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RE: Boiling Petrol? - Robert Leigh - 20-05-2018
(20-05-2018, 07:53 AM)Dave Wortley Wrote: Yesterday I did a 75 mile run in the chummy. It was a hot day.All was well for the first 30 miles but then the engine started running very lumpily and would not exceed 25mph in 2nd gear for a further 20 miles. On climbing out of lower ground onto the moors above Sheffield everything returned to normal. The same thing happened two years ago on the same run so I fitted a heat shield on the exhaust manifold and on the downpipe but these mods haven't done the trick. I checked and cleaned the distributor points,ap and all HT lead connections but all appear sound anyway. Any ideas re preventing this please? The chummy has a 4 blade fan and small SU carb and electric pump. Looked on the old forum but can't find anything.Presumably with the SU you are using a later type manifold. As I said in another post recently, I have found it helpful to cut through the hotspot. It was there to help vaporise old fashioned petrol, but it's more of a hindrance with today's product. The other thing which can help is winding the exhaust from the flange down to a point beyond the carburettor with the modern equivalent of asbestos tape.
Thanks,
Dave.
Robert Leigh
RE: Boiling Petrol? - Speedex750 - 20-05-2018
Hi Dave,
Yes; as Robert says, I forgot to mention minimising the hot spot. Hacksaw through between the inlet and exhaust in the middle and slot down above the manifold studs either side of the inlet. Make sure the heat shield bends under the float chamber to deflect warm rising air.
Keep your cool, Dave
RE: Boiling Petrol? - Dave Wortley - 20-05-2018
Dave, If it was my RN with the problem I would do the same.
The engine valance on the chummy hasn't a removable bit . I could cut it off but it would be a shame. Half way through making an even bigger heat shield round the manifold and an insulated piece between inlet manifold and carb
RE: Boiling Petrol? - Dave Wortley - 21-05-2018
I have a home made bunch of bananas manifold with integral buffer end inlet manifold ie all one fabrication. Now made an insulator piece between carb and inlet and tonight will finish extending the existing heat shield which currently only covers the actual bunch of bananas. I will try and shield the bottom of the float chamber as well as the side but keep airflow from the fan around the front of carb bowl. Thanks again for the advice.
RE: Boiling Petrol? - Dave Wortley - 22-05-2018
RE: Boiling Petrol? - Greig Smith - 23-05-2018
Hello Dave
You've done a very nice neat job there, lots of time spent shaping and fettling that aluminum, having just finished an entire single seater body in ali, I know just how long the details take.
I'd suggest that one of your problems is the location of your SU pump, basically it's being cooked by the exhaust so that any fuel flowing through it is pre-heated so to speak.
I'd suggest you did 3 things, firstly make up another heat shield for the pump, you can shape it up between the pump and the exhaust. Secondly find a little bit of modern car door pinch rubber and stick it onto the flange of the lower valance just next to your armoured wiring. This will hold the back of the bonnet flap open just a bit and effectively create a 'louvre' to draw air out and thus create airflow over the pump and carb to cool them down. Easily removed for shows etc and the bonnet side has enough flex to handle the small deviation. Thirdly, an optional step is to create a small aluminum air scoop under the car in the gap between the chassis and the side valance which cups cold air from the front, scoops and redirects it upwards to cool the pump and it can exit out the edge of the bonnet. Painted Matt black it would be almost invisible down there.
I had a similar issue with my '61 Alfa Ti a few years ago, issues with the mechanical pump led me to fitting an SU pump alongside the coil (power source). It ran very well until it stopped every so often.... A rag wrapped around it and soaking it at every stop was the answer until I could rebuild the mechanical pump whereby it hasn't missed a beat. The SU was a pusher type pump and relied on the fuel and airflow to keep it cool. When it got hot it got grumpy and boiled the fuel within it, despite it being on the opposite side to the exhaust.
As an aside I have some very neat stick on type heat shielding material that I use for my heat shields, very sticky stuff, aluminum layer on th visible side with a composite weave in between. Works extremely well and in your case would be invisible under or behind your heat shields. I'll get a picture of the box for you.
Aye
Greig
Sunny South Africa
RE: Boiling Petrol? - Dave Wortley - 23-05-2018
Thanks Greig,
I may have to put a heat shield round the pump. There is a clip-on heat shield around the exhaust downpipe which you can just make out. Will make a rubber piece to lift the back edge of the bonnet. All these mods will ensure that GB will not have any more hot weather this year! Stage 3 will be the almost invisible scoop under the side valance. Wait until I need to set the tappets when the air will turn blue.Cheers,
Dave.
RE: Boiling Petrol? - Colin Reed - 23-05-2018
HI Dave,
It would appear from the photo that the intake manifold is to close to the two centre exhaust pipes,
If you look on the Speedex site you will see that it is upswept a lot more if you do not wish to change it I would use exhaust wrap from the flange down.
Just a thought.
Colin
NZ
RE: Boiling Petrol? - Robert Leigh - 23-05-2018
(20-05-2018, 07:15 PM)Speedex750 Wrote: Hi Dave,
Yes; as Robert says, I forgot to mention minimising the hot spot. Hacksaw through between the inlet and exhaust in the middle and slot down above the manifold studs either side of the inlet. Make sure the heat shield bends under the float chamber to deflect warm rising air.
Keep your cool, Dave
(22-05-2018, 08:56 PM)Dave Wortley Wrote: This is what I finally did to prevent boiling of petrol. Need a long run in hot weather now!You have done a very tidy job of your shielding, but I have one small 'nit-pick': the fuel pipe loop should be horizontally set so that you have a steady rise from the pump to the carburettor, allowing any bubbles of air or vaporised fuel to rise easily to the float chamber. I know it is not likely to be a problem on the pump outlet, but in extremis it would be an improvement.
Robert
RE: Boiling Petrol? - Dave Wortley - 23-05-2018
Colin,Robert,
Good points for me to consider. The vertical loop is easy to correct so may have a go at that if my current mods prove ineffective. The inlet manifold is a little more difficult but very relevant but I’ll leave that as the last resort Colin.( Prefer cutting and rewelding to tape.)
Thank you,
Dave.