Briggs 17.5 single modifications.

I have been thinking about what to do with my Briggs 17.5 for a while now.
In general the engine works fine but I want to squeeze some more power out of it.

I don't have a clear plan but the idea is mainly to make it breathe better.
There's a few things that are on my list but I really need to fill in some gaps.

Firstly I have a Nikki carb that I want to replace.
There's a few options I can take like either a walbro or even a mikuni or keihin.
Iam not sure about what is best or what size I really need.

Secondly the exhaust I want to make I believe I should be able to size that up to 1 1/4 if I am correct.
Not sure so I could use some input on that.

I also might want to change some things on the valve train, tougher springs and also an exhaust pushrod on the intake side.

All in all I want it to run around 5k rpm without issues.
Not planning to push it hard all the time but I would still like to have it available.

Any help is greatly appreciated

- If you shave the head by up to .010" you'll gain compression which equals power. .006-.008 should keep you in the safe zone with stock internals.

- Port matching and smoothing the entire intake tract (including the carb) will make a big difference, sanding to a finish of approximately 400 grit is perfect no need to polish (you want a slightly rough finish)

- Max size for your exhaust should be somewhere inbetween the smallest ID portion of the the exhaust port and the exit "mouth", go too big and you'll kill velocity which will hurt low-midrange torque. Clean up the exhaust port while you're at it but don't remove material to make it bigger.

- Contouring the eyebrows (assuming it's a flathead) will aid in flow. Alot of guys shave them right down which I feel is too extreme unless building a high rpm racing engine.

- to hit 5k RPM you'll need to remove the governor while keeping the oil slinger.

- I'm not sure I'd upgrade the valve springs, they will be your new governer and should limit rpms close to 5000

- The stronger pushrod on the intake side you can't go wrong with!


There's a bunch of micro mods that in the end add up to a smoother, more powerful engine but I'll tell you, I've been very surprised and seriously impressed with what a basically stock, old 10hp flathead can handle without complaint!

This is not a flathead, it's an ohv 31 ci

I will remove the governor at some point mainly to also reduce clutter.
I just like a clean block.

Shaving the head is absolutely not a problem, we got a machine shop that has great prices.

I will leave the valve train stock except for putting an exhaust rod on the intake side.
I have enough lying around and it's a cheap way to be sure.

Careful shaving on an OHV, I've heard that if not done right or too much the valves will slap the piston.  Then again, just things I heard.  

Intake valve, definitely change out that push rod. Far as the port, clear obstructions, but leave rough.  Like @Wheelinhorse has mentioned before.  Help make the path less obstructive as possible for optimal air flow.

Exhaust side, polish the fool out of it.  You can make this side smooth as you want.  Get those gasses out of there as fast as you can.  Again, remove obstructions and castings that may be there.  

Being OHV, most improvements are already built in.  Since you are staying 5k or less, you should be ok not to upgrade your flywheel.

I'm sure you knew all that...

This an oil slung or pressure lube model.

It's pressure lube with an oil filter.

From what I read these can be shaved significantly but it depends on the internals and also the intended purpose.
I'll probably go up to .010 after checking clearance first.

Any options when it comes to cams? I have a lot of spare cams around that could be reground aswell but Iam looking for someone who knows his stuff.
Haven't seen any cams around so far.

If it were me I’d delete the syncro balancer also. They kinda like going “boom” as it were. Singles run just fine without them IIRC, it’s really more a gimmick IMHO.

Well as a sidenote I'd be much happier deleting the compression release those go bang even on a stock engine.

Seems to be an inherent but intermittent problem with the camshafts but I have replaced atleast 15 over the last 5 years.
That little pin just breaks shooting shrapnel through the block

Also sent precision cams an email just now to see what options are available

By shaving the head you'd have to shorten the pushrods on a OHV to keep proper valvetrain geometry so if you did have your cam reground to have a smaller base circle while leaving the lobe tip at its current height you would end up with more lift and the option for a little extra duration.

Just things to keep in mind.

If the grinding is small enough, could he not take up the difference in the rocker adjustment?

Probably but every engine is different, it could be fine or it could cause damage the only way to know for sure is to try I guess! Lol. The angle at which the rocker sits on the valve stem and where it "wipes" it would determine how close it is to start with, if the geometry is off it can lead to valve guide wear and possibly binding. If the cam base circle is ground the same amount as what was removed from the head then the valve geometry will stay essentially the same and is well within rocker adjustment settings. It gets more and more important the more a head is shaved. This is just general engine info and good practice in my opinion.

