A mixture question

[JerryL770]

My 4.2 S2 car was rebuilt in 2017. The engine was checked over but it had been rebored by the PO +30thou and we did not remove the pistons from their bores. I ran the engine in carefully.

Last year we did 1,700 miles round Scotland and returned about 21.5mpg. About to be expected maybe. However, if I'm driving at night and someone is following with headlights on and I hit the pedal to the metal and look in the mirror, I might think I'm driving a diesel and indeed my exhaust pipes are black inside and out and the back of my car becomes covered in blackness. In daylight I see no smoke in my mirror but yesterday I was being followed and was advised I issue quite a lot of smoke when I floor the pedal.

So, with fuel price going skywards, can I improve my mpg by weakening my mixture some and how?

As far as know all is standard. The carb needles are UM. Should I change them, to what? Just screw up the main jets a bit? Use thinner dashpot oil, currently engine oil? Tickover seems a bit quick at reported 900rpm by my re-calibrated tacho.

[malcolm]

Not necessarily your fuel mixture. MPG suggests that's about right. Not unusual for smoke when you floor it after driving gently. I assume valve guides and seals were checked when you had it rebuilt? If you didn't have the pistons out, could be a ring situation. What is your oil consumption like?

[christopher storey]

Jerome : UM are correct for the car and are the weakest practicable needles. The 21.5 mpg doesn't sound as though the overall mixture strength is far out of adjustment ( although I get about 23 but I am light footed ) . What definitely sounds abnormal is the black smoke on hard acceleration, and this points to the pistons rising too slowly . Too heavy damper oil is one possibility , and a change to light machine oil or auto trans fluid is worth a try, but the other possibility is that the springs are too heavy .They should be AUC2107 which have a light blue and black colour code ( for what it is worth because I have never been able to see the colours which should be on the end coil !) which have a load at compression to 3.875inches of only 4.5 ounces . It is worth measuring the free length of all 3 to see that they are the same , and load/length measurements can be done using electronic kitchen scales with a bit of fiddling about . At any rate, if your springs have a much heavier rating, this could account for the problem

If the springs and damper action are both normal , then it is worth looking at a more abstruse possibility , which is that of air leaks into the manifold . The effect of these can be that the overall mixture is set much richer than it should be . This does not show up under high vacuum conditions where the leaks compensate for the over rich setting, but once the throttles are opened wide and there is essentially no manifold vacuum, the mixture becomes very over-rich hence the black smoke

[JerryL770]

Malcolm, thank you. I think the head was done over fairly thoroughly, but just now, that is more a hope/expectation than firm knowledge I hold.

Christopher, thank you for those pointers, all fairly easy to check. I have to say that in daylight, I cannot see any smoke emission in my rear view mirror.

I think I can discount manifold leaks. I understand your point. I rebuilt the carbs with full kits from Burlens so I know they are in good nick, but I do not know which springs came with the carbs and are installed, so with your info, I can check that. I cleaned up the inlet manifold, port matching as much as possible. It wasn't a polished finish though. Changing the damper oil is easy.

I do wonder how much oil should be in the dashpots. I recently topped them up with Castrol XL to above the top of the damper tube and when I lifted the middle pot to check the needle ID, there was quite a lot of oil inside the dashpot piston - where the spring sits. Presumably sucked down the clearance in the tube. I do wonder about lubrication there if the damper tube is not ever filled.

Thanks for the info Gents. I will endeavour to remember to report back if I have any positive info.

[JerryL770]

Update on your advice Christopher.

I went to the Classic car races at Spa last weekend on a 5 night trip with Fasttracktours, an excellent operator for this type of trip.

I did some 1,100 miles including thrashing my E around the circuit for 3+ "parade" laps. I started the trip with an oil level just below the top mark and returned to find the level on the bottom mark this evening. So not really burning much oil. Calculated mpg = 19.03 and my rear numberplate has a brown film coat again. I was followed for the whole trip by my friend driving his open top S3 E and he reported a rich fuel smell the whole time.

