Stumbling Slant --what is left??

Slant 6 Engines

  1. MOPAROFFICIAL

    MOPAROFFICIAL FABO Gold Member FABO Gold Member

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    90 lbs is the absolute minimum for combustion...minimum.
    It's a long way from home ...
     
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    • my68barracuda

      my68barracuda Well-Known Member

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      I have a Briggs and Stratton 3.5 hp lawn mower that was not within striking distance of 90 psi when new, is now 50 years old and still runs. When new it took a pull on a rope wound around the flywheel to start it. Now I can almost spin it by hand. Not much power left but still starts and runs.
       
    • MOPAROFFICIAL

      MOPAROFFICIAL FABO Gold Member FABO Gold Member

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      Yes, lawn mowers run in low compression, a little more for riders.. but they spin a blade...and dont have to propel a 3000+ lb car to 70 mph...the crank shaft isnt near as big n heavy either. The compression is relative to application and rotating mass.
      One of the advantages I have in this hobby is that I was an actual line/diagnostic mechanic for years, Machinist Apprentice as well.. i started out working at little blue hole in wall shop next to savinos market in fresno starting out with a swedish merchant marine mechanic that owned it... I've wasted so much time and had return visits all for the sake of helping a poor womans car limp along a little more. They never ran good... they ran somewhat better...but only graduated to worse over short time. Like plugs... you can change them every week and feel a tiny difference/improvement....but Low cylinder pressure is like not being able to breath...like lung cancer.
      As soon as he said 90 lb we pretty much established that the cylinder head at the minimum would have to come off and be inspected. I'm not gonna feed this person false hopes aka piss down his leg and tell'em its raining.
      Pull the valve cover to check the stem heights ...then pull the head...its predictable after years of seeing it.
       
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      • volaredon

        volaredon Well-Known Member

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        Huh? Tapered seat plugs are peanut plugs, not used for drool tube heads
         
      • pishta

        pishta I know I'm right....

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        Ahhh, your right. I meant the non tapered head. Ill correct.
         
      • 67belevdere/225

        67belevdere/225 Well-Known Member

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        on older engines carbon built up on the intake valves acts like a sponge and and cause stumbling
        So much so that GM had a TSB on the fire engine , we would remove injectors and blast off the intake valves wit a coconut like (sand but not ) in the engine and that stumble would go away
         
      • AJ/FormS

        AJ/FormS 68 Formua-S fastback clone 367/A833/GVod/3.55s FABO Gold Member

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        While 90psi seems low, I would suspect the gauge is bad before condemning the engine.
        If the cylinder pressure really is low, then so will be the "suction" on the intake stroke. This might make the engine run rich, as the air is in no hurry to plow down that 1920. An intake leak would only make it worse. As would running the engine at say 170 versus 200.
        IMO all your problems are in the tune, with the possible exception of having an intake manifold leak. That engine should be able to tic over at 500rpm or less all the while smoking out every orifice and burning a quart of oil every other day. I had one like that so I know. Actually I have had two of them.
        >So then, here is what I would do if it was mine;
        If the engine is not running at or near 195, I would change the stat to make it so.
        I would prove that the cylinders are NOT getting air from anywhere but the carb.
        I would prove that the carb is being properly heated by the exhaust heating system.
        I would pick a cylinder and make sure the piston is coming up on the compression stroke but not yet at the top; then I would bop the closed valves a few times to crush whatever carbon may be stuck on the seats. You will know when the valves are sealing by the change in the sound as the valves hit the seats. Then repeat on the other cylinders.
        After that; I would run a chemical cleaner down the carb to get rid of that carbon , and put some in the gas-tank for good measure. Then warm it up and change the oil.
        Then I would very accurately reset the lash to .013intake/.023exhaust, because I know slantys like these numbers.. After you get the tune right, you can go back to factory 10/20 if you so desire.
        Finally, I would warm the engine up and repeat the compression test with a known-to-be-accurate screw-in gauge. and I would back it up with a wet-test.

