S3 "miss On Tickover. Baffling..........

[simon.mitchell]

Was the polarity of the coil LT leads corrected when the car was converted to -ve earth? The car will run with incorrect polarity but will require a slightly higher potential to be created in order to raise a spark, if I remember correctly. The coil + should go to the battery + and the - to the -.

Probably doesn't make a lot of difference in practice, but at least it should be easy to check and eliminate!

[type26owner]

Not to leave you hung up, I'd guess if the voltage drops to 10 volts or below is cause for concern.

I have had a similiar roving and intermittant misfiring from my DCOE_18 also. Found just recently that the cause was from air being sucked down the threads of the idle mixture screws while idling. Under some unknown operating conditions there is also fuel which leaks out of those threads too. I suspect this causes the different cylinders to act up at different times because when the thread is wet with fuel it probably won't leak in any air. Don't understand exactly the mechanisms that are affecting the variability of the mixture, just that it is real. Try goobering some grease around the idle mixture screws completely covering the compressed springs with the intention of getting an air-tight seal. I ended up replacing the springs with a length of fuel hose which is compressed and acts as the spring to keep the screw from moving. Just doing this one fix had a remarkable positive impact on the idling of my engine. I was plagued with a partial misfire which would happen randomly on different cylinders. After sealing those leaks it was a joy to linger (took me 8 years to find this problem) by the exhaust listening to the nearly perfect combustion. You can also cover the exhaust with your hand and feel the pressure bursts. It's easy to feel a partial or full misfire in this way even when it's too noisy to hear the engine because of background noise.

Here's a link which highlites the advantages of using a scope to do these types of diagontics. With OBDII all the older scopes are being sold off for really rather modest prices. I'm tired of not having the right tools so I'm looking to purchase a scope soon.
<a href='http://www.babcox.com/editorial/us/us30224.htm' target='_blank'>http://www.babcox.com/editorial/us/us30224.htm</a>

[elans3]

Hi Keith, Rohan, once again in order :-

Keith,

[elans3]

Hi Keith, Rohan, once again in order :-

Keith, no ballast resistor fitted, new battery, new wiring, dynamo checks out well, only unknown qty is voltage regulator. The same problem occured with the old coil, I've fitted the new one, no worse.

Rohan, It's not a proper full "miss", more like a weak spark. It drops onto the miss as it comes down through 1500, and going up, disappears after 1500 and it's off like the clappers. Its not affected by how fast you blip the throttle, or how quickly the revs rise, it's always there.
I would say it was slightly better when it's cold. When its at idle, it's as if its firing on 3 1/2 cylinders
You can hold it at 1500 and it is , for want of a better description "half a miss", and the more I think about it, the more I'm convinced it's a weak spark. Unfortunately electrics has never been my forte, and I'm in a process of elimination,
Thanks once again boys

[types26/36]

elans3
Just to change the focus of the thread slightly, I note the engine is newly rebuilt, have you rechecked the valve clearances ? How are you determining the miss changes cylinders ? I once had an engine do a similar thing and it turned out to be a valve insert had become loose in the head! Another possibilty.... could it have a broken valve spring ?? :huh:
Regards Brian

[elans3]

Simon, First thing I checked when I got the car, knowing it was a pos. to neg. earth conversion, were the coil connections. They're ok

Keith, Interesting theory, I read your comments in a previous thread. I'll try it at the weekend.

Brian, All valve clearances ok, no broken valve springs, and i'm determining the change of cylinder by removing the leads from each plug, one at a time, at idle. Last week it was no. 4 pot, this week no. 2......... There's no sparking from any of the (new) ht leads with lights off in the garage.

This is great chaps, because I certainly wouldn't have come up with all these possible causes and potential cures by myself.
Many thanks
Steve

[rgh0]

Steve

I to would believe it is most likely electrical. Most fuel / carb problems show some variation from hot to cold and with speed of revs change and have some slight time delay before they occur.

Electrical stuff tends to happen more quickly and consistently.

Unless you have proper diagnostic equipment many electrical problems are very hard to solve except by trial and error replacement of components. Findng someone with a scope who knows how to read it is probably easier than systematically replacing every electrical component that could be affecting the ignition.

However a few quick tests in the absence of any test equipment.

1. Wire the coil positive and negative and engine earth direct to and from a known good battery. If the miss continues then it is not in the power supply.

