Hot starting

[listernoble]

I have a 1969 Imp engined Ginetta G 17 single seater running twin 40's, which I use soley for hillclimbs.
Starts first time from cold but when hot is a so and so to start on button, but fires up first time from a bump start.
I know the battery is OK and anyway the same happens if I try a larger battery with jump leads.
The carbs sit very closely on top of exhaust,the exhaust is heat wrapped and there is no room for an alloy plate etc.
I do not want to chop the manifolds about as the car runs in historic class.
Previous owners say they always had this problem,although when I had it on the rollers recently it started easily with min throttle.
I assume it is hot or boiling fuel.
Any ideas on how to start ?
Lister

[Revracing]

If it is a fuel vapourisation problem you could try running a fuel return to the tank, you would have to experiment with the return line restrictor to get the fuel pressure right but it would always ensure that the fuel feed was cool and without vapour locks.

I beleive that the aircraft industry use this type of fuel system....

[Maisie]

This used to be a common problem in the MG Maestro 1600 too with fuel vapourisation in the carbs when hot. Is there any room for an electric carb cooling fan? That was the solution offered for the MG, along with an alloy plate, which you don't have space for.

[listernoble]

Maisie,
Thanks,but being a single seater there is no room for a fan and no power for a fan (small battery and no alternator)
lister

[Cameron Winton]

Had a similar problem with an MG Midget. It was running on SU's at the time and the exhaust manifold ran so close to the float chambers it was impossible to fit a heatshield. Car ran fine when doing hills & sprints as long as I didn't leave the engine running in the queue for the start. Also would miss when stopped on the road in traffic in warm weather. Partially cured it by insulating as much of the fuel system under the bonnet as possible. Eventually cured completely when began running the engine on a 45 DCOE which took the carb well away from the exhaust.
I would suggest the insulation route or looking for another period manifold that takes the carbs further away from the exhaust. Most Imp stuff was designed at the time of the G17 so you would not break with originality.
I take it your problem occurs after a run at the top of the hill?
Another thought. I have very vague memories of Alex Brown's G17 in the seventies ( i would have been around 7 or 8 years old!) and it may have have downdraught carbs which took the fuel well away from the exhaust. I have seen Imps in single seaters with (I think!!!) 38 IDF's - mini GT40 carbs

[listernoble]

Cameron,
Thank you,it is the ex Peter Voigt,Ken Macmaster ,Richard Homer,Russ Ward car and I don't want to mess around with it too much.
Two of my pals run another G17 and a Vixen,both Imp engines and both downdraught carbs without any problems so that would be the answer,but there is so much history with the car that I don't really want to change it.
But I may extend the inlet manifold to take the carbs away from the heat.A winter time job.
Lister

[BugEyed]

Lister

It does sound like fuel vapourisation, but it could also be spark related (it is painfull sitting on this fence! ).

What ignition system do you have, and do you run with a ballast resistor on the coil?

If it is fuel vapourisation, fitting a fuel return loop via a pressure regulator will keep the fuel in the lines cool, but will not do anything about the fuel in the float chambers. A decent heatshield or moving the carbs sounds the most likely solutions.

HTH

Duncan

[listernoble]

Ignition early Lumenition and I don't think the coil is ballasted.
I'm sure that the problem is fuel related and the long term solution is to move carbs away from the heat without destroying the appearance of the car.
I was wondering whether more throttle or less on hot start would be best,I can't get into the garage at the moment as we have the painters here but soon as I can I will experiment.
Lister

[MGDavid]

had you thought of ceramic coating the exhaust ? - this is often done to MGBGTV8s to cure high underbonnet temps and is thought to be more effective than simple wrap.

[listernoble]

David,
Thank you, it is something I will consider if moving the carbs off the exhaust doesn't work.
Lister

[listernoble]

Originally posted by Revracing
If it is a fuel vapourisation problem you could try running a fuel return to the tank, you would have to experiment with the return line restrictor to get the fuel pressure right but it would always ensure that the fuel feed was cool and without vapour locks.

I beleive that the aircraft industry use this type of fuel system....

