McCulloch SP125C replacement Carburetor

[Bubba Stump]

I am finishing up on a rebuild of a McCulloch Super Pro 125C. I got it running today and can not get it to run at top performance. Here is what has been done to it. I sent the original cylinder block to US Chrome and they NIKASIL'ed the cylinder. I replaced the original piston and con rod with a MC101 conrod and CP125 piston and rings. All new old stock seals and gaskets. New old stock decompression valve, spark plug, and NOS coil. I did away with the points and installed a NOVA II ignition module. I completely disasembled the stock hsh 124 carb and installed a NOS carb kit. The carb manifold boot was in good shape and no cracks so I just sealed the ends with gasket maker where the boot fit to the manifolds. All fuel lines and vacume lines where replaced with new ones. I am running premium 91 octane fuel with stihl 50:1 fuel mix which a local stihl shop told me would be fine to replace the 40:1 ratio for the mcculloch. The mix has also been treated with Stabil fuel stabilizer. Anyhow, When I run the saw it does not sound like it wants to rap up high enough. I built the saw to run a 50" cannon superbar. the saw cylinder was also modified with two large ports below the intake ports where the manifold is attached. the saw has tons of compression and no leaks. As a matter of fact, I sometimes forget to push the decompressor and about rip my arm off when I crank it over. it has new bearings, on the crank, rod, and piston. It does not seem to want to run the 50" bar. I am running carlton.404 pitch, .063 gauge, full skip chisel chain with a 7 tooth .404 rim sprocket. the saw also has a NOS clutch, and drum. I have pretty much put all new on it and it does not sound right when it runs. when I try and adjust the one adjustment on the hsh 124 carb it starts to backfire and sound funny. I have tried to find a sdc 20 carb for it but have not had any luck. Does anybody know of a fully adjustable carb that will run a modified 123cc engine good and won't take a buch of modification to fit properly?

 

[turtle561]

sounds like a timing issue. where do you have the nova chip mounted ? have seen them on kart motors but all are not compatabile.
i tried one on my 028 stihl points saw and it did't run right, cooking the chip in short order. try it with the points. make sure the new coil and flywheel are compatable.

 

[Bubba Stump]

I mounted the chip on the rear engine shroud just to the right of the flywheel. I bought the chip from lilredbarn.com and they showed that the chip was compatible with the sp125. I also watched a youtube video on rebuilding the sp125 and the guy on there had replaced the points with the same chip and he said that he always replaced the points with the chip. I guess I will have to do like you suggest and try the points. The way the saw was running, I thought it was a timing issue also but was told that the chip took care of the timing. the flywheel and coil are the right parts so I don't think that is the issue. I have actually gone throught two of the chips. the first chip I had mounted on the coil mount bolt and it quit working. I thought that having it mounted to the coil lamination was the cause of it failing but am not so sure now. I did some research on the web and found a youtube video from the place I had bought the chip and they were showing how to install the chip on a mcculloch 10-10. they showed mounting the chip where I have it now. has anybody else had any problems with the replacement ignition modules?

 

[sawfun]

Why not keep the points. Both of my stock sp125's run 50" without a sweat. Same with my 101b. All run points and have been in the rain of the PNW with no problems. Also the original cylinders were not plated except the sp125C. I imagine the ring material may be different and not sealing as well. I think you are trying to turn a sucessful old time design into a modern saw and good luck with that. You may accomplish it but why. Those saws are just right as they were made.

 

[turtle561]

i must add i have had several old big macs,1-71's a 795 and a 797 that all ran great on points with no problems and little maintenance. love that classic, fat, blue spark.
here's a untouched, very rough model 55 that still sparks great.

