Hello all. I had my 3.8 s type Tuned a few months ago using a dyno so I am very comfortable that they got the most output from the engine however since then I have noticed the cylinders 1, 2 and 3 spark plugs soot up but cylinders 4, 5 end 6 (that is the ones closest to the radiator) are fine and their spark plugs are nice and white whereas the spark plugs for the back three cylinders get sooty very quickly which I assume is from too rich a fuel mixture.
Is it the cas that the rear carby only supplies the rear cylinders? I would have thought the mixture would have been evened out across both carbies for all cylinders. If the carbies feed different cylinders then which cylinders are fed by the starting carb?
cheers

 

Yes, the rear carb feeds the rear 3 cylinders, and the front carb feeds the front 3 cylinders. The starting carb feeds all the cylinders through a system of pipes on the underside of the intake manifold that feed mixture to each cylinder.

If you want to spend some money, I have used an air fuel ratio gauge to tune the carbs and it works very well. I like the AEM brand, it has been quite reliable for me.You can be quite a bit off tuning by ear or looking at plugs.

I'm wondering if the shop that tuned the engine has the rear carb rich, the front carb lean and blended together at the tailpipes the exhaust gases were about right?

As a tuning method, what I have done is take off the dashpot and piston. Make sure that the scribe line on the needle is exactly even with the bottom of the piston. Then measure the depth that the jet is below the bridge, using a vernier caliper or other accurate depth gauge. I use a caliper good to thousandths of an inch. Measure both your carbs and see what the difference in depth is. Split the difference in half and raise one jet by that amount and lower the other by the same, so that both jets are the same height.

As an example, if one carb is at 0.110" and the other is at 0.092", the difference is 0.018", half that is 0.009". so raise one jet by 0.009" and lower the other to have them both the same depth at 0.101". Reassemble and see how the car drives. As a ballpark figure, on my car the happy place was the jet about 0.095-0.097" below the bridge of the carb. I'm at 3500' elevation, if you are at lower elevation you would want the jets lower to allow more fuel. When I took my car to sea level it was noticeably lean.

 

I'm pretty sure Jagboi is right. Same carb feeding those cylinders. While the Burlen Fuel Systems site gives you a lot of info buy this book. Presume both carbs fitted with correct TL needles & needles properly seated & centered in the jets. No air leaks anywhere. (false air ~ front carb). Under heavy acceleration do you see black/brown smoke out of one exhaust more than the other?





While we know your engine is worn. What you describe says it's not an oil passed rings issue.

Your dyno tuning folk should have picked this up!

 

SU carbs were about the best way to fuel and engine until electronic injection arrived. That's not only my opinion, but was given by a very respected fuel R&D expert. If they are OE carbs, they can be made to run very well. Don't believe anyone who claims otherwise.

Most of our engines are old, worn and we often don't know what previous owners (and some mechanics) have done to them. Before going to the dynamometer, it's advisable to be sure the engine has good, equal compression on all cylinders and that the carbs are in good, clean condition with the same needles correctly set in both pistons and centred in the jet. The throttle butterflys should be synchronised, though you might reasonably expect that the dyno shop would do that.

I've written that you should do all those things. I feel that the person who did the tuning should have checked with you about most of them before doing the work. Certainly, they shouldn't return a car with one carb running rich and the other weak. I'd be inclined to go back, show him the spark plugs and complain.

I fear that nowadays there aren't many people who have a chassis d and also understand SUs. If you were in the UK, I'd suggest that you tried Peter Burgess. In Aus, I've no idea.

 

Peter, I think the type of SU's on the S Type is an HD6 with the volume tuning screw to set the idle and balance, therefore balancing and idle is not set with the butterfly valves, they are both shut tight at idle.

 

Jeff, You are right about HD6s. HS6 are different for some reason.

 

Agree Glyn. I did buy that book on your recommendation last year.
I am suspecting that jagboi is right about the dyno shop averaging the mixture because the sensor was in one of the exhaust pipes.

i dont feel confident going to the detail that jagboi suggests with the carbs though i agree it would work.
Thinking i might just lean off the rear carb a quarter turn and enrichen the front by a similar amount, give it a bit of a drive and see if the plugs are looking better.

 

Yep! ~ give it a bash. Keep track of what you are doing so you can always return to present settings & go & crap on your tuning shop. Make sure both dashpots have oil in them.

 

Initially I found it easy to get confused about which way to turn the screws for rich vs lean, so that's why I just measured the height. I also found that was the easiest way for me to ensure the carbs were producing equal air -fuel ratios.

 

I use the lift pin on the side of the carb's to determine for rich/lean mixture.
Works well enough, but takes a bit of skill maybe ???
Just saying _ it wasn't mentioned here.

 

Yes, thinking it may be easier than removing the alloy air intake horn in order to manually lift the pistons a tiny bit to determine whether the mixture is too lean, too rich or just right.