Carb jet height/depth conundrum

[Gfhug]

I've always had poorish fuel consumption around 20mpg with sooty plugs. Have set up the carbs as per the book several times but have always ended up with quite high carb jet heights, yet still about 20mpg
So I eventually bought a Gunson analyser and followed the advice on the forum, e.g. here:
http://forum.etypeuk.com/viewtopic.php? ... lit=gunson
With tick over balanced using an air flow meter. But still ended up with jet height/depth of .025" for 5.3% CO

Any thoughts, please?

The carbs have been overhauled (by me) a couple of years ago with correct new jets, UM needles, etc. But one thing I did do this time was change the fuel float height to 1/2" as I had been advised that 7/16" could be fuel generous. Also tightened up the float lever pivot at the pin as they had been sloppy and may have contributed to excess fuel consumption through inconsistent float height.

The car drives like the proverbial scalded cat, but had done so before (don't you just love flooring it when Audi drivers insist on driving so close behind ) and the plugs are looking a better colour than before.

Thanks for any comments/observations

Geoff

[MarkRado]

I know that there are people out there claiming that you can do 25. Even with easy highway cruising, I really did never get a consumption better than 23, now with UE needles it is between 18 and 16. I adjust to a Lambda of 0,87/0,89, which results in a CO of between 5 and 6 and nice beige plugs. On my 3.8 the jets are set to 0,40 or 1,1 mm; I know from the 4.2 that it needs approx 1,5mm which is 0,60.

[malcolm]

Hi Geoff! I use to run on about .075", but after recently removing and re-installing the needles I'm up to .065". Runs well and I get an average of 22mpg on mixed (but mainly light traffic) running. Used to get 23mpg in same conditions. I'm thinking maybe the needles are slightly different in height since putting back resulting in this, as the plugs are still a tad sooty. Might go up a little further (to about .055") and try that.
Your .025" sounds very high to my untrained mind - are your needles set at the right level?

[mgcjag]

Hi Geoff.....just to make sure were all on the same page.....have you set the shoulder of the needle flush with the bottom of the piston....and is your jet measurement from top of jet to the bridge of the carb.......I rebuilt my carbs following lots of advice from David Lonsdale (Carb rebuilder well known on the forum)....his advice was to set float to 7/16.....set jet 2mm down from bridge........fuel should then be seen about 5/16 below top of jet....balanced up air intake and car run really well and over 20mpg......just needed slight adjustment after i had done a few hundred miles to let carbs bed in......Steve

[Gfhug]

Hi Steve, yes needles set properly and measurements all correct. It was at an E Type seminar that the 7/16" was mentioned as being too high, hence me trying 1/2". This change didn't make any difference to engine pick up when throttle floored.
As mentioned, the loose float pivots may have given incorrect float heights whilst engine running.
Haven't driven far enough to check the fuel consumption. Yes, David is a good man.

Geoff

[mgcjag]

Hi Geoff...well it you happy with how it runs and performs then leave well alone...if however you want to know why your jet height is different to others you will have to remove jets and needles and measure them....to make sure the dimensions are correct.....did u check the jet numbersbefor you installed them....even so measurements are the only way to find out whats different....Steve

[christopher storey]

Geoff ; 4 points


1.With modern fuels plug colour is no real guide to running mixture strength. The old days of a paleish grey/brown indicating correct mixture strength have long one

2.20 mpg seems about right to me if you are prone to "flooring it" . I am a relatively gentle driver but never get better than 23 mpg on either of my E types, and that is over a 25 year period. I get lower than that if short runs are involved

3.One thing that can affect carb performance is if the wrong springs have been fitted. Heavier than standard will produce excess mixture strngth under acceleration. However, since there is a very limited range of springs, this is unlikely

4.Are you sure your floats are in good order ? On my Mark 2 I developed fatigue cracking of the floats which allowed fuel inside and caused rich running

[Gfhug]

Christopher, thanks for your comments.

1. Not so much plug colour as being sooty, Angus uses plug condition (sooty or clean) as a way to set the mixture and I've tried to use that as one indicator

2. I don't floor it too often (my name's not Heuer )

3. All springs, needles, jets, etc. were new as part of an overhaul kit from Barratts

4. About the only thing that could be left are the floats themselves, but they seem to sit evenly in a bowl of fuel. The newer type of floats seem to be on back order at Barratts or Burlens, will try them next if I see no improvement in mpg. Even up to 21 or 22 would at least show going in the right direction.

