New project!

[RBrodnax]

Ok guys I've been lurking around here for a couple weeks so I thought it was about time I post. I thought I would share my project details and ask a couple questions about it. I have a freebie murray kart that was given to me. It had a 6.5 ohv tec on it when I got it. The engine needed carb work a pull cord and a new tank from what I could see but I haven't looked any deeper into it. I also got a freebie 5hp briggs flathead to go with it that needed nothing but a little carb work.

I decided to use the flathead with some upgrades so I would have enough power to possibly spin the tires for doing donuts. It would be fun with the live axle on it. This build is for kids. This is what I have done to the engine so far.

Ported and polished
Shaved the eyebrows
All new gaskets and a diaghram for the carb
Lapped the valves
Ground the valve rod a couple thousandths to allow for seating for the full compression stroke.
Milled the head.

Coming soon...

Air filter and adapter
Header
Arc billet rod
Copper head gasket
Governor removal
New clutch

Now I ran into a huge setback while doing the mods I have already completed which is why this project is currently at a standstill. The head bolt on the exhaust port corner of the head snapped off when I removed the head. While drilling the bolt for removal the drill bit snapped off in the bolt. I welded a nut onto the stub that was left and put a little heat to the block with a propane torch. It started to turn and backed out a little and binded up again. I started working it back and forth and the nut eventually broke off. I did this 3 times and it ended up broken off too far down to weld. I took it to a machine shop and they wouldn't touch it. I now have a bottle of nitric acid on the way. That should do the trick. If that doesn't work I will try to blow what I can out with a cutting torch and drill and tap it for whatever I can get in there. I plan on using a stud. It would be a good place to mount the header support anyways. So there's what I have currently... onto my questions.

First, about how much horsepower will I gain from the mods that I have listed? Second, I will be drilling the jet in the carburetor to a #34. Is there any more that I will need to do to the jetting? Thanks

 

[mckutzy]

I wouldn't use the acid as it will eat the block before the bolt is touched.
I think you may have to get a carbide drill bit( a solid carbide or even a milling bit will do).

 

[Poboy kartman]

It shouldn't be too short to weld with a small piece of steel tubing that fits in the hole.......about 1/2 the height of the nut?????

 

[mckutzy]

I'd reckon getting the gun or even the tig in there is tight.

 

[RBrodnax]

Even if I was to weld it a fourth time the weld would not hold. I have tried 3 times and it would unscrew about a quarter turn and break again. Unscrewing is not an option. I tried a carbide bit in a drill press it just started walking off the bit and going down the side of the hole. From everything I have read the nitric acid will not eat the cast aluminum block because it forms a layer of oxidation on it that protects it. The machines shop said they didn't have anything that would get it out. I tried a diamond bit in a dremel but it was eating the diamond bit faster than the drill bit.

 

[RBrodnax]

Thank you guys for the input about the bolt. However, the only 2 questions I had have not even been mentioned.

 

[firemanjim]

With a free flow intake and exhaust, jetted with no governor, you are probably getting in the 8.5-10 hp range. (This is all an assumption though. Or a calculated guess.....) And most likely closer to the high side of that ball park....

 

[Scout]

The predator 212 is said to be about 9.5 hp with just the intake, exhaust, and governor, so you're probably about there.

 

[RBrodnax]

Nice! Thanks! That's about what I was looking for! Now I just need to know more about jetting.

 

[Scout]

I ordered a jet kit from OMB I think, it was about $25 and came with two jets, a 26 and a 28, and an emulsion tube. I put in the 26 jet. My plug is nice and tan now, the cart has power everywhere, and drifts around corners. It is a hoot to drive.

 

[Poboy kartman]

Even if I was to weld it a fourth time the weld would not hold. I have tried 3 times and it would unscrew about a quarter turn and break again. Unscrewing is not an option. I tried a carbide bit in a drill press it just started walking off the bit and going down the side of the hole. From everything I have read the nitric acid will not eat the cast aluminum block because it forms a layer of oxidation on it that protects it. The machines shop said they didn't have anything that would get it out. I tried a diamond bit in a dremel but it was eating the diamond bit faster than the drill bit.

Well. ...IDK..what to say. ....anytime I try to remove a bolt....by drilling...I start with an 1/8" bit and work up.....and usually stop well before I get to the threads and start caving the bolt in on itself to release it from the threads..

Now. ...what occurs to me....why not dremel away material around the drill bit. ...weld on to the drill bit and back it out..... I mean really. ...it can't be stuck that bad.......

 

[firemanjim]

The predator 212 is said to be about 9.5 hp with just the intake, exhaust, and governor, so you're probably about there.

well, you forgot the bigger main jet, opened up idle circuit, and the emulsion tube, all got to be in the 212 to get in that power range. Free flow filter and exhaust are not enough. You got to get more fuel in there to make the power.....

 

[Scout]

Well. ...IDK..what to say. ....anytime I try to remove a bolt....by drilling...I start with an 1/8" bit and work up.....and usually stop well before I get to the threads and start caving the bolt in on itself to release it from the threads..

Now. ...what occurs to me....why not dremel away material around the drill bit. ...weld on to the drill bit and back it out..... I mean really. ...it can't be stuck that bad.......

Thats a good way to destroy bits. It puts all the stress and strain on the outer part of the point. Ideally you would drill a pilot hole, just a little bigger than the chisel point of the drill you want to use, then go right to that bit. Your bits will last longer and drill straighter. And do drill at the proper speed.

You could try to weld to the drill bit, but as it is likely hardened steel, it won't work well. If it's just rusted in there, you could try some Kroil/Aerokroil, P'Blaster, or maybe some white vinegar which will dissolve the rust.

 

[RBrodnax]

The bit that's broken off is my 1/16" pilot bit. The acid method would be a faster and much easier process than grinding around the bit and will also save the threads.

Here's some information regarding bolt removal with nitric acid for any of you whom this might help in the future.

http://www.practicalmachinist.com/vb/general/broken-tap-removal-chemical-methods-198135/

 

[RBrodnax]

Ok, recent discovery on my engine. I have a fixed jet carb that can't just be re-jetted. I thought about drilling just the first jet in the bottom out but I'm not sure that would do any good. What do you guys suggest with my listed mods?

 

[RBrodnax]

Ok quick update. After two weeks of dissolving the broken bolt with a broken bit in it, I finally removed the bolt. I reassembled the motor removing the governor and installing a billet rod. Put it on the cart. Pulled the cord a few times and bang... The engine fires. Pulled the cord again and it's locked up. Removed the head and there's a gash in my cylinder wall with a large lump of aluminum at the top jamming the piston in place.
I scrapped the flathead and ordered a brand new predator 212. Nice motor! I got on the cart after installing it and it had the power for me to do donuts in the yard. Anxious to see what it will do after a jet upgrade.