Running issues

[Mr Pharmacist]

I have running issues with my car and I'm ready to burn the bloody thing as I just can't get it to go properly. It's a 2056cc on twin 40idfs. It's got racing pistons, a fast cam and heads that have been ported.

The symptoms are a misfire over 4000rpm. The severity of the misfire varies from slight to major. Very occasionally it runs without a misfire - like this morning when I took it out for a 10 minute test run. It ran great and was flying along. I then took it out couple of hours later and it started to misfire. Took it out a couple of hours after that and it would hardly go at all to the point where it couldn't even idle without me blipping the throttle to keep it going.

The issue has been going on for at least a year now and i haven't been able to really drive it because of the issue.

these are things I've done to try and rectify it :

Changed the entire ignition system.
Checked timing
checked the valve clearances
Checked fuel flow and pressure
Cleaned the carbs
Replaced the fuel pump / fuel filter
Swapped the fuel pump relay
Carried out a compression test

I think that's it but there's probably more

Can anyone one give me some hope and explain what could cause a car to go from running great to running terribly like this? The hillclimbing season isn't far away now and I'm beginning to think I'm going to miss another season because of this

[jackstowers]

I cleaned my carbs half a dozen times before I found the tiny grain of rubber that was floating around in one chamber. It must've been getting stuck somewhere then reappearing to cause a blockage then freeing itself.
It looked like a bit of perished o ring or fuel hose and the size of a grain of sugar. Since finding it and having replaced all seals in the carbs and the fuel hose down stream of the filter, touch wood I've not had a problem. Sounds very similar.

[Mr Pharmacist]

I've only cleaned the carbs twice so 4 more to go yet!

Thanks for the advice - I will get on with doing them again

[Bruce M]

You might consider fitting a "jet doctor" kit. I've read they make a difference keeping the idle jets clean but I've not used it personally.

[chief]

Carb balance maybe.....

[Mr Pharmacist]

Carbs have been balanced - it idles better than it ever has at the moment. Lovely and smooth. Its just at the high revs where it plays up and then not always (but usually it does play up).

Will keeping the idles jets clean help with the higher rev range? I'm not fully up to speed with how carbs work!

[chief]

If you have the carbs balanced and only getting trouble at the high end, my feeling would be either fuel starvation or pressure.
I know every man and his bow wow says this but could be a route to investigate. How about the copper gauze filter in the bottom of the petrol tank?
Mine was three quarters blocked with years of sediment.....Just a thought.
Another thing that has just popped into my noodle. Have you got a fuel return line? I have had problems many moons ago with big carbs being over pressured at the high end and bleeding extra fuel straight into the manifold. Again just a thought....

[Bruce M]

Carbs have been balanced - it idles better than it ever has at the moment. Lovely and smooth. Its just at the high revs where it plays up and then not always (but usually it does play up).

Will keeping the idles jets clean help with the higher rev range? I'm not fully up to speed with how carbs work!

The idle jet are still important off idle. Is the issue with light or full throttle? Your first post mentioned symptoms when you had to blip the throttle (at idle?).
Poor flow of fuel is also a fair shout, would lean the mix if the demand was high and flow didn't keep up.
Also check the linkage geometry. It might be balanced at idle but open at different rates.

[Mr Pharmacist]

The 'usual' issue is at revs over 4000 rpm regardless of throttle, though at one point yesterday it was struggling to idle it doesn't normal struggle to do that. The fact that the issue is so variable is really puzzling me - on moment its running great then it starts to deteriorate. Leave it a few days and its ok again until after a few minutes driving where it starts to go bad again etc.

The fuel pump its running at 3.5 psi and there is plenty of flow (at idle - not sure how i can test flow whilst actually driving - any suggestions?)

I haven't checked the linkage geometry in any great detail so i will have a good look at that, thanks.

[mycar]

I'm not familiar with idf carbs Stuart but I was plagued with problems with The ids carbs.

I had them rebuilt twice over the years but it was only when I pulled the lead plugs did the secret gunk reserve reveal itself... and then only after much soaking and poking and blowing.

Turns out there's a fair bit of info on the idf over on the Registry, here's a taster.

http://forum.porsche356registry.org/vie ... =1&t=15475

[Mr Pharmacist]

Thanks Mike, I will do some research on the registry.

Just in case I decide to pull the lead plugs, where did you get replacements from?

[Spyder-finder]

Over 4000 rev you are running on main jets so I would defo check for crud in the jets. The float height is also very important so check that and also fuel inlet needles to make sure they are not sticking.

[914 KEV]

Just get rid of the carburettors and convert to fuel injection.

Last time my 914-6 didn't run very well is was down to what Spyder-finder has just mentioned a needle valve was causing a problem and affecting the float level.

[mycar]

Thanks Mike, I will do some research on the registry.

Just in case I decide to pull the lead plugs, where did you get replacements from?

Remove the plugs by drilling a small pilot hole in the lead, carefully screw in a fine woodscrew then pull out with pliers.

I soaked my carbs by totally submerging them in carb cleaner for a couple of days. (Carb cleaner is acetone, available ont net in 5 litre size) and gently cleaned everything I could get to with rather fetching yellow pipe cleaners and some compressed air here and there.

Replace old plugs with slightly oversized split shot from a fishing shop. (I know it sounds unlikely, what about the splits... but it was an Andy Haas recommendation and it worked perfectly.

[Mr Pharmacist]

Just get rid of the carburettors and convert to fuel injection.

Last time my 914-6 didn't run very well is was down to what Spyder-finder has just mentioned a needle valve was causing a problem and affecting the float level.

If I had the parts I would kev :D

Carbs are apart and soaking as we speak. I found a piece of wire in one and the floats weren't adjusted anywhere near to where they should be. So fingers crossed when they go back together it'll be OK.

I'll report back at the weekend once reassembly has taken place