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Hot!blowback through the card

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Dusty
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2019/11/11 18:12:36 (permalink)
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blowback through the card

Has anyone experienced subject on an 18hp briggs? (422437-1266-01). Timing marks are aligned, valves have been lapped. Also the only valve to tappet clearance can be measured at 1" BTDC on the power stroke, both cylinders - otherwise no clearance. Engine was rebuilt after #2 cylinder connecting rod replacement.  What bothers me is the intake valve is still open about 1/16" at 1/4 of the compression stroke...I think this is contributing to the problem. All tappets have been mic'd - all same length. I want to grind the valve stems but engine ran good prior to rebuild, and this would not correct the compression stroke problem. Any help would be appreciated.  

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    Roy
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    Re: blowback through the card 2019/11/11 20:54:00 (permalink)
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    Something is wrong. With a head off check the cam timing, the exhaust valve should be closing as the intake opens at TDC.
    kshansen
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    Re: blowback through the card 2019/11/12 08:37:25 (permalink)
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    Roy
    Something is wrong. With a head off check the cam timing, the exhaust valve should be closing as the intake opens at TDC.



    Something sure sounds wrong, if the valves were only "lapped" I would not expect the valve clearances to change very much. Sure lapping the valves will close up the clearance a little but only a few thousandth, unless the valves were ground as well as the seats.
     
    I took a quick look at the parts break down for this engine and did not see a compression release so think that is not the problem but not being there makes it hard to be sure.
    AVB
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    Re: blowback through the card 2019/11/12 09:08:05 (permalink)
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    The problem with L-heads is that they will run with near zero valve clearances so if the valve have been lapped then clearances can now be zero or minus clearances. I have seen this when carbon up valves are cleaned up.
     
    Once verify things are in time then re-gap the valves. With valve springs installed Intake should be .005 +-.001 and Exhaust should be .008 +- .001. Without valve springs installed Intake should be .007 +- .001 and the Exhaust should be .010 +- .001. If using the TDC compression adjustment method then you want the Piston 1/4" down past TDC.
     
    As for ACR the camshaft uses ramp decompression grind.
    post edited by AVB - 2019/11/12 10:47:51
    kshansen
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    Re: blowback through the card 2019/11/12 10:45:34 (permalink)
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    Good points, most of the engines I worked on never got to the near zero clearances. Easy to see that if the clearance was at the low side of spec's and the seat and valve face was worn a bit the clearance could get to zero with a lapping. Maybe even more so if the engine was run with good oil there by reducing wear on lifter and cam!
     
    AVB
    The problem with L-heads is that they will run with near zero valve clearances so if the valve have been lapped then clearances can now zero or minus clearances.
     
    Once verify things are in time then re-gap the valves. With valve springs installed Intake should be .005 +-.001 and Exhaust should be .008 +- .001. Without valve springs installed Intake should be .007 +- .001 and the Exhaust should be .010 +- .001. If using the TDC compression adjustment method then you want the Piston 1/4" down past TDC.
     
    A for ACR the camshaft uses ramp decompression grind.




    Dusty
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    Re: blowback through the card 2019/11/12 11:05:32 (permalink)
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    Thanks to all for the replies. Timing marks have been checked and verified numerous times - I'm able to pull the ball bearing back far enough to align the pinion with the cam. Also verified using mark on #2 crank pin, so I don't believe its a timing issue. The only reason I lapped the valves was to see if the seats had dropped back - they were lapped only until both seat and valve lost their shine. Before lapping, valves had dull surface - not burnt - seats were shiny. Valves maintain 1/16 margin. The blowback through the carb throat is substantial - after a few seconds, my palm was dripping gas. Carb was also rebuilt.
    Dusty
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    Re: blowback through the card 2019/11/12 11:17:06 (permalink)
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    The engine will run, but won't idle. Definite loss of power. It stalls immediately when PTO is engaged. I rebuilt the deck last year, everything turns smoothly. New belt.
    AVB
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    Re: blowback through the card 2019/11/12 12:14:54 (permalink)
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    Definitely sound like one cylinder not developing enough compression to produce the power stroke. I suspect the lack of valve clearances is your problem. I also think the idling problem is also related to this.
     
