21 replies to this topic

[Abbot]

After a very long delay and a complete rebuild of the engine on my LHD 998 Mini from 1998 the engine burst into life.

 

The engine runs but there is something wrong with the set up of the carb.

 

There is fuel OK

There is a strong spark  OK

I have set the timing to 8 deg BTDC using my strobe gun  OK

 

The engine starts with a bit of choke and runs smoothly

 

I noticed that the when I pushed the choke knob back in the the lever on the carb does not go fully back to its no choke position.

When i move it back the engine stops and is then difficult to start again.

 

When the engine was apart i did service on the Carb and fitted a new wax stat jet assembly

 

Any advice on what to try to solve my problem would most welcome 

 

Abbot

[sonscar]

Watch the linkages move carefully and look for binding.Do waxstat need the jet centering.Is the pipe to the jet free to move?Just thoughts,Steve..

[Abbot]

An additional point - I set the timing using my Gunson Timing light then I realised I set the timing to 8deg BTDC.

 

Lokking again at the instructions for the gun and Haynes manual it says dynamic timing should be done with the Vacuum Pipe disconnected.

 

If I disconnect the Vacuum pipe the engine stops   Any suggetions

[andyapanel]

You need to plug the vac advance when you pull it off the distributor. A small bolt/machine screw will do the trick. I also put gaffer tape over as belt and braces.

 

Loosen the distributor clamp slightly.

 

Increase the idle speed to 1500, move the distributor to line up the dots on the pulley and pointer.

 

Bon chance.

[Abbot]

You need to plug the vac advance when you pull it off the distributor. A small bolt/machine screw will do the trick. I also put gaffer tape over as belt and braces.

 

Loosen the distributor clamp slightly.

 

Increase the idle speed to 1500, move the distributor to line up the dots on the pulley and pointer.

 

Bon chance.

Thanks 

Just to be clear i need to block the vacuum pipe going to the manifold?  Is that becuse an open pipe leaks additional air into the inlet.

[andyapanel]

1. Yes.

 

2. Yes

 

Check that the vac advance tube does not leak, either. It happened to me last week.

[Abbot]

Right latest situation.

 

FYI i found a brass radiator bleed screw where the tee piece is exactly the right size to push into the vacuum pipe.

 

So reset the timing to 8 deg BTDC

 

However the engine will still not run with the choke pushed in.

 

It runs ok with the choke pulled out one notch.  (not sure if the choke pull button has notches but it feels like it comes out in steps)

 

As an experiment I tried my Colourtune to see what the mix looked like and it strangely appeared to be a nice shade of blue.

 

I cant see anything that is stopping the choke mechanism moving.  

[cal844]

I have never had to block the vacuum advance pipe when removing it from the distributor, perhaps because I have the engine running and clamp bolt loosened off before disconnecting it.

[timmy850]

Try increasing the idle speed screw

The first pull on the choke cable should activate the fast idle setting, and then further on the choke cable should drop the jet and add extra fuel

[NLinPEN]

Are you familiar with the attached document? I found it very helpful when I repaired, installed, and re-tuned my SU carburetor.

Attached Files

•  TuningSUCarburetors.pdf   1.64MB   23 downloads

[ADRay]

I’m in the same boat. I rebuilt my HS4 and I was having the same type of trouble. I spent some time yesterday fiddling with the jet adjuster nut and idle screw. Mine was dying after disengaging the choke too. It took setting the nut turned out more than I had it. I turned it in very slight movements. After about 30 minutes, I was able to dial it in so that it was idling around 1200 rpm, and it wasn’t dying when I hit the throttle. It needs more adjustment, but I was able to take it for a spin.

[Ethel]

As Timmy says, the choke raises the idle speed before it enriches the mixture. If you look at where the fast idle screw touches the choke mechanism you may see a little arrow - that should be aligned with the screw when you adjust the fast idle. If there isn't an arrow you should still be able to see the point just before the cable actually starts to move the jet.

 

You could hold the throttle on the fast idle then adjust idle screw proper to take up the slack. Go for a drive to get it warmed up, then back off the idle screw to get a regular idle speed. A bit under 800 rpm is good, depending om cam etc.

[Abbot]

Are you familiar with the attached document? I found it very helpful when I repaired, installed, and re-tuned my SU carburetor.

Thanks I haven't seen that.  I was using the info leaflet that came with the SU refrub kit.

[Abbot]

I’m in the same boat. I rebuilt my HS4 and I was having the same type of trouble. I spent some time yesterday fiddling with the jet adjuster nut and idle screw. Mine was dying after disengaging the choke too. It took setting the nut turned out more than I had it. I turned it in very slight movements. After about 30 minutes, I was able to dial it in so that it was idling around 1200 rpm, and it wasn’t dying when I hit the throttle. It needs more adjustment, but I was able to take it for a spin.

Thanks 

This morning I took the carb off so that I could check the initial set up of choke (2 times unscrewed)

 

In all of the videos I have found about tuning the jet screw I haven't seen any where the guy is doing the adjustment with the carb fitted in the car.  Which is why i took iy out to check initial set up

 

I will try your suggetion

[Abbot]

As Timmy says, the choke raises the idle speed before it enriches the mixture. If you look at where the fast idle screw touches the choke mechanism you may see a little arrow - that should be aligned with the screw when you adjust the fast idle. If there isn't an arrow you should still be able to see the point just before the cable actually starts to move the jet.

 

You could hold the throttle on the fast idle then adjust idle screw proper to take up the slack. Go for a drive to get it warmed up, then back off the idle screw to get a regular idle speed. A bit under 800 rpm is good, depending om cam etc.

Just adjusted the throttle adjusting screw and it seems to have improved things enormously.

 

Need to watch some rugby now before having another play