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Joined: 09 Jul 2019
Posts: 2

PostPosted: Tue Jul 09, 2019 10:53 pm    Post subject: 1969 - 911 - 2.0l - Starting Problems when Warm Reply with quote

Hope someone can help!

My 1969 911, 2.0L has a problem in that it will start no problem at all when cold, but I can run 5 mins to the petrol station fill up, but then just will not start. This is not associated with filling up as same happens if I run to the shop.

It is running on Carbs and I know engine to be in good condition, runs great on the road....just too scared to drive anywhere incase I cannot start.

If I wait 30 mins it will start back up. I read somewhere floor the pedal, crank till it fires, ease off and start but no luck.

Fuel pump seems to click away as per cold start, slows down I assume as pressure is built up till started.

Any ideas?
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Joined: 18 Dec 2018
Posts: 457
Location: Scotland

PostPosted: Wed Jul 10, 2019 8:11 am    Post subject: Reply with quote

Riisu... a circa 50 year old carb fed sports car can have all sorts of issues relative to the way it has been treated over the years. I also think that there were a few carb systems in use on early 911`s, some with electrical enrichment,micro switch and solenoid controlled (Zenith?) think Solex and Weber systems may have been used also and who knows how many non standard modifications/repairs may have been carried out to a 911 of that age...?

Could also be ignition related...

With no clue as to your mechanical capabilities...

Perhaps the simplest operation might be to run the car over the approximate time/distance it takes you to get to the shops/petrol station, but end up back at home, shut it off for a few minutes then try a re-start..I suspect your 911 will have accelerator pumps fitted to the carbs if the problem was lack of fuel then prior to, or during the attempt at a warm start perhaps a few quick full strokes of the accelerator will introduce more fuel (assuming the accelerator pumps are still operational..?)

If that does not work for you, then perhaps if you pull one of the spark plug leads off and using a spare spark plug, plug it into the lead, placing the spark plug somewhere that it`s metal body has contact with a metal part of the engine, and the electrodes can be observed to visually check if there is a spark while the engine is being turned over.... DO NOT TOUCH the plug while the engine is cranking as it can give you a nasty shock... could even kill you ..!!!

If that does not work, I guess another relatively simple option would be to buy a can of easy/cold start or similar from Halfords or a car accessory shop.... Follow the instructions on the can....... BE aware that the contents of the can are highly flammable and thus judicious use is required

Keeping my fingers crossed for your old 911.. Thumb
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Joined: 21 Mar 2015
Posts: 7268
Location: Brighton

PostPosted: Wed Jul 10, 2019 7:35 pm    Post subject: Reply with quote

Personally i think you need to find a local independent Porsche garage to have a look at this fault .

Off hand a coil breaking down , points closed or in need of replacement .. a carb float stuck and flooding .. incorrect setting up of the carbs .

Fuel delivery is a possible fault .. temp alters a lot of things .. parts get hot and stop working properly .

You certainly seem to have a heat related problem from your description .

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Joined: 09 Jul 2019
Posts: 2

PostPosted: Wed Jul 10, 2019 10:18 pm    Post subject: Reply with quote

Thanks for the suggestions. I hope I can be pretty confident that the Carbs, Coil and points are good.

The engine and carbs were completely rebuild by Pro9 in Birmingham as I struggled to find any garages who were confident to do this in my area.

So ancillary items are all refurbed/new. The item which was not changed was the Fuel Pump and I suspect this..but not knowledgable enough to know how this could be affecting either flooding or no fuel to the carbs?

There is only a plastic fuel filter between pump and carbs..should there be any additional pressure control vessels? or could a fuel supply air lock be happening..and best way to investigate is it flooding/starvation.

Are there any sensors which could be shutting off the supply till the engine cools?

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Joined: 18 Dec 2018
Posts: 457
Location: Scotland

PostPosted: Thu Jul 11, 2019 10:02 am    Post subject: Reply with quote

Thanks for providing a little more information Riisu. I am no expert but happy to share my limited knowledge.. From that which you type it seems you may not be mechanically in tune with your Porsche... but are keen to gain understanding..before handing it over to someone else to repair for you ...?

