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| Strange spark plug phenomenon https://www.ausmini.com/forums/viewtopic.php?f=2&t=89401 |
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| Author: | willy [ Sat May 09, 2015 7:45 pm ] |
| Post subject: | Strange spark plug phenomenon |
Hey all Just starting to sort out the tune properly on my 850. I fitted a new Powerspark distributor (25D) with electronic module, and changed it to a negative earth system while I was at it. New plugs, new leads, new coil. Everything sweet. What is not sweet, however, are the spar plugs. Cyl 1 and 2 are very rich. A very sooty plug. Cyl 3 and 4 are pretty spot on - absolutely clean. It has 19° advance at 1000 rpm - as per a workshop manual which seemed to me extremely advanced for an 850. .... What do we think? Any help appreciated. |
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| Author: | Harley [ Sat May 09, 2015 10:43 pm ] |
| Post subject: | Re: Strange spark plug phenomenon |
1 and 2 down on compression compared to 3 and 4? |
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| Author: | peterb [ Sun May 10, 2015 8:11 am ] |
| Post subject: | Re: Strange spark plug phenomenon |
Have you got twin carbs? |
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| Author: | willy [ Sun May 10, 2015 9:24 am ] |
| Post subject: | Re: Strange spark plug phenomenon |
I'm yet to do a compression test. And only a single HS2 on this wheezy girl. I sprayed WD40 around the inlet manifold to head area and there was no difference in noise, so I don't think there is a manifold sealing issue but I can't be certain until I pull it off and change it. |
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| Author: | aaron [ Sun May 10, 2015 10:51 am ] |
| Post subject: | Re: Strange spark plug phenomenon |
Rather than wd40, use carb cleaner. I would also check your gaps on the plugs along with swapping them over in the cylinders. I found a few plugs recently aren't quite doing what they are supposed to. |
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| Author: | willy [ Sun May 10, 2015 1:00 pm ] |
| Post subject: | Re: Strange spark plug phenomenon |
35 thou on each plug, which is about right for an elec dizzy I would have thought. I'll try testing new plugs to see if there is a plug fault. |
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| Author: | drmini in aust [ Sun May 10, 2015 2:39 pm ] |
| Post subject: | Re: Strange spark plug phenomenon |
If you get any misfiring during warmup, reduce plug gap to .030". |
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| Author: | willy [ Sun May 10, 2015 5:54 pm ] |
| Post subject: | Re: Strange spark plug phenomenon |
Cheers Doc I'll keep that in mind. Do you think 19 degrees (at 1000rpm) on a strobe is far too advanced? |
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| Author: | drmini in aust [ Sun May 10, 2015 7:33 pm ] |
| Post subject: | Re: Strange spark plug phenomenon |
willy wrote: Cheers Doc I'll keep that in mind. Do you think 19 degrees (at 1000rpm) on a strobe is far too advanced? Probably is, for a stock motor. My 1360 (has bathtubbed race head) was running 20deg static when I had the Pulsar dizzy in. Since fitting the Accuspark black box I've dropped that back a bit, but it still has 20deg @1000rpm. |
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| Author: | Irish Yobbo [ Sun May 10, 2015 10:19 pm ] |
| Post subject: | Re: Strange spark plug phenomenon |
Also be sure to check your rocker clearance. But I'd do a compression test as a first port of call. |
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| Author: | drmini in aust [ Sun May 10, 2015 10:23 pm ] |
| Post subject: | Re: Strange spark plug phenomenon |
Check your plug lead resistance. Old cheapo carbonized string ones increase resistance with age, eventually this reduces spark energy and plugs foul easily. I like Bosch (or similar) inductive core leads. They don't do this. |
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| Author: | simon k [ Sun May 10, 2015 10:37 pm ] |
| Post subject: | Re: Strange spark plug phenomenon |
Quote: new dizzy, new plugs, new leads, new coil what state was it in before you started working on it? were the old plugs in the same condition (2 good, 2 bad) when you took them out? have you checked the compression on all 4 yet? have you swapped the plugs and leads around yet? I've noticed that 850's are set up with a lot of advance as per the book - I assumed it was because they don't breathe very well with their little valves and a tiny bore, and then with the low compression don't get a very fast burn, so running a lot of advance gives it plenty of time to ignite and burn all of the fuel before the piston gets to the top... or something |
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