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Thread: Tune Up Trouble

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    Tune Up Trouble

    I need some help from the British Car Brain Trust.
    Been trying to tune my tricarb. It now accelerates well, but backfires out the exhaust while coasting. Loud popping, some warbles.
    What are the suspected causes?
    Valves: adjusted.
    Spark plugs replaced and gapped at 0.30.
    Timing was difficult to set, probably due to Pertronix. Or my Harbor Freight timing light.
    Engine struggles at 15 degrees; runs well over 20. I ended up rotating the distributor to get maximum RPM, then backed off 100 RPM and tightened the locking bolt. I read about that trick on another forum here.
    Linkage: set
    Carbs: adjusted with Colortune.
    I found the middle jet had vertical movement and would not return flush with the bridge when turning the mixture nut. How can this be corrected for now? I dropped the middle needle 0.18 to compensate.
    Carb rebuilds are planned for the off season, including new throttle shafts.
    The fast and regular idle are finally set. 1200 and 800 RPM.
    Topped up with premium fuel today.
    Placed on ramps and jack stands now, to check the exhaust system for leaks. After it cools down.
    I'm not an engineer or a mechanic, but I would like to learn how to properly tune my Healey.
    Suggestions welcome.
    Thanks.
    Douglas

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    Re: Tune Up Trouble

    Backfires can be caused by leaking between the exhaust manifolds an downpipes.
    Steve Gerow
    Altadena, CA, USA
    Maker of most complete Big Healey rear disc kit
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    Re: Tune Up Trouble

    I believe it is running lean.
    How well does it accelerate?
    If it is lean, pull the choke out while driving and see if it feels more powerful.
    About TV Shows-
    "...you really can't restore a car in 10 days. I don't want to watch a race where people have to restore it in a week. It's not going to drive, and those cars never work. In real life, it takes years to get it right. " Jay Leno.

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    Re: Tune Up Trouble

    I have 2 colortunes. In my experience if the carbs are adjusted for "bunsen blue" the carbs are too lean. Richer than bunsen blue toward the yellowish seems to run better.

    Des Hammill in his "SU Carburettor High Performance Manual" suggests using the tail of a dial caliper to set the depth of the jets so they are all equal - p.44 suggests for the 1-1/2" carbs, as a starting point, to use a jet depth down from the bridge of .040"/1.0mm. Then when you adjust them make sure they're all equal. You could also measure all 3 and average the jet depths.
    Steve Gerow
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    Maker of most complete Big Healey rear disc kit
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    Re: Tune Up Trouble

    Popping on the overrun is usually caused by excess fuel igniting on the hot exhaust. Popping while coasting is not a backfire.

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    Re: Tune Up Trouble

    I can see the value of starting the tuning operation with the jets all at the same height but am having difficulty believing they will all be at equal height when correctly adjusted?
    Have I misinterpreted what is being said?


    Des Hammill in his "SU Carburettor High Performance Manual" suggests using the tail of a dial caliper to set the depth of the jets so they are all equal - p.44 suggests for the 1-1/2" carbs, as a starting point, to use a jet depth down from the bridge of .040"/1.0mm. Then when you adjust them make sure they're all equal. You could also measure all 3 and average the jet depths
    .

    Danny

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    Re: Tune Up Trouble

    Thank you gentlemen.
    I will inspect the exhaust system today and put a socket on the 6 brass nuts at the downpipe flanges. There is a speed bump at the end of our road. The muffler sometimes drags over it.
    The car accelerates well, but I'll pull the choke to test a richer mixture. No pops or backfires on acceleration or steady driving.
    I tried to set the jets the same, with the butt end of my caliper. I seem to remember 0.058.
    I'm not convinced old carbs need to be set exactly the same. So many variables. Plus the balance tube should help average things out.
    I plan to go through the checklist again this week. Check timing, linkage, colortune, measure jet drop, etc.
    It starts OK and doesn't run-on as much now.
    Should I try thinner oil in the dampers? Moss SU damper oil is in now.
    Please keep the suggestions coming.
    Douglas
    Last edited by dcarlg; 07-22-2019 at 03:13 PM. Reason: typo correction

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    Re: Tune Up Trouble

    Thinner oil only allows the pistons to travel with less resistance and may help sluggish acceleration. I think Moss SU oil is okay. I tried Marvels a while ago and it disappeared in a couple of months.
    Be careful with tightening the 6 brass nuts as on the exhaust flanges as the can be broken easily.

