Briggs Flatty Low-Buck Buildup

Doc Sprocket wrote:

I might be short one cup of coffee this morning, but I'm not seeing how taller lifters are going to help anything. With lash correctly set, lift height is entirely determined by cam lobe height. If the cam lobe baseline is zero (valve closed) and the lobe peak height is 1/2" (example), then the valve will lift 1/2". Did I miss something?

He's talking about grinding the duration of the cam down, setting lash, and the taller lifters that make up for the added valve lash will provide more lift on the ramp, and peak, on the love of the cam.

redlinemotorsportts wrote:

Doc Sprocket wrote:

I might be short one cup of coffee this morning, but I'm not seeing how taller lifters are going to help anything. With lash correctly set, lift height is entirely determined by cam lobe height. If the cam lobe baseline is zero (valve closed) and the lobe peak height is 1/2" (example), then the valve will lift 1/2". Did I miss something?

He's talking about grinding the duration of the cam down, setting lash, and the taller lifters that make up for the added valve lash will provide more lift on the ramp, and peak, on the love of the cam.

No- that would be grinding the base profile down. "Duration" refers to how long the lobe holds a valve open. If the base profile (zero, valve closed) were reduced, and a longer tappet used, then it would increase valve lift height.

Doc Sprocket wrote:

redlinemotorsportts wrote:

Doc Sprocket wrote:

I might be short one cup of coffee this morning, but I'm not seeing how taller lifters are going to help anything. With lash correctly set, lift height is entirely determined by cam lobe height. If the cam lobe baseline is zero (valve closed) and the lobe peak height is 1/2" (example), then the valve will lift 1/2". Did I miss something?

He's talking about grinding the duration of the cam down, setting lash, and the taller lifters that make up for the added valve lash will provide more lift on the ramp, and peak, on the love of the cam.

No- that would be grinding the base profile down. "Duration" refers to how long the lobe holds a valve open. If the base profile (zero, valve closed) were reduced, and a longer tappet used, then it would increase valve lift height.

Oh, du-oh.

I saw a vid on youtube of a guy saying "just grind down the lobe and you will have more duration" I thought about doing it and then I thought " that can't be right, you would just have less lift"   I can't bileve people sometimes.

You can grind material off the base circle and get the lifters to match and it will give more lift, so in return if you gring the base circle and then grind the top of the lobe slightly wider/flatter you would have stock lift and longer duration.

I don't mind the concept of carefully re-profiling the lobe by hand, but the base profile? Seriously... How IMPOSSIBLE would it be to hand grind the base profile SO perfectly round that you could actually attain a reliable lash setting? I say "no way, Jose".

I welded a little extra material on the closing side of the lobes on one and got it to work pretty good. I was just looking for more duration not more lift. Takes a lot of time to do it by hand. Better to buy a cam if you wanted since they are still available for 5hp flat heads. You might even get a deal on a box lot of used junk on ebay or something. Most of the go cart racers are about done with these things and run those ohv motors now.

Thats just what i was told bu a racer friend of mine. I never tried it.

Yeah, I was talking about grinding down the base circle and then adding material to the lifters. Doc is right about being very difficult to get a smooth grind. It's possible to do, but not worth the time and patience now that I think about it that way.   

Yup im in the with Dirt Go-Kart Racers. I own a flathead, but now all in rules of AKRA you have to run a box stock 6.5 196cc Honda Clone OHV. Cheaper on performance parts. Thats why everyone is throwing them out. I honestly am gonna keep my flathead cause i think there better than a Honda Clone. I Have a Arc Billet Rod. 290 Lift Long Duration Cam. And A Alcohol Carb I Use On My Flathead

A built 6.5 will smoke a built flathead any day. its just such a technological advancement..

TroyBilt Pony wrote:

A built 6.5 will smoke a built flathead any day. its just such a technological advancement..

You'd be surprised. Every alcohol flathead vs a OHV in kart racing, the flat head has ripped its balls off. Their crazy engines

Im Keeping The Flathead To Show Them 6.5 Clone Guys That Ol Briggs Still Has A Name

I started the build last night. Everything went together well, except the head was a real P.I.T.A.... The 11hp head required different head bolts than the ones that were original to the motor. I used the 11hp bolts. The two bosss that are for mounting the fan shroud were shorter. I used longer bolts and stacked washers underneath.

Here is the pic I took just as I was beginning assembly, comparing the old and new rods and the dogbones. The new Con Rod is beefier than the old one, but not by a whole lot. Hopefully the folklore if true. It should survive though as I'm running stock valve springs as a rev limiter.



This is what the intake port ended up looking like after some massaging. I hope Doc doesn't look at this and tell me I went too far with it. I've ported many many 2-strokes and also OHV 4-strokes, but never a flatty!



More pics as I get them loaded.

AAAAAAA!!!! You went too far!!! Kidding. From what I can see, looks good.

Whew! I was a little worried about that port. I've learned so much on this forum, I really appreciate everyone's help. If I had built this motor two months ago, I would have ported it and that's all. I never would have known about the 11hp head, or the stronger rod, or the Billet Dogbone, or even about shaving the eyebrows. Oh, and eliminating the internal governor parts too! Yeah, This is going to be agood motor.   

Lookin' Good! Gunna tear my 11 horse down tonight!

 

T-Dub13 wrote:

Here is the pic I took just as I was beginning assembly, comparing the old and new rods and the dogbones.  The new Con Rod is beefier than the old one, but not by a whole lot.  Hopefully the folklore if true.  It should survive though as I'm running stock valve springs as a rev limiter.

yeah, on my 5 hp I just ran direct throttle and when the valves started to float it would slow down automaticly.

this thread got hardcore hijacked

TroyBilt Pony wrote:

this thread got hardcore hijacked

All i see is someone taking time out of there day to help someone else with a very similar engine rebuild. Not hijacked.

If I really had a problem with it, I'da done something. Some good ideas and info are being batted around, and I'm all for that.

TroyBilt Pony wrote:

this thread got hardcore hijacked

You're so right.    I feel bad. I tried to contain my enthusiasm and I tried to stay within the scope of the thread's title. And if I failed to thank anyone for all the knowledge I gained from this forum, I thank you all.

Hopefully anyone looking to build a 28 can look at this thread and use it for reference. I tried to pass along everything that I learned from Doc and from all the other threads I've read and followed etc... I was just trying to "enhance" or add to Doc's build thread since I was doing the same thing.

K its ok lol

Started it up last night. Runs great. Very strong motor. Seems like the carb is the rev limiter, but I'm not complaining at all. It revs high enough and makes tons of power in the mid range, right where it's supposed to!

I ran it with a cored out muffler and it didn't like it, but that was before I tuned the carb... so I switched to a newer fully intact can, then tuned the carb and it ran great. I will build a pipe for it next to see if it gains anything.

OK Doc, I still want to see you finish your build on this thread. I didn't take as many pics as I wish I had. And I know there's a lot more to learn!    

TroyBilt Pony wrote:

K its ok lol

lmao.   

Im really diggin this thread , it got me into upgrading my engine now, but im stuck at upgrading the connecting rod im not sure which im supposed to use. my engine series is this one 289700. i know T-Dub mentioned his connecting rod , im just not sure if thats the one i need aswell super lost here lol