Measurements for stock SU jet needles?

[Captain Obvious]

Sorry, but nope. Manifold vacuum never pulls the piston up. The vacuum that pulls the piston up is on the wrong side of the throttle plate to be considered manifold vacuum. The reason the piston drops when you decelerate isn't because the throttle plate keeps the manifold vacuum away from the piston... It's because when you decelerate, there is very little air flowing through the carb.

 

The pistons lift based on airflow. The more air volume that flows, the higher the piston. The less volume that flows, the lower the piston. And when you are decelerating with the throttle plate closed, there is very little air flow volume.

 

I'll dig up some theory of operation documents for the SU's, but I suspect Blue has several at his fingertips. I might just stall for a couple hours and let him post 'em up.   I'll let him get into all that Bernoulli stuff.

 

And as for your idea about measuring using a magnified picture... That's a great idea, but someone beat you to it. It's called the "Optical Comparator." You put the part in question on a table and a picture of the magnified part is projected on a screen along with calibrated dimensional lines for measuring.  http://en.wikipedia.org/wiki/Optical_comparator

 

 

I've toyed around with the idea of getting one for my shop, but can't justify the cost or the space it would take up.

 

In the meantime, I'll just have to resort to my calibrated Starrett, my good eye (with magnification) and my steady hands. So you want to send me your N-27's for me to add to my catalog? I'd love to double my sample size!

[Stanley]

I have plans for those particular stock N-27's. Going to do some experimental mods on those when I put the professionally modified (now re-modified) N-27's back in. Don't want to accidentally mess those up, since they work, if not perfectly. However I plan to buy a few more N-27 in the near future, also some more SM's or other British needles.

Been reading Des Hammill's book, way over my head at first but I'm learning. He explains in detail how to modify SU's to equal the performance of Weber's or Dellorto's, but it's not quick or easy. Needles are (almost) everything.

I see now that manifold vacuum relation to piston height is anything but linear. Seems proportional though; but that ratio varies depending on throttle opening and also on piston height. At high RPM with WOT, seems like the suction on the piston is proportional to manifold vacuum, since the Bernoulli effect is minimized due to high piston position.

Hammill talks about two ways (not including WOT at top end) a lean miss occurs on acceleration. First, the piston rises but A/F ratio can't keep up; this can be tuned out with damper oil and/or needle modification. Second, as the throttle is opened, A/F leans out and the piston will not lift because not enough suction is generated by the engine (Hammill, p. 20). This is tuned out by modifying the needle.

Simple in construction, not as simple in theory. I need to think about this some more. All I know for sure is that when manifold vacuum drops below 5 in/hg at WOT, it misses or pops through the carb. I was hoping I pull the air cleaners, try to get it to idle at 5 in/hg, mark the needle position and polish them down at that point. That idea is probably toast.

[Captain Obvious]

Actually the height of the piston is more closely described as inversely proportional to the manifold vacuum. Not quite that simple, but basically... The higher the manifold vacuum, the lower the piston position.

 

Here's a video that might help you get a better picture of how the pistons behave:

https://www.youtube.com/watch?v=tR63vrfhwr8

 

It's a motorcycle, but they are also CV carbs with vacuum controlled slides and the concept is identical to the SU's. You can hear the engine to get a sense of RPM and engine loading, and you can also see the throttle linkage moving around in the background. Note the rapid rise of the pistons when he gooses the throttle.

 

Basically, the piston will be highest at full load (WOT and higher RPM's) and lowest at lowest load (coasting).

I love that video...

[Captain Obvious]

All I know for sure is that when manifold vacuum drops below 5 in/hg at WOT, it misses or pops through the carb.

 

Is that 5 in/hg at WOT under load driving in a gear, or just goosing the throttle sitting still in neutral?

 

Different solution for those two situations.

 

[Stanley]

Under load. Revs fine at idle. Accelerates fine at part throttle. Was better with modded N-27's than the SM's in there now. The N-27's have been re-polished and will go back in as soon as I get a break with the work. Making plenty of torque with SM's but have to be careful how I drive it.

 

Using heavy 30wt oil. Adjusted amount of damper oil so each piston is equally hard to push up. 

 

Fuel pressure steady about 3.9 psi all conditions. Fuel in bowls set slightly higher than stock, checked with sight glass. Running too lean unless it's 4 turns down. 1/4 turn more and it fouls the plugs. Exhaust feels smooth, but it sounds slightly heavy like it's on the rich side. Passes the paper towel test though, except when I first start it up.

 

The link's not working for me.

Edited June 5, 2015 by Stanley

[Captain Obvious]

Well there's no reason you should have to go four turns down just to get it to run without it being lean, especially with modified N-27's or SM's. If you're running four turns down and you've got the bowl level higher than stock, you probably have fuel pooling on top of your nozzles, right? And if you go from too lean to fouls the plugs in just 1/4 turn, that's unusual as well.

