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| 1986 D-150 slant six misfire question https://slantsix.org/forum/viewtopic.php?t=51799 |
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| Author: | 1986slant [ Mon Apr 22, 2013 2:17 pm ] |
| Post subject: | 1986 D-150 slant six misfire question |
Hello All! just joined, far from new in the mopar world and have had a few slants in the past, this one has me puzzled though I ended up with the truck kind of by chance, buddy needed it gone and I was the first one he wanted it to go to was running rough when I got it but thought a tune up would do it, cleaned carburator; new plugs wires cap rotor button, made sure it was de-leanburned etc problem is it still miss's like crazy when warm and idles high, leading me to a vacuum leak I found the exhaust manifold is leaking where it bolts to the bottom of the intake, is the intake noted to crack in this spot as well? |
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| Author: | rsrls [ Tue Apr 23, 2013 3:44 am ] |
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have you put a vacuume gage on it? |
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| Author: | 1986slant [ Tue Apr 23, 2013 3:52 am ] |
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yes put vacuum gauge on and it reads 20" |
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| Author: | duval67 [ Tue Apr 23, 2013 8:20 am ] |
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Pretty sure 20 is about what you're supposed to be getting. I don't think the intakes are known to crack at the juncture between the two, but exhaust leaks there are common. Having an exhaust leak out there isn't grounds to assume the intake is leaking there--the exhaust just serves to heat the intake at that point. There is still plenty of solid metal around there...not a likely spot for a crack, IMO, but I could be wrong. Has the leanburn system been eliminated properly? I've never done it myself, but I know it isn't as simple as just snipping wires and pulling off components. Is the carb adjusted properly? Adjusting valve lash could help too...if it's off it can cause pretty significant idle quality problems, depending on how far off it is. Timing is okay? |
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| Author: | Reed [ Tue Apr 23, 2013 8:57 am ] |
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If the motor is the original 1986 motor it has hydraulic lifters, not mechanical. Hydraulic lifter engines have no provision for adjusting the valve lash. You say the motor has been de-lean-burned. Who did the work and do you trust them? What carburetor are you using? What ignition system are you using? What is the condition of the other emissions devices? Are they present and functioning? What else as been removed? High idle and misfiring sounds like at least a problem or two with the carburetor. Start by setting your curb and fist idle speeds and your curb idle mixture. Then move on to verifying your base timing (including verifying the accuracy of the timing mark). In the 1980s Chrysler used aluminum and cast iron intake manifolds on the slant six. The aluminum ones came in two varieties- a two piece unit welded together and a nice cast one piece. The one piece intakes are better. The two piece intakes are know to have porosity issues at the weld and are known to crack on the floor of the intake directly below the carb. |
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| Author: | duval67 [ Tue Apr 23, 2013 9:58 am ] |
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Ah yeah, didn't take note of the year. My mistake! I didn't know the two pieces were known to crack down there--I knew about the problems with the welds, but not at the bottom. A learning day for me too, hurray! |
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| Author: | Reed [ Tue Apr 23, 2013 10:50 am ] |
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The last four or five two piece manifolds I have inspected all had cracks right below the carb. |
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| Author: | duval67 [ Tue Apr 23, 2013 11:28 am ] |
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Guess I got lucky on mine...I didn't even check there. |
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| Author: | 1986slant [ Tue Apr 23, 2013 1:00 pm ] |
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the de-lean burn was done by me, someone half way eliminated it before I bought it and the computer was running the distributor but it had an older carter 2 barrel on it (believe Carter 2425) I replaced distributor with proper EI one, and done the swap to 4 pin ECU, all stuff I've done before truck has always run decent, runs better cold now, but it has also always ran crappy warm, what I changed didn't help and it sure seems like a vacuum leak the exhaust manifold either has a crack in it or a bad warpage as its leaking bad were it meets the intake, so they both have to come off to fix that I was thinking of having both trued on a milling machine and reinstalling it |
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| Author: | duval67 [ Tue Apr 23, 2013 1:07 pm ] |
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It's hard to get them to seal with the stock gasket--that little metal guy. I used a Remflex gasket on mine and haven't had any issues. But yeah, it wouldn't hurt to take them off and have them trued up. You can inspect for cracks much easier then too. |
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| Author: | Reed [ Tue Apr 23, 2013 1:19 pm ] |
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Have you adjusted the metering rods on the carburetor? Have you checked for throttle shaft bushing wear? The throttle shaft is a very common area for vacuum leas on a Carter BBD. |
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| Author: | 1986slant [ Tue Apr 23, 2013 2:08 pm ] |
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the carburetor is in excellent working condition but I tried another one I know is good on it and it ran the same, checked it with brake cleaner and it running, as well as with a propane torch un-lit. I put a kit in it not long ago, revs perfectly through RPM range but idles quite high with a miss I'm not new to mopars or slant six's and a auto tech apprentice by trade, but an intake leak on a slant is new to me, its a cast iron 2 barrel intake |
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| Author: | Reed [ Tue Apr 23, 2013 2:23 pm ] |
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OK, well, if you are 100% positive that the carburetor is correctly adjusted and has no vacuum leaks, then I recommend following the intake manifold installation procedure described HERE. However, an idle vacuum of 20 inches suggests that there is no manifold vacuum leak. I would still double check the metering rods in the BBD since that is an area often overlooked yet critical to good performance. An exhaust manifold leak typically will not cause a high idle or a miss. You have tried lowering the idle speed via the curb idle speed screw, yes? Is the choke linkage adjusted properly to allow the fast idle cam to drop away from the idle speed screws? "Missing like crazy" and idling high could also be problems with the ignition system. Have you checked the condition of the distributor? Are the springs disconnected or too light allowing too much advance at idle? Is the distributor vacuum advance hose hooked to a source of ported vacuum advance? Have you verified the timing mark on the damper is showing true TDC by using a piston stop tool and verifying? |
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| Author: | 1986slant [ Tue Apr 23, 2013 2:29 pm ] |
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idle screw and choke cam screw backed all the way out and still idles at about 1050-1100RPM distributor is a NOS one I had for years, everything in good shape on it, hooked to ported vacuum, new tuneup, (plugs, wires cap rotor button) I would consider it to be bad plugs out of the box but they all make a difference if you pull one at a time, and it still doesn't explain high idle maybe a video will help? |
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| Author: | Reed [ Tue Apr 23, 2013 2:40 pm ] |
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Is the throttle cable adjusted properly? The engine should not run if the throttle screws are back all the way out. I have run across people who do a Super Six swap and then have problems with the engine not dropping to curb idle (I have done this too) and the problem turns out to be the throttle cable was adjusted too tight and not giving enough slack for the throttle to close all the way. Loosen the throttle cable clamp and see if your idle drops. |
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