23hp intek v-twin backfires through the exhaust

[rfrench]

hi and thanks for adding me to the forums lots of valuable information here. 23 hp intek v-twin back fires through the exhaust when it warms up after about 5 minutes and the throttle is all the way up at fast. low idle it runs fine has new plugs and fuel filter

thanks for any suggestions

 

[motoman]

Check valve clearances and look at position of guides in head. Look at lift action of the valves. SB the same and adequate to admit and expel incoming and outgoing mixtures. Mine had a pushed exhaust guide and that cylinder ran and showed proper compression when the valve lift had been reduced to less than 1/8 inch due to grinding away of the cam lobe because of a guide pushed up into the head.

 

[Tinkerer200]

I can send you a Service Manual for your engine IF you like. Address below, put in proper fprmat and remind me engine model number and what you want. Excessive and premature camshaft lobe wear was because cam lobes of the built up camshaft were not properly hardened. Valve guide movement is usually caused by a cooling problem, plugged cooling shroud and or cooling fins. Do not forget the passage going completely thru the head around the valve guide castings. Guide should not extend more than 1/8" above the head casting. Some say ethanol fuel is causing valves to stick resulting in valve guide movement. I have no opinion on that.

Walt Conner
wconner5 at frontier dot com

 

[motoman]

The reduction of my Intek 24 exh cam to a circle was due to the guide moving up and not allowing the cam to freely follow its path on a free valve. My take was not necessarily that the cam lobe was not hardened properly, but that the "chilled" lifter was simply very hard and the soft gave way to the hard under pressure. I compared that lifter to the other 3 and could not see any wear under the harsh , constant contact. You can see a little pock mark on the lifter where its hardness was checked at the factory. That lifter is still in the engine 10 years later.

Before I bought a new Husqy I talked to my local dealer and he says some pros are adding 2 stroke oil to gas to avoid pushed guides. His explanation is that it counteracts the "dryness" that ethanol causes. I do not necessarily accept that, although it probably is like the old Wynn upper end additive car stores sell. My take on the valve stems is that it is puzzling Briggs puts a stem seal on the exh valve wiping down the little oil it get from the really anemic "mist" from the crankcase. I note that the "endurance" model Intek 24 in my new Husqy supposedly has chrome valve stems, which to me means a coating meant to wear better and so perhaps keep the valve stem from wearing and cocking . No one has explained why the guides always move up. Perhaps due to some action of the spring return that is working on the seating valve. Also somewhere I think I read the chrome is supposed to retain lubricant better. Microscopic stuff. The folk explanations continue.

Edit, Well, Briggs seal only partially puzzling because some reading suggest excessive oil on valve stem may be a recipe for "coking" of the stem, i.e., a build up of carbon on the valve stem which would defy the oil additives at high temp (280F-320F) and deposit hard coatings which would hang up the valve, resulting in bent pushrods, etc. Such heat can be reached , IMO, and shutting down such an overheated engine is an ingredient in the coke recipe. So go to the shade and cool down the beast after hot, strenuous workouts before shutting down. BTW this coking is what caused turbo engine guys to put water cooling on the turbo wheel bearing housing.

 

[Tinkerer200]

Well believe as you wish however, the valve guide on OHV engines is no where close to either the camshaft or cam follower, slipper guide has nothing to do with cam lobe wear. Push rods are bent due to either the valve guide moving up until the valve spring keeper hits it or by the valve sticking in the valve guide. Valve seals are used on Intake Valves as shown in the Service Manual.

Walt Conner

 

[motoman]

Walt, I may have not explained what I think happened clearly enough. As the guide moves upward the lift of the cam lobe causes the rocker tip to hit the guide. This stacks up the lobe against the lifter against the pushrod against the rocker tip and the valve cannot open fully. This is a constant overload situation. As the guide moves up the lobe is ground down. I have read about bad lobe castings and that these low end lobes are mass hardened on a line with gas or induction heating, but Briggs and others apparently has a lot of this problem.I never noticed any wear powder.

 

[Tinkerer200]

Good theory except the push rod will bend before putting enough pressure to accomplish this amount of wear. I have seen numerous rounded off cam lobes where the valve guide had not moved at all. Powder in a quart and half of oil over a few years and hopefully several oil changes?

Walt Conner

 

[motoman]

Walt, good to have discussion on this widespread problem. My theory says the stack-up load on the guide is constant , but not in large steps or increments which would bend the pushrod. Since you are a lathe man....the feed-in of the really dull cutter bit (lifter against lobe)is just engough to remove material slowly over time, but not enough to bend the pushrod, only perhaps flex it. Your theory of bad lobes is often stated in auto repair , even with hydraulic lifters. Perhaps both processes at work. Have you ever seen (casting) voids in the worn lobes? Ever had a hardness test done? Locally I got a quote of $50 for a Brinell test with no test documentation, motoman

 

[Tinkerer200]

I don't try to make a big deal testing etc. out of the obvious. This problem did not occur until B&S went from one pc. forged camshafts to built up segment camshafts. A friend and collaborator is a retired Sears Tech and many of these engines were warrantied for bad cams.

Walt Conner

 

[bertsmobile1]

A higher than designed for spring load will cause the cam lobe or follower, whichever is the softest to wear rapidly.
This become apparent then one fits stronger valve springs to a motor that is being tuned for more power.
A longer contact time will also promote excessive wear.
Spring bind or the rocker contacting anything will bend pushrods or spring them off .

I will fall in behind Walt on this one 100%
There is a massive difference in the wear properties between castings, forgings, pressings, rolled product & extrusion.
In order to make a mower engine cheap enough for Joe public to be happy to buy, corners have to be cut.
Forging is the most expensive way to shape a product, casting is the cheapest.
A built up cam, like a built up crank will never be able to attain the same strength & rigidity as a forged cam shaft.
The new cam attains all of its wear resistance from the heat treatment it gets.
What to gain from heat , you can also loose from heat.
Also because lobes are mass treated, not done one at a time, some will be a lot better than others.
It is the bane of heat treaters trying to attain consistency.
Then in comes plain dumb luck.
A not quite hard enough cam ends up in an engine with a slightly higher spring load and you get excessive wear.
Once the hard facing has worn through, the cam lobe will wear down to the base circle in no time flat.