Tuning Twin SU's

[g4zur]

Tuning twin SU's to me seems very daunting. But I will shortly be having a go. Stripped carbs down for service, changed waxstats to normal jets with full fitting kit.
Read a lot of topics on forum, all helpful, Bought colourtune and carb balancer. studied instuctions. Car running well over 2000 revs but rough below and not a good tickover so requires doing, Comments most welcome.
Question. Do I set mixture on individual carbs first or balance first, Sorry If I sound thick but must admit my mind is going round in circles.
Many thanks
Regards Gareth.

G.P.Albrighton

[Beans]

Before yo start, did you check the throttle spindles for wear inside the housing?
If they are worn, it will be very difficult to get a decent tick over.

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<font color="blue"><i>1976 TR7 FHC (needs some TLC ...)
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1981 TR7 FHC (Sprint engined a.k.a. 't Kreng
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[g4zur]

Hi Beans
Yes I did. also checked for wear on jet needles and float valve, I would have replaced anything required. Of course all gaskets were renewed.
Gareth.

G.P.Albrighton

[FI Spyder]

I think he meant the brass bushings that hold the throttle butterfly valve shaft. It is common for them to wear. This results in air leaking into the intake without having an effect on fuel being sucked in. This makes for a rough or a high idle that can't be dialed out. (This also applies to any seal/hose on the low pressure side of the throttle valve). If they are worn (play between the throttle shaft and bushings) they have to be drilled out and new ones pressed in. We are fortunate here in that we have a guy in out club that specializes in doing SU/ZS carbs. Not something I would want to do in getting the carb square to drill would scare the heck out of me.

- - -TR7 Spider - - - 1978 Spitfire- - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT

[g4zur]

The Idle was good prior to my overhaul so think the items you mention are ok. I wanted to get rid of the waxstats. Plus clean everything out. It wanted a good clean due to rusty tank.
Gareth.

G.P.Albrighton

[Stag76]

Set the balance first, making sure that the butterflies open at the same time, then adjust the tune. The best way to set the jets to a starting point is to remove the bells and slides, then, using the depth gauge of vernier caliper, set each jet .125" below the bridge. This is equivalent to 2.5 turns of the jet, but more accurate, as it is hard to tell when the jet is level with the bridge to start. You can do this before you re-assemble the carbies. The engine should start and run at this setting. Warm it up then adjust the mixtures (Looking from above the carbies, clockwise to richen, anti-clockwise to lean the mixture). After setting the mixture, reset the balance, using a vacuum gauge or tube to listen to each carby, and the idle screws to adjust them. If it goes wrong, start again.

Make sure you have the correct grade of oil in the dashpots...ATF works well in our warm climate. Thicker oil will increase the richening effect on acceleration, thinner will reduce it, so it's an experimental process.

TR7 Convertible
Sprint Motor
MegaSquirt EFI

[Maki]

Also check the butterfly poppet valves, these are known to cause issues they can stick open. Better still fit early solid butterflies.

[sonscar]

I use a different approach,first disconnect the carbs from each other and then set the mixture for each one individually,you have to revisit each carb more than once.When this is done then balance them and then reconnect the linkage again making sure the throttles open the same amount/time.I am sure that there are as many techniques as there are SU carbs out there.Have fun Steve..

[Hasbeen]

If you are serious about getting a good tune, first check the valve clearances, & shim if necessary. The absolute maximum variation from standard, 8 & 18 thou clearance acceptable is + or - 2 thou for satisfactory running.

Then after doing the carbs, check the ignition timing, & readjust the carbs if you have to change the timing.

Our carbs are blamed for much poor running caused by many other factors.

Hasbeen

[jclay (RIP 2018)]

<b>Steps for Tuning SU Carburetors</b>

1. Make sure the engine isd in good mechanical condition:

a. Compression

b. Valve lash

c. Ignition and spark plugs

d. Ignition timing set to specified advance

e. Carburetors functional (no leaks, all systems working properly)

2. Run engine until fully warmed up.

3. Remove air cleaners, check linkages, choke levers.

4. Top up dashpots.

5. Stop engine and remove suction chambers, pistons, and springs. Be careful not to

spill oil from dashpots, and to keep front and rear carburetor components together

in matched sets so that they can be replaced in their original locations.

6. Adjust each jet until flush with the bottom of the bridge, and then reassemble

pistons and suction chambers. Be sure each jet is pushed up as far as it will go.

7. Start engine and adjust idle speed with each idle stop screw. Use a Uni-Syn to

balance the airflow between cards.

a. Alternate method: Use a length of tubing to listen to the ‘hissing’ sound

from each carburetor (this is a crude method but can work if you do not

have access to a Uni-Syn or other air flow measuring device).

8. Adjust the jet nuts equally to get the fastest idle, then reset the idle speed to 600

RPM by turning each idle speed screw an equal amount. Recheck the air flow

balance with the Uni-Syn.

9. Check mixture by lifting each piston about 1/32 “. Adjust jet nuts or mixture

screw to achieve the correct mixture.

a. Engine speed increases considerably – TOO RICH

b. Engine speed increases slightly then settles back – CORRECT MIXTURE

c. Engine speed decreases immediately – TOO LEAN

10. Re-set idle speed as required to 600 RPM. Tighten throttle rod clamp.

11. Check choke linkage adjustment.

12. Re-install air cleaners

13. Start engine and check idle, no-load acceleration, etc.

<font size="2"><i>Not the way I used to do it, but I haven't touched an SU Carb in 40 years!</i></font id="size2">

Clay

[url="http://www.jclay.me/"]My Triumph Site[/url], [url="http://www.triumphtechnical.me"]Technical Stuff[/url], [url="https://docs.google.com/folder/d/0B8MWEvqOpX3udEF4SmFQUW9RS09hbU5uNW5Wd0xrUQ/edit"]My Public Folder[/url],

[Hasbeen]

Just one other thing. The manual says to give the engine a rev to 2500 or more, for thirty seconds or so, after every couple of minutes idling.

Hasbeen