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i am starting to get confused here, what should cyl head squish be i just check mine and it is running with factory oem 85 86 headgasket measured at .048. but i am still hitting around 210 psi in about 7-8 kicks. it is a 80over 250 motor
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sorry the squish measured at .048 not the gasket.
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Setting the squish clearance to a reasonable number does not guarantee the compression ratio will be correct.
Squish clearance is one design parameter and compression ratio is another parameter.
Squish clearance and compression have to be set simultaneously with the correct machine work.
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well i am runnin a coolhead with a 18cc dome, i talked with cometic and they are makin a new gasket with a 70 mm bore and .059 thick to try to help me drop some of the compression. i am hopin to get down to around 190, motor always ran good but on a hard long pull 3 4 wfo and then you would click into 5 it felt like someone smashed the rear brake. also the pipe turned blueish and purple. runnin sunoco gt110 and maxima caster927 at a 50 to 1 mix. hopin the new gasket helps and doesn't drop compression down to low
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If your squish is .048" it is already a little on the high side for a 250. Adding a thicker gasket will make your squish clearance too wide making the engine more apt to having detonation problems even when you are running 110 octane fuel.
210 PSI on a 250 is a little high but not that high if everything else is spot on with the 110 fuel.
Low compression does not eliminate detonation when the squish clearance are excessive
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if to thick of gasket makes the squish to big so i look into a bigger dome maybe a 20cc
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if to thick of gasket makes the squish to big so i look into a bigger dome maybe a 20cc
Unfortunately there is a lot more to cylinder head design than selecting a head volume and setting the squish clearance to this weeks most popular number found on the internet. Squish clearance and head volume are very popular topics on almost every forum. I have not read any threads in any of the public forums where the authors seem to have a very deep understanding of what they are writing about when it comes to the above mentioned topics.
When we use a cool head I just order a blank dome and then machine our own combustion chamber shape on it. It is very difficult for me to try to answer questions on the subject without having listeners that are also engineers.
Lowering the compression ratio raises the exhaust temperature and will usually make the engine rev a little higher. Lowering the compression ratio has a similar effect to the power curve as does shortening the exhaust pipe. Raising the compression ratio lowers the exhaust temperature and has a similar effect on the power band as making the exhaust pipe longer. (More low end at the expense of moving the power peak to a lower RPM.)
A chrome pipe should turn blue for the first foot or so. Lowering the compression ratio will add a little more blue to your pipe. How far the pipe turns the chrome blue depends upon the exhaust temperature, the RPM and how long the throttle is wide open at any given period.
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#7 Reply
Posted by
Iceracer
on 20 Sep, 2013 05:57
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I agree with everything jerry has said. TO properly set up you cyl and head you need to know the installed CC at TDC. This allows you to calculate Static and more importantly Trapped comp ratio. From there a couple assumptions need to be made on the Dynamic Ratio of the Engine. It would also help to know if your issue is actualy detonation or a overheating issue. I would guess without being able to look at it, its a combination of both.
Have you looked at your head off the cylinder? if so is there step machined into the Squish band? If there is, your looking at a new dome or Blank to get you where you need to be for Squish. If not The head gasket will need to be eliminated or the top of the cylinder machined to get proper squish.
Have you tried a better fuel, like C12 or C14 to see if its a detonation issue? also a couple sizes up on the main will cool the mixture and could point you to a detonation preignition issue.
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ok i guesse i have been informed wrong, i always thought higher compression higher temps, more low end power, lower the compression and you would lower cyl temps and also get more rpm. i will pull the head off tonight or tomarrow and get a pic of it. i did order a stock thickness gasket with 70 mm bore maybe the 72 mm bore the head is not hitting the fire ring enough and thats why i am blowing them out i will also get some pics of the gasket so you can see where it is burning threw at. the fire ring is usually still intack just seeping by and burning the graphite out and starts pushing into the water jacket.
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#9 Reply
Posted by
Iceracer
on 20 Sep, 2013 16:04
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No your right, as far as general assumptions go. But beyond that squish clearance and width have a an effect on head and piston temps along with head shape.
What size head studs?
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The .048 squish is close where it needs to be. Squish is another detonation preventative. It hides the a percentage of the fuel/air from the radiant heat from the spark plug. Squish should generally be between .015 and .060 to be effective. Sounds like you need a larger dome. Read the section on combustion in this book. It'll give you a good understanding of what needs to be done.
My mostly stock 87 motor reads about 160psi compression. I'm gonna hone and do piston and rings in it this winter. 210psi is a little high. but like what motorgeek said, if everything else is cool, such as bearings, crankshaft, piston, ect, you should be okay with race fuel.
Read this link. Great info.
http://www.datafan.com/Horsepower_Blog/gordon-jennings-two-stroke-tuners-handbook/
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here is a pic of how the plug looks
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pic of the head
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here you can see where the gasket is blowing in 3 spots
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well, spark plug looks good to me. The head looks funny how u can see that little swirl mark. Were u using pump gas? Or 110?
