Two stroke theory question

[brian kirby]

I am cutious if Nelson or any of the other two stroke wizards here might be able to answer a question for me. The Husky 125 I got came stock with a 38mm carb and a different cylinder port layout supposedly to take advantage of the big carb, prior years came with 32mm Bings. They also changed the piston to have two large oval windows on the reed side, while the earlier piston had a single window of the same size as one of the 82 windows, in other words they doubled the window area in 82. My question is, does the window area effect the intake velocity and low/mid power or does the carb size and reed valve control that? I have already put a 34mm carb on the bike and its better, but it is still seriously lacking in the midrange, so bad that I think my Penton 100 has double the midrange power as this Husky 125. It still feels like it has too big a carb, but I am hoping that buy going to one of the earlier single window pistons (which are identical to the 82 except for the intake windows) might help add some useable midrange power. Its also likely that the 82 cylinder just has too radical port timing and nothing but swapping to an earlier year less radical cylinder is the fix.

Brian

[Larry Perkins]

Brian,

I don't have the brain to answer your question but I do remember the 82 125 and 175 Huskys were pipey as hell and after riding one of the 125's I wondered how anyone could ride them in the woods.

Larry P

[linglewn]

Brian,
I'm sure you have properly tuned the carb and ignition and checked for adequate compression. In my humble opinion I doubt seriously that the piston window area is your problem. Although a smaller carb may help some on low end, the reed valve will, to some extent, vary the flow area as a function of the natural frequency of the reed and the negative pressure in the crankcase. The reed natural frequency can be tuned to a certain RPM by changing the reed material or thickness. The crankcase pressure, which you want to be as low as possible, is affected by crankcase volume, including transfer passages, port timing, and to some extent expansion chamber design. I think you are on the right track with the port timing differences. If you have access to a less radical cylinder, it would be interesting to measure the differences, and, of course, run them both to see if there is an improvement. I suspect you will find that the radical cylinder is ported higher (exhaust port opens earlier) and has less blowdown (less distance from the exhaust port to the opening of the transfer ports) than the non-radical cylinder. Good luck, and I hope this is useful food for thought.

Nelson Lingle
73 Jackpiner
74 Jackpiner
71 DKW 125

[454MRW]

Are you still running the 175 pipe? That may also make a significant difference in the point when the powerband comes on. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S
75 Can Am 175 TNT & 77 250 Black Widow
1976-78 RM & 77-79 PE Suzuki's
74 CR250M 07 CR125R & CR150R

[brian kirby]

Larry, your memory is correct, this thing is so pipey its basically unrideable for anything but MX. When I had my first one it was in California, not much in the way of tight woods in SoCal, plus I was a dumb kid so I wouldnt have known any better anyway.

Mike, I am running the 175 pipe, but it is better than the stock 125 pipe. The Husky Products 175 kit pipe was just a copy of the Pro Circuit 125 pipe from 80-81. I do have the stock pipe, and its an easy swap, so I'll try it just to eliminate one more thing.

Nelson, you confirmed my assumptions but it is good to have someone with real world experience let me know I am on the right track. The crankshaft of this bike has full circle flywheights but there are two very large holes on either side of the rod pin on both weights. Would it be a good idea to stuff those holes to decrease crankcase volume like was sometimes done with Sachs cranks? I am already going to split the cases to replace main bearings and seals so it would not be a lot more work to stuff the crank if you think that might help.

Brian

[linglewn]

Brian,
I would definitely stuff these balancing holes with something light weight. It won't solve your bigger problem, but it's certainly the right thing to do.

I had a mid-70s 175 Husky, and, as I remember, it had lots of mid-range torque (compared to my DKW).

Nelson Lingle
73 Jackpiner
74 Jackpiner
71 DKW 125

[Bob Bean]

Brian,
We had a early 70's 125 Husky. If I remember right, it was a 73 or 74 .   Rode some enduros with it.  The wife raced some MX with it.
It had a pretty decent power band that I remember.  ( I raced some RM100's, Hodaka 125's to compare to)

My Dad had a 175 husky that he rode enduros with. Pretty decent power band on that one too.

