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SU observations from a noob

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Leave the choke stuff for last. Get th' thing workin' on the carb first. Enrichment linkage and suchlike are simple later.
 
Update. Did first tuning session, by the book, several actually, all essentially the same. During warm up I noticed it was running almost entirely off the rear carb (by blocking intake with hand could feel big difference in air suckage).
Got flow equalized via throttle screws, linkage loose, used a hose to the ear (seems pretty imprecise, should get or mock up a uni-sync). On to mixture check, started both at 12 flats down, piston lift test on front carb got an increase in rpm but after moving jet adjusting nut up to lean out it had no discernible effect. Piston test on rear carb did nothing. Moved both nuts around seeking for some change but got nothing, returned both nuts to 12 and decided to proceed. Setting choke linkage had the front set noticeable "offline" with the rear but both jets start to drop at the same time so that was the goal I figured and both return up all the way when cable is slackened. My choke cable (set up for Weber) is a bit short, therefore tight and doesn't operate all that great, might have to redo the whole thing. I'm going to put on the air cleaners and housing and road test it (hopefully tonight if SWMBO doesn't give me stink eye). BTW I ended up putting four springs on as there seemed to be a tad of slop in the linkage so I have one spring to each side of the throttles, not the center and one spring each on the choke linkage. So far it's easily reversible back to the Weber except I might have ruined the choke cable by now.
Also wonder why this source says FRONT carb like that??
SUmixture.jpg
 
jvandyke said:
Update. Did first tuning session, by the book, several actually, all essentially the same. During warm up I noticed it was running almost entirely off the rear carb (by blocking intake with hand could feel big difference in air suckage).
Got flow equalized via throttle screws, linkage loose, used a hose to the ear (seems pretty imprecise, should get or mock up a uni-sync). On to mixture check, started both at 12 flats down, piston lift test on front carb got an increase in rpm but after moving jet adjusting nut up to lean out it had no discernible effect. Piston test on rear carb did nothing. Moved both nuts around seeking for some change but got nothing, returned both nuts to 12 and decided to proceed. Setting choke linkage had the front set noticeable "offline" with the rear but both jets start to drop at the same time so that was the goal I figured and both return up all the way when cable is slackened. My choke cable (set up for Weber) is a bit short, therefore tight and doesn't operate all that great, might have to redo the whole thing. I'm going to put on the air cleaners and housing and road test it (hopefully tonight if SWMBO doesn't give me stink eye). BTW I ended up putting four springs on as there seemed to be a tad of slop in the linkage so I have one spring to each side of the throttles, not the center and one spring each on the choke linkage. So far it's easily reversible back to the Weber except I might have ruined the choke cable by now.
Also wonder why this source says FRONT carb like that??
Click to expand...

What RPM were you using when you did your test? It's odd you had no difference with rear carb. When you screwed the nuts up, did you make sure the jets went up too? Sometimes choke linkage can make 'em stick.
Why not front carb? You've got to start somewhere! Doesn't really matter, though.
 
It was bout 1200 rpm I'd guess.
No, I didn't eyeball the jets as I did the mixture screws.
I just wondered why it mattered you'd start with the front one.

Update; did a road test. No tinkering first, well, I lied I dropped the idle a bit, both screws the same, no "hose test" after. No air filters, went for a short test hop. It was very good. At least as good as the Weber, or very nearly so (and I'm sure the SUs aren't tuned very well yet.
Less hesitation then the Weber. Much much quieter (air induction noise I guess)
I'm NOT running the filters yet though, just wide open carbs.
It was so successful, I don't know why I'd bother putting the Weber back on. I'm now hoping I can continue to tinker and get it better and better!!
yes, amazed to be honest with you. Who would drop big $$ on a Weber conversion? I don't get it? Couple weeks of testing and tuning and you may see my Weber kit on ebay.
 
How about another update?
Took another couple little runs, to bring kids away to friends. On the way home, the throttle cable pulled loose....oops. That was embarrassing. Good thing it's little, right in the middle of an intersection I had to jump out and push it over.
Quick cobbled it back together and got home. I don't like the look of that linkage, seems like a bunch of slop in it.
It definitely doesn't pull as hard but not very fair to compare with one complete rookie's first attempt at tuning.
 
I redid the tune, it's tough getting the idle low and carbs even. I had a hard time because the plates the throttle screws push on don't close tight on their own, I have springs going to each side AND one to the middle bell crank. I'm sure I don't have it very good yet it is now every bit as good as the Weber (it seems) but better in that it doesn't hesitate and is overall smoother operating. I did put the filters and housings on, just left the crank case vent (from rocker cover) open to atmosphere for now. I drove it pretty hard and thrashed her well and it was fine. Only issue is the idle really doesn't like to below 1100 or so.
 
