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hogboy52

SU carb tuning guide L16, L18

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True that. I spent a number of hours looking everywhere online, trying to get this info, and little to nothing came up.

 

I bookmarked this one, and am seconding the motion to sticky it.

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Grassy Ass!!! = spanish for Thank you!

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I like this one.

 

Maybe more specific for those of us with British SUs?

 

101_1764.JPG

 

Who's got a Unisyn I can borrow?

 

I think mine are running OK right now, but I still need to do the valve adjustment first.

 

Thanks for the beneficial links!

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The gusset at the top of the rear dome should be pointing at the filters like your front one...

 

Thanks for the links!

 

Later

Joel

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Here's a bit of info on the Hitachi-SUs Flat-tops.  I've been told that these are the L18SSS carbs that came on Japanese crate motors. Though there is lots of info in this post, I could not find any help specific to the high-speed balancing of to these carbs. Note that there is no "service screw" to raise the engine RPM.

 

If you have these:

IMG_1485_zpsz1ncqnpb.jpg

 

Screws circled in red are the high speed balance screws, the screws circled in blue are the typical idle adjustments:

IMG_1487%20balance%20screws_zpsah7zwuns.

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The blue circled screws are for balancing the carbs using an air flow meter, the red circled screws are to adjust the sync of both carbs so they move at the same time when you give it the pedal, BTW I have never seen two screws before, usually there is only one on one side, mine is on the back carb linkage, maybe one is to balance them and the other is for idle adjustment, I adjust my idle with the blue screws and a flow meter.

The fuel mixture is on the bottom of each carb center, if you don't know how to do any of this, buy a book or look it up on the internet and read the directions, it's not that hard to do.

Most SUs came to the USA on 30,000 mile used engines from Japan.

Flat top SUs are smog SU type carbs, you can alway tell because of the triangular pieces of metal next to the blue circled screws that delete the smog part of the cabs, other wise there are all these tubes hoses involved between the carbs. 

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I used a uni-syn to adjust the idle balance using the blue screws.  I then raised the RPM to about 2000 and used the red screws.  I read, on one of the links above, about syncing at the higher speed being more important than idle as the engine doesn't spend as much time there.  However, I found that having both idle and 2000 RPM sync'd provides for a smooth transition when accelerating from a stop.  I adjusted the red screw pairs in the same manner as the blue screw pairs.  I tweaked each pair to get the same reading on the Uni-syn at idle or 2000 RPM. Your right, after reviewing all the linked articles on this thread, I could not find an example with a linkage like mine.

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The glove box Driver's Manual for Datsuns used to be very comprehensive.  My manual for my 1967 RL411 has the proceedure for regulating the front and back Hitachi "SU" carburettors.  Maybe scrounge up an original or reprint glove box Driver's Manual for your vehicle?

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I use the old school mgb owners trick. Manually lift the rear piston and adjust the front carb to get the fastest, smoothest engine speed...repeat for the rear carb lifting the front piston and done. Its amazing how quick and accurate it is, I've checked with a tool and it's dead on.

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I use the old school mgb owners trick. Manually lift the rear piston and adjust the front carb to get the fastest, smoothest engine speed...repeat for the rear carb lifting the front piston and done. Its amazing how quick and accurate it is, I've checked with a tool and it's dead on.

 

That is how I do mine except I go for the steadiest low idle, I want them to chug exactly the same front and rear without dying, then I go by plug color after that.

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After I balance and sync I raise the idle enough to keep it running and pinch off a fuel tube from bowel to nozzle one at a time with a hemostat and set the CO to 1.5 using my Sun 5 gas exhaust analyzer then the other. For some reason 1.5 + 1.5 = 4.5 when you take off the hemostat and back the idle back down to 800 so now that my CO is balanced I just lean the nozzles out the same about ½ turn. The trick is to lean them out until the HC just starts to go up then back of until it drops back to its lowest point. I usually end up with a 3.5 CO and a 400 HC depending on how warn out the throttle shafts are which equals around 28MPG

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