Revracing,another thought and thank you.
I think the real problem is the carbs actually sitting a few mill above the hot exhaust and causing vapourisation through the actual carb body.
Lister

[Cameron Winton]

Miles off topic here but people may be interested.
Revracing mentioned using a recirculating system a la aircraft fuel system.
I have worked on and designed components for such systems and it is a completely different problem statement. aircraft gas turbine engines use the fuel for many functions other than for burning. One is for cooling the oil. The fuel likes being a little bit hot as it helps atomisation at the nozzles. The heat transfer from the oil is greater than that being carried away to burnt flow so some flow is bypassed back to the fuel tanks which are used as a heat sink.
Vapourization occurs under certain failure modes (eg backing pump failure and the engine pumps have to suck fuel from the tanks) and the combination of temp and depression creates vapour. Two solutions are either build in excess cooling capacity such as air cooled oil coolers or a system that detects vapour to vent it while simultaneously allowing liquid fuel continue flowing to the nozzles.
There is another bypass system but that involves the actual control of burnt fuel flow and that's another story
Hope this was interesting!

[Revracing]

Very interesting thank you Cameron, Although I was actually refering to normally aspirated internal combustion engined propellor planes.

[imull]

I have two twin 40 manifolds and a Tecalemit system on another for my Clan. All three of those have a thin (1mm max) plate that works as a heat shield and having seen this, I talked to the previous owner he never had vaporisation problems with it.

[Red Dog]

Every highly tuned race engine I have known has been difficult to start when really hot i.e. if you spin and stall it. Difficult, but not impossible. A pre-engaging starter is very good as an inertia one gets kicked out easily and using half throttle usually helps. A vapour lock indicating hot fuel lines is bad news and needs to be sorted. Lag the fuel lines or re-route them. You already have a lagged exhaust manifold which should keep the inlet and carb temp down and a bit of fresh air round the carb would be good in warm weather, but mind it doesn't ice up in cold.

[listernoble]

Thank you,I will play around with it today and report back
Lister

[listernoble]

Today I ran the engine up until really hot and switched off,then turned on and slowly opened to full throttle and spun engine over.
The engine either started more or less first time,if not I backed off the throttle a bit and it started then.
The open throttle seems to be the answer,but I can't quite see why, as I would have thought a hot engine and carbs would produce vapourisation and thus a weak mixture.
Am I being thick?
Lister

[StephenRae]

Red Dog...I wonder if the engine stalls when spinning because it floods and won't restart until the fuel level is up again. At normal tickover it doesn't seem to matter how quickly you dip the clutch it still stalls.

I've had most success starting hot engines with the throttle wide open.

[h4887]

Of course, if you open the throttle with DCOE's you get four big jets of fuel squirted in, which a hot engine is unlikely to appreciate.
Geoff

[Red Dog]

The engine stalls when I forget to dip the clutch Stephen!!!

You're right about the pump jets h4887 but the open throttle also admits lots of air and that's what they seem to prefer, I don't know why Lister.

My starter is inertia type and gets thrown out by the first cylinder to catch - the others don't catch when the throttle is closed. I have a single 40DCOE, the body is in cool air, the manifolds are same side but with heat shield between, the fuel line is well insulated. I did have exhaust wrap but the heat embrittled two proprietary exhaust manifolds very quickly so when I made my own 4 into 1 I didn't put any wrap on. It glows bright orange under load - very pretty!

[listernoble]

Originally posted by h4887
Of course, if you open the throttle with DCOE's you get four big jets of fuel squirted in, which a hot engine is unlikely to appreciate.
Geoff

Well I agree Geoff,but it seems to work.
I assume the accelerator pumps push out fuel, however slowly you press the accelerator?
How the devil are you?
Lister

[h4887]

Red Dog,
When my exhaust manifold glowed bright orange it was because the ignition management unit had no advance in it. Thanks very much Webcon...
Geoff

[h4887]

Lister,
I'm not too sure about the fuel vaporisation theory. The level in the float chamber would have to drop quite a way for the engine not to fire, and if you were boiling that much off it would stink of petrol. If you switch off when hot and try to re-start immediately will it go? It must take a certain amount of time for the heat to soak into the carbs.
Geoff

[Red Dog]

h4887 I think that you have good point, late ignition will put more heat into the exhaust for certain. Of course the flipside is that early ignition will cause detonation and melted pistons......