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Old 01-12-2011, 06:07 PM
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Default carburetor upgrades

I've been sorting out the carburetor situation for the extra engines I have lying around. I have a couple of vertical shaft Briggs engines that need to be turned sideways to work correctly, and I have a 5 HP Briggs horizontal with crappy suction carburetor that plugs up when you say "sediment". So, I got on ebay and bought some carburetors for them.

First I got a brand new Honda carb for the 5.5 hp GX engine, to put on the 5 HP Briggs. I paid $25 for that one a few months ago.

Yesterday I found this gem. http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWNX:IT Take note because there are several more for sale. I think that's a pretty good price, in case you need one. Mine will go on the 8 HP Briggs vertical shaft engine, which will end up on a kart made from the back end of an old Honda quad I got at the scrap yard.

Today I found this one. http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWNX:IT It will go on the 3.5 HP Briggs vertical lawn mower engine, with Comet 30, on a mini bike which I also found at the scrap yard.

I've always believed that the #1 factor in engine handling is the carburetor, assuming that the rest of the engine isn't a total piece of crap. I'm pretty sure the Briggs carburetors are not the best thing on the market, and the Hondas seemed like a step in the right direction. I need to wind a new spool for my welder, and get started on the intake manifolds. I'm going to try to keep the pipes as short as possible, without too many bends. I'll keep you guys posted on how they turn out.
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Old 01-12-2011, 06:09 PM
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I just noticed that the seller updated the listing on my first link. I got it with free shipping, but now it's ten dollars. It must have been a mistake!
$14.74 with free shipping was a pretty hot deal.
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Old 01-18-2011, 10:06 PM
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I decided to work on the Briggs 5 hp first. It looks easiest, and I can easily bolt it onto the 3 wheeler frame I got at the scrap yard.

I took off the old tank/carb combo, then I measured the hole sizes and spacing on the engine block and on the new Honda carburetor. I also used a protractor to measure the angle of the axis through all three holes on each end so I can make sure the carburetor ends up level. The first photo shows the diagrams and notes.

Marking the holes was easy. I just drew a straight line on the metal, then used a center punch to mark the centers of the holes the correct distance apart. The way to find the distance is to measure the diameter of the two bolt holes, then measure the distance between the closest edges of these two holes, then add one diameter, and that's the distance between centers.

Once the holes were there I marked the outlines of the two parts with a sharpie marker. Then I cut them with an electric carpentry saw with a metal cutting blade.

After that, I cleaned up the rough edges with a bench grinder. I tried the engine flange on for size and noticed that one of the bolt holes was just a little bit off, so I drilled it 1/16" larger. That fixed it.

I bought a short piece of 3/4" galvanized pipe about 4 inches long to weld in between these two flanges. Maybe there is a better choice for tubing. If I find something better I'll use it. But I like the fact that the inside diameter of the galvanized pipe is 1/8" bigger than the engine port and the carburetor diameter. Also, I like the fact that it's heavy pipe and should be easy to weld.
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Old 01-19-2011, 09:59 PM
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I put the manifold together this evening. It came out looking kind of lumpy, but it doesn't leak. I decided to cut the ends of the pipe at an angle so the pipe would lean away from the muffler just a few degrees.

I welded the engine port flange onto the tube first. The bolt holes are closer together on this end. I had to use my electric saw to cut away a little bit of the weld for a flat spot for the bolt head. I sprayed soapy water on the weld and blew in one end with my thumb over the other, and I kept finding little holes near the bolt cutaways. I got it sealed up after about 4 tries.

Then I had to get the flange welded onto the other end at the correct angle for the carburetor to be level. I had measured the angle of the axis of the holes on the carburetor and on the engine port with a protractor, but I assumed that I would just bolt the manifold on the engine and then set the other side so it was level, and draw a mark on it. But as it turned out this was not easy to do at all. So I ended up using the original data. I found the angle between the two sets of holes (40 degrees) and drew lines on a piece of paper at this angle. Then I set the welded end on the paper, with the lines crossing in the center of the pipe. Then I put the other flange on top and lined it up with my eye so the holes were parallel to the second line. Then I held it in place with two dots of super glue. I welded this flange on and got it sealed in one try, with no leaks.

