Puzzling misfiring from one piston

Semitone said:
Trying to figure out how this GTX side bar started. Incidentally, I’ve been using Castrol for many years in my bikes and cats, Can’t remember what I was that started my brand loyalty, but there you go.
Click to expand...
See post #26.
 
Semitone said:
Trying to figure out how this GTX side bar started. Incidentally, I’ve been using Castrol for many years in my bikes and cats, Can’t remember what I was that started my brand loyalty, but there you go.
Click to expand...
It's all there in the posts re cam break in, which was part of your discussion?

I sometimes make the mistake of assuming people want the same type of info I value, oil test results, oil analysis, chemical makeup, actual data vs anecdotal.
Sometimes they don't want the info or perhaps just find it to be boring stuff. It is a trifle boring I suppose.

Glen
 
marshg246 said:
If by "intake manifold pushrod" you mean that the intake valve pushrod was not properly seated in the tappet and rocker, there should have been a pretty terrible noise and I recommend you take the head off - surely the pushrod is well damaged!
Click to expand...
Agreed, but I did not hear anything out of the ordinary in the short time I had it running. Dunno if you read this in one of my earlier entries, but when I rechecked my tappet clearances after running it, I found that there was no gap on intake for problem cylinder. I had backed off the adjuster about 4 turns before I heard a “ping” and then I had loads of clearance. If intake valve had not be closing all the way, this could explain the misfire. Yet, I had good compression before I had tried to start it, which doesn’t make sense. Anyway, have not had chance to start it again since tappet adjustment. Will do later today. Thanks.
 
worntorn said:
It's all there in the posts re cam break in, which was part of your discussion?

I sometimes make the mistake of assuming people want the same type of info I value, oil test results, oil analysis, chemical makeup, actual data vs anecdotal.
Sometimes they don't want the info or perhaps just find it to be boring stuff. It is a trifle boring I suppose.

Glen
Click to expand...
Not at all Glen! I read the whole thing. It was really interesting, and I was reading I was thinking if I had your investigatory prowess I wouldn’t be in this forum exposing all my deductive shortcomings!
 
Semitone said:
Agreed, but I did not hear anything out of the ordinary in the short time I had it running. Dunno if you read this in one of my earlier entries, but when I rechecked my tappet clearances after running it, I found that there was no gap on intake for problem cylinder. I had backed off the adjuster about 4 turns before I heard a “ping” and then I had loads of clearance. If intake valve had not be closing all the way, this could explain the misfire. Yet, I had good compression before I had tried to start it, which doesn’t make sense. Anyway, have not had chance to start it again since tappet adjustment. Will do later today. Thanks.
Click to expand...
At very least I'd be having the rocker cover off, checking with a flash ljght while poking about with a long thin screwdriver to visually confirm rods are fully seated in rockers. I believe best done from exhaust rocker box side.
Perhaps issue was the rod was half seated, stuck on the cusp of the rocker seat. Giving some valve operation but not in proper sync with rest of valves. Thus poor cumbustion but still able to close, giving reasonable psi compression readings.
 
marshg246 said:
See post #26.
Click to expand...
Yes, I went back and found the segue. Thank you!
Tornado said:
At very least I'd be having the rocker cover off, checking with a flash ljght while poking about with a long thin screwdriver to visually confirm rods are fully seated in rockers. I believe best done from exhaust rocker box side.
Perhaps issue was the rod was half seated, stuck on the cusp of the rocker seat. Giving some valve operation but not in proper sync with rest of valves. Thus poor cumbustion but still able to close, giving reasonable psi compression readings.
Click to expand...
Well, something like this definitely happened. It started this eve on the second kick. It took 5 or 6 tries the other day. I could tell right away that left piston was firing normally because I still had idle screw set pretty high because it was essentially running on one cylinder last time, and it immediately jumped to about 2500 rpms. I backed off the idle screw and as rpm dropped back down to about 1200 rpms, I heard a metal on metal clanking noise, so I immediately shut her off. Don’t know why this happened at lower rpm, but it is clear that some damage was done the first time I started it. Guessing a bent pushrod or valve stem (as marshg246 had predicted). Going to pull the head later today. One problem solved, a new one is born.
 
baz said:
If the valve is bent you will lose compression
Click to expand...
You know after I had discovered the likely pushrod seating problem, and after resetting the gap, I did a compression test. I couldn’t get more than 50lbs from the cylinder. I removed the tester to make sure my hose connections were good and everything looked good, so I tried again and got 120lbs. Is it possible that valve gets stuck intermittently because, perhaps, the valve stem is bent?
 
