tell me what you think 16hp b&s

[tommypaul]

Hi I have 16hp b&s cast iron model 326400 mtd 990 I have been trying to tune. Let me give a little background. When I got it the machine was a basket case, here's what I have done. I changed to electronic points, put a new stock coil back on it. I have put a new carb kit in it now. I have tried champion, oregon & autolite sparkplugs. I have had it running good but at 3/4 to wot I have a miss. I'm thinking now that my float is high. Heres why, if I turn off the fuel and run it out then restart it, it runs great for a few minutes, then it starts to miss again. What do you think? I need some to brainstorm with me. Thanks!



note all igntion parts are new install at the same time carb kit and gas are new

 

[Dmace]

What is your air/fuel screw set on the carb? A good starting point is 2 turns out from lightly seated. What is the color of the spark plug after a couple of minutes running? A picture would be helpful to decide if you are running rich or lean.

If that is not the problem then I would move onto the float bowl adjustment. Also, is the main jet in the carb the same stock size. Running too large of a jet or to rich of a mixture will let an engine run good cold and flood when warm.

 

[Wis Bang]

How is it under load?

Dad had one on a simplicity tractor & it was just OK when moving even though it felt like it was missing. Put the mower in gear & it turned into a gutless wonder...

Sister traded it in and the dealer said it was a burnt valve.

 

[tommypaul]

it appears to have a rich cond. by the plug. here where I started air screw 1.5 out, fuel screw 1 out. if I go out past 2 fuel screw there is no change if I go in fuel it will run at wide open thottle (wot) but wil die at idle speed. air screw in or out I'm not really getting any change.the carb kit #394989 main jet appears to me to be a little smaller but it the kit that it call for. and the engine had the same problem before I install it. what do you think note I can't get it to run lean at all it will die before that. thanks agin tommy



wis bang it runs the same thanks tommy

 

[Dmace]

I have rebuilt many small engines and carbs but have yet to see one with separate screws for the air and fuel. Typically, the screw controls the amount of fuel allowed into the carb and the air is controlled by the slide or butterfly valve. I believe one of the screws you are referring to is the idle adjustment screw and the other is the fuel screw.

By unscrewing the idle adjustment screw you are lowering the idle speed and that is the reason the machine will not idle. Take what you are referring to as the "air screw" and set it at 1 turn out from lightly seated, then take what you are referring to as the "fuel screw" and set it 2.5 turns out from seated. The rich condition makes sense since you had the fuel screw only 1 turn out instead of 2.5. The idle will most likely be very high so be ready with a screwdriver to lower (counter-clockwise) or raise (clockwise) the screw to adjust the idle.

 

[tommypaul]

thanks dmace for your help. here is exploded view my carb i work alot on boats 2&4-storke maybe it is idle mixture#94 i just assume it was air mixture screw because it idle is condtrol by the govenor.and the screw for that is #98. #1227 is high speed or fuel screw. let me know if this is the carb you were thinking of.here what I did today lower the float(bent the tang up very little)same thing but the shut off on the tank is a screw in type so if I only open it about half way it good with a few pop at throttle up.note I'm useing 1/4" fuel line

 

[Dmace]

That's definitely a different carb then the norm but all the basics are the same. #94 is your air/fuel mixture screw that determines how much fuel goes in, that should be 2.5 turns out to start with then you can fine tune by plug color. #98 is your idle adjustment screw and should be set so that it is just keeping the throttle stop (#132) open a little. Since it runs good at WOT I say your high speed needle (#1227) is set correctly and you can fine tune that by ear and exhaust smoke if needed.

Bending the tab up on the bowl float will close the needle sooner and keep less gas in the bowl, this would help any flooding problems if that is the problem. Careful though because with little fuel in the bowl, you can run out quicker then the carb can refill at WOT and if this is a mower or something that may be used at steep angles, it could run out of gas also.

 

[bontai_Joe]

Some motors will run a little rich with no load on them. I've seen several tractors (usually Briggs vertical shaft motors) that sound like they have a slight hesitation when running until the mower deck is engaged and then they smooth out nicely.

 

[dqdave1]

Screw on side of carb is your idle (slow speed) adjustment; screw on govenor is idle set; screw on bottom of carb is high speed adjustment.

 

[peezer]

hi everyone im new to this thread/forum thing so please be patient with me... i bought mtd 990 recently.. ran great when guy dropped of went to star cant get to start... carb leaks bad looking to find a place to buy a replacement carb for it.. prefer new.. but willing to buy used if have to..