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MG TD TF 1500 - Vapor lock symptoms solved!

SU vapor lock symptoms cured! Step # 1 set the jet height @ .0625" +- .0025" and NEVER touch the mixture adjustment agIn!
2) Find a needle that has a richer needle off idle. I have had great results with an AO needle for my MG TD with H4 SU, but it does need some adjustments. Besides no vapor lock symptoms much more power thru the entire RPM range & great driveability. Len laf48@aol.com For help finding the proper needle for your engine!
Len
Len Fanelli

I don't understand your jet height setting?.

I always believed the top of the jet was set at the same height as the bridge?

Or are you referring to the fuel height in the jet (float height)?

What A/F setting do you get at idle and at 4,000r.p.m. on the road with those needles?

Cheers

Tony
A L SLATTERY

Hi Tony The top jet bearing should be level with the top of the bridge but may not be.
The jet height recommended is about .072" then adjust as needed for best idle. However the best fuel atomization is at .0625" +- .0025"!!!!
I tried to lean it out with the mixture nut 1/2 a flat and it was a disaster! I then neded to reset the jet height back to .062500"! Fuel level in the jet > from .1875- .245. I used .220" or almost 1/2" at the forks, in my case.
I have also needed to lower the needles in .007" incerements till I got to .028" in the piston.
A/F 13 @ idle 14 at cruise........11 @ WOT.
A/F ratio is still in progress stage as I am modifieng the AO needle to get the best ratio. so far I have made stations # 7 & 8 about .002"richer and will today do about the same to station # 10 and perhaps # 11
So far making 7 & 8 richer has been a fantastic improvement, like a much bigger engine. Now to get the wide open throttle A/F from 11, down to 9.5 is my target today....
Safety Faster
Len
Len Fanelli

Len, I've been trying to follow you, but I'm befuddled in some places.
1)I understand the top jet bearing at the top of the bridge.
2) I don't understand 'the jet height is ~ .072". Where is that measured from?
3) Fuel level in the jet--- where/how measured?
4)'lower the needles' Is that dropping them down in the piston so the needle protrudes that much farther?
5) What are these stations that you are talking about?

Thanks, Bud
Bud Krueger

Hi Bud, when the recommended initial mixture setting of 12 flats down = about .072" below the bridge. To confirm this remove the piston and dashpot and measure from the bridge to ‘Fuel should be visible in the jet tube (remove the carb dashpot and piston and turn on pumps to see where it settles. We actually set the jet tube height relative to the top of the bridge and NEVER subsequently touch the jet height (i.e. don't mess with raising/lowering the jet with the screw at bottom of the carb). We've found that top of the jet should be between /.060 and .065 below the top of the jet bridge. Best atomization happens with this "jet drop." With the jet higher than that, the fuel actually "mists" too much, and with the jet lower than that, it actually "puddles" and doesn't atomize well enough. Once the jets are set relative to bridge height, the next step is to set the fuel level in float bowls. You should see the fuel about .1875" to .245". below the top of the jet tube. Any higher and it's likely spilling over when the motor fires/vibrates. Blow down in jet tube (with pumps running) to see where the fuel level settles. This does require removing the choke springs and the choke rod between the 2 carburetors. From Manley Ford and other racers.
Len
Len Fanelli

Yes Bud the overall profile is great but too rich so dropping the needle works.
The "Stations" are each of the thirteen sections where the needles taper dome th the next section going richer as the piston rises.
Len
Len Fanelli

What is typical measurement for 'dropping' the needle in such circumstances, Len?
M Hyde

Merv, I have been dropping the needle in about .007" increments. Depending on the needle profile I have stopped between ,015" and ,030"
Again IF I did not mention it when it was slightly rich or lean and I tried turning the mixture nut 1/2 of a flat, instead of droping or raising the needles.
BIG MISTAKE!! Conclusion NEVER touch the mixture nut after setting the jet @ .0625" +- .0025" below the bridge!
Len
Len Fanelli

Okay, Len, here's what I'm hearing ---
1. I should assure that the top of the top jet bearing is flush with the bridge.
2. I should tweak the mixture nut so that the top of the jet is .065" (+/- .0025") below the bridge And, LEAVE it there.
3. Lower the needles in the pistons in .007" increments to obtain the desired richness of combustion.

