18hp B&S backfires & won't start

EZ-GOnewild · 04-20-2009, 01:48 PM

Hello everyone!

I have a Poulan rider with a 18hp B&S that just stopped. Now it will crank but it backfires through the carb and exhaust.

I took the valve cover off and the exhaust rocker was so loose it was about to fall off. Is this typical of this engine? I did find some reference to this problem elsewhere on the net.

Can I just set the lash at .004 and try to crank it or is there something else I should investigate?

Thank you in advance!

EZ-GOnewild · 04-20-2009, 01:50 PM

Oh, I'll add this info:

Model 31M777-0234-E1
Code 021228ZE
18HP Vertical Shaft

k2skier · 04-22-2009, 11:14 AM

Set valves to .004-.006 and retest.

Volt-Schwibe · 04-23-2009, 02:10 AM

I'd like to suggest checking for a broken valve spring. I know it's a rare failure, but if the rocker suddenly falls loose, there's not many other things to suspect.

EZ-GOnewild · 04-23-2009, 07:53 AM

Thanks for the replies!

I checked the valve springs and they are both ok. I adjusted the valve lash and tried to start it but no luck. It is still backfiring at both ends???

I then pulled the flywheel and checked the key and it was fine so now I'm lost again.

Anymore ideas that you all could suggest?

By the way, what is the little solenoid on the bottom of the float for?

Thanks

Volt-Schwibe · 04-23-2009, 01:45 PM

I suspect the solenoid on the float is for shutting off fuel flow. On a side note, some of these engines have an easy-start feature that partly opens both valves, making it easier to pull the cord. This may have failed, keeping both valves partly open all the time, causing the backfire through intake and exhaust.

I can't be sure without seeing it though :(

coleman mobile · 04-23-2009, 07:03 PM

Have you inspected the valve seat ? I have seen the valve seats pop out.

Volt-Schwibe · 04-24-2009, 03:42 AM

I've seen that too, in some of the larger engines, the valve seats are steel or a bronze alloy, press fit into the aluminum. If it pops out, i don't know of a specific fix to it, i've seen someone put it back in and it fell right back out.

That would cause the rocker to be loose, because the valve would stop at a point lower than what it should be, causing the rocker to show the slack.

Also, once i was running a minibike without an air cleaner, and actually sucked in a tiny rock, which jammed the intake valve open. Had to take the head off to pry it out, so it cost me a head gasket.

Try to check the height of the end of the valve, where it meets the rocker, and see if the two valves have drastically different heights. They should be the same, where as if the valve seat fell out, one valve shaft is going to be a good 1/8 to 1/4 inch shorter. rotate the engine slowly while watching it, and verify that it's moving. when i had a rock jamming a valve, it wasn't moving at all, but when my friend's tractor had the seat fall out, it was moving, but only at the highest point of it's travel.

Hope that all helps.

EZ-GOnewild · 04-24-2009, 07:14 AM

Thanks again for some good advice. I'll be out of town this weekend so I will not be able to work on it until next week. I'll let you all know what I find out then.

04-20-2009, 01:48 PM	#1 (permalink)
EZ-GOnewild Registered User Join Date: Apr 2009 Posts: 4 OS: xp	18hp B&S backfires & won't start Hello everyone! I have a Poulan rider with a 18hp B&S that just stopped. Now it will crank but it backfires through the carb and exhaust. I took the valve cover off and the exhaust rocker was so loose it was about to fall off. Is this typical of this engine? I did find some reference to this problem elsewhere on the net. Can I just set the lash at .004 and try to crank it or is there something else I should investigate? Thank you in advance!

04-20-2009, 01:50 PM	#2 (permalink)
EZ-GOnewild Registered User Join Date: Apr 2009 Posts: 4 OS: xp	Re: 18hp B&S backfires & won't start Oh, I'll add this info: Model 31M777-0234-E1 Code 021228ZE 18HP Vertical Shaft

04-22-2009, 11:14 AM	#3 (permalink)
k2skier Registered User Join Date: Sep 2008 Location: Oregon Posts: 313 OS: XP Home-SP3 My System	Re: 18hp B&S backfires & won't start Set valves to .004-.006 and retest.