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    Re: Tune Up Trouble

    Thoughts:

    Thinner oil will exacerbate a lean condition by allowing the pistons to rise too quickly. The rise needs to be retarded a little to encourage richness. I have on occasion used gear oil.

    Properly built (not worn out) SUs are precision instruments and should be tuned with the jets at the same height.

    I've found that, if the car runs OK but backfires going downhill, the jets are set slightly too lean. What's worked for me is to have the car just "burble" going downhill.

    I had a Harbor Freight timing light with the little knob for adjusting the advance - not worth a d*mn. Wouldn't trust it.

    If your carbs are worn out, you won't be able to satisfactorily tune them. Especially with butterfly shaft leaks. Ditto uncentered worn needles and main springs of different strengths and differing float levels.

    There is a lot of information regarding SUs on this forum. Suggest searching and reading up.
    Steve Gerow
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    Re: Tune Up Trouble

    Just my nature but I don't like vagueness. I would not trust the 'turn the dizzy to find the highest idle and back off 100 rpm' method, there are too many factors that can affect this so I think you need to know what you are setting it at. You either need to mark the pully for advance or get a timing light that has adjustable advance capabilities.

    A test for the other end of the timing spectrum is to make sure you don't exceed 35-36 deg. advance when dizzy is at full mechanical advance (I expect you will see full advance by 3000rpm but I usually run up to 3500 or so to be sure I'm seeing full advance).

    When trying for jets equal, I figure within .005 or so is probably good, a slight difference is ok but the closer, the better. You are right that the balance tube means all carbs affect all cylinders to some degree.

    Jet drop is only for cold starts so should have no affect on warm running.

    I don't think dashpot oil will have any affect on deceleration popping - the only time the oil affects things is very briefly when the throttle is first opened/blipped.

    Good luck,
    Dave

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    Re: Tune Up Trouble

    Back in my wild youth?, we used to cause a loud backfire by shutting off the ignition for a couple of seconds, then turning it back on. Not good for your muffler.

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    Re: Tune Up Trouble

    My apologies for incorrect nonemclature. I'm trying to be accurate. I will be happy with a warble instead of the loud popping.
    The carbs are well worn. Last rebuild kit installed was 1980; but only 10K miles since then.
    The needles look good. The springs are approx. the same length. Pistons move freely.
    Steve is right, as usual. This forum is a vast source of information. I study these posts constantly, and seek out other sources.
    Dave, I would like the timing to be precise. But the Pertronix seems to change the timing requirements. My Harbor Freight light may not be helping much either. The flywheel is marked at the notch. I will try to also place a white mark 3/4 to the right. Correct?
    After checking my jets again, I will enrichen the mixture by another flat and take a test drive. Repeat as needed.
    I will also try to eliminate the free movement of the middle jet inside it's tube so I can set it precisely.
    Pdplot: thanks, good to know...
    Tahoe Healey: I'll stay with the SU oil. Thanks for sharing your experience with MMO.
    Keep you all posted.
    Douglas

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    Re: Tune Up Trouble

    Since I used my old reliable Craftsman non-adjustable timing light, I marked the pulley with the following marks in inches to the right of the TDC notch as viewed from the front. Based on pulley diameter of 6.25". :
    14deg = .76", 15deg = .82"; 16deg = .87"
    32deg = 1-3/4"
    35deg = 1-7/8"
    Steve Gerow
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    Re: Tune Up Trouble