I ended up not using the galvanized pipe after all. I used a steel pipe with 1/8" wall thickness. I figured that it will probably rust if it remains unpainted, so I shot some yellow paint on it. It looks like crap, but it should run a lot better than the original Briggs carb.

I'll have to find a couple of long 1/4" bolts to connect the Honda carb to the manifold. If I can't find anything around here I'll have to go to the hardware store, but there's supposed to be 4 to 6 inches of snow by tomorrow morning. I don't think a couple of go kart bolts will be a good enough excuse to drive 5 miles on snowy country roads....

After the paint dries I'll post a photo of the manifold bolted on the engine. If things go well I'll also have the carburetor attached.
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Old 01-20-2011, 08:42 AM
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I'm sure the carbs you've got will work great, **** a squirt bottle full of gas would be better than one of those suction carbs, but why didn't you look for motorcycle carbs? Granted they're slightly more complicated, but a well sorted Mikuni or Keihin would run circles around a Honda industrial carb, and make more power. Are you retaining the governor on these engines? Your intake manifold looks good, but then again I'm a function over form kinda guy With a longer intake runner it should make more torque at low rev's. Keep it up!
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Old 01-20-2011, 11:13 AM
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Good point...... I'm no expert on motorcycle carburetors. Is there a particular one that you could recommend for a 5 or 8 horse engine? Keeping it in the $20 range would be ideal.

And no, I'm not going to keep the governors.
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Old 01-20-2011, 02:59 PM
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Any 4 stroke bike with similar displacement to the engine you're working with will have a carb that's roughly the right size. A Briggs 5 horse should be around 200cc's, while a Honda GX160 displaces 163cc's (hence the GX160 nomenclature). Honda XR's came with 100, 125, 150, 200 and 250cc engines throughout the years, just for starters. You could also use a carb from a 2 stroke dirtbike, like this one -->Suzuki RM 125 carb on eBay, but since 2 stroke carbs are designed to meter fuel & oil mix to an engine with a power stroke on every revolution, the jet sizes tend to be larger than necessary for a 4 stroke of the same displacement. It would be reasonable to assume that a carb from a 2 stroke half the size of your engine would be about the right size (eg; a carb from an 80cc 2 stroke would work pretty good on your GX160).

Ideally, you'd want to visit a bike wrecker and salvage not just the carb but the throttle cable and possibly even the twist grip in addition to the carb. Any bike wreckers in your neighborhood?

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Old 01-20-2011, 05:14 PM
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There's a really good scrap yard here, and they usually have quite a few dead motorcycles. But most of them have really large engines.
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Old 01-21-2011, 01:42 PM
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Even if you're looking at 750's or 650's, depending how many cylinders they have, the carbs may still be usable. For example, a 750cc 4-cylinder bike will have 187.5cc cylinders. Check the displacement (which is usually cast in or stamped somewhere near the base of the cylinders on the engine cases) and divide by the number of cylinders to determine the displacement for each cylinder. The carbs on a 750 will probably have 28mm to 34mm bores, depending how old the bike is. On multi-cylinder bikes they're always bolted together in some way too, but they can be disconnected from each other. The throttle linkage may give you some trouble as well, but you'll usually get at least one usable throttle assembly from the whole mess. Good luck! I love hunting for stuff in scrap yards, brings back memories of my wasted youth
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Old 01-21-2011, 05:33 PM
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I wouldn't think it would do any good to put a carburetor with a 30 mm bore on an engine with a 22 mm intake port.
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Old 01-21-2011, 05:41 PM
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I'm under the impression (I'm still learning about carbs) that the donor engine's size is only a part of the puzzle- Here's my illustration. I have a carb from a Yamaha 250 motorcycle, and the bore is quite a bit bigger than the carb on my GX340. The 250's carb was feeding two 125cc cylinders (engine's a twin), at a revvy 8000 RPM, as compared to my GX340 carb which feeds an engine designed to run at less than half that speed. So my thought is a carb has to supply a much higher CFM to a revvy bike engine than an industrial engine, so basically, cubes don't necessarily equal cubes. A 350cc bike carb is too big for a 350cc industrial engine, assuming things are still relatively stock.

