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202 popping through exhaust at idle.


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#1 D-Train

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Posted 11 June 2021 - 06:04 AM



G'day guys, I need some help.

My engine is popping through the exhaust at idle. When I give it some throttle, it stops popping until it goes back to idle.
Its a newly built black efi 202 at 30thou with a m90 supercharger. I'm using a megasquirt 3 gold box for the ecu with a 36-1 trigger wheel.
9.2:1 compression roughly.
Xu1 grind cam
Running 100% e85, usually only 80% ethanol at my stations.
Yella terra 56cc alluminiumt head with  1.6 rockers.
Compression is rouggly 165 psi on all pistons.
I've checked that its getting spark with a check light. No 6 plug is black while the other 5 are normal. I swapped coils but no change, new leads amd plugs, but nothing.

I thought the rocker arm on no 6 exhaust might be hitting the rocker cover as it'd originally cover but I can't see any marks. I was thinking of.starting it without the cover but thought it might make a huge mess.
I'm running out of ideas. Any thoughts

#2 yel327

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Posted 11 June 2021 - 08:19 AM

Cut the lid off an old rocker cover and use it if you want to run it with the rockers exposed.
How come you used a big cam designed for triples with a blower? Just curious.

#3 jd lj

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Posted 11 June 2021 - 09:12 AM

It's potentially running too lean at idle. Do you know what the AFR is at idle. Try to richen up the idle circuit and see if it helps.

#4 D-Train

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Posted 11 June 2021 - 09:36 AM

Sorry, that was the wrong one. Cam is 208 at 50 420 lift. Cm203d.

I pulled it all tge way down to 12:1 to check that but that's total afr at the collector. It's odd it is just one cylinder.

#5 warrenm

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Posted 11 June 2021 - 10:53 AM

Could be a dodgy injector in #6, move the injector to opposite end & see if the problem moves.

#6 axistr

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Posted 11 June 2021 - 02:16 PM

Pull one spark plug lead or injector electrical plug of at a time and check which cylinder isn't working. Check for a dribbling injector, vacuum leak and spark at idle on that cylinder. Have a good look at the spark plug it should tell you part of the story. If you have a vacuum gauge put it on and make sure its not flickering at idle. I found a very weak valve spring on a engine that still had good compression on that cylinder, took me a while to find it as it was probably the most unlikely fault in my mind. However The first thing I would be looking for is a vacuum leak at the inlet manifold on that cylinder.  






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