squish clearance

[John Ehrhart]

Trying to get my Jackpiner back together and running into an issue with the base gasket. The manual says to get a clearance of (trying to remember here at work, don't tell anybody) of .7mm and says it is stamped on the cylinder. I can only find a "2" stamped near the part number on both cylinders I have. Am I looking in the wrong place?
Secondarily, would there be any benefit, in light of earlier posts, to using a little more clearance, i.e., a thicker base gasket, to compensate for present-day fuels?
Thanks to all in advance,

John in Baltimore

[john durrill]

John,
Our 75 , 175 manual says ( thats if your engine is the 7 fin cylinder) you are looking for .7mm or .028-.030 on the distance from the top of the cylinder to the top out side edge of the piston and ring at TDC. It also says not all cylinders were marked .
  We use .040 on our 175 and it works well. I run .060 on my 125/175 A motor and it runs well.
 Ron , Doug you folks please jump in here and help the folks with this.
.030 deck heights are used in very high output short life motors like road race and drag bike engines. You gain some power but its at a price. My understanding of the squish are is that it controls the burn in the combustion chamber to keep detionation from happening. To use a .7mm deck height a 175 piner has a very unique ( at least to me ) combustion chamber. Its not the normal squish area like we see in a 125 Sachs engine.
On the 5th lap of an  MX i would bet your making less HP than a 175 piner running a little more deck height. Too much compresion = heat which at some point will = less HP if not engine failure.
 Cranking pressure is a better indication on deck height to run. Anything over 160psi on an air cooled 2 stroke motor is more than is safe most times.
 Carl built engines that lasted and were faster than most. I would take his lead on this to heart.
 My motor with .060 deck height is  running a cranking pressure of 145 psi with new seated rings. It runs well and starts well like this.
 Just my 1/2 cent based on my experiance with several types of 2 stroke engines.
hope it helps some,
John D.

[tomale]

John, I just saw this post and I am going to check the compression on my 250, thanks a bunch, this maybe helpfull on my question of Ping...

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
75' GS400 (project bike)
72'sixday (project bike)

[Chakka]

Sorry to dredge this old post back up but I was setting the deck height on my 175 tonight and read this post with interest. I used the thickest base gasket in the set supplied by Al and came up with approx .035 after tightening the head and then removing to check the dimension. John, are you still running your Jackpiner at the .040 dimension and are you still happy with the performance? I might need to add a second thin base gasket to achieve the .040. I also need to grind the tail end of my dial calipers to ensure I am actually touching the top of the ring.

Chris

[john durrill]

Chris.
We have not changed the deck height on either motor.
Can you measure the base gasket you are using and let us know which one you have in place?
 If you have a 7 fin cylinder the manual calls for something like .028 to .032? for deck height. We get that usually with the 1 mm base gasket on most cylinders.
 Double check the clearance if you can. Use some electrical solder, rosin core. Check the deck height with  the head on ( we use an old OEM head gasket for this) and torqued down.  That compresses the gaskets .  You can get it in different diameters of solder . I have a roll of .063. Strip off enough so you can slip it in the spark plug hole and touch the back of the cylinder when you move the piston to TDC. We usually double or triple up the strands and twist them together. They keep there shape better when we bend it to fit.
 We just added one of the thin base gaskets + the 1 mm to ours and that bumped it up to .040 on Peters 75 ,175.
.040 is a good rule of thumb for deck height. Over .060 and you can loose the advantage of the squish area.
 This has worked for us on several different engine types.
let us know what you did and how it works please.
John D.

[Dwight Rudder]

I remember we used to set squish on the 250 Husky at .035 per Dick Burleson.
Dwight

[Chakka]

John,

I am reporting back in to let you know what I went with on my deck height set up. I used 2 gaskets for the base, the first .035" and the second .005". I determined that when I torqued the nuts on the base the gaskets compressed .005". This left me at around .030-.032 on the ring measurement. After this initial reading I removed the .005 gasket and went with an .015" thk base with the original .035". After torque the ring measurment was .045. I am going to use some playdoe tomorrow sent down the spark plug hole to double check my figures.

Chris

[Merlin]

.......unless you have first knowledge (I do) about drag or road race engines I would be careful quoting squish dimensions. If you follow the KTM shop manual and use the proper brass (Mikuni needle jet air correction/Bing vaporizor #40-404) in your carb your engine will run with high output and no ping.

I posted this on another board for folks to see what "air correction" has a profound effect on.
.............the air correction jet is just that, to correct the mixing of the air with the needle jet metered fuel, too large an air jet with the proper needle jet and jet needle will at best produce a weak power spot in the fuel curve. Pinging is what happens when the air is too much and the fuel mixing weak in proportion to the total emitted compared to mixure strength, a small change in air available to mix can make a dramatic difference. For the sake of reference because it is so dramatic I have a Bing equipped 175 Penton that I vintage MX race. When I first had the bike it had only 400 miles on it and was in very good condition, I used it to race in the local series in the 0-200 class. The first time I raced the bike it ran perfect for about 2 laps until up front while smoking the field then all of a sudden anything between full throttle and idle produced pinging so severe it sounded like the engine had swallowed metal and was about to start flinging it. To combat this for a couple of events I talked myself in to getting out front and cooling it, this only made the problem less severe but none the less still present. After much thought and experimenting with jetting out the ping only to make the bike an unrideable pig when it heated up a thought came to me about mixture strength vs air correction. In the Bing carbs the air/needle jet curcuit are made similar to a Mikuni except no air correction changeable jet is provided but a vaporizor that has fixed air metering in it. I decided to go back to the jetting that ran best until the engine that had been worked hard and became heat soaked started to ping, I remove the vaporizor and soldered all of the holes shut, drill one small hole, problem cured with no other changes. Small changes can have a great effect on many things..............

