Author Topic: Need help w/ cylinder head  (Read 1235 times)

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Offline Dancer

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Need help w/ cylinder head
« on: July 03, 2018, 03:10:24 PM »
I have a Murray rear engine w/ 12 hp B&S.  It needs a valve job anyway so I'm hoping to get some more hp and torque as I do the repair and as a nookie I have a few questions:  1)  Can I replace the stock valves with larger diameter?  2)  Is it possible to do a 3 angle grind on the valves and does it do any good.  3)  After porting the "eyebrows" I'm going to lose some compression.  Is there an available "how to" about welding material to the roof of the cylinder head to raise compression?  (And yes I did search before posting my questions)



Offline Dancer

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Re: Need help w/ cylinder head
« Reply #1 on: July 03, 2018, 03:12:22 PM »
Sorry, that should read, "... as a noobie..."

Offline rotten ron

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Re: Need help w/ cylinder head
« Reply #2 on: July 03, 2018, 10:48:54 PM »
A three angle valve job is the only right way to do a valve job and it can help a lot and it is the only way to move the seat to the outside of the valve face where it should be and to narrow the seat when it is too wide . Just doing a good valve job you will be able to feel that the engine has more power.I have never tried to weld a head on a flathead but have welded lots of overhead valve heads to make more compression and sometimes to repair damage. Once you have moved the seat out on the valve you can backcut the valves to make them lighter and flow more.


Offline Dancer

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Re: Need help w/ cylinder head
« Reply #3 on: July 04, 2018, 10:17:51 PM »
Thanks for the information, I have zero experience building a flathead engine; some experience with traditional over head valve car engines but none with B & S flatheads.  BTW, the pop riveted engine tag says it is model # 283707 and type 1139-E1.  I'm hoping to find a source that will definitively tell me how big I can go on the intake/exhaust valves and from what I've read, there is an alternative to modifying my cylinder head; apparently a 10 or 11 hp B & S cylinder head will bolt up to my 12 hp model.

Offline rotten ron

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Re: Need help w/ cylinder head
« Reply #4 on: July 04, 2018, 11:01:28 PM »
If you are going to do all this stuff to your engine you need a flywheel that can take the higher rpm that it will go after all this work and the new flywheel will have ajustable timing that should help for more power ,If you put bigger valves in it the valves will be shrouded unless you machine out the head and that will lower the compression too

Offline Dancer

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Re: Need help w/ cylinder head
« Reply #5 on: July 06, 2018, 10:19:40 AM »
Again, thanks for the valuable advice; I figured some of the engine internals would need to be upgraded.  What I didn't think about is that I could spend a fair amount of money on machining and larger valves, only to end up with less hp because of reduced compression.  Because of my ignorance about B & S engines, what I really need is a how-to-hot-rod-Briggs-&-Stratton-engines book.  I've searched on amazon and google; there are several youtube videos, some wiki how-to sites and lots of already build race engines but I haven't been able to find "the book".  If anyone knows of a book that I can buy, that would be greatly appreciated; something that tells me specifically what I need to do to reach my goal of a 30 - 40% increase in hp and torque.

Offline PJG56

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Re: Need help w/ cylinder head
« Reply #6 on: July 06, 2018, 06:09:36 PM »
Again, thanks for the valuable advice; I figured some of the engine internals would need to be upgraded.  What I didn't think about is that I could spend a fair amount of money on machining and larger valves, only to end up with less hp because of reduced compression.  Because of my ignorance about B & S engines, what I really need is a how-to-hot-rod-Briggs-&-Stratton-engines book.  I've searched on amazon and google; there are several youtube videos, some wiki how-to sites and lots of already build race engines but I haven't been able to find "the book".  If anyone knows of a book that I can buy, that would be greatly appreciated; something that tells me specifically what I need to do to reach my goal of a 30 - 40% increase in hp and torque.

Sorry,but no such book exists...
If you put a little effort into it you can find most anything you need to know on this site..
Paul Guptill  (oppy) #238modX
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2013 slmra mowdified champion(builder/owner)
2013 ARMA national champion Mod-X(builder/owner)
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2014 All American Champion Mod-X/CP(builder/owner)
2014All American Champion FXT (engine builder)
2014 All American 2nd place SP(builder/owner)
2015 All American 2nd place BP (builder/owner)
2016 OutlawTriple Crown FXT Champion (engine builder)


Offline Dancer

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Re: Need help w/ cylinder head
« Reply #7 on: July 07, 2018, 05:06:03 PM »
Progress!  Buddy came over and helped me remove the engine from the mower chassis (after we first removed that big round thing under the mower).  Those briggs engines are heavier than I thought.  I also took off the cylinder head to see what is going on.  The cylinder looks good; still has some cross hatch.  The exhaust valve is a nice brown color but the intake valve is black; not sure what that means.  Anyway, I'm searching this site to see what I'm supposed to do next.

