Dual SU Adjustment

[BrandonS]

I picked my car up a month or so ago and it has been pretty rank behind it; windows down and I smell horrible, my wife complained when driving behind me.  I finally got my inlet flowmeter in so I can make sure to match the carbs.  That part seems fine and they were matching right off the get-go.  The problem I'm having now is that to get the car to idle at a normal RPM 800-1000, it's too lean and the RPMs drop when actuating the lift pin.  If I adjust it to the point the RPMs don't drop with the lift pin, it then has an idle of 1500-2000.  I'm not sure if the lift pin even works correctly at this point since it's off idle.  I started at 2 turns down out from flat inside the carb.

 

All the idle screws are completely out and the chokes weren't holding it up either so there goes that for an easy kill.

 

Any suggestions would be much appreciated.

[wayno]

OK, so you have the SUs balanced with the flow meter, now what you do is lift the pin on the front disabling that carb, then you start closing the fuel mixture adjustment screw on the bottom of the back carb till it is almost going to die, it will kinda lope/chug but continue to run, then you lift the pin on the back carb disabling that one while you close that fuel mixture screw on the front carb till you get the exact same lope/chug, when you let the pin go the engine should smooth out, you then may have to adjust with the flow meter again till you get the idle back down to where you want it, then disable each carb again and adjust them till they are the same again, they need to lope/chug exactly the same.

If the engine dies while you are adjusting the fuel mixture on each carb, then it is too lean or the idle might be too low, I would say balance the carbs at around a 1000rpms(idle), and yes the idle should drop when you lift either pin on either carb, you just don't want it to die, you want it to idle at a lope/chug.

[BrandonS]

I'll try that approach.  I might be doing it wrong.

 

What I was doing was adjusting both to make the jets flat in the carb, turning 2 turns down since I have round nuts not hex ones.  Once I did that I was just bumping the lift pin and seeing if it raised/lowered the idle.  Then I adjusted the mixture knob.  I kept doing this on the front carb until it stopped lowering the idle when the lift pin was bumped.  The mixture knob would end up getting turned out pretty far, so then I would start to do the same on the back carb and in very short order I was a 2000 rpm idle.

 

So you are saying to hold the front lift pin in and adjust the back carb to jsut before it dies.  Once that's done do the same with the front (hold the back lift pin up).  Then just bounch back and forth until they are CFM matched?

[MikeRL411]

The above seems to jibe with the advice that I got from the ex mechanic on the Datsun USA Datsun Race team [before he retired and dissappeared off the face of the earth],  The dual "SU" Hitachi carbs should not ideally synch earch other, the front Carb should slightly lead the rear carb!  I have followed his advice for many many years!  BTY, this is on my RL411 with the BW Automatic transmission, but I think the basic advice is germaine to all R series engine 1600 liter engines with Hitachi "SU" type carbs,  RL411 sedans or 1600 cc engined Roadsters! 

[BrandonS]

See post below... maybe found my problem.

[BrandonS]

These dudes look like crap on the PCV line that goes from the block to the intake manifold; so I would guess that'd be a "vacuum leak"?  I'll have to run and get some new hose.  Would this be a large enough leak to cause this problem do you think?  Those clamps look OG too.

 

 

[flatcat19]

Listen to Wayno about this. He's on point with running his SUs.

[Doctor510]

That is a water line not the PVC line.

[BrandonS]

That is a water line not the PVC line.

Thanks, I don't know how the hell I didn't see that; literally took a pic of it going into the coolant outlet.  For some reason I had searched PCV before and found a picture showing the block vent highlighted wrapping around the block.  I googled again and see it comes off the same side the carbs are on.

[flatcat19]

You're running an electric fuel pump. Is it putting out proper pressure?

[wayno]

I'll try that approach.  I might be doing it wrong.

 

What I was doing was adjusting both to make the jets flat in the carb, turning 2 turns down since I have round nuts not hex ones.  Once I did that I was just bumping the lift pin and seeing if it raised/lowered the idle.  Then I adjusted the mixture knob.  I kept doing this on the front carb until it stopped lowering the idle when the lift pin was bumped.  The mixture knob would end up getting turned out pretty far, so then I would start to do the same on the back carb and in very short order I was a 2000 rpm idle.

 

So you are saying to hold the front lift pin in and adjust the back carb to jsut before it dies.  Once that's done do the same with the front (hold the back lift pin up).  Then just bounch back and forth until they are CFM matched?

 

Yes, push and hold the pin in, this disables that carb, then you can adjust the other carb the way I described, you want it to lope/chug but not die.

I balance them with the air flow meter and then tune/adjust them, then adjust the idle again with the flow meter(sync both carbs), then check the fuel adjustment again and call it good for a couple weeks of daily driving, then I pull the plugs and look at the color, if the front two plugs are tan and the back two plugs are black I close the back carb mixture nut a little(quarter turn or less) and check again in a couple weeks, I want the front two plugs and back two plugs to look the same tan color.

If the plugs are white, then it is too lean and you need to richen/open the fuel mixture nut on them plugs.

If you can disable the either carb and completely close the fuel mixture on the other carb and it doesn't die, then the carb is not serviceable anymore(it's wore out), and it will need to be rebuilt.

The fact is that my back SU carb has not been serviceable for several years now, I can turn it all the way closed and it keeps right on running(doesn't die when tuning it), but I just leave it a little open now and drive it, the back two plugs are always black, but it runs alright, so I leave it alone, once it starts to back fire or some other issue appears, I will have them rebuilt, I bought them new from Nissan back in the mid 90s when they were having a clearance sale and have been daily driving them since.

[BrandonS]

Thanks Wayno... that worked out very well.  There is no longer a slight miss/rock to the car at idle, off the line driveability has gone way up and it seems to pull a little better/more evenly.  The exhaust note has changed quite a bit now.  It sounds more like a performance exhaust now, before it sounded like 400 exhaust leaks/tapping with now growl.  Oh and after my test drive, the car actually shut straight off like a normal car instead of sputtering on for a rotation.

 

The only problem is I can't seem to get the idle below 1200 at the moment.  Maybe the timing is to advanced or the choke linkage is binding a little (i can push down on the arms on each carb and drop the idle a little) all the linkage is new though; I have a new distributor coming in the mail to get rid of the points so I'll end up going through all this again, but until I get it the car is running way better than when I got it.  

 

Thanks a lot for breaking your method down.

[wayno]

Check to make sure your throttle cable is not tight or if you have an idle adjustment screw on the linkage itself, if the throttle cable is tight you will have to adjust that looser also.

You have to do a few things to get SUs dialed in, the air flow balance, I adjust my idle speed with this, I can idle it so low it will die but one cannot adjust the fuel mixture with the idle that low, you need to be sure that both carbs move at the same time instead of one or the other moving first.

The choke linkages can also effect the idle, they need to move up at the same time, and they need to drop all the way down when not being used, if one gets stuck on, the engine tends to idle rough.

Timing isn't likely an issue on the high idle speed, if you cannot get the idle down using the screws that you use to adjust the air flow balance, then you likely have an air leak, I would say turn the air balance screws counter clockwise till your idle goes down, then re-balance them at the lower RPM.