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| 3 cylinders lean - 1 cylinder rich https://ausmini.com/forums/viewtopic.php?f=2&t=102582 |
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| Author: | Barkfast [ Wed Jul 30, 2025 10:10 am ] |
| Post subject: | 3 cylinders lean - 1 cylinder rich |
I've recently had the head refurbished (clean, crack checked, new rocker shaft and bushes, valves reground) and I'm getting a significant difference in the colour of the spark plugs. Cylinder 2 appears to be running rich while cylinders 1, 3, 4 all appear to be quite lean. Set up is a 1275, hif44, minispares manifold, lcb extractors, nissan electronic distributer, new NGK BP5ES . I have the PCV plumbed into the manifold. Troubleshooting to date is: -checking spark plug gap (set to 32 thou) -switching spark plugs from cylinder to 2 to another cylinder (issue seems to be isolated to cylinder 2) -checking spark plug lead to cylinder 2 (I got 6.1k ohms - seems in spec) -compression test (all cylinders around 185 psi dry test, wet test had a difference of 0-5psi) -tightened manifold nuts (both for studs that connect to the head and carb) -inspected breather hose and tightened clamps from tappet chest cover to manifold -checked for signs of head gasket issues (no signs of bubbles in coolant or milky oil) -checked and cleaned PCV (no obvious tears or holes in diaphram but its likely to be 15+ years old) From what I rather the issue seems to indicate air leak somewhere in the intake? Keen for ideas on what next.... |
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| Author: | timmy201 [ Wed Jul 30, 2025 10:15 am ] |
| Post subject: | Re: 3 cylinders lean - 1 cylinder rich |
It more sounds to me like a misfire rather than an air leak. Have you re-checked the valve clearances on all the cylinders? What ohm coil are you using? |
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| Author: | Mick [ Wed Jul 30, 2025 11:00 am ] |
| Post subject: | Re: 3 cylinders lean - 1 cylinder rich |
Thats a big spark plug gap, it may contribute to a misfire and a darkened plug. Perhaps try bringing them back to a stock 25 thou and monitor. You won't notice the difference at this point in a more closed gap. |
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| Author: | drmini in aust [ Wed Jul 30, 2025 12:51 pm ] |
| Post subject: | Re: 3 cylinders lean - 1 cylinder rich |
6K ohms is a lot for a plug lead, new Bosch inductive core ones were 1.8K. Bend the lead about whilst measuring resistance, if it varies toss the lead out. Cheap leads are just graphited string inside. |
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| Author: | 68+86auto [ Wed Jul 30, 2025 3:19 pm ] |
| Post subject: | Re: 3 cylinders lean - 1 cylinder rich |
Switch the lead with another cylinder. |
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| Author: | Barkfast [ Wed Jul 30, 2025 4:01 pm ] |
| Post subject: | Re: 3 cylinders lean - 1 cylinder rich |
Seems the issue was indeed ignition related (thanks for all the pointers and help - VERY much appreciated!) After removing oxidisation on the distributor cap contacts (there appeared to be a build up of whitish crud that scraped away with a knife) and setting the plug gaps to 25 thou... and taking it for a 10 min round the block run.. the colour on the plugs are all roughly the same. DrMini - I'll look into Bosch leads, but maybe not til after Rylstone. I tried moving the lead while testing resistance.. the reading only varied slightly (possibly due to the probe moving). |
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| Author: | drmini in aust [ Wed Jul 30, 2025 4:36 pm ] |
| Post subject: | Re: 3 cylinders lean - 1 cylinder rich |
I'm running a set of Karcraft leads after selling my peak ignition system to a mate. They were only around $30 for the set and are working well. 1.5 ohm NGK coil, 43D dizzy with a Powerspark module, modified advance curve, Plug gap 0.030". Edit You used to be able to get loose Bosch inductive core cables from supercheap etc but no more. |
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| Author: | 68+86auto [ Wed Jul 30, 2025 8:46 pm ] |
| Post subject: | Re: 3 cylinders lean - 1 cylinder rich |