    Quick Update:
    Looks like I have bolts (instead of studs and brass nuts) at the downpipe/exhaust manifold flange. Yellowish. Grade 8? 2 needed some tightening.
    I opened the 3 carbs and attempted to set the jets to 0.060" below the bridge. The middle jet is at 0.071, which seems to be the minimum obtainable.
    There was slightly less popping during test drive. I pulled the choke out some which reduced the popping more. Guess I am still running lean.
    So, I'll enrichen the mixture another flat on carbs 1 and 3 and test again. And recheck with the Color Tune.
    Maybe I can find another timing light.
    I'm curious. Anyone using Pertronix? What is your timing set at?
    I feel like we're making progress.
    Thanks.
    Douglas

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    Re: Tune Up Trouble

    I'm using a Pertronix and go for the maximum advance of around 35deg. When I had my distributor overhauled by Jeff Schlemmer of Advanced Distributors, he told me to set it at 16deg at idle with disconnected vacuum. That also results in the 35 max.
    Steve Gerow
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    Re: Tune Up Trouble

    I'm with Steve and while I may be guilty of oversimplification I set timing at 3500 and let the distributor take care of everything at lower rpm's. I have found my car to be prone to overheating with too much advance and 32-34 degrees is what I use as a max.

    Best--Michael Oritt
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    Re: Tune Up Trouble

    Quote Originally Posted by Michael Oritt View Post
    I'm with Steve and while I may be guilty of oversimplification I set timing at 3500 and let the distributor take care of everything at lower rpm's. I have found my car to be prone to overheating with too much advance and 32-34 degrees is what I use as a max.
    I'd like to try that technique but I've always been loathe to rev an engine that high without a load. I don't think it's good for an engine; am I just paranoid?

    I also cringe when one of the 'hot rodders' on TV car shows jumps into a car with a cold engine, starts it and immediately romps on it (my dad gave me an earful anytime I drove fast before the temp gauge at least came alive). It's a definite no-no on an air-cooled aircraft engine.

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    Re: Tune Up Trouble

    Quote Originally Posted by Bob_Spidell View Post
    I'd like to try that technique but I've always been loathe to rev an engine that high without a load. I don't think it's good for an engine; am I just paranoid?
    It's the only way I've set my timing for over 25 years now. And, if you know your distributor advance curve, you probably don't need to rev that high. Mine is a Mallory dual point with a Pertronics unit and it achieves full advance about 2800 rpm, so anything above that is all I need to set timing.

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    Re: Tune Up Trouble

    If you think your jets are close then instead of just turning the jet nut one way or the other which to me is a guessing game, I instead put a quality tach on the distributor or coil and turn the jet to get the highest rpm. That should mean that you have the best mixture. I have found that doing this is sometimes a tech to rich. but after doing this rpm adjustment to the jet I go for a brisk ride, pull back in the garage quickly shutting it off then read the plugs. Whatever the plugs show is how I final adjust. (make sure all adjustments to the carb are on a very warmed up engine.)
    About TV Shows-
    "...you really can't restore a car in 10 days. I don't want to watch a race where people have to restore it in a week. It's not going to drive, and those cars never work. In real life, it takes years to get it right. " Jay Leno.

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    Re: Tune Up Trouble

    Quote Originally Posted by dcarlg View Post
    Quick Update:
    Looks like I have bolts (instead of studs and brass nuts) at the downpipe/exhaust manifold flange. Yellowish. Grade 8? 2 needed some tightening.
    I opened the 3 carbs and attempted to set the jets to 0.060" below the bridge. The middle jet is at 0.071, which seems to be the minimum obtainable.
    There was slightly less popping during test drive. I pulled the choke out some which reduced the popping more. Guess I am still running lean.
    So, I'll enrichen the mixture another flat on carbs 1 and 3 and test again. And recheck with the Color Tune.
    Maybe I can find another timing light.
    I'm curious. Anyone using Pertronix? What is your timing set at?
    I feel like we're making progress.
    Thanks.
    Douglas
    Have you tried to do a static timing yet?
    Patrick
    '67 Metallic Golden Beige/Red
    Owned since '72

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