Technical questions

[AustinWood]

I have an aluminium heat shield over the exhaust and the down pipe is lagged and have no problem with heat in the carburetor.

[Malcolm Parker]

I've always believed that a longer inlet manifold is beneficial for torque and low down power but perhaps one of our racing forumists could elaborate.   My Ulster rep has a down-draught SU on a long manifold which puts the carb well above the engine.  Fuel is supplied by an SU electric pump.   The benefits are that it is away from the heat of the exhaust and it is easily accessible for adjustment.

[Dave Mann]

Ah now I understand. I hope you have removed the plate which fits under the exhaust down pipe and traps all the heat in and proves to be a winter heater.

[Dave Wortley]

Derek, Glad you have taken the yellow cardboard arrow off before you started the engine!
Cheers,
Dave.

[Tony Griffiths]

Like these.....a lot easier to do in Photoshop than on a milling machine and welder.  I have to say that, in many miles of A7 motoring I've never found a problem with the standard updraft inlet manifold.
As already pointed out, the theory says that a shorter induction pipe is better for high r.p.m. power while longer gives enhanced low-speed torque. So, take your choice...

[Derek Sheldon]

Well just reporting back. Not had the really hot weather again to fully test the modifications. The fuel is now not boiling. 
I drove today 5 miles to Barry Mulroy’s house ,I considered the adaptions to be doing their job. Barry however was concerned about the temperature of the inlet manifold and subsequently the top of the carb. He suggested making a heat barrier between the manifold and the carb using a Bakelite type material. 
Having returned home I made the said spacer, 10mm thick and fitted it. I took the car for a run and the difference In temperature between manifold and carb is quite noticeable. 
It was mentioned in an earlier post that the longer the inlet tube the more toque obtained, could 10mm about 10% make much difference, because it feels like it does.
I’ll take this opportunity to thank the input received even if some of it was tongue in cheek.??

[Bob Culver]

Theory about pipe length applies mainly to the final not signifcantly branched run to the inlet valve.

[Dave Wortley]

Derek, I had problems with my chummy which I thought I had resolved with various heat shields and a red fibre insulator between carb and inlet manifold. It’s on here if you search “boiling petrol”. BUT Tony Griffiths now has the chummy and he had a different solution involving wrapping copper wire round the manifold/carb junction. Note the chummy has a sidedraught SU carb. Tony may be able to show you his modification on here. Cheers, Dave.

[Tony Griffiths]

When I bought the car from David he explained about misfiring on hot days and how it would vary with altitude and temperature. As I'd always run my previous vintage Seven with a standard updraught manifold, and never had a moment's trouble, this was somewhat puzzling. When I examined the neat arrangement of a side-draught SU and 4-branch exhaust (complete with a number of beautifully shaped aluminium shields to deflect heat) that David had cleverly engineered, I noticed that there was no hot-spot - and wondered if this might lead to ice forming in the manifold. Indeed, this proved to be the case for, after a run with the temperature at a mild 15C - and the car running normally - I discovered the manifold covered in condensation and ice cold. A subsequent journey in much hotter weather brought about the misfiring problem with even more condensation and an extended area of cold on the manifold. As a crude fix, I annealed some copper wire and wrapped it around two of the four-branch exhaust pipes and over the inlet manifold, as in the picture. The result was an instant fix - though I'll have to conjure up something that looks a little less inelegant. This solution is probably only relevant with an unheated inlet manifold when used on the road where the throttle opening varies; on a racer with full throttle all the time, it probably would not be necessary. Remember: in a carburettor, it's winter all the time...

[Bob Culver]

Some observations on carb icing may be of interest. Javelin cars have the carbs bolted direct to the top of the heads with the exhaust remote on the other side and the engine is forward of the radiator. The carbs have small venturis and are subject to very strong suction pulses so atomisation is thorough and very prone to icing. Occurs in damp conditions (ie fog) at low temps above freezing (which dries the air). Around town shows up as stalling etc as a sheet of ice on the butterfly blocks the transfer outlets. On the open road very high consumption (due choking), lack of power, may stop, and often the throttle locks open! I imagine an SU would be very confused. If sit in the car pondering the external ice melts and there is no clue as to cause.
It is easy to partially blank off a Seven radiator so the air passing throgh is more heated.