Sanity Check Please - Cold Start Issues

[azr]

I agree with electrical since it's 1 & 4 that are not firing (they are controlled by the same coil). What does the spark look like on those two cylinders compared to the other two that are firing properly?

[tom203]

Sure makes you suspicious of ignitor circuit that fires 1 and 4 at cold start. Also, is bike typically on kickstand during start ?- even on "prime" spot maybe #4 doesn't get enough fuel initially.... BUT. the black #4 plug says otherwise.

[nater]

I would check the ignition system first since it is both 1 and 4. How is the condition of the coil and spark plugs on those cylinders?

I feel like an idiot not checking this before, but you're right this makes the most sense to check. The resistances of both coils check out, and to double check this i swapped the 1/4 coil, wires, and igniter pin onto the 2/3 cylinders and it idled on those two cylinders just the same. Also, I've got a solid 12.2v to each coil (12.5 at the battery), so coils, wires and plug caps should be ok.

Tom - I'll do the igniter test tomorrow, but based on everything so far i think it makes a lot of sense.

I'll have an update shortly!

[althomas101]

I had a nighthawk that did the same thing when I got it. A good carb cleaning fixed the issue. Maybe you should revisit the carbs.

[nater]

Well...igniter looks like it's working just as it should and i've got nice blue spark on all four plugs when I jump the corresponding igniter leads. Also, I have 157 Ohms on both the pickups, so looks like my ignition system is working as it should. Was good to go through, but sucks not finding the culprit.

It's really interesting that if i stay off the throttle and just idle, even below a few krpm, cylinders 1 and 4 NEVER catch. Oh well, back to the carbs...

[nater]

Well its been another couple weeks but I've got a few things accomplished. Re-dipped and rebuilt all the carbs again. At this point I am confident in their operation, but still no improvement in cylinders 1 and 4. In fact, they seem to be getting steadily worse to the point they almost never catch anymore. So with air, fuel and spark leaves compression...my compression test results:

Dry test:
1 - 82
2 - 78
3 - 85
4 - 80

Wet test: (teaspoon of oil in each cylinder)
1 - 155 dropping to 125
2 - 125
3 - 155
4 - 110 dropping to 85

Tests were done on a cold engine (not much choice lol) at WOT with plugs removed. I'm not all that confident in the compression tester, but it's pretty clear that the head needs to come off. Does such a significant difference between dry and wet tests indicate rings? Does the pressure drop in 1 and 4 indicate head gasket or valves as the wet test should be sealing the rings? There seems to be a lot of bad things going on here...

[MisterCinders]

Yeah, those look pretty tough. Sounds like a top end adventure for you.

[Nessism]

Yes, rings.

If the bike hasn't run much as of late I'd focus some more time on trying to get it to run better. I'd unscrew plug caps, snip off a 1/4" or so of lead, then screw them back on. Also check voltage at the coils. Most likely you have a spark issue since both 1 & 4 seem to be affected, and they share the same coil. If you can get it to run then start thrashing it pretty good so see if you can get the rings to seal better. Many times some good hard miles will help with compression on bikes that sat inactive for a long time.

Good luck.

[nater]

Well I started digging into the top end, and I'm a little nervous. I've done a number of head gaskets, but never had jugs off an engine and not exactly sure what I'm seeing. A pic of the intake side of one of the jugs:



And a pic of one of the pistons:


All four pistons/cylinders have the same scoring on the intake side. If I run my nail across them, i can feel the grooves but they do not stop my nail. Is this something I should be worried about?

My other question is about the service limit measurements. I've measured the cylinder bores for all 4 cylinders and everything seems to be in spec. However, the two pistons I've measured are 66.75mm +/- 0.05mm, and the shop manual says 66.88mm minimum...based on all of this, do I need to bore out and do oversized pistons, or can I run these pistons and clean up the scoring?