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Triumph TR3 - Too Rich?

My 1959 TR3A seems to be running too rich. The exhaust pipe is black and the plugs are black/sooty. I can see the black smoke when I rev up the engine and the exhaust smells rich. I have tried to lean the SU's by raising the jets but they seem to have no effect. In fact I have them as high as I can get them now. The book says raise them to the bridge "if you can" or as high as you can. I am not able to get them up and flush. My needles are SM's which are standard. I have rebuilt these carbs so the jets and gaskets and such are new. The jets are the standard .100" dia. Should I buy the "lean" SL needle or is something else more obvious wrong? Is this a common problem? The car has plenty of "get up and go" but I think I ought to address this problem. Oh, if important, I have a TR4 engine using TR3A carbs.

thanks
Howard
Howard Petri

Howard-The jets should be about .060" (2 turns) below the bridge. Referring to the Moss catalog, look at the cork seal(#97). It should be compressed to the extent that it is barely visible. If it isn't compressed enough, it will be impossible to raise the jet high enough to get the correct mixture. Try soaking the seal in oil before installation to make it more pliable. Other things to check would be the float level and verify that the jet fork is contacting the underside of the adjusting nut. The inability to get the mixture lean enough just about drove my nuts (not a very far drive) until I discovered that the cork seal was affecting the jet heighth. Good luck.
Berry Price
BTP Price

Thanks Berry; that's the most useful definitive information with numbers. The books really don't give a hint about what might be the solution to this problem although they are pretty thorough. And yes, it's driving me crazy as I know it's wrong but didn't know what to do about it. I will try this today.

thanks
Howard
Howard Petri

Well I tried this and the cork is thick alright and hard to compress. In fact I can only get the jets to sit about .060" below the bridge when I have the jets screwed up all the way ie, not 2 turns lower. But I can get them up there. My mixture still seems too rich though. Maybe I need to try the smaller SL needle. I would think one should have a range of settings that would allow a too lean to too rich range which I don't think I have. But, the car seems to run great.
Maybe I should buy one of those colortune spark plugs that allow one to see the color of the flame in the combustion chamber. That ought to tell me for sure when I have the right mixture setting.

Howard
Howard Petri

Howard - Maybe your needles should be lower by 0.060" where they are secured in the upper suction piston. If you lower them, you will have the same effect that you are looking for.

Don Elliott, 1958 TR3A
http://www.tr-register.co.uk/images/memberscars/trusty.jpg
Don Elliott

Howard-The SL needle is made for high altitudes and if you compare the profile with the std. SM needle, there is only .0005"-.001" difference in dia. and is the same for the first 2 stations. I believe your problem is caused by not being able to get the jet high enough. You can lower the needle as Don suggested, but I would prefer to keep the shank flush with underside of the piston as recommended in the manuals. Make sure you have only 1 thin copper washer (Moss #90) fitted to the upper jet bearing. If you have access to a lathe, machine about .015" from the shoulder of the upper jet bearing. It is possible that some of the aftermarket jets are slightly shorter than the originals as measured from the top of the fork to the top of the jet tube. The Colortune gizmo is useful in providing visual proof of the mixture and is more sensitive than the lift the piston routine.
Berry Price
BTP Price

AH, a lot of good advice again. I sure can lower the needle if I want to and see what happens; I hadn't thought of that. I do have only one brass washer under the upper jet bearing and I do have a lathe so I can machine the bearing down if I have to. What about eliminating the washer?
I guess I'll forget about buying the SL needle. That doesn't sound like they way to go if it was meant for high altitude and is so slightly different. I fly and all carburetors in planes have a way to lean the mixture for high altitude. I wonder if the Brits ever used carbs like these on aircraft engines? I don't think they did. I'm thinking of Gypsy Major engines on DeHaviland Tiger Moths and such. But I'm not familiar with all of their flying machines.

Thanks for the further advice.

Howard
Howard Petri

Howard-I believe the washer is supposed to act as a seal between the upper jet bearing and the carb body. SU carbs have been used in a lot of applications, but might not work in an aircraft because of the need to be upright. Maybe someone else can chime in on that issue. Good luck.
Berry
BTP Price

OK, I tried Don's suggestion of lowering the needles .060" as that was the easiest and quickest change I could make. This has finally had the desired effect. I no longer see any black smoke and the rich smell is gone; the mixture is clearly leaner. Now of course I'm wondering if I have it too lean; gotta keep worrying. I don't think so though.
I suppose now that I can adjust it through it's operating range I ought to buy one of those "color tune" devices. I think I will pull a plug in a few days and see of the carbon has burned off. The car still is running nicely.

thanks
Howard
Howard Petri

Howard:

Clean off your plugs. Start the engine and take a ride around the block. I had my car on jack stands and ran it for about fifteen minutes and noticed a difference in the spark plug color. seemed to work for me and I got real good at removing and installing spark plugs at the same time.

Harry
HDW Ward

Interesting discussion after I spent a few nights to get my mixture just right. Since the completion of my 58 tr3A last summer, I could not get the mixture right from the screw range as indicated in the manuals. After consulting a friend of mine who is a Caterpillar mechanics ( after all this is a tractor engine !!!)we realized that my problem was with the float level. I sure did the trick adjusting the float level with the 7/16" shaft, but its is not precise enough. My friend just blew in the intake tube and bent the level until he could not get air through with the shaft still in position. Simple but it solved all my problem. I was then able to use the ColorTune to get the right "blue" mixture using the screw with 2 turns as a base line. No more black smoke and black plugs !
Louis Boucher

Well after using the car for a few weeks I'm not convinced that I have made any real progress. I still have black plugs. I think I will pursue the float level approach next as this can be a real problem and while I have set the level with the 7/16 shaft, I never checked to see if the flow actually is closed off. The saga continues. Thanks for the new tip Louis.

Howard
Howard Petri

Howard-Just a few things to check comes to mind: once again make sure that the jets are fully returning after using the choke. Make sure that the needle is centered in the jet and that the piston moves freely, hitting the bridge with a satisfying "click" when dropped. Sometimes the guide rod is galled or the piston is contacting the dome. I haven't found that the float level is very critical, but make sure the needle valve isn't leaking. Check fuel pressure-it should be 2-1/2 lbs. max. Incidently,if I remember correctly, my jets were set about .054" below the bridge to provide a satisfactory mixture. If all else fails, call Joe Curto, the SU guru.
Berry Price
BTP Price

This thread was discussed between 10/05/2005 and 19/06/2005

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