2HP Briggs engine trouble and mods

[Dale]

Hello friends. I'm in the process of making a cheap little run around go kart.

I've been having some trouble with the engine. She starts fine, idles alright. However, when I rev it up, it begins to backfire like mad, and the engine speed slows down. I can minimize this by messing with the mixture screw but it does not eliminate it. The points are brand new, as is the condenser. Could a weak spark cause this? Or a vaccum leak?


As for mods. I have an aluminum flywheel, much lighter than the cast iron (I think) flywheel that's on there right now. Can I swap them, and will I see any improvements in performance? I also have the head from a 3 horse motor.

[jr dragster Tyler]

Do you have the ajustable ARC racing flywheel if so you might want to play with the timing...Ohhh you dont have a coil you have points.

[Dale]

Right, points ignition. I thought timing too, until I realized that the timing is set by the flywheel. And yes, the flywheel key is still good.

[Russ2251]

Did engine ever run right since you acquired it ? If so...what changes to engine were made since then?

[Dale]

It always ran rough. I drained the tank and cleaned the carb out with some carb cleaner. The intake gasket was homemade and blocked the intake port, so I widened it up a bit to let the air through.

[Russ2251]

Backfire could be lean carb setting or possible debris stuck in something that carb cleaner missed or dislodged and is now partially blocking jet(s) creating the lean problem.
Do you have high and low speed adjustments or just low speed?
If it has a high speed needle, try backing it out completely (while it's running) and remove it and see if engine still runs. Also, are you sure homemade gasket is not leaking creating lean out?

[2or3wheels]

it could be your engines carb was made to run at a simgle rpm and will not run on higher rpms, try and see if anything is gumed up in carb and if there is a vacume leak. i use carb spray and look to see if it get sucked in anywhere. Once you find a leak(if you have one) fix it.

[Dale]

Backfire could be lean carb setting or possible debris stuck in something that carb cleaner missed or dislodged and is now partially blocking jet(s) creating the lean problem.
Do you have high and low speed adjustments or just low speed?
If it has a high speed needle, try backing it out completely (while it's running) and remove it and see if engine still runs. Also, are you sure homemade gasket is not leaking creating lean out?

Just the low speed. Its one of those extremely simple carbs without a bowl and just the one pick up. I thought about the gasket causing a leak, I'll definately check that out.

So you don't think it has anything to do with spark?

[Russ2251]

So you don't think it has anything to do with spark?Possible but unlikely. Bad ignition will affect (but not always) entire throttle range. If possible try a different carb and/or coil. I strongly suspect gasket leakage or warped mounting flange. The latter due to over torquing carb mounting bolts. This can be checked with a straight edge.
If you plan on keeping this engine you might want to look into getting a fully adjustable carb.

Mr. Wheels suggested: i use carb spray and look to see if it get sucked in anywhere.This works but might be hard to spot. I carry this a step farther by using starting ether. Spray suspect area (target carefully and sparingly as it can explode) and listen for engine rpm to increase which would indicate leak. Do this at your own risk.

[2or3wheels]

i do that also carb spray and less explosive.

[Dale]

Possible but unlikely. Bad ignition will affect (but not always) entire throttle range. If possible try a different carb and/or coil. I strongly suspect gasket leakage or warped mounting flange. The latter due to over torquing carb mounting bolts. This can be checked with a straight edge.
If you plan on keeping this engine you might want to look into getting a fully adjustable carb.

Mr. Wheels suggested: This works but might be hard to spot. I carry this a step farther by using starting ether. Spray suspect area (target carefully and sparingly as it can explode) and listen for engine rpm to increase which would indicate leak. Do this at your own risk.

I think you may be right about the mounting flange being warped. I removed the gasket today, to try and use some of that instant gasket stuff instead. I noticed that the carb will not sit flush even with the bolts torqued.

It's all starting to make sense now. If I tune it rich enough, it will idle, but backfire when throttled up. It doesnt idle nice, but it does stay running longer.

Is it possible to have the flange machined? Is it worth it? Are there any cheap solutions?

This was originally destined to be a cheap piece of junk thrown together in my spare time for fun. But with the cost of the clutch, sprocket, chain, and now engine repairs, I'm beginning to have second thoughts.

[Russ2251]

I have had warped carb flanges and repaired them easily but can be time consuming to do so. Place a piece of emery cloth (not sand paper) on the flattest hard surface you have. On this surface rub carb flange over emery cloth in a figure 8 pattern. Check flange as you do this and you should see a pattern working its way from outer edges of flange toward center of flange. Once you have a uniform 'shinyness' you are done. Avoid getting metallic particles into carb. Keep an air hose or vacuum cleaner handy and blow/suck out particles as you work.
Hope this helps.

