Hesitation above 70mph

SeanP · Post by **SeanP** » Wed Jun 14, 2017 6:53 pm

neilbardsley wrote:
wildtexas wrote:They're supposed to do 110mph but I think it wouldn't be a comfortable place to be.
I got pretty close to that in my 912 once before I noticed! Mind you, it's a targa so there is always some wind noise. 70+ should be easy for a 912.

Dunno about that..... have you been out driving with Marcus lately. Apparently the Leicester constabulary had him clocked at 115+ slowing down for a left hander!

Kneeslider · Post by **Kneeslider** » Wed Jun 14, 2017 10:08 pm

Mike (Smith) mentioned checking for full throttle. Get an assistant to press the throttle pedal to the floor while you look down the throat of one of the carbs, and make sure that you have full movement. Check by popping off a ball link, and seeing if the arm on the carb will travel any further.

This could be several things. Is there a throttle stop on the floor which you can screw in and out to set the pedal travel?

If it is like a 356, then there will be a bell crank on the gearbox, and another behind the fan shroud. There are adjustments at every point, and all need to be set correctly. If the car is idling properly, then pop off the ball link that comes into the engine compartment, and adjust the length of the vertical link behind the fan shroud till you have 50mm between the fan shroud and the centre of the ball at the link you have just removed. Then adjust the length of the 'coat hanger' rod that goes up from the gearbox into the engine compartment. This ought to get you close. There is an article on the Registry forums about this process, and pictures speak a thousand words.

Next, do you have a 'soft' link in one of the throttle rods? Is it in tact?

Is the petcock bowl full of crud?

You can't be that far away from having everything working!

AndrewSlater · Post by **AndrewSlater** » Wed Jun 21, 2017 11:53 am

I had hoped to have changed the remaining rubber fuel lines by now, but I've been sent the wrong hose.

So I decided to take the fuel tank sump guard off, and decant some fuel in preparation for the job.

I put a hose on the engine compartment line and decanted about 3 gallons or so using a hand bulb primer.
As I was managing to pump 5 litres of fuel in under 5 minutes, this got me thinking that surely I was hand pumping the fuel out much quicker than the car would ever consume it.

The hand primer bulb is clearly sucking the fuel out, so should replicate what the fuel pump is doing.
The only difference to the normal setup is that I bypassed the fuel filter and connected at the hardline in the engine bay.

Before I go for the all out rubber fuel line change, I think I will investigate whether the fuel filter is restricting flow.

AndrewSlater · Post by **AndrewSlater** » Sat Jun 24, 2017 7:35 pm

So my replacement fuel line arrived and I set about changing the two rubber lines, tank - tunnel and tunnel to engine compartment line.

First thing was to take the tank out.

The front pipe was slightly soft but nothing particularly bad - shown in the top with the replacement line below.

The rear was a different make and had become quite hard but not yet brittle.

I used a 1.5m cleaning line/brush to clean the tunnel line from front to back and back to front but didn't find any debris.
I did the same for the rear line and there was a little brown dirt but after a good clean with carb cleaner it all looked OK.

I noticed one of the blanking plugs on my tank had been slightly weeping so decided to refit that.

After refitting everything the tank was filled with about 4 gallons of fresh fuel and we went for another test drive.

Unfortunately the performance seems identical so the problem seems to be still there.

At least I know all my fuel lines are clean and fitted with fresh rubber.

I think the next step is to look more closely at my Solex carbs.

AndrewSlater · Post by **AndrewSlater** » Wed Jun 28, 2017 9:00 am

I have almost completed rebuilding a good set of Solex cores, so I can swap them out with mine to see if anything changes with the engine performance.

I decided to rebuild them without referring to my current ones - as if there is a problem with my current carbs I didn't want to repeat it.

The 'new' set are pretty much built and the float levels have been set on the workshop bench.

The one question I have is what the latest recommended pump injector quantity is?

At one place 356CarbRescue suggest 0.35ml per two strokes, but also reference the Duane Spencer guide that suggests 0.1cc per strong pump.

Does anyone have any recent experience or suggestions as to what to set to.

Thanks
Andrew

mycar · Post by **mycar** » Wed Jun 28, 2017 9:14 am

The Solex 40P11 tuning article in the Registry's Resource says..

"Place the vial in the throttle bore under the squirter to catch the gas as it comes out of the accelerator pump nozzle. Check manual for correct amount of fuel per stroke -- sometimes it's more accurate to use a number of strokes and divide by the number of strokes. When stroking the throttle, open the throttle smartly and hold open until squirting has stopped before attempting another stroke.

The injection quantity can be varied by adjusting the length of the stroke using the nuts on the accelerator pump adjustment threaded pull rod (#12).

Note: Many believe that the factory settings are too high for todays gas, and can cause a flat spot upon acceleration. You may want to experiment with using a somewhat lower volume."

https://porsche356registry.org/article/47

Perhaps the info is in the 912 Technical Specification book or Spec's Book ? Maybe someone here has a copy handy, or Design 911 sell them.

http://www.design911.com/Technical-Spec ... /prod6124/

AndrewSlater · Post by **AndrewSlater** » Thu Jun 29, 2017 9:10 am

Thanks for the setup info - this is the procedure I have been following.

