20 replies to this topic

[lxondecks]

Hello All, I’m hoping somebody will be able to offer a little assistance.

 

I have a 1980 1275 GT with single HS4 carburetor.  I’m attempting to adjust the air/ fuel mixture as I believe that the car is running rich, due to black plugs and very sooty exhaust pipe and bogging down when accelerating.

 

I have not changed any of the settings on the carb yet.

However I have had the timing checked and confirmed to be okay.

 

I read in the Haynes manual to use the piston lifting pin on the carb to check the mixture.

• So I started the car and got it up to temperature
• Adjusted the idle to 1000rpm
• Lift the piston lifting pin until I feel the piston and then lift a further 1mm  (hard to judge)

 

At this point the engine drops in revs and wants to stall until I release the pin and the revs pick back up.

 

From the Haynes manual it states

If the engine RPM decreases when the pin is lifted your mixture is too Lean

If the engine RPM Increases when the pin is lifted your mixture is too Rich

If the engine RPM initially raises as the pin is lifted, and then settles back to the original RPM, the mixture is right.

 

So from this I would take that the engine is actually running too Lean.

 

However I then accidentally noticed that if I blocked the breather hose connection on the carb (which does not currently have the hose from the canister to the carb connected) the revs drop and the engine stalls

I understood from this that that by blocking this connection and reducing the air intake into the carb I am effectively richening the mixture too much causing the car to stall.

Which to me would say that I am running rich. Can someone please clarify that I am understanding this correctly?

 

I am now confused and have no idea if I’m running too rich or too lean. Are there any other checks that can be done?

 

Cheers

[hhhh]

If you have an open hole after the butterfly (the disconnected breather) you will certainly be lean. This must be connected or blocked for the mixture to be correct. The lifting pin will only indicate your mixture at idle. If your needle profile richens too much off idle, it's quite possible that you will be rich everywhere but idle even though your idle is set lean.

[carbon]

Breather hose should be before the butterfly on the HS4. Is this the brass tube at 45 degree angle?

 

As hhhh says it will affect the mixture, and the breather tube should be reconnected. Worth checking your oil filler cap has a small hole about 4.5mm diameter and there are no other vents or air leaks into the engine.

 

Would recommend taking dashpot off, clean up inside and the piston very carefully (no abrasive) and also check there is no signs of wear on the needle (look for tell-tale ridge about 3mm from face of piston)..

[lxondecks]

Thank you for your replies. I've taken a look at the carb and the breather connection is the 45deg brass pipe which is before the butterfly.
I've removed the carb piston and it's a little grubby so will give a clean up and replace. Couldn't see any obvious wear on the needle.

I'll look for vacuum leaks and probably try a different oil in the dashpot to see if that makes any difference to the running.

[carbon]

For purposes of setting up the carb I would suggest using SU recommended SAE20 straight oil to start with, the nearest multigrade equivalent to this is 5W30.

 

The dashpot oil only affects mixture strength for short times during heavy acceleration, if you have black plugs and sooty exhaust there will be something else amiss.

 

Do you know if the correct needle is fitted? There have also been occasions when the wrong jet has been installed, which gives super-rich running...

[lxondecks]

Okay will give that a try.
I have know idea what needle or jet are fitted as I've never changed them.
Will have to do a little research.
After making adjustments how long of a run would I need to do to see any changes in the colour of the plugs?

[carbon]

Before going any further I would suggest taking the dashpot off and checking exactly which needle is fitted, as it may have been altered by a previous owner.

 

Also if you have a set of twist drills you can use the 2.5mm drill to check the main jet size. If the blunt end of the drill can fit into the main jet then you have a 100thou jet fitted which is incorrect. If it does not fit then it should be 90thou, which is right.

 

And when dashpot is off measure how far the top of the main jet sits down from the bridge, and operate the choke several times and make sure the jet is returning cleanly up to the same point every time (they can stick). The top of the main jet is normally between 1-4mm below the level of the bridge, can be higher up to almost flush but not often lower.

[lxondecks]

Okay, so I removed the dashpot to check the needle type. Unfortunately the needle collar is seized in the piston so I have this soaking in some oil at the moment in the hope it will come free.

 

I could not fit the 2.5mm drill bit in the jet so this must be the correct .09" jet.

I have operated the choke and the jet seems to be moving correctly and returning to it stating position.

The jet is sitting around 1-2mm below the level of the bridge.

 

Hopefully I will be able to remove the needle tonight to find out the type.

I read online that the standard needle should be the ABB type. Does this sound correct?

[lxondecks]

So I managed to get the needle out after soaking it in oil. The type is ABB which sounds correct to me.
What would you suggest next? Adjusting the carb mixture nut?

[lxondecks]

After finding out I have the correct jet and needle. I managed to get a piece of hose to pipe from the cannister to the carb.Using the piston lifting pin the revs picked (opposite to last time) meaning the mixture much be rich. So I've adjusted the carb but by 2 flats to lean it until the piston lifting pin made no difference.
I took it out for a run and it feels much better.

When I got back I checked the plugs to see if they had clean up.
Can someone confirm if the plugs look okay?
The concern I had was with cylinder 4. Half looks okay but the other half is black and crusty. Not sure what that means?

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[Mini Manannán]

Did you chop it or just pootle to a halt?  They look OK to me, a tad on the rich side but nowt to worry about. 

[Northernpower]

They look reasonable to me. Regarding the one that looks a little dark, what milage have the valve stem oil seals done and how old are they?

[lxondecks]

I have not changed the valve stem oil seals in the time I've had it. So not sure when the last time they would have been done.

[lxondecks]

Did you chop it or just pootle to a halt?  They look OK to me, a tad on the rich side but nowt to worry about.

Not sure exactly what you mean. But I took it for a run on the motorway for a good 10 miles.

[Northernpower]

I have not changed the valve stem oil seals in the time I've had it. So not sure when the last time they would have been done.

If the seals have some age to them they will not give a good seal. Is there any sign of smoke on the overrun?

 

 

Did you chop it or just pootle to a halt?  They look OK to me, a tad on the rich side but nowt to worry about.

Not sure exactly what you mean. But I took it for a run on the motorway for a good 10 miles.

 

The best way to check the plugs is to drive it hard and then stop the engine immediately and take the plugs out. If you drive it and then drive back home the colour of the plugs will change.