    Side note: Many twins will run on one cylinder until loaded by such as a deck. This especially true of v-twins.
    Roy
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    Re: blowback through the card 2019/11/12 12:30:34 (permalink)
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    Check the valve seats, they have a habit of coming loose on that engine series. If the seat rotates when you are trying to lap the valve you have a problem.
    Dusty
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    Re: blowback through the card 2019/11/14 10:15:39 (permalink)
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    Checking the gaps at 1/4" before TDC on the compression stroke, believe it or not I get .005" and .008" on both cylinders. So I guess that eliminates a valve problem. Also checked the seats and they do not rotate. The engine is all buttoned up and as soon as we get a break in the weather I'm going to push it outside and let it run for a half hour or so just to see if this break-in period will help solve the problem.
    Mikel1
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    Re: blowback through the card 2019/11/15 20:05:36 (permalink)
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    I seen AVB mention it but are both cylinders running?
    Dusty
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    Re: blowback through the card 2019/11/16 09:14:47 (permalink)
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    Yes, checked both cyls. #1 misses occasionally, #2 steady. Got good spark - blue, about 1" long from cap to plug when I pulled the wires on each, so the magnetron is still good. Ran it constantly at different RPM for about 15 minutes, then i pulled the air cleaner (while running) to see if gas vapor was coming out of the carb throat - no vapor! Previously, I had been trying to run the engine without the air cleaner attached, but noticed it started and actually ran better with it on.....
    I'm glad I checked, the inside of the cleaner housing was soaked with oil as well as the air filter from #1 breather tube. #1 breather only fits one way, baffle is in place - I couldn't determine if the check valve(?) was operational or not. How do I fix this?
    Mikel1
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    Re: blowback through the card 2019/11/16 19:31:47 (permalink)
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    If oil is spraying out breather hose then compression is leaking into crankcase, this is assuming oil level isn’t high. I may have missed it but has a leak down test been done?
    AVB
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    Re: blowback through the card 2019/11/18 08:51:22 (permalink)
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    The only problem with a leak down test is that it only tests the upper cylinder for problems. If the cylinder is tapered or out round at mid to bottom of the stoke it will not show up during the test.
     
    Plus not only a worn cylinder or rings will cause excessive crankcase pressure but worn exhaust valve guides can too. I have several single cylinder L-heads were most techs would think it was a worn cylinder causing the engine to smoke to only find an exhaust valve guide worn out causing excessive crankcase pressure blowing oil thru the PVC valve into the carburetor.
    Dusty
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    Re: blowback through the card 2019/11/18 11:08:25 (permalink)
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    Thanks, both make sense. The dipstick oil lever is where it should be. This engine was overhauled, bores mic'd, new #2 piston and con rod. I need to find my old compression tester and check both cyls just for information now anyway. What am I looking for regarding valve guide wear? The answer seems obvious so I'm guessing no up/down, side/side movement? I see that the exhausts guides are available for this engine....how difficult is that to remove/replace? I want to thank all of you guys for helping me with this. I don't think the engine is beyond repair and it seems like I'm getting closer to getting it running again.  
    AVB
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    Re: blowback through the card 2019/11/18 12:02:35 (permalink)
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    Although the Briggs valve guide repair kits have a GO/NO GO gauge I have found if valve wiggles excessively side to side the guide and possibly the valve stem is worn.I have replaced numerous guides that the GO/ NO GO gauge would pass otherwise. I usually compare the exhaust side to side in reference to the intake side to side.
     
    Now as replacing the valve guide I usually tap the guide and use a heavy washer, a screw, and a nut to pull the old guide as it to be casted version if the ipl is correct.Now of course the new guide in pressed into place and finished reamed. As the finish reamer it depends if the valve is 1/4 or 5/16. I don't remember which it is.
    Dusty
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    Re: blowback through the card 2019/11/24 10:45:51 (permalink)
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    I found the repair manual for this engine on the internet. Another thing I noticed when I had the heads off is that the notches in both pistons are facing forward, not toward the flywheel as illustrated in the manual. I'm sure there's a reason for this, probably doesn't contribute to the carb issue but may have something to do with the oil in the air cleaner?  Looks like I'll have to open her up and check the lube holes in the connecting rods while I'm at it. You're probably wondering why this repair/rebuild was wasn't done right in the first place. That's another story.......let me know if you're interested.
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