How should I troubleshoot this?
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I'm going to order a vacuum line. Since shipping is more than the part, what else should I add to the order while I'm at it? New engine? (Just kidding.)
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The usual location for an electric pump is under the car in the front left corner of the rear subframe. This is just below the front end of the fuel tank.
Question for Deb:
Is the condition worse when the fuel tank is about half full or less?
Reason I ask: A Mini fuel system is pretty much self-priming - the top section of the fuel tank (except on estates and vans) is higher than the fuel inlet to the SU carbs. I discovered this when I parked my Mini with a full tank and a faulty fuel inlet valve. Gas trickled right through the carb, down into the cylinders and right past the rings into the sump/gearbox. Not the whole tank - just until the level dropped to the height of the carb throat. My engine oil was very over-full and smelled weird. Although I haven't tried it, the engine could probably be fed by gravity without a pump, though it wouldn't accelerate well.
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"Hang on a minute lads....I've got a great idea."
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For the fuel pump location and appearance, go back to page 2 of the thread and look for Dan's message where he posted a picture showing the location and top of the mechanical fuel pump. On that page I suggested that if you do not have a mechanical pump you trace the fuel line back from the engine bay to the boot looking for an electric pump. Factory pumps are cylindrical with a sort of disk flange and they were mounted on the rear subframe. Aftermarket pumps take a variety of shapes and may be mounted in the factory location or inside the boot. Google for images of "SU auf201 pump". For the aftermarket pump shapes, Google for images of "Facet pump". You will see a variety of shapes and configurations that are possible... and there are other brands.
You asked why the engine might run OK at 3k RPM or higher if this is fuel starvation. This goes back to my earlier question. I thought you said you were downshifting a lot to get/keep the RPM up? The amount of fuel consumed in a high-gear with low engine RPM will be identical to some low-gear running at higher RPM. In short, if you are downshifting and able to rev the engine to a higher RPM but NOT accelerate to a higher road speed, that more or less confirms a fuel supply problem.
That being said, as Jemal pointed out, if using the choke made things "worse" when the engine is having problems, that sounds like too much fuel, not too little.
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According to Malsal, the car probably has a HIF type carb. That suggests a few ideas.
1. The "choke" mechanism does not pull the main jet down, but opens a secondary fuel passage to enrichen the mixture. Ethanol in fuel might have affected its seals, leading to a rich condition at various rpms, and the problems being noticeable in the intermediate range. Or the choke cable may not have any slack, leaving the choke slightly 'on'.
2. The idle mixture screw moves a bi-metallic lever to adjust the height of the main jet. The mixture is not easy to adjust because the bi-metal is intended to lean out the mixture as the engine warms up. If not done quickly, the person tuning is chasing a moving target and can't get it right, resulting in a rich tuning once on the road.
3. The dashpot may have an inappropriate weight of oil, making it respond to quickly or slowly. Too quick may result in a momentary lean condition, or too slow would result in a momentary rich condition.
4. The dashpot may be sticking or may have its air ports blocked.
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"Hang on a minute lads....I've got a great idea."
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If in doubt, flat out. Colin Mc Rae MBE 1968-2007.
Give a car more power and it goes faster on the straights,
make a car lighter and it's faster everywhere. Colin Chapman.
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(Why would it run well until last month? I didn't change anything.)
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Jemel--When I pull on the choke, it runs rougher. Ditto when it was a cold day, earlier in the month--starts fine with choke, and then after a few seconds runs smoother as I push it back in.
MiniMike--if it's starving at higher rpms, why would it run great above 3000? It's not happy until that point. Today, after replacing the plugs (yes, correctly gapped), I drove around the neighborhood. A very clear shuddering between 2200 and 3000, then smooth above that point. The street I was driving on has some grades, but I noticed this even on level ground. BTW, it idled okay while I got out to shut the bonnet. The idle speed is lower than I like, but it didn't try to quit or stumble.
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If it makes it worse, check your fuel pressure, needle and seat(s), float levels etc.... If any question about leaking vacuum hose, disconnect and plug off as a way to test.... enough of a leak and you will "go lean"!
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"Hang on a minute lads....I've got a great idea."
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I replaced the plugs today, no change. Will order a vacuum line. Around here, there aren't too many options for auto parts stores...
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We're going to replace the vacuum line, because we are suspecting leaks, so that's a given.
As for when it started running poorly, it was after I got the last tank of gas. Again the question--bad gas at the station, or junk in the tank stirred up by running too low? Since I don't have a very good gas gauge, I've been filling up by mileage. The last fill-up was 7.07 gallons. The previous one was 6.1. I think there was a tank before that, but I've only had the car since late October. It ran fine then. First hint of problem was with the last tank. BTW, we (Villagers) use non-ethanol around here for older cars, and my husband has filled up from the same pump in the 356, never had a problem.
Not sure if it's been brought up, but could the fuel in the tank be old? My own Cooper S coming out of winter hibernation was rough. Probably last run just before thanksgiving, and was unhappy when I took it out to a local gathering last week. So in less than 4 months, fuel in the tanks was noticeably off! Modern fuel is just not up to modern standards, specially in old cars with vented tanks.
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If in doubt, flat out. Colin Mc Rae MBE 1968-2007.
Give a car more power and it goes faster on the straights,
make a car lighter and it's faster everywhere. Colin Chapman.
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I'm back home, and ready to work. Going to do the plugs tomorrow. Spouse went to the local auto parts store today to get a vacuum line, but didn't know what to get. Any hints? Is there a universal fit line, or is it Mini-only? If the latter, what would a 1275 engine take? On this site, I see Part No: VAC001. Looks right--is it?