Help needed: Z Stumbling when under load - suspected carbie problem

[ritchie]

Hi all,

 

I've got a Datsun 260z (purchased a few months ago) which seems to be fitted with twin British SU carbies (bell top shape, 4 bolts to manifold, suspected HS6 model).

 

At the moment the car is really struggling when accelerating - like, struggling to the point where 20km/h I now consider fast.  It seems to stumble - revs up, then drops all of a sudden, revs up, bogs down, etc etc.

 

It isn't running ideally at idle, but it certainly is a significantly better.  In neutral revving up the engine by opening the throttle works fine - revs all the way up to 6,000+ without bogging down.  The problem is only really apparent when under load.

 

We first thought it was electrical, so replaced the sparks/leads, checked the points (fine), checked the dizzy (fine) and checked the coil (fine).  So now we think that it's fuel/carbie related, especially as the problem is only present under load.  Fuel filter looked fine, we replaced it anyway - no change.

 

Before I take apart the carbies, does anyone have any advice on specific things to look for?  I'm thinking there might be an issue with one of the floats, or there might be some crud in the float chamber that got past the fuel filter. Alternatively the damper might be getting caught... but this is just speculation.  The only other observation I have is that the car isn't blowing smoke, which indicates to me that there's a fuel supply issue, as opposed to a problem relating to air supply.

 

Any advice at all would be so much appreciated!

 

Cheers,

 

Ritchie

 

(ps. It seems odd to me that the problem doesn't appear when revving in neutral, but does appear when in gear - anyone got any ideas on why this might be?)

 

 

 

[oldmates260z]

Some questions:

- Does it run any better with the choke on?

- Have you run it without the air filters?

- Have you checked the fluid in the carbies?

 

Had a problem many years ago with a Ramflo style air filter without the inner mesh on the filter material and when I accelerated it sucked the filter into the intake. Would only do it on the highway under load.

 

Also reving in neutral does not use the same amount of fuel as same rev under load. Maybe fuel starving.

 

What about the fuel? Could have water in it?

[chris240]

I reckon the oil in the carbies is too thick causing the needle to get stuck....suck out the old oil with a syringe you get from the chemist and replace with the proper "thin" viscosity. other than that I dont have a clue !! 

[ritchie]

Thanks for your advice guys.

 

1. The engine idles at higher revs with the choke on, but regardless of choke on or off, the engine always is bogging down at ~ 2,000 rpm.  The engine then has a distinct "click click" sound, my guess is that it's because it's so lean it's only running on 3 cyclinders (one carby working, one not) - although I could be way off.

 

2. Took off the air filters, no change.

 

3. I've taken apart the bells / damper domes, and checked the fluid. One did look quite low - any idea exactly how much fluid to put in? (i.e. with the bells on, should the damper screw thing face 'resistance' when plunging?) I've given the bells & piston a good clean, replaced with transmission oil, sounds a little better than it did but no real change.

 

I've also added ~ 10L of premium petrol to the tank - it was running low, so I thought maybe some crud had got in somewhere.

 

I'm going to experiment with more / less transmission oil, but if that doesn't work, I guess I have to move onto the floats chambers...

 

Cheers,

 

Ritchie

[260Coupe]

Richie

 

sounds like your mixture is too lean if your engine is pinging under load (acceration)

Use a light machine oilor auto trans fluid in the carb pistons, they should rise smoothly (use tour finger to push up) and drop down with a nice "thunk".

 

Set you choke heights to be the same on each carb and then wind out the mixture 1/4 turn at a time and see how you go.

(this is assuming your ignition and fuel is Ok of course)

 

 

[ZD44TT]

are you running electric fuel pump?

 

 

[Scoota G]

I'm feelin it is a damaged fuel pump or a blocked fuel filter

[Zedman240®]

Have you checked if both carbies are balanced? ie both sucking the same amount? while its idling, place your hand in front of the carb (to partialy block the intaking air) on both and you may feel one is sucking more than the other. Or if you have a balancer, even better.. Have you got a timing light? Would be good to make sure the timing is spot on.

[ritchie]

It's an electric fuel pump, and I've already changed the fuel filter.

 

Suction feels relatively the same with my hand, but that's not very scientific.

