Several time now I have tried to tune the carbs in my 3 1/2 ltr Mark V.
I have tuned other engines with great success but this one refuses my advances.
I have always believed it has run rich and on purchasing a colour tune spark plug the yellow combustion flame would back this up. However I can not tune it to a point where the flame will go blue??
I have fitted service kits to each carb and noticed it had FW control jets compared to my book that has them as FL so I purchased a pair and fitted them. No noticable change to the flame colour but now on engine over run (down a hill) it backfires badly.
What is the difference between the FL and FW jet needles (both I’m told are correct for Mk V’s)
Could I have fuel getting in via the starting carb when it’s off? (converted to manual control and operates correctly)
Any thoughts?
Regards, Jordy
622228
Hi Jordy,
I think back firing is much more likely to be due to wrong ignition timing but to answer your questions on carb needles:
The standard needle for the 3.5Litre MkV is FW with the option of a richer one FL
The MkIV standard for 3.5 Litre in 1948 was FL or richer AQ
If the back firing has only started since a major engine overhaul then check your valve timing. Check it with the rocker cover off and turn the engine until the inlet rocker is just opening and the exhaust rocker is just closing and the two are at exactly the same height. The cylinder for the rockers you chose should be exactly at TDC.
Peter
Thanks Peter.
Looks like I made it richer while it makes me poorer.
The backfiring only began after changing the FW for the FL.
Regards, Jordy.
Hi Jordy, the F.W. needle is the standard needle called out in the Mark V service manual for the 3 1/2 litre engines. I have not found where the F.L. richer needle is mentioned for consideration in the factory service manual but it is in the charts as shown by Peter if tuning is considered beyond the service manual F.W. recommendation.
Sounds like you might wish to revert to the F.W. needle to verify the backfiring goes away due to too rich mixture. Good luck.
Fuel can get in through the starting carb, but usually only if the sealing faces are damaged or the swivel stopper hanging up crooked.
How about your vacuum advance, maybe time for a new one? It is the same as for XK120 and is available from some of our favorite vendors like XKs Unlimited. I replaced mine and I recall discovering that the mounting studs on the new one were not BSF thread, they were UNF. So if you get one be sure to ask the vendor to supply new nuts for the studs.
Some guiding comments from Jaguar service manuals:
1946-48 All Models - p.17 “it is inadvisable to change the needle supplied by the makers, except in cases were the needle has been damaged, and then it must be changed for another of the same type.” p.18 on carb piston damper “The only attention necessary is to keep this supplied with engine oil of the recommended grade. Indication that the oil chamber requires filling is given when spitting back is experienced when the throttle is opened quickly.” p.19 Carb Jet Needles - 3 1/2 litre uses FL for LHD and DY for RHD (also shown on p.14).
Mark V service manual - p.C.7 “Observe that lack of oil will cause weakness of mixture on snap throttle openings and consequent falling off in performance.” p. C.8 3 1/2 litre needle is F.W… p. C.11 “It will thus be seen that on acceleration and snap throttle openings a slightly enriched mixture is obtained due to the control exerted on the piston by the hydraulic damper preventing the piston from rising with undue quickness.” and “Absence of oil will be indicated by poor acceleration, loss of power and spitting back through the carburettors on rapid opening of the throttle.”
SU Workshop Manual - p. A.3 “To prevent the piston from rising too quickly as a result of brisk throttle opening, an oil-damped plunger unit (26) is positioned inside the hollow piston rod, and this puts a fluid brake on too rapid a piston rise but exerts no restriction on its fall. It provides an appropriate degree of enrichment for acceleration and improves cold starting and driveability from cold.”. p. A.4 “On the bulk of carburetter installations, which are car manufacturers’ original equipment, the type of needle (of which there are many) fitted to a specific car has been settled only after prolonged tests, and therefore it is not usual for the recommended type to need changing in practice.”
Haynes SU Carburettors - p. 30 “A richer mixture is required for smooth acceleration. This is achieved by increasing the depression at the jet for any given piston position by retarding the rate at which the piston lifts, thus temporarily increasing the air velocity. The hydraulic damper, in conjunction with the oil filled piston rod, controls the rate at which the piston rises. Note that the damper does not limit or reduce the height to which the piston rises, it merely controls the speed at which it rises, causing an immediately richer mixture to be delivered when the throttle is snapped open, which gradually weakens as the piston lifts to its correct position. As the damper is only effective for upward movement of the piston, the piston falls immediately the throttle is closed, preventing an over-weak mixture being produced.”
A couple of thoughts. First, the FL needle was called for in '46-48 cars and was replaced by F.W. in Mark V cars. The FL needle is richer earlier in the profile and richer overall than the F.W… Maybe the FL needle is way too rich on throttle closing for the fuel in use. Second, there are varying interpretations of the term backfire. Note the above quotes talk about spitting back through the carbs. United States terminology may have “backfire” as the noise of burning fuel reaching into the exhaust line.
Hi Roger.
Thanks for the info.
I may have inadvertently increased the richness of my mixture with the jet needle change and I will look at changing back.