 

[Bubba Stump]

I got the saw off of ebay a couple of years ago. It came from washington and was complete but not running. When I got it, it would turn over but there was no spark. I thought that it was a points problem and i ordered the chip replacement to confirm it was the points. the chip is easy to install and you don't have to take the flywheel off so I thought this would be the best way to get the saw spark. I installed the points and got spark but the saw would not run. I decided to do a complete restore on it and upon breakdown found the seal on the clutch side had gone out and the saw was worked hard sucking in dirt and cedar dust. the saw is a SP125C with the chrome bore. the dirt had sandblasted the chrome off the bore in spots and the rings were not sealing, the bearings were rough also. I watched ebay for two years and could not find a suitable cylinder for my rebuild. I finnally found a place in wisconsin that is affiliated with US CHROME and they said they could replate my original cylinder for $180. I took a chance and they replated it with nikasil. when I got it back it looked like a brand new cylinder. as far as the rings not seating, that is not a problem as I have checked compression and it is around 180psi and I even got almost 200psi once on the gauge. US chrome said that the ring material would not make any difference in cunjunction with the nikasil bore because nikasil is far superior to the old chromed bores. I made a point to ask about what kind of rings I would have to use for a good seal and they assured me the chrome rings that I was going to use would not be a problem and I sent them the new piston and rings so that they could match the nikasil plating to the piston I would use. I am not close to the PNW so these saws and parts are very hard to find. my sole source for parts has been ebay and that is the reason for the mc101 piston and rod . I could not find a good sp125c rod or piston that was a decent price so i went with the more available 101 rod and piston. dimensions where the same but of a differnt desighn. I am not trying to make it like a new saw, just trying to get as much power out of it as I can. I have another sp125 and it runs my 50" bar no problem. I was just hoping to get this rebuild to outdo my stihl 084 or at least come to a draw. my 084 with a 50 can run circles around my other sp125 with a 50. it would be nice to get my american made saw to outdo the german saw, both worthy apponents being 123cc engines. if this project does not work out I have been gathering MC101C parts to build another saw with. the best thing with the mcculoch saws is that you can modify, and mix and match lots of parts with kart racing parts to build a one of a kind musclesaw. the new saws don't have that option unless you want to pay big bucks to have a machinist build you custome engine components. it must have been great to live back in the old school days when american products ment power and superiority.

 

[sawfun]

A ported 088 will run circles around a 125 but a stock one will not. Thats both saws running 50" bars buried. And the 088 and 084 will handle better as well. If you are getting 180 + compression then yes, the rings have seated nicely. Most 125's have far less than that.

 

[Bubba Stump]

Thanks for the input guys but I figured out what was going on with the saw today after I got back from a job. I had taken the pull start cover off and was about to start taking the module off to replace with the points and had a premonition. The instructions had a small box off to the side of the diagram. It said that if the saw will not spark to switch the wires on the module around. it said that the polarity might be opposite and the saw will not spark if it was. I really did not pay any attention to this before because it made it sound like if the polarity was not right the saw would not even attempt to start because there would not be any spark. My saw was running, just not running right. well, I decided to swap the wires just to see what would happen. After a couple of pulls on the starter, the saw came to life and sounded awsome. I adjusted the high and low on the carb and now it will tear through a log without even a slight sputter or hickup. Now the only thing I have left to do is put it up against my 084. My brother said he was up to racing me, me with the 125 and him with the 084. Anyhow, thanks again for the input and advice. It is always some stupid little thing that has to mess with me and have me second guessing my builds.

 

[leeha]

I'm not sure what you consider modified. A 101 rod and 101 piston
don't make it modified but i sure would like to see where you drilled
holes in the block below the intake ports.
I was also unawhere that US Chrome could do those blocks. I will
give Mark a call tomorrow as i'm a dealer with them. They have done
well over 30 cylinders for me in the past on various saws.
I'm with the others and would go back to points and see how it runs.
Keep in mind the one adjustment on that HS Tilly carb is the low speed
only, the High speed is fixed.
Also sealer is not recommended on intake boot installation. Mcculloch
never used any. I've rebuilt or restored over 20 of these 125's and never
used any sealer on an intake boot.
That saw should pull a 50 incher with no problem at all.