Am I reading things right?
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Old 01-21-2011, 06:14 PM
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Quote:
Originally Posted by toystory_4wd View Post
I'm under the impression (I'm still learning about carbs) that the donor engine's size is only a part of the puzzle- Here's my illustration. I have a carb from a Yamaha 250 motorcycle, and the bore is quite a bit bigger than the carb on my GX340. The 250's carb was feeding two 125cc cylinders (engine's a twin), at a revvy 8000 RPM, as compared to my GX340 carb which feeds an engine designed to run at less than half that speed. So my thought is a carb has to supply a much higher CFM to a revvy bike engine than an industrial engine, so basically, cubes don't necessarily equal cubes. A 350cc bike carb is too big for a 350cc industrial engine, assuming things are still relatively stock.

Am I reading things right?
Correct. One carb off something like a CBR250RR would be more than enough (probably too big) for the little Briggs even though 250/4=62.5.

A carb off a 90-125cc 4 stroke would be good for the Briggs. Grab one for a Chinese bike in that size range off eBay, they perform well.

IMO anything is better than a Briggs lawnmower carb, they're about as sophisticated as a bucket with holes in it, perform much the same too.

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Old 01-21-2011, 06:44 PM
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Thank you for contributing to my education. There's things I know, and things I don't. I'm trying to lengthen the first list, and shorten the second.
And now back to your regularly scheduled thread. I swear I didn't mean to jack it!
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Old 01-21-2011, 08:01 PM
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Quote:
Originally Posted by sideways View Post
A carb off a 90-125cc 4 stroke would be good for the Briggs. Grab one for a Chinese bike in that size range off eBay, they perform well.
Good idea, never would have thought of that, working with brand new parts is so much nicer than buying something used that you have to clean first

As for the displacement question, it's really a flow-over-time and volumetric efficiency question. In toystory_4wd's example, a GX340 (which displaces 389cc's) turning 3600 rpm is displacing 2472.73 cfm. By contrast, a 250cc bike engine @ 8000 rpm is flowing 3531.45 cfm. Both of these calculations assume 100% volumetric efficiency since both engines in this example are at or near their respective power peaks. Obviously this is not the case, since the Briggs is a flathead design with a small intake port and a badly shrouded intake valve, as compared to the 250cc bike engine with overhead cams and short, steep intake ports designed to flow well at high revs.

So yes, it's an apples & oranges comparison, but my point was that grabbing a carb from a similarly sized engine and/or cylinder will get you in the ballpark. To return to the 250cc example, if it were turning 5500 rpm it would flow exactly the same as the GX340 does at 3600 rpm.

I don't mean to come across as a self-righteous butthead, I just wanted to prove my point I'm more than willing to admit that I'm wrong, but my statement isn't based on theory (although to be fair, I only did it once! Used a bike carb on an industrial motor I mean...).
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Old 01-21-2011, 08:12 PM
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Quote:
Originally Posted by toystory_4wd View Post
There's things I know, and things I don't. I'm trying to lengthen the first list, and shorten the second.
No worries, I try to do the same thing, and help others to along the way.

Thanks

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Old 01-22-2011, 03:43 PM
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Latest update: I tried running the engine today, but it must be sucking air around the gasket. It starts and runs a little bit, but as soon as the throttle is opened it bogs down and quits. I'm going to put some silicone sealant on both sides of both gaskets, then try it again and see what happens.
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Old 01-22-2011, 03:45 PM
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Quote:
I'm going to put some silicone sealant on both sides of both gaskets,
Should not need any kind of sealant.
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Old 01-22-2011, 03:51 PM
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It shouldn't if my parts are flat, but I can't be sure of that.
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Old 01-22-2011, 04:04 PM
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Listen to sideways and buy a 125cc china pit bike carb.Nice job with the manifold.Show us finishing result when done mate
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Old 01-23-2011, 12:42 PM
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I put the silicone on both sides of both gaskets, let it sit overnight, and tried it again today. It works a lot better.

It's about 35 degrees out there, so I had to choke it a little bit and it started up on the second pull. It still likes to run with just a little bit of choke, but that's probably because of the lack of air filter. That's my next project. I'll probably use the original air filter from the Briggs. It's a box with a rectangular paper accordion element. Of course that means I'll have to make another adapter tube......

Here are a couple of photos of the engine with the new carb in place.
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