Su Tuning Help Needed

[altezzaclub]

I've fitted the twin SUs, along with a set of extractors and a cam to the KE70. I bought ram tubes and made a custom airbox.

 

The car runs fine except at very light throttle loads or just lifting off a constant throttle, when it hunts or surges.

 

The timing is 10deg, the carbs are balanced at idle, richening the mixture doesn't cure it. I picked the best of the throttle shafts of the 4 I had access to, and its running ATF in the piston tubes. The floats are plastic, so are not adjustable. I will pop the carb tops off this weekend and see if they are filled to the same level. The needles are not catching in the jets, the pistons drop with a 'clunk' OK and they still pass the 4second test when you let the piston slide in without a spring.

 

It will do it all around town, coming up to a traffic island or turning into a side street, and it will also do it at 100kph on the open road. Give it a decent boot and its fine, goes like a rocket, but very small throttle openings seem to trip it up. I tried sealing off the PCV tube without changing it, and it still has the vac advance and charcoal canister fitted.

 

To me it feels like one carb is opening before the other, or opening faster for a moment then the other catches up.

 

Any ideas?

[Felix]

Dynoed it to find the correct needles?

[Bamboo]

Just a thought. You may have check this already, but the springs on the mechanical advance?

 

If they're shot, they give low down weirdness.

 

Was the dizzy ok in its previous incarnation?

 

Boo

[raleigh]

e the

 

I have had the same issue on a Austin Healey when i fitted ram tubes.The only thing i could put it down to was not enough differential over the bridge/jet area of the carbys to draw the fuel out of the jet.I mention this because when i put back the round ram flo 300cfm filters the light throttle stammer went away and still pulled 8000rpm with out any trendy air box.Another thing to look into is the type of fuel bowls used.Eg are they for level use or inclined ? They are different and this will change the fuel level.Check the part number tags on one the 3 screws on the fuel bowls. We have fitted twin, tripple and single SU carbys to many cars over the years with great results.I only fitted a DCOE weber to my own KE20 because i couldnt get a SU Manifold.Just my 1 cents worth.Regards,Raleigh.....

[altezzaclub]

Thanks Team-

 

Dyno.. planned for this week sometime, thought I'd give it one more weekend before I tried that. I'm hoping a low-throttle run with an oxygen meter will help.

 

Dizzy. It was working fine, but I did bearings at the same time so motor out & stripped, oil pump checked and dizzy given a once-over. The dyno should so that with a timing light as we throttle off.

 

Bowls... not sure actually, no tags left on the carbs of course... They run vertically on the Lynx manifold as it is.

[altezzaclub]

Well, two things today...

 

The needles are the floating type so they sit in a little cylinder with a spring on top pushing them down. The cylinder is held in the carb piston with the usual grubscrew. One needle was at an angle, every time I pushed it straight it jumped back to be offcentre. A careful look showed a slightly raised edge on the cylinder that the needle sits in, just where the flange sits. It was only a couple of thou, so 5minutes with a honing stone had that off an the needle now sits dead true. I remember seeing that the cylinders were different bwteen the two carbs, it had the look of a bodge job rather than originals.

 

So the needle must have been touching the jet and that might have impeded its movement. It moved up and dropped with the mixture pin OK, but that has a lot of force behind it.

 

..and I dug out the dwell meter that showed me the points were too far open, even though the timing was 10degrees. That's because I used feeler gauges and a timing light last time.

 

So it is smoother on the 'slight acceleration' part, but still bad slowing down.

 

The hunt will continue! At least on a test run in the hills around Orange I can appreciate that it hold 100kph no problem now. The stock engine used to die terribly going uphill!

[raleigh]

Well, two things today...

 

The needles are the floating type so they sit in a little cylinder with a spring on top pushing them down. The cylinder is held in the carb piston with the usual grubscrew. One needle was at an angle, every time I pushed it straight it jumped back to be offcentre. A careful look showed a slightly raised edge on the cylinder that the needle sits in, just where the flange sits. It was only a couple of thou, so 5minutes with a honing stone had that off an the needle now sits dead true. I remember seeing that the cylinders were different bwteen the two carbs, it had the look of a bodge job rather than originals.

 

So the needle must have been touching the jet and that might have impeded its movement. It moved up and dropped with the mixture pin OK, but that has a lot of force behind it.