I do have oil in the dampeners. I also see at one stage there were also springs employed, within the dampener chamber, for controlling the rise on acceleration. Must have been deleted at some point. Mine does not have them.
My reference to backfiring is, as the US terminology has it, ignition in the exhaust system and not the spitting back out the carbs.
Thanks and regards, Jordy.
Spitting back into the intake and carb is cited on the snap throttle in the quotes above (this is technically called backfire in many places since it is coming backwards in the intake path).
Backfire in the exhaust system (this is technically called afterfire, but in the US pretty frequently is called backfire, perhaps since it is at the back end of the flow path) may occur on the overrun, throttle closing rapidly when too much fuel is in the mix. The SU and Haynes quotes above show concern to keep the mixture from going too lean on overrun. The switch from FL to F.W. by Jaguar in the quotes above for the 3 1/2 litre engine suggests that overrun on FL went too far rich when trying to avoid weak mixture on overrun, which could cause exhaust backfire.
Also, both distributor and valve timing can cause these issues. Since your case coincides with needle change, I would revert to F.W. needles before examining timing issues, but then would double check timing under the conditions where backfire was observed. Sometimes problems are a blend of more than one cause.
Hi
A friend had the same symptom (non-Jaguar).
Needles were correct, timing was correct, plugs colour was good, and he had a mystery backfire on overrun.
By trial and error, while checking everything, we found backfire ceased when we fitted colder plugs (colder than standard).
We concluded it had to do with the quality of modern fuel, in conjunction with the plugs grade.
Who knows.
Francis Thibaud
Hi,
Yes, moderns fuel is different and especially for the 2" HD8 SU’s on the E-types you need different needles than original to have the correct AFR at all engine speeds.
However the only times I have wittnessed “backfiring” or what we would call “afterfiring” or “afterburning” in the exhaust was when the mixture was too rich and the distributor advance was not working (weights were stuck inside the distributor) or once when a garage has set my timing 30 degrees retarded! Surprising the engine ran fine until ca. 3000 rpm when it started to sputter and burn fuel in the exhaust.
On the MKV I would suspect all of them, but you have to debug it one by one. Making sure the “hisser” aka staring carburettor closes properly and your valve timing would be where I would start. For checking the ignition timing I used a white paint pen to make 0 deg, 5 deg and 10 deg marks onto the side of the crank damper/pulley so I can check timing with a strobe light. You can also fiddle around with the timing adjuster on the side of the distributor, adjusting it should also change idle speed which could give you an indication towards the problem you may have.
Cheers,
Pekka T. - 647194
Fin.
Ps. Just started my 3 1/2 Litre for the first time in almost two months, great how the engine now works, after warm up nice and steady burble at the 450rpm idle…
If the cause is not carb related and given that it spits back on the overrun this might suggest that the ignition is too far advanced. (On overrun the vacuum connection will advance it further.) You could also check whether it still spits back with the vacuum connection disconnected.
In the UK the term back firing is normally restricted to firing in the exhaust system. This gives an explosive bang and can explode silencer (mufler) boxes.
Peter
That’s interesting!!
I tried the colortune plug thing once on a Cortina. It was pretty useless. My suggestion would be to get an inexpensive CO meter, or pay a shop to tune it.
Agreed: unless used on a chassis dyno, they are a cute toy.
The story is that originally the dampers were made of brass. Postwar,SU found they could be made cheaper in aluminium but as these were lighter , the springs were added to compensate.
Thanks for everyone’s input.
Yesterday I changed the jet needles back to FW (even though I have a SU service brochure specific for the Mark V that says they are FL)
and it is running a lot better. I cleaned out the starting carb and made sure the seat below the solenoid was sealing fully and checked the mixture again with the colour tune plug. It is nearly blue which is better than what it used to be so not sure of the original rich running issue I had. Perhaps that seat wasnt fully sealing? On a test run it ran a lot smoother and quieter without the backfiring on overrun but still had a splutter or two at times. I may take it in for an exhaust gas diagnostic just to fine tune it and hopefully this will be an end of this problem.
Next issue, with summer approaching here, is to address the way it runs after being restarted when hot. The engine heat rise when stopped transfers into the carbs (and fuel) and affects the running when restarted until the car has driven some distance and the carbs have cooled with the airflow.
A local solution with other Mark V owners here has been to thread a fitting into the front fuel banjo and run a return line back to the tank allowing cooler fuel to be available on hot restarts. I’m told this works.
Regards, jordy.
A bleed off does work. it only needs to be a small 1/8" capillary.
Chrysler did it on Valiants years ag by using an on line petrol filter with a take off.
I did it on the MK IV and VII and never had a problem afterwards
I didn’t mean from VE Day , but the alloy pistons were the reason for the springs and the change was made post war.
Another input on the Hot Soak problem, when a restart is difficult after sitting a while with the engine compartment hot, is that ethanol blends have a lower boiling temperature. As Ed notes, circulating fuel back to the cooler tank addresses this. So does utilizing gasoline without ethanol. Dave