Into the unknown: Tuning the 1275 A-series engine

Discussion in 'Classic Mini' started by ImagoX, Nov 21, 2014.

  1. ImagoX

    ImagoX New Member

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    This is my follow-up thread to the epic and tragic "Service Project" - see that thread for a first-hand account proving beyond a shadow of a doubt that Murphy's Law is indeed alive and kicking where classic Minis are concerned...

    OK, so here's the situation:

    I have a newly-rebuilt 1275 engine from Seven, currently fitted with twin-SU carbs and an electronic dizzy. The engine was just timed by a garage known for good work, the carbs are balanced, and all the mechanical linkages (choke and throttle) are adjusted. The engine starts right up and idles fine in the warm shop. The garage reports that the timing interval set by Seven appears to be 15-degrees in advance, which seems too high but they left it assuming the builder set it that way on purpose (I'll need to confirm w/ Seven).

    When the car's taken outside, however, it wants to totally bog out and die unless the choke is left ALL THE WAY OUT, which, of course, causes the engine to race. Push the choke in after warming = car sputters and dies. I can drive the car (with the choke open) and it runs and pulls HARD: Seven seems to have done a bang-up job on the rebuild and I've never felt such power from the old gal. The garage warned me of the issue and wants to see the car again when the weather warms for final adjustments.

    However, me being... well, ME... I'm wanting to see if I can do anything to make things right on my own, now that the stuff I can't do (timing) is done. Problem is, I'm not sure if the issue is being caused by the high degree of advance (15 degrees), the mixture, or a combination of both.

    Where to start?
     
  2. Minidave

    Minidave Well-Known Member
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    #2 Minidave, Nov 21, 2014
    Last edited: Nov 21, 2014
    Sounds to me like your carbs are just too lean, but one easy thing you can check first is to see if there's any oil in the tops of the carbs - use something light like ATF to top them off.

    I think the timing is too far advanced too, back it off to about 8* either at idle or static. It's easy to do a static check of the timing - you car should have marks on the lower pulley and a corresponding tab on the timing cover. Put a test lite on the minus (-) side of the coil, turn on the ign and pull the center coil wire out so it won't try to start, put the car in 4th gear and roll it forward till you see the mark on the pulley, as you continue to roll it forward the light will light when the points open, at that instance check the timing marks. If it's too far advanced, loosen the clamp on the dist. and turn it counter clockwise a bit, then back the car up and roll it forward again watching for the test light to light. The light should go on just as the pointer lines up with 8* or so.

    However, the timing being off will not cause the symptoms you're describing....
     
  3. MCS02

    MCS02 Moderator
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    I really don't know much about SU's but have lots of experience with Weber's. All the stuff I worked on were Alfas, Lancia and Fiats not Minis. But I have always liked Weber carbs. Have you thought using Webers?
     
  4. Minidave

    Minidave Well-Known Member
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    Here are a couple of easy things to check on the carbs.....take the air cleaners off, then loosen the caps on top of the suction chambers all the way. Stick a finger into the throat of the carb and lift up on the piston all the way, it should go up easily and drop right back down again when you remove your finger. It should not drag or scrape either direction.

    If that's good, put the caps back on top (make sure there's oil in there) and do the same test, there should be resistance going up and it should free fall down again.

    If it doesn't pass these tests, something is wrong in the assembly of the piston and dome, it could be related to those screws or it could be the jet is not centered or the needle is bent or incorrectly installed. The piston should move cleanly and smoothly.

    If it does pass these tests then I'd bet on the carbs being set wrong, which surprises me if you took it to a shop that's used to working on these cars.

    One other possibility - make sure no vacuum lines are off, especially the one for the brake booster.
     
  5. ImagoX

    ImagoX New Member

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    Re: Webers - I'd love to but I'm so far over-budget on this so far that new carbs are simply out of the question. The SUs I have now are (I'm told) a set that came off a Cooper S prior to their rebuild, so they should be more than adequate for a daily driver like Fiona.

