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DIY Home Built Motorized Bicycle (non kit) Post all about your home built rides here. Weedwacker motors, lawn mower engines ect. This area is for non kit builds

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  #11  
Old 12-03-2016, 07:39 PM
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scratchbuilder scratchbuilder is offline
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Default Re: Adapting Briggs Build to accept bigger engine

I know what you mean about taking apart good runnig engines. Mine too has that short shaft and I had to also run a pully with belt lever clutch. Maybe I to will someday see if the crank's are swappable.
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  #12  
Old 12-04-2016, 08:45 AM
curtisfox curtisfox is offline
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Default Re: Adapting Briggs Build to accept bigger engine

If the model # are the same series it should it should change over, Just have to try. Even if they look different, Briggs prolly used the same crank for a lot to save coast............Curt
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  #13  
Old 12-04-2016, 03:38 PM
MEASURE TWICE MEASURE TWICE is offline
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Default Re: Adapting Briggs Build to accept bigger engine

I have checked the crank shaft where it steps up in size right by the crankcase wall before it meets the seal. Both the 5/8 and 3/4 step up to 7/8 inch at the seal. If they call that the journal (think supporting in place surface) for that side by the output shaft, then probably the same 7/8 inch on the side as it leaves the crankcase on the opposite side by the flywheel.

My newer used engine has no points and I have seen the space where the point cover would have been under the flywheel if it would have had points. I don't think that would have any consequence except where I have not yet compared is the area where the crank shaft is shaped to work in conjunction with points movement if it were of that kind.

With 1/2 a hp difference I was wondering what made the difference. It will be later for these answers as I have a good used 3/4 shaft 3.5 hp and only if it does not work with my bike will I be checking more to swap parts.

The height of the cylinder looks to be just a bit taller, but only due to the taller fins on the head. That does not give any extra displacement though as it is outside the cylinder. The diameter and piston throw which is created in conjunction with the connecting rod and crankshaft and the head space gives the compression.

If it turns out the compression is the same with both the 3 and 3.5 hp, I have heard of this in other engines where the cam shaft have a bigger bump to allow longer intake of more fuel and for example an 8hp becomes a 9.9hp.

I think if the camshaft is something as well that can be swapped along with the taller fins on the two Briggs engines a 3 may be a 3.5 hp.

If you look up Easy Spin Start there is the opposite in what I believe is the bump on the camshaft for the intake valve that means a longer intake opening, but during the compression stroke. This leaks out some of the compression and makes it easier to start. It was created when law imposed lawn mower blade to stop if you release a handle. When it stopped with this brake it could have the crank in a compression stroke making it hard for some to start.

I myself would take the high tension lead off and turn the blade back to get some slack and then put the high tension lead back on to start easier. Anyway this reduced the rated horsepower I think. Conversely I heard that at starting speed the engine only looses the compression mostly, as the compression cannot escape as quickly at running higher speeds.

When I get this engine that had seized, but freed by only taking off the head, I'll see what damage must be in the crankcase. This is where I can't otherwise. Then I'll be measuring the crank of this older engine and also going to see about the bump on the intake cam that I hear might be there as part of Easy Spin Start.
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  #14  
Old 12-04-2016, 04:14 PM
curtisfox curtisfox is offline
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Default Re: Adapting Briggs Build to accept bigger engine

A lot of times the hp difference in hp is just the carb size, Like Kohler engines K 321 ( i think that is the number ) The block is the same, the only difference in 12 and 14 hp is the carb size...........Curt
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  #15  
Old 12-04-2016, 04:45 PM
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cannonball2 cannonball2 is offline
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Default Re: Adapting Briggs Build to accept bigger engine

Lotta times the difference is the decal. Snapper was known in the past to "derate" Larger engines to lesser HP to compete in certain niches. This was legal but not the other way around. While working at a Snapper dealer an entry level 28" rear engine rider showed up with a 9hp L head engine. Was actually a 12hp. Never was sure the reason but it forced a bunch of buyers to 33" 12hp models for just a bit more which were overstocked at the distributor. Next spring we received 12hp decals to be applied to any of the "9hp" remaining stock. With a price hike of course.
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  #16  
Old 12-04-2016, 10:17 PM
MEASURE TWICE MEASURE TWICE is offline
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Default Re: Adapting Briggs Build to accept bigger engine

That is interesting about just for show numbers HP rating and psychological effect for sales.

