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Dual SU cold start issues and running rich when hot


mojojojo78

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Just a little history:

I bought my 510 back in January and have pretty much been daily driving it. A few issues have arisen that I was able to take care of, but I am not that familiar with SU carburetors. I understand the principle and how they work, but have never fiddled with them.

Its a Z22 block with an A87 head and a lumpy cam. I don't know the specs. It has the round top SU carburetors, a matchbox distributor and stock exhaust manifold. The matchbox was facing the exhaust and would start to run erratically when the engine would get hot. So I did the GM 4 pin module conversion. It runs good. I removed the ballast resistor and have a MSD Blaster 2 coil. It actually runs really good 95% of the time. The idle seems high at about 1000 RPMs. I tried checking the timing and I believe it was 10 BTDC. I have electric fans and an aluminum radiator with a 160 thermostat. It tends to run between 160 and 170.

So here are the issues:

Occasionally when it is hotter out like in the 90's it tends to run hotter, like 175 to 190. Especially when I have been driving it hard. When that happens it doesn't run as good and smells like it is running rich.

Then I noticed this just starting to happen as the weather really cooled down here. In the mornings it is really hard to start. Once it does it runs rough till warmed up. There have been a couple of times where it just won't start. I keep cranking it over but it never catches. Like it is out of gas. But it is not.

So it never had the choke hooked up and it doesn't have the heat shield behind the carbs.

I am not even sure where to get the choke cable. I believe it has two cables that hook up to one pull knob.

Also, I was looking around and read that the vacuum advance is better to hook up to manifold vacuum rather than ported vacuum. I believe right now it is hooked up to ported vacuum right at the front carb. Would it hurt to run it to manifold vacuum? There is a port on the top of the manifold that is capped off I could use.

1972 Datsun 510-Engine 03.jpg

1972 Datsun 510-Engine 08.jpg

1972 Datsun 510-Engine 15.jpg

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First - SUs are notoriously hard to start when cold if you don't have a choke. Trick - when cranking the motor over, crack the throttle a tad. This lifts the needles and allows the cranking vacuum to pull fuel in from the raised needles/well jets.

 

Second - you could rig up a choke system using non-oem parts. Look at how the British cars (like MGB or Austin Healey) connect the chokes. Worst case, you hook one carburetor choke up. It will help, but yeah, that would be weird.

 

Backing plate - get one or make one. It will help on hot days.

 

Vacuum advance - As a rule, I disconnect them on all Datsun L motors and modify the curve of the distributor to make sure the timing is correct at all engine RPMs.

 

Running hot - is your radiator a 3-row? If not, have it re-cored.

 

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Check for oil in the tops. They use the oil up and will run rich when low. Add oil each oil change.

Try to  leave the balance alone. More often than not imbalance is from valves needing adjustment.

Vacuum advance plates in the distributor will wear quickly. Manifold vacuum is much less than venturi vacuum and will not work well, but then again it may be the same as no vacuum at all. Unfortunately as the advance plates wear, the advance may stick somewhere and totally mess up the advance curve function. I find a carefully set and fresh vacuum advance is only worth about 10% fuel economy and not much worth the effort.

Idle advance is much less important than the advance at torque peak and full advance. Ping setting is trouble, first ping happens at torque peak and that will not be audible. Cam changes alter all of this. Larger displacement does too. There is a big difference with 2200ccs instead of 1600cc. Big bores burn slower, high compressions burn faster, big cams fill the chambers slower, and so on. Like Matt says, recurve the distributor for the newly reengineered package.

Free advice on the internet isn't of value here. Maybe mine too.

"Lumpy" cams are trouble with SU carbs. They are CV designs that work with manifold vacuum more than throttle position. Reduce the manifold vacuum and they will not open as soon, just the opposite of what the cam is trying to get. Leaving the oil out is a poor work around on street cars. It works OK on an IT spec racer but all you want is WFO there. Where angels fear to tread......

1000rpm idle speed with "lumpy" cams are about right. Stock is about 800rpm.

Datsun Roadster choke cables were the original part, they may still be available. Pretty close to MGB parts. 240Z parts can be used if you want the lever by the shift lever.

These motors like to run warm, 170 is usually too cool. 

