Self ignition

[lotusbzz]

Tomorrow is Saturday, my Elan is back from the local Guru properly tuned in timing and air+fuel mix, I?ll fit colder sparks plug a tank of 100 octane petrol and see what?s happen, then one at the time I?ll go through all you useful advices
Please note that the engine after the restoration runs just a couple of miles.
I?ll keep all you posted
Have you all a nice week end
Paolo

[HCA]

The same guru who passed it off as the compression earlier...?

Best not to over compensate at the outset. Keep everything as standard to start with. Fill the tank with SP95. If the dieseling still persists in standard trim, and all the settings are correct, then you can start looking for the culprit and compensate if necessary.

I have a 500bhp Audi RS3 that runs pretty well as good on SP95 as it does on E85. I therefore think it should be easy to have your car running sweet on 95 as well

[lotusbzz]

Just to avoid any misunderstanding, I fully trust in the guru because he has 50 years in servicing racing cars Ferrari porche jags Aston Martin and any classic cars for 1000 Miglia
He said that the timing is ok and there is no dieseling, for sure the fixing is a thicker head gasket, but before to go for such extreme action, I want to try something more affordable
You all gave me a lot of suggestions that now Is going to evaluate

[lotusbzz]

Just to avoid any misunderstanding, I fully trust in the guru because he has 50 years in servicing racing cars Ferrari porche jags Aston Martin and any classic cars for 1000 Miglia
He said that the timing is ok and there is no dieseling, for sure the fixing is a thicker head gasket, but before to go for such extreme action, I want to try something more affordable
You all gave me a lot of suggestions that now Is going to evaluate

[gjz30075]

If it's not dieseling, maybe you can post a small video of your symptoms.

[HCA]

runs great but .... he doesn?t want to shut down!
I switch the key off but he goes on to run for awhile sometimes with awful noise and a bit of knocking too.
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He said that the timing is ok and there is no dieseling,

I am sorry, but now I am completely lost.

Now it appears to be alright and this 'guru' is contradicting what you say?

All this is now a waste of our time?

[lotusbzz]

May I ask you all whick kind of sparks plugs do you fit?
Currently mine are Champion N9YC that are normal, neither cold nor hot.
this is a pic of the sparks plug today, it's a bit oily

it's not very nice, maybe the mixture is too greasy
online the Lotus-dealer offer the NGK BP7ES that is 1 grade to the cold of the NGK
maybe these spark plugs and better fine tuning of the Stombergs could help.

[HCA]

Now you have oil on the plugs?? Are you sure it is oil? Are they maybe wet and you are mistaking it for oil? Is there blue smoke from your exhaust? This is a whole new game if you have oil on your plugs and will take you back back to your valve guides or piston rings.

A mixture cannot be 'greasy' (outside of two stroke). You must check if the plugs are oily or just wet with petrol. If the former, you have other issues, if the latter then mixture too rich which is a classic cause for dieseling.

Surely your guru, with his weath of experience should be telling you all this and finding the root of your problem

[HCA]

I was on my iphone earlier without a good view of your photo.

The centre of the plug is a bit too grey and slightly damp - certainly running rich I'd say.

As to the wet on the threads, yes it does look oily. Do a test - rub some of the wet from the threads between two fingers. Does it feel oily or can you feel some friction there?

N9YC plugs are the equivaqlent to BP6ES, so should be ok. But you are right to be concerned by the wet threads. Are the other three plugs the same? Maybe lets see a photo of all?

[rgh0]

As stated by others Strombergs are prone to running on. This is due to their ability to supply a combustible mixture at very low air flows compared to Weber's. I had an early SAAB 99 many years ago that had an electric solenoid device retract the idle throttle stop when the ignition was tuned off so that the butterfly's fully closed when the ignition was tuned off to stop run on. The source of ignition will be a hot spot in the engine. This hot spot may be due to a combination of a number of possible causes and each needs to be analysed to see if its contributing. If the engine has only done a few km since a full rebuild the rings may not have been seated properly which may explain the oily plugs.

Options are:

1. Too high a compression ratio. You quote 9.6 but how has this been determined. Has it actually been measured or have you at least measured the head thickness. You would only need to fit a thicker head gasket if the head has been shaved and you want to lower the compression. If its truely 9.6 then it does not need lowering and the engine would probably run Ok on 91 Octane if everything else was right !

