SU Piston test

[jvandyke]

HS2 from a '63. The original carbs I'm told, obviously been tinkered with in the past but look pretty straight.
I've been teaching myself about SU's a bit. Got my 'Tuning A Series Engines" book from Santa. That and the video by John Twist make me question my piston/vacuum chamber fit. If I do the test (I cleaned them pretty good I thought) Block the air holes and do the drop, one takes 8 seconds, the other doesn't want to come down at all. With holes open they both drop straight out pretty quick. So, do I need to address this or just press on and "make note of it".
I'll have questions coming about the float valves too, they look like the ball type, I read those are troublesome?

 

[dklawson]

If I remember the video correctly doesn't John Twist suggest if the drop rate is radically different that you try moving the piston from one vacuum chamber to the other and repeating the test to see if they are closer?

 

[glemon]

The ball type float is called a Grose-jet, I think they are great and have never had trouble with them, enough people have said there was a bad batch produced at one time or another that I believe it, but I have never had trouble with them. The newer float valves have a special tip as well and are less likely to have problems. I would not worr about that aspect of it unless you start to have problems, usually it comes in the form of gas coming out of the float chambers, if you do have these problems the float needles are easy enough to replace at any time.

On the first issue--I don't know abou the special test with holes blocked, always have just lifted them up and if they drop steady and about with some retarding action from the oil in the dashpots figure they are doing their job right.

 

[jvandyke]

Yes, but even so, one piston takes forever to drop, in either chamber and even the "fast one" is way beyond the 5 seconds suggested by Vizard (p.89) I'll go back and clean thoroughly again before making a big deal out of it.

 

[dklawson]

I'll go back and clean thoroughly again before making a big deal out of it.

Don't forget to try swapping the pistons between the two vacuum chambers as I mentioned earlier. Unless you have owned this car since new there's no reason to think that a previous owner didn't mix the parts accidentally in the past.

I am in the anit-Grose Jets camp. I have removed them from my cars after I experienced two very different scenarios. I had a set installed in the GT6 many years ago. Those stuck shut, starving the carbs of fuel. I also had a set on the HS2s in the Mini. Those failed to close and created both an overly rich mixture at idle AND got so bad as to start leaking fuel out of the float chambers. (Low pressure Facet pumps on both cars... not a pressure related problem.) I now run the Viton tipped float valves on our cars.

 

[Hap Waldrop]

Mathcing drop rates sometimes requires a bit of craftmanship, I often use a variety of tricks to get them dropping like I want, I use burgundy scotchbrite
(3M 7447) to clean up the inside of the daskpots, then I've chucked the piston up in a drill pressed and use the 3M 7447 to polish the outer most part of the piston ( the portion that comes inot contact with the ID of the daskpot) then I use samll flex hones to hone the ID of the piston spindle and the ID of the dashpot spindle bore, so every moving surface is clean and burr free.

 

[JamesWilson]

I've been told that the pistons are <span style="font-style: italic">supposed </span> to be matched to their pots when originally assembled, so if they've been "swapped" in the years since....

 

[jlaird]

I had one that was ok and one that was tight. A bit of crocus cloth and rubing by hand solved that.

 

[dklawson]

I went back and found the YouTube link for University Motor's demonstration of piston/suction-chamber matching. See:
https://www.youtube.com/watch?v=IfU47Oqq9wA
(cut and paste if necessary... I seem to have lost my ability to create direct links)

As I mentioned above, John Twist recommends trying the pistons in both suction chambers to get a "drop rate" that is as close as possible. He cautions against thinking that the parts are still properly matched as they were when the left the factory.

 

[jvandyke]

After more cleaning and some light polishing the drop rates are getting better. Got one set nice and needle centered and working well. The other I can't get centered, if I tighten the needle bearing nut the piston (needle I assume) rubs and goes tight, slacken off that nut a tad and it falls good. More fiddling I guess, possible new parts? When I started neither piston would rise or fall for squat when pushing up through the intake. Off center or gummy or both I don't know but I'm getting there. Vizard seems to say the 1 1/4" SUs are barely enough carb but I'd think that on a stock (other than header) 1098 they should be a good carb, maybe if I do some of the mods he mentions (butterfly thinning, piston bottom shaving, K&N filters) it'll flow plenty for 1098. I'm still pretty convinced I'm better off trying to get the SUs working well then the Weber, since I've got both set ups, may as well tinker, eh? Will learn a lot even if I don't improve anything much. Good performance AND good mileage seems more attainable from the SUs, they seem so simple compared to the Weber.
Are K&N filters in the stock housings considered a bonus and worth the investment? Vizard seems to like the K&Ns a lot. I'm still wondering about worn throttle shafts, is there an easy way to check for wear there?

 

[Pythias]

Since you ask about K&Ns: There is a nearby hill that is on one of my "routes". One day I went up. I had to downshift from 4th to 3rd gear to make the climb. The next day I put on K&N's with stacks inside. When I tried the hill I accelerated in 4th gear going up.

The stacks are 1 1/2" tall so I bought the 3" deep filters from APTFAST.com to fit them inside and still have plenty of room around the horn for proper undisturbed airflow. I've since gone to billet stub stacks, still inside those same filters. Worth every penny. .. Oh, I'm running a 1098 too.

 

[Roger]

I too have heard about the pistons and dashpots being "matched", but I really wonder what that meant in practice. Not a lot, I reckon, and certainly not as much as you can do with time, care and patience.
My method was usually to drop the dashpots off the pistons, rather than the other way round, but that's probably because I was brought up on the old, solid, heavy pistons; in either case, dropping them onto a soft, yielding surface to avoid damage.
The objective is to achieve a steady, uniform drop on each carb and then equalise them with judicious use of very, very fine abrasive - I used wet-or-dry, wet, with soap as a lubricant, on the piston in a drill-press.
One other thing to check - make suer the pistons and dashpots are still round! If they've been dropped a few times, they may not be.

