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Rough running BT7 3 carb HELP

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dvu101

Jedi Hopeful
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I have been chasing this problem for 2 years now. Its gotten better with all I have done but I do not trust driving the car.

Problem
first it idles at 1500 to 1700 the three idle screws are backed almost all the way out.

second
when driving down the road 20 to 50 mph NOT under load the engine seems to sputter or miss
when excelerating or under load it drives fine

What I have done
About 2 years ago I had Jeff S rebuild the dist. less then 1000 miles ago
new vacuum adv too. it is working
switched out the wires, points, conditioner, cap rotor, same, so i put Jeffs good stuff back inside, same
switched out the dist, same put Jeff one back
replaced the spark plugs several times

Checked timing a bunch of times, set to 15 BDC did with vaccum plugged

Compression test
cyl 2 was 125, the others were in the 140 range, forgot where i wrote them down
checked valve adj they were ok, loosened a couple a little

Carbs
new filters, before fuel pump and before carbs
fuel pump is a facet pump from Moss. about 5 years old
replaced float needles, gross jets, cleaned fuel bowls, and adjusted, not really dirty
cleaned dash pots and needles look ok to me
checked floats, not leaking
Followed the Book and adjusted the carbs to initial setting, then used a color tune for final adj. Only changed setting some
Sprayed DW40 on card linkage. I think I have some throttle shaft ware. Idle increased some on 2 carbs

The whole problem started with a backfire coming down a hill a couple years ago.
for the last six months the engine backfired a lot but I seem to have gotten rid of that.

I checked the balance tube rubber connections only on tri carb. replaced them. same

I cannot think of what else to do at this point besides getting the throttle shafts fixed and rebuild the carbs.
I was hoping to drive this summer and maybe do that for a winter project.

Any suggestions would be appricated

Thanks Scott
 
Remove carbs , pack in a box , ship to apple hydraulics , get full rebuild .
They set them up balance them and flow test them .
Bolt them back on start car set idle drive the wheels of it .....done .
 
I forgot. I switched to sports coil 40K this winter. same. Just to be sure I switxhed to another known coil, same. so I put the sports coil back on

Scott
 
Test manifold for air leaks at head and where exh manifold joins down pipes. The latter can cause backfiring.

Check balance pipe for air leaks.

Compression test sounds low - did you block the throttles open?
 
If it's idling that high, either too lean or timing wrong. Runs well under acceleration only suggests too lean at low speed. Try raising the main needles or lowering the jets.

I don't trust the colourtunes - my car runs right in the orange range on the richer side of bunsen blue.

I would want to make the car too rich, then work back to the correct point.
 
If it's still idling at 1700 rpm with the idle screws all the way backed out, I think you must have a vacuum leak. It's got to be getting air from somewhere. It's probably your throttle shafts. It's not all that difficult for a machine shop to either ream them out for an oversized shaft or bush them to fit the regular shaft.
 
Initial reaction same as Steve's: too lean.

It's been pointed out to me, that most, if not all the wear is taking place on the throttle shafts themselves, so before getting the bodies rebored/rebushed, try new standard sized throttle shafts.

Have you balanced the throttle shaft linkage so that all three (3) react at exactly the same moment? This is a mechanical function, regardless of air-flow through the bores (you can balance the airflow with a Unisyn-type device with the throttle interconnecting linkages completely loose, THEN snug the linkages so act on each butterfly at EXACTLY the same time/same travel).

Revisit the possibility of vacuum leaks, and try setting your timing with total advance__centrifugal all in__at about 32*-34* BTDC at whatever RPM Jeff S. says the max comes in at (+ a rippums).

Sounds like you're doing almost everything** the same way/same order I'd do them in, and keeping track of the results of each test/change is a mark of professionalism. You're narrowing it down, you'll get there!




** My Color-Tune hasn't been out of its box in about thirty-two (32+) years; save yours for the archeologists too! ;)
 
Thanks guys for the advice. Scott.
 
Well:
I'm not keen on them Fruit Orchard Carby people,but Nisongner can do an excellent job on your Carbs if you need service.
 
If the idle speed is way too high with the idle screws right out then the timing is over advanced, so do as the Healey workshop manual suggests and set the static timing correctly before you go near the carbs.
 
My two cents: you've got a vacuum leak. With the engine idling, mist some water around the carbs and hoses. If the engine slows down, you've found the leak.

Tom M.
 
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