36 replies to this topic

[Monteemini]

Specs

Austin Mini Mayfair 1985

998cc with Stage one kit fitted.

HS4 Non-Waxstat carb

 

 

Hi,

 

I am having real trouble getting my new ignition system/rebuilt carb setup,

 

 

I have installed a new Powerspark electronic dizzy, HT leads, Coil and Plugs aswell as rebuilding the HS4 carb with a genuine SU kit (I replaced the needle with a new like for like as i accidentally damaged the original)

 

I set the timing using a strobe to TDC

Set the HS4 jet to '12 sides' down from the top position on the adjustment nut

Wound up both the fast idling and the standard idle screws so neither were making contact.

 

The engine fires perfectly on the first turn of the key every time, but is idling about 2000RPM.

So I raise the jet adjustment nut, this lowers the revs but still only to about 1200RPM, however at this position it is unable to recover after acceleration.

So I 'devance' the timing to lower the revs but this causes misfires with i can't tweak with the mixture nut

 

Has anyone any ideas on where i am going wrong?

 

Thanks

 

 

[petey81]

Check for air leaks.

[Monteemini]

Check for air leaks.

Ive checked the Vac line and replaced it with a new tube, Re-sealed the carb with new gaskets to the inlet manifold.

Anywhere else i should check?

[petey81]

Did you replace the inlet manifold gasket?

[Monteemini]

Did you replace the inlet manifold gasket?

Yeah swapped it out, also used gasket sealant for a good seal.

[petey81]

What timing settings are you using?

[Monteemini]

What timing settings are you using?

4 Degrees from TDC with Vac Pipe Off

[Spider]

If you replaced the butterfly spindle and / or the butterfly itself, or disturbed the old ones, the butterfly will need to be seated properly in the closed position.

 

You'll need to take the carb off, back the Idle Screw right out, loosen the butterfly screws, wiggle the spindle about with gently pressing on the butterfly until you see no gaps at the top or bottom of it. If you hold it up to the light, that can help, though, you'll usually see a tiny bit of light coming through, but not much.

 

If it is seated, it could be the the SU Body has worn from the Butterfly around the Spindle, if worn, it tends to only do it on one side.

[Monteemini]

If you replaced the butterfly spindle and / or the butterfly itself, or disturbed the old ones, the butterfly will need to be seated properly in the closed position.

 

You'll need to take the carb off, back the Idle Screw right out, loosen the butterfly screws, wiggle the spindle about with gently pressing on the butterfly until you see no gaps at the top or bottom of it. If you hold it up to the light, that can help, though, you'll usually see a tiny bit of light coming through, but not much.

 

If it is seated, it could be the the SU Body has worn from the Butterfly around the Spindle, if worn, it tends to only do it on one side.

 

Okay thanks I will check this, i did indeed replace the butterfly spindle and the valve itself.

[carbon]

The HS4 carbs can have either 'coarse 26tpi' or 'fine 40tpi' threads on the main jet adjuster. The coarse type was only fitted to older HS4s, and this is the version which uses the 12 flats down setting for initial start. For the 40tpi version you need about 18 flats instead of 12 for the initial setting.

 

Easiest way of checking which you have is to set the jet flush with the bridge and then turn main jet down 18 flats. Then measure height of the jet below bridge with vernier caliper:

- if it drops by about 2mm it's probably fine thread adjuster

- if it drops by about 3mm it's probably coarse thread adjuster

[Spider]

The HS4 carbs can have either 'coarse 26tpi' or 'fine 40tpi' threads on the main jet adjuster. The coarse type was only fitted to older HS4s,

 

I didn't know that - cheers

[Monteemini]

 

If you replaced the butterfly spindle and / or the butterfly itself, or disturbed the old ones, the butterfly will need to be seated properly in the closed position.

 

You'll need to take the carb off, back the Idle Screw right out, loosen the butterfly screws, wiggle the spindle about with gently pressing on the butterfly until you see no gaps at the top or bottom of it. If you hold it up to the light, that can help, though, you'll usually see a tiny bit of light coming through, but not much.