The problem I had with the Easy Spin Start is even if the HP was same at higher speed, I was seeing some reverse flow of fuel past the venturi at the airfilter. Paper airfilter after getting wet would stop flow of air into engine and choke to stop. It could have been that the valve wearing into the seat lost all of its gap and combined with Easy Spin Start it was over board with 1/2 the upward compression stroke and the intake valve then just closes.

I made a larger clearance gap at the valve tappet to stop that, but another way may be if I see a little nub of metal and relate it to this Easy Spin Start on the in take cam (aside from a normal lobe), I may grind it away.

But if it then has too large a gap, I'd have to cut the seat as the valves adjust by grinding either the end of the valve stem or the valve seat to adjust.

Overhead valve engines with real adjustable valves are nice and I have a generator that may give up and salvage the engine for a motor bike, but it is only 2hp. I'd remove the governor from that engine as it probably only has a range of adjustment to keep the 60 Hz and runs fast with no idle at start up.

Last edited by MEASURE TWICE; 12-04-2016 at 10:20 PM.
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  #17  
Old 12-05-2016, 08:25 AM
curtisfox curtisfox is offline
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Default Re: Adapting Briggs Build to accept bigger engine

You will find out that generator engines have special crank and parts that make it hard to salvage for normal use. The newer small ones have the charging system right in the flywheel, so look it over good before you start. ..............Curt
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  #18  
Old 12-06-2016, 12:15 AM
MEASURE TWICE MEASURE TWICE is offline
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Default Re: Adapting Briggs Build to accept bigger engine

I might have had you thinking I was taking apart the generator's engine from the set. But the cranks were just from very similar Briggs engines.

The gen is a Colman and is Chinese made. The engine is probably good, but the tank is rusty. It is hard to start even after cleaning carb and tank but runs.

I bought a new magneto when the old one was intermittent which had my arm almost come off. The guy who gifted if to me said it works see. Yea he pulled the cord like 50 times.

Still the generator part which is separate has its own issues even after making new connectors all around and cleaning it.

Probably not part of motorized bicycle stuff, but if you know what could be going on I'd appreciate it. I already looked for answers at Smokestack website where there some real informed folks there that work on portable gens and ones the size of Mack Trucks.

See what happens was at times the overload light comes on, but I did not draw excessive amount. It is also alternator type without permanent magnets. This presents some strange thing that sometimes is necessary called flashing the coils to put a minute amount of magnetism back in them so the gen can excite and put out the full load. I've not risked doing that yet. It requires turning by hand or by drill the gen and actuall having voltage applied to the output plug from another source. Yeah.... strange!

The diodes I checked and seem OK. There is a capacitor I bought a new one as it had some contacts burnt also so connections would not be good.

At most I spent 25 dollars on parts. I used it to power some 120 vac super bright 500 watt halogen work lights and so for a while it works. I forgot what it is rated at but it like 700 watt gen.

Last edited by MEASURE TWICE; 12-06-2016 at 12:20 AM.
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  #19  
Old 12-10-2016, 11:34 PM
MEASURE TWICE MEASURE TWICE is offline
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Default Re: Adapting Briggs Build to accept bigger engine

I could have used an even smaller pilot drill, or at least find a small pilot drill bit that has a more pointy end. This was brand new out of the package, so it is just the style that is not optimum for staying put in a center punched metal. My center punch made a mark that had the bit wobble around. Then I had to correct it or use the other end of the jackshaft and start over.

I only need one end with a thread on the jackshaft so I could have started over, but it is fairly well centered, not exact though and it will work.

I forget what they call the bits, but I bought some at Sears that self center. There is a pointy section below the rest of the normal diameter of the bit and I may look for one like that in the future.

Multiple pilot drilling with smaller bits using maybe 3 before the tap drill bit could have likely avoided the wobble. To set up a drill press would have been ideal, but that is hind sight.

The tap worked great, I only used it once before and it is a 1/4-20 with a 7 drill bit set by Vermont brand.
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  #20  
Old 12-11-2016, 07:20 AM
curtisfox curtisfox is offline
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Default Re: Adapting Briggs Build to accept bigger engine

https://www.google.com/search?q=doub...0ePsSp8SS9M%3A,
This is what you should get for starting holes like you are doing. MSC or others have them, all different sizes, and different tapers 60 degree is most common. I got a package at swap meet cheap, from tool guy vender. ............Curt
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