Clearing SU carb jets

[Timothy Keith-Lucas]

The engine acted like it was running out of gas and then died. Spark plugs are the right shade of brown but dry after attempts at starting. Spark is present on all cylinders. Fuel level is correct in both bowls, and the electric fuel pump is delivering fuel to both bowls. Compression is above 150 PSI on all cylinders. I'm down to suspecting the jets are clogged. Yeah, I delayed putting a filter in the line and one is now on order. 

 

If I've made the right diagnosis, what is the easiest way to clear clogged jets? I could come from the top, remove the needles, and run a fine wire through the jets. I could detach the fuel feed at the bottom of each carb and blow air upward while holding the needles up. Maybe there's a standard way of doing it and you folks know the secret.

 

Is must be a pretty common problem, but it doesn't earn any comments in my SU carb manuals.  It certainly is on other small engines. Some mechanics just blow carb cleaner up the jets and hope for the best. 

[MV8]

You could remove the bowl tops, drain and clean the bowls, remove the needles, then with a rubber tipped air hose nozzle and a small air tank and/or mini-compressor, blow from the needle side back into the bowl, then clean the bowls again.

 

I always add filters to small engines and valves to shut off the fuel flow so they can burn most of the gas out of the bowls instead of shutting them off electrically. Minimizes ethanol problems.

[jbcollier]

SU carbs have (relatively) huge jets and seldom get restricted unless very large chunks indeed are in your system -- peeling tank liner for example.  If the fuel in the float bowels looks clear, this is not your issue.

 

I know nothing about your engine but try a NEW set of plugs.  Ordinary NGK spark plugs can be prone to clean fouling.  The plugs look perfect and fire just fine when removed from the cylinder.  Pressurize them though, and they will fail.  I bought a spark plug pressure tester for just this reason.  I also avoid the basic NGK offerings.

 

I spoke to a NGK technical chap and they acknowledged the issue.  Something to do with the basic plugs having a slightly porous ceramic insulator.  Their iridium and platinum plugs are just fine.

 

After that, replace the condenser if you still have a distributor with points.

[Drakman]

Just a thought, take the fuel cap off and try, if the cap is vented and the vent blocks it creates a vacuum in the tank and a no fuel flow issue results.  The other thing is a few years ago there were quite a few fake NGK spark plugs being sold, mostly through Ebay, Chinese copies that produced the symptoms you are describing.  A quick rough and ready SU flow check, take the float bowl top and the dashpot and piston off, while looking down into the top of the carb pour a little fuel into the float bowl, you should see the fuel overflow out of the top of the jet.  In fact if the float level is right you might even see the fuel in the jet just by rocking the car, the fuel level will obviously be the same in the bowl and the jet.

 

Dave

[Timothy Keith-Lucas]

Found it! The problem went back to my buying a set of thin wrenches that inexplicably lacked a 5/8 inch open end. The locking nuts on the jets were as a result not as tight as I should have made them, and the two carbs self-adjusted to the lean side, particularly the rear one. Too lean to fire. Setting them back to two turns from the top wasn't enough either; these carbs need 2 1/2 or so.  Thank you for all your suggestions - they were not wasted, because I now know a bunch about SU carbs that I didn't know before I started. It is a bit of a challenge to master this car when there is no nearby source of expertise. 

[jbcollier]

The jets need to be centred properly or the needles will rub.  Just google SU jet centring for the procedure.

[speedwagon]

I really like the SUs. Got 350hp out of a single 2" 164ci Corvair 9 minute engine ( did get to the top of pikes peak on 3 cylinders). mixture was slightly rich at 5000 feet and burned clean at 14000 (the high boost wasn't the carbs fault)  They are hard to learn and I never completely understood them. Still have several in one of my containers) john