 

If it is seated, it could be the the SU Body has worn from the Butterfly around the Spindle, if worn, it tends to only do it on one side.

 

Okay thanks I will check this, i did indeed replace the butterfly spindle and the valve itself.

 

 

Ive stripped the carb down again and checked the positioning of the butterfly valve. It was slightly out of place (i could see a slither of light through it when closed)

Now when i reassembled it i really struggled to centre the jet in the jet housing, the piston would not drop all the way to the bottom, even with the jet bearing nut loose

 

So i figured the needle must be slightly bent out of shape, the first 80% of travel on the piston was fine but is always got stuck on the final 20%.

 

I can't see any damage/bending of the needle but i assume its only a little off centre, could this be causing my problem?

[Monteemini]

The HS4 carbs can have either 'coarse 26tpi' or 'fine 40tpi' threads on the main jet adjuster. The coarse type was only fitted to older HS4s, and this is the version which uses the 12 flats down setting for initial start. For the 40tpi version you need about 18 flats instead of 12 for the initial setting.

 

Easiest way of checking which you have is to set the jet flush with the bridge and then turn main jet down 18 flats. Then measure height of the jet below bridge with vernier caliper:

- if it drops by about 2mm it's probably fine thread adjuster

- if it drops by about 3mm it's probably coarse thread adjuster

 

Cool thanks i will check this on re-assembly

[Spider]

 

 

If you replaced the butterfly spindle and / or the butterfly itself, or disturbed the old ones, the butterfly will need to be seated properly in the closed position.

 

You'll need to take the carb off, back the Idle Screw right out, loosen the butterfly screws, wiggle the spindle about with gently pressing on the butterfly until you see no gaps at the top or bottom of it. If you hold it up to the light, that can help, though, you'll usually see a tiny bit of light coming through, but not much.

 

If it is seated, it could be the the SU Body has worn from the Butterfly around the Spindle, if worn, it tends to only do it on one side.

 

Okay thanks I will check this, i did indeed replace the butterfly spindle and the valve itself.

 

 

Ive stripped the carb down again and checked the positioning of the butterfly valve. It was slightly out of place (i could see a slither of light through it when closed)

Now when i reassembled it i really struggled to centre the jet in the jet housing, the piston would not drop all the way to the bottom, even with the jet bearing nut loose

 

So i figured the needle must be slightly bent out of shape, the first 80% of travel on the piston was fine but is always got stuck on the final 20%.

 

I can't see any damage/bending of the needle but i assume its only a little off centre, could this be causing my problem?

 

 

Try removing the Needle altogether to check that the Piston will drop all the way.

 

If that checks out, wind the Jet up all the way, refit the Needle, or preferably and old one if you have it, drop the Piston all the way down (with the Bell on) and then Centre the Jet.

[carbon]

 

 

If you replaced the butterfly spindle and / or the butterfly itself, or disturbed the old ones, the butterfly will need to be seated properly in the closed position.

 

You'll need to take the carb off, back the Idle Screw right out, loosen the butterfly screws, wiggle the spindle about with gently pressing on the butterfly until you see no gaps at the top or bottom of it. If you hold it up to the light, that can help, though, you'll usually see a tiny bit of light coming through, but not much.

 

If it is seated, it could be the the SU Body has worn from the Butterfly around the Spindle, if worn, it tends to only do it on one side.

 

Okay thanks I will check this, i did indeed replace the butterfly spindle and the valve itself.

 

 

Ive stripped the carb down again and checked the positioning of the butterfly valve. It was slightly out of place (i could see a slither of light through it when closed)

Now when i reassembled it i really struggled to centre the jet in the jet housing, the piston would not drop all the way to the bottom, even with the jet bearing nut loose

 

So i figured the needle must be slightly bent out of shape, the first 80% of travel on the piston was fine but is always got stuck on the final 20%.

 

I can't see any damage/bending of the needle but i assume its only a little off centre, could this be causing my problem?

 

Strange, would have expected your carb to be fitted with swing type needle.

 

When you have the piston out of the carb does the needle have any side-to-side movement at its end?