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Briggs 21 hp over revving


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#1 jhn9840 ONLINE  

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Posted August 19, 2010 - 11:40 AM

Hello all,
Have a 407777 0121-e4 030325 that is over revving upon start up. Put choke all the way on and it slows down to normal. Start to release the choke any at all the rpm’s shoot way up with a lot of black smoke coming from the exhaust. I’ve checked the governor linkages and the adjustment of the governor itself. It all appears to be functioning as it should. I took the carb apart and cleaned it out. There was some dirt inside, it helped a very little but was not the solution. Any suggestions are much appreciated.
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John

#2 olcowhand ONLINE  

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Posted August 19, 2010 - 11:45 AM

Sounds like an intake vacuum leak to me. Without a load on the engine, with a vacuum leak the engine can still get enough fuel to just take off, with the governor not able to stop it. When they run with a leak like this, they have little power & can be shut down easily with a load applied, but you don't want her shutting herself down due to a rod through the side! I think you'll find a vacuum leak almost for sure. If you get the engine running slow, you can spray WD40 around the intake area & when you find the leak, the WD sucks in & you'll know you found it.

#3 ducky OFFLINE  

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Posted August 19, 2010 - 12:36 PM

Just a thought.
Is the throttle plate in place and are the screws tight?

#4 jhn9840 ONLINE  

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Posted August 24, 2010 - 06:29 PM

Thanks for the replys guys. Sorry it took me so long to get back. Job keeps getting in the way of all my fun. Anyhow, I did a little work on it today and discovered a screw gone from the throttle plate. I suppose this is most of my problem. Nothing seems to be hammering away inside and the screw is brass. So I am not sure what other damage I may be looking at just yet. I am about tempted to just put another screw in it and see what happens, but I know that at the least I should pull the heads and have a look. I'll keep you posted on the outcome.

jhn9840
John

#5 olcowhand ONLINE  

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Posted August 24, 2010 - 06:47 PM

Well, I struck out on my guess! Sure hope that screw passed on through without damage. I've seen air filter housing screws get sucked in & damage valve seats on twin cylinders. The owner keeps running on one cylinder trying to diagnose & hammers the seat. You'd have known this already if it were the case. These plate screws are small enough to pass on through, but worth pulling heads to be sure.

#6 jhn9840 ONLINE  

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Posted September 15, 2010 - 10:31 AM

Finally got around to pulling the heads from this motor. No sign of the screw from the throttle plate. Heads are carboned up pretty good. Just wondering what you guys soak them in to help cleaning them up?

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#7 olcowhand ONLINE  

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Posted September 15, 2010 - 12:38 PM

Steam is the best thing to pop the carbon off the heads, but in all honesty, I scrape it off. I know it's advised against, but if careful, you can scrape it off without gouging the surface.

#8 jhn9840 ONLINE  

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Posted September 15, 2010 - 02:21 PM

Steam is the best thing to pop the carbon off the heads, but in all honesty, I scrape it off. I know it's advised against, but if careful, you can scrape it off without gouging the surface.


Thanks for the advise olcowhand, Don't have any way to make steam so I guess that I'll be scrapping.

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