Offline rotten ron

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Re: Need help w/ cylinder head
« Reply #8 on: July 08, 2018, 01:14:18 AM »
If you thought you were going to have low compression,my brother has low compression,he restored a 1942 Harley 45 cubic inch motorcycle ,he said it has 23 horsepower and is between 5 or 6 to 1 compression ratio. I might be getting old but I don't want to ride something that slow

Offline Dancer

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Re: Need help w/ cylinder head
« Reply #9 on: July 09, 2018, 09:57:56 AM »
I don't know what the compression ratio is for a stock 12 hp B & S but it probably isn't much more than a 1942 Harley.  I'm still wondering if the reduced compression from porting the eyebrows is worth the marginal increase in flow.  Today I'm calling local machine shops to see if I can find one that will do a valve job on a B & S engine... and I'm still searching this site for ideas and specs. 

Offline rotten ron

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Re: Need help w/ cylinder head
« Reply #10 on: July 09, 2018, 08:29:11 PM »
Do any of the shops build race engines,they are the ones that might be able to do a performance valve job,the black on the intake valve is from burnt oil

Offline Dancer

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Re: Need help w/ cylinder head
« Reply #11 on: July 10, 2018, 10:33:42 AM »
I called an automotive machine shop yesterday and they suggested that I bring my engine over to them; they think they can do a performance valve job on my 12 hp B & S.  I think I'll be compelled to keep the stock valves; I've searched and searched this forum but I just haven't been able to find the information I need to know about what size I would buy for enlarged valves.  Plus there is the issue you raised about valve shrouding; I could end up spending a lot of money on machine work and valves for no gain in hp or torque.  Maybe I can get more flow with a new camshaft which has higher lift cam and / or longer duration.  I'm going to try to call ARC Racing; perhaps they are willing to share some ideas with me about prudent engine mods, including a proper camshaft for my application.  Anyway, one of the next steps is to take the engine to the machine shop and have them compresses the valve springs and remove the keepers so I can do a mild port job on the intake and exhaust passages.  One of the other areas I'm pondering is carburation.  I've already rebuilt the stock Walbro carb but I'm wondering if there is a better option, i.e., after I slightly increase the air flow, will I need to change the jetting and does the Walbro carb give me that option?

Offline PJG56

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Re: Need help w/ cylinder head
« Reply #12 on: July 10, 2018, 05:57:08 PM »
The walbro lmt 56 is the carb of choice for modified 12hp flatty,dont waste your money on bigger valves for what your doing and I have plenty of nice used cams that would work well for you ,I also have the arc rod and flywheel you will need and the 28avs piston as to go with the rod as well,you will also need the arc brass weights for crankshaft,you will also have to upgrade valve springs and retainers.
Paul Guptill  (oppy) #238modX
2012 slmra mowstock champion(builder/owner)

2013 slmra mowdified champion(builder/owner)
2013 ARMA national champion Mod-X(builder/owner)
2014 ARMA national champion Mod-X(builder/owner)
2014 All American Champion Mod-X/CP(builder/owner)
2014All American Champion FXT (engine builder)
2014 All American 2nd place SP(builder/owner)
2015 All American 2nd place BP (builder/owner)
2016 OutlawTriple Crown FXT Champion (engine builder)

Offline rotten ron

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Re: Need help w/ cylinder head
« Reply #13 on: July 10, 2018, 07:49:56 PM »
It is too bad you don't have an overhead valve engine  like a Vanguard .When I helped Ryan Kerr in 2008 he had been racing all summer and ruined most of his engines but he wanted to go to the All American so we had to get an engine together (he was using 38 cubic inch Vanguards) He only had one head that was usable and the other head was smashed and he said it was junk.I welded the head and put new seats in it,soon it was good again,I had some Toyota intake valves that I put new keeper groves in and shortened,they were a lot too long .the engine ran ok and would rev 13000 He went to the race and won in FXT  He said it had lots of power and said he only drove it hard was at the start of the race and on the restarts .He said he could have lapped George Herron but he didn't want to get him upset
We didn't have low compression,it was 14.5 to 1 and had about 80 HP

Offline Dancer

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Re: Need help w/ cylinder head
« Reply #14 on: July 11, 2018, 11:13:13 AM »
A big thank you to PJG56 and rotten ron for all the great information.  And PJG56, l am interested in those high performance engine parts so please let me know how much you want for them.  I guess we would initiate all that via email (but if that doesn't work I can just give you my mobile number).  My only hesitation about changing the cam, piston, crank etc. is that I have zero experience building a "flattie".  Truth is, I've never rebuilt any engine.  With car engines, I always drop off the short block at the machine shop, do my own cylinder head porting, then drop off the heads for a performance valve grind and have them re-assemble the cylinder heads.  Once the short block is done and the heads are rebuilt, I bolt everything together for a complete longblock and get a buddy to help me install the engine.  I guess if I get in over my head, I could have a small engine repair shop re-assemble everything.  Again, thanks for all the help so far; I'm looking forward to this process (and finding a local racing venue).  BTW, about 20 years ago I did about a third of a sprint Kart racing season.  I actually bought the Kart for my son but he was intimidated with mixing it up on the track so I raced it.  Because of the kind of engine I had (a two stroke Italian high revving mill), I had to race against 12-16 year old kids!  I thought I would do very well because they all had Briggs & Stratton engines and I thought a 2 stroke would crush a flathead 4 stroke.  I was the one who got crushed and I learned to respect those B & S engines. 

 

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