Starting Problem??

[wculbert]

Hi,
I have a problem with starting my Rochdale GT, it has a triumph 1500 engine and SU carbs. When I turn the key the engine turns over, starts and then cuts out. I try repeatedly and does the same, eventully I get it started. When it runs first it trys to die but when it gets going it is fine and there are no problems. Does anyone know what the problem could be? Thanks William

[grahambell]

Hi William,

Is this just a cold start problem or does it do the same when it's hot? If just a cold start problem it could point to a faulty choke, though must admit I don't know what sort of choke your SUs have. More of a Weber man myself.

Might also be worth a look at the fuel pump if it's an SU electric one as they're notorious for the points sticking when they start getting on a bit. A couple of good taps on the pump body with something solid usually effects a quick - though temporary - fix.

[Martin_S]

As Graham says, if it is a cold start problem (which I'm guessing it is, from your comment that 'when it gets going, it's fine'), then it sounds as though the mixture is not rich enough to fire.

Is this a problem which has just manifested itself on an engine that was running fine before or has anything been changed/rebuilt on the engine recently?

Depends which type of SU carb you have. Like Graham, I'm more of a Weber man, but IIRC there are two types of cold start enrichment used on different types of SU's. One drops the needle seat to let more fuel flow past the main needle, the other has a separate enrichment circuit.

Either way, check that the linkages are correctly adjusted (particularly that the throttle is not held part open and that the choke linkage is working correctly), the piston in the SU is moving freely and the needle seating correctly, and that the oil level is correct in the dashpot.

Could also be an air leak somewhere(inlet manifold?) that is causing the problem.

SU carbs are pretty simple and are easy to strip down and adjust, but need to be set-up correctly to run right. Your best bet is to invest in the Haynes Manual for SU carburettors and work through the tuning and set up procedures shown in that.

[wculbert]

Hi,
Thanks for the replies. The problem just started, I changed the jets for gouse jets a while back but with no problems. It was running fine but just started doing this the other day. Thanks William

[Ribol]

If it is Triumph 1500 based I would have thought it would have an AC mechanical type fuel pump on the engine block?

If it has, it could be that the fuel is draining back to the tank. Car starts on fuel in SU chambers when it runs out, engine dies, cranking engine a while drags up fuel from tank again, runs alright when started.

How does that sound for a guess?

[Martin_S]

But the engine (hence the fuel pump) runs much faster once it has fired than when it is cranking on the starter.

So if it fired on the fuel in the float chambers, the pump would drag up fresh fuel almost instantaneously to keep it running?

[Ribol]

Martin_S said:

But the engine (hence the fuel pump) runs much faster once it has fired than when it is cranking on the starter. So if it fired on the fuel in the float chambers, the pump would drag up fresh fuel almost instantaneously to keep it running?

Yes but when fuel drains back to tanks you get an air lock and that has to be cleared before the fuel gets to the pump. Then the carb chambers have to be filled to the right level before it will run properly.

[wculbert]

Thanks for your email. That sounds possible. How does one fix it?
Thanks William

[Ribol]

wculbert said:

That sounds possible. How does one fix it? Thanks William

First you need to prove it is.

Assuming the fuel pump is mechanical(?) then it will have a flexible pipe going to the body work. Clamp the flexible pipe up as soon as you switch off so the fuel does not have the time to drain off. You can use something like Mole grips with cloth to do this. In the morning unclamp and try straight away, if it sorts problem you know what is wrong.

Like I said before this is only a suggestion, that may not be the problem but it is what I would try given what you have said so far.

[BliarOut]

Silly thought, but worth a check as I lost hours on a similar problem. When it's happening, release the fuel filler. Could be a vacuum in the tank. I only cured that one when I heard a hissing. If she runs ok with the filler cap off, check the breather.

[Martin_S]

Ribol said:

wculbert said: That sounds possible. How does one fix it? Thanks William First you need to prove it is. Assuming the fuel pump is mechanical(?) then it will have a flexible pipe going to the body work. Clamp the flexible pipe up as soon as you switch off so the fuel does not have the time to drain off. You can use something like Mole grips with cloth to do this. In the morning unclamp and try straight away, if it sorts problem you know what is wrong. Like I said before this is only a suggestion, that may not be the problem but it is what I would try given what you have said so far.

Alternatively, disconnect the fuel pipe at the carb and insert the end into a suitable receptacle (empty pop bottle?). Turn the engine over on the starter. if fuel does not start spurting out of the end of the fuel pipe within a few seconds, you have a problem.

[IOLAIRE]

wculbert said:

Hi, I have a problem with starting my Rochdale GT, it has a triumph 1500 engine and SU carbs. When I turn the key the engine turns over, starts and then cuts out. I try repeatedly and does the same, eventully I get it started. When it runs first it trys to die but when it gets going it is fine and there are no problems. Does anyone know what the problem could be? Thanks William

William,
your car is fitted with the Spifire engine which, depending on the year, will have either inch and a quarter or inch and a half SUs fitted; either way they will be virtually identical.
These carbs have only one jet, which is both the strength and weakness of the design; if anything goes wrong with the jet or needle the fuel flow to the engine is interrupted throughout the whole operating range of the engine and the car stops. But on the other hand the simplicity of the design makes it easy to repair and modify.
You say you changed the jets for "gouse" jets, what on earth are those, I've never heard of them?
These carbs operate by having a fixed jet in the base of the carb which is adjustable up and down thus allowing mixture control. A damper is fitted to the top of the carb holding a needle that runs down into the jet: the needles are available in differing thicknesses and shapes for varying uses.
They are not simply tapered although to the naked eye it looks like that; they actually have a contoured shape. On the base of the needle are code letters that tell you what size they are.
As you open the throttle the vacuum created by the increased airflow through the venturi lifts the damper up and, as the needle rises, more fuel is sucked from the jet.
The action of the fuel being sucked past a needle tip atomises it into a more easily combustable vapour.
On your car the initial mixture setting is effected by turning a brass nut on the bottom of the carb that lifts the jet up or down; lifting it up will weaken the mixture, down the reverse.
If you get your car fully warmed up and it starts and performs perfectly, then the problem is lack of fuel on cold start up.
There are various systems on SUs to enrich the mixture required for cold start, but they are all vital.
If an engine running on SUs is correctly set up, it will not start from cold unless the mixture is enriched. If you are lucky enough to get it to start, it will run like the proverbial pig until warm.
On your car there is an extra linkage bar running between the two carbs that is attached to the choke cable; this has tiny nuts and bolts at each end that attach to the jets in each carb.
When you pull the choke cable the linkage pulls the jets down inside the carbs, enrichening the mixture.
There is also an attachment to the throttle linkage to give you fast idle; all of this is adjustable at each carb and frequently these adjustments come loose and you lose choke.
To check it get someone to sit in the car and pull the choke cable right out whilst you watch the jets on the bottom of the carbs; they should move down about three quarters of an inch at full choke; if they don't, you've found your problem, and it's simply a matter of making the required adjustment to the linkage setup to get choke.
One other common fault is dirt in the tiny supply tube that runs from the float chamber to the bottom of the jet, this supplies the jet with fuel on demand and sometimes can be partially blocked so that it won't supply enough for an enriched and hence higher fuel demand when cold.
Remove the float cover and check if there is dirt or corrosion of the alloy in the bottom: to clean it out effectively, you'll need solvent and compressed air.
Now ALL of the above is on the understanding that you have been sensible enough to check out your ignition system and compressions first, MOST important!!
Good luck.

[DeltaFox]

Do yourself a favour and check....has it got points perhaps? Change them and the CONDENSER. It sounds like its gone awol to me.