It is a big valve head, changed as original had leak. Even if that might account for increased fuel consumption (?) why would the jets be at this depth?

It's that the jets always seem to need to be at this depth whatever method is used. Might be time to ask David Lonsdale to apply his magic touch this winter

Thanks for all the observations from everyone

Geoff

[mgcjag]

Hi Geoff...see my post above...just becaus the jets/needles were in a kit they may be incorrect....needles have letters on them...how about the jets they do come in different sizes...Steve

[Gfhug]

Hi Steve, it says UM on the needles, didn't check the jets, but as they came in a standard for the car/SUs kit they should be correct ( ). Don't really fancy taking the carbs to bits to check what it says on the jets. Will wait for a decent drive before going much further with the investigation

Thanks

Geoff

[christopher storey]

Geoff : I suspect that the big valve head may have something to do with this ( presumably from an S3 XJ6 ? ). The effect at relatively small throttle openings will be to lower the gas speed, which lowers the torque produced, which necessitates a larger throttle opening at any given constant torque requirement .This would perhaps explain why you need atypically high jet positions to produce the fairly standard CO of c.5%, because with a lower gas speed the venturi effect on the jets will be diminished . If you are really worried about this ( frankly I wouldn't be ) your next line of investigation might be to put a lambda sensor in one or both downpipes so that you can monitor mixture strength on the move as it were

[abowie]

At the end of the day if the car goes well, all is probably well. A few thoughts...

I have never measured the fuel consumption of any of my cars. If I cared about it I wouldn't be driving a 4.2l hemi headed straight six designed in the 1930s. I wouldn't focus on it as a measure of tune as how you drive and where will probably affect economy more than anything else.

The actual measured jet height you end up with is not as important as the CO which is the final product of tuning. I agree that 25 thou is less than I usually end up with but I wouldn't focus on it as a problem if your engine goes well, and all 3 jets are of an approximately consistent height. A different head may also affect tune. If you have checked where your needles are in the piston and you are satisfied they are correct then I would leave well alone.

Similarly, plug colour as mentioned above is not reliable with new non lead containing fuels. Even with 1960s fuel, a proper plug chop involves stopping the car dead after running at constant speed for a while and then pulling the plugs, which no one does anyway. Inspecting the plugs in the garage after a drive was of no benefit in 1960 and is of even less benefit now.

As far as I can remember the jest aren't marked, so removing them to check won't be fruitful. They come in 90 thou, 100 thou and 125 thou diameters. There was a 3/16" jet but as far as I know they were only used in old motorbikes. E Types use 125 thou jets and an UM needle would jam into any incorrect jet as it would be too small, so if your car runs you have the correct jets.

[Gfhug]

Christopher and Andrew, thank you, both. It's all a learning process and I'm not concerned more trying to understand what might be the reason for differences. Yes, XJ6 S3.
OK, Andrew, you're right regarding fuel consumption in an old design

I've really appreciated the comments and advice this forum produces and wouldn't ask if it wasn't for knowing the help that would be forthcoming. And it's great to have the assurances there's nothing to worry about.

[MarekH]

I have a spare wideband lambda sensor that can go in tailpipe if you want to pop round and test yourself.

kind regards
Marek

[Gfhug]

Marek,
Thank you, that'd be an interesting test. Does it need a gauge or is that part of it?

I've seen some lambda sensors that can be screwed into a hole in a tailpipe. I've toyed with having my old tailpipes adapted to take that sort of sensor as temporary fit to drive and check the AFRs.

It'll be a couple of weeks before your offer could be accepted, thanks

Geoff

[MarekH]

It's already wired up to a gauge. You just need to connect up 12v and earth. It is a spare I made up, left over from the fuel injection project.

kind regards
Marek

[Gfhug]

An update after driving the car with the above jet height/depth set up using the Gunson. The fuel consumption has improved to circa 23-24 mpg and that's motorway cruising at the national speed limit, no loss of get up and go, either. I'm pleased with that and will accept the conundrum regarding the jet height.

Thank you to everyone for your comments, advice and help, and Marek, I might still take you up on that lambda sensor at some point, thanks.

Geoff