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General Tech Tuning the SU carbs

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KVH

KVH

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I’ve only been tuning these things for about 52 years, so maybe I’ll finally get it in a few more. Today I tried resetting the mixture because I was told my SUs were running with a fuel mixture too rich. I used the Shop Manual techniques listening for all the tell tale signs when you lift the pistons 1/8 inch and all that. No way anything seemed to follow the book. When I finally thought I had it right my car starting chugging and just shut off. ‘66 TR4A. HS6 carbs. I pulled the plugs, and they were all black and sooty.

I assume I was way too rich. My SUs are really new, like 14 months. I opened the adjusting screw about 19 flats. Obviously, too much. But now the car seems to run a lot better (just in the garage; not on road yet) and my plugs look normal. My mixture setting is 14 flats open. So, here’s my question: Is there some tight range whereby you’re fine with one adjustment, but stalling and chugging if you just change by as little as 5 flats? Is it like an inverted V curve where you hit it correctly in a narrow range and fail miserably going either leaner or richer?

Thanks all.
 
I have a tr3 that has SU carbs, but they are not the SH6 version and I have never worked on the SH6. Maybe SH6 is basically the same SU I just have no experience.

My tr3 can idling at 6k and runs on its own for maybe a minute or two at that idle speed, with an electric Sun tach hooked up to double check tech calibration. I am not sure how tight the window of operation would be in terms of 3 or 4 flats effecting that idle, but I think it will change the engine performance at idle by moving the nut 4 turns either way. So yes kind of a small window

Driving fast or almost open throttle, I would say the needle has pulled away from the jet so much that counting the amount of flats is not as critical as at idle. Sure the overall tapper of the needle is important and MD suggests finding a needle that suits the overall spectrum. I am sure he is correct and I need to that Joe guy in NY.

Anyways the low idle is tricky, but the old saying what is wrong with your SU carbs is your Lucas ignition is real. The distributor needs to return the spark back to the idle speed or naturally the engine will idle higher. Engine temp is important for a low idle also. The hotter the engine gets past, say 170 the engine idle goes up a little. Vacuum leaks are another engine idle riser.

I would say set your SUs to have an idle speed at 6 or 7K and the richness setting will be there also.

steve
 
I assumed 600 was intended. Even then I’ve never achieved less than 900 and run smooth.
 
I use the HS6 carbs on all 3 of my TRs. That is a TR3 and 2 TR4s. As to adjusting them they are the same as the H6. The big difference is that they have a lot fewer internal parts to mess things up. I use the SM needle in all 3.
That is the recommend needle for the TR3. The TR4A needle is a leaner needle. I suspect due to smog concerns.
I am not a good person to recommend fine tuning. As I do not obsess over fine tuning. Once I have them set , I do not mess with them unless there is a problem. And I rarely have a carb mixture problem.
Keep in mind that idle speed is largely dependent upon a balanced engine, timing set properly and the carb setting. Along with a whole bunch of other things like worn carb or engine parts. One of my cars can idle at 600 while another's are more like 900 too 1100.
Charley
 
14 flats is a WHOLE lot!! I usually start the tuning with a fresh setup at 5 flats and adjust from there. It has never taken more than 2-3 flats to get the mixture spot on.

Remember that the first step is always to synch the carbs for equal pull, then set the idle speed while ensuring the equal pull remains...and finally adjust the flats to best idle plus one flat rich. Repeat as necessary until you get it spot on. Always adjust the carbs to equal flats to prevent a split.

In general...a rich setting will slowly reduce idle speed until it stalls. A lean setting will cause the idle to roll back and forth a couple hundred rpm.
 
Oh gosh. Sorry all, I goofed. My ignition wire was defective, holding by two stands. That was the cause of stalling twice and cutting out repeatedly, making me think I needed a new fuel pump. It manifested with other problems as well. It’s all running great right now, but the fuel mixture is still in need of refinement. FYI, the adjusting nuts and size/threading changed with the new SUs I bought 2 years ago. I spoke to technical support about it. Even requires a different SU wrench because the nuts/screws are larger. So, the standard 6 flats, etc. has to be adjusted in the field, so to speak. But the jets definitely aren’t dropping down far enough with just 6 flats. More later . . . .The never-ending project proves itself again.
 
The problem with SU's is they are so simple people can't wrap their heads around them.
IF you have the float level set correctly (with the piston out you see fuel in the jet and the
jet is just lower than the bridge). Half the battle is won. Install the correct needle and adjust
per the manual. DO NOT overthink this or believe in magical ,mystical stuff. Start out with properly set
floats.If you don't see fuel, or it runs over the bridge, fix it.
Mad dog
 
I hear you Charley my other tr3 idles at 9 hundred to 1K (thanks KVW). My thoughts are the throttle shaft leaks too much air or the distributor is not falling back completely or maybe both contribute a little adding up to those 3 hundred RPMs

steve
 
sp53 said:
I hear you Charley my other tr3 idles at 9 hundred to 1K (thanks KVW). My thoughts are the throttle shaft leaks too much air or the distributor is not falling back completely or maybe both contribute a little adding up to those 3 hundred RPMs

steve
Click to expand...
One other problem I have is the throttle linkage not doing a full return when the throttle is eased off. It will do a full return if I let the pedal snap back up.
Charley
 
There is an easy fix for worn throttle shafts that reuses all the original parts.
Metal bonding epoxy will repair the worn spots in the shaft.It is easily worked down
to the fit required to stop leaking air. When it fits to spec, lube with synthetic grease.
I have done this repair for myself and customers for years.It lasts and is DIY friendly.
Replacing the shafts and re-bushing is the sledgehammer approach (IMHO ) to a simple
problem.
Mad dog
 
You repair the shaft. Do NOT try to use anything but metal bonding epoxy. It will cling to
any sanded metal forever.You only need to fill in the wear spot mostly on the sprung end of
the shaft. Once you have dressed and sanded it smooth to a perfect fit, re-install the shaft making
sure that it works as new..After it is lubed with synthetic grease(stays put). install the butterfly
and double check the action again . Done and done.
MD
 
Did the SU original have bushing in the body? I could go did around and find out. No, probably no bushing

Guess I am asking, do the oversized shafts need much fitting to go through the body. I putt some shafts in years ago and I need small refresher. I think the kit had bushings to install into the body.

steve
 
mctriumph said:
You repair the shaft. Do NOT try to use anything but metal bonding epoxy. It will cling to
any sanded metal forever.You only need to fill in the wear spot mostly on the sprung end of
the shaft. Once you have dressed and sanded it smooth to a perfect fit, re-install the shaft making
sure that it works as new..After it is lubed with synthetic grease(stays put). install the butterfly
and double check the action again . Done and done.
MD
Click to expand...
I’m looking at this old post. Really? Forget buying new shafts? Epoxy? Sounds fun. What about the bushings?
 
From my experience there is very little wear on the bushings. and it is corrected
when the shaft is worked down. You sand and fit ,till it is just right. Doing
a set now, may post a picture, not sure if it will be very photogenic.....
Mad dog
 
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