        Now the engine is ready to be tuned.
        Firstly, you should know that a stumble is almost always caused by a lean tip-in.
        Here are some of the causes;
        > a cold-running engine; hence the 195 stat
        > tight valve lash, hence .013/.023
        > engine already sucking air Not from thru the carb, and carb adjusted to compensate. hence the test
        > Lack of carburator heat on a carb that was engineered to run it.....
        > A low WET fuel level, and/or stale or contaminated gas.
        > Insufficient transfer-slot exposure causing the transfers to stop or nearly stop flowing. This is nearly always brought on by too much Idle-Timing.
        > insufficient pump-shot or a faulty pump-circuit.
        > wrong and uncompensated air filter house.
        > pollution controlled and delayed spark timing.
        > a poorly functioning bowl-vent
        > a poorly functioning tank-vent system.
        > fuel-pump sucking air.
        there may be more, but I got a brain-stall cuz it's lunchtime, and mabelly is distracting the concentration right outta me.

        I can tell you that, even at 90psi, I am very confident that your engine can be tuned to eliminate that stumble; unless
        1) your TC is defective
        2) you are running a sub 2.76 rear gear
        3) your brakes are locked, or
        4) you are hooked up to 10,000 pounds of cargo trailer,lol.
        The upshot of this is that from a stop, when you gas it; the rpm has to jump up to the stall-rpm and the car has to start moving, If it doesn't, then you can have problems, even with a well-tuned system.
         
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        • MOPAROFFICIAL

          MOPAROFFICIAL FABO Gold Member FABO Gold Member

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          Our imported gasuckoline is dirty garbage.
          All intake valves are gunked after a messily 3000 miles or less and just get worse over time. The back of the valve is hot..every shut down leaves some on the back of closed but more so cooks and the residues build whether running or off and just hot. The only part of the valve that stays clean is the max lift flow path that across the valve on the far side of the bowl. That's the fuel 'liquid' that separates because it weighs more than o2. The greater broader the splash thrown the better if in the right direction..which is off across the apex turn to the far side of the bowl. The majority of the ssr air flow stops after .150ish lift 'debatable but in a hair split fashion'.
          What kind of reverse eddies you may have can also keep liquid passengers on the inside of the head as well.
          Heres a valve from a well ported 915 . Look at the clean half moon section of flow that's a fingerprint of max lift/wet flow.

          20211009_172354.jpg

          20211009_172334.jpg
           
          Last edited: Apr 4, 2022
        • 67belevdere/225

          67belevdere/225 Well-Known Member

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          my valves were way worse than that , the engine had 5000 on it
           
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          • volaredon

            volaredon Well-Known Member

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            I remember during my days of working at the dealer, ('90-92ish) we had an 89 D150 with a 318 come in. I did not draw that ticket, the guy across the shop did. major pig, no balls was the complaint. He ended up pulling the heads and decarboning everything.... the intake valves were so bad, the carbon hanging from them dang near filled the whole area under the head of the valve..... fuel and air had no room to do anything in the intake ports. Any that did find its way in definitely couldn't have passed thru the valves into the chamber! I have never seen an engine so carboned up, before or since. as I remember, this thing had like 35-40k on it at the time.
            the guy who drew the ticket to do that job, had previously worked at a Jap car dealer, Nissan I think it had been.
            I remember him telling us that they had a setup to blast coked up valves in the chamber without pulling the head..... just the manifolds and rocker arms had to come off (so the valves would be shut during the process) to deal with vehicles that came in carboned up like that.
             
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            • MOPAROFFICIAL

              MOPAROFFICIAL FABO Gold Member FABO Gold Member

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              This one had a bottle of octane boost & cleaner in it every tank. Still sucks.
               
            • wrock

              wrock Member

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              These are some great ideas. I think I have addressed some of them already. I checked with a propane torch to look for leaks around the carb and intake, and did not find any. It appears that the engine is/was running rich, as new plugs look like this after less than an hour of operation: 20220417_122013.jpg

              Six is on the bottom, and #1 on the top of the image. So the time on these plugs was before I replaced the carb. Doing a wet float check on the new carb, it seemed a bit high (2 or 3 32nds above the 27/32 level) so I adjusted the float a bit. And while I was at it, put the float spring in correctly; it was in at an angle. I would guess this is something that might happen in shipping.