 

About the piston oil, you don't change the amount of oil to adjust how fast the pistons rise. You're always supposed to fill them to the line. If you want to change the rise rate, you mess with the viscosity, not the level.

 

For the link, you can go to youtube.com and search for "XS1100 Carbs Vacuum Slides UNDER LOAD" that's the video I tried to link to above. It sure would be interesting to see a video of what your pistons are doing when you drive your car.

 

So your float level is OK and your fuel pressure is good. Remember though that the bowl level you measured was created while there was very light load on the engine. Maybe you've got a partially clogged needle valve or banjo filter(s). Pass enough fuel at idle to run OK and maintain bowl level, but not enough capacity to keep up under high load.

 

In theory, you could drive the car WOT until it stats to sputter and then push in the clutch and kill the engine in one quick motion and coast to a stop in neutral. Would be interesting to see what your bowl levels look like at that point. Like a "plug chop" for your bowl level.

 

In other words, just because your bowl levels look OK sitting static on the driveway doesn't mean they aren't getting sucked dry at high load.
 

[Mark Maras]

At WOT under a load, when it craps out, pull the choke a bit (richen the mixture) & see if it improves. If it does, you have enough of fuel in the bowls. If it doesn't, I would say it is fuel starvation. This is assuming the float level (critical) is correct.

[rossiz]

been reading your thread with great interest, as i've recently installed a set of su's on my 78 280.

 

i set float levels on the bench running the fuel pump into shot glass bowls, then re-set them on the car with a sight glass but i was still lean on both carbs, significantly more at the rear carb. this site: http://www.zcarz.us/technicalinformationpagecarburation.htm had some good info re setting fuel level at the nozzle vs. focusing solely on the float bowl - makes sense since the the fuel actually draws from the nozzle. his measurement shows 1 cm down from top of the bridge, so 10 turns down and the fuel should be right at the top of the nozzle. remove domes & pistons and look straight down and you can see it. i would keep it at 8 turns down, adjust the float, run the pump to fill the bowl, then dial in the last 2 turns to see if i had it. when it's right, the meniscus should hit the top of the nozzle at 9 turns and the fuel just flows over the top at 10 turns.

 

this method worked for me and when i was done the float bowl levels, measured with the sight glass, were pretty different (both higher than 23mm, the rear quite a bit more than the front) but the nozzles were dead on and it runs well in the 2-3 turns down range.

[Stanley]

I've tried the choke test a few time but it was inconclusive (so I'll try it again). It helped but there was still  some weakness.

 

Last time I changed needles and tuned carbs, I was refilling the dampers and noticed one piston was easier to push up than the other. When I put more oil in that one it was harder to push. By adjusting the oil (apparently), I got both pistons the same. Looking at the drawings, though, I can't see how that would work. Maybe something else affected it, like one damper stalk not being screwed down tightly.

 

Maybe the oil is too thick for the richer needles. When I said it was too lean above 4 turns down I don't mean it  was lean overall, just that leaning out on acceleration was worse. 

 

I have flexible tubes for the sight glass, so I hold them right in front of the nozzles to check, which eliminates effect of different fuel height in front and rear bowls. Fuel level is high but not pooling on top of nozzles. My sight glasses are leaking slightly at the tube fittings so I need to fix that before doing the shut-off test.

[Mark Maras]

Normal oil level, both carbs. Do the pistons drop at the same rate? If not try swapping either the pistons or the chambers & try again. If you need to really get into the details of matching the carbs,(no they aren't identical) get the ZTherapy DVD "Just SUs". Best money you can spend this side of a Uni-syn.

[Captain Obvious]

I've seen some intermittent issues with the check valve on the damper stalk where if you reach inside the carb and lift the piston sometimes it's easy to lift, and sometimes it's not. Make sure the oil levels are right and try lifting the pistons a bunch of times on both carbs. Is one carb more consistent than the other?

 

So were you able to find that video?

[Stanley]

Have the video, haven't watched it for a couple years though. Good stuff. IIRC there's a shot of the carb pistons in action.

The pistons seem to raise and fall at the same rate. Made sure, last time the air cleaners were off.

Modifying the N-27's made a big difference, tuned out most of the miss, We'll see.

Wondering about the springs. I see 8 oz. (yellow), 4.5 oz. (red) and 2.5 oz. (blue) are available. But FSM says stock springs are #23's ? I'm might try a set of 2.5's but wonder which ones if any are stock. Bottom end of my engine is stock, top end and exhaust not so much. 2 1/2 turns down was OK (as was regular gas) until I put the '81 JDM head assembly, guess it flows more air.