Running rich, engine tilted. Should I adjust floats?

The 4.2 in my Mk2 is running well but it runs rich despite trying to lean it out with the mix adjusters. I have 2x HD-8s, UB needles, and 02 sensors in both pipes. At full temp it runs in the low 11:1’s at idle and steady cruise, but creeps into the 13s / 14s under heavy accel. I put thicker oil in the dampers. Made a little difference, not much.

The engine is tilted down towards the rear but it’s as high as it’ll go before the shifter case on my T5 hits the floor. When I pull off the domes and pistons, the front carb jet is almost level with the bridge, while the rear is much lower. I’m wondering if lowering the front float and raising the rear one might help stabilize my mixture? I’m trying to lean out the idle while keeping accel rich.

Been trying to solve this for awhile…

I think you may be looking down the right road , can remember reading about a E-type with a 3 carb set up , something along the same lines , there was quite a bit of diffrence between the first and last float bowl level , you have nothing to lose in trying !

Pulled the floats, they were set evenly to each other. I lowered the front float about 1/16 and raised the rear about 1/8. Adjusted the mix screws … preliminary results are promising.

A quick run around town resulted in 12.5:1 - 13.5:1 idle and regular driving. Had a weird moment where one float stuck and dumped fuel from the overflow but a few wacks on the bowl fixed that (never happened before…). Further testing required but optimistic!

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Although I do not have the same setup as Theo, I have seen similar behavior.
I have been running some exhaust gas tests on my Mk2 with a freshly rebuilt 3.4L set up with standard carbs. I have 1000miles on the engine and have limited the engine to around 3000RPM so far.
I see the same thing that Theo mentions which is that at light throttle I have lower AFR than when the throttle is opened up say climbing a hill.
I don’t really understand why but I think it may be due to poor fuel burning at low cylinder pressures.
With hard throttle I see the canonical 14.7 whereas at light throttle I get in the 12 -13 range.
I have a wide band sensor fitted to the header pipe near the start of the horizontal section on cyl’s 4,5, &6.
I have recorded the readings on a local substantial hill followed by a fairly flat section of road.
When climbing the hill I see a steady 14.7 or so but once on the flat the ratio clearly drops down and fluctuates as I have to negotiate traffic. The hill climb was at a pretty steady speed.
I also notice that when cruising at say 60mph on a flat road, the instant I press on the gas the AFR jumps to about 14.5.
The motivation for doing this was to make sure that my engine was not running too rich at higher powers and indeed I think it is working rather well. I would like to have recorded RPM but I am having difficulty getting a signal from the ignition to trigger the readout device. I will continue to experiment to try and get an RPM readout on the recordings.
FYI I am using an ALM-LED made by Ecotrons together with their GUI on a laptop.

Route 2 Test Oct 20.pdf (465.1 KB)

Very interesting. The heavier dashtop oil helped a little bit with the instant leaning when you hit the throttle, but for me it still goes lean after a few seconds. What readings are you shooting for?

I think that if I have 14.5 or so I can’t ask for better than that. At high power the mixture is ideal with no danger to the engine from overheating due to weakness.
At lower power, having to accept a lower AFR which implies unburned fuel is just something I have to live with. I do have weaker needles but have not tried them yet. And I would expect that they would weaken the higher power case which is not what I want.

When the engine has some more miles on it and I have got the RPM signaI working, then I will get more precise measurements with both sets of needles.

I’ve always read 13-14 is good for idle and cruise, you want 12’s under heavy load for max power and protecting the motor. That’s my goal, but to be generally in the 12-14’s is the right range. I find myself having too high expectations for 50 year old carb technology.

Here’s my E Type engine - the car is on the level floor in my garage. It’s as near as sh*t is to swearing - level. I rekitted all carbs and turned with a UniSyn and Colour tune. I don’t have your science but car runs really well.

Our XJ6 is nowhere near level. Similarly rekitted and tuned. Also runs very well.

In my youth I had a Dolomite Sprint. I remember having the same concern as you, so I made tube “packer” for the rear float bowl, to level up the fuel meniscus in each. No noticeable difference.

FWIW, no gas analysis, so blissfully ignorant. Paul

Maybe I’m missing something here but…? Liquid seeks it’s own level - the level of the fuel in the float chamber is the same level as in the jet - they are like a plastic tube with liquid in it held in the shape of a U. It doesn’t matter that one carb is higher than the other, as they are not connected in the jet area. The height of the jet effects the size of the opening around the tapered needle, and hence how much fuel can pass by it. The higher the fuel level in the jet the more that flows. A needle too far into the piston increases the size of the hole. So the fact that your jet in front is higher, maybe because your float level is too high, a worn jet or needle, a miss positioned needle. But it should have nothing to do with heights of the carbs.

You should be able to see the actual fuel level in the jet with the piston out. It should be the same on both carbs everything else being equal.

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If you have genuinely never exceeded 3000 rpm in 1000 miles, you need to hurry up and give it some work to do before it turns into a bore-glazed smoker IMO, especially if you fitted chrome top rings.

Red lining briefly in the lower gears is in order from about 250 miles and working it gradually harder until you’re basically finished by 1000.


Well, I was guided by the words on the Jaguar running in decal which instructs " 3000RPM max between 1000 & 2000 miles".

Yes I have occasionally revved up to 3500 so far.

I would think it’s more important to work the engine hard so that the sealing forces on the rings are high to promote bedding in, rather than just rev high.

I recognize that these instructions reflect the technology of the 60’s

I don’t know if the top piston rings are chrome plated since I just fitted Mahle pistons with rings installed. They did not appear to be chromed, i.e. not shiny.

I think the car is being worked adequately hard at the present mileage.
I have just put the car in winter storage and on the 50 mile run, mainly on the Mass Turnpike, I was running at 80 mph (along with other traffic) which is 2800 RPM in O/D.
I have a 3.54 rear axle which gives a nice cruising with an overdrive.
As yet I do not do hard acceleration in the lower gears

Yes, good, The key word is work, not miles or revs exactly. Mr Average decal reader, or some shops in the boonies, couldn’t be trusted to judge workload and sealing pressure concepts, so the factory ran each engine on a testbed and each car had a road test and oil change before despatch. After that, the running-in guidance used easy-follow numbers to minimize abuse and warranty expenses, whereas we run our rebuilt cars from first start and are prepared to judge effort and duration of use more wisely. Carry on.

I understand the carbs are not connected which is why the float levels were not quite right, the front carb was overly rich while the rear was running lean, despite the front jet being almost level and the rear one being quite deep. I adjusted the floats and now they run equally and consistently. I have pushed the engine to 3k but it started to go lean, now it doesn’t, so I will push it more. But I also drive the car pretty hard at times so it has done work even if not at particularly high revs. Also, never a whiff of smoke or oil consumption, so seems ok.