I have just removed my dashpot springs and tested them. All the same length. I measured and marked a pencil with a 3.875" mark, placed a spring on my scales and zero'd (tared) the reading. With the pencil held inside the spring with the mark level with the top and pressing the spring down till the pencil was just about to touch the table recorded about 3.3 - 3.5 oz, so they seem a bit light.

[abowie]

Check the CO level in your exhaust. I run my cars between 4 and 5% CO.

I agree with Christopher that 21mpg is pretty reasonable.

[Tom W]

What colour is the smoke from the exhaust?

[JerryL770]

Ah, now, putting my brain in gear and thinking on the spring strength question properly I realise that of course the spring installed in the dashpot would still exert its dead weight upon the piston, so the proper way to assess it, is to include that . . . . I presume.

In that case, having actually measured in that way, the load imposed by the springs at 3.875" length was pretty close to 4.5 oz. So looks like I have the correct springs and needles for a standard engine.

Time to get my CO measured but I guess that needs a rolling road to do it properly. Looks like a trip to Project Heaven which is nearby.

21mpg maybe close to optimum but I think 19 is low for a 1,000 mile average, even if I did thrash it for about 15 mile round Spa Most of the journey was conducted within the required speed limits.

[Philk]

This may be too simple an answer given the technical depth of some of the posts above, but have you pulled all six spark plugs and checked they are an even colour (across all six)? My car was running too rich recently and I found #1 and #2 (nearest the bulkhead) were a little too lean, #3 and #4 were slightly rich and nos #5 and #6 were way too rich (very sooty). Running a CO meter in each exhaust will only give an indication of richness as opposed to which Carb is at fault (particularly given Carb #2 spans both exhaust manifolds).

Having richened carb #1 (neared the bulkhead), leaned off fractionally the middle carb and then leaned off more significantly the front carb, brought things more into alignment. At the same time I also rebalanced the carbs using a meter so they were all drawing the same air intake at tickover.

The above adjustments helped a lot. Re mpg from forum member to forum member, I would also add that someone running a 3.07 rear diff will see improved economy over someone running a 3.31 rear diff given the higher gearing at cruising speed (even if the carbs are set up and performing identically). If you are running a US spec 3.54 then the difference is even more marked. 3000 RPM equates to approximately 65 mph on a 3.54 diff and 75 mph on a 3.07 diff.

[mgcjag]

The difference between 19 and 21 isnt much..over an 1100 mile trip...especially when youve thrashed it and dtiven it hard......now try just a 100 mile trip driveing steady and braking gently......you will probably be up in 24mpg area.....Steve

[christopher storey]

Jerry : I agree 19 is a bit heavy if you are on standard needles . Since you have established that needles and springs are correct, there are a few further things you might check

1.Take the dashpots off and using a depth gauge measure how far each jet is down : each car will vary but i find with UMs that a usual setting for correct mixture strength is between 60 and 75 thou down with the median being about 65, and there should be near equality between the 3 jets

2.Float levels : these have a big influence on mixture strength if they are too high : an easy rule of thumb check with the dashpots and pistons off is that you should be able to see fuel in the jet but it should not be at the top of the jet and certainly not overflowing it

3. Get a micrometer and measure as accurately as you can the needles at various stations : over time both the needles and the jets can wear and richen the mixture ( of course if the carbs are recently rebuilt this can be ignored )

[Phil P]

Hi Jerome. Probably not the case but when you rebuilt the carbs did you check the time of fall of the piston within the dashpot. This should be between 5 and 7 seconds. It's measured with the dashpot and piston assembly upside down, holding the piston and allowing the dashpot to fall away. This will affect how quickly the piston reacts to manifold depression.
Yours,
Phil

[Woolfi]

The mixter is depending on the circular cleft betwwen the needle and the jet.
If you adjust the mixture lean at throttle,this ich changing the mixture at part or full throttle not so much.
Maybe (?) the needle or the jets are worn. Then the cleft is becommimg bigger and the mixture is becomming too rich.
Drive yuor car ten minutes with 70 - 90 mph. Stop the motor immediately, roll on a parking place and check the colour of the plugs. If all 6 are bright grey, the mixcture seems okay. A light brown is still okay, but not good. Darker than light brown is too rich.