My suggestions were related to simple tests, to establish whether the issue was down to lack of sparks or lack of fuel.

As you seem to think for whatever reason your issue is fuel related we will go with that given as STANDARD your fuel system would have been pretty basic... You have not identified the carbs fitted to your car and whether the fuel pump is mechanical or electrically operated. As I had suggested, given the passage of time much could have changed relative to your 911`s original specification..

In terms of sensors, again dependant on the carbs you have, there may be an electrically operated Auxiliary enrichment device that relies on the operation of a micro switch to energise an solenoid (electrical valve) to act as an automatic choke to assist cold starting...Your car may still start from cold with this system not operating correctly but stuck in the situation where it is enriching the mixture, and in this situation it could be that your mixture could be overly rich when trying to re-start when hot and the extra fuel is not required but is now inhibiting a start..... But then that is nothing more than a possible theory so do not grab at it as being the answer..

Rather than waiting till the engine will not re-start.. with the engine still running and warmed up if the "choke" is stuck "on" the exhaust may be smoky and the tail pipes black and sooty.... providing a hint as to what the issue may be..?

A general outline of the fuel system I might expect to be fitted to your car is that each carb body has a float that operates on a hinge as the fuel fills the carb.. float bowl.. the float rises with the fuel level and when it arrives at the ideal fuel level for carb operation, the float contacts and closes the valve supplying fuel to that carburettor, avoiding over filling the carb and overly richening the mixture or at worst spilling excess fuel out of the overflow.... As the fuel pump is obviously running to supply fuel for this process, with the float having reached the required level and the valve having shut off the fuel to the carb, the pressure will rise in the fuel line between the fuel pump output connection and the carburettors, at which point the fuel pump`s internal pressure sensing mechanism will cut the electrical circuit to the pump/ or operate a mechanical system to stop pumping, dependant on which type of pump is fitted..... until the engine uses up a measure of fuel in the float bowl at which stage the float drops and opens the valve, the pressure in the line drops, the pump switches back on, and the cycle continues...

In a situation where the float or the valve fails to shut off fuel to the carburettor there is an overflow system that generally dumps the excess fuel on to the road away from exhaust systems and the likes, such a situation could create a possible non start situation caused by overly rich air/fuel mixture, the smell of fuel could indicate such a possibility...? Looking under the engine may also provide some clue...Though I have no memory of the actual overflow system fitted to early 911`s, but I did once own a late 60`s 912.

My thinking behind the simple introduction of the easy start was to determine if you had a fuel issue or not.... But strange as it may seem engines can become hooked on that stuff, thus my suggestion was to use it as a one time test, and not as an alternative to a proper repair process, should your engine have re-started.

If as it seems you do not have a grasp of fuel system basics, then I suspect the very wise Mr DeMort`s suggestion would be the safest option for you, for I suspect it might even be too much to expect the average garage "mechanic" today to be up to speed with triple downdraught carbs..Though if there is a classic car specialist in your area that might be the next best bet if no Porsche specific mechanical skills are to be found locally..

While I understand so much information is available on the interweb it might still be worthwhile to try to obtain the Haynes Porsche 911 owners workshop manual No 264 in the Haynes series. Covering models from 2.0L through to 3.2L ISBN 1 85010 243 0 Which may not provide the answer to your issues, it may at least give you further insight to the nuts and bolts of your 911...?

Hope this helps in some way Riisu. Question
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Joined: 03 Mar 2007
Posts: 673
Location: North Yorkshire

PostPosted: Thu Jul 11, 2019 10:15 am    Post subject: Reply with quote

post up on ddk ( www.ddk-online.com) . Lots of owners running the same car with carbs ) depends what actual carbs youre running & also dont' assume because theyve been rebuilt that they are set up correctly or good ) . Hot start issues has been a relatively common question over the years

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Joined: 16 Mar 2015
Posts: 181
Location: south wales boy bach

PostPosted: Thu Jul 11, 2019 6:35 pm    Post subject: Reply with quote

Had this sort of issue with cars and bikes over the years....I would say coil,ht leads ,doubt it would be carb probs,
MARK-english lad in wales....where men are men and sheep are scared

84 carrera 3.2
And a couple of old bikes
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