[Dale]

I have had warped carb flanges and repaired them easily but can be time consuming to do so. Place a piece of emery cloth (not sand paper) on the flattest hard surface you have. On this surface rub carb flange over emery cloth in a figure 8 pattern. Check flange as you do this and you should see a pattern working its way from outer edges of flange toward center of flange. Once you have a uniform 'shinyness' you are done. Avoid getting metallic particles into carb. Keep an air hose or vacuum cleaner handy and blow/suck out particles as you work.
Hope this helps.

I'll definately give this a try, Thank you!

[2or3wheels]

thats a good idea, time consuming.

[JerryAssburger]

In regards to the head choices, you might want to stick with the 2hp head. The 3hp has a larger intake valve, and so the head is cast with a bigger area around the valve- the problem is, this drops your compression ratio (more volume in the head.)
***Yes I've been accused of more empty volume in the head, thank you!***
I think Russ is on about the intake tract leaking somewhere. Also, did you pull the mixture needle, and the "mixture nut" it threads into out? If you did, you'll see the other part of the jet screwed into the carb. Take it out with a regular screwdriver and peer into the hole it came from... there will be two small holes that act as fuel-bleeds into the carb. Blast some cleaner through both of them and then put it all back together. If you have the right size screwdriver handy, it all takes less than 5 minutes to do this. Also, hunt around for newer B/S 2or3 hp engines with the solid state magnetos... they'll fit right on to most of the old B/S engines and that'll eliminate your points for good. I had an old 6b (early 60's) engine that ran and ran for me... I finally had to retire it when it would foul the plug with oil about every other tank... it still ran, though!

[Dale]

In regards to the head choices, you might want to stick with the 2hp head. The 3hp has a larger intake valve, and so the head is cast with a bigger area around the valve- the problem is, this drops your compression ratio (more volume in the head.)
***Yes I've been accused of more empty volume in the head, thank you!***
I think Russ is on about the intake tract leaking somewhere. Also, did you pull the mixture needle, and the "mixture nut" it threads into out? If you did, you'll see the other part of the jet screwed into the carb. Take it out with a regular screwdriver and peer into the hole it came from... there will be two small holes that act as fuel-bleeds into the carb. Blast some cleaner through both of them and then put it all back together. If you have the right size screwdriver handy, it all takes less than 5 minutes to do this. Also, hunt around for newer B/S 2or3 hp engines with the solid state magnetos... they'll fit right on to most of the old B/S engines and that'll eliminate your points for good. I had an old 6b (early 60's) engine that ran and ran for me... I finally had to retire it when it would foul the plug with oil about every other tank... it still ran, though!

Thanks! I'll leave the head as it is then.

You seem to know exactly what this engine is all about. Good thing too! It's been giving me such a headache.

I did as you said, pulled the mixture needle out, the nut, and the jet. I saw the two small (different sized) holes, and shot em up with carb cleaner. I poked a guitar string through them to dislodge anything that might be stuck, and I put it all back together. The intake is now flat and mated nicely to the engine. However...

My problem still persists. If I adjust it rich enough, it will idle nice and smooth, but backfire when revved.

If I adjust it while revved, so that it does not backfire, it will be too lean to idle, run rough, and soon stall.

I noticed the needle has a small indentation on one side. You can just barely feel it with your fingernail. Would this cause my problem?

:mad2: This is me right now!

[Russ2251]

I noticed the needle has a small indentation on one side. You can just barely feel it with your fingernail. Would this cause my problem?Absolutely, yes. Needle should be flawless.
Just for the heck of it...what is color of porcelain around center electrode of spark plug?

[Dale]

Absolutely, yes. Needle should be flawless.
Just for the heck of it...what is color of porcelain around center electrode of spark plug?

Aaaaahahha, blacker than the night. I fouled it once, cleaned it up.

Where can I pick up a needle?

[Russ2251]

Aaaaahahha, blacker than the night. I fouled it once, cleaned it up.

Where can I pick up a needle?
Any lawnmower repair shop should carry it or they can order it. You could also order it direct from Briggs & Stratton. Get a new plug also.

[Dale]

Any lawnmower repair shop should carry it or they can order it. You could also order it direct from Briggs & Stratton. Get a new plug also.

Yessir, will do! Thanks again!

[JerryAssburger]

....sorry about not helping solve the problem. Between everybody, somebody's going to figure it out!

[Dale]

....sorry about not helping solve the problem. Between everybody, somebody's going to figure it out!

Hey, it's cool, You did help pinpoint the problem! Hopefully this solves it once and for all.