Briggs High RPM Poor Boy Special

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bowtiebutler

Member
#1
#1
I don't know about the rest of you, but it seems like I always end up spending way more on my engines than I intended too. That isn't going to be the case with this one. The point of this build is to have an engine I can safely spin to around 7000 rpm without breaking the bank. I was recently on ebay looking for a shroud, for 5hp I/C that I was rebuilding, and I found one for $12, and the listing also said he had the rest of the engine, and if you needed other parts to ask, well I asked. He said it was an I/C engine with compression, and he would take $20 for it. Sold! When it got here I was very pleased as it was a true I/C with cast iron bore, and dual bearings. The cylinder looked fantastic, so I gave it a very brief 5 second hone just to check for any scratches, and there were none to be found. I also picked up a used Wiseco STD. forged super sock piston for .99 cents, and a used ARC billet rod for $25. My plan is to use a 03-ss Dyno cam, and a billet flywheel, which OldMiniBikes sells an ARC billet non-adjustable for $99, but I'm hoping to find a used one. I know some of you may laugh at this build, but I thought it might be fun to try, and make some horsepower on a tight budget. Thanks

Matt
 
M

Mikez3202

New Member
#3
#3
I'm always putting a lot of money into my engines! Probably way more than I should. It's always the parts that cost the most for me. Nice find. Ill be following your build it'll be interesting to see how much power you can make on a budget. Good luck. :thumbsup:
 
B

bowtiebutler

Member
#4
#4
Here are a couple pics of the used Wiseco piston, and my homade torque plate for honing the cylinders on my 5hp engines. (Don't Laugh!!):lol: I got the idea from Jimmy Glenn's "Building a race ready 5hp Briggs" book. :laugh:

Matt
 
J

Jay727

Banned
#5
#5
Use a stock flywheel.
Take two forward clutch return springs from a 700R4 or 4L60E gm transmission and use it as an inner valve spring. Use normal retainers, get rid of any exhaust valve rotator. This inner spring sits close to the center so it won't break the retainer. I've seen 7500 with this.
Advance the timing, take the key out and throw it away, adjust it and tighten the nut/starter clutch.

What about a carb, I've bored out a fun kart style four bolt carb and got 7500 rpm out of it, not sure on the power, and you have to drill the air bleeds to make it run right throughout the entire rpm range.

Otherwise get a tillotson, sounds like you can get one for $20 :1orglaugh:
 
B

bowtiebutler

Member
#8
#8
Jay727 said:
Use a stock flywheel.
Take two forward clutch return springs from a 700R4 or 4L60E gm transmission and use it as an inner valve spring. Use normal retainers, get rid of any exhaust valve rotator. This inner spring sits close to the center so it won't break the retainer. I've seen 7500 with this.
Advance the timing, take the key out and throw it away, adjust it and tighten the nut/starter clutch.

What about a carb, I've bored out a fun kart style four bolt carb and got 7500 rpm out of it, not sure on the power, and you have to drill the air bleeds to make it run right throughout the entire rpm range.

Otherwise get a tillotson, sounds like you can get one for $20 :1orglaugh:
Click to expand...
That very interesting about the forward springs on a 700R4. About the carb, the words Tillotson, and Budget don't go in the same sentence together!:laugh: However I did pick up this 22mm Mikuni for $35 awhile back. I can make an intake for it. Have you ever used a Mikuni for a Briggs build? Thanks

Matt
 
J

Jay727

Banned
#11
#11
bowtiebutler said:
Have you ever used a Mikuni for a Briggs build? Thanks

Matt
Click to expand...
I've used whatever was on a 1979 RM80. I took my tillotson off my kart when I was 15 and put that carb on, it had an intake manifold that fit the tillotson 5hp manifold.

I remember it ran just as good but didn't want to idle.

I just put the tillotson back on it.
 
125ccCrazy

125ccCrazy

Well-Known Member
#12
#12
the mikuni will run great on the engine but you will want to start out with a 125 to a 130 main jet and a 17.5 to a 20 pilot... I'd get a few jets.... The stock jetting will be too lean...
 
H

Havasu Dave

Moderator
#14
#14
bowtiebutler said:
Thanks, I appreciate that info.:thumbsup:

Matt
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Hey Matt, I got one of the 5HP BS that 125ccCrazy built. It has a 22 Mikuni on it. I played with the jets a LOT. Finally got it dialed in, and it purrs with excellent throttle response.

IIRC, 110 highspeed jet, and 15 low speed jet, throttle clip in the middle. It took a long time to get there, lol. 900 feet, very low humidity, high temp desert environment, no muffler.
 
B

bowtiebutler

Member
#15
#15
I pulled the block out of my chemical dip this morning, which removed all of the grease, and grime, and most all of the paint, although there is still some stubborn paint I need to remove. I then bead blasted the IN/EX ports to get them cleaned up. The exhaust valve guide had some play in it, so I tapped it to a 5/16 thread, and used a bolt to remove it. The original guide was steel, and very prone to wearing out, but the new replacement guide from Briggs is bronze, which is much better. The guide was $4. Some of the new aftermarket performance guides are much better, but they require machine work, which doesn't fit into this budget build. I then ported the upper short turn radius of the intake port, and clean up all sharp edges in both ports. I did contour, and smooth out the eyebrow area, but I did not go into the bore, I just smoothed it out up to the edge of the bore. Here are a few pics. Thanks

Matt
 
B

bowtiebutler

Member
#16
#16
Here are a couple pics of the guide installed. These guides do require honing them to fit the valve stem. I use a 1/4 Aluminum Oxide Flex Hone for the job. I also put the valves in my drill press, and used a Scotchbrite pad followed by some 0000 steel wool to polish the area directly bellow the seat area. I realize there is probabally next to no HP gain from this, but my labor is free, which fits very well into a budget build.:laugh:

Matt
 
old-timer

old-timer

Scamming Member
#17
#17
Wow the motor cleaned up really nice Matt :thumbsup: What did you use for cleaner ? I've got the small HB parts washer, but can't seem to find a good wash solvent to use in it.

Vic
 
B

bowtiebutler

Member
#18
#18
old-timer said:
Wow the motor cleaned up really nice Matt :thumbsup: What did you use for cleaner ? I've got the small HB parts washer, but can't seem to find a good wash solvent to use in it.

Vic
Click to expand...
Vic, I have a 5 gallon bucket of chem dip carburetor cleaner I soak them in over night. The next day, I just rinse them with water, and this is how they turn out. It really takes the work out of it. :thumbsup:

Matt
 
B

bowtiebutler

Member
#20
#20
I've decided to use a set of 1.485 tall valve springs in this engine. You can see in the pic below, these are considerably taller than an OE Briggs exhaust spring. These are used, but you can buy them new from Dyno Cams store for $2.45ea.. I'm also going to use spring locaters on top of both spring to keep them centered on the valve. Some Briggs 5hp engines came from the factory with these on the exhaust side, but not most of them. I'm sure those of you who have worked on several of these before have taken the valve cover off, and seen the spring sitting out of center before, as well as a groove wore in the block from this. I included a pic of what happens when an engine has this problem. This is not my IC block, its just one I have laying around. Thanks

Matt
 
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