lean or bust

I've been puzzled for months about why I couldn't get the mixture lean enough. SU H6, standard jets and needles. Regardless of how high up I screw the adjusting nuts, the engine ran rich. Warm idle always lumpy, plugs always sooty; slight dark smoke on acceleration.

So this afternoon I cleaned the plugs, checked the gap, and lowered the needles so the shoulder is about 1/16" below the bottom of the piston.

Started up, engine is idling faster and smoother.

Went for a 50 mile drive; no loss of power, good smooth acceleration as well as idle. Came home and checked the plugs. Slight grey coating on the tips - absolutely no soot.

If at first you don't succeed ...

Tom

There were dozens of needle profiles available, and perhaps you have non-stock ones. In the 60s/70s, I had a little booklet showing all the available sizes. The measurements were taken every 1/8" or so along the needle in order to get the right mixture just where it was wanted. There was a lot of swapping going around then, mostly by those of us who had no idea what we were doing. But now it seems you found the magic spot for the needles you have.

NutmegCT said:

...and lowered the needles so the shoulder is about 1/16" below the bottom of the piston.

Started up, engine is idling faster and smoother.

Went for a 50 mile drive; no loss of power, good smooth acceleration as well as idle. Came home and checked the plugs. Slight grey coating on the tips - absolutely no soot....

Click to expand...

You might well be just fine now, but I'd continue to monitor the situation. Even with today's gas forumlations, I expect to see some more reddish or brownish color on the plugs. Greyer or whiter, and I begin to worry about the carbs being TOO lean. Over time, this isn't good, as it can lead to burnt pistons and such.

Paul - I've got new standard mixture SM needles and .100 jets. Also new jet seals, etc. inside.

Andy - thanks for the suggestion. Believe me, I'm keeping an eye on the plugs now.

Is there a good color picture of what the plugs should look like with correct mixture? I've heard brown, tan, brownish red, even "yellow ochre" and "burnt sienna"(!) etc. I've found dozens of b/w pictures, but so far nothing with good color. I've also heard some guys call grey exhaust "black smoke", so being typically male color blind I need all the help I can get.

Tom

Here's one such guide from Champion . Interestingly, it notes A grayish tan to while [sic; I assume they meant white] color indicates the correct heat range spark plug is in use, the fuel and ignition systems are in good shape and overall engine mechanical condition is good. So maybe a whitish plug isn't so bad nowadays?

Wow - that IS interesting. "Grayish tan to white ..." can indicate correct (plugs, mixture, ignition, etc.).

This discussion could get *very* interesting.

Thanks Andy.

Tom

There have been several software programs written with databases for all (or most anyway) of the SU needles available. 'WinSU' used to be free, but apparently costs 10 GBP now and I lost my old freeware copy in a hard drive crash. But the old DOS Haystack program is still free, and can be found for download with Google. Probably the freeware version of WinSU is still out there too, somewhere.

Think I mentioned before, the additives in the fuel definitely affect plug color. MMT for example leaves a pinkish deposit, while Amoco Premium unleaded (I think it was) used to leave a purplish color. Most of the old plug color guides assume the fuel is leaded, and without any of the more exotic additives, so may not be entirely accurate today.

Good article about needle selection at
https://www.team.net/www/morgan/tech/tuning.html
(although not really from the TR point of view, the information is still applicable).

Much better to err on the rich side. Might be a slight loss of power & fuel mileage. Too lean & it is all done.
D

Andrew Mace said:

Here's one such guide from Champion .

Click to expand...

Here's another, originally from NGK .

TR3driver said:

There have been several software programs written with databases for all (or most anyway) of the SU needles available. 'WinSU' used to be free, but apparently costs 10 GBP now and I lost my old freeware copy in a hard drive crash. But the old DOS Haystack program is still free, and can be found for download with Google. Probably the freeware version of WinSU is still out there too, somewhere....

Click to expand...

Randall and others, this might be the one you're referring to. Go to the Team.Net FTP site and look for suneedle.exe

Andrew Mace said:

Go to the Team.Net FTP site and look for suneedle.exe

Click to expand...

Yup, that's the 'Haystack' DOS program I was talking about. "suneedle.exe" is a self-extracting archive of it.

I also just found the ZIP file for the freeware version of WinSU. Since Basil doesn't seem to want to let me upload big files to his server, PM me your email address if you would like a copy. It's about 1.25 Mb.

Hello Tom,

I believe the Winsu programme only deals with single carburettor set up.

You have by-passed, not corrected the problem, there must be something else wrong if you have had to move the needle in the piston. Also, as I have said many times, at idle all needles (of the same group) are the same so if the idle is wrong it is not due to the needle.

If the Champion advice regarding colour is current advice, bear in mind that modern cars with fuel and ignition management can run much leaner mixtures than the older cars so I would be very wary of going to that advice with a 60's car.

Alec

Fuels are being changed every day now. Summer blend, winter blend, 10% ethenol, etc. I try to use the same station all the time to reduce the variables.I also tune to the rich side on a color-tune just for insurance.
When I was young I worked in a cycle shop and saw a lot of burnt pistons where guys would lean out their carbs for that extra little fuel savings.
Trust me on this, gas is cheap compared to pistons and that extra little bit of power you get when it's perfect isn't worth the headaches.

Just to clarify ... I'd been following the "lift the piston 1/16 inch" technique from the factory manual, trying to get the mixture right on my H6's. Regardless of how high up I screw the jets, lifting the piston 1/16" would raise rpm, indicating rich; letting the piston drop to the bridge would return rpm to original speed. Even with the adjusting nut all the way up, "lift the piston" indicates rich. Plugs remained sooty black.

With the needles lowered 1/16th inch, the plugs show light grey ashy covering, and piston test shows correct mixture (raised piston - brief rise in rpm followed by return to original rpm).

Has anyone compared the "raise the piston" test in the factory manual to setting mixture with a Colortune? If mixture is set using Colortune, does the "raise the piston" test also indicate mixture is correctly set? And vice versa?

Thanks.
Tom