Hesitation above 70mph

[AndrewSlater]

We took the 912 to the Beaulieu Simply Porsche gathering at the weekend (great event).
Rather than stick to the back roads we decided to brave it on the Motorway to give it a good run out.
It's a 4 speed and happily reached 70 on the motorway ( to the dismay of a number of cars we overtook ), however beyond this speed progress is a bit of a struggle.
I normally run it on A-roads so haven't really spotted this limit before.

So anybody got any suggestions as to what the problem is (if there is one)?

My thoughts are:-

Ignition advance, Carb jetting or fuel delivery.

So I've checked my 022 distributor and I'm getting approx. 35 degree total advance at 3000rpm ( all seems good - but I could retard it a little to better match modern fuels - but would this make a significant difference). I have a second 022 I have refurbished that I could substitute to try ( or even a boxed NOS 031 ). Somehow I don't think the distributor is the culprit.

I did rebuild my Solex 40PII-4 carbs when I restored the cars a few years back, so in pretty good state. They do have a little throat wear but nothing that stops me getting a good idle. I researched this all at the time and put new stock jets in.
If my main jets were a size too small would I get this effect.

I'm running on the original mechanical fuel pump ( rebuilt ) and the float heights were recently set on the carbs. How would I know if I had a fuel flow problem, or better still how to test it with a mechanical pump?

So in what is your guess as to what is causing the high speed hesitation/limit?

As always any suggestions would be much appreciated.
Thanks
Andrew

[Nick Moss]

How old are your various rubber fuel lines? Modern fuels destroy the integrity of old hoses and as your fuel pump is a "sucker" it tends to collapse the weakened hoses.

[AndrewSlater]

Hi Nick,

thanks for the prompt response.

A new tank was fitted about 4-5 years ago with a new tank filter and all new flexible lines.

All the flexible fuel lines in the engine bay were replaced with a new fuel filter the day before the Beaulieu run.

On reflection I think I saw this issue when the car was first re-commissioned 5 or so years ago, but I tend to avoid motorways so not really seen the issue.

Fuel starvation still makes sense, so I'm wondering if either

the rigid line through the tunnel is restricted in some way.
or
the fuel pump can't achieve the flow rate I need.

I'm really looking for a simple test to try and narrow down the problem - any thoughts?

[100702mark]

Andrew

Had similar problems on my 912, above 3000 revs it was running very uneven. I solved it by retarding the timing a bit, I seem o remember about 32 degrees was recommended.

Mark

[Mike Smith]

Your first check is to ensure that you are getting Full Throttle on Both Carbs

[AndrewSlater]

All good points gents - thanks for the assistance.

Your first check is to ensure that you are getting Full Throttle on Both Carbs

This was all carefully setup a few years back and I definitely had full throttle on both carbs then - but it is definitely worth a recheck.

Andrew

Had similar problems on my 912, above 3000 revs it was running very uneven. I solved it by retarding the timing a bit, I seem o remember about 32 degrees was recommended.

Mark

Does anyone think 35 degrees instead of 32 is likely to cause this problem?
I'm currently running super unleaded ( Sainsburys finest )

[AndrewSlater]

I am lucky to have a lot of the original documentation with the car, including the original owners notepad.
In here he recorded the cars usage and any work that was carried out during his ownership from 1966 to 2007 - which is a fascinating read.

According to the notebook, on March 14 1971 he made a round trip from Las Vegas to Bullhead Arizona and back (240 miles in total), at speeds of 80-90mph, returning 23.8mpg. The car had 52000 miles on the clock.

So I have evidence that it used to go faster than it does now - albeit 46 years ago

Mind you in the months after he wasn't happy with the performance and adjusted the timing twice. He also rebuilt the carburettors later in the September as he still wasn't happy with the performance - and the Beck rebuild kits were an extortionate $2.50 each.