              I'll need some education on what "bopping" a valve entails. Is this tapping on the rocker arm over the valve with a hammer, or brass hammer, or plastic mallet; or with a wood block in combination with any of those? is it pulling off the rockers to hit the valve directly?

              I don't think the fuel system is a problem; I tried running it with a 2-liter bottle of known good gas plumbed directly into the fuel filter and it seemed to run the same. I also dosed it with Heet to try and eliminate that as a cause; back in '86 that fixed a similar problem that came down to a load of bad gas I got while I was hauling a 2 axle U-Haul cross country. But that's a story for another time.

              I'll get a 195 stat and try that also. What I heard "back in the day" was that to de-gunk an engine, you poured ATF into the intake till the engine stalled, then let it sit overnight. But your bopping method seems like a more reliable method of removing deposits.

              Thanks for your thoughts and time.

              Bill
               
            • ESP47

              ESP47 Well-Known Member

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              I'd start with a simple valve adjustment and go from there. Cover the valve cover gasket in vaseline so you can remove it multiple times without it tearing if you need to.
               
            • AJ/FormS

              AJ/FormS 68 Formua-S fastback clone 367/A833/GVod/3.55s FABO Gold Member

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              With ATF, flooding the engine to a stall is not required, nor is letting it sit overnight, as there is no chemical action taking place. There are better chemicals available.
              I have had good results even with water, which, in the chamber converts to steam, rather explosively, at a very-hi idle and does a great job. Just don't feed it too much too fast, no big deal. With water you don't need to bop the valves they will be clean as clean can be. What I do is get a kitchen atomizer and blast the primaries.

              To Bop the valves, Here is what I recommend for a first-timer.
              remove the rocker shaft.
              install a LeakDown tester set to ~90psi, (yes you can use less but there is no need to go higher) and slowly pressurize, blowing the piston down to the bottom. Now you are set. I use a small hammer. The trick is NOT to hammer the valve.
              The trick is to bop it such a manner that the hammer slides off the stem, and let the combination of air pressure and spring, slap the valve back onto the seat, crushing any carbon that might be there. You only need to bop it say .25 inch Two or three shots will blow the carbon off, and the sound will change to a deeper-sounding thunk, and the hissing of the escaping pressure will cease.
              If it does not, and they all sound the same, then either;
              1) this was a colossal waste of time, or
              2) your intake valves are all bent or
              3) your exhaust valves are all burned.
              Next is a proper LeakDown test, which, with the Rocker shaft off is so simple.
              Just crank the pressure to 90psi and blow the piston to the bottom then read the % leakage. This tests the valves, more so than the rings. After all six are done interpret the results. If all are similar, then yur done. But if some are quite a bit higher than others, then something is up. At that point, you need to squirt a little oil into the cylinders with low readings, to test the rings. After the oil goes in, (about a tablespoon), you need to crank the engine a bit to distribute it around the ring. Then reduce the test pressure to 30psi, and hit the LD tester, while watching the gauge. The number should, initially, be higher, then drop as the air pressure pumps the oil into the crankcase thru the ring gaps. or other gaps, lol. If it does not act like this, you will need to listen at the carb or at the tailpipe for hissing, which would point to a leaking valve(s). If you hear it in the CC, the engine s sorta doomed at 90psi cranking compression. Well if it hisses anywhere but into the CC it is doomed anyway. Some hissing into the CC is normal, but it cannot begin until AFTER the pressure is turned on and the previously injected oil is blown down.
              IMO, and others may argue,
              as for ring leakage only, exclusive of hissing valves, at a test pressure of 90psi which is what I use;
              less than 4% leakage is pretty good.
              over 10%, she's getting lazy, and seems to be using a lotta gasoline especially around town
              over 20% she's a turd in desperate need of attention. Sure it runs. Sure it smokes. Sure it fouls the plugs. and sure it gets lousy fuel economy; but it still runs.