I believe the original spec is 0.45cc warm weather / 0.65cc cold (two strokes) - which is claimed to be way too high for modern fuel.
I'm pretty sure my current Solexes are set to 0.35ml for two strokes - but that was four or so years ago.

Just wondered what other people find works well these days.

Thanks
Andrew

AndrewSlater · Post by **AndrewSlater** » Wed Jul 05, 2017 8:59 am

I haven't quite finished rebuilding my second set of carbs, as I have run into some quality issues on the rebuild parts that I ordered from Italy.

I'd built up the carbs only to realise that the pump jets weren't fitting and sealing correctly. Turned out it was a tolerance issue in both the part and also the supplied gaskets.

In the meantime I started to strip off the airboxes in the car in preparation for a swap and to do some further investigation.

Turns out the throttle plates aren't opening 100% on full accelerator travel.
After taking out a lot of slack around the rubber throttle coupler I'm now getting 90% or so, but still struggling to adjust the last 10%.

The rubber throttle couple seems to have a few mms of slop in it, so maybe time for a new one.

With the increased throttle opening I went for another test drive. The car seems quite a bit more responsive to drive, but oddly the top end wasn't improved as much as I was hoping.

I think the next step is to adjust the linkages again to get 100% throttle and see what that does.
I then plan on swapping the carburettors over one at a time to see what differences I see.

I will keep you updated.

Mike Smith · Post by **Mike Smith** » Wed Jul 05, 2017 11:24 am

Mike Smith
Post subject: Re: Hesitation above 70mph

Posted: Thu Jun 08, 2017 10:36 pm

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

_________________

Further Note - The emphasis is on FULL - That last 3mm to 6mm makes a HUGH difference"

Mike at P.R.S.
http://www.prs356.com

AndrewSlater · Post by **AndrewSlater** » Wed Jul 05, 2017 1:34 pm

Thanks for the advice Mike - and I gladly acknowledge that this was the first bit of advice you gave me - so spot on there.

Can I ask how this is best done as visibility down the bores even with a mirror and torch is difficult and the angle difficult to judge?

Am I aiming for the throttle arm to simply bottom out, or do I need to check the butterflies in detail?

The reason I say that, is that some sources say to ensure that that butterflies don't open too far ( which I take to mean over-rotate past 90degrees ).
I'm pretty sure that with my newly built carbs the throttle plates are parallel to the walls of the carburettor when the arms bottom out, so can't see how they would ever over-rotate.

Finally is there a linkage adjustment guide anywhere that explains how to set the travel, from pedal to carb?

As always any help much appreciated.
I am hoping to get to the bottom of this this week before a family holiday will stop play.

55ovalcharlie · Post by **55ovalcharlie** » Wed Jul 05, 2017 1:52 pm

the positions of the crank behind the fan shroud is particularly important to get full throttle (the one labelled "1" on this image swiped off google images)

Mike Smith · Post by **Mike Smith** » Wed Jul 05, 2017 7:37 pm

Andrew,
As much as I would like to help further - setting up carbs, although simple and logical, represents too much typing for me and I really, really, do not have the time. This is why I seldom get involved with the website on a regular basis.

The illustration that Charlie has posted will certainly help you considerably and the 2 x Dimensions quoted are IMPORTANT
Start with the 50mm one on the back of the fan housing - basically you are working out from that dimension in two directions

Remember when balancing the carbs that the linkages to each carb. should be removed - they are the last thing that you adjust for length when everything else has been adjusted

The other important item is the pedal stop
Make sure that you have one (No pedal stop means that eventually all the linkages will end up distorted and one of the ball ends will pop off when you have your foot to the floor on the outside lane - ask me how I know)

Also make sure that it is not this pedal stop that is actually stopping you getting full throttle
For full throttle both the Butterfly`s need to be vertical in the Carb. bore

Of course - you may also be trying to get too much speed out of a knackered engine ?

Look how much I have already typed and I have only given you a couple of tips - must stop
I have to go to the pub and defend my Darts record

AndrewSlater · Post by **AndrewSlater** » Wed Jul 05, 2017 11:00 pm

Thanks for the wise words Mike, I appreciate the effort you and others have made in helping me out.
Hopefully the darts went well!

I spent a fun evening crawling under the car and into the engine bay slowly adjusting the linkages.
I now have full throttle opening on both carbs - and the car is running much better for it.

I suspect there is still more to be gained from some more fettling but the car is now much more enjoyable to drive.
Once I get some time I will start fresh over from valve clearances all the way through to fitting and setting my rebuilt carbs.

Thanks to everyone for their contributions - I will update as I make any other improvements.

Mike Smith · Post by **Mike Smith** » Thu Jul 06, 2017 8:51 am

What are you achieving now at the top end ?

AndrewSlater · Post by **AndrewSlater** » Thu Jul 06, 2017 2:55 pm

Hi Mike,

by the time I had finished and warmed up the engine around some A roads I didn't get much time to test it on the local motorway.
From the brief m/way test my first impressions are that top end is a bit better but I still think there is more to solve.

Obviously I'm not worried about the top speed, but the car drives much better all round (as you would expect).

My next step is to have another go at tuning the carbs, before swapping to the newly refurbed ones.

All the changes are giving incremental improvements so it is definitely getting better.

Thanks again for the help - will update with more info when I get some free time.