Regards Wolfgang Gatza

[christopher storey]

Unfortunately the " plug cut " check advocated by Wolfgang is no longer reliable with modern fuels , which are sootier than the old leaded fuels even at normal mixture strength . It's worth giving his method a try, but do not take the result as a firm indication of running mixture strength

[Philk]

Unfortunately the " plug cut " check advocated by Wolfgang is no longer reliable with modern fuels , which are sootier than the old leaded fuels even at normal mixture strength . It's worth giving his method a try, but do not take the result as a firm indication of running mixture strength

That's interesting (I was not aware). I would still advocate checking the six plugs for consistent colour across them since that will highlight whether (or not) a carburettor is running leaner/richer than its companions.

[christopher storey]

PhilK

I agree that plug colour is a good relative check, in that the richer the mixture on a given carb, the furrier the deposits will become. What is disappointing with modern fuel is that the old silver grey exhaust pipe deposit check no longer works , and the pale brown plug colour is now in my experience something of a rarity . In some ways colourtune is still the most reliable , and I find that on XK engines which need a richer than stoichiometric mixture i.e about 12.5 rather than 14.7 a blue flame with a slight but definite yellow tinge is usually about right seen at say 1200 to 1500rpm under no load

[JerryL770]

Hi Jerome. Probably not the case but when you rebuilt the carbs did you check the time of fall of the piston within the dashpot. This should be between 5 and 7 seconds. It's measured with the dashpot and piston assembly upside down, holding the piston and allowing the dashpot to fall away. This will affect how quickly the piston reacts to manifold depression.
Yours,
Phil

Thank you for all the responses and advice, more is unexpected but welcome.

I don't understand that Phil P.
Dashpot (assembly?) removed and held upside down - the oil runs out surely?

Granted there is not much difference between 19 and 21.x mpg but a 1,700 mile trip round Scotland last year gave me the 21.x mpg. Given that I might have thrashed it a bit more to/from Spa and round Spa track, I still think 19 is too low - for me. BUT, on leaving the Track on Sunday, I had to stop and look under the bonnet for a fuel leak as the smell was too strong. I could not find anything wrong so we drove home, never more than 70mph on motorways with some windows open for ventilation.

On emptying my boot of stuff, I found the pop bottle of oil which wrapped in a cloth and lying was beside the spare wheel, was soaking wet! Not surprising really as part of the journey home was in monsoon conditions. However, this prompted me to remove the spare wheel and I noticed petrol smell and looking low found the fuel pipes to the pump were wet with petrol. The outlet banjo was leaking!! On removal, I found it had ill fitting aluminium washers and was loose!! I'm awaiting delivery of proper fibre ones from SNGB - enough to do all the banjo's.

Christopher. I had all the dashpots off and cleaned them out. The rear one, the piston had an oily black coating in a pattern and inside the pot (a little). The centre one was pretty clean and the front one slightly black and oily. On the whole trip the oil level went from 1/6mm below the top mark to the bottom mark. I didn't measure them but all the main jets seemed to be about the same level and about 1.5mm down (~60thou). I need to measure to be sure. I have replaced the engine oil in the dashpots with cycle oil.

I did remove the rear plug (in the garage only), and it is light brown.

I can't go further till I can reconnect the fuel pipes.

In the meantime, I will get on with fitting Marek's electronic fuel pump conversion and the Retronic intermittent wiper kit I've had sitting round for too long. Wished I'd had the latter for my trip to Spa!!

Thanks again for the advices, I will follow up later, maybe after a rolling road trip.

[mtnjag]

Top stick to bottom stick mark. Isn’t that 2 US qts in 1,100 miles?
Seems like highuseage if that’s the case.

[malcolm]

If it is 2 US quarts, then that's 330 miles per uk pint - not too excessive i wouldn't think, particularly as some of the miles were a thrash round SPA

[mtnjag]

Really? I went through another discussion on this site and the general consensus was 800-1000 moles/US qt would be considered normal while 500/US qt excessive.
But if plugs are not oil fouled then that is in error I guess.

However, rather than pull only one plug to check condition i would pull all to confirm there is not one bad cylinder for either fuel or oil.