Carburettor or ignition problems

[Martin Nash]

Hi

I am renovating an Austin Sevon with an Ulster chassis and more recent ulster replica body. The car was in poor condition however it did start and run quite nicely. It doesn’t now! Nearing the completion of the renovation the engine was replaced .The engine is fitted with a 11/4” SU carb. I did a top side overhaul , re ground valves etc. I have though had enormous problem getting the car to run properly.  I set up the ignition pretty accurately I think. First I couldn’t get the car to start at all but by advancing the ignition managed to get her to run. Despite all efforts though she would not accelerate when opening the throttle   I undertook various adjustments to the SU (float height, centralise jet etc) and rechecked the ignition timing. Now when you start the engine it accelerates to almost maximum revs with the throttle fully closed!!! I have found by retarding the ignition somewhat the revs will drop somewhat. What could  possibly be happening? Does anyone have any ideas?
Thanks

Martin Nash

[John Cornforth]

Hi Martin

Lots of possibilities, and without seeing the car that makes diagnosis difficult but here goes...

The SU usually has a fast idle mechanism linked to the choke control, so pulling out the choke knob a little bit gives you just fast idle, then pulling more gives you fast idle plus a lowered main jet for enrichment. Check this mechanism isn't cracking open the throttle and raising idling revs.

If the throttle is genuinely nearly closed, high idle revs are a sign of uncontrolled air entering somewhere in the inlet manifolding downstream of the throttle plate e.g. manifold to head gasket, carb to manifold gasket. If the SU has a vacuum port (for vacuum advance distributor) this should be blanked off. Sometimes squirting WD-40 onto a suspect joint will cause a change in engine note and aid diagnosis.

Don't fret about exact float level, SUs are tolerant of this as the "suction" on the jet is quite significant.

If the car won't accelerate on the road when the throttle is opened, either the mixture is excessively weak (spitting back and jerking) or you have a failing condenser giving a weak spark. The weak spark isn't enough when combustion pressures increase, and you get misfiring.

If you have an auto advance distributor set the static timing to TDC to begin with. Check that the distributor weights are being completely closed by the internal springs at zero/low revs or the timing will be all over the place - If you have a Lucas DK4A they are quite sensitive to accumulated wear in the various pivot points.

[Chris KC]

As John says, it's hard to diagnose when there are so many variables and so little we know about the car.

I'm just going to pick up on one thing - " by advancing the ignition managed to get her to run".
What sort of distributor do you have? (DK = auto advance type, or DJ = manual advance - or something else?)
And what static timing did you set initially?

[Robert Leigh]

You don't mention checking that the carburettor piston in the dashpot can be lifted all the way and will fall under its own weight when released, making a distinct click when it stops. If the piston does not rise and fall freely you will have problems. Also if the carb is HS type it must have oil in the damper, if stuck engine oil wwill suffice. Carburettor and ignition faults frequently get confused because they can show similar symptoms.

[Howard Wright]

Hi

I agree with all the previous comments.  May I add the following…..

On the bright side, you have it running at high revs so there can’t be too much wrong with the engine itself!

How was TDC established? Flywheel marks are not always reliable. Best to check with a marker through No 1 spark plug hole (of course ensuring you are on the compression stroke).

Fast running smacks of an air leak.  What manifold are you using and were the faces true before bolting it up? 

Good luck, you’ll find the problem and when you do let us know.

Howard

[Martin Nash]

Guys thanks for all the useful comments. The distributor is a DJ4-7 manual advance retard (the mechanism long since discarded by previous owners). Ignition was set at 1 and 1/4 inches on the flywheel before BTDC. Piston works perfectly in the SU which is mounted at an angle to the manifold. The vacuum port is blanked off. Latest development is that she starts but with throttle fully closed runs at fast idle. This can be adjusted a bit by retarding the ignition.
When I originally dismantled the carb there were two fibre washers under the 3/8th nut that secures the jet assembly. I think this was to reduce the level of the jet assy. Even though it runs at fast idle it is very unresponsive to the throttle. i.e. if you open the throttle the revs pick up very slowly. I am beginning to think about the condenser breaking down under load. Curious thing is that the engine ran pretty well before the strip down so it's something I have or have not done. Thanks again Martin

[John Mason]

Hi, it’s a long time since I had a vehicle with a SU carb so I am a bit rusty on them. However following on from what you said about sluggish opening up. Is there anything preventing the piston lifting easily. Does it slide up and down easily. Is the vacuum working correctly. Do you have a bent needle.

John Mason

[Martin Nash]

Hi all,

With your input we have much improved the problem. Engine now runs and accelerates freely but with a slight stall early on. We changed the jet assembly and piston using the same needle. This enabled piston to "clunk" normally. Removed washers between jet assy and body of carb. Also found air leak between carb and manifold. Ignition timing critical as it doesn't have auto advance/ retard. We found that by manually advancing ignition by hand as we opened throttle and manually by hand retarding it as we reduced throttle all went well. We will replace the old manual advance/retard mechanism.Time to move on with the next challenge!!. Many thanks Martin

[John Cornforth]

Hi again

It took me a while to realise that you are presently running fixed advance, which is bound to narrow the power band. You might find it useful to know that the flywheel is 10.25 inches in diameter, so 1.25 inches at the rim represents 14 crank degrees. As I'm sure you will appreciate, that's a bit much at idle/low RPM and not enough at the top end.

The later Ruby engines used a starting setting of about 5 degrees static advance and another 16 degrees centrifugal advance to give a total of about 21 degrees, which is still fairly conservative. If you want to set a manual mechanism for 21 degrees max, that's 1.875 inches at the flywheel.

[Charles P]

I recently bought a modern timing light with the ability to “dial in” the advance. This allows you, with a single pair of TDC marks, to understand the advance at any level of revs. Very useful for diagnosing where the distributor advance is. Not much use with a fixed distributor but when you reinstate manual advance/retard would be very handy for sorting things out. Great when sorting recalcitrant auto advance distributors too.