MFI hot start problems...fuel lock?

[IJD]

Hello, I'm a DDK newbie so hopefully this lands with the right audience.
I've just finished full rebuild of my '69S with MFI RS spec engine/running gear along with the larger 93mm Mahle barrels and pistons. Slowly going through the snagging process, but the one that's causing most head scratching is a definite reluctance to hot start. Before rebuild it was fine and running exactly the same spec, with no need for long cranking or fuel circulation prior to start.
Car runs the standard cold start system which is working fine, along with the overrun solenoid that is currently disconnected awaiting a new Bosch RPM transducer. CO starts high then drops down after a while so assume thermostat is working fine, but will fine tune following CMA once I'm happy all's operating as it should.
I did replace all the fuel lines during the rebuild, aluminium through the tunnel with braided back to the fuel filter and regular pipes at the front along with new electric fuel pump. These pipes were off the roll and not a Porsche assembly.
Searching Pelican, it seems as though the factory may have fitted a 1mm restrictor in the 7mm ID pipe between the tee and the fuel pump return to prevent fuel lock. A fix could be to use an old carb jet or similar crimped into the pipe. However before I start messing around with that mod does anyone have any more knowledge about these restrictors and whether they significantly impact hot start/fuel lock issues?
Indeed does the Porsche fuel pipe assembly include such a restrictor?

Also read using a manual switch to the cold start solenoid can overcome the problem by kicking the engine into life, but would like to keep as factory as much as possible.

Many thanks for any advice.

[majordad]

I’ve usually used a manual switch. Even without it, my hot start technique is slowly depress accelerator to floor, turn key and crank engine til it fires up. I forgot this one time in Zell and it would not start, had to resort to a push start down a steep hill.

Welcome BTW

[210bhp]

I agree.

If you are happy with what you have done with your build I would definitely question yourself first rather the car.

The hot start technique is definitely different to cold start for MFI.

Regards
Mike

[hot66]

Hot start for me is to not touch the throttle at all when cranking

Have you tried disconnecting the power to the enrichment just incase it thinks it’s a cold start

[IJD]

thanks for your comments and suggestions, should have mentioned the injection pump is an RS spec one to match the engine i.e. no enrichment solenoid.

It may just be a question of how i start it when hot, but am intrigued the possibility of a restrictor in the return fuel line.

[Ian 2.2S]

I have been experiencing a hot start issue for a couple of years now - 2.2 S.
Always starts from cold (using manually wired enrichment solenoid if particularly cold).
When warm it will start straight away if only stopped for a minute or two. Any longer and it will be a PIA. It seems to develop some form of fuel lock, because no matter how much throttle and how long you crank for, there is no fuel smell from the exhaust. One technique that works sometimes, is to switch ignition on, leave fuel pump running for 20 seconds or so, then 'tickle' the cold start solenoid a few times. Then with pedal flat to the floor crank it.
I have replaced all the flexible hoses throughout, in case there was some deterioration, but no change. Latest plan is fitting an inline fuel gauge to see what is happening to pressure in supply when problem occurs. There is a non-return valve in the filter console, but I think this is OK, but not sure how to check.
Ian

[IJD]

Thanks Ian, sounds very similar to mine in its behaviour. I'm just going through CMA at the moment getting everything else spot on in case there's another issue, but once I've exhausted that I'll fit the notional 1mm restrictor in the 7mm ID return line between the tee and the electric fuel pump....maybe it keeps some residual back pressure in the system?

Looked at replacing the front pipes with the Porsche assembly that costs >£70, but think i'll try a carb jet in return line first.

It does seem as having manual control on the cold start solenoid is a common theme to address the problem though.

[IJD]

Thanks for the answers so far, can I ask if you all run a properly functioning check valve at the fuel filter console?

I've tested the flow rate, but not yet been able to test the fuel pressure specifically when hot starting. Be interested in Ian's results once fitted a fuel pressure gauge. I'm convinced its something to do with lack of system fuel pressure when hot, found the specs for the check valve on Pelican so I'll try and test mine. Bit more research suggests the elusive fuel return restrictor is only in the assembly when the pump is rear mounted, mines still front mounted.

[IJD]

Just to close this thread out, I found the standard check valve on the fuel filter console just wasn't shutting off properly hence leaking any residual pressure. It was clagged up with what looked like fuel residue. Managed to clean it with carb cleaner an airline and exercising it with a small screwdriver. Once clean and controlling fuel pressure correctly the hot start works just fine.
So no need for carb jets or any other restriction in the return fuel line.