Building a Flathead

[Montecruz]

Happy New year to everyone in the forums helping out us MiniBike enthusiast,

For my first engine build I have decided to go with a Briggs 5hp Flathead from an industrial fan. I am sure there are a couple of threads on flathead builds on this forum already. Although all are great, I still have unanswered questions.

I should start with what I am trying to accomplish. I am trying to maximize the potential of the engine without having to use a hand held starter, as well as run high octane fuel. My questions are concentrated near what parts match together that work in unison. From what I am understanding the raptor block is a little different from the standard 5hp briggs. The only noticeable difference that I see is the extra bolt hole for the aftermarket carbs that require fuel pumps. I am deciding to stick the the original style carburetor, so that should not be a problem.

If I were to go with an open/limited cam what set of lifters and valve springs should be looking for? The end result of my build is to be somewhere around 12-15hp.


Any advice or links to other threads would be great,

 

[CarPlayLB]

original carb and 12-15HP should not be used in the same sentence! There, I said it! ...and here I go again. 12-15HP without an electric starter? I dunno 'bout that!
The later blocks all had the 3 hole intake mount. The Raptor blocks had the governor hole left undrilled...'bout the only difference! I have used a Dyno 94SS cam with great results. Probably made 6HP with a stock type tank mounted carb! Keep your expectations real. If you want to make that kind of HP, you need a lot of cam and carb. It will be a dog when taking off! No down low power to get you rolling. So you set your clutch at 4000RPM stall, wind the stuffin' out of it to get it rolling...power comes in with the large wheelie that knocks you off the bike,...fun over!
The 94SS cam has stock type .230ish lift and more duration. Still have to get it to the compression stroke and then pull the rope! If not, the sensation of feeling in your hand will take a couple hours to get back!

 

[Montecruz]

Haha thank you from bringing me back to reality. I was reading online about blueprinted carburetor and carbs race prepped by shops and figured I would be able to run the stock looking tank mounted carbs. If I do not have the third bolt hole, what are my options? Do people get an intake made for it? or should I take the time to find a raptor block?
Another question, about the starter, are you referring a engine mounted started or the hand held starter?

Thank you for the speedy response.

 

[joshua. c.]

I put a 107ss cam that has 315 lift in mine with a billet rod and flywheel port work a shaved head a mikuni vm22 carburetor and a header and that combo seems to work well for me, its more power than I need most of the time. I did retard the timing a bit for easier starting. and I always pull start it with the throttle closed so it wont brake my hand. I don't think its 12 hp maybe 6-8. I had to clearance the crank and the lifter boars to get the cam to fit. I also welded the lifter boars for safety and the head wasn't shaved a lot so I didn't have to clearance the head for the valves. I have herd of people going for a much higher lift cam and still using a pull start. if you do go bigger, larger valves and a larger carb would be a good idea. iv herd a 1 inch carb is the max you would want for a high lift big valve raptor.

 

[Montecruz]

Having trouble responding will Private message you.

keeps saying
Post denied. New posts are limited by number of URLs it may contain and checked if it doesn't contain forbidden words.

Thank you J.C

 

[David wulf]

My opinion is this for a good rider run a 94ss dyno cam . Lift specs are 268 lift and 241 duration, this is a pretty good cam for a rider and will have plenty of power. I'am getting ready to build a flatty with this cam and some other work . The 107 as is also a good cam but it starts getting kinda wild , it will rev to the mid 8's with the recommended springs .
I built one for my brother with a lot of port work about .30 off the head and a 334 tilly and the 107 ss cam . That motor is hard to pull start like it is and with the tilly not a good rider , no part throttle just wide ass open . But the motor screams on the big end , gotta be making more than 6-8 hp .
A limited flatty makes around 17-18 hp I've been told , the cams in them are in the 450 lift range and no pull start on them .
If you want a pretty bad ass motor go with the 107ss and a slide carb like josh has . The carb will make it rideable.

 

[David wulf]

 

[chipper]

Re-jet the carb on a clone and you can get 22.8 hp :laugh: Listen to these guys they know their stuff :thumbsup:

 

[Montecruz]

I do plan on having a big cam engine, but small enough to pull start.
I am leaning towards stroking the engine. I was on (Turk Brothers) website forward to me by J.C and was looking at the Wiseco pistons. Is their any known combinations of a flatty with a big cam pull start? Or am I asking for too much? haha

So far, I know I must purchase:

Crankcase cover with bearing
Billet Flywheel and rod
Milled Raptor head
Should I go ahead and swap out the crank, for a crank w/ bearing?

Thank you for all you guys help and pictures.

 

[creia]

You really need to purchase this book and read it cover to cover:
Jimmy's Book
My son, who was only 17 at the time, read this book before he performance-modified a Briggs Raptor 5 Hp for his Bonanza 1200. It proved to be invaluable in not only telling you WHAT modifications produce the most power for your $$$ (as well as which do not), and also WHY they do- he explains the mechanics behind the modifications. Jimmy Glenn is THE GURU in all things Briggs 5 Hp and is an ICON in the racing circles with these engines. My son ordered the book directly from Jimmy and he was kind enough to sign and write a few nice words in the inside jacket to my son.
Michael

 

[Not so mini bike]

You really need to purchase this book and read it cover to cover:
Jimmy's Book
My son, who was only 17 at the time, read this book before he performance-modified a Briggs Raptor 5 Hp for his Bonanza 1200. It proved to be invaluable in not only telling you WHAT modifications produce the most power for your $$$ (as well as which do not), and also WHY they do- he explains the mechanics behind the modifications. Jimmy Glenn is THE GURU in all things Briggs 5 Hp and is an ICON in the racing circles with these engines. My son ordered the book directly from Jimmy and he was kind enough to sign and write a few nice words in the inside jacket to my son.
Michael

That's awesome. :thumbsup: thank you for posting the link to get the book. I think I'm going to have to add that to the tool box collection

 

[Montecruz]

I ordered the book. Should coming within the week.

As far as parts go, I believe I know the route I am going with. My concern now lies with the inner modifications. reinforcing the block for my power plant.

I have only found one thread with pictures of reinforced housing for the lifters. Can someone either post pictures step by step on "how to" or if their is a good thread someone can send me the link to.

All help is well appreciated
Thank you,