 

[dklawson]

In the University Motors video the pistons are not actually allowed to fall out of the vacuum chambers. John Twist places his thumb at the bottom of the vacuum chamber to act as a stop.

There are a couple of products our firm manufactures that are also stocked/assembled/sold as "matched sets". I'm sure it means the same to us as it did for SU. The parts are very carefully measured (100%... not just one piece from a lot) and "grouped" according to their ID or OD range. Pieces are taken from these production groups and "kitted" together with a mating part having the proper/desired running clearance.

The point that John Twist and others are trying to make is that over the years owners may not have paid attention to this and may have taken the piston from one carb and put it in the vacuum chamber for the other. That's why he suggests measuring the drop rate of each carb and swapping the parts around until you get a pair of carbs whose pistons fall at close to the same rate.

You obviously want all the parts clean and free from deposits, but I would only consider extensive cleaning with ScotchBrite or similar AFTER you've tried moving the pistons to the other vacuum chamber. I'm sorry that I can't seem to post an HTML link, but cut and paste the link to YouTube I supplied in my last post. Watch John Twist as he explains and demonstrates his process. It's a lot more entertaining than reading my posts.

 

[losmorob]

I converted to K&N's on my B. Such a breathing iprovement that I had to run richer needles in my carbs. Good bang for the buck.

 

[Westfield_XI]

Regarding K&N's, the increased airflow comes from the reduction in restriction caused by a reduction in filtration. There is no such thing as a free lunch...... Also once the oiled filter picks up a little dirt it flows no better than a stock filter, although it then will filter just as well as the stock unit. The main advantage is the reusability factor, but I also think it's a real pain to wash, dry and correctly oil K&N's and would rather just use throwaway filters for a street car.

 

[Hap Waldrop]

After more cleaning and some light polishing the drop rates are getting better. Got one set nice and needle centered and working well. The other I can't get centered,

I assume we are tlaking about non baised, solid needle carbs? Do you have the jet centering tool suppiled with the SU tool kit, sometimes this is the only thing other than swapping out a jet bearing that will fix the problem. Some of the early jet bearings have a gap around the OD of the jest bearing of it once tightened in the carb body, this kind can give you fits cnetering them smoetimes, the later style fits into the carb body much tighter, and have less chance of misaligning. YOur desription definately sounds like a misaligned jet bearing casuing the needle to stick, and until it is centered you won't be able to find your true drop rate.

 

[dklawson]

I have recently been working on a set of HS4 carbs. When I took them apart there were a couple of jet related parts missing. On the HS2s I've worked on (and Strombergs as well) there is supposed to be a washer between the jet bearing and the jet lock nut. This was missing on my HS4 carbs, perhaps it's missing from those you are working on. The washer will help the jet stay centered as the lock nut is tightened.

 

[jvandyke]

I assume we are tlaking about non baised, solid needle carbs? Do you have the jet centering tool suppiled with the SU tool kit, sometimes this is the only thing other than swapping out a jet bearing that will fix the problem.SNIP and until it is centered you won't be able to find your true drop rate.

Yes solid needle carbs I believe original HS2 from a '63 (I'm 90% sure anyway).
No, no centering tool, I'll look into that. So far all I've done is loosen and retighten repeatedly but no joy on that one carb, BTW the whole float chamber wiggles a bit more on this carb then the other. It seems like there's a bit too much slop to me, if I move the choke linkage, which pulls the jet tube down it isn't exactly nice and smooth and straight. Not sure how it's supposed to be. I can apply a little pressure to the jet tube and the piston will fall down. I'm going to fiddle a bit today, just swap various parts back and forth but the needle seems straight (although I can't get that one out of the piston body for love or money). The jet bearing has some curious score marks on it's top, hmmm.
BTW drop rates for piston/chamber matching were done off carb. Now back together; the one good one falls down with a nice solid hit (like it should I believe) the other always grounds out just before it makes it to the bottom.

 

[jvandyke]

Alright, got carb 2's needle centered:
I eyeballed the bearing as I tightened the nut (piston and chamber removed), lots of slop in there, could easily see how the bearing gets pushed over on tightening so I fiddled until I could see the bearing had equal clearance all around and the nut was tight. BUT
When I tighten up the chamber, piston gets tight, in fact if I just apply moderate pressure (without screws) it does, let off and it drops freely. What could be wrong with the chamber fit? I suppose something is tweaked, I swapped piston/champer combos to opposite carb and BOTH wouldn't slide with chamber tightened up. Put it back as the one carb works great.
BTW one float chamber is looser than the other, is that a big deal, little bit of play in it. Someone welded the intake back together too and the weld is pretty ugly, not that I care but I should probably clean up the inside for good flow.
Either that or start working on tuning that Weber?

 

[jvandyke]

Okay, I'm learning more. It was still a centering issue. I've learned the bearing needs to be centered where the combo of piston and chamber (tightened) want it, not necessarily dead center in the carb's bore. I got it by making piston and tightened chamber happy with bearing nut fairly loose, removing chamber/piston, eyeballing bearing, reassemble, tighten, test, repeat. Somewhere along the way I put a bit of oil on the bearing/washer so it wouldn't want to move so much with the last snug of the nut. So now both piston rise and fall (all the way down) with ease, although one seems a bit "tighter" than the other. So maybe I'll throw them on the car sometime after all.