1991 250 TXRP no spark Cyl 5

[florchak]

Help... odd problem!!

No spark in cyl 5 on #1 engine in twin engine application

moved around spark plugs, coils, wires, and even the ecu to the other engine 1 at a time. NOTHING changes the problem. Motor runs at 4000 rpm and then won't go any further. Runs ok but a little rough from start to 4000 rpm. #5 plug is clean and very wet so we tried with the plug wire off cyl 5 and motor runs exactly the same.

So, the engine must run they way it is without cyl 5.

Could any sensors be causing #5 to not fire? Like O2 or knock sensor??

What do you think is causing this?

tks!!

[florchak]

Any ideas? This has me stumped...

[jb123]

no O2
test spark with motor rpms over 1500
are you pulling wire test or plug out visual test ?

most likely clogged carbs @ 5 or more

[florchak]

We actually went further and ran engine without number #5 plug wire attached. I know this is bad but it proved the engine ran the same without the number 5 cylinder. The engine preformed exactly the same and only reved to about 4100 rpm. We really think the #5 cylinder is not getting any spark after checking everything we can think of. We did try to see it with the plug out and could not see it.

What could cause just one cylinder to not have spark?

tks in advance!