Hesitation above 70mph

[AndrewSlater]

So I've carried out some more tests.

Firstly I reduced the total advance from 35 degrees to 30 degrees on my 022. At the top end it may have made a small improvement, but certainly no eureka moment.
At low rpm there is an occasional pop now, so probably need a tad more advance at idle.

I then broke out my NOS 031 distributor. I think the 031 has a slightly lower total advance than an 022, so should lend itself to the 912 engine with modern fuel better than the 022.
Again I set the total advance to 30 degrees. Again possible a small improvement all around - but I think that is because I swapped a used distributor for a new one.
Possibly a little better at the top end but still no eureka moment. Much better at the bottom end though. I measured the dwell to be 49 degrees.

To answer a few questions, I pulled the jets from the carbs and I am running 57.5 idles and 120 main jets. I noticed a tear in one of the washers on one of the main jet carriers so replaced that.

The tank was changed to a new one less than two hundred miles ago ( new style with no internal filter ), and all other checks within a similar mileage, valve clearances, tune etc.
The engine bay fuel lines were changed last weekend as was the inline filter. The one to the tank was new when I replaced the tank 3 years or so ago.

I've yet to rule out fuel starvation as per Timo's suggestion.
If I had to guess I would say there is some debris in the main rigid fuel line.
Any thoughts on how I clean this out? ( very long pipecleaner )
Alternatively I could set up a second fuel tank somehow to test the theory ( any thoughts ? )

Another 'easy' option would be to change the carb jetting, I have some spare 115 and 125 mains - but which way to go?

The more I look the more things are good and correct, so I must be narrowing down the problem.

As always any suggestions much appreciated.

Andrew

[mycar]

Camelbak do a long cleaning brush which is good for fuel lines.

https://www.amazon.co.uk/Camelbak-Clean ... B01MXSZNTO

Try swapping your 57.5 idles for 60s.

[AndrewSlater]

Thanks for the suggestion on the cleaning tools - Ideally I would need one that will reach at least half way down the pipe and then use if from either end - better still one I could use the full length

I'd be interested to understand your logic around changing the idle jets.
I would expect that in the mid-rpm range, only the main metering system would be supplying the fuel, i.e. the main jets ( not the idle ones ).
I may try changing the main jets up or down a size but I don't expect the characteristics of the engine to change drastically.

Keep the helpful suggestions coming - Thanks Andrew

[55ovalcharlie]

Have you double checked your float levels? This is exactly what mine did when I had a low float bowl.
Can't remember but think mine was caused by a sticking needle valve. Just run it up to speed for a minute and whip the tops off the carb to have a look or use the p75 tool.

[AndrewSlater]

Have you double checked your float levels? This is exactly what mine did when I had a low float bowl.
Can't remember but think mine was caused by a sticking needle valve. Just run it up to speed for a minute and whip the tops off the carb to have a look or use the p75 tool.

The float levels were checked less than 100 miles ago with the P75 tool - and here is a photo.



Something else to check again ( if I can find the P75 tool that is ).

[Nick Moss]

What fuel line was used? It would still be my suspicion as modern fuels destroy some hoses in hours.

[AndrewSlater]

Hi Nick, thanks again for the response.

I think I used R6 rated pipe at the time, but can't be sure.

I think the plan is to remove the front suspension cover, drain the tank and then inspect the lines.
I have two rubber lines (tank to tunnel and tunnel rear to engine bay rigid line ) to check and replace.

At the same time I will try to clean out the two steel lines at the same time.

I think this is the only way I can be certain.

I'm guessing your recommendation would be R9 rated hose or is there something better?
Do I have to worry about manufacturer?

Regards
Andrew

[mycar]

I'd be interested to understand your logic around changing the idle jets.

Yep, I'm grasping at straws here but you seem to have every other base covered. On page 200 of the 'Technical and Restoration Guide', Vern Lyle discovered that increasing the size of his 'pilot jets' to 60s got rid of a troublesome flat spot between 3000 and 4000 revs. I thought it might be worth a try.

Perhaps 912s don't go any faster than 70mph ? Maybe that's going to be it.. and if you want to go faster you have to buy a 911 ?

[AndrewSlater]

Perhaps 912s don't go any faster than 70mph ? Maybe that's going to be it.. and if you want to go faster you have to buy a 911 ?

Thanks for the feedback - I will keep the idle jet theory in mind as I try more things.

I'm not really looking to drive at high speeds in the 912 but I think the lack of top end pace is indicative of a problem - and the engineer in me would like to get to the bottom of it. I'm sure that once solved it will improve the overall driving experience.

The Sept 1965 Autocar road test, managed an average top speed across a few runs of 119mph, so there should be some more oomph in there somewhere.

[wildtexas]

Just a thought . What RPM are you pulling at 70mph.

I find at motorway speeds of 70-80 it is noisy and pulling 3000rpm plus on a 5 speed box.

They're supposed to do 110mph but I think it wouldn't be a comfortable place to be.

Matt

[AndrewSlater]

I think the final drive on the 4 and 5 speed boxes is the same ratio.
The figure I have seen quoted is 19.8mph per 1000 rpm in top, so that's pretty much spot on 3,500 rpm at 70mph - which sounds about right.

I suspect if I can sort out the issue, then general acceleration will be smoother across a wider range.

Agree it is not a quiet ride at 70+

[mycar]

Ok, my guess for today.... you have a flaccid diaphragm.

Your fuel pump's not up to the job and just not capable of delivering the sheer amount of fuel required to propel a 912 beyond 70 mph.

When was the last time you rebuilt your fuel pump ?

[AndrewSlater]

How dare you say I have a flaccid diaphragm !!

Joking aside - and here is my stock answer - the fuel pump was rebuilt less than 200 miles ago with a new rebuild kit!
However that's not to say I rebuilt it correctly - so the fuel pump is added to the list of suspects.

I suspect if I fitted an inline fuel pressure gauge after the pump and filmed it whilst driving, I would observe the pressure drop off at mid-high rpm range.
I think this would still point to clogged fuel line/obstruction, fuel line collapsing under draw, or poorly pump.

I don't expect I will get another chance to look at it until the weekend - but please feel free to keep the suggestions coming.

[mycar]

Did you pre load the diaphragm spring when you re-assembled the pump ?

[neilbardsley]

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.