Car will not rev past 4000 rpm

[lpraun]

Hi, as the headline states my car will not rev past approx. 4000 rpm.

My car is a 1973 240z converted to 3-screw SU carburetors (long time ago) and electronic ignition (recently) no emission control – everything is the original setup. The carburetors are worn with air- and fuel leaks etc. – bottom line is that they were in pretty bad shape! Despite of that the car ran quite well – revving all the way to 7000 rpm. Anyway I decided to fight the recession by spending some money on a refurbished set from ztherapy – and I was very happy when they arrived this week – they look great!

Now I have switched to the new set (4-screw) and tuned them according to my workshop manuals and numerous write-ups on this forum and elsewhere on the net – which basically tell the same.

Now – does anybody have any idea about what could be wrong – the ONY thing I have done is to switch over from the old to the new carburetors. I mean – these SU carburetors is freaking simple – float level, fuel mixture, idle, fast idle – make sure they are balanced – what else is there? What could be my problem – please help!

I have searched, found and read a lot of other threads about the same problem but have not been able to solve the problem. Another thing I have noted is that the pistons in the carburetors is quite difficult to push up – I am using ATF in the dampers. Also, they do not go up very high when I rev the engine – only around 5 to 10 mm.

Help me!

Lars Peter

[GreenZZZ]

Things I can thing of

One carb has a stuck needle valve in the float bowl and you are running on 3 cylinders, you can tell when this is happening by getting lots of change when tweak the mix on one carb but little effect with the other.

Bad vacuum leak leaning out the mix (brake booster, distributor, other crankcase leak)

Unrelated ignition issue popped up somehow

[yetterben]

Fuel Filter.

[Gary in NJ]

I was thinking vacuum leak too, or perhaps the gasket. Can you try spaying the mounting gasket to see if the rpm increases? If nothing else, it rules out one possibility.

Is the linkage assembled correctly?

[bigoak]

If it hit a RPM wall so to speak, the cause is likely electrical. Either the vacuum advance is not working or your points are worn out (likely the latter).

Fuel restriction (clogged filter, etc.) is a possibility but will usually occur across a range of RPM levels.

Check the points.

[lpraun]

Last year I swopped to a later z-car electronic ignition distributer with the E12-80 ignition module, so points and vacuum advance is not an issue. The linkage is assembled correct. It idles fine and starts with little choke and pulls fine until about 4000 rpm. The fuel filter is also quite new and I had no problem with it before.

Everything was working fine (I know it sounds stupid) before I switched to the new carburetors! Therefore am I fairly convinced that this is a problem with the setting of the carburetors – the question is what!

Could this be a classic problem regarding incorrect float level setting – the new carburetors is right out of the box from ztherapy so I trusted the level setting to be correct?

[Gary in NJ]

the new carburetors is right out of the box from ztherapy so I trusted the level setting to be correct?

Bingo! The floats can flop around during shipping and should be rechecked.They both should be set for 14mm between the lid and the top of the float. Make sure the float is level with the top as well,

[lpraun]

Okay – the float level then!

The 14 mm distance you mention – is that from the underside of the “roof” of the lid or is that from the underside of the edge of the lid that protrude about 3 mm further down? I hope it make sense.

[ChrisZ]

Lars Peter, do you have your car at home or at your ol' folks ?

When reading your describtion, i think you hit the nail when mention the piston don't move more than 10 - 20 mil, being it's a variable venturi, you will most likely run out of air, if the pistons don't rise more than that.

A quick test you could preform is this, engine shut off, lift a piston and let it go, it should come down with a "clonk" if not, i wil bet you have a bend needle unless the piston is sticking to the dome, anyway, do you have your car at home, you could drop by during next week (Thuesday) and we can have a look at it.

Chris

Oh, one more thing, when you bolt on the piston dome, you will have to push the piston up and down while you fasten the screws, if the dome doesen't seat correct, it can cause the piston to stick.

[lpraun]

Hi Chris – long time no hear! I’m at my parents place on Fyn, but maybe I will bring The Datsun to Copenhagen sometime during May if I can make the car run properly – then we can meet up and… you know – look at it!

I’m positive that the pistons is not sticking – they move freely but it take a lot of force like if I there is a very heavy grade oil in the damping thing – but I have used standard ATF.

Well – it’s getting late and I’m off to bed. I will adjust the float level tomorrow and let you know how it goes!

[ChrisZ]

Yes it have been some time, long and cold winter is no time for cars.

Search for float level, IIRC there is something about the 4 screw have different settings, something like 12 mm on the rear carb.

I'm out of the SU business for the time being, just finished my triple install and that my freind is WOW, the sound is so sweat, just wait until you see and here it.

Bringing my car back home tomorrow, time to enjoy, do you know DVK have a gathering at "dragør fortet" some time in June.

Chris

[Gary in NJ]

Okay – the float level then!

The 14 mm distance you mention – is that from the underside of the “roof” of the lid or is that from the underside of the edge of the lid that protrude about 3 mm further down? I hope it make sense.

14mm is measured from the bottom side of the float cover, to the top of the float. One of my 14mm wrenches has a handle that is exactly 14mm. Check your wrenches, it's nice to have a measuring stick to stick between the float and the cover.