Head gasket thickness falls in the same category because they have the same affect on a pushrod engine as shaving the head, all things must be considered and accounted for if you want a smooth running and reliable engine.

I could shim 0.010 on the cam bolt, not really cause for concern I guess
Just make sure to use some Loctite on these guys.

Talked to precision cams and they can hook me up with a new cam for 175

If that is a single cylinder 31 or 33 series you are modding, that is already subject to failure due to camshaft compression release failures which if at high rpms destroys the engine internally.

Just re serch on line Briggs 31, and 33 series compression release camshaft failures.

Okie743 wrote:

If that is a single cylinder 31 or 33 series you are modding, that is already subject to failure due to camshaft compression release failures which if at high rpms destroys the engine internally.

Just re serch on line Briggs 31, and 33 series compression release camshaft failures.

I already mentioned this in the thread, I am a lawnmower mechanic and do know of all the weaknesses of this engine.

In the end it's rather a hit and Miss, I noticed that the engines that go on the snappers have less issues with it than most engines on John Deere for instance.

Quick fix is to just remove it and put a heavy duty starter in or just clock the engine after the compression stroke before you start it.

My Clone gx440 is shaved .100 and I just did a set of heads for a gx620 and took .050 off them, I’d definitely recommend shaving the head, big gains there for cheap.
Also definitely do a atv type carb, they just run so much smoother and better throttle response with them.

Yeah I think I'll go with a different carb as ec has now discontinued the jets.

I have the option for a 26 or 30mm at a decent price. Anyone have an idea on which to go for?
I think the 26 will give me some better power down low

Slightly offtopic but speaking of the devil I just had a customer's 31ci AVS engine roll in yesterday with the broken compression release.

Low hour machine too

Are you guys sure .010 is the limit?
Looking at the head there's plenty of space.
Looked on heymow and someone said they were running 0.06

Thoughts? I'd appreciate an answer asap as iam bringing the head tomorrow

Flat top piston? You could take more then I'm sure but might want to calculate what your new compression ratio would be at different amounts removed. You mean .060? .006 is less material removed than 10 thou (.010). 10 thou is generally a safe bet to get a perfectly planed surface, a little extra compression and still have a reliable engine.

Removing .060 is going to drive the compression ratio way up which will require higher octane fuel, definitely require shorter push rods and you'd want better con rods to handle the stress, the stock internals are only designed to handle so much. There's always a chance of having to shorten the head bolts a little as well depending on how much they stretch and how much space is left over.

Your budget is what ultimately determines how far you want to take her!

Flat top yes, I heard on heymow that some even said .125 and under .10 as a safe limit for internals which also seemed excessive to me.
When it comes to octane, 91 minimum comes here straight out of the pump so that's pretty good
I'll measure some things today and make a decision

Wow that seems like a heck of alot but hey, if it works for them and is reliable who am I to say nay!? Lol. No matter what you choose to do I hope it all works out well, keep us updated!

Like I said I did .100 off mine and .050 off another, I would do .050 at least if you want a noticeable gain. Also I’d go 30 vs 26 on the carb

I still whent with the 0.01 as it leaves the cooling fin intact aswell.
I could always look for a higher piston sometime in the future.
Taking more off would be nice but yes I also need some reliability.
Only cost me 50 bucks so that's decent
I am going to put a thinner head gasket in the future as well.

I did go with the 30mm carb, D slide for a better throttle response.

I guess the biggest gains are in the intake and exhaust for now.
Looked at the ports and they are absolutely flawless from the factory aswell.

Already ripped out the compression release and governor.

Iam just throwing this in here to follow up on the saga, maybe there might be someone out there that can use this.

I found a write up about Rocker ratios and their relation to pushrod length.
The steel pushrods are slightly longer and will alter the movement of the intake valve.
Theoretically this will mean that the intake will be faster off the line but might slow down slightly at the end.
I will do some research and simply need to play around with it.

I do have some rockers from a vanguard but they would require some machining to fit.
Again, need to look into that.

If all else fails I'll just cave in and order 1.6 ratio rockers, having them open slightly agressive doesn't really bother me if they actually open nice and full

The vanguard or 1.6 rockers both have their adjustment on the top of the pushrod making this less of an issue, the intek basically only allows you to set clearance on the pivot point which is useless