The V8 Owners Forum -

Hi everyone,

Got a P5B at the moment that's refusing to play ball - basically when it's cold it starts up right away, idles well and will run indefinitely, even when hot and driven hard.

The moment you turn it off, it's nigh-on impossible to restart - it doesn't even cough.

I gave the carbs the quick once over, cleaned the dashpots and needles, check the fuel filter and flow (have good flow to the float chambers) and cleaned up all the plugs.

Ignition system looks OK and I have a visible spark - just wondered if maybe it's not strong enough once the engine's hot and trying to restart?

I seem to remember a lot of problems on these with fuel vaporisation but it seems a bit extreme - either that or low compression - but the engine runs very well otherwise.

Any tips or guidance appreciated



Lewis

Finally found the right combination of keywords to get some decent search results......so will plug away at those tomorrow - mostly ballast resistor, fuel float (sticking needles) and various other bits...

It might be running too rich. So it starts and runs fine from cold, but when you try a hot start, it's effectively flooding it.

Have you had the HC's / CO measured? What's the fuel economy like?

Chris.

I don't know, it hasn't been used for 19 years so isn't really road legal yet

I will pull the plugs tomorrow (I ran it for a good 15 minutes today at varying speeds) and see what they're like. It does seem a little black/sooty behind the exhaust so you could be on the money there, thinking about it!

Did use a fair bit of fuel in that idling session alone...

Have a gratuitous engine shot, for the hell of it:



Have to say, after fiddling around with this one, really developing a bit of a soft spot for the old 3.5 again. Sounds lovely and turbine smooth (at least, this one is ).

Hello Lewis,

I am inclined to think that even if the idle mixture was on the rich side, that would not be the cause. You can check if they are too rich by lifting each piston via their lift pin or with a screw driver lifting the piston slightly and see it the revs rise without dropping back down.

The air filters if dirty, blocked flame traps and hoses, dirty engine breather filter all play their part in reducing air into the engine so the ratio in favour of fuel increases.

You could change the coil and the spark plug leads and see if that makes a difference.

Vapour lock is also a high possibility. Is the engine fitted with a mechanical fuel pump? If so a new kit would probably be a good idea too.

Next time you stop the engine when hot, and try to restart and it won't...release the top nut on the fuel filter, watch for bubbles. Tighten and restart. If she fires, you have found your problem.

Ron.

Some progress today, I think it's definitely the carbs that are out of whack somewhat.

I took off the flame traps and pipework, gave them a good clean out - whilst they were off, out of my own curiousity, I turned it over and it ran really well. Overtly rich mix leaned by additional oxygen/air?

I put about half a turn on each mixture nut and reattached the breather hoses, at which point it maintained a steady-ish idle.

Looking at the bridge in the carbs where the dashpot and needle come to a halt - where the fuel is drawn in - there is a noticeable step from the bridge flat to that of the choke assembly/fuel inlet hole - with the choke off. Strikes me as too much of a step, i.e. too rich.

So, I think it just needs a real good tune now - balance the carbs and set the mixture up. Still too rich I think, it dies sometimes if left unattended.

Will keep on going and let you know! Will also check for vaporisation (thanks for all the tips Ron, much appreciated) and the other odds and sods - unfortunately the weather is against me today!

...forgot to say, part of this 'rich' conclusion came from the fact that if you had your foot depressed on the pedal when starting warm/hot today, it'd run.

Take your foot off though and it'd eventually slow, stagger, then stop. Seems to be improving with every little tweak though so hopefully a good session will straighten it all out.

Will let you know! Stand to be corrected on any/all of the above conclusions, mind

Lewis,

Just a wild thought, have you had the air cleaner assembly apart to ensure some little beastie has not made a nest in there whilst it has been lying dormant and partially blocked the air filter.

Looks a nice old thing by the way.

Kevin.