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Discussion Starter #1
As you all sit with you evening whiskey reading the BB, here's a problem I have just found. and would appreciate your intellectual assistance.
2 litre Spider, fast trip to the South of France 800 plus miles, 80-90 all the way...perfect, 27mpg. Return..half way home drops onto three...still idles on four and starts perfectly. Have changed..coil... points... condenser... distributor cap.. plugs...substituted leads...blown out the jets and passages in the carbs.. checked needle valves...checked all fuel supply..rebalanced carbs. Fuel consumption drops alarmingly. All plugs are grey. Leads are copper cored. Tried direct feed to coil from battery to eliminate internal wiring...the same. Spray WD40 on carb/manifold junction to see possible leak..nothing doing.
However, for five seconds, it goes back to four and surges away beautifully just as usual. Then back to three. Change from three to four..well anytime really ..uphill...full throttle...downhill...light throttle...constant speed. Lightest of right foot and I can get four at 1500 revs.
Ideas chaps?
 

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My experience that that the root cause is electrical 80% of the time, 15% fuel related, 5% mechanical. You have pretty much eliminated the electrical (did you replace the plugs?). You are positive that the firing order is right? I ask because my last mystery motor problem turned out to be because I swapped # 3 and #4 wires. Idled fine but no power.

I don't know much about carbs but it seems that you have done the right things.

That leaves mechinical. Since only one is misbehaving, the timing chain is not likely. The only thing I could think of is a sticky valve and/or carbon buildup at a valve seat. Try doing a compression test and see if #3 is off relative to the rest. Better yet, do a leak down test. You could also pull the cam cover and look for anything unusual. Is it making (or did it make) any strange noises?

What puzzles me is that you say all the plugs look grey. That would suggest that #3 is firing efficiently. But what you're describing suggests just the opposite.
 

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Now that I reread your post, perhaps I misunderstood what you are saying. Is #3 cylinder missing or are you saying generically that sometimes you are running on three cylinders and sometimes four? If the latter, are you saying that it "feels" like you lost one cylinder or is there concrete evidence that you are loosing one cylinder? Is there any eveidence to suggest that it is one particular cylinder or is it random?

Sorry, I am probably confusing the issue now...
 

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I think most carburation problems are electrical, too. And the solution to most problems of either kind are usually simple. You've been through most of the electrics with no luck at curing an intermittent high speed miss. Clean and tighten the positive and negative connections at the battery. Is the battery healthy? Alternator? Try installing another known good ground between the engine block and chassis. Start going through electrical connections at the fuse block and at the ignition switch (ugh!).

Lest the fuel side be totally ignored is a fuel filter plugging up? Ailing pump?

Keep plugging on the simple stuff. That's my guess where the problem is.

Mike Hollinger
Atlanta
 

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Discussion Starter #8
Thank you all for your input. The problem appears to be found and it is so simple that I have been kicking myself.
On DCOEs there is a small grub screw and locknut on the underside which holds the auxillary venturi in place. This screw had "fallen out" which allowed the AV to rotate and move backward and forwards within the bore of the carb.
The result was that on start-up the slow running jets were not affected as they are in front of the AV but when the throttles were opened, depending on the position of the AV, fuel would either flow from the main jet or not thus three or four cylinders would fire. Hence good starting as usual. As I drove along the AV would vibrate and sometime allow fuel into the cylinder. Grub screw replaced and the other three tightened up. Interestingly the others were all locked solid but the AVs loose in the carbs. So the tappety noise which I thought heralded an clearances check has also disappeared.
Why should all the plugs be the same colour? Can't answer that as yet
 

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Discussion Starter #11
Rich,
I can claim no glory here, my mechanic after hearing all the things that I did to find the problem lent his hand on the carburettors and idly touched the end of the SV...it was loose. He check the others and two more were loose and one was totally out of position and so loose that it was revolving within the barrel of the carb.
No moral to this story? Would have been delighted to say that after three glasses of J & B the solution appeared before me. No so! I now have the task of retro-replacing all the original kit too!
 
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