Datsun Roadster Forum-311s.org

Hi All:
Im seem to be having a bit of trouble getting my carbs dialed in.

The rear carb always seems to run rich compared to the front.
I can get the engine to rough idle idle off the rear carb, but the front carb has to be richened quite alot to get it to run when the rear is disabled.
Carbs are right on the money for air flow balance at 600-650 rpm.
The difference between the nozzle setting ends up about .085" (which I think is too much) to get each carb to rough idle.
Only thing is the exhaust is always black as heck when at idle and lower rpms.
Both float bowl levers were set to .55-59" and both nozzles to a depth setting of .090" when starting fuel adjustment.
With the domes and suction pistons out , I noticed that the fuel levels in the nozzles were different even thou the nozzles were both set to the starting depth of .090".
The front nozzle had fuel about .100" below the nozzle top and the rear nozzle fuel level was almost flush with the nozzle top.

My questions are......
Will the difference in fuel levels in the nozzles affect affect the syncronization of fuel mixtures between carbs?
Should I re-adjust the float levers to obtain a matched fuel level in each of the carb nozzles to enable both carbs (balanced for air flow) to pull fuel from the same height?
Anything else I should look out for?

Ray B

Ray,

I would think you would want to shoot for getting the floats as equal as possible. It sounds like everything else is spot on. The SUs are great once you get them dialed in-sometimes it takes several attempts to get there!

Steve

Hopefully soon they will dial in.
After a few attempts I have gotten nowhere, so I think I'll go and hit a few golf balls and give it a break.
This weekend I'll give it a few more tries and it that fails, I will have to try jury rigging two funnels to the manifold and use a long ladle to pour the gas in while driving.
It might be the easier way to go.

Ray B.

I would use only 1 funnel and a "T" fitting to maintain proper balance.

Balancing and mixture problem now solved.
Arm got sore from holding ladle, so an IV drip was installed and hung from the mirror.
There is still little bit of throttle lag when I open the petcock during passing speeds.
I'll have to sort that out later.

Thanks
Ray B.

You need to get the float levels even. float level is the gross mixture adjustment on an SU while the idle mix screw is the fine adjustment. .10 below the nozzle should be right. Even with the top of it is too rich. Also, make sure the needles are set to the same depth in the suction piston. The shoulder of the needle should be even with the bottom of the aluminum part of the suction piston - I usually use a razor blade or other straight edge to set them even with the bottom and then tighten them.

Brian:
I managed to get the levels fairly even. Both float valves ended up being replaced as one of them had a worn and leaking seat and was making the rear carb run rich.

The front carb was a pain to get it running on 2 cylinders at a steady beat.
I ended up having to disable to the rear carb (blocked the intake), set the mixture at 2-1/2 turns (.087"nozzle depth) and then adjusted the idle screw to open the butterfly to get it to idle at 600 rpm.
Since the throttle shafts are fairly worn, I put a temporary piece of rubber hose over the end (opposite of the tunbuckle side) and plugged the end to reduce throttle shaft air leaks while tuning.
Then I attacked the rear carb ,which was set at same initial mixture, and the balancing seemed alot easier.

The carbs are now balanced and the fuel mixture is pretty close.
Engine seems to run smooth at idle at about 750 rpm.
When I removed the plugs after a highway run, all electrodes indicated that the mixture was pretty close as they had a very light tan colored. Since the color was a little too light, I think that the mixture should be tweaked a bit richer so that I don't end up burning a valve.
#3 plug was a little darker that the rest, so I will have to re-check the valve clearances for that cylinder.
Hopefully thats not an indication that I've broken an oil ring on #3 piston.

Soon it will be time to put the car away for winter, so I wont have to worry about it until spring.
Maybe by then I will have saved up some cash to get some new or re-built carbs.

Thanks for responding
Ray B