TR2/3/3A TR3: carb adjust mixture ie. how many flats...

[karls59tr]

My plugs are badly carbon fouled. I'm going to start from scratch with the mix adjust. Is it turn up the adjustment nut till resitance is felt(do I bottom the nut out? dont want to strip it) then back down 10 to 12 flats? This is supposedly the baseline then road test and check the plugs right?

 

[aeronca65t]

Yes, just as you say.

Turn the nut "up" until it meets slight resistance and then turn down 12 flats (2 full turns). This is the usual baseline on most SU carbs.

Be sure you have a decent ignition system (wires, points, cap, rotor, etc), since this can mimic a rich mixture.

If mixture is right with car warmed up and idling at normal speed, try the following:

With air filters off, lift each damper piston a small amount (about 3/16") with a thin screwdriver. The engine *should* speed up for a moment and then return to normal idle RPM.
Some SU carbs have a small "lifting button" for this test. I don't recall if they're on a TR3.

 

[Number_6]

Here's a pic of where the lifting pin that aeronca65t mentions in his post. I found it when I recently rebuilt the carbs on my TR3. It seems to have been eliminated from later SU's.

 

[TR3driver]

Just to clarify; do the "lift the piston" test first and get it right, before commencing with road tests. You should get a clear indication when it's right, IOW moving one flat either way will cause the rpm to either stay up (indicating too rich) or fall below normal (indicating too lean). If not, there is likely some other problem like valve lash incorrect.

You should also check that the piston moves absolutely freely except for the action of the damper. Remove the damper and check for absolutely no binding or rubbing at any point in the piston range, then install the damper, lift the piston to the top (against the damper resistance) and let it drop. It should land with a soft but distinct click. If it doesn't click then the piston or needle are binding.

AFAIK, all TR2-3B had the lifting pin.

 

[billspit]

One other hint, when screwing the adjustments home, look in to see if the jets are both flush. Sometimes one will screw home slighly more or slightly less than the other.

 

[karls59tr]

Yes the SU's on my 3 do have the lifting button. It's odd but when I've done this test in the past I've never had any change in rpm by raising the pistons up a small amount as you say I/8" or 3/16". I always have to raise them up much higher to get a reaction. Wonder what that would indicate? One piston falls with a distint click the other drops a little slower with a small thud. I've had the pistons out years ago and polished them with a non abrasive jewelers cloth and I dont think there is any significant binding. In regard to valve lash I dont remember if I posted here before to inquire about what the valve gap should be with a mild street cam and headeras opposed to sstock setting. Anyone know? I should get one of the older valve adjuster tools with the wheel knob. Would valve adjustment being out cause plug carbon fouling???

 

[TR3driver]

One piston falls with a distint click the other drops a little slower with a small thud.

IMO the thud indicates a problem; possibly the damper is bent or the jet is not centered quite properly. Having jets off-center (so the needle rubs the jet at idle) can lead to serious problems with mixture. Do the springs appear to be identical?<div class="ubbcode-block"><div class="ubbcode-header">Quote:] I've had the pistons out years ago and polished them with a non abrasive jewelers cloth and I dont think there is any significant binding.[/QUOTE]Doesn't have to be the piston at fault. The domes are very soft and thin, it doesn't take much of a blow with (for example) a slipped wrench to dent them enough to cause binding. It also doesn't take much binding at all to mess up the mixture. The forces on the piston are fairly small and even tiny changes in venturi size (and hence air velocity through the venturi) have fairly large effects on mixture.<div class="ubbcode-block"><div class="ubbcode-header">Quote:]In regard to valve lash I dont remember if I posted here before to inquire about what the valve gap should be with a mild street cam and headeras opposed to sstock setting. Anyone know?[/QUOTE]As I recall, my "3/4 race" cam called for something like .015" intake and .018" exhaust. It really depends on the cam grind, so it's hard to generalize. There's an interesting table posted at https://www.tildentechnologies.com/Technical/TriumphCams.html which shows figures all the way from .011" to .026". If you poke around on that site, he also has instructions for how to 'degree' your cam, which should allow you to identify it from that list.<div class="ubbcode-block"><div class="ubbcode-header">Quote:] I should get one of the older valve adjuster tools with the wheel knob.[/QUOTE]IMO, if your rockers are worn enough that you cannot get an accurate reading with feeler gauges, it would be better to have your rocker shaft rebuilt. Besides, just .001" or even .002" shouldn't make all that much difference.<div class="ubbcode-block"><div class="ubbcode-header">Quote:] Would valve adjustment being out cause plug carbon fouling??? [/QUOTE]It could, especially after adjusting the mixture at idle (as per the book). I'm actually kind of in the middle of this with my 57 TR3; many of the valves were set way too tight when I tried to adjust the carbs, and the resulting setting was way rich. Now that I've run the valves, I've got to go back and redo the carbs.

