SU Carbs

[Glen]

Hi Guys hope you can help me out. I have a 2.8 Litre motor with 1.3/4" SU's and the car runs really badly at low revs 700 to 2,000. Once the car gets up in the revs, say up to 5,000 it seams to run quite well. When the car gets warm some of this low rev problems goes away. The car has a lumpy cam in it and I dont know if the problem is due to the cam or the SU's, but have a feeling its a fuel problem. I live in Australia, so can anyone tell me a place to get SU rebuild kits or are other nissan models such as the 240k, 280C good places to look for another set of SU's.

[Bruce Palmer]

Glen,

When you say "runs badly", describe what that is like.....

[Glen]

Hi Bruce

Badly is that, particularly when cold, the car coughs and splutters when idling and when I accelerate the motor fails to take off (like it is suddenly starved or flooded with fuel?), I need to be on and off the accelerator a bit until i find a "sweet spot" and the car finally gets over 2000 rpm, then it accelerates away reasonably well.

The car is running a bit rich at the moment. The car has no trouble starting with a bit of choke when cold it will sit there and splutter away for as long as I want it to.

Hope you can shed some light on my delema.

Glen

[wishihadaz]

Hey Glen,

I'm not not going to trouble shoot on what I don't know about but, if you're running 2.8L with unmodified SU's they probably can't feed that displacement plus the cam lift and duration.

In my research on this site, most people have to install larger needles.

Additionally intakes are also swapped.(Somebody chime-in on which one...N36?)

You might try www.rebelloracing.com for advice as they do SU rebuilds here in the states.

[Zedrally]

Hi Glen,

Can you advise what SU type you have?

Are they Hitachi Round top, flat top or some euro/british model?

Not all SU's are the same and if you have the flat top then I can understand why it stumbles.

That said, rebuild kits are available. Try Repco/Super Cheap or Auto Barn.

Good luck.

MOM

[jmortensen]

Are you aware of how the SU is oiled? The brief story is that the black knobs on top are dipsticks and they have a low and full level on them. The carbs need periodic oiling, and I used to pull the domes and clean the pistons every 7K or so. You should do that, and then maybe try different weight oils. My old SU's liked thin oil, like ATF or Marvel Mystery Oil. My friend's needed 20 weight or thicker.

Also check for nozzles stuck down. Search and you'll find lots of info on SU's.

They're about the simplest carb ever. You should take some time to check them over. Your problem might have a very simple solution.

[beandip]

I recently installed a 280 ZX engine with a moderate cam and headers etc, the engine pull great from the line . My cam is stated to be in the power band from 2000 to 5500 RPM. At the top end of this range she is still pulling strong . The head was milled and 9.5 to 1 is the result. As this is not a car for the track , I didn't wish any higher RPMs . What has been passed on to me during my research , is the more lift and overlap the less vacuum the engine is drawing in at low RPMs . SUs are vacuum dependant to function properly . You may find also for the same reason the idle speed must be raised . With a ''lumpy '' cam , as you say , the engine will require a higher compression as well . As to the size of needles I have been running 54s and they have been good but I recently switched to 27s and the engine seems to be smoother and perform some what better . Time will tell on this when she goes on the dyno . What needles are in your carbs ? Gary

[Glen]

I pulled the front carbie this afternoon.

Before this I removed the air filter and lifted both cylinder and let them drop and they seemed to both drop at about the same rate. The oil in the top of the carbies is red and thin, sort of like transmission fluid, didn't know how to check the level. Top came off su easily and was very clean inside, spring looked ok. Puleed needle out and it has the number 27 on it, neddle was straight and free from any score marks. As a side note the needle shoulder was sitting flush against the SU cylinder before I took it out but when I reinstalled it the needle could actually be pushed up a few millimetres inside the cylinder before it bottomed out. Anyone know the correct position for the needle. Only markings on the SU's I could find was a casting imprint of either IS0 or 150. SU's are round top.

any suggestions as to whether I should change needles or what oil I should use as I will have to put oil back into the front SU, how do you check the oil level.

Any advise would be appreciated.

[jmortensen]

The dipstick has a low level and a high level marked by tiny lines close to the bottom of the stick. The carbs don't take much oil at all, I used the cap on the bottle of oil to fill mine. If you accidentally overfill it's not a big deal. When the piston goes all the way up the first time the excess oil will spill out the top and get burned in the engine. I'd try some 20 weight and see if that helps. 20-50 if you can't find straight 20 weight. The needle height is pretty critical. You can set it "better" than what I'm about to tell you, but I'd set it flush with the bottom of the piston for now. If you really want to get it dialed in, get the video from Ztherapy and that has details on how to dial them in just perfectly. Flush with the bottom of the piston and then screw the mixture knobs on the bottom all the way in, then back them out 2.5 to 3 turns and you should be in the ballpark. If you don't have a synchronizing tool, that would be a good thing to get.

Make sure when you put the domes back on that you push up on the pistons before you tighten the screws. The pistons need to be able to move freely so you might have to move the dome around until you get to where everything moves nicely.

[beandip]

I pulled the front carbie this afternoon.

Puleed needle out and it has the number 27 on it, neddle was straight and free from any score marks. As a side note the needle shoulder was sitting flush against the SU cylinder before I took it out but when I reinstalled it the needle could actually be pushed up a few millimetres inside the cylinder before it bottomed out. Anyone know the correct position for the needle. Only markings on the SU's I could find was a casting imprint of either IS0 or 150. SU's are round top.

any suggestions as to whether I should change needles or what oil I should use as I will have to put oil back into the front SU, how do you check the oil level.

Any advise would be appreciated.

The spring is a sensitive part as it controls the rate of rise of the piston so handle with care . the needle needs to be set flush to the bottom of the piston. I found 20wt oil at the local cycle shop . Ask for fork oil , it is used for the front forks on Motor Cycles .

[kenz240z]

Glen, the symptoms you describe can be caused by out of balance carbs, where only one carb is actually working at idle speed.

[Glen]

Pulled rear SU down today and found that needle was slightly off centre when rolled along bench. This is probably part of my problem, Can someone tell me where I can get a set of N-27 needles in Australia. Called SU Midel but they only do English SU's.

Glen.