    Re: Lean - I'm thinking that, too. The supplier of the carbs confirms his "starting point" for the main jet screws is only 8 flats, not the traditional 12 (...why? only God knows...), so that could be it. Sounds like a good place to start. FYI - the shop that rebuilt the carbs also used a slightly RICH needle for both since he knew I'd be using a LCB manifold with a Maniflow straight-through exhaust (no cat).

    last, for what it's worth, Seven confirms that their build sheet lists they set the engine for 8 degrees of advance, not 15, so I'll look to dial it down when I get access to or buy a timing light. Seven advised the only problem with a 15-degree advance may be pinging if I run regular fuel (which shouldn't be an issue as I always run Premium) and should not cause the bogging issues I'm experiencing.

    Re: removing brake booster - really? I've never heard that in any of the videos or threads I've researched. I know about taking off the vacuum line to the dizzy while setting the timing (I've never hooked it up, even when I ran a single HIF carb), but never thought to take off the brake vac line. Iiiiiinteresting... Or are you saying to make sure that line is CONNECTED? maybe I'm not following, sorry.

    Definitely things worth looking into this weekend, if the weather won't turn me into a popsicle when I go outside. I really need a win this week... something... anything.

    Thanks! --M
     
  6. Minidave

    Minidave Well-Known Member
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    Matt, feel free to call me if I can walk you thru anything that might help. You can get a test light from any auto parts store for only a very few Bux, you don't need to buy a timing light.....you don't need webers, you just need these set correctly and I'm sure you'll be happy with the performance.

    913-648-6339 is my number
     
  7. ImagoX

    ImagoX New Member

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    I have a basic clip-on test light probe - is that all that's needed for static timing checks, or something different? Thanks, Dave - gonna try tackling this over the weekend and see if I can improve things on my own. (<--- sucker for punishment, obviously) :p
     
  8. Minidave

    Minidave Well-Known Member
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    Yes, the clip-on 12V test light is all you need for static timing.

    Clip one lead to the minus (-) on the coil and the other to ground. Then just follow what I wrote, if it's confusing call me and I'll walk you thru it.
     
  9. ImagoX

    ImagoX New Member

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    PS - if anyone finds it helpful:

    [ame="https://www.youtube.com/watch?v=Ush9Zc-hNIA"]Good video on twin SU carbs on YooToob[/ame]

    This is the best vid I've found yet explaining the position of the adjustment screws on twin-SUs, how to achieve "default" starting-point carb settings, where the location of things like jets, needles, and oil are, etc. This shows the units on an MG, but they're basically the same on the Mini (just flipped L-ti-R looks like).

    Hope it helps.

    PS - Thanks, Dave for the timing video - that REALLY helps! :Thumbsup:
     
  10. old81

    old81 Club Coordinator
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    The above post is why I like this MINI community. Hell, Dave Thank you and it is not my Classic. :)
     
  11. Minidave

    Minidave Well-Known Member
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    As more of us old farts are retiring and dying off, a lot of this knowledge is being lost to the ages....so I like to try and teach young people the old ways.

    I take my Jag and my old Mini to school to show my students old tech, but I know they will probably never encounter any of this is their careers.
     
  12. Crashton

    Crashton Club Coordinator

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    This so true & oh so sad. :(
     
  13. ScottinBend

    ScottinBend Space Cowboy
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    .....but then again, so are the cars that these talents can be used on. :sad:
     
  14. MCS02

    MCS02 Moderator
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    [ame="https://www.youtube.com/watch?v=EkaKwXddT_I"]https://www.youtube.com/watch?v=EkaKwXddT_I[/ame]

    Seems fitting
     
  15. ImagoX

    ImagoX New Member

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    Well, THAT was anti-climatic...

    Started the car this morning and it simply wouldn't run, even at idle, without full choke. Opened the mixture screws a bit (4-5 flats) to let it idle w/o racing and warmed the engine. Touching the throttle would bog the engine, so no bueno.

