# Briggs 446777 21hp backfire thru carb



## Madcat737 (Aug 2, 2009)

*Briggs 446777-0246-E1 21hp backfire thru carb*

I'll try to make this question short as i can. I built up this engine with a new block,cam, rings,valve seals and gaskets. Also i cleaned the carb real good and lapped the valves till they would hold acetone.I set the rocker arms at 
.005 1/4"ATDC. It should run like a brand new motor. Wrong!
When i first fired it up it started right up ,but it continuely backfires thru the carb totally at random. I have checked for vacume leaks,checked the gap on flywheel ,checked the flywheel key a few times. I am seriously suspecting there is something wrong with the new cam i got from magneto power. Has anyone ever gotten a bad cam? I am trying to get the cam out of my head as being the culprit ,but i am running out of ideas. The carb is so clean inside you could eat off of it. I was wondering if the valve guides could have slipped but they look the same on both sides and they dont seem loose. Also my compression is 150 on both sides

Imade a video to speak for its self. If you look closly in the carb you can see the fuel being sprayed back into the intake area
www.youtube.com/watch?v=exHoopNLwMI


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## manic (Aug 2, 2005)

You sure its in proper time?


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## Madcat737 (Aug 2, 2009)

I started doubting my self about the cam timing so i yanked it off and reopened the case. Timeing marks are a good time to take a picture by the way. They are dead on .The dot on the crank gear is in the V and the cam dot is on a tooth . As far as i know you just mesh them up dot to dot
I dont have a compression release so to speak. I am guessing its built into the cam lobe?


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## manic (Aug 2, 2005)

Might be 180 degrees off, got the classic signs of being out of time..
Im no certified mechanic, but Ive worked on alot of different motors.
Heres something stupid, do you have the plug wires on the right
plugs?


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## Madcat737 (Aug 2, 2009)

I don't think there is a way to get it 180 degrees off. The pistons are at a mid position when the dots line up between the cam and crank timeing gear.
As far as the plug wires go ,they wont reach across the motor to get them crossed.
Its definatly acts like its out of time but its kinda idiot proof.


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## octaneman (May 13, 2009)

Hi Madcat737

From your description of what you did so far it was done by the book, but looking at the video I'm suspecting a vacuum leak and judging from the angle that the video was taken the choke is too far open. When you were cleaning the carb it's also possible that idle jet is blocked with a spec of dirt somewhere causing the problem also. The first step I'd take is check for vacuum leak and then re-do the carb, check the idle port and re-adjust all the carb settings.


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## Madcat737 (Aug 2, 2009)

I have checked for vacume leaks several times and could only find a small leak at the throttle shaft. I am not sure how sensitive these carbs are to small leaks like that. This carb is non adjustable except for the idle.I may spring for a new carb if i could only be positive that thats is what causing the flash back.
With all that said, i opened up the valve covers and re-adjusted the valves
a little different way which seems to work out the same as the briggs method . Can anyone tell me which is a better way or do they achive the same results? Here is a video of the adjustment process i used this time and has seemed to cure alot of my flashback in the carb. I am still getting the flash but it is not nearly as often

YouTube - Lawn Mower Repair Valve Adjustment V-Twin


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## octaneman (May 13, 2009)

The carburetor should not leak at all, any extra air getting in throws the air\fuel off balance. On your first post you said that you cleaned the carburetor, if it's the diaphragm type it also acts as a fuel pump, over time the diaphragm hardens causing the engine to run rough or not at all.


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## Madcat737 (Aug 2, 2009)

This carb is a double barrel non adjustable carb. The fuel pump is external to the motor and runs on crank case vacume via tube on the valve cover. This carb is completely run on orfices and tubes. I am going to disassemble it again today and try to find something i can call the problem. If it wouldn't pop back thru the carb it would run perfectly. It seems to idle smooth until a pop comes along and screws it up. It seems to run fine at high rpm and doesn't surge until it backfires thru carb.


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## octaneman (May 13, 2009)

Here's a link that has your type carb with eploded view of parts list and from the diagram the carb is adjustable.

http://www.partstree.com/parts/?lc=briggs_and_stratton&mn=446777-0246&dn=51950005


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## Madcat737 (Aug 2, 2009)

Yes, that's my carb. It does have an idle adjustment but thats it. I took it apart down to the casting and checked all the passages. I rigged up a small hose to slide over the tubes so i could blow them backwards if that makes sense. The tubes have tiny holes drilled into the side of them,and are sealed at the bottom. I heard a pop when i was blowing one out. I am hoping that was a piece of debris blowing out.I was able to see down into the small tube to make sure they were clear. I will be putting it back on today to see if i made any difference.


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## Madcat737 (Aug 2, 2009)

Well after the 3rd time cleaning the carb it runs exactly the same. I am positive the carb is clean and am starting to doubt the carb is the issue. Has any one had any issues with the fuel pumps? Could a pump cause this or a magnatron. I am going to have to start putting parts on it at this point. I just hate to buy a new carb and it do the same thing. I have always been able to save a carb,unless its pitted and corroded.
I did put it all together and mowed with it for about an hours. It ran fine and smooth. It did start to pop in the exhaust a small bit after it got good and hot. Which sounds like a lean condition. Back at idle it is still surging and poping as well. This engine is kicking my butt


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## Madcat737 (Aug 2, 2009)

I finally got some time to get back on this project. After running a leak down check and ignition check i finally broke down and bought a new carb. This fixed my backfiring /surging problem nicely. The back firing is what was throwing me off as a valve issue. 
I still to this day cannot find anything wrong with the defective carb. It has 176 hours on it and was used regularly. This is one of those low emission,non adjustable carbs,except for the idle screw of coarse.
I can only come to a conclusion that 1 of the cylinders was hogging all the fuel and starving the other. Something in the carb was causing the inbalance.
So here it is ,watch the video above and if your Briggs V-twin runs like that and your carb is clean,it can still be a bad carb.


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## Basementgeek (Feb 7, 2005)

Thanks for getting back to us. Does not make much sense does it. :4-dontkno

Nice job on the video.

BG


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## Madcat737 (Aug 2, 2009)

There is a valve adjustment video above also. I have used this method and it work excellent


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## Basementgeek (Feb 7, 2005)

I have the valve adjustment video book marked, it is good.

BG


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