setting that behaved as stated but couldn't get it rich enough to cure my problem. So have moved to the H22 idle jets which have solved the problem, I haven't tested all the air hole setting yet and only adjusted the idle mixture by ear, it is running well but might be a bit rich.

I need to pull the engine to investigate a gearbox issue so not expecting further tuning for some time. Also don't think peak of Texas summer is best time to tune the carbs.

If I was you I be tempted to just buy the Hypo jets and try them out you get a lot more configuration than the standard jets and Keith will probably swap them out if you need larger ones. So probably more cost effective than buying multiple standard jets and then equivalent Hypo jets.

Cheers,

Mark

[seniorchristo]

Mark,
Thanks for the reply. I will give the hyper jets some thought. Do you have a stock configuration engine? Also, have you experimented with Keith's "airleaks"? I am not sure what these do but I believe they can vary mixture through the progressive stage. Being able to measure air fuel ratio seems to be crucial to getting accurate results. I think it's safe to say most engines would greatly benefit from proper carb tuning.
Later
Chris

[mbell]

Hi Chris,

As far as I know my engine is standard Big Valve. I've not looked at the air leak devices, but I think your right on there purpose.

Cheers,

Mark

[Ian T]

Well thanks to the advice and information found here, I have been tinkering.

Firstly, I have reset my float levels. With the top off the carb and rotated to dangle the floats against (but not depressing) the float needle ball, the setting looked correct at 8.5mm, but reassembled, the fuel level was 31.5mm below the top of the main jet bores. I made Keith Franck's optical dipstick device to check the levels after dangling the plunge part of my digital caliper into the well until it touched the surface.

With that, I adjusted the floats using a 5mm drill bit, ran the engine up again and re-checked the levels. Sorted - they are now consistent at 25.2mm.

I went through the best lean idle procedure to adjust the idle mixture screws, fiddled with the balance until the engine felt smoother and went for a drive. Not fixed, but better - the flat spot had pretty much gone, but it was still lumpy in transition. Plugs black. I'd deduce that the changed float levels brought the main jets in earlier and so narrowed the starvation band. I thought it might be a good idea to see what my old Crypton exhaust emissions analyser made of it -approx 7.2% CO, which would mean a very rich exhaust, but with a very high HC count.

After several recommendations I bought a STE synchrometer (lovely bit of kit) and an elbow to fit the Elan from Eurocarb - a good price on the Synchrometer of ?35 and the elbow was an eye watering ?25. They also sold me a set of 50F8 (as opposed to the 45F8 fitted) idle jets, which, when fitted made it run lumpy until I had set the mixture screws again. Back through the lean idle procedure and a quick check of the balance, which was more or less spot on. Interestingly, the procedure wound up with all four mixture screws set equally at 5/8 turn out from fully in - the first time they have all been the same. The Crypton now reads 6% CO and lower HC at idle, which, in my book is too high, as my target was around 4%.

It might be relevant that while checking balance between carbs, the front pair are spot on the same, but the rear pair are slightly mismatched, with no1 marginally lower airflow than no2. When I adjusted the balance, I set no1 to match the front pair (no1 and no2), expecting to see no3 looking the leanest.

The idle seemed a little rich, but interestingly when I took it for a run, there was no evidence of any flat spot. In fact it seemed pretty smooth and tractable, right down to idle. Feeling happier with myself, I drove home and pulled the plugs - 1, 2 and 3 were a browny colour, with 1 showing a tiny bit of cleaning of the arm. 4, however was still black, so evidently that cylinder is still running rich.

I have a couple of theories:
1) my original 45F8 jets have probably been in the car for nearly 40 years. I wonder if they've been overlooked in more recent tuning. I have cleaned them (soaked for a couple of hours in carb cleaner and paint brush bristles passed through the openings to tease out anything that might be stuck there. I really wonder if one or more had some partial blockage resulting in some difficulty in settings and performance.
2) I think the 50F8 might be a bit too rich overall and while it was a standard fitment for some models, the 45F8 was obviously a correct size for my engine at some point. I may buy a set of new ones just to be sure.

I think my next steps are to get the colourtune plugs out and have a look at cylinders 3 and 4 to see whether I can clean no4 up by adjustment, then perhaps go out for a longer drive to see if that helps clean it up.

I'm also seriously considering a wideband lambda gauge and sensor, so that I can measure mixture in all conditions. I don't want to fit the gauge to the car though, just use it for testing and then keep it as test equipment. I've read that by recessing the sensor, it is possible to fit it much further back in the system than would normally work because the cooler gas flowing over it doesn't cool it too much - that might make it possible to do something near the back of the exhaust to fit the sensor temporarily.

I need to look at the fuel and temperature gauges as they seem to misbehave a little - fuel gauge reads 1/4 tank until the engine warms up, then it reads empty. Similarly, the temperature gauge (with less than a full tank) reads lower than the thermostat temperature. I suspect a dodgy earth, or supply connection, but where?

Getting there I think.

Ian.