Briggs & Stratton 6S - A Beginnig

[Spurry]

Brilliant job, Bill. FWIW, I think the second one looks better.

Pete

 

[b.lindsey]

Thanks Pete, I'll just chalk the first one up to a senior moment, or judgement lapse, or practice part or something ;D.

Bill

 

[BillC]

Bill,

Are you going to use the "vacu-jet" carburetor? With diaphram pump?

The real thing works really good - but is the normally the most common reason alone for most B&S engines to refuse to start....

This project is very interesting, cannot wait until you are ready to crank it up! :bow:

BillC

 

[b.lindsey]

Bill,
I am not sure if they were calling it the vacu-jet back then or not. It does not have a diaphram pump. It does have the manual slide choke and a straight suction tube going down from the carb into the tank with a small check valve. I remain concerned that the carb may turn out to be an issue though I patterened it as closely as possible to the original. I am planning on doing some testing here shortly to see how well it will lift fuel from the tank but for now it remains a rather huge question mark!!

Bill

 

[BillC]

Bill,

The issue was the changing volume of the fuel tank and the amount of lift (vacuum) that the crankcase vacuum could exert on the column of fuel, and then the efficiency of the check valve responsible for holding that volume of fuel. A full fuel tank was mandatory for starting. Once started all was good and there was ample increased crankcase vacuum with the crankcase check/filter system that was used. And that is where my memory goes....I know that B&S had trouble with the initial designs of the lift type (top tank) fuel delivery systems and used the Vacuu-jet with rubber flapper check valves and a diaphram lift pump to decrease the dependency on just the crankcase vacuum on starting. The diaphram and checks are a vacuum (increasing) differential membrane for the initial lifting of fuel and of course for a continued supply as the tank fuel volume dropped away from the available 'lift' vacuum of the crankcase.

B&S engines (and other small engine manufacturers) were a key element in the mechanization of the small family modern era, most especially with the average small plot land owner having the ability to turn some soil for the production of a food garden. Sometimes, the efficient give way to the popular. I have a deep love for these small engine manufacturers and their great contribution to our society. Though, seem inconsequential today where a box of milk in the grocery store cooler is its origin.

A carburetor with these small engines is usually the most trying and exasperating portion of making a successful 'scale' engine. My hope is that you will overcome this small portion and most important phase.... Best of luck (with) your skill to you, Bill

BillC

 

[BillC]

Only trying to help Bill.....don't intend to discourage at all....basically just sharing a thought and what I remember from working on small engines.

BillC

 

[b.lindsey]

Bill, I appreciate all the input I can get. I was doing some online research yesterday after your initial post and apparently these problems were pretty common. I haven't made the check valve yet but as you noted, it will need to seal well to keep the gas in the tube from seeping back down at least until the engine is running. The other good point that you made as well as seeing it elsewhere is to start with the tank full to minimize the lift reguired. That is easy enough to do. My main concern is how well the whole carb is sealed, meaning drawing air in through the air filter and past the venturi without leaks from other places. The only obvious one I am aware of is the choke slide, but it seems fairly tight thus far. The other concern is whether or not there will be enough venturi effect to actually work in a scaled down version. All questions that remain to be answered for now, but I remain hopeful that all will work as planned or can be modified to work with minimal changes. With something like 20 odd paprts to the carb, I really hope not to have to do any major mods...lol.

Bill

 

[Rustkolector]

Bill,
As you know, model engine displacement doesn't scale down from a prototype engine. As a result, the air flow of a model engine doesn't scale either. So designing a model carb using true venturi effect for fuel lift could be a real exercise. I suggest that you not worry about venturi effect, and go for an air flow restriction that will give you the needed fuel lift. Use the choke (since it is easily adjustable) as a restrictor to get it running well with the widest opening that allows the engine to run well. Then calulate the optimum choke opening area, and make a corresponding restricting orifice that you can put under the air cleaner. The choke can then be used carefully for cold starting. The same fuel draw should be available from either a properly sized venturi, or a properly sized restriction orifice. The latter is much easier.

Jeff

 

[b.lindsey]

Jeff, good point and one that should be easy enough to check/implement using the choke first as you suggest. A restrictor under the air filter should be an easy thing to do as well. Not having worked with IC engines before, much less a scaled down version of an existing design, I am more than open to all ideas...so thanks for this one.

Bill

 

[b.lindsey]

A few more little steps on the crankcase. First the inside area has been pretty well hollowed out leaving a lot more clearance for things now. Photos 1 & 2 show this with the crankshaft and the axel pin for the camshaft both in place. The other task finished was to get the enclosed area where the tappets and valve stems meet milled out to final depth. Its a little hard to see (photo 3) but there is a ridge running down the center of this cavity where I still have to drill and tap for a 5-40 stud which will hold the cover on.

 

[b.lindsey]

I finally got around to making the small check valve for the bottom of the gas pick-up tube. The body is 1/4" dia brass turned down to 1/8. the through hole is .035" and the top .250" is counterdrilled to the 1/16" dia of the picl up tube. The tube was then inserted into the check valve 3/16" leaving a small gap for the check ball to work in. The ball was robbed from a ball point pen and is about .040" in dia. As noted the through bore of the check valve and the ID of the tube are both .035 so the ball should remain captured in the gap. In the third picture below I inserted a solid 1/16" rod and tapped it to try and make a seat in the bottom of the valve body.

