Vacuum advance hitachi SU's

[blackdeth666]

So I decided to while I was doing the head gasket job on my 1974 620, I might as well put on my set of su carbs from an old project on while I was in there. Everything is hooked up in what I think is the correct order, but I cannot find where to hook up the distributed vacuum advance! Is there supposed to be an inlet here where the pencil is pointed?

My other truck did not have a brake booster, but the 620 does so the barb on the back runner of the intake is hooked to that. That's the only spot where I think I may have had it hooked up to on my old truck. But from what I've read that isn't even the correct spot.

 

It fires right up and the timing is set, but it lacks power under acceleration, which seems about right for no VA

[Stoffregen Motorsports]

If performance is your game, then I would recurve the distributor and delete the vacuum advance. I am a one trick pony in that regard.

 

Most guys here on Ratsun will tell you to run a vacuum advance, but in my experience, doing what I suggested above is well suited to the average Datsun owner and the way they like to drive.

[MikeRL411]

On the rear carb, close to the firewall, is a vacuum line connector.  That's where the vacuum advance connects.  Original vacuum advance connector is a push on rubber hose.  Aftermarket and late roadster connection is a screw on connector with mating fitting on the carb.

[MikeRL411]

The Nissan Performance Parts catalog used to list a non-vacuum advance mechanical only advance for this engine.

[blackdeth666]

Not really going for all performance all the time, I just cruise with the occasional 'give er hell' moment. But what do you mean by recurve the distributor?

 

MikeRL411- I'm looking high and low and not seeing anything back there, I don't believe it is that banjo bolt I just have that running into the atmosphere same with the front for a breather.

Here's the view between the carb and firewall

[datzenmike]

I have late L16SSS or later L18SSS SUs. The vacuum advance is on the rear of the front carb.

 

Vacuum advance is almost non existent at full throttle, so really it only affects part throttle driving and cruise speed . If you have poor performance, maybe the jets are poorly matched for your engine size. Take it for a hard drive pull over and read the plugs. If very light brown or white you are way too lean. You might ask what someone else with the same engine/carbs are running to get you closer.

[blackdeth666]

Datzenmike thank you for the insight. I've done nothing in the sort of tuning yet I was just looking to get everything hooked up first. There's a very likely chance that they just need tuning to get optimum performance! I was told they were from a Japanese import motor, any they are on a stock (other than matchbox dizzy) L18

[wayno]

I have never seen a set of SUs without a vacuum advance orifice except on British SUs.

It is on the front carb on the very bottom almost under the bore behind the fuel bowl pointed forward, it's placed in a great spot for the L block dizzy.

[blackdeth666]

Well I've gone as far as taking the front carb off to look for this elusive port and still can't find it...

Here's a bottom view

[wayno]

Second photo, see that hole, that is it, but it doesn't have the fitting sticking out, that is where mine all are.

[datzenmike]

I wouldn't piss on photobucket if they caught on fire. Mine is behind the front flattop. Has a little red plastic plug on it.

[LenRobertson]

Since I love a mystery I had to go look at my SU/220 manifold setup which isn't mounted on an engine (although the urge to stick them on my 510 is almost overwhelming. It is supposed to get near 100F here this week so that is probably when I won't be able to restrain myself. Oh well, keeled over in the engine bay of a 510, wrench in hand, isn't a bad way to go. :lol: ).

 

Right where Wayno said, the hole in your second pic is where I have a short piece of brass tubing sticking out which had vacuum hose on it. I think this is pressed in not threaded. Possibly your tubing fell out. If you can press a piece of plastic tubing or vacuum hose against that hole and blow through it, listen for air blowing out inside the manifold. I can hear it on mine but can't see the hole inside the manifold. I can measure the diameter of the brass tube if you need that spec.

