Briggs Twin - Hesitate from Idle, carb backfire?

Messages
576
Location
Madison, Wisconsin
My Briggs 18 hp opposed twin has been running a bit rough, hesitated, and doesn't seem to have the power it did. I swapped out spark plugs, fuel/air filter and no improvement. Warm compression on both cylinders was between 110-120 on both cylinders IIRC. I figured the carb was the next option, and since I'm trying to learn small engine mechanics by doing, I ordered a rebuild kit. I'm a rank novice here. Got the carb all tore down and cleaned up. Ran into two issues 1) I could not get the fuel inlet needle seat out. Evidently you need a self tapping screw, which I didn't have on Sunday AM. 2) The kit did not include the main jet (?) or whatever the name is for the brass jet that sits behind the bolt on the side of the carb. Figured I'd leave the old seat and jet in and replace it later if needed. Fired the engine back up and adjusted the idle screw. The engine purrs a lot nicer now. There is no more hesitation at acceleration - with one exception. When I quickly go from a dead idle to full throttle there is about a one second delay before it catches. But if I slowly run the throttle up, it speeds up like I would expect. So, is the first scenario where it hesitates where I quickly accellerate from idle something to be concerned about? Or is this typical on Briggs twins? I noticed I get a puff of air out the air intake on the carb when it happens. Thanks for the help!
 
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Messages
882
Location
North Carolina
What follows is just a wild guess..... Sounds like it is a little lean on the power mixture. Lean can make for delayed ignition. Delayed ignition can back fire out the carb. A quick check might be possible.... If you slightly can restrict the air intake on the carb this will richen the mixture. If this makes the backfire situation improve, then it is probably too lean on the power mixture. It is possible for an incorrect float level to cause this. (fuel level in the bowl too low.) Good luck. Rickey.
 

TWG1572

Thread starter
Messages
576
Location
Madison, Wisconsin
Good call. I hadn't thought of that trick. I'll give it a whirl this weekend. After mowing this week I did determine that the carb was set to lean. It idled ok, but started surging at higher speeds. I turned the idle screw out about a 1/2 turn and full throttle smoothed out and feels like it has the power it used to. So, I assume (hope) that the idle jet helps @ full power? Or else it shouldn't - I missed something cleaning the carb and making the idle richer helped. Still a bit of hesitation with the change - didn't have time to check on the backfire but will this weekend. I may try to isolate the issue and not use the throttle on the dash, but the lever on the carb just in case something is sticking.
 
Messages
4,310
Location
Central Wisconsin
Is there a intermediate or transitional jet in that carb? On some older Carbs I'm familiar with, they had such a jet/port the gave a little extra fuel as the throttle was opened toward full power. Easily gummed up as I recall, small orifice.
 
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