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Went for a decent 40 mile run last weekend in the special. About 4 miles from home I felt the engine change and a slight loss of power. Upon getting home and a little investigation I found that the plug in #3 cylinder was fouled up completely. The other plugs were pretty sooty too. The car doesn't burn oil (noticeably) and I believe it is running pretty rich.
It has a 26VA zenith side draught card on that has been cleaned and rebuilt with standard innards.. I've read on the forum several threads on the difficulty of adjusting the mixture and that the air screw doesn't have much effect? Is this true? What would be the best method of weakening the mixture a bit?
Thanks in advance
PS, anyone doing the "Soft top Sunday" at Goodwood this coming weekend? Plan is to take the special and I've managed to get entrance to the track/pits.. If so, see you there!
may try and drop in carless on Sunday...
Float level correct?
Location: Piddle Valley
How do I check, and alter the float height? From memory, the valve at the top of the float chamber is not adjustable is it?
The usual problem is that cannot sufficiently richen the idle.
A sunken float will give somewhat rich mixture but accompanied by leaks. Ditto very high pump pressure. If fitted a very blocked modern filter causes richness. Or the choke flap stuck. The small air drilling at top of emulsion block on front of float chamber should be clear. Also the other air feed to the adj screw; hole at 10 oclock looking into carb. Otherwise an unscrewed jet (4 of). If of long standing, wrong jets or drilled jets a possibility. I dunno but presumably blockages of air bleed inside or leading to the emulsion block could richen. A homemade gasket blocking off capacity well would richen. Free flow exhausts, low resistance mufflers etc richen mixture.
Jet sizes can be compared or even measured using the taper part of a safety pin as a gauge. Bores may be tapered so check from same end.
Location: Auckland, NZ
From my experience it seems that a large number of A7's fitted with 26VA run rich to a greater or lesser extent, the float valve and seat are usually worn, and the the jets may well be oversize through wear or abuse.
Edit, Perhaps Bob you could explain for us how a blocked fuel filter or free flow exhaust system will richen the mixture?
It is my understanding that the slow running mixture adjustment has no effect on the mixture while driving. Opening the throttle disables the slow running operation.
The mixture while running is set by jet sizes and fuel level. As mentioned already it is likely that the fuel level is the cause of the problem. Maybe the float has developed a leak or the float valve is leaking.
My RP had a leak on the needle valve not the valve itself, but the sealing washer was not seating well. I tinned the washer which cured the leak . I noticed the float was wearing where the needle rubs on the top, so, as the mixture looked a bit rich, I tinned the wear dimple, this seamed to cure the richness. I have now fitted an ex-mini SU carb on a home made adapter, giving better torque and adjustment.
Location: Piddle Valley
It might be worth trying spark plugs with a different heat range, fit plugs which are hotter running.
My engine showed black sooty plugs until I fitted hotter running plugs.
( NGK lower number runs hotter, if you run with 6 try a set of 5 )
Ruby heads are well known for plug fouling.
Richard, thanks for the tip re the plugs... I'm running on champion plugs from the seven workshop at the mo, might try a different type as per your suggestion...
Or is it just stale fuel which behaves like kerosene?
Sorry, missed the word air. (For others, with externally vented carbs, any air filter functions as the choke. On other makes export models with air filters were fitted weaker jets than versions without.)
I dont know the exact mechanism but on my car a simple silencer with little obstruction dropped mmpg from 50 to 40. I have also read of the effect in tuning books.
(The 1937 heads are prone to oil on plugs, the earlier very resistant.)
The idle circuit contributes notably at part throttle. Tinkering with slow adj, prog jet etc effects mpg. I run a 1600cc 1960s car and and the slow run (idle) jet is prone to blockage. As well as stalling, it is apparent as slight irregualr running up to 50 mph on the flat)
Location: Auckland, NZ
I am afraid your tendency to speak in half sentences makes you rather hard to understand Bob, so my apologies for thinking you meant fuel filter. I think you will find that a free flowing exhaust system improves the volumetric efficiency of an engine which in turn increase's the amount of fuel used. However volumetric efficiency and the fuel air mixture are different things, it is more usual that the mixture weakens when air flow is increased.
It is likely that you made such a small improvement to your cars performance that it was un noticeable other than in fuel used. In my experience removing restriction in the silencer of a seven, unless seriously restricted, makes little appreciable difference to top end, but does improve low down torque. As you seem to love an armchair theory I am sure you will come up with another answer. There were some replacement Silencers on the market in the UK a few years ago which were very poorly designed and had quite a marked negative effect on performance, I have come across a couple here.
I would strongly suggest that you try hotter plugs.
I have found that with 18mm plugs the Champion D21 and similarly with 14mm plugs NGK B5HS.
Gives great results in Austin Sevens.
All available from Green spark plug Co.
I personally believe it is a change in the composition of fuel that has created this problem.
Hope that succeeds for you.
Location: The Centre of the Universe
Nick, just ordered a set of NGK's from the green spark plug co and will feedback results..
Thanks for the advice 😀
I have a similar problem. Approx how many turns anticlockwise when screw is tight should I give to air mixture screw
Location: rural northants
I installed the new plugs and this certainly made a difference. Running a lot lot better!!
My air mixture screw is 1/2 turn anticlockwise from tight, but to be honest it doesn't make much difference other than at idle