Datsun Roadster Forum-311s.org

I got a the unisyn to measure the carbs. I think I am rich cuz I only get like 18mpg on a stock 1600. Plugs are somewhat sooty with brown centers.

Anyway, when I try to do the Unisyn following Keith's guide it wants to choke the carbs out if I set it restrictive enough to get enough movement from the little ball. Seems like I must be missing something. Maybe this should go in stupid questions.

Nope, no stupid Ones! Using a unisyn takes some practice. It's an on and off affair... you cannot leave it over the throat for any longer than a moment or two before it will effect the airflow. So, set it by adjusting the dial in the center so it only has to raise the ball about 1/3 the way up... that will give you a little more time. Then, on/off, on/off... you'll get good at it.

This is why I like the "Snail" meter (mine is different actually, but works the same)... it does not interfere with the airflow, so you can leave it over the throat for as long as you want. Google snail airflow meter or something like that...

Gregs672000 wrote: Sun Mar 06, 2022 4:44 am Nope, no stupid Ones! Using a unisyn takes some practice. It's an on and off affair... you cannot leave it over the throat for any longer than a moment or two before it will effect the airflow. So, set it by adjusting the dial in the center so it only has to raise the ball about 1/3 the way up... that will give you a little more time. Then, on/off, on/off... you'll get good at it.

This is why I like the "Snail" meter (mine is different actually, but works the same)... it does not interfere with the airflow, so you can leave it over the throat for as long as you want. Google snail airflow meter or something like that...

Great info, I was not aware if this type of meter. I do now and on my wish list.


https://www.amazon.com/Latest-Rage-5385 ... B00CMC57R0

When I had SUs I would stick my finger into the carb, lift the piston and see if each one had the same effect on the idle.

Gregs672000 wrote: Sun Mar 06, 2022 4:44 am Nope, no stupid Ones! Using a unisyn takes some practice. It's an on and off affair... you cannot leave it over the throat for any longer than a moment or two before it will effect the airflow. So, set it by adjusting the dial in the center so it only has to raise the ball about 1/3 the way up... that will give you a little more time. Then, on/off, on/off... you'll get good at it.

This is why I like the "Snail" meter (mine is different actually, but works the same)... it does not interfere with the airflow, so you can leave it over the throat for as long as you want. Google snail airflow meter or something like that...

Ok makes sense probably should have watched a video first

Yep, air flow is one part of tuning, the other is the air/fuel ratio, which is controlled by the needle, its mounted position in the piston, the adjustment of the dial underneath that adjusts the fuel nozzle position, and the oil weight in the dampeners. What you see on the spark plugs gives you an "overall" indication of A/F. What Spyder described will give you an idea of what A/F ratio is at idle mainly, but any adjustment to the dial changes the ratio to some degree. When you lift the piston (there's a push pin to do that) you are turning off THAT carb and testing the ratio of the other and vise versa. You want the rpms to increase initially for a second then drop off and act like it's on 2 cylinders but still running... chug chug chug. There are a number of extraneous variables that can influence that behavior including the condition of the carbs and leaks past the throttle shafts, other intake leaks, the condition of the two cylinders each carb services, and the valve adjustment. For example, a difference in airflow on #4 throat in my car (I have Weber-like throttle bodies, so individual runners for each cylinder) noted on the flow meter was showing the broken top compression ring in that cylinder that I later discovered.

All serious carb tuning or adjusting requires the rest of the engine (plugs, valves, wires, compression) to be in as best setting and condition as possible. So, consider checking those things as best you can and recognize their potential influence. If the plugs are darker than you want and you're using more fuel, then the dial on the bottom is the place to start once airflow is balanced. Reading plugs takes time, so have a set or two around that are clean and can be swapped in to check the overall result of your changes over time/drives. Check your timing too... power is in the ignition timing, so make sure it advances normally (requires a timing light) and the vac advance is working correctly (can have a significant effect on fuel economy).

Keep playing and posting!