drmini in aust wrote: You used to be able to get loose Bosch inductive core cables from supercheap etc but no more. With how difficult they are to find online now, I'm not sure they are actually still being manufactured at all. I haven't liked the quality of any of the other leads I've bought. Most don't even seal properly. There was a supplier of quality wire and terminals but they closed down. I've been looking for a source of the wire and haven't really found anything. There's a local parts store who used to make leads and they still would except for the fact that they couldn't find any wire. They said they'd have another look. It's a proper type of parts store, not one which sells beach balls etc. |
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| Author: | drmini in aust [ Wed Jul 30, 2025 8:52 pm ] |
| Post subject: | Re: 3 cylinders lean - 1 cylinder rich |
Graham Russell still gets race quality ones made, they are great but not cheap. |
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| Author: | lomin [ Wed Jul 30, 2025 9:45 pm ] |
| Post subject: | Re: 3 cylinders lean - 1 cylinder rich |
Graham Russell get his ignition leads from me at Peak Ignition. .They are tailor made precisely from the best iinduction core available ( centre from USA and outer made here in australia, with the best quality boots. The ohms resistance is not even 1000. They are excellent value at $90. Having tested spark output with different leads I know you get what you pay for. If you think about spark, your ignition produces only a certain amount , then you lose some of it. You are producing very little with points, losing a bit with long leads, losing some with high resistance rotor buttons, ignition leads and carbon brushes, losing some more with excessive spark gaps….there is not potentially a lot of spark left to burn the fuel. Your car cannot perform well if you dont burn the fuel. Cheers Lindsay Siebler |
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| Author: | Barkfast [ Thu Jul 31, 2025 8:07 am ] |
| Post subject: | Re: 3 cylinders lean - 1 cylinder rich |
lomin wrote: Graham Russell get his ignition leads from me at Peak Ignition. .They are tailor made precisely from the best iinduction core available ( centre from USA and outer made here in australia, with the best quality boots. The ohms resistance is not even 1000. They are excellent value at $90. Having tested spark output with different leads I know you get what you pay for. If you think about spark, your ignition produces only a certain amount , then you lose some of it. You are producing very little with points, losing a bit with long leads, losing some with high resistance rotor buttons, ignition leads and carbon brushes, losing some more with excessive spark gaps….there is not potentially a lot of spark left to burn the fuel. Your car cannot perform well if you dont burn the fuel. Cheers Lindsay Siebler All makes sense.. always found high resistance of leads a bit of an oddity in terms of electrical efficiency. Wondering if there is a particular resistance value in the lead, where spark plugs with resistors should be used to prevent electromagnetic/radio interference? |
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| Author: | Steam [ Thu Jul 31, 2025 9:34 am ] |
| Post subject: | Re: 3 cylinders lean - 1 cylinder rich |
6k is the figure that silicone leads are, that is carbon string silicone oiter that were all the rage 10 or so years ago. The latest spiral wire leads are around 1k per meter but typically measure 2 to 3 k a lead. The high resistance is to allow the spark to fully develop as well as helping with lowering interference. |
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| Author: | cooperess [ Thu Jul 31, 2025 9:46 am ] |
| Post subject: | Re: 3 cylinders lean - 1 cylinder rich |
Is using copper wire leads better than the carbon core leads? Also where can you get Peak Ignition leads? |
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| Author: | timmy201 [ Thu Jul 31, 2025 1:01 pm ] |
| Post subject: | Re: 3 cylinders lean - 1 cylinder rich |
Copper leads have essentially zero resistance compared to normal leads, so they can give some electrical noise, especially with standard plugs Peak Ignition can be bought from Lindsay (lomin) |
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| Author: | Steam [ Thu Jul 31, 2025 4:51 pm ] |
| Post subject: | Re: 3 cylinders lean - 1 cylinder rich |
Any of the major brands spiral wire leads will work well. I use Eagle 8mm but there are others. |
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