38mm carbs on a cammed 2200 seems small, but I have not been happy with 42mm Z carbs there either.... 

Dennis

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Thanks for the replies. I have been looking at making my own heat shield at work.

What is a good temp for it to run at? It is a three row.

The sticking vacuum advance plate would make sense.

Are there any points on the throttle linkage or even in the distributor that should be lubed regularly? I am just thinking that when it does get up past 175 maybe something is sticking

I have looked at this for another car:

https://www.cbperformance.com/product-p/2013.htm

Any body ever heard of this or used one?

38mm might be small, but that is all I have. It does seem to lose breath past 4000 RPM which is around 90 MPH. Which I do not do that much. I usually keep it around 75-80 on the freeway.

Edited by mojojojo78
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An engine should run at or around the temp of the thermostat rating. ie - if you have a 190 degree thermostat, it will run up to the temp of the t-stat rating, then the t-stat will open, allowing cooled liquid to flow into the engine and hot coolant to escape to the radiator. That's the theory anyway.

 

Datsun motors like to run at around 190 degrees F, so if you've got a cooler t-stat in there, consider replacing it with a 190 degree. Just for reference, an L motor is overheating at 230 degrees.

 

I would avoid installing modern electronics when you can get the job done with a simple re-curve.

 

The backing plate doesn't need to be fancy. Just a large oval that connects the two carbs, and is large enough to shield the intake air from the heat of the exhaust manifold. Racers build heat shields that go under the carbs to keep the float bowls cool too. Our MG Midget racer has adhesive heat barrier wrapped around the float bowls.

 

Note - when cutting the holes in the backing plate, make sure you cut the breather holes too. Those are the holes above and to the sides of the the main throttle bore.

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1 hour ago, mojojojo78 said:

 

https://www.cbperformance.com/product-p/2013.htm

Any body ever heard of this or used one?

 

Well, sure but....

I have spent many hours on dynos to try and reengineer distributors and ignitions, "Tune them" as Matt calls it. The CB piece looks great, I had Brian Rebello do this sort of thing with a Microsquirt controller some time back on a sweet L20b with Fuel injection and electronic ignition, but it is a costly process. It cost as much as the built engine did. I talked to Electromotive at SEMA a few years ago about my fancy 2 liter Cosworth Ford and they started at about $10K to "tune" it. It is a museum piece now I guess. Peggy calls them rabbit holes. I just get a little enthusiastic when I do things I guess, but without the "tuning" equipment it is kind of a disappointment. Like Matt says, if simple works, do it.

Those round holes on the front of the carbs are the things that actually allows the slides to rise and function more or less like throttle plates so you need them open for sure.

Dennis

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As far as a choke, there really isn't one. The mechanism drops the jet and makes a richer mixture. The SU slides are always a choke.

On my ITS spec 240Z I just use a piece of copper electrical. #14 solid, and pull it up, hook it onto the heat shield until warm, then release it and go hammer the old girl for an hour or so. The wire has lasted about 20 years now....

Dennis

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The SUs should have a vacuum advance on the front one. It's located under that red plug. Use it, NOT the intake vacuum. 

 

Maximum advance at WOT is around 32-34 degrees for an L series. At part throttle the cylinder is way less than completely filled and the compressed mixture takes longer to burn. In these cases (which is 99.999% of the time when driving) more advance can be added to get the maximum cylinder pressure to happen at just the correct position after TDC. This may be as much as 12.5 distributor degrees... 25 crankshaft degrees on a '79 matchbox distributor. Giving a total advance of over 55 degrees, again under certain part throttle conditions.   

 

CTik9TI.jpg

 

L series like no lower than a 180F thermostat. If you run hotter then it's not the thermostat's fault. You're running a 9.8 compression 37+% larger displacement on a 1600cc capable radiator.

 

Dash pot oil in the SU's slows the opening of the slide when throttle is suddenly opened causing a rich condition. This in effect acts like an accelerator pump.

 

The choke manually lowers the tapered needle in the jet allowing more fuel in for a rich start.  

 

1972 Datsun 510-Engine 03.jpg

 

1/ Those small open pipes sticking up from the float chambers are the overflow tubes. Get some hose on them and run down past the engine mount. If a float should stick you don't want a gasoline fountain spraying all over a hot exhaust!!! Better it blow under the car.

