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Wedge I can't get my SU carburetors running right [TR7]

Hegg

Jedi Hopeful
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Yay! I finally got my TR7 back together and got it started! Now I gotta get the gremlins out so I can drive it around reliably and fun-ly.

It starts and I have driven it about 10 miles so far, but I can't get the carbs running right. Seems like I have a few oddities that hopefully someone can put all together and tell me what I'm doing wrong.

First off, lifting the pistons and letting them drop does make a "thunk" sound as it hits the bottom. If I tighten the mixture screws all the way UP, then the car will start (no choke) and BARELY idle, missing probably half the time. Opening the throttle doesn't help at all, it just lowers the already low RPMs to where it nearly dies.

If I lower the mixture adjustment down several flats while it's running, the engine starts to perk up just a little bit. However, if I put my hand near the intake now, the engine runs very smooth and jets up to probably 6,000 RPMs and won't come back down. So then I quickly run to the key and shut it off because I certainly don't want my fresh engine running that fast (yet).

Playing with varying adjustments of the mixture nut, I also experience the symptom that the car does not idle back down once the accellerator is pressed. Other times, I have to keep opening the choke a half inch or so in order to get the engine to run, but then it idles at 2,500 or so. Closing the choke makes the car nearly die and pressing the throttle kills it more. All of this is done when the engine is warm, not cold.

My distributor is turned counter-clockwise as far as it can go, and a timing light indicates around 18-20 degrees BTDC (again, idling at 2,500). When the car is off, pushing the piston up in the carb will sometimes cause fuel to leak from the carb intake.

Oh, and by the way, my throttle stop screws are unscrewed all the way so they aren't even touching the throttle stops. My throttle plates have that spring valve thingy on them.


This is my first experience with SU carbs, so I'm fairly unfamiliar with them. I've tried going through my shop manual, but it doesn't seem to make any difference what I do.

I've read somewhere that the spring valve on the throttle plate can be worn and basically stay open. My rebuild kit included two throttle plates without a spring valve on them, but I opted to keep the existing ones.

Does anyone have any suggestions? I appreciate everyone's willingness to read this and share their expertise.
 
Re: I can't get my SU carburetors running right [T

I like it. To the point.

Thanks!
 
Re: I can't get my SU carburetors running right [T

Let me preface that my SU experience is on the TR3A so some of this may not apply or differ on the TR7.

I would also reset everything to spec that has been tweaked as you tried to sort this out.

You say the distributor is turned as far as it can go... not sure what that means but it should be able to move in either direction for final tuning. If the book gives a spec for static timing then that would be your starting point (along with assuring the weights if any move freely and the vacuum line if any is attached).

Have the throttle stops screwed to where they again make contact and maybe a turn or so in from there.

If you have lost track of where we are with the mixture nut run it in all the way then back out 2 full turns (12 flats) for a starting point. On early SUs you can get the piston and needle out of the way and actually see & measure how far down the jet is but don't know if later ones allow this.

Take comfort that once you get these sorted out they should give years of trouble-free service.
 
Re: I can't get my SU carburetors running right [T

Geo Hahn, what does the term 12 flats mean. I'm taking that it has to do with the true # of turns.

Thanks, Achtung
 
Re: I can't get my SU carburetors running right [T

It's like your drum brake adjusters...kinda. It's a screw, but it has 12 flat sides on it, and it clicks into one of those flats, rather than resting on its edge and being infinitely variable.
 
Re: I can't get my SU carburetors running right [T

SUs mixture (mine anyway) are adjusted by turning a nut at the base of the jet. It's a hex nut of course so it has 6 sides or flats. Thus, turning it 12 flats would be 2 full turns.

Some speak of this in flats as once you get into the final adjustment only small increments of turns are needed and counting the number of flats can help you keep track of where you are.
 
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