8 replies to this topic

[YankWithAMini]

The engine:

998 A+, completely stock. HS4 carb.

 

Background:

Reviving my Mini after 2 years in storage. I though it was a good time/good idea to pull the carb and give it a good cleaning. I did about a 95% disassemble and cleaned it all. I replaced the jet, the needle (with the same needle as it had), gaskets as required. There was what I would describe as very very slight play in the throttle spindle, not enough that I thought would cause an issue (but I have no reference for that).

 

The jet was sticking when I first put it together, to the point that it wouldn’t return when the choke was pushed in. It didn’t look bent, and after some light sanding it is OK (still sticks a tad right at the end if you push the choke in slow, but a quick choke out/choke in frees it).

 

I installed the carb and set it up as per recommendations I have seen, which was to screw the jet all the way in, and then back it off two full turns. This got the car running with the choke out, but as the choke was pushed in, it would get rough under 2000 rpm and die at around 1500 rpm. I now have the jet turned almost five full turns down, and the idle is where it should be. If i stick a finger or screwdriver into the elbow and attempt the piston lift test, the engine dies with even the slightest movement of the piston. Bumping the throttle produces a good response. If i pull the choke out fast, the engine sputters and very slowly increases RPM; if I pull the choke out slowly, it does not sputter. There is oil in the piston cylinder.

 

The dwell seems OK (54 deg), and the timing is close, probably 2-3 degrees low (reads 7 deg or so, I think the book value is 10 deg).

 

Questions:

Does the number of turns down on the jet seem reasonable? What is the max number of turns down you can do on a jet? The “piston lift test” seems to indicate the mixture is still lean, but without knowing the max turns I am worried about going too far and the jet coming off or something.

 

Any ideas or help is appreciated!

 

 

 

[PoolGuy]

Did it run ok the last time it was run prior to you taking the carb off? Have you made any other changes, however insignificant they may seem?

 

 

 There was what I would describe as very very slight play in the throttle spindle, not enough that I thought would cause an issue (but I have no reference for that).

 

IMO any noticeable movement is too much.

[Mr Piggy]

If you don't have a spring loaded needle you may need to centre the jet? Without the engine running and the damper fitted, lift the piston and allow it to fall under its own weight. It should drop right to the bottom with a clunk. Gently lift it again to check there is no extra resistance/sticking in the first few millimetres of lift. Any slight extra resistance means the needle is rubbing against the jet. This will keep the piston in a slightly raised position at idle and weaken the mixture. But if you adjust the jet to compensate you'll have a very rich mixture at higher revs.

[red81]

Have a look at "Burlen"   https://sucarb.co.uk/technical/   There is a good section on servicing SU Carbs.  Also I put "valve guide seals" on the outside of the throttle spindle, seen it on the net somewhere, you have to cut them to fit but it does help seal and stop air leaking. 

 

Have a look at the needle spec that your carb started life with, then if different compare with the one you bought to see how far out of spec you are, and work out what it means.  Again I found comparison sites on the net.

 

A Gunsons Colour tune kit is good to see what's going on in your cylinders.

 

Steve I

[YankWithAMini]

Thanks for the replies

 

PoolGuy:

Yes, it ran fine the last time we drove it. Only "issues" was that it much preferred idling at the higher end of the quoted range I have seen (600-800). The question about other changes is interesting... After the last drive I fogged the engine prior to storage. It was a spray in product, where you spray it into the intake while running. I also put it direct into the cylinders. I would have expected this to burn off and things would get back to normal. I obviously dissembled and cleaned the carb now. Other than that, no changes. In hindsight, I should have just tried to start it straight away before cleaning the carb...

 

Point taken on the bushings.

 

Mr Piggy:

It is a spring biased needle. The piston gives a clunk when lifted and dropped with the damper in. Interestingly, there is a "dead" zone at the bottom before the damper kicks in. It lifts freely maybe 1mm before the damper starts to have an effect. I did not notice any difference in resistance over the piston travel range.

 

re81:

That specific link didn't seem to work, but I have used info on that general web site. I will look again to see if I missed anything. I replaced the needle with the same one it had (AAC). To be honest, the old needle/jet didn't look all that worn to me, but I replaced them anyway. I had not had any adjustments done to the needle, but I can't speak for any former owners. Gunson's is on the list now, I think!

 

 

I will fiddle with it tomorrow some. I might take it out for a spin around the block just to see how it goes, but first I have to get a wheel brake cylinder replaced. I will ignore that pesky expired registration thing....

[panky]

If you're concerned about air leaks down the carb spindle try this little mod.

 

[Ethel]

What you describe as a dead zone is normal. The damper has a sort of 1 way valve so it resists lift more than dropping, you're taking up the slack in the valve to its more resistive position.

 

If you haven't, give the ignition the once over. A couple of years is long enough for advance mechanisms to get corroded, gummy & sticky; rubber bits in the vac advance to split; contacts to oxidise & insulation to break down....

[YankWithAMini]

Been a while, but I got some test driving in finally. I ended up at about 6.5 turns down to get a stable idle. The car seems to run fine, other than a brief run-on event which I will try and address. The first drive it seemed to buck a bit driving steady at ~30 mph, but it didn't do that more recently.

 

If the 6.5 turns down isn't normal, I can keep troubleshooting.

[Designer]

Hi YankWithAMini,

 

I have had the same problem with the jet assembly sticking when pushing the choke back in. Sometime in the past I had my HS4 Carb serviced by a local garage and it would appear they fitted an after market jet which was not true speck. The length of tube from the bottom of the bowl to the end of the jet assembly is too short thus pulling the whole unit side ways.

 

Read the post headed Su Hs4 Carburettor post started by me on April 6th 2023.

 

The problem arose after I had changed the placing of the linkage back to its proper position.

As I am planning to have the carb fully serviced by SU Carbs at the end of the summer I have set it so it has passed it's MOT and is driveable. With the bad state of the roads in this country and the Mini's suspension if it sticks it soon gets knocked back in. If not I just stop and push it in once the engine reaches near normal running temperature.

 

Regards Paddy