If the carbs are adjusted properly and balanced you have a bit of an air leak. Check with some carb cleaner.
 
I agree with Jack, though I'm puzzled where you write "the plates the throttle screws push on don't close tight". Are you talking about the throttle butterflies or something else?
 
Let's see if I can describe this, I should go shoot a video. the throttle linkage, extreme outboard pieces, as pictured, without springs the throttle can be closed but there's enough slop in these that idle is high, I can push down on them and the idle drops, so I added springs (as pictured, are they all supposed to have three springs on just the throttle pieces?). Maybe I have to set the linkage so that when closed, it pushed down on these plates and ignore the clearance spec in the manual? It seems overly sensitive too. I can have it idle at 1300 and slowly turn each throttle screw down a bit at a time blipping the throttle every once in a while and it suddenly fell to 500. Brought it back up to 1000 or so. Rode around and idle wants to return to 1200 which a bit too high for my liking.
throttlelinkage.jpg

I should be able to fiddle through it. Another couple runs yesterday and I'm not planning on putting the Weber back on, nope. I like the SUs even before they're "dialed in".
 
I'll try the air leak test. Now that I think about it, maybe the throttle shafts are worn and leaking and when I put pressure on those throttle stop arms (number 7 above) I'm pushing the throttle shaft and stopping the leak? Because I can put pressure on there and the rpm falls but it stays low after I let go too though...hmmmm I checked the butterfly fit when I had them apart. When closed I held the carb up to a bright light and looked through the bore, no light = good fit I thought. I'll leak test it with carb cleaner as you say. Any change in rpm indicates an issue I assume?

I joy rode again yesterday and was pretty satisfied with performance but for an idle a bit too high and maybe, just maybe not as overall "fast" as with the Weber, but again, much smoother operation and no annoying hesitation on initial acceleration (from a dead stop or off cruise as was there with Weber).
 
Did you put new throttle shafts in the carbs? I think they are pretty cheap and may slow the leak sig ificantly.
 
No, I didn't replace ANY parts in the carbs, (okay, I lied, some of the screws with mangled heads got swapped out) just went through them cleaning and adjusting, everything looked so good (to me). This pair of HS2 came off the car in '86 and have sat since. There sure isn't any "feelable" slop in the shafts as far as for and aft motion but I imagine if you could feel it it would be a horrendous leak. I'll do a leak test before I get too far down the part replacement path. I have to tell you though that other than not seeming to respond "properly" to the "by the book" adjustment procedures, the SUs are performing pretty darn nice.
Something must account for the unwillingless to idle low and stay there though.
 
"Something must account for the unwillingless to idle low and stay there though"

Does it idle faster when hot? If so, then I'd put money on worn throttle shafts.
 
Honestly I haven't fiddled extensively with it yet. I got a decent idle around 1100 and it was time to drive and drive I did, as much as I could all weekend. I could push down on those throttle plate linkages and the idle would drop drastically, I'd adjust the screw up a bit and next time I'd cycle the throttle, the idle would pop back up and stay there. I'll fiddle some more, spray some carb cleaner around and see what I can come up with. Worst case scenario, I'll put the Weber back on and rebuild the SUs next winter. Even so, I'm psych'd to be running the SUs. Pretty fun.
I'll take this opportunity to say it's just too dang fun to mess with these little cars. Perfect hobby. Perfect mix of fun driving and tinkering, as long as it stays operational 90% of the time, I'm lov'n it! I need to get a couple more.
 
Are the springs on the throttle linkage? If you can get it to drop to below 1K RPM by pushing the linkage, the springs must be mis-attached somehow.
 
I started with one spring to the center bell crank, then I added one to each of the two sides, so everything is getting pulled back to shut pretty well I thought. I'll look it over again and report back.
 
Make sure you loosen the bolts 7+8 before you make adjustments. Also a feeler gauge under the throttle cable arm to heat shield contact point. I forget the amount tho but I don't think you want it to hit tho when everything is adjusted and tightened back up.
 
Yep. I had those bolts loose so each carb was independent. BTW has anyone ever installed a port on the spacer between carb and heat shield? The Mikuni carbs on my motorcycles had them there for the sole purpose of hooking up vacuum gauges for synching. I know the balancing chamber negates this a little but no more difference than a uni-sync. Just wondering aloud. I still have a 4 gauge Kawasaki carb balancer....hmmmmmmm Spot on syncing anytime with air cleaners in place. Tempting.
 
Not here. Always used a Uni-Syn. Still have my fathers.
 
Wow, throttle shafts, eh?
If I had kept spraying, it would have stalled.
This is a movie , might have to wait a while for it to load.
 
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