Briggs and Stratton Engine Trouble

[331kimmel]

I have a Briggs and Stratton animal engine on my go kart and I am having trouble when the engine comes to full rev. It seems to “bog” out when I hold the throttle wide open. I cleaned the carb out and all of the jets and everything seems okay there. My only thoughts are that it’s getting to much fuel and running fat. But not really sure what to do to fix this issue. Smaller main jet ? Any help would be appreciated.

Thanks

 

[itsid]

hm..
Animals come with PVL ignition IIRC..
and digital rev limiting (4.1k 6.1k and 12k options available)

you don't happen to know what rpms exactly the engine bogs down at, do you?


'sid

 

[331kimmel]

Unfortunately no I do not know the exact value of the rpm when it bogs out. Just know that when I hold the throttle wide open is when the problem occurs, otherwise it’s fine

 

[itsid]

and an approximate value?
.. that shouldn't be too hard.

cent clutch I guess (since it's a racing engine)
road speed (phone gps) -> wheel size, gear ratio and you're back at rpms

on google playstore there's an audio tachometer (free last time I checked)
not the most exact thing in the world I guess but a good clue perhaps

anyways...

go WOT and wait for it to bog out.. keep it at WOT for a bit (at least a good second)
then immediately shut the engine down, and remove the spark plug (don't wait for the engine to cool down!).
if it's wet chances are you're running too rich indeed,
if it's not... well not so much.

A dark plug can mean running a bit rich in general of course .. and should be adressed,
but for the engine to stop firing you need a wet plug in such a case.
Show us a pic of the plug if you're unsure.

But frankly.. I'm assuming you're just running into the rev limiter.
Oh wait.. can you see a B&S partnumber on the ignition coil (starts with a triple five IIRC)
yes you'll need to remove the shroud in order to find out I'm afraid.
if you do with that we can check what max rpm it's limiting to

'sid

 

[331kimmel]

I haven’t had a chance to get back to work on the kart yet or look at the model #. But I will do so this evening and get back with you. Again thanks for the help I appreciate it.

 

[KartFab]

loss of power at WOT is typically a lean condition FYI.

 

[JTSpeedDemon]

If it's "bogging" in the sense I'm thinkin of, the engine just kinda goes BLEH and won't rev anymore. Rich would be breaking up (crackling and struggles to rev up).
This is my personal experience with my Briggs flathead, which has probably seen every AFR from 1:2 to 1:50!
But seriously, I have screwed up the adjustment several times, and it's always an exercise in trial and error.

---------- Post added at 06:54 AM ---------- Previous post was at 06:53 AM ----------

Here's the million dollar question to see if it's the coil or not:
When you floor it(WOT), does it bog off the line, or is it only at high RPM?

 

[331kimmel]

Itsid:
I checked the spark plug and wasn’t wet but did look a little dark. As far as the model # I’ve looked and can’t seem to find it on this engine.

JTSpeedDemon:
Yes the engine is doing as you said where it’s breaking up crackling and struggling. Not sure what you were saying when talking about AFR??? But when you WOT it bogs right of the line.

 

[JTSpeedDemon]

Your engine is running too rich. Without a doubt. You need a smaller main jet size, or if your carb is adjustable, turn the main mixture screw in a bit(clockwise).
Your coil is fine, it's just running rich.
AFR stands for Air Fuel Mixture.

 

[KartFab]

ok please listen here. I tuned an engine to run perfect on a $13 carb. You wont have the same tools here but this is the KEY

The carb controls how much air and fuel comes in (also know as air fuel ratio AFR). Without buying anything, looking at the plug is the best way to tell visually.

There are different parts of the carb that control the air fuel mixture at different slide heights. If your carb is not adjusted right (or is dirty, which is USUALLY THE CAUSE of all your problems), then it could run rich at idle, lean at part throttle, and lean at wide open. crazy right? So what do you do ? Well first, you really have to make sure that your gaskets are not leaking (e.g. head gasket, intake manifold gasket etc)... then actually physically disassemble your carb and thoroughly clean it. There may have been a tiny hole somewhere that you completely missed, and if it ran before just fine, sat, and now doesnt run so great, theere is a pretty good chance you just need to CLEAN IT RIGHT. So do that first, then what? Well if it never ran well, then you gotta tune it. If it ran great before, and now doesnt, tuning wont help you if your carb orifices are gummed up with varnish. So lets ASSUME that the problem is the carb/tune and lets ASSUME that your air fuel ratio is caused by the carb not being tuned correctly. With these ASSUMPTIONS, how do you tune the carb?

TUNE FROM THE BOTTOM UP
1)THROTTLE POSITION mark the throttle position (e.g. twist grip or cable on pedal) at idle, 1/4 throttle, 1/2 3/4 and full (wide open) positions. With no air filter on the carb you can look at it to see the slide position aka throttle position.

