SU questions - wear and needle interchange

[DatsunDave]

Hi all,

Firstly, sorry for not owning a British car...

I own and race an old Datsun 1600 (or Datsun 510 in the USA) which is fitted with Hitachi "SU clone" carbs - basically a pair of Japanese-made copies of HS4 SU's. I'm currently upgrading these to genuine British HS6 SU's and have a few questions I was hoping someone could help with.

Firstly, how much wear is "too much" in the throttle shaft ? Is there a specification or some sort of empirical measurement (ie xxx thou deflection) ?? I am rebuilding the carbs before fitting them, and have never had them running to know if there were issues with them. Throttle shaft play seems to be fairly minimal, I'd like to know for sure before deciding whether to get them rebushed.

And secondly, I have had someone with a near-identical setup to mine suggest "BAZ" needles as a good starting point for tuning purposes, but my carbs are fixed needle rather than the "swinging" BAZ ones. Is there a way to convert the swinging BAZ needles to be able to be used in the fixed-needle carbs ? Otherwise, what is the best way to determine a close equivalent - I've downloaded a few of the SU needle comparison programs, but I'm not sure how much variation in needle profile is OK before there are noticeable performance changes?

Thanks in advance,
Dave

 

[eschneider]

throttle shafts - the real question is, "do you have a vacuum leak"? Simplest test is to squirt carb cleaner at the bushings with the engine idling -- if there is a dramatic change in engine RPM, then the engine is sucking the flammable stuff through a vacuum leak. Be sure to test both at idle, and just off-idle (shafts might not leak at idle, but leak like heck once they turn a little)

If you're gonna play around with needles, the best way to go is to acquire or build a wide-band oxygen sensor setup, and prepare to buy a few sets of needles to play with. It's a delicious game if you're a test-n-tune kinda guy.

I've re-profiled some needles using a drill press and wet-dry sandpaper, but I am certain it wasn't very accurate.

Hope this helps.

 

[DatsunDave]

Thanks for the reply. I guess that is the best way to do it - I was hoping to be able to "measure" somehow without having the carbs on the car.

I might end up re-bushing them while they are dismantled just to be sure , there were bushes included in the rebuild kit I bought. Better than rebuilding them without doing the shafts, fitting them to the car, then finding out I really should have rebushed the shafts.

 

[JPSmit]

The guy that just rebuilt my carb said if you can do Morse Code with the shaft it needs rebuilding. You could definitely feel the wobble and make it click - you can't do either any more. Another way to consider is the mileage if known. I remember reading somewhere that if the carbs got more than about 40k miles on them they were ready for a rebuild.

 

[GregW]

My opinion: If you're using them in your race car, re-bush them. You'll want the carbs in top form.

 

[texas_bugeye]

Dont know if this holds true for your clones but the SU HS2 has a habit of wearing the shafts and not the body at least not much. Now if yours has actual bushings instead of riding directly on the aluminum body than this pattern is not present. Most HS2 carbs can get by simpley replacing the shafts with new ones opting out on installing bushings at least on the first go around. Installing new bushings not a bench top fix .

 

[DHC120]