    Then I pulled off the dash pots and dialed the jets up to flush, then dialed then DOWN 12 flats, just like my video said. Reassembled everything aaaaand:

    https://www.facebook.com/video.php?v=10202826503333147&ref=notif&notif_t=like

    Runs like as TOP! Idle needed a bit of tweaking but the garage balanced them so I just went slow and adjusted evenly until I had solid idle. NO BOGGING and man does she pull hard! :cornut::drool:biggrin5:

    Still a few small issues (baffling electrical issues/no tailllights/no wipers/oil leak from somewhere - gah!) but I'm gonna officially call the engine "fully running"!!!
     
  16. Minidave

    Minidave Well-Known Member
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    See?

    Knew you'd get on top of it.....
     
  17. Angib

    Angib New Member

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    General thoughts:

    - Get to love your SUs - maybe not the highest performing carburettor, but one of the most durable, will be working fine after a nuclear hit. Webers may give more horsepower but will stay in tune for several minutes, max. Expect to adjust your SUs at least once every decade, max.

    - Balancing two SUs by taking off the air cleaners and running a small plastic tube from your ear to the carb inlet works surprisingly well - and you don't need 30 years' experience to use this method.

    - Everywhere it says to check/top up the oil level in the SU dampers but you will never, ever see or feel any improvement from doing this, so treat it as some weird semi-religious rite that everyone does but no-one can say why.

    - Your A-series engine should leak oil - this was the advanced rust-prevention method used by the manufacturer. Plus if you fix the oil leaks, your remote gearchange will become stiff from lack of lubrication.....
     
  18. Minidave

    Minidave Well-Known Member
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    #18 Minidave, Nov 23, 2014
    Last edited: Nov 23, 2014
    As I said before, balancing the air flow can be done with a hose, but if you want to get it accurate you use a Unisyn. Does it make a difference? Yes, the engine will be more responsive, especially immediately off idle.

    The oil in the tops of the pistons does have a function, it keeps the piston from moving too far or too quickly, which can cause large fluctuations in the mixture, as the fuel metering needle is directly attached to the piston. Not having oil in the dampers can cause the engine to backfire especially when cold, and it will go thru a lot more fuel too. However, it doesn't need attention every time you fill up with gas, once a year or so is plenty.

    I agree with the durability of the SU and yes, once set correctly it needs virtually no maintenance to keep the engine in tune. Although like any mechanical thing parts do wear out eventually - in the case of the SU the throttle shaft bores will wear, causing idle fluctuations. The needle and main jet will wear too which will make it start to run rich, and the dashpots need periodic cleaning but these issues only come after many miles of driving.

    Webers also will stay in tune once set up correctly, as the only moving part is the throttle, no pistons, moving needles or other parts to wear on them.

    My A Series does not leak oil, not should they IMHO.....comic comments aside.
     
  19. Crashton

    Crashton Club Coordinator

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    My experience with Weber carbs is once set up correctly they tend to stay that way. My experience with SU carbs is the same as the Webers. Folks like to blame the SU's for everything, but it usually is not the case. Car doesn't run right? Look at the ignition system first. Just my .02 shillings.
     
  20. ImagoX

    ImagoX New Member

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    Car started right up this afternoon and continues to run well. I'll get a Unisys balancer to make sure, but they should be really close, if not perfect - I tried turning both idle screws equal amounts and went slow until idle was correct. My tach isn't re-connected yet so I'm not sure the exact idle speed, but it sounds right.

    Went out and drove about 15 miles just to see how she'd do - gas and temp gauges work (yay). Still leaking oil - I suspect the crush washer on the oil galley plug at the front of the block (where the oil pressure sender goes) - right now I have the adapter for that system plugged in with a set of bolts plugging the holes and it looks like it's leaking pretty bad under pressure from there. Will just get a simple bolt to plug the hole until/if I install an oil pressure gauge next season.

    Acceleration with 20-weight oil in the dash pots is snappy and the car pulls HARD. I don't want to thrash it during break-in though, so I'm trying (semi-successfully) to take it easy. Heh...

    Thanks again guys!! --M
     

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