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balancing air SU carbs

Posted by hymodyne 
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hymodyne Avatar
james king
salisbury, MD, USA   usa
1972 Triumph GT6 MkIII "Baby Blue"

Took the dashpots off the carbs tonight to check the level of fuel in the jets. Previously it had been level with the deck, and drooling out the rear carb at times. now the fuel appears to be around 1/4 of an inch below the deck. After warmup, an effort to balance air intake on the carbs at idle revealed that the front carb was operating at a much lower suction level than the rear. inspecting the piston level showed the rear to be higher than the front, which was barely off the deck. attempts to adjust this with the front carb idle screw improved the draw on that carb just a little. These carb bodies had new shafts bushings and butterflies installed by Mr. Curto. The tube to ear test showed a strong, noisy suction on the rear, and a barely audible suction on the front. Both dashpots have oil. I am going to check the condition of the piston and dashpot on the front carb tomorrow.

Manifold temps 1-5 were pretty uniform, high 200's, low 300's, with the exception of #6, around 280.

Other suggestions as to why the level of vacuum is so different, and how to resolve it?


James

Marcus Clayton
Melbourne, Ivanhoe, Australia   aus

OK James, this is a pretty normal situation. I just went through it again balancing the 3 carbs on the MG just yesterday after an engine out. Same procedure just one less carb for you.

First warm the engine to operating temperature, then disconnect the carbs from each other, and from the throttle linkage.

back off the screws on the fast idle cam. These activate the throttle when the choke is applied.

Back off both idle screws until throttles are completely closed, and then turn them down equally, about a half turn.

Start engine (you will have no foot throttle as it is disconnected). you may have to turn another bit to get engine to idle.

Adjust screws equally, until you get desired idle speed, you can then make fine adjustment using the tube noise as a guide.

Turn idle mixture screw nut down until engine revs drop (too rich) and then screw up again until you hear the engine speed pick up and run at highest RPM

Repeat on the other carb.

Re adjust idle speed if necessary, (turn both screws same amount and check again with the tube)

I usually then turn another 1-2 flats rich from this point, whatever works.

Turn off engine

Connect everything up again. This is usually where it goes wrong. Check that linkages open both carbs at the same time, and that throttle is not hanging one carb open.
Wear and alignment in the linkages is a major cause of driveability issues with multiple carbs.

When everything is together an linkages are relaxed and engine idling, check with the tube again. You know the base settings are correct, so if there is a problem it will be the linkage causing it, and then you can spot and fix it.


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Jediscuba Avatar
Steven Spandorf
Bayside, New York, USA   usa
1963 Triumph Spitfire "Pussycat"

James,

a key point that many miss when first learning about synchronizing SUs or other multiple carbs is that you MUST unlock the linkage connecting the throttle plates. Until you do that, fiddling with one will effect the others.

On SU's especially, make sure that your float bowel is set correctly. If the level is too high or too low, it will be impossible to adjust the carbs correctly.

The fuel level then should appear to be about 1/8 of an inch down the jet tube.

Here's a good site to check out about the SU carburetors:
http://www.mintylamb.co.uk/?page=sutune.htm

Steve

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Dave Braun Avatar
Minnesota, USA   usa
1952 MG TD "Tommy"
1970 MG MGB "Maggie"
1974 Triumph Spitfire 1500 "Sammy"

I like a lower fuel level than 1/8 inch below the bridge. I generally start about .20 below the bridge, measured by dropping the jet with the enrichment lever, until it is level with the fuel, and measuring the depth of the jet. Takes some practice. Then, set the jet .070 below the bridge with the enrichment lever full 'off'.

X2 or 3 on loosening the linkage between the carbs while balancing he airflow. Once they are balanced, lock the shafts together and make all further idle adjustments identically to each. There is a small amount of torsional twist, so work precisely.

Here is the tuning information I send my customers... Oops, can't do that from an iPad. It's on my website though.



Edited 1 time(s). Last edit at 2013-12-05 06:58 AM by Dave Braun.


Member Services:
dbraun99 LLC provides complete bench services on SU Carburetors. We also provide advice, repairs and restorations of both systems and complete cars.
hymodyne Avatar
james king
salisbury, MD, USA   usa
1972 Triumph GT6 MkIII "Baby Blue"

carb air flow is now balanced. still running rich (3 full turns) but idling steady.

James

Jediscuba Avatar
Steven Spandorf
Bayside, New York, USA   usa
1963 Triumph Spitfire "Pussycat"

In reply to # 927637 by Dave Braun I like a lower fuel level than 1/8 inch below the bridge. I generally start about .20 below the bridge, measured by dropping the jet with the enrichment lever, until it is level with the fuel, and measuring the depth of the jet. Takes some practice. Then, set the jet .070 below the bridge with the enrichment lever full 'off'.

X2 or 3 on loosening the linkage between the carbs while balancing he airflow. Once they are balanced, lock the shafts together and make all further idle adjustments identically to each. There is a small amount of torsional twist, so work precisely.

Here is the tuning information I send my customers... Oops, can't do that from an iPad. It's on my website though.

Dave,
The difference between 1/8" ( .125" ) and .20" is so small that to an untrained eye, they may look almost the same, especially when looking down the throat of a narrow bore.
Besides, most non machinists have little idea what .20" looks like. That said, I agree that .20" would be possibly a better starting point.

Steve

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carChips Avatar
Victor Harnish
Kelowna, BC, Canada   can
1933 MG Magnette
1973 Triumph Spitfire 1500 "Chip"
1989 GMC Sierra 1500 "Bush Truck"

Or, .1825



'S all for now
Vic

Dave Braun Avatar
Minnesota, USA   usa
1952 MG TD "Tommy"
1970 MG MGB "Maggie"
1974 Triumph Spitfire 1500 "Sammy"

In reply to # 927648 by Jediscuba
In reply to # 927637 by Dave Braun I like a lower fuel level than 1/8 inch below the bridge. I generally start about .20 below the bridge, measured by dropping the jet with the enrichment lever, until it is level with the fuel, and measuring the depth of the jet. Takes some practice. Then, set the jet .070 below the bridge with the enrichment lever full 'off'.

X2 or 3 on loosening the linkage between the carbs while balancing he airflow. Once they are balanced, lock the shafts together and make all further idle adjustments identically to each. There is a small amount of torsional twist, so work precisely.

Here is the tuning information I send my customers... Oops, can't do that from an iPad. It's on my website though.

Dave,
The difference between 1/8" ( .125" ) and .20" is so small that to an untrained eye, they may look almost the same, especially when looking down the throat of a narrow bore.
Besides, most non machinists have little idea what .20" looks like. That said, I agree that .20" would be possibly a better starting point.

Steve

That's why I measure the depth of the jet when level with the fuel with a dial caliper. smileys with beer

I used to use 1/6th inch (0.16) but to get the range of adjustment for mixture lately, here in Minnesota, I've been using the deeper .020. Not challenging you, just offering my two cents.

Warmly,
Dave


Member Services:
dbraun99 LLC provides complete bench services on SU Carburetors. We also provide advice, repairs and restorations of both systems and complete cars.
. You can hide this ad & support the site by upgrading to a Gold Membership ~ click here for more info

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