IF someone raised the exhaust port on that model even slightly, it kills the bottom and midrange.  My buddy, Tony, raised the ex. port 1 mm to get some more power.  Well it had more power.  At 12000 rpm
Below that, ziltch.....



1986 ISDE Italy
1987 ISDE Poland
1989 ISDE Germany

1973 Jackpiner 175
1974 Penton 250 Harescrambler
1976 Husky 250CR
1985 Husky 400WRX
1985 Husky 400WR

[firstturn]

Brian,
  Everything Nelson says is true and plugging the holes on the crank is fine as long as it is done with very lightweight material.  I am more concerned about the balance of a crank when doing this more than anything.  If it was my bike I would find a older cylinder and measure the exhaust port and go with one that opens later (like Bean mentioned).  On to the carburetor you are on the right track and 32mm 0r 34mm (for reeds should work).  What Nelson mentioned about the thickness of the reeds was something I leaned in Go Karts and it does make a difference.  I do not remember the configuration of the piston ports (round or square) so you might send me a picture to my email when you tear it down.  Sincein '82 I was worrying for Honda I do not ever remember looking inside on of these engines...but I liked the Engineering Design from back in the '70.  Keep us posted.

Ron Carbaugh

[brian kirby]

Nelson, Unfortunately the '76 175 has nothing in common with the 125 engine, its based on the Mag 250 with a 54mm stroke and the 82 has the same bore and stroke as the 73-74 125 with 55mm bore 52mm stroke. My Uncle has a '76 175 and it is like an electric motor, super smooth with lots of torque, basically the polar opposite of this engine.


Bob, this bike was known in '82 as being all top and zilch below that in stock form, I think Husky went too far on the port timing trying to keep up with the Japanese. Its actually pretty darn fast in stock form and if it was only going to be a motocross bike I would leave it that way, but its completely unrideable as a CC bike.


Ron, I will figure out a way to stuff the holes with a light material if I decide to do it. I have not pulled the cylinder off yet, but I will post some pictures here when I do. I am sure that a 79-80 cylinder will have a milder port layout than this thing, plus I will do the case/cylinder transfer port matching that Clark mentioned in the first thread about the bike.

Brian

[firstturn]

Brian,
  I believe that by 1982 the crank case areas were already very close to being stuffed so I am not worried about the crank area except that you are fretting up the bearings.  If you get another cylinder try it with the smaller carburetor and try both pipes.  I am going to take a shot in the dark and say you will do best with the older cylinder with stock pipe 34 carb with smaller  port holes in the piston.  The only other thing I would do with the stock engine is to trim some meat off the head and move the compression to about 150 to 160 PSI.  If you can find a hot rod pipe for that era you can use the pipe for MOTO X and the stock pipe in the woods.  Like I said this is a shot in the dark since I haven't worked on this particular engine.  You have my email address.
  You are a really good rider so it you get a bike with a nice powerband that is that part of the equation.  You indicated that you want this for woods ridding so my next cause would be to lower the bike on both ends.  Just my thoughts.
  Great posts.

Ron Carbaugh

[Ron]

Brian,
I had an 83 125XC back when and rode SoCal desert.
I tried a smaller carb and felt like it only lost top-end rpm's, without a noticeable gain in mid-range power. Ended up going back to the 38mm.
You might play with advancing the timing a bit to get a harder hit off the bottom, but be careful screaming it.
One other thing, the 125 Husky is the same size as the larger bikes. Frame, wheels, forks, etc and it feels like a big bike.
That's going to make it feel slower than your Berkie, even though you may be going just as fast. Just a thought.
Ron

[auto]

Brian,I have a 82 125WR and it has a very strong mid-range.The WR cylinder reed block is smaller.Four pedal,I believe the CR/XC is six and a much larger intake track.Check parts #'s on a 82 WR cylinder and see if it's the same as a 79-81 or find a 82WR cylinder.When I got the bike it needed reeds,I replaced them with a set I had laying around and they really made it work in the mid range.I couldn't tell you what they were for,but they fit. I have to pull the top end for some maintance.If you shoot me a PM I'll set something up with you to compare porting and reed thickness.