BTW, as I recall if you push up the pin and the engine does not initially increase rpm before dropping off, it's likely rich. If it completely dies then it's probably too lean... that's what I remember from back in the day when I had SUs on my L-series powered 620 PU.

ok good thing I got in there. idle was balanced but at 2000 the front carb was 11 and the rear was 31. I like the snail guy.

Got to where raising or lowering the slide both dropped engine speed. I kind of get to a middle ground where adjustments don't seem to change the rpm, vacuum, or slide movement response either way so it seems a little imprecise. I guess the next step is testing with plug checking.

Excellent work! Â difference in airflow like that is a significant find. Were you able to get them both flowing the same at various rpms? Are you understanding the linkage and what does what? That's a big part of this process. I would set both ratio dials the same number of turns or clicks on both carbs if you have not already. When you get some drive time we can discuss what it acts like. Start with a new set of plugs if you can. If the car bucks or the engine pings that could be lean. Rich and it will just be boggy. I hope you're having fun and enjoying the process!

Gregs672000 wrote: Mon Mar 07, 2022 11:40 pm Excellent work! Â difference in airflow like that is a significant find. Were you able to get them both flowing the same at various rpms? Are you understanding the linkage and what does what? That's a big part of this process. I would set both ratio dials the same number of turns or clicks on both carbs if you have not already. When you get some drive time we can discuss what it acts like. Start with a new set of plugs if you can. If the car bucks or the engine pings that could be lean. Rich and it will just be boggy. I hope you're having fun and enjoying the process!

yeah, I had loosened up the dog bone because it was too tight at idle a week or two ago, but I didn't realize how sensitive they are and didn't check them. now they are both even. I cleaned the plugs up and will check them after driving tomorrow.

Ok just sharing for anyone who is playing with SUs:

Driving today didn't feel good. Sluggish and cold start didn't want to idle. So dug back in when I got home.

Pulled off the pistons and set depth to .087" with caliper. Was waaay too rich (verified by lifting the pistons and idle sped up). I don't really know the history on the carbs other than that they were done by ztherapy many years back and stored.

I found out they have the little plunger that raises the slide for you (missed it while doing it with my fingernail all night..)

I also replaced the fluid in tops with motorcycle fork oil cuz I spilled one.

This time I was adjusting with the idle low. I just wasn't getting enough feedback trying to adjust at 2000.

I ended up adjusting both until lifting the pistons would be almost a slight pause/bump and then a drop instead of an immediate drop. It also seemed to give the best throttle response.

Drive felt great, probably best it's ever run. More revvy and more power. Pulled domes off again just to check and one was .02x one was .032 so I set both to .032. I don't know how anybody gets even turns on the little nuts from above without seeing them

Not sure why my 'good' height is so far off from the .087, maybe float level? But was very happy with the idle and drive so I'll see how it feels tomorrow.

It has been nearly 40 years since I played with SUs, but the idle mix "lift the piston" test with the pin (glad you found it... it's consistent) sounded right as I described above. There are a few experts here, Keith being one, who can guide you better than I. With flow and the needle position set right, you just need to monitor/read your plugs. One thing I did was swap out new plugs, take pictures of the old ones (keep them in order so you can see how each cylinder is firing), drive for a while (enjoy it!) then see what the new ones looks like. Differences between the pairs of 1/2 and 3/4 is likely a carb difference, variations between 1 and 2 or 3 and 4 are cylinder specific of course since they are being fed by the same carb, suggesting a valve, spark or compression difference in that cylinder(s). The other thing to consider is trying different weight oil in the tops. The oil acts as a dampener to keep the piston from flying up too fast and (from my recollection) leaning out the engine. Issues here will show up when you hit the throttle hard and it stumbles (lean). Several owners have talked about the results they have had with different oils. You can suck out the oil with a syringe so you don't have to remove the domes.

These SU experts do things I never knew about, like setting the needle position as you indicated... good stuff! Glad you're getting good results!