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Question on fuel consumption of my BN6

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The reason for vacuum advance is to start the combustion process earlier at part throttle/light load; i.e. cruising at highway speeds. By starting combustion earlier, more power is generated for the same amount of fuel or, conversely, less fuel creates the same amount of power, which increases fuel efficiency (throttles can be a bit more closed at part throttle/light load to maintain same speed). You don't want additional advance at idle because, well, the engine is running slower (the combustion event duration does vary, depending on mixture, etc., but relatively less than engine speed). You don't want additional advance at wide-open throttle (WOT), since the load and combustion pressures are greater, which can cause detonation (aka ping or knock). If your vacuum advance isn't working, your highway mileage will be down (typically, a Big Healey should get 23-25MPG at 60MPH, depending on road grade, loading, atmospheric conditions, etc.). In short, the distributor's job is to time the spark event to maximize efficiency and power--both related--under various conditions of load, speed, etc. If the distributor isn't working properly you won't get the power, hence efficiency, that's possible (compared to modern, electronically controlled ignition systems, all distributors are defective).

As someone suggested, you should 'read' your plugs under various conditions and/or get some Colortunes to check your mixture. If mixture is correct, you might be losing fuel somewhere (a leak in a fuel line or joint might not be obvious). Or, if your engine is worn you'll be using more fuel to get reasonable performance. Have you checked your compression? In general, higher compression yields greater efficiency (up to a point).
 
BN6_2197 said:
Bob, my compression is about 6,5 bar (94,27 psi) on each cylinder.
Click to expand...

That's pretty low compression. Could well be the cause of poor mileage (or one of the causes). Some of your engine's power is escaping into the sump (and back into a carburettor), and/or past the valves.
 
It's up to you. If you've exhausted all other possibilities, the low compression could be the cause of your bad mileage (I guess you've checked timing, for leaks, distributor advance working, mixture, etc.). It depends what you want out of your car ... you can drive it like it is for as long as you like, or bite the bullet and do a rebuild. If the engine has a lot of miles--100K or more--it's probably time. The low compression alone is sufficient reason to do a rebuild, IMO.

It's a bit of work, and no small expense, but very gratifying to have a solid powerplant. Welcome to Healey ownership; that is, the car now owns you!
 
If compression is low, try to determine if it's because of the valves or the rings. If it's valves, you can just pull the head and not do a complete rebuild; just a valve job. Ring compression problems can be determined by squirting oil in the cylinder and seeing if the compression goes up. If it does, the rings need replacement.
Good luck, Bob
 
Good point, Bob. Since the compression loss is even across all cylinders, there's a good chance it's rings, or both; a leakdown test can confirm. Volker, if you're not familiar, a leakdown test provides a constant air pressure to a cylinder at TDC and measures the pressure held by the cylinder. For example, if 80PSI is applied, but the cylinder only holds 40PSI, that cylinder is weak. If you hear air escaping from a carburettor or the exhaust pipe the intake or exhaust valve is leaking, respectively. If you hear it coming out of the dipstick tube or crankcase vent it's rings. Try oil in the cylinder--just a teaspoon or two--with a standard compression test; that may tell you all you need to know (expect a lot of blue smoke next time you fire the engine up). If not, see if you can rent or borrow a leakdown test rig, or get a shop to do the test for you.

Since you're not sure if you want/need to do a rebuild, doing these additional tests may help clarify your assessment while you mull it over.
 
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Gents, I will do some tests and maybe neccessary adjustment of the carbutors and the distributor tomorrow. Any recommondation to find the correct adjustment of the carbs and the distributor? In particular how can I determine the correct ignition time sequence. In addition I will do a compression test.


If the the carbs and distributor settings are fine I will continue with the engine test as suggested by Bob.
 
Gents, I checked the carbs, sparking plugs and the distributor yesterday. Everything ist fine. BUT: there seems to be fuel in the oil, at least it smells like this. At the top of the pistons there is burned oil (in Germany called "oil coal") and there are horizontal groovings out. I conclude that the cylinders are worn-out, isn't it? Any other ideas?

Volker
 
Volker

On my BJ7 engine the pressures were around 116 PSI and I was putting oil in at the rate of 1 imperial pint per 100 miles. I had the engine reconditioned last year and the pressures are around 160 PSI and now I am not burning any oil. On my old BN4, I was getting 13 MPG around town and 23 MPG on a good run after the engine was reconditioned.

By the sounds of it, you are desperately in need of an engine recondition especially with scored bores.

:cheers:

Bob
 
It's possible you're leaking gas from around the fuel pump, or from the carbs. Either place wouldn't show up on a CO test. Do you ever notice smelling fuel while driving?
 
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