 

2/ Where's that fuel hose going after the rear SU????????

 

3/ Make your own heat shield. I bought a $5 aluminum serving tray and cut holes in it. Not rocket science.

 

YwDun0Y.jpg 

 

lIgVZ7q.jpg

 

Under dash choke pull has dual cables. This one may be from Roadster.

 

 

1972 Datsun 510-Engine 08.jpg

 

22100-?????. (just below the D4K8-08) Get the last 5 digits and I can tell you the year and application for this matchbox.

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9 minutes ago, datzenmike said:

L series like no lower than a 180F thermostat.

 

1/ Those small open pipes sticking up from the float chambers are the overflow tubes. Get some hose on them and run down past the engine mount. If a float should stick you don't want a gasoline fountain spraying all over a hot exhaust!!! Better it blow under the car.

 

2/ Where's that fuel hose going after the rear SU????????

Thanks for the timing numbers.

That is a good idea about the overflow tubes.

That fuel line goes to the last carburetor and ends there. I do not have any type of return line to the tank.

I like that heat shield idea. Simple and effective.

20221024_181233 (2).jpg

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Have a look on the right side near the fuel lime for a return line. Note that there is a tank vent line that ends on the driver's side strut tower, you don't want that.

 

You could easily source an L20B fuel line between the pump and the carburetor that includes a return. This had the restriction on the outlet that makes the pump push against to raise the pressure so the fuel will go into the carburetors rather than take the easy path back to the tank.

 

This is the return end with small hole to let fuel back into the tank. The return allows cool fuel from the tank past the carburetors and returns stale hot gas. Makes hot restarts much easier. In addition the tank fuel is slowly run through the fuel filter over and over.

11aMYIJ.jpg

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8 hours ago, mojojojo78 said:

Occasionally when it is hotter out like in the 90's it tends to run hotter, like 175 to 190. Especially when I have been driving it hard. When that happens it doesn't run as good and smells like it is running rich.

 

 

How does it smell rich? gas smell?? Different if you have an air/fuel gauge reading or the plugs are caked with soot. A gas smell could be anything. Such as gas boiling in the carburetors from the heat and escaping out those two vent pipes.

 

 

 

 

8 hours ago, Stoffregen Motorsports said:

 

 

Vacuum advance - As a rule, I disconnect them on all Datsun L motors and modify the curve of the distributor to make sure the timing is correct at all engine RPMs.

 

 

This is fine for racing. Vacuum advance is for part throttle where the static and mechanical timing is not enough. 

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One trick I did for the return line incase you don't have the metered orifice line....  Use an old air corrector from the carburetor and stuff it in the rubber line going to the return...

I have no idea what size, I used the smallest I had and drilled it out slightly.... you want just enough to flow past but not an open tube or your pressure would drop....

I used a fuel pressure gauge to make sure I still had consistent pressure. 

 

 

 

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I tried the part throttle when starting and that helped a lot. It only took a couple of tries to get it started this morning. I am going to run by the Autozone by my house and see what universal choke cables they have in stock and at least get something hooked up.

I didn't see a return line at all. It comes up by the fuel line right? Did the 72's have a return line? I had seen the vent line by the drivers strut tower and never knew what it was actually for, now I do.

How exactly does the return line plumb into the SU Carbs?

Also, another member mentioned the distributor number. I looked and it is pretty worn down. I am going to see if I can get a better look at it in better lighting. What does the last 5 start with? It looks like B9512 right now.

I do have the two vent holes lined up right under the air filters.

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Try your Datsun/Nissan dealer for roadster choke cables.  They come in various lengths to accommodate 2- or 1.6-liter engines.  get the longest one they have in stock and trim length to fit.     Tin the cut end before installing [and don't forget to neutralize the acid core solder which you will mistakenly use] and the cut end will fit the carb "socket" just fine. Also, they will not fray.

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A hint to ease cold start problems-Jiggle the protruding wire on the bottom of the float valve casting.  It is there to clear the valve when it gets stuck by debris, but jiggling it splashes just a bit of fuel into the main body and eases starting ,

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16 hours ago, datzenmike said:

 

This is fine for racing. Vacuum advance is for part throttle where the static and mechanical timing is not enough. 