2. To low octane fuel - unless you have over 11.5 compression ratio 98 Octane fuel will be OK if everything else is right. With your quoted compression ratio of 9.6 then should not need 100 Octane

3. To advanced ignition timing - Advanced timing can contribute to hot spots. You're guru may have set it to what he thinks is correct but does he know whats correct and does he understand all the errors around ignition timing in various Lotus manuals and which curve applies to your engine. For trouble shooting purposes the timing needs to be per the curve in the manual which for the non emissions Stromberg engine is actually correct i.e. 9 crank shaft degrees at idle moving to no more than 33 degrees at 6500 rpm, the details of the actual curve is in the manual.

4. To lean a mixture - Assuming your guru has the need wide band O2 meter and knows how to use it and knows how to adjust Strombergs and you have the correct needles, diaphragms and springs etc to properly service them. He should have the mixture in an acceptable range but it is easy to get Strombergs running "well" but too lean. This will encourage hot spots from carbon deposits or sharp edges on the cylinder head or on the spark plugs. Strombergs are not common in Italy so maybe he has missed something. Even experts can get it wrong sometimes which is why you always need to ask for specific numbers not just assurances that its OK.

5. Sharp edges in the cylinder head - if the head was machined to fit new valves and valve seats and a sharp edge left from the machining operation that edge can overheat and glow. Normally this goes away with time a the edge gets burnt and rounds itself off. But if oil is present the oil will burn onto the spot and leave carbon that will continue to glow.

6. To hot plugs - Champion N9YC plugs should be OK. You could try a colder plug such as the NGK BP7ES or Champion N8YC but if that fixes it then it says something else is wrong such as too lean a mixture and or to advanced ignition

7. Carbon deposits - With the oil in the engine carbon can be quickly deposited which then is heated and glows red hot and provides an ignition source after the engine is switched off. If everything else above checks out OK this would be my best guess.


Does the running on occur only after the car is driven and not occur if the engine is started, warmed up at idle and then turned off ?

regards
Rohan

[Slowtus]

my solution is ev. too practical or simple ------------------------ too many of you are indoctrinated!!!! sandy

The difference here Sandy is that the OP stated the car runs well. This would not happen if he had the plug leads incorrect. If he had rough running, then definitely a case of sorting it out!

To a point...when I piked up my first Lotus (an S2) I thought it ran (subjectively) well.

Until I found it was running on 3 cylinders.

Once I corrected that it ran really (subjectively) well.

Head before hands is needed here, then a logical approach, eliminating one item at a time - unless you prefer the "split-halves technique"...

[StressCraxx]

Wet threads:
Some of us put a drop of oil on the spark plug threads to prevent the plug threads from seizing in the head.

Agree about the electrode shows rich mixture. Suggest checking the needles and seat for wear.

[lotusbzz]

I thank Rohan for the list!
this is what I'm doing step by step to fix the problem.
since other people iis involved in this job, I'll need time to go on all the points but for sure I'll report the result of the investigation for future help of anybody with same trouble.

P.S. Facebook is great because you can post pictures easly, but a Forum like this with experienced people that can write properly a letter/email and explain in understandable way is much much better!
Thanks you all

Have a wonderful Sunday!

[2cams70]

This was a very common problem on many different types of engines when fuel mixtures were leaned off for the first round introduction of stricter emission regulations around 1975-76.

The cure that was adopted was fitment of an anti run on solenoid. The Ford crossflow engine for example having a Heron head design with combustion chambers in the pistons was particularly prone to run on because of high piston crown temperatures.

The electrically activated anti run on solenoid vents the intake manifold to atmosphere when the engine shuts down. This immediately weakens the fuel/air mixture entering the engine to an extent that it can no longer support combustion so the engine immediately shuts down.

Whilst it is a solution that perhaps addresses the symptom more so than the cause it may be an option in your case and is easy to retrofit. I for one would only be reducing the compression ratio as a last resort if the engine runs well otherwise apart from the run on. See link below for an example of an anti run on valve.

https://www.ebay.co.uk/itm/MGB-ANTI-RUN ... SwPl1dLjZn

[69S4]

I had a lot of trouble with run-on with my Stromberg engine to the point where I considered an anti run on solenoid but a combination of three things seemed to sort it. First, a complete rebuild of the carbs. The needles and jets were worn beyond the point where adjusting them worked so there was a rich tickover mixture. Second, replacing the distributor. The original was worn to the point where there was considerable idle speed timing scatter. Third, keeping the tickover speed low. IIRC it's meant to be around 900rpm. Anything from 1100 upwards and run on was close to certain. As a 'fringe benefit' doing that helped no end with hot starting (which was actually my primary goal).