What else could it be?

[jcrews]

CBH
This may sound crazy, but did you use the same head bolts that were in there or did you replace them with new ones? I had the same problem with my 104 and learned that bolts will stretch which happened to mine and I replaced with grade 8 bolts from Tractor Supply and they also streched even though they were brand new so I bought grade 9 bolts and they did the job.

[CBH]

Yes, I used the same bolts, torqued to a little over 25 Ft. Lbs.

[Matt G.]

Originally Posted by CBH

Today I pulled the head and de-carboned everything, but it was actually more like soot than any carbon build up. Installed with a new head gasket, mowed for about 1/2 hour,... stopped and let it idle,... turned it off ....BANG!
If it is gas getting into a hot muffler, I'm wondering if an exhaust stack style muffler would keep it from banging. What do you think?

A different muffler isn't going to change the fact that gas is still getting in there and exploding.

This may sound crazy, but did you use the same head bolts that were in there or did you replace them with new ones? I had the same problem with my 104 and learned that bolts will stretch which happened to mine and I replaced with grade 8 bolts from Tractor Supply and they also streched even though they were brand new so I bought grade 9 bolts and they did the job.

This should be unnecessary. Are you guys RE-torquing the bolts as per the manual after 30 min. run time? Did you check the head for flatness and correct any warpage as necessary? I have reused head bolts all but once, when I replaced them all in a tractor because one twisted off. I have NEVER reblown a head gasket, to this day, in the past 6 years.

[CBH]

There were no signs of a leaking head gasket. I torqued the bolts as the manual said.
I guess I'll just have to live with it and end my mowing sessions with a bang! :biggrin2.gif:

[RChristensen]

Since I don't know all the things you have done to your tractor, have you checked the adjustment of your valves lately.

[oldcubowner]

My model 86 did that until I slowed down the idle. I have all but elimanated my carbon build up by adding some Marvel Mystry oil to the gas every time I fill it up. Most of the cubs I have been around idle to fast from items wearing out like the throttle shaft etc etc.
Rod

[jcrews]

Originally Posted by Matt G.

A different muffler isn't going to change the fact that gas is still getting in there and exploding.



This should be unnecessary. Are you guys RE-torquing the bolts as per the manual after 30 min. run time? Did you check the head for flatness and correct any warpage as necessary? I have reused head bolts all but once, when I replaced them all in a tractor because one twisted off. I have NEVER reblown a head gasket, to this day, in the past 6 years.

Matt
I used to reuse the headbolts also but Roland told me about the bolts stretching and I compared them side by side and sure enough the bolts on the engine had gotten longer. Talk about retourqeing the head, try 5 times and they still did it. I'm not saying they all do it but this could be the problem. Grade 8 bolts on exhaust manifolds on cars and trucks do it sometimes after long term high heat exposure.Roland told me some people relace the head bolts every time the replce the head gasket.

[jcrews]

Matt
One more determing factor for me with the bolt is that it gets very hot down here in Ga(this week average high temperature 97) and that probably plays more in to my bolts stretching than in Illinois, where the summer temperatures are generally more moderate.

[Matt G.]

Originally Posted by jcrews

Matt
One more determing factor for me with the bolt is that it gets very hot down here in Ga(this week average high temperature 97) and that probably plays more in to my bolts stretching than in Illinois, where the summer temperatures are generally more moderate.

Ever been to Illinois in August? It routinely gets in the mid 90's here in
August; in fact, we have lower 90's forecast for the next 3 days, which is really unusual for June...

A 10-15 degree difference shouldn't make that much of a difference in the grand scheme of things. Oldcubowner has a good point...The idle is set too fast on most of the tractors I buy. I think they left the factory set at 1000 rpm but I've seen a lot that sit at 1300-1400. I'd check that too if you haven't already.

Use studs not bolts next time you change head gaskets. There is torque loss in throughout the length of the bolt. Also saves the threads on your block.



Scott