[oldmates260z]

Pinging and breaking down at revs definately sounds like running too far advanced timing. If it idles well, actually a bit high, also advanced timing. Mine had that when i bought it and drove it in the open road. I just retarded it a bit and it was a completely different car

[ritchie]

Thanks for everyone's advice/input.

 

Come to think of it, it is idling a bit higher than normal.

 

I will check the distributor rotor, points etc and check the timing tomorrow.  The following threads seems to describe a similar problem, and in the end it was always ignition related:

 

http://www.viczcar.com/forum/index.php/topic,2297.0.html

http://www.viczcar.com/forum/index.php/topic,2448.0.html

http://www.viczcar.com/forum/index.php/topic,1848.0.html

 

I'm struggling to understand exactly *why* this would occur though - surely if it was a spark timing issue, the issue would be apparent at idle too? Or is it to do with the speed of rotation - issue becomes more apparent when revs are higher?

 

Cheers

 

Ritchie

[oldmates260z]

Ritchie

 

I beleive pinging is a result of the fuel pre-igniting (too early). If this is the case the fuel is going off too early which would cause a lack of power and this would be come more evident under load. I thing it can also happen with high compression engines too.

[ritchie]

Alright, timing light indicated we were at about 18 degrees BTDC.  I experimented with various levels of "retardation". Setting it to around 12-ish made idle smoother & lower revs, but the same stumble happens on acceleration (it's an electric distributor, by the way, so it can't be points related either).

 

Now, we replaced the spark plugs about a week ago as the first step.  I decided to look at them, and interestingly, the front most plug is brown, maybe a little on the lean side.  The rear most spark plug is charcoal black already.

 

So now I'm thinking maybe the carbies are out of tune.  Would that cause the stumbling under load we're talking about?

 

I don't believe the car is "pinging" - there are definitely odd sounds, but it's more of a rattle that increases in frequency as I accelerate.  My best guess is that 3 cylinders are firing (the rear three, considering they're rich), and three cylinders aren't.  Hence I get a good sound out of three, and low/empty clicking sounds (maybe sparks going off with no fuel?) on the front.  But this is just a theory.

 

The other thing is that this problem happened all of a sudden.  Everything was fine, could get the car up to 140km/h no problem when we were in Tasmania (doing a fair bit of driving).  Then came back, the next day drove 10 kms absolutely fine, got out of car, got back in 15 mins later, then this stumbling thing started.

 

Basically I'm trying to say that it happened all of a sudden, rather than a gradual decline.  Hence I'm thinking something somewhere is blocked or busted.

 

Any advice appreciated... at wits end!

 

Cheers,

 

AR

 

[Scoota G]

K here is a long shot. Does your car have an expansion tank. Mine didn't have one when i bought it and the fuel tank is completely sealed. It is a problem i need to address but just every so often the pressure in the tank needs to be released. Just check when you open the gas tank if it draws air in or blows it out. If i go on a long drive sometimes it happens but more often in the winter.

 

Just one more question where are you located. Maybe you can get a local member to drop around a take a Squizz at it.

[ritchie]

Ok everybody, definite improvement happening.

 

After noting the front 3 plugs were lean, I decided to check the float bowls.  Interestingly, the float bowl associated with the lean plugs had a lot less fuel in it than the other float bowl.  Anyway, didn't see any foreign matter in there, so screwed back on.  Started the car and wallah!  Idling sound far better, opened up nicely on acceleration!

 

Can't really explain it, but maybe there was some obstruction of the float that I fixed (somehow) by simply removing and replacing the lids. Weird! It would explain the problem though - float bowl obstruction could keep float low, letting less fuel in.  Under acceleration, float bowl empties before it has a chance to refill And hence can't deliver fuel to front three carbs.

 

Only issue now is the carbs are backfiring every minute or so when I really get going (starts around 60km/h).  I adjusted the timing (reduced advanced), it now accelerates even better but there is still the carb backfiring/popping.

 

I hear this is because of a lean mixture.  I was thinking about trying to adjust the SU's myself, but I figure it's probably worth getting a pro to tune them - will probably save that much on petrol in the long run...

 

Thanks again everyone for your suggestions - definitely a learning experience!!

 

Cheers

 

ritchie