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Discussion Starter · #1 ·
See attached photo. I picked up this 5.5hp White (MTD model 31AH6BHE790) unit at a great price… Has the Tecumseh 5.5hp Snow King engine. Yes, it is your basic 5.5hp MTD but it has electric start, scoop out shovel, headlight AND hand warmers. Everything works. It starts easily but doesn’t run great. I can tell someone has been into the carb. Bowl was spotless. It can stall a bit if the throttle is moved. If I pull the throttle down to stop it, it will usually backfire blue flame from the exhaust. Thoughts? Valve issue?
178196

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If it was mine, a new carb is like 13-18 dollars ... I would start there, especially when you say someone has already taken its carb apart and messed with it. Might have some crap gas as well?

Machine looks well taken care of ...

fwiw ... all my 5 gallon gas cans get 2oz of Stabil and 2oz of SeaFoam on fill up ... this way all my machines have treated full at all times .. winter, summer, 2 stroke ....
 

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Agree with oneacer, sounds like a dirty carb. Buy an adjustable carb.
 

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definitely make sure the main jet is clear. popping out the exhaust when you throttle down is normal but it still wouldn't hurt to check the valve clearance. it is not unheard of to have exhaust valve issues
 

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Discussion Starter · #5 ·
Thank you all. I figured replacing the carb would be the best advice.
 

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See attached photo. I picked up this 5.5hp White (MTD model 31AH6BHE790) unit at a great price… Has the Tecumseh 5.5hp Snow King engine. Yes, it is your basic 5.5hp MTD but it has electric start, scoop out shovel, headlight AND hand warmers. Everything works. It starts easily but doesn’t run great. I can tell someone has been into the carb. Bowl was spotless. It can stall a bit if the throttle is moved. If I pull the throttle down to stop it, it will usually backfire blue flame from the exhaust. Thoughts? Valve issue?
View attachment 178196
View attachment 178197
Its the valves. My buddy had a blower he got free from his neighbor with the same engine. Wouldn't run right, adjusting carb did nothing. He popped the breather and sure enough valves were out of spec. Once you reset the valves it will run good. A little more tedious on the L head, because you have to remove the carb to gain access to the breather and the side of the valves, than you have to pop the caps to release the springs to get them out, a valve spring compressor makes that a lot easier, also heads gotta come off to pull the valves out. You gotta grind the stems on a bench grinder until their back in spec. You should lap them back in while your at it and clean any carbon on the head and top of piston while at it. Still overall not too bad. Its usually the exhaust valve, the heat causes it to stretch, so it stops closing all the way. After you do that it'll run good again and have a lot more power.
 

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you don't need to remove the head to adjust the valves. you can do the job with a file and valve spring compressor. i have done it to a few machines. if done right i find most of the metal that is removed stays on the file.
 

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you don't need to remove the head to adjust the valves. you can do the job with a file and valve spring compressor. i have done it to a few machines. if done right i find most of the metal that is removed stays on the file.
Are the valve stems too hard for a file? Grease the file, the filings will definitely stay put.
 

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no. they file just like any other metal

straight from briggs and straton website under how to adjust tappet clearance
  • Step 3: If clearance is insufficient, remove the valve and grind or file the end of the valve stem square to increase the clearance. Check the length frequently as it is easy to remove too much metal.
 

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Discussion Starter · #10 ·
Thanks Dusty, I am thinking the exhaust valve as well. I think I can raise the valve and file a bit off without removing it if it measures wrong. I may give that a shot first before I replace the carb.
 

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Well this was good info, I have a Souix valve and seat grinder so I will have a couple of backfiring 8hp's to check out this summer. Easy fix grinding the valve tips. Now I can see that the clearances they have are for a cold engine and being it's an aluminum block they will get looser as it heats up.
 

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I will have a couple of backfiring 8hp's to check out this summer
just making sure the valve clearance is correct makes a huge difference. i had one that was bad enough that you could see the muffler glowing red during the day. fixed the valve clearance and it has been fine since. run good and almost always starts on first pull. should the head be pulled and the valve lapped sure but i don't care about the machine enough to mess with removing the head and hope i don't snap bolts. runs good enough and doesn't usually bog under load but it is a small machine with big engine
 

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Discussion Starter · #13 ·
Well, I checked the valves. Spec says intake .004 and exhaust .008. My intake was .004 and my exhaust was .003. A bit of grinding later, exhaust is now .008. I wouldn't say it is tons better but I think my back fire is gone. I have surging with choke fully off. One click on it goes away. I checked the carb again and noticed something I have never seen before. I removed the bottom nut/jet and bowl. I look up at the emulsion tube and their is a black rubber o-ring up there. Looks like is seals the bottom nut/jet to the tube. Is that supposed to be there?
 

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Well, I checked the valves. Spec says intake .004 and exhaust .008. My intake was .004 and my exhaust was .003. A bit of grinding later, exhaust is now .008. I wouldn't say it is tons better but I think my back fire is gone. I have surging with choke fully off. One click on it goes away. I checked the carb again and noticed something I have never seen before. I removed the bottom nut/jet and bowl. I look up at the emulsion tube and their is a black rubber o-ring up there. Looks like is seals the bottom nut/jet to the tube. Is that supposed to be there?
sounds like its lean,
 

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just an fyi, I believe the valve clearance specs for the HSK engine are .004 min and .008 max for both intake and exhaust. That is what the TEC L-head handbook calls out. Not 4 for intake and 8 thou for exhaust. Where did you find the spec that you mentioned?
 
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Well, I checked the valves. Spec says intake .004 and exhaust .008. My intake was .004 and my exhaust was .003. A bit of grinding later, exhaust is now .008. I wouldn't say it is tons better but I think my back fire is gone. I have surging with choke fully off. One click on it goes away. I checked the carb again and noticed something I have never seen before. I removed the bottom nut/jet and bowl. I look up at the emulsion tube and their is a black rubber o-ring up there. Looks like is seals the bottom nut/jet to the tube. Is that supposed to be there?
Yes, the carbs with plastic emulsion tubes have two one at the base you can see and one more towards the top, just before the tip sticking out into the throat of the carb. Its good to pop those down with a flathead screwdriver without breaking it, make sure to clean out the passages in that tube and put it back in. If the O rings are shot, which they usually are, replace them. Their usually the second size up in the harbor freight o ring kit. You get about 15-20 that size in those o ring kits they sell. Worth it, pays for itself 10 times over just in that size o ring.
 

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Discussion Starter · #18 ·
How quickly I forget. A year ago ago I was given an 26" MTD with an 8hp Tecumseh. I had to replace the emulsion tube and the two rubber gaskets. It works great now.

I did order a new aftermarket adjustable carb for this White snow blower for $18 delivered. Makes a world of difference. Starts easy, runs strong, and still no backfiring. Thanks for all of your help!
 
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