kyle Posted March 26, 2007 Share Posted March 26, 2007 Hi guys- this is sorta cross posted from zcar.com, so apologies to readers of both. I have a L28/N42 with SU/SM, header, mild port/polish, MSA stg 1 or 2 cam (prob 2, but I can't remember) and GM HEI. The engine was rebuilt about 5K ago, and has seen a couple track days. Recently, it's started missing, performing poorly, just being grumpy. Compression is 140-150 across the board, Spark jumps 1/2"+ from wire to plug, SUs are cleaned and balanced, HEI module has been swapped for another, Ign coil seems okay (I forget the Ohm readings, but they were reasonable), Ignition timing is 16 base, 36 total, Plugs are clean and stay that way, Valves are .011I .012E. I did some non-related work (welded up a leaky gas tank fitting), fired it up, and it ran 100% perfect. Put the air cleaners back on after checking carb balance, and when I went to drive it, the miss was back. Wound up checking the vacuum at the brake booster fitting on the balance tube: needle waves around like mad from 0-5. Even with the engine running 1500RPM, the vac only got to about 10, still fluctuating madly. At this point, I'm getting the suggestion that there is a sticky valve or dropped seat. I'll see about doing a leakdown test; any other things to do before pulling the head? Quote Link to comment Share on other sites More sharing options...
oinojo Posted March 27, 2007 Share Posted March 27, 2007 Hi guys- this is sorta cross posted from zcar.com, so apologies to readers of both. I have a L28/N42 with SU/SM, header, mild port/polish, MSA stg 1 or 2 cam (prob 2, but I can't remember) and GM HEI. The engine was rebuilt about 5K ago, and has seen a couple track days. Recently, it's started missing, performing poorly, just being grumpy. Compression is 140-150 across the board, Spark jumps 1/2"+ from wire to plug, SUs are cleaned and balanced, HEI module has been swapped for another, Ign coil seems okay (I forget the Ohm readings, but they were reasonable), Ignition timing is 16 base, 36 total, Plugs are clean and stay that way, Valves are .011I .012E. I did some non-related work (welded up a leaky gas tank fitting), fired it up, and it ran 100% perfect. Put the air cleaners back on after checking carb balance, and when I went to drive it, the miss was back. Wound up checking the vacuum at the brake booster fitting on the balance tube: needle waves around like mad from 0-5. Even with the engine running 1500RPM, the vac only got to about 10, still fluctuating madly. At this point, I'm getting the suggestion that there is a sticky valve or dropped seat. I'll see about doing a leakdown test; any other things to do before pulling the head? perhaps your crank pulley outer ring spun... resulting it improper ignition timing? You should check it by pulling a plug and use a screwdriver to determine proper TDC. Quote Link to comment Share on other sites More sharing options...
schwiplarkin Posted March 27, 2007 Share Posted March 27, 2007 a low jumping vacuum could also be caused by a vacuum leak, that could cause a lean missfire. I hope its a disconnected hose and not valve springs. Quote Link to comment Share on other sites More sharing options...
Daeron Posted March 27, 2007 Share Posted March 27, 2007 try hunting around the engine bay with a mechanic's stethoscope (or a piece of tubing stuck in your ear) Run the leakdown test, too (always good to know your numbers) but if that checks out, then make sure you don't have a vacuum leak somewhere. IT *would* cause a miss, and low vacuum.... Quote Link to comment Share on other sites More sharing options...
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