2) IDLE MIXTURE when the engine idles a tiny bit rich, its perfect. The "optimal" idle mixture is the one that when the throttle is not moving, it allows the engine to idle highest. This is acheived through an idle pilot/air/mixture screw, not the screw that forces the slide height higher.

3) MID-RANGE at 1/4 to just before 3/4 throttle, the throttle response should be crisp. Run at 1/2 throttle (preferably under load) for 1-2 minutes, then IMMEDIATELY kill the engine. check the plug. should be brownish, not white, not black. The needle height controls mid-range. if you put the needle clip to the top position(forces needle lower), it will make it run leaner ad mid range, if you put the needle clip to the bottom position (forces needle highest) it will run richer mid range.

4)WIDE OPEN this is the LAST step. if you get a poor response just after 3/4 throttle, and especially at wide open throttle, you need to change the main jet. If you can get it to run at wide open, and it shoots black soot out the exhaust, it is way too rich, and you need to get a smaller main jet. If it bogs, it could be lean, but just get it to run for 30 seconds or so right before bogging, and do a plug read. If its lean (after performing steps 1-3) at wide open throttle, you need to drill out the main jet to the next biggest size and try again.

Based on what I have personally experienced, you can be anywhere from 9.8 to 15:1 AFR and still have the engine RUN. If it bogs, and it is fuel delivery related, you are OUTSIDE THESE LIMITS

 

[BigWes]

I must agree with the above poster....loss of power at WOT is most like a lean condition. It will still run under a rich condition at WOT. It will just blow black sooty smoke. If it leans out the engine will start to get excessively hot and the rpms will back off. Not so much with a rich condition. A rich condition will waste fuel and end up with a cooler combustion chamber. It will also eventually cause upper cylinder wear because the fuel will at some point wash the lube off of the cylinder walls...but as a rule you may see less than spectacular performance but usually a longer engine life.

 

[JTSpeedDemon]

It seems like what Grant's saying is conflicting with my experience.
So, let's try to figure this out without blowing up on each other. Cool? Ok.
With my flathead, I can effectively shrink the main jet with the high speed adjustment screw, and I also have a idle jet adjustment screw. The engine wants to have the perfect combination of fuel from both jets at idle, WOT and everything in between.
I have screwed up the tuning at least 5 times, as recently as yesterday.
Yesterday, I screwed up the tuning again only to realize it was out of gas.
So I managed to get it started and make it peel out, and then it would just go BLEH in the yard when I gave it any gas.
No crackling, it just wouldn't go. So I backed out the main needle and it ran great again. That means in effect, I INCREASED jet size, richening it. And it makes sense that if it's lean, more air would want to kill the engine.
On the contrary, multiple times have I been out in the neighborhood, and it would crackle like Rice Krispies and struggle to rev up any more when I gave it gas. So I screwed IN the main needle, leaning it out. Then it ran great again.

As for smoke, when it's reeeeaaally rich, I might see a little bit of white smoke, and when it's really lean or at such a low RPM that it doesn't want to stay running, then I'll see black smoke.

This is all my experience, and I don't want to start an argument, but I am genuinely curious why your experiences say otherwise.

 

[331kimmel]

Thank you everyone for your replies and advice. With taking all of this into consideration I think it needs a smaller main jet so that will be my next step to try. Once I try this I will let you know the result. Thanks again!

 

[JTSpeedDemon]

Thank you everyone for your replies and advice. With taking all of this into consideration I think it needs a smaller main jet so that will be my next step to try. Once I try this I will let you know the result. Thanks again!

That's what I would do.

 

[331kimmel]

331kimmel

I have a couple of questions for you.

Did it run OK before?

Was it possibly set up to run on methanol /alcohol?

Here's a tuning guide.

https://www.briggsracing.com/sites/default/files/carburetortuningguide_ms10429_0.pdf

This is the condition that my brother purchased the kart so I do not know how it ran before or if it was set up for methanol/alcohol.

 

[JTSpeedDemon]

It could've even been set up for E85. E85 needs about 30% more fuel to the same amount of air as a gas tuned carb. So that would mean a bigger jet.
Or of course someone just may have screwed up when jetting it, or maybe it had a different exhaust or something.
There's no way to know, but for sure it needs a smaller jet.

 

[331kimmel]

So I did a little research and was able to find out that the current main jet it has is a 555537 which I believe is for a methanol set up (correct me if I’m wrong).

 

[KartFab]

So I did a little research and was able to find out that the current main jet it has is a 555537 which I believe is for a methanol set up (correct me if I’m wrong).

this might be helpful when choosing jet sizes (see end of manual)
www.briggsandstratton.com/engines-racing/~/media/Racing/carbtuningguide.pdf