Questions about tuning datsun SU carburetors

OK, TL:DR; my car has an L24e engine from a maxima and a 240Z intake with a pair of SU carbs.  It idles OK, and runs alright if I rev it up strongly.  In between, though, it is sputtering and trying to die, especially if the engine is not warmed up sufficiently.  I would like to drive it on the street without sputtering/backfiring/dying. I need to diagnose and fix the issue.

long version:

Here's a video of problem occurring yesterday.  

This morning it was also backfiring badly.  Here's the same issue without the air filters obscuring the backfire:

Later today when both the weather and the engine were warmer, the symptoms were greatly reduced.

The reading I have done about carburetor tuning says that there are many other things to make sure are correct you before you tune the carbs.  I don't know if any of these could cause the symptoms described above:

• could these fouled spark plugs cause the problems described above?
• Could the symptoms be caused by vacuum leaks cause these symptoms?
• What about throttle linkage?  Could this happen if one carb is activated before the other

This is the same thing I was describing over on my build thread but I thought a new thread might attract more attention, and also, be easier for Datsun owners to find in the future if they have similar issues.

In reply to JoeyM : most carb problems are really electrical problems.  Fix the ignition system first,  replace all old gasoline with clean fresh gas. Make sure vacuum hoses are good and not loose or cracked. 

Then start cleaning the carbs if it's not running right. SU carbs are dirt simple.  

frenchyd said:

In reply to JoeyM : most carb problems are really electrical problems.  Fix the ignition system first,  

Could fouled plugs produce those symptoms?  That's electrical, and I know that if plugs were the problem it would be a simple fix.

replace all old gasoline with clean fresh gas. Make sure vacuum hoses are good and not loose or cracked. 

Then start cleaning the carbs if it's not running right.

I know that the carbs could be cleaned.....

....and that the air flow through the carbs is not different.....the front carb is doing more, and I can hear more noise from it.  I think that's from air going through, but I'll be honest; it could be that the front has a vacuum leak.  Anyway, here's a front and back  comparison:

SU carbs are dirt simple.  

It's lean.  Drop the jets a bit.

Streetwiseguy said:

It's lean.  Drop the jets a bit.

Thank you.  I will try that if I can.....I don't know if the carbs are Hitachi HJG46W or Hitachi HBM46W.  It looks like the former model has movable jets and the latter model does not. (Zparts.com)   I will start looking for adjustment screws and such.

The second carb you listed has a flat top, yours are the earlier and better round tops.

Vacuum leaks can cause some weirdness, and if your plugs and wires are fresh, I think the issue lies in the carbs.

Here's the balancing tool I used on my Datsun : https://www.ebay.com/itm/Dual-Carb-Sync-Uni-Syn-Carburetor-Synchronizer-Weber-Austin-MG-Jag-Edelbrock/162890319257?epid=655364391&hash=item25ed050599:g:F58AAOxyjxlTPlpI

If they're out of balance, one carb is doing more work than the other. Similarly if mixture is off on either.

JoeyM said:

Streetwiseguy said:

It's lean.  Drop the jets a bit.

Thank you.  I will try that if I can.....I don't know if the carbs are Hitachi HJG46W or Hitachi HBM46W.  It looks like the former model has movable jets and the latter model does not. (Zparts.com)   I will start looking for adjustment screws and such.

The screenshot of the video you linked shows the knurled knob that moves the jet, right at the bottom of the carb.

Brotus7 said:

Here's the balancing tool I used on my Datsun : https://www.ebay.com/itm/Dual-Carb-Sync-Uni-Syn-Carburetor-Synchronizer-Weber-Austin-MG-Jag-Edelbrock/162890319257?epid=655364391&hash=item25ed050599:g:F58AAOxyjxlTPlpI

Thank you.   It is ordered. :)

Streetwiseguy said:

JoeyM said:

Streetwiseguy said:

It's lean.  Drop the jets a bit. I will start looking for adjustment screws and such.

