I've just finished the first moderate engine run and would like
your opinion on how the spark plugs look. Plug on the left is
number 4 cyl, number 1 cyl on the right. Running a HS6 carb
with BDE needle on a 1293 engine. Engine heater valve open,
All comments, even humorous, would be appreciated provided
they help in some way
plugs.jpg
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Strange...... Not a daft question, but What did the engine sound like?
If the plugs were weak, rich, rich weak there'd be a simple answer. But those plugs, in that 1 to 4 order indicate to me that cylinders 1 and 2 are being almost starved of fuel while 3 and 4 or rich to the point of being subsidised by 1 and 2 cylinders.
Is there something in the inlet manifold that is restricting the flow to 1 and 2 cylinders? An untapped vacuum ? This is a classic case of how an old fashioned (but I swear by them) colourtune plug would give the answer
Is the engine modified? HS6 on a 1293 might be too big....because an SU is a variable choke carb it can run, even if its wrong, if you see what i mean.
Thanks for great info, I do have a colortune and will check the
manifold. The engine is moderately tuned with mild porting
and 1.5 roller lifters. Will check and report what I’ve learned.
In my opinion, it'd mean that the oversize, centrally mounted carb was over fueling the two centre cylinders at the expense of the outers. Path of least resistance that affects all depressed air flow
Out of interest, how did it run when you were out for your 10 mile drive, in terms of throttle response etc?
In your situation, I'd try to go through the basics and eliminate any potential issues one at a time before attempting to make any mixture adjustments on the carb, as it does look like you've got an imbalance between the two pairs of cylinders.
My mind immediately goes along the lines of vacuum leakage which may be somewhere to start; vacuum leaks are simple enough to check for while the engine is running, if you spray a bit of carb cleaner around any potential leaks (eg the manifold mating surface to the cylinder head), if the revs rise, you know you've got a leak there which needs fixing. Obviously, take care when spraying flammable liquid around the hot exhaust manifold!
If that doesn't come up with anything, as others suggested, maybe check for other sources of potential blockages or differences in air/fuel flow between the two exhaust ports, along with a general check-over of the carb itself to ensure everything operates correctly.
It's also worth eliminating any possible problems on the ignition side, for instance swapping the plugs over to validate they all perform consistently, measuring the primary resistance of your coil to ensure it's correct for your system (AC Dodd has a great video that covers this on YouTube), validating all of your parts are in good working order (as even brand new stuff can be faulty, this is becoming more and more common in the Mini world!), and checking your ignition timing with a timing light (if you have one).
Personally I think the single HS6 should suit your setup quite well as it's an appropriate size (basically the same as a HIF44 which is very popular for big bore engines), once you've eliminated any problems that are playing havoc with the mixture, you should hopefully be able to get it dialed in and running sweetly. I know a lot of people use a BDL needle as a starting point on similar engine setups to yours, which is slightly richer than your BDE - but every car is different and I'm not sure what camshaft you're running, air filter setup, exhaust system etc etc which all play a part in the fuelling requirements of your under varying throttle conditions. The only way to get it 100% spot on (or near enough) throughout the rev range is to measure the air to fuel ratio and select or tailor a needle, spring and damper oil combo that provides optimal fuelling - many people opt for a rolling road session for this reason.
The rings are nowhere near sealed yet and so any such checks will likely be all over the place at this early point.
Check again after 300 + miles.
This is a very valid point, I'd completely overlooked the fact that this is a fresh engine when I read the original post!
I've sorted out the valve clearance at .015 thou for the 1.5 lifters and will drive it some more to
seat the rings and see what it looks like then. I mentioned in another post somewhere
that I started out with a BDL needle but I'm at 4500 ft altitude and the car was running
fairly rich before disassembly hence the reason to go for a slightly leaner needle to start.
I might do a compression check just to see if I'm close on all cylinders. I would have never thought
there could be that much difference between cylinders using a single carburetor. Will post what
I find when I find it
Thanks everyone for the valuable information, really appreciate it