Lee

 

[Bubba Stump]

Lee the 101 piston and rod was not all of the modifications that I made. I made the crank into a full circle crank with the help of some info on mcbob's website in australia and the help of a automotive machinist that I have do alot of work for me. I also modified a 101 stuffer so that the manual and automatic oiler would work. it was a little fat and caused some alighnment issues with the mounts so I had my machinist mill alot of material off the flat side so that it would fit better. the ports that I cut in the cylinder are just like the two extra ports the kart guys put in the 101's. I don't know if it will help anything since there is no boost port but I thought it might make the fuel mixture flow better into the case. I widened the intake ports a bit wider with a dremel also and smoothed the exhaust port out. All of the cylinder mods were done before I sent it to US Chrome of wisconsin. I initially contacted US CHROME throught there site and they told me that since the cylinder was a dead cylinder that they could not help me. I somehow came across US Chrome of wisconsin on the net and they told me they could do it. I don't know for sure but I just asumed that they were part of the original us chrome but maybe they are not related. I do beleive that the person I coresponded with was named Mark like you named. As far as the sealer, I only put it on the lips of the intake maifold and the tank maifold so that when the boot poped over the lips it would squeez the excess sealer out toward the manifolds. I made sure ther was not any noticable sealer inside the bore of the boot as I did not want any to enter the crankcase when fuel reacted with it. I wish that I would have taken pictures of the build at different stages but I guess I was just to impatient to get it built. I guess my next sp125 project with the 101C kart engine will have to be documented with pics so that I can show and tell. I will try to get some pics posted of the 125 I just finished for you all to see. I do have one question for you mcculloch 125 kart engined builders out there. I have a GEM slanted intake maifold with the West Bend 820 reed cage and a tillotson HL 292G carb off of a West Bend 820 that I want to use on the 101C kart engined SP125 that I will work on next. I want to keep the stock fuel tank setup on the saw, so would I need to cut out the carb area on the tank housing or is there some other way to fit the setup on there?

 

[jeff taswelder]

How about a video when you run it against the 084
opcorn:

 

[sawfun]

Yes, I too woild like to see the battle of the 120cc saws with 50" bars. I did my own so I kinda know already, that is unless that 084 has a good deal more power stock than an 088 or 880. Stock for stock with 42" & 50" bars, a Mac 125 has more torque and is much harder to pinch or stop than a Jonny 111S, Homie 750, 166, or 088 & 880 or even a 090 direct drive. Those 125's just don't work up a sweat. No need to hold it up to gain rpm before a cut. Just lay 50" of bar on a big log and pull the trigger. Almost like a gear drive. If you port a 088, THEN you will have the Mac torque combined with modern high rpm. the best of both. And both weigh about the same, HEAVY.

 

[jeff taswelder]

Yerp very heavy but the old girl is a lot of fun

 

[leeha]

First of all a full circle crank in a saw will take away it's torque. It will make
it rev higher but in a saw without torque your going backwards. This is what
Terry Ives explained to me a few years ago. The holes you drilled in the block
are only for piston lubrication. Seen it done before. Widening the intake ports
hurt you unless you did the exact same job on the opposite side intake ports.
The stuffer didn't hurt you but you will never see or feel any gains with your
setup. On a fully modded and ported kart motor you would.
I feel you should have left the motor stock, It would have run great.
As for a stock appearing 125 with a 101 motor, it's a huge challenge and alot
of work, not for a novice.



Lee

 

[watsonr]

I'm with Lee on this one... you need an SDC20 carb as the 124 is a single low adjustment, why it won't rev, well part of it anyway. Adding the 101 crank and a CP piston didn't do anything as the 797/125/101 use mostly the same stuff. The real deal is the cylinder porting, why 101s crank out the power.

Most of the 101 cart motors using a bigger intake have the tank cut out or like Lee's saw, a different tank set-up. It's a lot of work and having the right tools and knowledge make a huge difference. I own nearly 20 or so saws in this same grouping 797/125/MC100 and I've not done one yet because of those challenges.

There are NOS parts out there, knowing where to look is the real deal. Good to see some of them old girls still earning their keep! Lets see a video of it, should be able to tell if its a carb issue just by listening to it run in and out of wood.