2. Check the static timing and advance. Avoiding pre-ignition / knock by ear you can do but this is not precise enough to get the timing right IMHO. Set the static timing at 10 to 12 degrees based on the marks. You really should get a timing light to check that the advance curve is in the right ball park.

3. From here on without more diagnostic equipment ( scope , multi meter) you are really just guessing. Work through coil, condenser, points ( or electronics if using), rotor, cap, leads, plugs and if its electrical it will eventually go away or at least change in line with one of the component changes.

One potential area that is not electrics that has not been mentioned is bent valves. It is very easy to bend the inlet valves especially due to their thin stems when timing the motor on a rebuild. The bent valves do not seat properly and this can lead to a low speed miss due to lack of compression that goes away at high speed where the small amount of leakage is less critical.

regards
Rohan

[type26owner]

Steve,
Have you been using the same cap and plug wires on the different dizzies? You know that the wires must be spacially separated and not bundled together with tie-wraps? Carbon tracks on the inside of a black cap are invisible. Seems highly unlikely that the voltage would go much above 6kv at an idle and any sort of intermittant miss would result. There is a small chance there is another problem. That would be if the reluctor is clocked into the wrong position by about 10 degrees in a counter-clockwise direction. That would create a large airgap between the rotor and the contact in the cap for the spark to jump across. That could manifest itself as a miss at idle. The rotor and cap would burn up quicker though. The only way to check this is to partially cut the cap away below a contact and shine the timing light onto the rotor through hole and look at the actual phasing of the rotor to the cap's contact. Remember to hook the inductive probe of the timing light to that contact's plug wire.

You never answered all my questions. Are the leather seals on the throttle spindles new? There might be a casting flaw in the carb body where the retangular dust cover that encloses the actuating lever for the accelerator pump clamps against. That flaw will leak in air which inturn gets sucked through the close clearance hole for the spindle. How much in degrees of turning has the idle speed screw opened the throttles from fully being backed off at the idling rpm? What rpm are you idling at?

I suspect you've got all the regular airleaks and the dreaded wrong butterflies syndrome. To quickly tell if you got the lean hole just above an idle do this test. Raise the idle speed to 2000 rpms and close one of the idle mixture screws. Slowly reduce the idle speed listening for when that cylinder stops firing. If the desired idle speed is 1000 then the acceptable range it cuts out at is 1100 to 1200 rpms. Do this test to each cylinder. They should all behave in the same way. Bet they cut out around 1500 to 1700 rpms though. These problems absolutely kill the idling. BTDT.

[elans3]

Hi Rohan, Keith :-
Rohan, Will try the wiring of the coil and engine earth to another good battery, but the battery on the car is almost new.
I've checked the static timing to 10 / 12 BTDC with a test lamp and it's spot on.I advanced it a couple of degrees after a run to get it hot, and if anything it's slightly better in the advanced state, but still there.
Been through the coil, new magnetronic, (and older Proper Lumenition Optronic), rotors x 3, cap x 2, (inc a new one), new coppper cored leads and old leads with both caps,, and just changed to BPR7ES plugs, with no difference.
Don't think it's a bent valve as there's absolutely no popping back through the exhaust on overrun, nor any spitting through the carbs either. In fact when you depress the throttle from idle to blip it, it seems as smooth as anything. Just happens at idle and when you hold the revs at any figure under 1500.
Keith. as above tried all permutations of cap / leads / rotor on all 3 distributors, but no change. All ht leads separated with "combs" as much as possible, don't believe they touch each other at any point. All leather seals in all three sets of carbs replaced, and all 3 sets tried, there won't be a casting flaw in each set.
Idling at 850 - 900.
All butterflies correct item.
I'm going to put another engine earth on tomorrow, and change the voltage regulator.
Many thanks
Steve

[hatman]

Er, chaps, in view of all the erudition displayed on here I'm a bit reluctant to make myself look a complete chump but it couldn't be something as simple (and normal) as the cammy effect on tickover of the big-valve conversion could it? No? OK - I'll get me coat.

[type26owner]

Hey Hatman,
It's been my limited experience with high lift, long duration cams on the twincam that regression is a bunch of poo. Our BRM phase 2 race engine will merrily idle at 400 rpms with TJ fuel injection without ever missing. My younger brother (the driver) doesn't like it idling there because it creeps him out for some reason. Personally I think it's cool to have a 4 cylinder race engine idling below what most V8s will do.

