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Engine misfires at idle, coil only getting 8.5v
http://slantsix.org/forum/viewtopic.php?f=28&t=64214
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Author:  Plymouth_Ruster [ Wed Nov 13, 2019 2:58 pm ]
Post subject:  Engine misfires at idle, coil only getting 8.5v

So now that I've fixed the transmission issues with the help of this forum, and the horn works, and so do the reverse lights - it's time to fix something I've been putting off for no reason.
My car misfires fairly often at idle, and it seems like it does it on all cylinders. I've ruled out everything besides an electrical issue, and today I measured voltage at the coil - 8.5v. I'm reasonably sure it's supposed to be a bit higher than that. I'm wondering if wiring the alternator backwards would do this? The car runs absolutely fine at any speed other than idle.

Author:  Joshie225 [ Wed Nov 13, 2019 6:17 pm ]
Post subject:  Re: Engine misfires at idle, coil only getting 8.5v

So you've already set the valve lash and done a compression test?

Author:  Plymouth_Ruster [ Wed Nov 13, 2019 7:12 pm ]
Post subject:  Re: Engine misfires at idle, coil only getting 8.5v

Joshie225 wrote:
So you've already set the valve lash and done a compression test?


Yeah, a few months ago. But it's done this since I put the engine in.

Author:  MadScientistMatt [ Thu Nov 14, 2019 5:53 am ]
Post subject:  Re: Engine misfires at idle, coil only getting 8.5v

Unless you've swapped in a HEI ignition or something more exotic, it's normal to see a significant voltage drop due to the ballast resistor.

Author:  Plymouth_Ruster [ Fri Nov 15, 2019 2:07 am ]
Post subject:  Re: Engine misfires at idle, coil only getting 8.5v

MadScientistMatt wrote:
Unless you've swapped in a HEI ignition or something more exotic, it's normal to see a significant voltage drop due to the ballast resistor.


What would be causing the misfire then? Plugs, wires, cap, rotor, ignition controller, ballast resistor are all new. Carb was rebuilt a few weeks ago. Engine had 120psi on all cylinders last I checked, and the previous engine was running way below that on four cylinders and never had this issue.

Author:  DadTruck [ Sat Nov 16, 2019 10:58 am ]
Post subject:  Re: Engine misfires at idle, coil only getting 8.5v

Can you post a video of the idle / misfire?
That would give us a bit more to go on.
Did the misfire change at all after ignition components were re newed?
Does the misfire change as the engine goes from cold to hot?
As just a guess but it could be a sticking valve, which I think would be worse with the engine cold, then as the engine warms and loosens should get better.

Another thought is with the engine ideling, remove the sparkplug wires one by one, see if you can get a connection between a dead cylinder and the misfire.

Author:  Plymouth_Ruster [ Sun Nov 17, 2019 11:17 pm ]
Post subject:  Re: Engine misfires at idle, coil only getting 8.5v

DadTruck wrote:
Can you post a video of the idle / misfire?
That would give us a bit more to go on.
Did the misfire change at all after ignition components were re newed?
Does the misfire change as the engine goes from cold to hot?
As just a guess but it could be a sticking valve, which I think would be worse with the engine cold, then as the engine warms and loosens should get better.

Another thought is with the engine ideling, remove the sparkplug wires one by one, see if you can get a connection between a dead cylinder and the misfire.


So the engine misfires quite a bit when it's not under load - I.E. when the car's in Park or not in gear. When you put load on it, there are no misfires whatsoever when the engine is between 120 degrees and 160 degrees fahrenheit (but it will still misfire in Park, etc). After the engine gets to operating temp (in my case, about 185 degrees because I'm not using the stock thermostat) it will often have a "soft miss" at idle - you can't really hear it when standing next to the exhaust and the engine barely jumps, but it's definitely not firing correctly. I suppose it's better to say it's firing late or something but I'm not really sure what's going on. At operating temp it will still misfire irregularly while idling and not in gear.

The only thing I've noticed that changes this is the idle mixture - if I lean it out, the car will have a hard miss more often while not in gear and will sometimes have a hard miss while in gear as well, although this is rare. Richening the mixture more only has the effect of making the engine run worse and bog at idle.

I'm going to retard the timing a little when I get a chance, as it seems to have slowly advanced since I adjusted it last. It now sounds choppy while cranking, as if it's firing too soon. Hopefully I'll have enough time to make a video on Wednesday.

Author:  Plymouth_Ruster [ Mon Nov 18, 2019 1:13 am ]
Post subject:  Re: Engine misfires at idle, coil only getting 8.5v

Forgot to mention; Reasonably sure it's not a compression issue as I tested it, and the previous engine (which read 110, 120, 110, 80, 85, 90) never had this issue, and this engine tested at 120psi on every cylinder because someone actually changed the oil at some point.

Author:  Joshie225 [ Mon Nov 18, 2019 3:56 pm ]
Post subject:  Re: Engine misfires at idle, coil only getting 8.5v

I'll bet you $1 it's the carburetor.

Author:  matv91 [ Mon Nov 18, 2019 4:53 pm ]
Post subject:  Re: Engine misfires at idle, coil only getting 8.5v

viewtopic.php?f=19&t=60065&hilit=1975

Author:  matv91 [ Mon Nov 18, 2019 5:00 pm ]
Post subject:  Re: Engine misfires at idle, coil only getting 8.5v

viewtopic.php?f=19&t=60036

Author:  matv91 [ Mon Nov 18, 2019 5:15 pm ]
Post subject:  Re: Engine misfires at idle, coil only getting 8.5v

What is your carb number? Look down the venturi during the warm up period and hot idle, choke off. Venturi should stay dry, no drips. If there are drips,just even 1 or 2 over 2 minute time period, high liquid fuel level, heavy float etc, brass or plastic float?

Author:  Plymouth_Ruster [ Mon Nov 18, 2019 8:46 pm ]
Post subject:  Re: Engine misfires at idle, coil only getting 8.5v

matv91 wrote:
What is your carb number? Look down the venturi during the warm up period and hot idle, choke off. Venturi should stay dry, no drips. If there are drips,just even 1 or 2 over 2 minute time period, high liquid fuel level, heavy float etc, brass or plastic float?


Plastic. We rebuilt the carb a few weeks ago, float had no fuel in it and was set to the correct level. Will check for drips when I next have a chance to work on the car.

Author:  matv91 [ Tue Nov 19, 2019 2:02 am ]
Post subject:  Re: Engine misfires at idle, coil only getting 8.5v

viewtopic.php?f=19&t=63479

Author:  Plymouth_Ruster [ Wed Nov 20, 2019 11:19 pm ]
Post subject:  Re: Engine misfires at idle, coil only getting 8.5v

Good news! I've discovered that this basically new slant 6, with just 53,000 miles on it, has somehow developed an issue with the wrist pin in cylinder 5. Either that or it's the rod bearing going out but I think my oil pressure would be a bit lower if that was the case. Either way there's a knock every time the cylinder fires. It's not constant, and will sometimes go away for a minute or so. The previous engine came with a bad wrist pin, and I'm thinking that either I'm doing something wrong here or I just have exceptionally terrible luck. It hasn't gotten any worse in the last few months so I'm hoping the engine lasts until I can get my other car running.

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