TR2/3/3A TR3 Front Carb Running Rich

[RedTR3]

My story is I have owned my '58 TR3 for about 8 months now, I have been going through it mechanically and it is now a really reliable driver. It has the stock H6 SU Carbs. I have got the timing set right, new plugs, wires, coil, points.. etc etc. I recently replaced the carb float bowl valves. Actually the car starts and runs really well. The only thing that bothers me now is that I am pretty sure that the front carb is running rich. The front plug is sooty and I put a colortune on it, and according to the colortune it is rich. I cannot get it to run any leaner - Even if I adjust the jet adjusting nut all the way up, the colortune indicates that it is rich. The rear (#4) plug looks perfect.

I do not have much experience with SU carbs so I am not sure how to correct this or if it is within my ability. Even though the car seems to run well I would like to fix this. Any advice would be welcome!

 

[AweMan]

RedTR3:
It could be that your front carb needs a "Jet Berring kit"
This kit contains all of the seals and copper washers ETC. to make the needle seat seal properly. {It could be leaking past the needle} The needle can also be adjusted up or down, whichever needed. There are people on this forum that are miestros when it comes to these su carbs, I`m sure some of them will be chiming in.

 

[Deleted member 451]

My first thought is that your needle is positioned to high in the piston. Try lowering it about 1/16 of an inch at a time until you can lean the carb.
This could also be an indicator of a worn jet from the needle not being centered properly. If so you need to replace both.
Remember when using a ColorTune that you want it to be blue but closer to orange side instead of the yellow. Gas is cheaper than pistons.

 

[Harry_Ward]

Even if I adjust the jet adjusting nut all the way up, the colortune indicates that it is rich. The rear (#4) plug looks perfect.

I do not have much experience with SU carbs so I am not sure how to correct this or if it is within my ability. Even though the car seems to run well I would like to fix this. Any advice would be welcome!

Did you adjust the connecting linkage between the front and rear carb as well to allow the front carb jet to seat fully up against the adjusting nut? If you did then the needle itself may need to be repositioned as mentioned earlier.

 

[DrEntropy]

Yup... Sounds from here like the jet/needle need attention. Centering the jet, checking that the needle is not rubbing it. If there's wear evidence (ovality in the jet orifice) it will need replacing along with the needle. With the damper out, raise the piston full height (from the inlet, with yer finger) and release it. It should strike bottom with a noticeable "tunk!" sound. Compare the two, front to back. If it doesn't "click!" the way the rear one does, your needle is likely rubbing the jet.

The linkage between the carbs will not noticeably affect mixture, just balance/idle speed.

 

[Harry_Ward]

Not the throttle linkage between the carbs. I meant the lower linkage, ie Jet lever connecting rod for the choke engagement. If the threaded rod forks are not adjusted correctly to the jet levers the front carb jet lever may not seat the jet up tight against the adjusting nut even though the nut is up tight against the carb body. In other words the rear jet may seat first before the front jet even gets there if the rod is not adjusted correctly.

 

[TR4]

Have you balanced the air flow so each carb is drawing the same? Maybe the front carb simply is not getting as much air as the rear carb. That could happen quite easily if the throttle linkage between them has slipped.

 

[RedTR3]

Yes, I balanced the carbs using a a Uni-Syn.

There was a comment above that I did not understand:

"...gas is cheaper than pistons..." Does that mean that a leaner mixture can damage the engine pistons or does it refer to somehting in the carb pistons?

 

[bobh]

Red,

Take a look at this;
https://www.jcna.com/library/tech/tech0006.html

There is some good information about SU carbs including some things that can cause the carb to run rich.
I've read most everything I can find on SU's. This article includes some information I have not seen anywhere else.

BOBH

 

[AltaKnight]

Yes, I balanced the carbs using a a Uni-Syn.

There was a comment above that I did not understand:

"...gas is cheaper than pistons..." Does that mean that a leaner mixture can damage the engine pistons or does it refer to somehting in the carb pistons?

Running an engine in a lean condition for extended periods will surely ruin the engine. Lean conditions cause the engine to run VERY hot and can cause pre-ignition destroying the pistons (and other bits also)

 

[Simon TR4a]

Bill's guess seems like a good point to check.
Just to check more simple stuff, do you have the same needle in each carb?
If you are not the original owner of the car you can't take anything for granted...someone may have "profiled" the needles or one of them, or the needles or even carbs may have been changed at some point in time.

Simon.

 

[RedTR3]

Thanks for all your suggestions...I plan to work on it this weekend.

I agree that the first thing to check is the needle type and height setting. I will do that and report back here.

Thanks again, Tim

 

[Roger]

I can't remember if your carbs have the little push-up button under the dashpot or not.
In any case, using either that or a thin-bladed screwdriver, lift the front piston a fraction while the engine is idling. If it speeds up, and/or your colourtune shows the right mixture strength, you are indeed idling rich.
That doesn't necessarily mean you're rich at higher revs or under greater load (depression) but it's a start.
Check all the things above, including that the needle height (the protruding length) of the needle from the piston) is the same on both carbs.

 

[DrEntropy]

Not the throttle linkage between the carbs. I meant the lower linkage, ie Jet lever connecting rod for the choke engagement. If the threaded rod forks are not adjusted correctly to the jet levers the front carb jet lever may not seat the jet up tight against the adjusting nut even though the nut is up tight against the carb body. In other words the rear jet may seat first before the front jet even gets there if the rod is not adjusted correctly.

Ah! Now it makes sense, Harry. Somehow I was thinkin' balance linkage. DOH! /bcforum/images/%%GRAEMLIN_URL%%/wink.gif