A few SU tuning questions

[seerex]

glad to hear its getting better , if all if the parts are working right , and once you get a bit time getting the diffrent settings right its a great feeling to tune them yourself

[Murph]

Well I've just given it the full tune up by ear, and I'm quite happy with the results!

I actually had a go at making my own flow meter, just a simple U tube manometer type design, but unfortunatly didn't get the sort of resolution needed. Maybe another time I'll sit down and design it properly.

Anyhow, I balanced it by ear which wasn't too difficult at all but I can see that it's never going to be quite as accurate as doing it with a flow meter.

Adjusted the mixtures, rebalanced and reset the idle, checked mixtures again. At this stage I kept it quite conservative (ritch) and while it ran strongly on the road it did blow a good bit of black smoke. Leaned them up a quater turn, still on the ritch side of things, but you could feel on the road that it had lost some power. Came back and ritchened then a 1/8 turn and like this feels great and doesn't blow any smoke.

Took it for a 15min fang in some nice open areas near my place and it performed quite strongly. When I got home I checked all the plugs and they look great too, nice browny sort of colour as a pose to the jet black they have always been.

So does this sound about right to everyone? Anything I've done wrong or should do differently?

Thanks to everyone for your help! I'd be buggered without you all!

Justin

[zeiss150]

Hey Justin

If it runs good and your happy with it ... then good times my friend! Brown spark plugs and no black smoke is also a good sign. I had to lean mine front carb up a bit because my #2 cylinder was always black, but I think i have that fixed. I have a question for you, how is your idle is it nice and smooth or is it a little "bumpy"? Mine seems smooth as slik at about 700 rpm's. enjoy your finely tuned Z. Good times in the land down under my friend.

Matt-

[Murph]

Still a bit lumpy, but it always has been. Mind you it is a lot better than it was, eg it will comfortably idle at 700 rpm now whereas before it seemed quite unstable.

[Murph]

Just found out it aint run quite so well when fully cold.

It was hesitating quite badly for the first few minutes....maybe even 5 minutes or so. Drove it for about 20min or so like that and even then it still felt a bit weak. Ritchened them up 1/8 turn, ran better but seemed a bit on the ritch side again. Lean them off just a tiny bit again, seemed ok. I'll have to see how they go in the morning when it cold again.

So why is it that it will hesitate when it's cold? Is it running leaner when cold?

[beandip]

Dont you use the choke untill the engine warms up? This is what it is for to richen the mixture untill the engine is up to normal temp. If you adjust the mixture for a cold engine , it will always be too rich when it warms up. There is one more thing you can do is to use 20wt oil in the carbs . This will give a richer mixture for a secound off the line and help eliminate any stumble. This will not effect mileage or performance other wise only helps off the line from idle or low RPM.

[zeiss150]

Hey justin, My Z runs a little rough when cold, but I just use my choke and it works great, It can take a good 5 to 10 minuets of stop and go driving to get it nice and warm. I also have to crank it a bit in the morning to get it started. Bean dip had a great tip about the piston oil for the carbs 20 weight oil works great. By the way i've been running my idle at about 750 to 800 and it runs really smooth. Isn't it summer where you live ring now? In the summer time In sunny california where I live I don't use choke at all.

Matt-

[Murph]

Hehe....it's summer all year round up here.

I recon I'll lean them out a little again, to where it originally had them, and try giving it some choke if it's hesitating too much.

I always use the choke to start it when it's cold pretty much out of habit, because it always needed it. No idea if it does now. Next time it's cold and I remember I'll see how it goes with no choke. With choke it always fires imediatly.

[Bruce Palmer]

Something that can get in the way of adjusting mixtures "equally" between both carbs has to do with needle & nozzle wear. If one carb has a worn needle nozzle combination, you essentially have a bigger hole in that carb for fuel to exit through. If this is the case, the carbs will never adjust the same.

With the domes off, look down into the orifice in the brass fuel nozzle to see if it appears round. If it's round and the needle in the poiston doesn't appear shiney (worn) on one side then you're probably okay.

Sorry,a little late with this I know but like they say in the funny papers, better late than never....

[Murph]

These carbs were complelty disassembled, cleaned and kitted a few months ago, so needle/nozzle wear shouldn't be an issue.

Got another question. What's the effect of taking these carbs that were meant for a 2.4L motor, and then chucking them on a 2.6L?

Is it simply a case of the engine using more air, therefore the carbs will provide more fuel and always keep the correct A/F ratio? What if you put these carbs on a motor that was considerably larger or smaller?

[zeiss150]

I did some checking on this one Justin,(cause im a nerd and I am always looking for more power) The SU's are bitch'n carbs they will easly power a 2.6 or even a 2.8 with out any modification. I talked with several guys that built 3.1 liter strokers and they used the SU's with SM needles. You can modify the stock SU's for a suped up 2.8 that is pulling 200 pluse horse power. The only modification needed is thoes SM needles which you can get from Ztherapy for about $30. If you find that you need more "juice" I've heard of guys using Jaguar SU's which are designed for a much bigger engine. Vrooooooommmmmm!

Later Matt-

[Murph]

Cool cool.

I'm going to start building a mild L28 soon still using the SU's. The SM needles are definantly on the list of things to get.