The screenshot of the video you linked shows the knurled knob that moves the jet, right at the bottom of the carb.

Ok, do you mean this thing?

That's the one.  

frenchyd said:

In reply to JoeyM : most carb problems are really electrical problems.  Fix the ignition system first, 

New NGK plugs are due to arrive at O'Reilly tomorrow....

JoeyM said:

Brotus7 said:

Here's the balancing tool I used on my Datsun : https://www.ebay.com/itm/Dual-Carb-Sync-Uni-Syn-Carburetor-Synchronizer-Weber-Austin-MG-Jag-Edelbrock/162890319257?epid=655364391&hash=item25ed050599:g:F58AAOxyjxlTPlpI

Thank you.   It is ordered. :)

....and I now have the UniSyn 

This tuning guide also recommends having a gunson color tune (a special spark plug with a clear glass insulator that light passes through) so you can see how the mixture is burning in the cylinder and use the color of the ignition to tune your mixture.  Do you guys think it's worth getting one?

Vacuum advance hooked up? Ported or manifold vacuum match (ie did the distributor and carbs come from the same car?)

Robbie said:

Vacuum advance hooked up? Ported or manifold vacuum match (ie did the distributor and carbs come from the same car?)

No, they're not.  The distributor is an aftermarket pertronix unit.  The carbs and intake are from a 240z, probably around a 1971.  The engine is from a 1982 Maxima.

In reply to JoeyM :

Then I'd leave the vacuum advance disconnected for now until you get it a bit more dialed in. Remember to plug the line to the carb!

Plugs are here.  I can change those now while I wait for the 14 mm color tune that I ordered to look at the color of the flame burning in the cylinder to see how rich or lean it looks.

Don't know if the product has been re-calibrated since, but word on the Brit car scene at one point was that the Gunson Colortune was not very effective with todays clean-burn fuels. There was not much of a color spectrum from lean to rich.

NOHOME said:

Don't know if the product has been re-calibrated since, but word on the Brit car scene at one point was that the Gunson Colortune was not very effective with todays clean-burn fuels. There was not much of a color spectrum from lean to rich.

Ooh! Good to know. Well, it is already ordered. We'll see if it works

Surprised nobody mentioned checking the dashpot oil

triumph7 said:

Surprised nobody mentioned checking the dashpot oil

Good point, and yes, I did add oil (well automatic transmission fluid) to one of the carbs.  I should have mentioned it in this thread; that was earlier, and I mentioned it in the build thread for the car, but for the sake of thoroughness, it should have been here as well

JoeyM said:

frenchyd said:

In reply to JoeyM : most carb problems are really electrical problems.  Fix the ignition system first,

I put in new plugs last week, and drove the car last weekend. I still had missing, sputtering, and attempting to die. I have new plug wires, and that will probably be my next attempt.  

By then the gunson color tune should be here

The timing is correct (both at idle and at full mechanical advance), correct?

Robbie said:

The timing is correct (both at idle and at full mechanical advance), correct?

Excellent question, and I don't know.   Should that be my next check after swapping the plugs, and before I start messing with the unisyn/colortune?

Float level is next on my list.

Here is a test I do to every SU carb before I start mucking with it.

Pull the bell off and remove the piston with the needle.

Make sure float bowls are full by turning the pump on for a few seconds

Using a straw or a small diameter tube, blow a puff of air directly down the jet hole. NOT Big Bad wolf style of puff, just enough to make the fuel deek down into the jet and bounce back.

The fuel in the jet should dip down and jjjjjjjjjuuuusssstttttttttt trickle back over the top of the jet when it bounces back. If it floods the carb the float level is too high and if it does not make it over the top the float level is too low.

  Yes it is a bit subjective, but once you see  it, it will make sense.

On an HS4 carb, I would do this with the jet screw turned down the typical 2.5 turns, on the Nissan carb, I don't know that you have the option?

Pete