Quote: Thomas Jefferson, We are all born ignorant, some work to remain that way.
 Quote:Merlin, "it is impossible to teach those that wish not to be taught".

[tomale]

I am not sure I understand... first you used a couple of terms I am not familiar with. what is a air correction screw... I know what a air mixture screw is but I though it only effected the Pilot circuit not the Needle jet circuit... and second what do you mean by vaporizer.... two things come to mind... there are a bunch of small holes on the out side of the pilot jet... or do you mean the the small hole in the throat of the carb that leads to the pilot circuit. I have heard of people changing the jets to change the powerband of a motor but I haven't a clue how to do that.... Making jet changes because of altitude is one thing changing a power curve is another.... this all intrest me to no end....

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)
72'sixday (project bike)

[john durrill]

Thom,
It sounds like Merlin made his own 40-404 Vaporizer. A lot of the early CMF Pentons did not come with them stock.
 Here is a link to John P's Carb article. It may help.
http://autos.groups.yahoo.com/group/PentonRestorationandTechGroup/files/JP%20on%20Carbs/
There is a small passage drilled on the air boot side of the venturi of your carb.In the center and at the bottom. It passes air to help with mixing the needle jets fuel delivery. Its a different circuit from the idle jet.
 Mikuni uses jets of different sizes that  fine tune this circuit.They are listed as Air Jets if you have a Mikuni parts list and blowup.  Bings just have a hole with no jets and different vaporizers with a hole or multiple holes drilled in them to do the same job.
Keep asking about this till your satisfied with the answers. It will help you a lot and can save you money in parts and  a lot of time in getting the carb right for you.
John D.

[tomale]

Thanks John I will.... The other day I was reading an artical about one of the factory mechanic working on a factory bikes back in the day and it being "pipey" and that there were able to jet some of that out and that took me by surprise. It got me to thinking and I think the light is begining to dawn.... Too much air in the middle of the power band from a secondary source could cause pinging and lower the power out put and increase heat.... hmmm.... I was talking with a friend a couple of months ago who was using a completely different needle jet. by doing so he could effectively eliminate the changing of pilot jets for altitude differences and only needed to change a main jet. If I understood right the stock needle jet was too small and made it much more depended on having the exact pilot jet to maintain the correct fuel/air mixture through out the powerband range.... I am not saying I understand I am only thinking out loud.... correct me if I am wrong....

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)
72'sixday (project bike)

[Merlin]

quote:Originally posted by tomale

I am not sure I understand... first you used a couple of terms I am not familiar with. what is a air correction screw... I know what a air mixture screw is but I though it only effected the Pilot circuit not the Needle jet circuit... and second what do you mean by vaporizer.... two things come to mind... there are a bunch of small holes on the out side of the pilot jet... or do you mean the the small hole in the throat of the carb that leads to the pilot circuit. I have heard of people changing the jets to change the powerband of a motor but I haven't a clue how to do that.... Making jet changes because of altitude is one thing changing a power curve is another.... this all intrest me to no end....

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)
72'sixday (project bike)

 Please point out my reference to the "air correction screw"(?), if you would please go back and slowly read what was written and consider the effects as noted. Once the proper pilot is selected it has little to do with anything over 1/8~1/4 throttle, the mid range (above 1/4 throttle) is where "air correction" takes effect on mixture strength, a balance must be made between atomization and signal to the vaporizor or needle jet.

Quote: Thomas Jefferson, We are all born ignorant, some work to remain that way.
 Quote:Merlin, "it is impossible to teach those that wish not to be taught".

[tomale]

Merlin, you are correct... there is no air correction screw, I misunderstood.. but I still do not know what you mean by a air correction jet.....? I have read what you wrote several times and I am sure I am missing something but I am not sure what...
This subject reminds me of what was happenig to my 78 Maico a couple of years ago. It was loading up if the revs dropped off but on the top end it was running hot and pinging like mad. I had not change the jetting and had up to that point raced it many times and it always ran great. what I discovered was a number of things needed attention. the needle and needle jet and the slide were all worn. to top it all off all of the gaskets were drying out and were leaking. and the rubber boot around the cable was cracked and allowing air into the carb.... once I replace all of the worn parts it ran great once more... there is always much to learn...
question how does a mikuni deal with some of these midrange jetting issues....?

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)
72'sixday (project bike)

[brian kirby]

Unlike the Bing the Mikuni has an Air Jet in the intake side of the carb that works with the needle jet to regulate the mid range mixture. The Bing does this with the Vaporizer alone.

Brian

'72 Six Day (on loan from Ernie P.)

[Merlin]

I am sorry, I made the assumption someone at play with jetting (and possible engine damage) would be a bit more in tune with the terms used around metering. My mistake not explaining Mikuni vs Bing exacutions more clearly.

 Another topic related to posts of this thread question if more squish space has a benefit, the short answer is no, the fact the squish band stops the end gases from burning is in itself the reason it prevents pinging. The squish concept is a product of WWII aircraft, in a quest for more power with limited fuel quality it was discovered this "Squish" type combustion chamber allowed higher compression ratios needed for higher output with the marginal fuel  available during the war.

Quote: Thomas Jefferson, We are all born ignorant, some work to remain that way.
 Quote:Merlin, "it is impossible to teach those that wish not to be taught".