Is that what I need to do? Bud
Bud Krueger

Hmm.--Personally I'm not a real fan of dropping the needles but in saying that i agree that there is a magic spot for jet height
I usually set the jet height at .065" and needles level with the piston and find the mixture is usually real close if the needles and jets are in good condition- one or two flats adjustment either way sorts out any adjustment needed and always leave the needles level with the piston base-
Just out of interest I have CD Strombergs on my elan, these came std on it and being one of the last of the elans it was set up to comply with emissions rules--idle mixture being a major part of that---I've looked into what Stromberg did to meet the polution specs and one part of this was testing for the best atomization to get the best idle control needed----Stromberg, in doing this set the jet height to the best height from testing and it's actually pressed into the body and non adjustable--Idle mixture is done with a long allen key down through the centre of the damper tube via a grub screw adjusting the height of the needle-It's a super fine thread down there and only 3-4 turns of adjustment available so probably approx .060" max.adjustment overall. I've never actually measured the jet height as it's permanent but yesterday I couldn't help myself so pulled the top off one and measured it---the answer--.059" set by Stromberg as the ultimate--------But that's a Stromberg not an SU but interesting that that is the height they went for.
End of useless info
willy

Never ever had an issue with vapour lock with this car but it'll certainly ice the carbs up if you blast off too quickly before the engines thoroughly warmed up--I take that as a bit of a -told you not to do that thing and live with it.

Today's job is to put the car back together and go for a drive---just an excuse for a roadtest----lol

William Revit

Bud, the top jet bearing may or may not bee flush with the bridge due to stack up height and shim washer sizes, not an issue. Each flat = about a .0063 jet height change.Oh and the setting is .0625" +- .0025"
Not any higher or lower for best fuel atomization. When I first used this setting, with the needles flush idle was still fine but the part throttle tourque was fantastic, just touching the throttle at a cruise speed resulted in much more power/acceleration. At that time I was not using a wide band A/F ratio sensor & meter. Later when I did install an A/F ratio kit I found how much leaner it was. That was the time that I sarted to look for alternate needles. By the way it seems there are many more needles, in this range for the H4 than the H2, so IMO a few H4 needles could be tried in the H2 carburetors.Then after picking another needle like a GS instead of the GJ the GS worked well enough for most people,but I thought it could work better from 2000-3000 so I leaned out a part of this needle for better performance as well as dropping the needles .015". It worked so well that I have done the same with AO needles then now RO needles, with BO oil in the dashpots.I does takes a lot of combinations to get the A/F ratio to 9,5 at wide ope throttle and idle, pick up anc cruise in the acceptable A/F range for power/ torque. Bottom line is install a A/F ratio kit and richen up the off idle mixture by raising the original needle for a stock or almost stock engine, and using a different needle that is richer for any modified engine and lowering it as needed.
Len Fanelli

Air/fuel at 9.5? I thought ideal for power was 13?

Jim
J Barry

Okay, Len, I'll give it a try this weekend. Now, if you can give me an ideal setting for idling along in a parade for a couple of hours, I'd be delighted. Bud
Bud Krueger

Jim-i agree 9.5 is way too rich 12.5 - 12 is about it, If it's supercharged or turbo'd then you can creep into the 11's but there'd be a marked loss of power at 9 unless there's a high dose of ethanol in the fuel
Bud
For a good stable idle I'd suggest 12.5:1 ,you might sneak back to 13 maybe, but 12.5 is the most stable and reliable.
William Revit

Willy The Strombergs jets can and do drop due to excessive heat, there is a tool to press them back into the proper height.
Len
Len Fanelli

Jim, Willy & all, Yes most but all recommend an A/F ratio of about 12-13 :1 @ WOT. However for what ever reason A race or almost race T tyoe engine, especially with custom swirl polished valves,like the ones that I sell, require 9.5 :1.At Wide open throttle, above 3000 RPM!
All the racers use the RO needles for normally aspirated engines that give an A/F ratio of 9.5:1 and on a cold day go even richer at WOT.
Len
Len Fanelli

Is that on petrol Len or an alcohol blend
William Revit

E 10
Len Fanelli

This thread was discussed between 18/09/2023 and 01/10/2023

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