Misfire on cylinder #4

ryanmlee78

New User
Hi All!
I can turn wrenches but I'm not much of a mechanic. I have a To-30. I've had it for about 5 years. It has had a "miss" in it for some time now, but it has always started and ran well, it was a little weak but it doesn't smoke (maybe a very small blue puff on start up) or backfire (never). You can hear a "tick tick tick tick..." while it is running. Recently I went on the hunt for the miss, I noticed I could pull the plug wire on #4 without changing the cadence of the engine. I do have spark on that plug. I stuck my finger in the spark plug hole and there is not enough compression to push my finger out of the hole (I know that isn't exactly a real compression test but the other cylinders will push my finger out in a hurry). So I pulled the valve cover and turned over the engine to make sure the valves moved - they do. I pulled the cylinder head and honestly the valves and cylinder look pretty good but I don't really know what to look for. Any thoughts?
Thanks!
Ryan
 
(quoted from post at 02:32:44 10/28/14) Hi All!
I can turn wrenches but I'm not much of a mechanic. I have a To-30. I've had it for about 5 years. It has had a "miss" in it for some time now, but it has always started and ran well, it was a little weak but it doesn't smoke (maybe a very small blue puff on start up) or backfire (never). You can hear a "tick tick tick tick..." while it is running. Recently I went on the hunt for the miss, I noticed I could pull the plug wire on #4 without changing the cadence of the engine. I do have spark on that plug. I stuck my finger in the spark plug hole and there is not enough compression to push my finger out of the hole (I know that isn't exactly a real compression test but the other cylinders will push my finger out in a hurry). So I pulled the valve cover and turned over the engine to make sure the valves moved - they do. I pulled the cylinder head and honestly the valves and cylinder look pretty good but I don't really know what to look for. Any thoughts?
Thanks!
Ryan
I'm guessing you didn't check the valve clearances before you pulled the head. There is a good chance the #4 exhaust valve wasn't closing fully. If that was the case, the valve face and seat will not be "clean" from mating. Probably should lap the valves before putting it back together.
 
No I didn't check the valve clearances before I pulled the head, so I
should look to see if the valve face is clean to diagnose? Do you
mean I should have the valves ground prior to putting it back
together?

Thanks for the advice!
 
Irv's answer is a definite possibility. When I said "lap" the valves it presupposes the valves and seats are in good shape. If they are eroded or otherwise messed up you should probably have them ground by a machinist. Lapping is done by hand using graded abrasives you can get in most any auto parts store. You have to spin the valve lightly against the seat until you have a uniform band around the face of both the valve and the seat. The shop manual has pictures and gives specifications - if you don't have a manual it is one accessory you really need. Of course, when you have the valve out to lap it, you can check out the spring. If the #4 spring is shorter than the others it has the same effect as if it were broken.
 
While you have the head off lay it level and pour
gas into each one of the valve chambers with spark
plug replaced and check how fast #4 leaks down
compared to the others.
 
(quoted from post at 02:32:44 10/28/14) Hi All!
I can turn wrenches but I'm not much of a mechanic. I have a To-30. I've had it for about 5 years. It has had a "miss" in it for some time now, but it has always started and ran well, it was a little weak but it doesn't smoke (maybe a very small blue puff on start up) or backfire (never). You can hear a "tick tick tick tick..." while it is running. Recently I went on the hunt for the miss, I noticed I could pull the plug wire on #4 without changing the cadence of the engine. I do have spark on that plug. I stuck my finger in the spark plug hole and there is not enough compression to push my finger out of the hole (I know that isn't exactly a real compression test but the other cylinders will push my finger out in a hurry). So I pulled the valve cover and turned over the engine to make sure the valves moved - they do. I pulled the cylinder head and honestly the valves and cylinder look pretty good but I don't really know what to look for. Any thoughts?
Thanks!
Ryan

Also look for a bent pushrod. If the intake pushrod on #4 was bent it wouldnt let the valve open and draw any and fuel in. Although it is highly unlikely to have happened to your engine, a flattened cam lobe can also cause the symptoms you describe. I would say more than likely you have a burnt valve or bent pushrod.
 
All good advise, if in doubt take the head to a shop and have it gone through.

Be very careful (if not too late) not to turn the engine over with the head off! The cylinder liners will lift up out of the block. If any have moved they will have to come out, the liner and seat in the block will have to be thoroughly cleaned and new orings installed. Otherwise they will leak coolant into the oil!
 
You have some great answers but being fat and lazy
as I am, I would have put some oil in the plug
hole to make sure the rings were sealed and then
do the thumb compression. This would help to
eliminate bad rings. You pulled the head.
Scratches on the cylinder walls from a busted
ring? Can you see down past the edge of the piston
and see the top of the rings? Piston to cylinder
side clearance is how much? Now you have the head
off and you get a valve job and reseal it all, the
new valves will build more pressure and new
pressure past old rings...
As was suggested, gas in the back of the valves
and see if it leaks past the valves. Broke spring
and the rest of the story you already know.
This is just something to keep you entertained
over the holidays.
 
I can think of a few things to check, head gasket, rings, bad valves, hole in piston, and maybe a cracked head? some of these may only show up when the engine is hot, but I believe they would all be fixable.
 
I'm certainly learning as I go, looks like I should have done some more pre-work prior to pulling the head. I really appreciate the advice.
No, I don't see any scratches on the cylinder walls. And it looks like a pretty tight fit I certainly don't see a gap. Should I be able to measure the gap between the cylinder and piston?
 
Well the pushrods passed the "roll them on a flat surface test". They look pretty straight. I'm going to try the gas in the valves test and report back. I really hate to think about a rounded camshaft.
 
Generally on a used engine if you can't see the top of the rings it's good enough. The only problem you may encounter is the possibility of a collapsed compression ring that might lower your compression.
If your piston has three rings, the bottom is oil control. The second ring is about 80% oil control and the top ring is the compression ring.
You could measure the side wall clearance but unless your planning on an overhaul...
Not really worth the effort.
Oil pressure is probably a better indicator of the need for overhaul more than compression, unless your compression has dropped to zero as might, might be your case. Until the valves get straight best to leave this alone for now unless you just want to get it over with.
 
Finally got the valves off the head today. Looks like I have a bad
seat (or bad valve?) on the exhaust valve on that cylinder. The seat
has a ridge that you can feel all the way around instead of being a
perfect, consistent bevel. Neither the valve nor the seat have any
shine to them which I think means the valve has not been fully
closing. I also noticed the inside of the intake manifold of this
cylinder is oily (wet). I don't notice this on the other cylinders. I'm
going to take the head to the shop to see if they can cut me a new
seat on the head. I don't know if there is a limit to how much they
can grind off but this seems like a sizable ridge in the seat.
 
Finally got the valves off the head today. Looks like I have a bad
seat (or bad valve?) on the exhaust valve on that cylinder. The seat
has a ridge that you can feel all the way around instead of being a
perfect, consistent bevel. Neither the valve nor the seat have any
shine to them which I think means the valve has not been fully
closing. I also noticed the inside of the intake manifold of this
cylinder is oily (wet). I don't notice this on the other cylinders. I'm
going to take the head to the shop to see if they can cut me a new
seat on the head. I don't know if there is a limit to how much they
can grind off but this seems like a sizable ridge in the seat.
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top