 

[b.lindsey]

At the same time I played around with the carb some, applying some moderate suction the the outlet end with the pick up tube immersed in water. This was done with the choke both closed off and open. To my great surprise in both cases water was sucked into and through the carb with a noticable difference between the open and closed choke positions. How this will translate based on the vacuum generated by the intake stroke of the piston remains to be seen, but it still seems to be a good sign for the moment.

Bill

 

[BillC]

At the same time I played around with the carb some, applying some moderate suction the the outlet end with the pick up tube immersed in water. This was done with the choke both closed off and open. To my great surprise in both cases water was sucked into and through the carb with a noticable difference between the open and closed choke positions. How this will translate based on the vacuum generated by the intake stroke of the piston remains to be seen, but it still seems to be a good sign for the moment.

Bill

That sounds very promising. Water is so much heavier than gasoline - larger molecule - so the fuel should respond much easier to the vacuum that you describe. Darn shame you couldn't find a plastic or an aluminum ball for your check ball.

When you first start your engine it will suffer from ring leak-by until the they seat. After the rings seat in, the carburetor will need to be leaned out and at that time you will know for sure that your system is a success. The most trying time with these little engines is the initial starting and adjusting to run steadily and respond correctly to the carb. I usually start a new engine on a carburetor that has known characteristics, but it's just as much fun to make her go with the 'stock' carb. Sounds like you're on the right track!

We have faith!

BillC

 

[b.lindsey]

Thanks for the vote of confidence Bill. Since the suction was being applied by yours truly, I hesitated to try it with gas :big:

I did try blowing and sucking on the check valve end of the pick up tube with again, a noticable difference in air flow. I'm not sure it seals perfectly but it is providing some checking action. Now i'm not even sure it will be needed but it looks more original and I don't see that it can hurt anything and may help. Geeze that ball is small too...took me three pens to hang on to this one and I about lost it a couple of times today.

Bill

 

[Deanofid]

A resourceful way to salvage a tiny ball, Bill. That's got to be one of the smallest shop made
check valves I've seen made here.

Dean

 

[b.lindsey]

Hey Dean, as I recall it was Marv that suggested that so I credit him with the idea. Anyone have a clus as to what they are made of? If it happens to be stainless then eventual rusting or corrosion wouldn't be a problem. After the water test maybe I should put a drop of oil in it just as a precaution.

Bill

 

[b.lindsey]

This morning I wanted to check how the con rod and crankshaft were fitting into the now hollowed out case, especially now that the camshaft axle is in place. There was MAJOR interference between the axel and con rod cap and the camshaft would only add to it once installed. So with some major head-scratching and wondering how I could be THAT far off, i took the restored 6S off its base, turned it over and in one of those ah-ha (how stupid can you be) moments, found out that I have had the piston and con rod backwards in previoius fit-ups and in my head as well. The first picture below is one i previously posted showing the wrong orientation. The second one taken today shows the correct orientation. The conrod cap does clear the axel now as shown in the third photo, but just barely. A little milling off on that side of the conrod opposite the oil splash finger should take care of that even with the added diameter of the camshaft. Even in the original engine this is a very close fit. The other thing I noticed was the reason for the flat releived area just on one side of the conrod on the crank end (on the upper right in picture 4). As it turns out that flat provides just enough clearance for the intake cam lobe as it rotates into the path of the con rod every other revolution. Fortunately it had been modeled true to the original, but I am only now realizing why.

That being said, I will accept my dumb spell knowing that the fits are still ok with the one minor modification noted, and with an even greater respect for the B&S engineers who designed this visually "simple" engine with all its hidden complexities in the days when drafting boards and pen and ink ruled and when CAD referred only to someone with less than gentlemanly tendencies.

 

[Deanofid]

and with an even greater respect for the B&S engineers who designed this visually "simple" engine with all its hidden complexities in the days when drafting boards and pen and ink ruled and when CAD referred only to someone with less than gentlemanly tendencies.

Even more amazing, the same kind of fellows designed things like the P51 Mustang using the same pen and ink, and they did it in a matter of months.

Glad you got your unforeseen problem figured out, Bill!

 

[walnotr]

Even more amazing, the same kind of fellows designed things like the P51 Mustang using the same pen and ink, and they did it in a matter of months.

Glad you got your unforeseen problem figured out, Bill!

Not to mention the P-38! It just goes to show that throwing enough well organized engineers at a problem can get it solved pretty quickly. Ditto the moon shots.

Steve C.

 

[b.lindsey]

I apologizr for the lack of recent progress. As I have noted before this is just a busy time of year at the school...lots of rapid prototyping jobs, work in the machine shop, and so forth for all the projects going on. Its fun to watch, with only 5 weeks to go until all the senior projects are due to be finished, many of the groups are at that "oh $hi*" moment. All the components they have ordered have arrived with the greatest expectation that things will fit together perfectly and then oops ???, it looked good on paper but the reality is another story. That's what learning is all about though. Anyway, i found a few minutes today to remove a few chips of alumiunum from the crankcase. The first two are of the front of the engine. The gap is where the case necks down above the main case and below where the fins begin. The third picture is the back side, i needed to outline the tappet/valve stem cage so I can work on the cover over the weekend. I hope to have some more progress to report tomorrow or Friday. It seems to come in spurts.