Semitone said:
You know after I had discovered the likely pushrod seating problem, and after resetting the gap, I did a compression test. I couldn’t get more than 50lbs from the cylinder. I removed the tester to make sure my hose connections were good and everything looked good, so I tried again and got 120lbs. Is it possible that valve gets stuck intermittently because, perhaps, the valve stem is bent?
Click to expand...
Did you use any lube other than oil when fitting the valves?
I once heard of someone using chain lube and as the engine heated it locked the valve up
 
Yes that is possible I've been told with any rotating valve. If you can replicate the 50lb. again by several additional tests stop there so the valve can clearly display the condition when water testing as recommended is performed with the head off. On that topic find GregM's 850 rebuild diary at his site for great information on head rebuilds. Good luck!
 
baz said:
Did you use any lube other than oil when fitting the valves?
I once heard of someone using chain lube and as the engine heated it locked the valve up
Click to expand...
No. Just 20w40. Got the head off this afternoon. The problem cylinder pushrod showed a little bit of scarring just below the cup, so it was not seated at some point and rubbed against the rocker or casing. Thinking this is what I heard on last running. Then I noticed a huge difference in rocker adjuster screw positions between left and right intakes (see pic). Clearly, when I thought I had the left side pushrod seated properly, it wasn’t. Luckily, there wasn’t any damage to intake valve, although as I think of it, wouldn’t it take in incorrect cam position or pre detonation to cause this? Anyway, no evidence of valve/piston collision, and water did not leak past the valve. Don’t know why I had such trouble seating the pushrod this time around. I’ve taken the head of at least half a dozen times in the 30 years I’ve owned the bike! Putting head back on today and will post an update.
 

Attachments

  • Puzzling misfiring from one piston
    4341F225-895D-4C5A-99FD-B41D2D130F07.webp
    125.7 KB · Views: 111
Jdub said:
Yes that is possible I've been told with any rotating valve. If you can replicate the 50lb. again by several additional tests stop there so the valve can clearly display the condition when water testing as recommended is performed with the head off. On that topic find GregM's 850 rebuild diary at his site for great information on head rebuilds. Good luck!
Click to expand...
This makes sense, although I already have the head off. I’ll check out the diary, thanks!
 
Semitone said:
No. Just 20w40. Got the head off this afternoon. The problem cylinder pushrod showed a little bit of scarring just below the cup, so it was not seated at some point and rubbed against the rocker or casing. Thinking this is what I heard on last running. Then I noticed a huge difference in rocker adjuster screw positions between left and right intakes (see pic). Clearly, when I thought I had the left side pushrod seated properly, it wasn’t. Luckily, there wasn’t any damage to intake valve, although as I think of it, wouldn’t it take in incorrect cam position or pre detonation to cause this? Anyway, no evidence of valve/piston collision, and water did not leak past the valve. Don’t know why I had such trouble seating the pushrod this time around. I’ve taken the head of at least half a dozen times in the 30 years I’ve owned the bike! Putting head back on today and will post an update.
Click to expand...
Its easy to make a mistake when installing the pushrods and head, it seems to be very hard to see one pushrod is seating on the rocker arm when looking, a thin screw driver usually puts it in position, I always check that the pushrods and rechecking once the head is down and are in position before pulling the head down on the bolts and slowly rotating the motor to make sure they are all seated before tensioning the head.
 
ashman said:
Its easy to make a mistake when installing the pushrods and head, it seems to be very hard to see one pushrod is seating on the rocker arm when looking, a thin screw driver usually puts it in position, I always check that the pushrods and rechecking once the head is down and are in position before pulling the head down on the bolts and slowly rotating the motor to make sure they are all seated before tensioning the head.
Click to expand...
That’s exactly what I did when I put the head back on today. I must have checked a dozen times as I started to torque down the head. I kind of go cross eyed trying to see the rods and back ends of rockers. When I finished torquing the head bolts, I made sure that all adjuster screws were in the same ball park with each other when setting the gap. I rolled it over a few times and chech gaps again. I started it and it voila. No misfire, no clanking. Didn’t run perfectly but I haven’t done a dynamic timing yet, and suspect carb needs to be tweeked as I rebuilt that as well.

So, serious user error on this one. I hadn’t put the head on the bike in probably ten years, and forgot how fastidious you need to be when doing it, to do it right. I also made some poor assumptions. I really appreciate all the input and hope you all don’t feel like I wasted your time. I learned a lot nevertheless.
-Paul
 
Paul you aren't wasting anyone's time, as I said before it's easy to make a mistake even if you done it many times before, we all do it, sometimes the brain doesn't work right and your thinking goes way ward, I know I have done it before, lucky when I do it hopefully no one around to see it lol.
 
Semitone said:
That’s exactly what I did when I put the head back on today. I must have checked a dozen times as I started to torque down the head. I kind of go cross eyed trying to see the rods and back ends of rockers. When I finished torquing the head bolts, I made sure that all adjuster screws were in the same ball park with each other when setting the gap. I rolled it over a few times and chech gaps again. I started it and it voila. No misfire, no clanking. Didn’t run perfectly but I haven’t done a dynamic timing yet, and suspect carb needs to be tweeked as I rebuilt that as well.

So, serious user error on this one. I hadn’t put the head on the bike in probably ten years, and forgot how fastidious you need to be when doing it, to do it right. I also made some poor assumptions. I really appreciate all the input and hope you all don’t feel like I wasted your time. I learned a lot nevertheless.
-Paul
Click to expand...
Did you see and damage on the offending pushrod?
 
Back
Top