Finally started this McCulloch BP-1 project, need some parts and advice....

[Hinerman]

Was at a GTG in ft. Scott ks at an old farm and tractor show. Older gentleman walks up and we start talking. He tells me he used to work for McCulloch doing r&d. Tells me he worked on the BP-1. You should have seen his face when I pulled out awol’s BP-1. Alan started it and he made a few cuts...Priceless! What are the odds?

 

[Scotty Overkill]

Was at a GTG in ft. Scott ks at an old farm and tractor show. Older gentleman walks up and we start talking. He tells me he used to work for McCulloch doing r&d. Tells me he worked on the BP-1. You should have seen his face when I pulled out awol’s BP-1. Alan started it and he made a few cuts...Priceless! What are the odds?

I think I remember you telling me that story last year. Yes that is way cool!

 

[Al Smith]

I don't know if this will work or not .Pics of the carb diagram taken in macro setting from a micro fiche transfer .

 

[Al Smith]

If you could copy and send these to a photo gizmo on your computer it might be easier to read .If this works out I could take pics of the entire service manual if that would help

 

[cus_deluxe]

I got to run a bp1 that mark brought to iowa last year. Had a pretty angry sound to it for the smaller displacement. Strange feeling to have a saw kick sideways when its revved up,

 

[Al Smith]

I thumbed through some stuff as I took those pics .Specs 4.2 HP at 10,000 rpm, goverened to 12,000 rpm .I had thought 14 thou .7.2 to 1 comp ratio .Double diaphragm fuel injection ,jet spray primer,no choke .Self mounted chain grinder ,auto sharp .Most likely didn't work well .Odd little duck this one .
The induction goes by way of the small scavanging piston up through the transfer via the crankcase .In a way acting some what but not exactly the same as a supercharger .More like the old Cooper Bessemer two cycles used as oil well pumpers about 1910 .
With all this I would have thought it needs a reed in the induction .I either over looked it or it's part of the carb .Maybe It doesn't need one,don't know .

 

[Scotty Overkill]

I thumbed through some stuff as I took those pics .Specs 4.2 HP at 10,000 rpm, goverened to 12,000 rpm .I had thought 14 thou .7.2 to 1 comp ratio .Double diaphragm fuel injection ,jet spray primer,no choke .Self mounted chain grinder ,auto sharp .Most likely didn't work well .Odd little duck this one .
The induction goes by way of the small scavanging piston up through the transfer via the crankcase .In a way acting some what but not exactly the same as a supercharger .More like the old Cooper Bessemer two cycles used as oil well pumpers about 1910 .
With all this I would have thought it needs a reed in the induction .I either over looked it or it's part of the carb .Maybe It doesn't need one,don't know .

There's no reed, I know that for certain. It'll be cool to try put a different carb and see what it does. This will be an interesting build, regardless. Glad to have yinz along for the ride....I appreciate all the information and interest, fellas.

 

[Scotty Overkill]

Come hell or high water, this thing will run (be it schitty or not) one way or another. I'm committed now (or at least I probably SHOULD be committed..)

 

[heimannm]

Carburetor is the same as the one used on the 170 welder that is (if I recall) 105 cc. The BP-1 is a thirsty little devil when it is running.

The BP-1 is piston ported through the balance piston which act as a "sliding intake valve". With the balance piston the engine is very nearly perfectly balanced, and the added displacement in the crankcase puts a bigger charge into the cylinder for more output. The result is nearly instant acceleration when you hit the throttle.

The governor was an air vane governor, as the engine speeds up the air flow from the flywheel would push a vane connected to the throttle and slow it down. In fact, the throttle is spring loaded to open, the throttle is not connected to pull it open but just backs away so the spring can open it, and the governor can push it closed based on engine speed.

The BP-1 carburetor, not sure who took this photo, maybe Mweba but the penny is there to give you an idea of the size of the carburetor.







Mark

 

[Al Smith]

I would assume with a pressurized crankcase that thing would almost have to short circuit a bunch of fuel out the exhaust port on transfer .It would certainly blow the exhaust gasses out with gusto .No wonder it uses a lot of fuel .

 

[drf256]

I too ran Mark’s at Randy’s gtg. He had quite a few rare saws there he let us all try. What a great guy.

The BP1 was certainly odd in that it twisted your hand along the long axis of the saw when you blipped the throttle. That saw cut extremely well for 44cc. I woulda guessed near double that.

As for the carb, I think the second piston acting as a supercharger in a way throws normal displacement rules for sizing out the window. If you make your spacing for a Stihl carb, you could try any carb in the lineup.

 

[Al Smith]

So with this crankcase charging evidently from the OEM comments on the service manual it stated a double diaphragm carb /fuel injector .Could that be associated with a rather long intact duration ? It doesn't look like a very large butter fly according to the penny in the picture .
Then too I would assume with a replacement carb you'd need something with a shaft long enough to attach the air vane to .It would be a huge dissappointment to do all that work and the first time it fires it goes to about 20 thousand rpm's and comes apart like a dollar watch .That would really suck to say the least .

 

[Scotty Overkill]

Wonder if I should incorporate a throttle stop on my experimental carb, something I could fab up with an adjustable stopper screw that would allow me to limit how far open the throttle butterfly could go? And start out with a low setting, gradually adjusting it until the desired RPMS are achieved?

 

[Scotty Overkill]

As for the carb, I think the second piston acting as a supercharger in a way throws normal displacement rules for sizing out the window. If you make your spacing for a Stihl carb, you could try any carb in the lineup.

That's what I've kinda figured all along, and also what led me to figure on using an HS carb. Stihl used them on the 041's (62cc) all the way up to the 076 Supers (111cc), if I recall correctly.......

The low profile of the HS will also allow me to use the factory air filter and cover, keeping the saw stock-looking, where a larger HL carb will not.

 

[Deets066]

Some of the HL carbs are setup for the air vane governor already though. If you could fit an 070 or 090 carb in there.....

 

[Steve]

You could also use a walbro SDC. Mac used them from the 10-10 (54cc) to the SP125 (123cc). Plus it would look factory as the SDC with the cast pump covers had the McC logo on them!

 

[Deets066]

I don't believe you really need the governor anyway. Why can't you limit rpm with fuel like every other saw?

I ground the vain off of my 090, and just make sure it's tuned right

 

[Steve]

I don't believe you really need the governor anyway. Why can't you limit rpm with fuel like every other saw?

I ground the vain off of my 090, and just make sure it's tuned right

The more I read the thread the more I think I'm over thinking the governor on this saw. The only running bp-1 saws I have seen were all original. Using common thinking on an uncommon platform it could be done safely. It may only rev to 14 or 15,000 rpm. Hell, a good ported 346 does that!

 

[Deets066]

The more I read the thread the more I think I'm over thinking the governor on this saw. The only running bp-1 saws I have seen were all original. Using common thinking on an uncommon platform it could be done safely. It may only rev to 14 or 15,000 rpm. Hell, a good ported 346 does that!

Yeah, just give it more fuel until you like the rpm

 

[Al Smith]

The rumor was McCulloch feared if for some reason the gov stuck they would self distruct from over reving and were concerned about law suits .If one ever actually flew apart I have no idea .It's timed at 27 ahead which is one more degree than a standard McCulloch at 26 ahead . I suppose you could back off the timing a tad bit which might hold it back if it runs away .
Now after all that and in reference to the saw you don't suppose this thing runs a freak chain ? Everything else is a one of .Wouldn't that be a kick in the pants .