Rohan would know better than I though since he's really into keeping accurate records of cams.

[rgh0]

Hatman

I have not got any low speed miss with a proper carb setup and ignition working properly with the cams I have tried up to 300 degrees duration and .450 lift and bigger than big valve valves and ports to match so I suspect its should not be the cam or head modifications provided the cam is timed right.

Steve
You appear to have tried pretty much everything. I would run a slightly hotter plug I prefer BP6ES for road use but I dont think that should be the cause of your problem. From your description it sounds like the miss dissappears immediately the throttle is opened from idle and only comes back at steady continuous revs below 1500 rpm. You could try disconnecting the throttle connection between the 2 carbs and independently adjusting the carbs throttle stop to see if its affected by throttle position changes in one carb while he other is left stationary. But having tried 3 sets of carbs and got the same miss with them all when they dont cause a miss on other engines its hard to see how its in the carbs.

Rohan


Rohan

[type26owner]

Well I have to point out that the Weber problems I've been describing are universal IMHO. If you swap 100 pairs of carbies onto that engine, all 100 will probably cause the same missing. Remember there is always some sort of bell curve distribution for the tuning response and you don't know where this engine resides on it. Here's hoping you don't have one out on the extreme end of a wing and it's a lemon.

My son made me smile the other day. He's not riden in my Elan lately and when he did his only comment was that I must have the best tuned twincam on the planet. I earned that compliment by devoting many weekends to understanding the carbies well enough to make significant improvements. Now he wants me to tackle the carbie on his muscle car. It should be easier since the induction vacuum doesn't pulsate much since it's bolted to a common plenum. The total advance has a HUGE affect on the induction vacuum level of a V8 with radical cams. In fact the smart tuner sets the timing with a vacuum gauge. His 383 is truely an engine were the regression effect rules bigtime.

The key to success is to dance with the two-phase flow effect of mother nature and not have it trip you up. Two-phase flow dominates the idle, progressive hole and the main jet mixture behavoir. Two-phase flow is a fascinating branch of science I'm finding. Unfortunately most of the info is not to be found in the free public domain.

I'll p*ss off now....just pulled my engine out for the third time to try another lipseal solution in place of the rope one. I hope the third time is the charm.

[rgh0]

Keith

I agree with you that the standard Lotus Weber setup is poor, especially the 30 and 32 mm choke version, the standard 33mm choke jettings I find the best but that may just be a peculiarity our local fuel.


They all can have problems with flat spots and problems getting off idle and onto the main jets smoothly. However you can generally get them to Idle OK at constant revs without a miss.

Air leakage at idle from all the paths you have pointed out and patiently worked on sealing on your engine can create idle problems. Bu if you have a couple of sets of carbs that dont cause a miss on another engine then it is probable that all the problems due to wear or misalignement or poor maintenance that can cause a miss are absent in them at least to the degree that you dont have a problem on a average engine (whatever that is !).

Rohan

By the way I came across a guy who runs a Lotus Cortina when getting my alternator replaced yesterday. The vibrations in the nose of my engine at 8000 rpm shakes the internals of the alternator to pieces every couple of years. Turns out the guy at the auto electrical shop is working on getting a harmonic balancer made for his car because he has the same problem, a few other people he knows who run similar engines have apparently done the same conversion with good results.

[type26owner]

Rohan,
We're on the same page about the probabilities. I concur. What I'm kinda pushing though for causing the idling fits is the airleaks down the idle mixture screws has a 50/50 chance of being the problem though. Since the density of the fuel is nearly 600 times that of air, what do you think is going to be the prefered stuff that gets sucked into the idle mixture port? This only makes perfect sense if you understand the physics like I suspect you do. This is why I keep stressing that two-phase flow can kick your arse at every level with the Webers. Weber finally installed seals on the later carbies for a damn good reason.

Only if the cubic feet per minute consumption of air at an idle is identical between the different engines will I concede that it's not the a suspect. By goobering the grease onto the all the idle mixture screws that can be eliminated as the cause in just five minutes of testing effort.

Yup, I read that about the harmonic balancer too. The twincam needs one. All the knowledgable builders I've talked to are flabbergasted to hear Lotus never installed one. Besides it's a very fine product of Oz.