[Knute]

Hey Brian,

Congratulations on finding your 125 XC.  I've been collecting Husky's for several decades and love the 1981-82 era bikes.  I've attached a few pictures of one of my 125's for you...a 1981 CR in this case. Historically, the 125 Husky's always got the previous year's parts and considering how expensive they were, many people chose to go the Japanese route instead.  Using your bike as an example, the "new" tank, seat, rear hub, and 40mm forks were introduced in 1981 on the bigger bikes, as well as the Ohlins piggyback shocks.  
 
Regarding your engine dilemma, I spent some time with Kevin Brown today working on our 495 and mentioned your situation to him.  He rode for Burleson on the Husky team from 1981-83 and spent a lot of time on the 125's.  He also rode a 1981 125 WR at the Italian Six-Days, won the final moto, and came away with a gold, which is pretty impressive considering the 125 Husky was underpowered compared to the other 125's of the era.  On the other hand, it was a tried and true setup and it was dead reliable.  

As you have found out, for 1982 the 125 top end was radically reconfigured and the result was a bike that was both unrideable and unreliable.  The ports were so big they would wear very quickly and start snagging rings.  These problems were quickly recognized and Kevin ended up using a one-off engine with a 36mm Mikuni during the 1982 season.  As a side note, neither of us are aware of any primary kick 125 Husky's from that era...even the pre-production 1984 water-cooled 125 he rode in Wales was not primary kick.

As far as your engine goes, I think you should revert to a previous 125 top end and, assuming the ports match the bottom end reasonably well, your bike will run fine.  It won't be the fastest setup out there, but it will be reliable...something I value much more than peak HP.  I would also strongly recommend spending the majority of your efforts on getting your suspension sprung and valved correctly, and you should have a very reliable and competitive machine.

Kent

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[brian kirby]

Auto,

Only the CR and XC got the "new" cylinder for 82 and the WR still had a Bing, according to the brochure I have so your bike should have a nice strong midrange. I also have the 175 kit accessory bulletin and it says "If you want to adapt any 81 or the 82 WR model to a 175 you must order a larger carburetor and reed assembly".

Knute,

Great information! You confirmed a lot of things I had wondered about this 82 cylinder. I see your 81 has the same Koni remote res shocks as my bike too. Sounds like I definitely need to find an earlier top end. I'll gladly give away some peak HP to gain some midrange and reliability.

I dont know what to say about the primary kick, but this bike and the one I had new in '82 both have it for sure. In '82 I didnt even know there was such a thing as a bike with non-primary kick as I had only ridden Japanese bikes, so I know I would have noticed back then if the bike didnt have it. I checked when Clark said something about it in the first thread, and I double checked again last night, the bike I have now 100% sure has primary kick.

Brian

[SouthRider]

Brian,

You may end up chasing your tail trying to get the 125 setup to run the way that you'd like it to.

If at all possible put the 175 kit on it and THEN begin tuning. It will be a lot more fun to ride. Though even the best running 175's were still mild hitting easy (& fun) to ride mid range bikes.

If you are determined to make the 125 run try to find a 34 or 36mm Mikuni or Lectron (preferred), do the bottom end stuff we discussed, and then start jetting. I've never bothered to stuff the cases on one - seemed like a lot of work & risk for moderate reward.

Oh - by the way - I'm sure you have checked, but - make sure that no one has left a rag or shop towel in the exhaust port that is now halfway down the pipe clogging things up.

Remember - I have the parts & service manual as a pdf file. Let me know your email address if you want them.


Clark