 

..and I dug out the dwell meter that showed me the points were too far open, even though the timing was 10degrees. That's because I used feeler gauges and a timing light last time.

 

So it is smoother on the 'slight acceleration' part, but still bad slowing down.

 

The hunt will continue! At least on a test run in the hills around Orange I can appreciate that it hold 100kph no problem now. The stock engine used to die terribly going uphill!

I just had another thought about your SU issue.Standard fuel pressure for self centering needle type su's is 2.7 psi.

have you put a guage on it? Regards,Raleigh....

[altezzaclub]

Good thought Raleigh- I forgot to mention I fitted an adjustable regulator when I did the work.

 

Currently set at the lowest pressure, I think its 2psi. It feeds OK up to 5000rpm on what, which is all I need.

 

The exhaust is not at its best. The new extractors go into a 2" pipe to a 2" resonator, and then a joint back into a perfectly good stock system. I didn't feel like paying out the extra few hundred to replace all the rest, I thought I'd leave that up to the daughter if she wants to.

 

I don't think it has any effect at low revs, it will no doubt be contrictive up high.

 

We did a few timed runs each way over a piece of country road with a hill, going from 20kph to 100kph.(bascally 2000rpm in 2nd to 4000rpm each gear) When the car is sorted we will repeat it and see how much quicker it is. Starting downhill near the bottom and going out along the flat took around 19seconds, but finishing uphill took around 33!! Its hard to get up to 100kph uphill with a stock motor!

Edited May 22, 2010 by altezzaclub

[Sloth]

Had a very similar problem with the Twin SUs on my 7K when I first put it in (except I got problems with I opened up the throttle, but it was ok when not open too much). Turned out (like Felix suggested) the the needles were wrong (and it need a little machining).

 

Everyone that has seen the twin SUs have commented that they are a pain cause they get out of tune with each other so easily, but I've never really had any problems on mine yet.

[altezzaclub]

Yeah I'm hoping the dyno will show the A/F ratio through the rev range-

 

 

I had SSS Datsun 1600s for years in South Africa and they had twin Hitachi copies of the SU. Once we had them sorted they never gave problems, so I'm trying to get the 'rolla the same..

 

There are two different systems for joining the two throttles, and the older, more complex one was better I reckon. I have the later spring steel junctions where the throttle shafts become a solid entity, and I reckon they move slightly as you tighten them up so changing the synchronising.

 

The old system had U-shaped brackets with pins in the hollow of the "U", and each was individually adjusted.

 

Still, it must be able to be sorted out..

[altezzaclub]

Well, the word is that the needles are running lean all the way through once they get off idle. The points he replaced but thet hasn't solved the jerking over-run.

 

So, off to SU-Midel for needles and maybe new throttle shafts & discs, just to make sure the air leaks are minimised.

[rob83ke70]

your hs2's were fitted to austin healey sprite mkII or similar... 948cc.... I would imagine your 1290cc engine might need a bit more fuel lol...

 

Robert.

[altezzaclub]

Well, it ran down to Sydney at 6.4L/100km, took the usual 3hours in traffic, but it hold 100kph up hills much better now!

 

I couldn't really do a measure on the return trip last night with richer needles as some dude rolled his container truck in Mt Vic and thousands of cars spent a couple of hours with their headlights on and motors idling just to inch forward every 15minutes!

 

However it hasn't stopped the lurching over-run, although its not there on a country run of course as the throttles never get back to idle with overrun at 2000rpm in third, which is when it is the biggest problem.

 

Just slowing down to turn at an intersection in town is when its most jerky.

 

So far I've disconnected the PCV, the brake booster, the vac advance, and the charcoal flter, none of which have changed it so its not air leaking in through any of those hoses.

 

What I'd like now would be a pair of colour-tune plugs so I can watch the colour of a couple of cylinders at once as it slows down.

[tojo2]

Good to see you got it going Keith

 

Just curious, since the manifold is for hs4 and you got hs2 carbs, did you fill in manifold for port matching ?

[altezzaclub]

No, I opened up the head to match the manifold, but have a step out from the carbs to the manifold.

 

I figured that at low revs, like up to 4000, its not going to be crucial.

 

Its certainly quicker! The new needles are about 10% richer and that helps.

 

I'll do the big writeup soon!