04-23-2009, 02:10 AM	#4 (permalink)
Volt-Schwibe Asst. Manager, Automotive Forums; HJT Trainee Join Date: Jan 2003 Location: Behind you, watching you as you type. Posts: 7,372 OS: Click "My System" to view details My System	Re: 18hp B&S backfires & won't start I'd like to suggest checking for a broken valve spring. I know it's a rare failure, but if the rocker suddenly falls loose, there's not many other things to suspect. __________________ <signature> TSF is funded by our Admin's pocket, care to help? Power Tip: Subscribe to your thread (Thread Tools) to receive an instant email notification when you get a reply. New Members: Creating a single new thread in the correct section is the best way to assure your thread will receive a reply. </signature>

04-23-2009, 07:53 AM	#5 (permalink)
EZ-GOnewild Registered User Join Date: Apr 2009 Posts: 4 OS: xp	Re: 18hp B&S backfires & won't start Thanks for the replies! I checked the valve springs and they are both ok. I adjusted the valve lash and tried to start it but no luck. It is still backfiring at both ends??? I then pulled the flywheel and checked the key and it was fine so now I'm lost again. Anymore ideas that you all could suggest? By the way, what is the little solenoid on the bottom of the float for? Thanks

04-23-2009, 01:45 PM	#6 (permalink)
Volt-Schwibe Asst. Manager, Automotive Forums; HJT Trainee Join Date: Jan 2003 Location: Behind you, watching you as you type. Posts: 7,372 OS: Click "My System" to view details My System	Re: 18hp B&S backfires & won't start I suspect the solenoid on the float is for shutting off fuel flow. On a side note, some of these engines have an easy-start feature that partly opens both valves, making it easier to pull the cord. This may have failed, keeping both valves partly open all the time, causing the backfire through intake and exhaust. I can't be sure without seeing it though :( __________________ <signature> TSF is funded by our Admin's pocket, care to help? Power Tip: Subscribe to your thread (Thread Tools) to receive an instant email notification when you get a reply. New Members: Creating a single new thread in the correct section is the best way to assure your thread will receive a reply. </signature>

04-23-2009, 07:03 PM	#7 (permalink)
coleman mobile Registered User Join Date: Apr 2009 Location: wisconsin Posts: 7 OS: vista	Re: 18hp B&S backfires & won't start Have you inspected the valve seat ? I have seen the valve seats pop out.

04-24-2009, 03:42 AM	#8 (permalink)
Volt-Schwibe Asst. Manager, Automotive Forums; HJT Trainee Join Date: Jan 2003 Location: Behind you, watching you as you type. Posts: 7,372 OS: Click "My System" to view details My System	Re: 18hp B&S backfires & won't start I've seen that too, in some of the larger engines, the valve seats are steel or a bronze alloy, press fit into the aluminum. If it pops out, i don't know of a specific fix to it, i've seen someone put it back in and it fell right back out. That would cause the rocker to be loose, because the valve would stop at a point lower than what it should be, causing the rocker to show the slack. Also, once i was running a minibike without an air cleaner, and actually sucked in a tiny rock, which jammed the intake valve open. Had to take the head off to pry it out, so it cost me a head gasket. Try to check the height of the end of the valve, where it meets the rocker, and see if the two valves have drastically different heights. They should be the same, where as if the valve seat fell out, one valve shaft is going to be a good 1/8 to 1/4 inch shorter. rotate the engine slowly while watching it, and verify that it's moving. when i had a rock jamming a valve, it wasn't moving at all, but when my friend's tractor had the seat fall out, it was moving, but only at the highest point of it's travel. Hope that all helps. __________________ <signature> TSF is funded by our Admin's pocket, care to help? Power Tip: Subscribe to your thread (Thread Tools) to receive an instant email notification when you get a reply. New Members: Creating a single new thread in the correct section is the best way to assure your thread will receive a reply. </signature>

04-24-2009, 07:14 AM	#9 (permalink)
EZ-GOnewild Registered User Join Date: Apr 2009 Posts: 4 OS: xp	Re: 18hp B&S backfires & won't start Thanks again for some good advice. I'll be out of town this weekend so I will not be able to work on it until next week. I'll let you all know what I find out then.