Hot Starting and Manifold Spacers

[chris68]

Hi All,

Has anyone fitted phenolic spacers under their inlet manifolds to improve hot starting? Did it make things better and does it necessitate longer head studs?

Mine is a 2.0 litre car running Webers. It refused to start the other day after stopping briefly following a 20 mile run. I've read that it could be due to heat soak from the cylinder heads.

Thanks,
Chris

[mycar]

I've fitted them recently Chris, PMO carburetors supply them as a kit with the longer studs, though I bought them from Pelican because it was easier.

http://www.pelicanparts.com/cgi-bin/kse ... -INSULATOR

They are intended to prevent heat soak to the carburetors which then boils the fuel in the float chambers.

Not sure if this is the problem you're having, what are the symptoms of your hot starting problemos ?

Mike.

[chris68]

Hi Mike,

Sometimes when hot starting I crank the motor but it just doesn't fire. It doesn't happen often fortunately, just occasionally when I try to restart the car 15 minutes after a good run.

I wondered also about fuel getting vapourised in the float chambers then pushing past the metering jets and flooding. Don't really want the trouble of swopping all the head studs but I guess the spacers have to be reasonably thick to provide a useful thermal barrier.

Do you think the spacers made an improvement for you?

Cheers
Chris

[Berny]

It's not the head studs, just the manafuld studs you have to change.

[mycar]

Swapping the studs is pretty straight forward, enjoyable even. Remove the manifolds and carbs as one piece, then double nut the studs and wind them out, then screw the new longer studs in using two nuts again. The perfect excuse to buy a nice 1/4 inch socket set.

Probably best to temporarily plug up cylinder throats with paper towel.

Carbs eh ? God bless em, it's like dealing with an eccentric pair of uncles in charge of your engine. Before you go down the phenolic spacer route have you tried the starting the hot engine with the accelerator fully depressed ? As time has gone on I've learned a few different ruses for coaxing the old girl into life.

Cold start :- wait for the pump to stop ticking, 2 pumps of the accelerator, accelerator half down, turn over.
Warm engine :- Off the acceleratot, just turn it over like a modern car.
Hot engine :- Allow the pump to prime the carbs then accelerator to the floor, wait.... then turn the key and as the engine catches, lift the accelerator pedal.

All this works even better if the timing is as it should be.

Fuel will vapourise from the float bowls of a hot engine but will be replenished by the fuel pump when you turn on again.

If you do have heat soak problems the fuel doesn't so much vapourise from float bowls but percolates like a coffee machine and flows over and into the cylinder(s). From what you say, it doesn't sound like this is your problem... but there is a way you can tell. Drive the car till it's good and hot then park up and wait and listen around the engine bay. If you do have a heat soak problem it can take 15 to 20 minutes for the heat to transfer up into the carburetors and then... there is an unmistakable gurgling noise. Whip the air box of and there'll be an unmistakable stink of petrol, peer down the carb throats to witness the flow of fuel.

Fuel it seems is far more volatile these days, but that's not really a good reason why you engine should start behaving unreasonably all of a sudden. There's more to this fuel percolation business than meets the eye but work out if this is actually the problem first.

Regards, Mike.

[911hillclimber]

I too have a pair of crazy old guys running my 3.2 on BIG PMO's and the above advice is spot-on.

cold start is 'lucky' on the day, allowing the carbs to fill is key.
hot start is very good, warm is too.

Get them tuned first, I have the PMO heat resisting spacers, but heat still gets in via the studs but to a much lesser degree.

I tune mine with an air meter (£45 off ebaY) and a Halfords Guyson Colour tune. Takes about an hour all told.

[haasad]

I agree that the spacers are a good mod and easy to do, including the above good advice bet you note an improvement, you may also consider a return on the fuel supply to allow circulation, bit of a pain to retro fit but good pratice assuming you currently just run 3.5 psi straight to the carbs and use the float valves to hold ?

[haasad]

I agree that the spacers are a good mod and easy to do, including the above good advice bet you note an improvement, you may also consider a return on the fuel supply to allow circulation, bit of a pain to retro fit but good pratice assuming you currently just run 3.5 psi straight to the carbs and use the float valves to hold ?

[chris68]

Thanks to everyone for the replies, all good advice. I'll try to identify whether I have a percolation issue or not, although it sounds like the spacers are a good idea anyway.

Mike, your reply suggests the spacers fit below the inlet manifolds and hence it's the head studs that need to be longer. Is this corect and does the increased height make it more difficult to get air cleaner in?

I realise also that I've been using different fuel recently, and not the normal Tesco 99 stuff. Wonder if that's part of the equation.

Cheers,
Chris

[mycar]

Evening Chris, yes I have the spacers between heads and manifold but of course they will fit between manifold and carbs, which offers up a bit of a conundrum... if there are two ways of doing a thing, there's usually the right way and the not so right way... so which is it ?
My thinking was to stop the rising heat as low as possible, no point having the manifolds at the same temperature as the engine block if you could avoid it.

The spacers aren't very thick, maybe 6mm, I haven't had clearance problems with the air box.

Maybe your change in fuel is having an effect but probably not. Easy to check though with a trip back to Tesco.

As to whether the spacers improve running, they seem to for me. Deep within these 911 forums there's a description of how lifting the carbs away from the combustion chamber improves the progress of the fuel/air mixtures but I can't remember where. Maybe someone could chime in. (Andy ?)

Regards, Mike.

[Lightweight_911]

Chris - if your 2.0 engine was originally equipped with Webers you shouldn't need to fit longer studs to the heads as it should have had Factory phenolic spacers when new.

Mike – engines with more aggressive cams like 906/RSR which have lots of overlap can suffer from fuel reversion at low-to-medium revs caused by pulses up the intake which hit a resonance and create a cloud of fuel droplets floating above the mouth of the carb.

By making the intake track longer (911R/906 inlet manifolds were 98mm tall compared to 68mm for the std 2.0 production manifolds) the frequency of the intake no longer matches that resonance and so the cloud disappears - at higher revs this tends not to be a problem since the air/fuel mixture is traveling at a higher speed into the cylinders.

[mycar]

That's the info I was talking about, thanks Andy, not really applicable to our little engines then.

Somebody else mentioned Porsche phenolic spacers for the Webers on my car but I couldn't find any reference to them on PET and neither set of studs I have would be long enough to accommodate them. From what I understand, Porsche introduced the spacers on the 1971 T for the first time.

Regards, Mike.