Slant Six Forum :: Strange worsening misfire at idle

Are you running a choke? If so, that may need some adjusting>

Its sounds like you have a vacume leak somewhere. Check all your lines and spray some WD40 or carb cleaner around your intake and vacume lines and see if the idle changes. Also you might want to check your
vacume advance to insure its working properly. I think whats happening is when you rev up your engine your producing more vacume but at idle
there is not as much to keep you vehicle running if there is a small leak.

Are you running a choke? If so, that may need some adjusting

What is the voltage at the out going side of your current resistor?

It should read exactly the same at the coil.

If not your engine harness plugs may be corroded or the harness along the valve cover may have gotten hot and melted a few wires inside.

The input side of the resistor should show 12 volts.

With the MP box you really should use a higher ohm coil so the the voltage drop is across the coil and not the ballast.

The MP ignition should be able to take more amps than the pertronix (better heatsinking), so with a 3 ohm coil you wouldn't need a ballast.

Author:	otherroutes [ Wed Apr 15, 2009 1:29 am ]
Post subject:	Strange worsening misfire at idle
Overall the slant is running like a dream, at least compared to the way it was when I bought it. Now at the top of my list is a strangely behaving idle. The engine starts well both cold and hot. It idles well AT FIRST. It drives well and pulls hard. However, when left at idle, after a little while, 30-60secs I'd estimate, a misfire starts. Just random, but it gets worse and worse, revs dropping as you'd expect, until eventually it'll stall. However, if I give a good hard blip on the throttle the smooth idle returns (without misfire) - but only for 30-60secs again when the misfires start over. I'd welcome any suggestions on how to go about diagnosing this one. I've searched the board but cannot find symptoms like this described. Engine details: 1986 225 (in a Dodge B250) - MP electronic ignition (with relay and resistor) - Blaster 2 coil (USA made) - New plugs, leads, cap - NOS dizzy - NOS Holly 1945 - New manifolds and the Aussie gaskets Thanks in advance.

Author:	sandy in BC [ Wed Apr 15, 2009 3:51 am ]
Post subject:
Idle mixture needs a tweak.

Author:	Doctor Dodge [ Wed Apr 15, 2009 9:13 am ]
Post subject:
Are you running a choke? If so, that may need some adjusting> DD

Author:	otherroutes [ Wed Apr 15, 2009 9:24 am ]
Post subject:
Quote: Are you running a choke? If so, that may need some adjusting> Yes, the electronically controlled / electrically assisted choke. I thought I'd checked that the choke was fully pulled off once hot, but I will certainly check that again. I'm afraid I don't get much time to work on the old van, so I try to research between work sessions to try to do the best work I can when I get to it.

Author:	uberjoebob2000 [ Wed Apr 15, 2009 10:30 am ]
Post subject:
Its sounds like you have a vacume leak somewhere. Check all your lines and spray some WD40 or carb cleaner around your intake and vacume lines and see if the idle changes. Also you might want to check your vacume advance to insure its working properly. I think whats happening is when you rev up your engine your producing more vacume but at idle there is not as much to keep you vehicle running if there is a small leak.

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Author:	otherroutes [ Wed Apr 15, 2009 1:44 pm ]
Post subject:
Quote: Its sounds like you have a vacume leak somewhere. Check all your lines and spray some WD40 or carb cleaner around your intake and vacume lines and see if the idle changes. Also you might want to check your vacume advance to insure its working properly. I think whats happening is when you rev up your engine your producing more vacume but at idle there is not as much to keep you vehicle running if there is a small leak. Thanks I will do that and measure the manifold vacuum at the carb base as well as the #6 runner.

Author:	otherroutes [ Sun Apr 19, 2009 3:21 pm ]
Post subject:
I've now had a chance to do some further investigations. Vacuum at both the tree and the carb base is 20.5 inHg. I believe this is quite reasonable and doesn't indicate a vacuum leak? In terms of mixture: that was adjusted by an old school mechanic very recently to get it to pass our UK emissions tests. So I'm thinking that it's not too far off. However, I've not yet had time to get the colortune into it. However, I did measure the voltage at the coil primary - 4.5V. This seemed low to me so I measured the resistance of the ballast resistor. The engine had been running a few minutes so it was probably warm. It measured 2.6ohms. Given the Blaster2 coil has a primary windings of 0.7ohm I think the ballast I have is too high resistance. My (totally imagined) theory is that the lower voltage to the coil is leading to lower spark energy and this is causing the plugs to foul (and misfire), and that the revving clears it out. How daft is this theory? I'm looking to get the MSD ballast resistor and will fit and report any change, but would be grateful for any further thoughts or advice. Regards, Richard.

