running lean?

H

heapy

Member
hello i have a 1973 auto and i have replaced all fuel lines, filter, cleaned the manual pump and valves its a low mileage car 23k so not worn? absolutely all electrics have now been replaced.
the car was stood for 24 years so there was a little residue in the first carb and it had a sticking float (all good now)
as i have thouroughly cleaned it (have not had a chance to do the second but this would make it run lumpy and rough surely if something was amuck??) it runs nicely and smooth but its still running very lean and only gets some oompf when the choke is pulled the majority of the way out. The carbs are capped off at the mixture screw so myself or the prevoius owner could not have fiddled??
nothing is missing ie pipes and there does not seem to be any gaps around where the carbs bolt on though there is a loud whistling when accelerating hard this i thought to be air whistling in past the elbows going to the filter?? or is this something more sinister?
is this a common fault?
ps what is the correct grade of oil for the dash pots i have looked through all my manuals to no avail, it has a very light oil in them at the mo as it did not seem to like 20/50 which is what the marina su's state.

so any suggestions on what i may have missed would be appreciated.
Cheers Keith
 
The whistling could be an air leak so have a good look around, but first I'd set the dwell and timing, then prize out the tamperproof plugs over the mixture screws and set the mixture correctly. The cars were set up weak fro the factory, and the plugs were there to stop people messing about with the mixture. If you can't set the mixture correctly you'll have to start looking for a reason why, which goes back to rechecking for an inlet air leak.
 
Hello Keith,

The original factory recommended oil for the dashpots is SAE20. If this in not available, then you can use 20W-50 engine oil, the only difference being as the temperature rises, the straight 20 weight oil will thin much more noticably compared to the multigrade oil. The thinner the oil, the quicker the piston can rise and the leaner the acceleration mixture will be, assuming all else remains unchanged.

When you say that you need to use the choke a fair way out, is that during normal running at normal running temperature, or during the first 10 mins or so after first starting the engine?

Ron.
 
ok, ,ill definatelly have another check, a very thorough one for leaks as it was a quick check before. i have not checked the timing yet, was waiting for the timing light to turn up. thats here now so will do.
as for the choke, it does run but gutless, takes forever for the choke to be pushed in when warming up and when hot does run smooth but goes much better when pulled out hence lean. also dies off and does nothing when trying to pull away quick at a rounabout untill the choke is pulled out. i forgot this once and nearly had a heart attack!! all is fine obviously at a gental pace. tho i think that part may be down to the near water like viscosity of the oil thats in the pots now.
thanks for that.
 
Hello Keith,

Just a thought, have you rebuilt your mechanical fuel pump? After all those years standing, maybe the diaphragm is starting to perish and the valves may be sticking a little, so as a consequence fuel pressure is less than what is should be and so the float bowls are not filling with fuel hence a lean mixture will result.

If you do adjust the mixture screws, be sure to do so only while the engine is idling. The adjustment essentially changes the idle mixture only, while the taper of the needle effects the mixture off idle, although a richer idle mixture will have a flow on effect as the jet will be further below the bridge deck.

You can use the lift pins as a guide to check the present mixture setting.

Ron.
 
The whistling does sound like the key to this. Have you got air filters in place in the housing?

Assuming there are no obvious sources for the whistle then it's back to basics and do the whole thing from scratch. First off is to verify and set the ignition. Start by replacing everything that could conceivably be called a consummable including plug leads, plugs, distributor cap, condensor etc etc.

Once on to the fuel system cleaning on its own is not sufficient. Replace eveything you can. 23 years is ample time for things to quietly corrode to the point of sticking. So fuel pump service kit, floats and float needles in the carbs and have the carbs off the manifold to clean and fit new paper gaskets eveywhere. A good tip to clean any deposits from the inside of the dashpots and pistons is a metal cleaner such as Duraglit.

Then check the piston fall times - take out the return spring and the damper and with dry pistons and dashpots check that both pistons take the same time to drop out of the dashpots. Swap pistons and dashpots to achieve the best match. Check the springs have the same free length before refitting.

Next check the throttle linkage setup. Make sure you have a rubber bush at the bulkhead for the fore and aft accelerator shaft to land in! Then check that pressing the throttle pedal to the floor achieves full throttle at both carbs. Adjust and repair the linkage until it does. Then with the idle screws wound right off check that movement of the throttle linkage from closed cracks both throttles open simultaneously. Now adjust until you get a good compromise between cracking together and full throttle together!

Only now are we on to mixture setting etc. Ideally you would need two colourtune plugs so that you can see what is going on inside the cylinder on cylinders fed by each of the two carbs simultaneously. You are also going to need a means of assesing how much air flow is going into each carb. I used to use the end of a bit of heater delivery hose as a sort of sthethoscope for this.

As stated before, the adjustment on SU's is for idle throttle opening and idle mixture only - not what happens at large throttle openings. That's why we've spent so much time on the linkage etc. The objective now is to set a fairly fast idle - say 900rpm and get the right mixture coming from each carb (blue flame just hinting to orange) against equal air delivery.

Finally you can set up the choke linkage so that both chokes are engaged at the same time.

At any point in this process other faults might become apparent, eg dislodged needles, stuck jets etc. Please don't think that a) the car must have been right when it left the factory or b) just because it's low mileage no deterioration will have occured.

Other possible sources for the whistle might be a leak between the manifold and the cylinder heads or a leak on the vacuum side of the brake servo.

Hope all that helps.

Chris
 
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