Good luck.

 

[stinkbait]

Those chips do not work on the 125s. I have tried two different chips on two different 125s. The best thing that you can do is switch the saw back to points. The chip will make the 125 hard starting and backfire. It also won't let the saw rev correctly.

 

[sawfun]

Yes cutting out the tank is a lot of work, I can tell you from recent experience. If you run a tilly it will require less cutting. Fitting a BDC requires very extensive cutting and fitting. However fabricating a throttle linkage is well, not so fun and is the hard part of these conversions. Still, if you want a 101 to really run, this is what needs to be done. I have a 101 with the SDC carb and it sounds wicked and rpm's, but was a bit of a dissapointment. It isn't much I more saw than a 797 or cp125. and is noticably less than a ported 088. Also, Lee is correct on the lower torque output with a full circle crank. My 101 with one has
less low end torque than my sp125's off idle. And I ran the 101 & 125 & super 797 side by side using 36 & 50 inch bars buried in Doug Fir.

 

[Bubba Stump]

Thanks for all the great info. I really don't know much about the mc101 kart engines but am quite interested in learning more about them. is there any kind of literature out there that discuses the power modifications? I added the full circle crank, stuffer, and porting thinking that if it was done to racing karts that I would be gaining power but I guess I was wrong. I did port both sides on the intake ports. the ports behind the plugs were kind of a pain. all I really did was kind of make the round ports square, I did not remove any material from the tops or bottoms. installing new plugs was a huge desaster, I ended up messing up the first set and finally got the second set in but did not feel very sure about them sealing so I filled in the plug area with JB weld as I don't think I will ever be messing with that block again. since I switched the wires on the module, I have not had any more issues with sputtering or backfiring anymore. I guess time will tell on that. I have not put the saw to the test real hard yet but it seems to be running nice. I was wondering if I would gain any better performance if I drilled the high speed jet on the carb to a little bit bigger hole. The elevation where I am at is a little over 5000 feet and the mountains where I do alot of my jobs is all the way to around 8000 feet. I am guessing that because the sp125 was built for the PNW that mcculloch built it for running at around sea level. I have a spare NOS high speed jet that I had bought to experiment with drilling the hole bigger. As far as a video, I have never put one on the internet so I will find someone to show me how to go about putting sending one as an attachment or however you do it.

 

[Bubba Stump]

Lee, this was just kind of a learning experiment for me on the SP125 and I plan on building more. I have been buying and saving up some Mc101C kart parts for my next 125 build. I want to see if I can build one that will run a Cannon 72" rollernose bar that I picked up in like new condition. Right now I have a Mc101C block that has been bored and honed out twenty thousandths with matching 101 piston and rings, a mc101 crank and con rod with nothing done to them, a GEM sloped intake maifold with WEST BEND 820 reed cage and high performance Gem reeds, A tillotson HL 292G Carb that is NOS and configured for gas, a Velocity stack/filter mount, a mc101 super stuffer, and a mc101 high compression head and copper blowproof gasket. What would you recomend For getting as much power and torgue as possibly feasable for this build with what I have have in the parts dept?

 

[leeha]

If your plans are to build a stock apearing 125 with a 101 motor
you have a job to do. Not realy for a novice saw mechanic but if
you insist go ahead. Sounds like you have most of the correct
pieces but the sloper intake may not be the correct angle. You won't
know this till you get to that point. I had to make my own intake.
If you insist on porting i strongly recomend sending to some who
is in the know about porting 101 motors. I would recomend talking
to Terry ives in Ca. A novice porter can ruin a block in seconds thats
why i don't port. I know little about it. If you plan to run a 72 incher
then i would leave the block stock. If you were going to run a short bar
for racing and cutting cookies then port it.
The carb you have is i think a 1 inch bore. The 101 won't get the full
benifit with that carb. 101's need more along with the right exhaust.
A stock SP125 will pull a 72 incher with the right guy behind the throttle.



Lee