View Full Version : carbs too rich

04-20-2009, 08:19 PM
Like to get some info on carb adjustments. On my 75 TR6, with ZS carbs, seems to be way too rich from looking at the plugs and that smell. I have them adjusted as lean as i can get them by means of the mixture screw. I have come across some forum post that if the float is adjusted to high this could give you rich setting. So going from the 17mm to 15 or so could fix this problem. Thoughts? Any are appreciated. Trying to gather as much info for that next sunny day to get out there and tweek. Thanks.......

04-20-2009, 08:39 PM

I would bottom them out (with the proper tool, of course) then come back two and one half turns and drive it. After putting a few miles on, as in 20 to 30, then check the plugs and start tweaking from there.

I am of course assuming that you've checked to see that the choke is opening properly and all of that good stuff?

04-20-2009, 08:40 PM
Are you sure you're turning the mixture screw the right way? It never stops going towards the lean side, just unscrews and keeps turning.

Checked for splits or holes (even tiny ones) in the diaphragm?

Having the float level too high can be a problem, as it will let raw fuel dribble out of the jets even with the engine not running. But it should have relatively little effect on mixture with it running.

You might also check that the cold start valve is returning all the way. I think by 75 they had deleted the spring that pulls it closed, and relied on the choke cable to push it.

Check out the articles at the Buckeye Triumph web site, there's lots of info there about these and other topics (including how to modify the cold start valve with a spring).

04-20-2009, 08:50 PM
Make sure your ignition system is in good shape.
A weak spark can give you a false rich condition.
A cracked cap, bad plug wire.....
I have made this mistake before!
The carbs were fine, just a small crack in the cap when it got hot.

04-20-2009, 08:55 PM
Yes sir on the choke settings and on the correct way of turning the screw. As far as i can tell the diaphragms are in good shape too. Alittle confusing at first but got it right, I have not done any adjusting since last summer so I would have to refresh myself on that before i start again. I been have using the article from the Buckeye site to guide myself, very useful. I know there are prbably a million things that could cause my problem, just trying to rule out the simple ones first. thanks again.
Yes sir on the wires,cap,coil, etc. All new and hopefully still performing

04-20-2009, 09:13 PM
Possibly the needle valves in the float chamber are stuck and not closing, allowing fuel to flood the carbs.
When you set or check the level of the floats, remember that the carbs are upside down when you measure them. Therefore, increasing the height lowers the fuel level

04-21-2009, 03:30 PM
Therefore, increasing the height lowers the fuel level

Boy am I confused.......Is that increasing the height measurement, or height of the float as it sits upright in the chamber? or it that the same? So is going from say 17mm to 15mm the right direction?

Just looking at the tech. article from buckeye and their cross section of the float chamber show the fuel line being almost clear at the top. What the deal here?

04-21-2009, 04:37 PM
floyd, when you measure the float height, the carbs are upside down, with the tab on the float resting on the needle valve.
Here's where you make the measurement. As you see increasing the measurement makes the floats sit lower. Remember, the carbs are sitting upside down when you do this.
I think it converts the same, but I recommend .627 inch.
Note: if I had taken the picture on the other side of the carb, you'd see the tab resting on the needle valve. Also the caliper should just touch the bottom of the UPSIDE DOWN float; but I think you'll get the idea from this picture.

04-21-2009, 04:41 PM
When you set the "float height", you turn the carb upside down (so the floats fall to the closed position) and measure from the bottom of the float to the gasket surface. Thus the larger this measurement, the lower the fuel level will be when you put the carb back together.

The floats don't float very well, in fact they are mostly submerged by the time they push on the valve hard enough to hold it closed. So the actual fuel level is considerably higher than the measurement you make when setting the float height; and does vary somewhat depending on things like the density (and hence temperature) of the fuel; as well as how much pressure your pump produces.

But usually it all works out because moderate changes in fuel height have a relatively small effect on mixture.

04-21-2009, 04:42 PM
So is going from say 17mm to 15mm the right direction?In a word, NO.

04-21-2009, 05:24 PM
oooook, I'm getting picture now. I don't mean to sound dumb, but this upside down stuff had me all baffled. Now......i hate to ask this question again, but, if my needle valves and all are working correctly, and my heights are correct (16-17mm), would the float setting have anything to do with it running rich? sorry.......