 

[Geo Hahn]

...lift each damper piston a small amount (about 3/16") with a thin screwdriver. The engine *should* speed up for a moment and then return to normal idle RPM...

FWIW -- I find that by using a decent dwell-tach to look at the RPMs I can more easily see the rise & fall (or maybe I'm just to deaf to do this by ear).

 

[karls59tr]

Ai-yi-yi I was hoping to just get out and drive the beast as the Summer's are so short here. It's interesting that you mentioned the carb springs as I have a new set and havent installed them yet. I'll swap those out first and then try the adjustment. I have two new needles as well that I could install.I ordered them quite awhile ago because I read somewhere that they were more suitable for my engine set up than the standard ones but I'd have to check the package to see which ones they are. Hope their not the "rich" ones ....that sounds counter productive at this point! Would I have to re center the jets if I installed them. That wiuld mean the carbs would have to come off.

 

[TR3driver]

Would I have to re center the jets if I installed them. That wiuld mean the carbs would have to come off.

Nope, shouldn't be a problem changing needles without recentering the jets; unless maybe the old ones were bent or made wrong. Normally they are all quite concentric, so the jet is centered on the bore in the piston.

I find it helps to also twist the dome back and forth a bit before putting the screws in, to check if there is any grit or FOD between the dome & body. That will mess up the centering.

 

[karls59tr]

Hi Randall I checked out the TR Cam site you mentioned but found it a little confusing. Here is my cam spec. Would you be able to hazard a guess as to what I should set my valve lash at? M518 duration: 227 degrees @.050"....280" camlift........ .420" valve lift on 109 degree lobe c/L Is my cam similar to the Kastner D cam on the list? Thanks for all help. Karl

 

[TR3driver]

Beats me, sorry Karl. Looks like your cam is not on Larry's list.

Also, a modified camshaft (and any other mods you have made) may well alter the needle profile & spring that your engine needs. For example, more overlap may result in reversion (reverse air flow) through the carbs at low rpm, causing part of the air to pick up a triple load of fuel.

Without information from your cam designer; all I know to suggest is try opening it up a bit and see what happens. Same goes for mixture, if you think it's too rich then lean it out a bit and see what happens. That's one of the problems with modifications, they upset the original calibration points.

But let me repeat, I'm no expert in this area nor do I play one on television. When I need cam advice, I go to people like Larry.

 

[MadMarx]

When I adjust the SU carbs I remove the domes and measure the depth of the jets.
A starting point is something between 1.3 - 1.5 mm. SM needle.

To find the right mixture I do it this way:

I hold the engine speed at 2500 rpm and then go off the throttle.

If the mixture is right, then the engine find its way to idle speed.
If it is too rich, the engine remains a second at speeds above 200 rpm above idle and then is coming down to idle speed.
If it is too lean then it is going below the idle speed and after it rises to idle.

Simple, isn't it?

To react to the testing I remove the domes each time and setting the jet depth to a new value. Leaner, 0.1 mm up, richer, 0.1 mm down.



Cheers
Chris