Author:	Aggressive Ted [ Sun Apr 19, 2009 3:58 pm ]
Post subject:
What is the voltage at the out going side of your current resistor? It should read exactly the same at the coil. If not your engine harness plugs may be corroded or the harness along the valve cover may have gotten hot and melted a few wires inside. The input side of the resistor should show 12 volts. Let us know what you find.

Author:	emsvitil [ Sun Apr 19, 2009 4:48 pm ]
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The 2.6 ohms sounds about right. You want about 3 ohms resistor + coil. Since the resistor was warm, the resistance would have gone up a bit.

Author:	otherroutes [ Mon Apr 20, 2009 12:35 am ]
Post subject:
Quote: Are you running a choke? If so, that may need some adjusting Apologies DD, I also checked the choke and once warm, when the symptoms occur, the choke is held hard wide open. Quote: What is the voltage at the out going side of your current resistor? Voltage at the outgoing side of the resistor is also approximately 4.5V. I say approximately because measuring both here and at the coil gave readings from 4.3V to 4.8V, but I'm assuming the variation is due to the temperature effect on the resistor. Quote: It should read exactly the same at the coil. It does Quote: If not your engine harness plugs may be corroded or the harness along the valve cover may have gotten hot and melted a few wires inside. When I fitted the EI, I put all new wiring in with power taken via a relay. My wiring diagram is here. Quote: The input side of the resistor should show 12 volts. The input side has the following voltage: 12.8V with engine off, 14.0V with engine running. I've now seen and read your thread asking essentially the same question about ballast resistor values. I saw that Eric W uses the MSD 0.8ohm ballast with the Blaster 2 coil too. Theoretically this would give around 6V to the coil which, from what I've read, seems about right. May I ask what you went for in the end and how well it worked?

Author:	emsvitil [ Mon Apr 20, 2009 1:08 am ]
Post subject:
You want to adjust your ballast so that you don't have too much amps running thru the mopar ignition control box. I think the max that the mopar box can take is 4-5 amps continuously. So 14v / 3.3 ohms = 4.24 amps Using erics setup 14v / 1.5 ohms = 9.33 amps Which is a bit high for the MP box.......... With the MP box you really should use a higher ohm coil so the the voltage drop is across the coil and not the ballast.

Author:	otherroutes [ Mon Apr 20, 2009 1:38 am ]
Post subject:
Quote: With the MP box you really should use a higher ohm coil so the the voltage drop is across the coil and not the ballast. Excellent, thanks for the explanation. It looks like the stock coil might have to go back on and eBay may see the Blaster2. Hey ho, another lesson being learned

Author:	otherroutes [ Mon Apr 20, 2009 3:47 am ]
Post subject:
I see the Pertronix Flamethrower comes in a 3ohm version. With 2.5ohm through the ballast this would provide 5.5ohms overall giving 2.5A through the EI module and a primary coil voltage of 7.6V. Does this make for a healthier option?

Author:	emsvitil [ Mon Apr 20, 2009 3:57 am ]
Post subject:
I have a pertronix with their 3 ohm coil and no ballast............... The MP ignition should be able to take more amps than the pertronix (better heatsinking), so with a 3 ohm coil you wouldn't need a ballast.

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Author:	otherroutes [ Mon Apr 20, 2009 4:11 am ]
Post subject:
Quote: The MP ignition should be able to take more amps than the pertronix (better heatsinking), so with a 3 ohm coil you wouldn't need a ballast. No ballast = no spare ballast required. And of course it'll only break on a cold, wet night when we're late for a gig. Sounds like a good thing in itself! So ... new toy (Pertronix Flamethrower 3ohm in understated chrome) now ordered and paid for. Fingers crossed on this one. Thanks for the help, I'll update when fitted and checked through.