Dave;
I, also, own a Datsun "1600". (Datsun 510).. I bought mine new in April 1970.. I am a "Factory Trained" Datsun Mechanic.. worked on the 1600 & 2000 (Two Liter) Roadsters (did a LOT of Five Speed fixes), saw the first 521 Pick-up roll off the truck, a month after we put an L-16 OHC Engine into a 520, in place of the 1300cc pushrod.. Also worked on the first 240 and 260 "Z-Cars", as well a few "Fairlady's (1500 Roadster) 410's, 411's, and the odd Nissan Patrol, etc.
I "converted" my 510 to the "SSS" Set-Up, using a set of SU HS-4's off an MG-B... These were easier to find, rebuild, etc. than the Hitachi metric "SU Clone" carbs.
With respect to you fitting the SU HS-6's.. If you haven't done a BUNCH of "Speed/Power Modifications" to the L-16 engine, you are going to find, after spending a LOT of money, that the HS-6's are WAY too much carb for normal driving (My 3.5L Six Cylinder DOHC Jaguar has twin H-6's, my '62 Volvo 122S (w/B-18 4cyl Engine bored to about 1.9L) has HS-6's.. The L-16 just doesn't have the displacement to use, much less "need" the 1.75" Carbs.
In my Datsun 510, I'm using the "#5" fixed Needles in the 0.090" Main Jet and it's been working fine for several years of daily driving.
As the "swinging needles" were fitted to the "HIF" SU Carbs used on "later" (early) Emission Control engines, I feel you are shooting yourself in the foot using "smogger motor" carbs.
"Tuning" via needle changes.. Been there, Done that, back when it was cheap to buy needles (and working at the dealership, I could "try" and not "buy" until I found what I needed).. Each needle "number" has between 13 and 16 "stages", depending on it's length, usage/application, etc. These stages are 1/16" apart.. and hard to determine "where" one stage starts and another ends because the needle is tapered. The stages are different diameters, sometimes varying by only 0.001", sometimes 2 or 3 thousandths. To "tune" the SU's (even the clones), you MUST be able to "hear" if the engine is running lean or rich at each throttle setting (while driving!!).. If, as an example, you are "cruising" at 1/4 throttle and you discern an occasional "pop" back thru the carbs, then you are running lean at THAT stage of the needle.. depress the throttle a "tad more" so the Piston rises another millimeter or two, and the engine runs smooth, then the mixture at THAT stage is "just right".. so, you go back, measure the stage that is "hanging" in the jet at 1/4 throttle, reduce the size of ONLY that stage by 0.001" and try again, see if that enriched the mixture enough at the 1/4 throttle stage..
Now the rub comes in.. At exactly what point is the Piston in the Dashpot when your are cruising at 1/4 throttle? Depending on the level of vacuum being produced, it is not impossible to have the piston "floating" half open while cruising at 3/4 throttle or more.
Enter the Dashpot/Piston Springs. Generally speaking, the MG-B stock springs in the HS-4's are just fine for the Datsun 1600, but now and again, too much piston "flutter" can happen resulting in a need to change to the next heavier spring.. Since this will hold the piston (and needle) "down" more, the mixture (stages of the needles) are now going to show lean across the entire throttle spectrum.. A simple "mixture adjustment" by lowering the needles (enrichening the mixture) is generally all that's required.. But, a lot of WOT running might show lean on top (risk of "holing a piston"), so it's back to tweaking the needles..
If you are lucky (and really good), you can sort out the carb mixture problems before you need to rebuild the engine from all the test driving you had to do..
Just remember, Too Rich is better than Too Lean! Sparkplugs are a helluva lot cheaper than pistons!!
Lastly, and others have answered this, but just to emphasize, if there is ANY lateral movement (up/down, left/right) of the Throttle Shafts in the carb bodies, then replace the shafts AND have new Bronze Bushings fitted into the Carb Bodies (there in place here in Ft. Worth, Texas that charges $5.00USD per bushing, installed.. Takes less than ten minutes to do! There is an SU carb place in New York (city? or state?) that was charging $45USD PER Carb Body for JUST installing the bushings.. Total rip-off for no more work than it is..
A loose (even "barely loose") throttle shaft in the carb body WILL result in a burned valve!!(Pull the Throttle Shafts and visually inspect the rubbing surface.. may times, the throttle shaft has heavy wear as a result of a heavy carb return spring!) And it doesn't take long, about three months is all, then it's valve job time!!
Don't know what suppliers/vendors you have "down under", but Moss Motors (in California, USA) offers all the "nit-picky" SU Carb Parts that you would ever need, and having four daily drivers fitted with SU Carbs, I've bought a LOT of Carb Parts from Moss!! I buy certain "perishable" (rubber) items "in bulk" just to have on-hand.. just in case! Check-out their website.
Didn't mean to get so windy, but, if I may be so humble to say, I'm pretty good on SU Cars (had a great teacher and mentor back in '68).. and I started out on the Hitachi SU Clones fitted on the '65-'70 Datsuns.
Charles.