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THRUMMER Starting Member
United Kingdom 20 Posts | Posted - 07 Aug 2017 : 16:23:06
| Well, what a to- do this one has been, just driving along a couple of weeks ago and a misfire started which got worse on the 3 mile journey home, whatever could it be......
Despite a Malpassi Filter King being in the fuel line, after checking for detritus in the Malpass bowl and finding only a limited amount of finely divided precipitate, I stripped and blew through the visually clean carb and checked the already perfect float level settings. On refitting, the misfire was still there with blowback through the carb.
Oho thought I, years of running on unleaded has resulted in a burnt valve so.... lets check the valve clearances cold and all were perfect to specification. Right, lets check the compression, all AOK and balanced between 130 - 135 psi cold.
Hmmm, must be ignition then. lets check to Powerspark electronic ignition and associated rotor arm no, the rotor arm shows a 1k resistance, the Powerspark seems to be AOK too but we'll buy and fit another one and guess what, the misfire is still there.
Is the timing OK, yes, does the distributor vacuum advance work and is it to spec, yes, are there air leaks around the carb body, mounting block, vacuum takeoff for the servo, well no...... not according to my spraying with 2-butoxyethanol ( carb cleaner!)
Right, let's go out in the dark and use t'dragons plant spray to spray on the plug leads and see if there is any HV tracking - none whatsoever, just as you might expect with modern resistive leads. It can't be the distributor cap as that's a nearly new Beru item. Maybe its the coil sop lets check the primary and secondary impedance and the presence of any shorts but no, everything is OK OK, lets test the plug leads and king lead for impedance and guess what, all are in spec even when wiggled and bent. OK, lets test the plugs, all NGK BP6ES with less than 2000 miles on em ex Euro Car Parts , no shorts to earth, no continuity break to centre electrode.
Whatever the hell is it........ tell you what the plugs are cheap, lets put another set of BP6ES in, it won't be them but you can get a set on EBAY for £6 delivered and fit them in 10 minutes.
And guess what....... misfire gone, only the second time in 40 years I've had a bad plug and for 35 years since I swopped from Champion to NGK I had ever had a problem. Guess there's a first time for everything.
I am just glad that after 10 days the problem has been solved.
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RhysN V4 Fanatic
United Kingdom 411 Posts | Posted - 07 Aug 2017 : 17:19:03
| not being smart, but sometimes it pays to start at the simple solution first. Glad you got it sorted. Maybe they had been dropped sometime in the past (in the post?) and eventually gave up. | Edited by - RhysN on 07 Aug 2017 17:20:27 | |
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melle V4 Guru
United Kingdom 3830 Posts | Posted - 07 Aug 2017 : 17:55:23
| quote: Originally posted by THRUMMER [...]burnt valve so.... lets check the valve clearances cold and all were perfect to specification.
Drawing the conclusion that the valves are OK from measuring the clearance can be misleading, burnt valves can have clearances that are perfectly within spec.
quote: Originally posted by THRUMMER It can't be the distributor cap as that's a nearly new Beru item.
Oh yes it can. I once had a brand new faulty cap. It was a Beru...
I swear by NGK BP6ES, never had an issue with them (I replace them every 15k km), but as with my Beru distributor cap, even parts from reputable manufacturers sometimes don't meet expectations. Apart from that, as Rhys said: distributor caps and spark plugs are both items prone to failure if dropped. This is not likely to happen in the factory or in your workshop, but it's not unlikely it happens at least once in between.
www.saabv4.com | Edited by - melle on 07 Aug 2017 17:56:08 | |
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john-saab Administrator
United Kingdom 2679 Posts | Posted - 07 Aug 2017 : 18:00:40
| I bought a large number of NGK BP6ES from GSF when they were on offer about 7-8 years ago..paid about 80p per plug. Sold a few and used them on my cars. Almost every one failed in a very short time with the exact same issues as you describe. GSF were good and gave me a re-fund on what I had left but the few I sold were hard to track down.
'73 96 (Vernon),'74 95 (Veronica)plus 4 other 95's and 2 96's | |
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THRUMMER Starting Member
United Kingdom 20 Posts | Posted - 07 Aug 2017 : 19:54:29
| Thats interesting John as I had always considered NGK to be the paragon of spark plugs - I'll be more wary now. | |
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THRUMMER Starting Member
United Kingdom 20 Posts | Posted - 07 Aug 2017 : 20:10:43
| quote: [i]O]Drawing the conclusion that the valves are OK from measuring the clearance can be misleading, burnt valves can have clearances that are perfectly within spec..
Indeed, and that's why you follow with a compression test to verify further!!!!!!!!
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melle V4 Guru
United Kingdom 3830 Posts | Posted - 07 Aug 2017 : 21:45:18
| Oops, missed that for some reason! That's the correct procedure indeed.
I also buy spark plugs in bulk (for personal use only) as I use them in the 900 as well, never had any issues.
www.saabv4.com | |
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