[neilbardsley]

I don't think 35 degreed is the cause. Do you have the same issue at 3000 rpm in the 3rd gear? That size idle/main jets do you have. Consider buying a cheap pressure gauge to measure the fuel pressure above 3000 rpm

Sent from my X5pro using Tapatalk

[sennen]

I had the same issue on my 12'

Jez @ GCS solved it with hotter plugs, as excess fuel was not being burnt -it kind of went boggy at 70 mph.
that was last year, no heat problems and now runs great

[AndrewSlater]

I don't think 35 degreed is the cause. Do you have the same issue at 3000 rpm in the 3rd gear? That size idle/main jets do you have. Consider buying a cheap pressure gauge to measure the fuel pressure above 3000 rpm

Sent from my X5pro using Tapatalk

Thanks for the suggestion, I have measured fuel pressure previously at fast idle and it all looked good - but something else to try again.

I had the same issue on my 12'

Jez @ GCS solved it with hotter plugs, as excess fuel was not being burnt -it kind of went boggy at 70 mph.
that was last year, no heat problems and now runs great

Thanks for the info, 'boggy at 70mph' sounds exactly like what I have.
I have just bought some NGK B6HS to see if the hotter plug works better than the B7HS I have fitted. Something else to try that should be a quick test. Thanks again.

[55ovalcharlie]

Float bowls in carb(s) too low?

[mycar]

+1 on the float levels but it got me thinking.... if you posed the same question over on the 356 Registry (your engine is just a super dooper 356 engine after all) a certain Ron La Dow would harangue you and suggest you 'tune your engine'..

He has a point, with wild guesses and/or suggestion you might drop on a cure for the problem but a more methodical approach will assure your engine is in the best of health and the chances are you'll regain the ability to break the upper speed limit with ease.

Briefly the tuning procedure is, A, set the valve clearances. B, tune the ignition system. C, tune the carbs.

Details can be found here, it's important you do them in order. It takes time but is rather rewarding.

https://porsche356registry.org/article/44

https://porsche356registry.org/article/45

https://porsche356registry.org/article/47

Bon chance !

[AndrewSlater]

I changed the B7HS plugs to B6HS plugs tonight and again went for a motorway run.
There may be a slight difference but still gets bogged down at 65-70mph in 4th, so I think there is another underlying problem.

Float bowls in carb(s) too low?

I have previously bought the P75 tool and set the float bowls - so I think it is unlikely to be that.

+1 on the float levels but it got me thinking.... if you posed the same question over on the 356 Registry (your engine is just a super dooper 356 engine after all) a certain Ron La Dow would harangue you and suggest you 'tune your engine'..

When I restored the car I did rigorously go through all of the tuning steps and bought all the tools I needed along the way. The car was also final tuned by Max at Revival, however I suspect the current issue has always been present.

Although the car was 'restored' a few years ago it hasn't been driven a lot since then, so I would expect all the adjustments to still be valid.

A fuel starvation issue would make sense but I can't think of a simple test to establish this.
In the meantime I think I will reduce the distributor advance next and then possibly swap out the distributor to see if I can see any change.

Keep the suggestions coming as I determined to get to the bottom of the issue.

[mycar]

Say you're speeding along at 70 in 4th, how many revs is that ?

Do you have trouble in 3rd beyond that same rev value ? If not, it's unlikely to be you distributor advance or timing.

How many miles since doing the valves ?

Do you have inline fuel filters?

Have you checked the petcock and tank gauzes ?

Has your fuel tank being restored/lined ? Is the fuel supply squeaky clean ?

[Timo]

I'm thinking fuel delivery personally, my car has had this exact same issue on 2 occasions, the first time was down to some rubbish in the tank being pulled into the pickup when accelerating, obviously your car has had a tank refurb so this is probably not your issue.

The second time I had this problem it was down to an overly tightened fuel hose clamp on the inline filter which was causing the plastic inlet tube on the filter to be slightly squashed which restricted flow enough to cause a problem when higher flow was required. Definitely worth a check.