It's not just fine for racing. No engine needs vacuum advance. It's completely optional.

 

Adjusting the  spring tension allows more centrifugal timing, just like vacuum advance does, except without the variables introduced by engine vacuum. It's done in a more controlled manner. Limiting the movement inside the slots puts a dead stop on the advance so it can't go over a prescribed amount.

 

After all these years, I feel like I'm still trying to convince you that this is an acceptable modification for street driven cars. I'm only one guy and I've set up dozens of street Datsuns without a vacuum advance. And there are many others doing it too. It's even discussed in the popular Datsun modification books. Nissan even sold the parts to lock out the vacuum advance, for street cars and race cars alike.

Edited by Stoffregen Motorsports
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47 minutes ago, Stoffregen Motorsports said:

It's not just fine for racing. No engine needs vacuum advance. It's completely optional.

 

Adjusting the  spring tension allows more centrifugal timing, just like vacuum advance does, except without the variables introduced by engine vacuum. It's done in a more controlled manner. Limiting the movement inside the slots puts a dead stop on the advance so it can't go over a prescribed amount.

 

After all these years, I feel like I'm still trying to convince you that this is an acceptable modification for street driven cars. I'm only one guy and I've set up dozens of street Datsuns without a vacuum advance. And there are many others doing it too. It's even discussed in the popular Datsun modification books. Nissan even sold the parts to lock out the vacuum advance, for street cars and race cars alike.

 

I still have some old Nissan parts catalogs.   If anyone wants it, I can post the part number for vacuum advance delete part number

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2 minutes ago, MikeRL411 said:

 

I still have some old Nissan parts catalogs.   If anyone wants it, I can post the part number for vacuum advance delete part number

I don't think the parts are available anymore. Not new at least. I'm sure there is a stash of NOS parts out there somewhere.

 

But even so, a stock distributor can be set up the same. It just takes a bit more time.

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17 minutes ago, Stoffregen Motorsports said:

 

I don't think the parts are available anymore. Not new at least. I'm sure there is a stash of NOS parts out there somewhere.

 

But even so, a stock distributor can be set up the same. It just takes a bit more time.

 

I was referring to the old NISSAN speed parts catalogs. The replacement part I believe included a recalibrated vacuum advance cam.

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4 hours ago, Stoffregen Motorsports said:

It's not just fine for racing. No engine needs vacuum advance. It's completely optional.

 

Adjusting the  spring tension allows more centrifugal timing, just like vacuum advance does, except without the variables introduced by engine vacuum. It's done in a more controlled manner. Limiting the movement inside the slots puts a dead stop on the advance so it can't go over a prescribed amount.

 

After all these years, I feel like I'm still trying to convince you that this is an acceptable modification for street driven cars. I'm only one guy and I've set up dozens of street Datsuns without a vacuum advance. And there are many others doing it too. It's even discussed in the popular Datsun modification books. Nissan even sold the parts to lock out the vacuum advance, for street cars and race cars alike.

 

And yet every Datsun and carbureted engine with a distributor has vacuum advance. It's there for the reason I mentioned. Yes it will run fine without it but part throttle efficiency needs more than the static and centrifugal at part throttle. Vacuum advance tailors the advance in elation to engine load.

 

Yes you can alter the centrifugal advance to come in sooner, I agree. I only don't know why it is so delayed by the car makers in the first place.  

 

 

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17 hours ago, datzenmike said:

 

And yet every Datsun and carbureted engine with a distributor has vacuum advance. 

 

I only don't know why it is so delayed by the car makers in the first place.  

 

 

So what? Vehicles used to come with smog pumps too. And road draft tubes. And updraft carburetors.

 

That's a separate issue. Your not knowing why is not germane to the discussion.

 

 

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So not to throw gas on the fire, but I disconnected the vacuum advance hose and plugged up the ported manifold connection. Drove it to work this morning and it seemed fine. I only noticed that it ran slightly different at slower speeds when under load. But it wasn't that noticeable. This makes me think that I need to replace the vacuum advance.

I am going to see how it drives home in worse traffic. Hopefully this will give me a better idea.

I did hook up some choke cables to see if that would help and they did. So now I need to find a more permanent nicer solution.

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