04-21-2009, 05:48 PM
I'd put it this way, floyd. If you can't lean out your mixture by lowering the metering needle (with counterclockwise turns of the adjusting screw) then the only other thing to try is to lower the fuel level in the float chamber.
There are some versions of ZS carbs that have neither an adjustable needle or jet and the mixture is regulated by the float height.
SU carbs also respond to changes in float levels even though they have adjustable jets.
So, it is in the realm of possibilities that you may have to take that approach as well.
I have mine set at .627.

04-21-2009, 07:36 PM
You answered my question and I thank all of you for your help. Tweaking my floats will be next.......

04-21-2009, 08:23 PM
You know, really, there are other possibilities...the wrong or worn needles. B1AF is the needle #. A needle worn to the point that it can't lean out (IMO )would be rare in a ZS carb, but not impossible

04-21-2009, 08:37 PM
here is something I ran into

expired fuel pump had never been changed 40 plus years old
and found after running rich and then installing my heat shield that my # 3
was by gravity leaching fuel

check your carb nearest the firewall look for fuel setting in the throat of carb .

if so replace ASAP fire hazard

TRF provides the best available

good luck

04-21-2009, 09:08 PM
You know, really, there are other possibilities...the wrong or worn needles. B1AF is the needle #. A needle worn to the point that it can't lean out (IMO )would be rare in a ZS carb, but not impossible

Yup, Ken. I've seen needles (mis-aligned) wear into the jets to a point where they oblong the hole and skein the needles. Result is: rich run no matter what.

If the jet orifice looks at ALL oblong, it's fubar. Likely the needles as well.

As for the float height, the larger the setting from 17mm, the less fuel to the jets. It cuts off the supply sooner if set lower when right-side-up...

go look in th' crapper. Set th' float lower and ya can't flush proper! :devilgrin:

04-22-2009, 07:09 AM
I notice Moss is now selling replacement jets. (Has anyone out there tried installing these?) Part of the wear problem I think would come from the way the needles are spring loaded off center (?)

04-22-2009, 11:54 AM
One other remote possibility; the jet is a light press fit into the carb body. If it has somehow gotten moved too low, it may be impossible to lean the mixture enough by lowering the needle. I can't imagine how it would happen, but I've heard simiar reports before.

Also on the subject of jet/needle wear; we are literally talking just a few thousanths of an inch, which is difficult to detect with the naked eye. If all else fails, it might be worth just replacing them.

Many years ago, I struggled with a TR3A that always ran hot at freeway speeds. Took a long time (and a blown engine), but I finally figured out that the problem was worn jets in the carb. We had replaced the needles on general principles, but not the jets. On a TR3A there is a lot more range of mixture adjustment; and the jet wear affected mostly idle mixture. By setting the idle mixture "right" with the worn jets, the cruise mixture was way too lean.

Probably should have realized the problem when I discovered the exhaust manifold glowing red after a freeway run; but I was young and foolish then and didn't realize the significance.

My buddy's MGA was even worse, it ran rich at idle even with the jets all the way up. But you could do a "plug cut" and see that it was lean at speed (which is part of what led me to the problem with Dad's TR3A).

The spring-loaded needles are actually supposed to solve the wear problem rather than aggravate it. Earlier carbs (like the SU H6 on the TR3) used hard-mounted needles and centering the jet to the needle was an important step in adjusting the carbs. Not getting it right would result in fairly rapid wear because of the large amount of force applied when the piston fell and wedged the needle against the side of the jet opening. The spring-loaded needles aren't supposed to rub hard enough to wear.

But that doesn't mean it can't happen, after enough decades of operation.

04-22-2009, 05:04 PM
It is not the same car or carb but I had a similar "no way to lean the mixture enough" problem with my TR4A. After much trouble shooting and problem solving that lead nowhere fast I discovered the thermostat was not closing and as such I had a cooler engine then I should have and in order for the thing to run right it had to be too rich. A new (and properly installed - an incorrect installation was the original problem) thermostat and the carbs adjusted just fine. Very remote possibility but thought I would at least let you know about it.