              Happy Easter
               
              Last edited: Apr 17, 2022
            • wrock

              wrock Member

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              I like the thought of "lean tip-in" except that this thing misses at all RPMs and throttle positions. When I get it up to highway speed, it is not as noticeable at those RPMs (well, most of the time not), but It has some really crappy missing that happens at part throttle, and when goosing it from part to WOT at lower RPMs. So maybe I misspoke when I said "stumble"; let's call it an intermittent miss at all RPMs and throttle positions. The fact that it does this from stone cold start would, in my mind, eliminate thermostat and heat valve as possible suspects. I guess I can throw some seafoam or whatever down the carb and see if I can blow away any carbon; that is certainly easier than taking the head off, which the only option I have left in my big bag of ideas.

              Thanks again for the engaging dialog.
               
            • Bewy

              Bewy Well-Known Member

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              I would be looking at electrical......
               
            • AJ/FormS

              AJ/FormS 68 Formua-S fastback clone 367/A833/GVod/3.55s FABO Gold Member

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              IMO
              Your miss is coming from the ignition system, in response to the sooted up plugs, which are leaking voltage to ground. Fix the sooting and the ignition will get back to business. A missfire like you describe; speaks directly to incomplete combustion.
              We are conditioned to think that;
              rich means too much fuel. But
              rich can just as easy be not enough air.

              Now, lets just talk for a bit;
              You say the compression is about 90psi.
              90psi by itself is not bad. But;
              in your case; the same rings that only make 90psi, are not "sucking" very hard on the intake stroke, so it takes a large amount of piston-travel, to actually get the atmosphere interested in entering the carb..... where it finds the butterfly almost closed. So it's real lazy about it. So then, when you open the throttle, the already lazy air stream finally gets moving, and pulls fuel. By this time, the piston is slowing down getting ready to stop, turn around, and go back up on the compression stroke. So the "suction is decreasing rapidly. Hopefully the valves close and seal, so that the pistons cannot drive any of that just barely inducted mixture, back into the intake. Which would upset the next cylinder in the firing order.... and maybe another.
              What this means is that for any given speed, the throttle always has to be opened further, to get sufficient air, to maintain that speed. It also means that to accelerate, the throttle will again have to be opened further than is "normal", to get the required air.
              Now; because of the overly large throttle opening, the vacuum advance has shut off, Leaving your timing retarded.
              So now, not only do you have a slow moving column of air, arriving late to the party; but when the fire finally gets lit, the piston is well past where it ought to be, so the engine is down on power, so you open the throttle even further to get the power you need.
              But because the finally burning and expanding gasses are chasing after the piston, the gasoline molecules are having a real hard time finding oxygen to react with, because all the molecules are so far apart and getting ever further apart with each passing millisecond. And so, a lot of fuel just never burns, or never burns completely, inside the combustion chamber, and a lot of it finishes burning in the exhaust manifold. Of course that means the exhaust system now has pressure in it, and the pistons have to pump that down to the tailpipe. Furthermore,if there is any air getting into the exhaust manifold or the downpipe, you may at times have multiple afterfires at various rpms, as the unburned gasoline molecules find fresh oxygen to react with.
              Ok so that would be the scenario if the ring-seal is bad.
              Now, throw a restrictive exhaust system into the mix, and you get an under-powered, lazy, after-firing, plug-fouling POS.

              If your engine was designed to only have 90psi, and your rings and valves were working perfectly, the engine would run just dandyfine, but you would need more than 225 cubes to propel your car decently.

              Ok no more talking.
              Your engine has exactly one right time, measured in crank-degrees, for all the air and fuel to have finished reacting, and the hi-pressure gasses to be pushing down on the pistons. IDK what it is for a slanty with a 4.125! inch stroke, but I do know how to find it at idle, and at cruising rpm, and at WOT. All your timing systems are factory engineered in an effort to start the fire at the right time, to achieve max pressure, at the right crank position.
              In order to set your starting point (base-timing) in relation to the piston, you first need to know exactly where TDC-Compression is. Your balancer has a mark on it to help you, but unless you prove that mark is true-TDC, you cannot know how accurate it is. So this is job #1..
              Job #2 is to set the True timing.
              Job #3, is to Measure the vacuum, and interpret the behavior of the needle under various conditions; which will reveal a lot about the condition of your rings and valves, and even the exhaust system.