 

Len

[blackdeth666]

Datzenmike hahahaha I was almost loosing my mind trying to upload those pictures. Between the pos site and my mystery on hand. Geez. It's bad. any recommendations on image hosting? I concluded that if I'm going to run the SUs I need to know more about them and how to properly set them up. Or get rid of them so I'm not tempted to complicate my life agin lol. So... I put the stock carb and manifold back on. Not very exciting but I just need to get my truck back on the road. Thanks guys for helping me figure that out!

[Lockleaf]

Theres lots of threads about pic hosting now. Look in the general discussion area

[blackdeth666]

Thanks lockleaf I'll look into it. I guess I never think to look on here for non Datsun stuff

[blackdeth666]

Len, you and Wayno are correct! I was able to run the same test as you and got the same result. I guess I need to make a new pin to press in there, or possibly tap it? Any measurements would be great!

[wayno]

Just find a brass tube that fits in the hole snugly, or any other metal tube for that matter.

[Draker]

If you can't find a brass tube that fits... Call z therapy. They can get one in the mail to you. Salem, or.

[distributorguy]

Hobby shops that sell RC cars and planes will have that tube.  

Ported vac like you have WILL increase drivability when you're using parts throttle and accelerating/decelerating a lot.  It'll have more "life, and spirit." 

 

Recurving is when a guy like me alters the timing curve that's built into your factory distributor to make timing optimal at ALL rpms.  The factory setup is not only worn out by now, but it never really was ideal.  Compromises had to be made to ensure every vehicle could pass emissions testing.  The truth is that with a recurve, you can EASILY pass emissions testing because it'll be in a better state of tune all the time, at all rpms.

[Stoffregen Motorsports]

Recurving is when a guy like me alters the timing curve that's built into your factory distributor to make timing optimal at ALL rpms.  The factory setup is not only worn out by now, but it never really was ideal.  Compromises had to be made to ensure every vehicle could pass emissions testing.  The truth is that with a recurve, you can EASILY pass emissions testing because it'll be in a better state of tune all the time, at all rpms.

Yes, grooves get worn into the distributor cam over the years also. Sometimes these can be fixed with a small file. And you can weld up these slots a little to limit the total mechanical advance. Nissan Motorsports used to sell a "SSS" distributor cam that had the limited mechanical advance built in. I wonder if the solid breaker plate and distributor cam are still available.

 

The way I was taught to recurve a Datsun L series distributor was to either install a SSS dist cam or weld the stock cam's slots, then remove one of the springs that hold the centrifugal advance. With a solid breaker plate (no vacuum advance) the initial timing should be around 12-15 degrees, and total timing should be around 32 degrees. I probably built 100 or so like that.

[LenRobertson]

Len, you and Wayno are correct! I was able to run the same test as you and got the same result. I guess I need to make a new pin to press in there, or possibly tap it? Any measurements would be great!

 

I tried a couple of different mics on outer diameter of the vacuum advance tube and got a reading of 0.196 inch. According to my decimal equivalents chart that is just under 5.00mm. I don't know if that comes close to any standard tubing size (metric or imperial). On the equivalent chart, 0.196" falls about half way between 3/16" and 13/64".

 

As for the inner diameter, a 5/64" drill bit fits into the tube with a slight amount of wiggle room. You will probably be okay with whatever tubing you can find that is close to these dimensions. A bit of a press fit seems best since the hole in the aluminum manifold will expand with engine heat.

 

There is 15mm of tubing sticking out of the manifold.

 

I measured the vacuum tube on the base of a stock Hitachi 340 2 barrel carb and got 0.199 inch, only 0.003 thou larger than the tube in the SU carb. If you have an original Hitachi and can heat the base enough to extract the vacuum tube, you may be able to use that.

 

It is worth checking the hole in your SU to be sure there isn't any broken off piece of the original tube still in there. Unlikely there will be but easy enough to check.

 

Len

[datzenmike]

wayno... Len, you guys are spot on. Just for giggles I went out and checked mine. I was told it was on top but it has the small brass tube sticking out just where you said it was. I stand corrected.