SU Carburetor Issues

[mojojojo78]

I have been having an idling issue when it gets hot out.

I have a Z22 with an A87 head. I am not sure what cam profile is in it, but it is pretty lumpy.

Distributor is the matchbox style but with a GM module and MSD Blaster 2 coil.

Dual HS4 SU Carburetors.

160F thermostat. Aluminum radiator with dual electric fans (manually operated for now).

So when it is 75F or cooler my car runs nice. The idle is lumpy but smooth. RPMs are around 700-800. Engine temperature stays between 160-165F.

Once the temperature gets up above 80F the idle starts to drop to around 500-600 RPM and the engine temp will rise to around 170-175F.

I don't have any experience with the SU Carburetors. I know how to set timing and adjust a normal carburetor, but these are like voodoo to me.

Could the dampening oil be getting to hot changing the viscosity and letting the plunger rise too quickly or not rise enough?

Is there something else that could be going on?

[Stoffregen Motorsports]

No, not the dampers. I'd say it's running lean once the temp comes up. There is an adjusting nut on the bottom of the carb (called a well jet or main jet) that can be turned by hand. Adjust it down to richen the mixture. Adjust it up to lean it out.

[mojojojo78]

Thanks, I will try that. Is it like adjusting the idle on a regular carburetor where you turn them 1/8-1/4 turn at a time?

[Stoffregen Motorsports]

Yes, but keep in mind, this is not an idle mixture adjustment. It's an entire spectrum mixture adjustment. It will affect the entire RPM range at any level of throttle input. And at idle.

 

You'll want to make sure the balance is good too. And it can be tricky. Best way is to loosen the tie rod between the two carbs and set each idle on each carb individually, then tighten the cross bar (or tie rod) clamps. Here - https://www.mgexp.com/forum/mgb-and-gt-forum.1/su-dual-hs4-throttle-idle-adjustment-69.4039601.4039615/

 

Are you sure they are HS4 SU carbs? Not Hitachis?

Edited June 23, 2022 by Stoffregen Motorsports

[mojojojo78]

I was able to take a look at them over the weekend. I am pretty sure they are HS4. There are no markings that I can see, but they have the round top and not the flat top.

What I did notice was that it looked like there wasn't oil in the damper piston. How exactly do you check and fill these? What is the best oil to use in these?

Would not enough oil make it run rich somehow? It just has that too much gas smell sometimes.

I haven't messed with any of the idle since I don't think the oil level is correct.

[MikeRL411]

49 minutes ago, mojojojo78 said:

I was able to take a look at them over the weekend. I am pretty sure they are HS4. There are no markings that I can see, but they have the round top and not the flat top.

What I did notice was that it looked like there wasn't oil in the damper piston. How exactly do you check and fill these? What is the best oil to use in these?

Would not enough oil make it run rich somehow? It just has that too much gas smell sometimes.

I haven't messed with any of the idle since I don't think the oil level is correct.

 

I use 3 in 1 or Singer sewing machine oil. Whatever you use, don't use multi grade [e.g.,10W30] oil.

[jagman]

I use a non-detergent 10Wt in mine.

[Stoffregen Motorsports]

Marvel Mystery Oil

 

HS4 SUs and round top Hiachis look very similar, but they are, in fact, different carbs. The SUs are the OG, made in the UK carbs and Hitachis were Japanese speific to Datsun. Tuning will be more or less the same, but parts are vastly different, or at least, not interchangeable.

 

Read these pages -

https://www.ztherapy.com/technical_stuff/spotterguides/zcar/240spotter.htm

https://zparts.com/index.php/resources/su-carburetors-explained/

[mojojojo78]

That was an interesting read. I will take a look at the carbs more when I get home. One thing I noticed was that mine have a two bolt flange at the manifold and not a 4 bolt square flange.

What would cause the oil to leak out or get sucked out thru the intake? The wrong oil? I am not sure what the previous owner had been using or if they even did any real maintenance on this car.

[Stoffregen Motorsports]

Some tuners remove the oil so the jet opens more quickly, allowing for more fuel right away. Race cars don't use oil.  Sometimes even the springs and inner pistons are removed.

[mojojojo78]

17 minutes ago, Stoffregen Motorsports said:

Some tuners remove the oil so the jet opens more quickly, allowing for more fuel right away. Race cars don't use oil.  Sometimes even the springs and inner pistons are removed.

That is very interesting. So running with no oil won't hurt it, but it won't help normal driving performance either. How does that affect idling?

[Stoffregen Motorsports]

Lack of oil, or the wrong oil won't affect idle. It's the spring that keeps the dashpot closed at idle.

 

EDIT - assuming your idle speed adjustment and balance are correct.

Edited June 27, 2022 by Stoffregen Motorsports

[datzenmike]

The damper oil delays the slide or piston from opening too soon. The throttle is wide open but the piston is slightly delayed in opening. Beneficial at low speeds. Think of it as the equivalent on an accelerator pump on a regular carburetor. The throttle open but the slide delayed causes a slightly richer mixture to prevent lean bogging from the sudden opening. 

 

 

[620slodat]

Interesting! When I had a Roadster (many, many years ago) I wish that I had Ratsun around.

 

Don

[datzenmike]

I have a set of flattop SUs and intake. Was told late L16SSS and L18SSS had these. Also have a pair of HS6 SUs from a '70s Volvo. They don't have the external float chamber

[Stoffregen Motorsports]

HS6 is a big carb. If it is internal float, it is an HIF6.

[datzenmike]

The L16/18 SUs were 38mm or 1.5" correct?

 

The Volvo was a 2 liter? engine. These are only 40mm or 1.57"

[RyanC]

I always found with my sus years ago on l16and l18 using a balancer to get them set the same made a big difference to idle and response.  I think I used some light engine oil on the pistons sparingly.

 

I ran her out of fuel one day and I think it was hot and it cracked one of the brass floats so replaced with newer plastic floats after that.  

 

I also found heat kept killing my ignition module on the dizzy so built a heat flange to help protect it from no1 header pipe but hot days sitting in Sydney traffic were never fun.