              Ok so back to the toolbox and Happy Wrenching, lol.

              BTW;
              I wouldn't trust those plugs in the condition they are in. I know they are brand new, but that soot is guaranteed to drop your coil voltage, and if it drops too low, then you get missfires.
              Speaking of which, your points have to be gapped pretty close to exactly right, and they have to be clean; oily points are trouble. And the point-gap has to be the same on each of the six cylinders because the gap controls the timing. If the gap is changing while running, then so is the timing. Furthermore, a bad condenser will sometimes do what your engine is doing, but IIRC, you already replaced it.
               
              Last edited: May 11, 2022
            • MOPAROFFICIAL

              MOPAROFFICIAL FABO Gold Member FABO Gold Member

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              You hold the throttle to 2500 rpm and drizzle trans fluid OR WATER down the carb and watch the smoke cloud out the tail pipe
               
            • RustyRatRod

              RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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              What in the WORLD plugs are THOSE? What to they extend into the chamber......1/2"? Somehow, I think those are wrong.
               
            • ESP47

              ESP47 Well-Known Member

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              I think they're NGK ZFR5Ns. If they are then they're a very common plug for slants. One of the better ones according to .org.
               
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              • RustyRatRod

                RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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                I'm not talking about the brand. NGK is fine. I'm questioning the mile line extended electrode.
                 
              • slantsixdan

                slantsixdan =..=

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                They're not. This kind of projected-electrode plug was originally designed for AMC and Chrysler in the late '70s when they were trying to make engines run reliably with extremely lean and stratified charges—and they did a real nice job of that, so much so that Chrysler kept on using them well into the '90s on motors including the 4.0 Jeep Six and 3.5 V6. The extra-long electrodes put the spark point a lot closer to the center of the combustion chamber and moves it away from the quenchout zone near the chamber walls, which in turn results in a reduced chance of misfire. They are of the same heat range as the stock Slant-6 plug.
                 
                Last edited: May 13, 2022
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                • slantsixdan

                  slantsixdan =..=

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                  Check and see if the vacuum advance pod is faulty (can you suck air thru the vacuum advance hose?).

                  Low. This motor sounds like a tired pony—as also evidenced by your worn-out distributor.

                  Tired motors do this. Suspect you've got a lot of blowby coming into the intake tract via the PCV line, and the oil vapours are crappin' up your plugs. Speaking of the plugs, those ring washers don't belong on 'em in a '63-'74 Slant-6 head; take 'em off the plugs.

                  Tune-up parts and technique suggestions in this post.
                   
                • RustyRatRod

                  RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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                  Thanks, Dan! I actually thought I tagged you in that post, but I've felt really crappy today and must not have. Quench out zone. Yeah, with the piston down .180" in the hole. LOL

                  Anyway, I don't believe I'd try those plugs on a long rod 225 with the 2.2 flat top. lol

                  Thanks for chiming in Dan. It's always appreciated.
                   
                • slantsixdan

                  slantsixdan =..=

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                  "Quench" is sometimes used to mean the shape of the combustion chamber/piston top. "Quenchout" refers to dead(ish) zones created by relatively cold metal and poor mixture access that tend to impede the fire: the space between the top ring and the top of the piston…spark plug electrodes shrouded by nearby cylinder head metal…that kind of thing.

                  Yeah, you have to have room for these plugs. They'll work in a stock/ish Slant-6 or LA motor…in a 2.2/2.5…in a 3.0 Mitsu V6…probably in A-motors and others, too, but I haven't checked or tried. Less negative deck height or extensive head milling or other changes will require checking to see if these plugs will still clear everything, but if you're making those changes, you'll probably wind up wanting a colder plug anyhow, and these projected-electrode plugs come in one step colder, but not two.
                   
                  Last edited: May 13, 2022
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