Here's what I would do:
1)Get a brand new precise metal ruler and a set of feeler gauges. Sit at a well lit table or bench and lay that ruler on the head at every angle possible and try sliding in the small feeler gauges to check for warpage.
2) check the piston rings. Particularly the gap between the ring and ring groove.
3) check and make sure wristpin and rod bearings still feel right. I'd just pull up and down to feel for play
4) read the link I sent. You can use that math to figure out your compression ratio. If you are not sure how many cc's the head is, then you can fill it with water then measure the water in a graduated cyl to find out the cc's
5) If the ruler shows the head is warped, don't worry, just have it cleaned up by a machine shop, but tell them to take exactly what you need off (whatever the largest feeler that fit under the ruler was) as long as it's not more than .030. That would leave your squish at .018.
6) decide what compression ratio would best suit you and order the corresponding dome per the formulas in the link I sent. Buy a cometic gasket kit, put a little dab of never seize on your head studs, properly torque the nuts, then retorque after the next few rides.
This is just what I'd do man. I'm not a champion engine builder by any stretch, but I'm no dummy either.
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here is what i have done, i use sunoco gt110 fuel maxima castor 927 oil at a 50 to1 mix, new piston with 3 moto's on it since last head gasket, new wrist pin, i always check ring end gap gap was .012 on both rings, new wrist pin bearing and thrust washers every top end. i did check my head on my counter top witch is granite. it did wobble slightly thou ?? not sure, used a 320 grit with breakaway oil for lube and lightly rubbed it down without the dome, dome was flat did slight rub and no witness marks or being warped, head differant story. but still have a over heating problem or even just thought of it fuel bowl is running out, i am using zilla fuel petcock with the 5/16 fuel hose and 38 airstriker. on long hard pulls 3 and 4 ok but when i get into 5th it starts fallin on its face like i grabbed the rear brake, thought i was having brake problems at first, so got new caliper, caliper mount, rotor and ss brake line and rebuilt master with no help to the problem. to where either getting to hot or running out of fuel. i guess once i get it back together i should look into a coolant temp gauge to see actually how hot it is getting, the pipe is purple and blue all the way to the stinger. i do know i bought a 18 cc dome for the head but someone told me they had to cut the dome to get the right squish, so the cc now ?? i will find that out soon. thanks for everyones help.
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I had a similar problem with mine when I was tuning my carb. I run a 39mm PWK. I got it used and it's a sudco racing like yours, bug not air striker. It measures 39.75ish mm
I run a 160 main jet with my needle at the second notch down. 2-1/2 turns on the air screw with a 58 slow jet (I think) can't remember slow jet for sure.
Anyway, mine was leaning out like that and I found that I had set the float height incorrectly. It has to be set with the needle down, but the little spring loaded button NOT depressed! I had done everything you've done, bigger hose, I drilled out a fuel petcock (ruined it), heck I even tried a freaking flutter pump! Then it turned out my float height was wrong. When I made sure the needle valve was just seated, and button NOT depressed, that baby would rev 6th gear to the moon 70mph all the way! (at least that's how fast it felt)
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Float height 16mm, needle valve seated spring loaded button not depressed. I know it's 16mm for the 39mm, pretty sure same for the 38. I had to hold mine kinda on it's side, and support the float with my left middle finger while measuring with the calipers with my right.
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yeah i havent checked float i will do that tomarrow sound like that may be the issue, running a 50 slow and a 165 main. i was even thinking goin to a boysen bowl or even the edelbrock oversized bowls next. but i will check the float first.
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Good luck man! I'm betting this does the trick, and with the high compression your running, getting too lean like that could sure be the cause of all your frustrations.
I'd still get the head checked out. I think when I get my factory head milled, I'm gonna get the squish band as wide as I can. I really want it like 10-12mm wide and then the combustion chamber deeper, to get the cc I want, but I havent worked the math out yet and I don't really know if that's possible with the factory head, but I hope so. I'll be keeping an eye on your build and how this pans out.
With that head, you have options. You just gotta have your numbers spot on. If it's gotta be milled, you gotta know how much. You can order a custom dome to your specs.
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Hmm I wonder if a raptor 660 petcock would flow more fuel then the zilla.. I have one bolted up to my 250 tank right now just haven't had a bike to put it on.
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#21 Reply
Posted by
Dezsled
on 22 Sep, 2013 06:15
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What needle are you running? What clip setting?
Nice race qwads you have there
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i belive its the way i bought the carb NEW. i did lower the needle 1 notch so raised the clip one groove, basicly bought the carb and bolted it on and ran it. the slow jet that came in it was a 50 it seem to run good down low and doesnt load up so i left it alone i did have to jet up on the main the plug was a bit white think the main was a 158 might have been a 160 i do remember i was running almost a 175 on the main for a while but when i went to a track that was mainly sand and she was wfo on long pulls in the straits it was spittering out on the big end so i ended up leaning it out to a 165 or 162. that got rid of the sputtering. also found the bolts holding the coil on started to come loose that could of defintly been a grounding problem.
What needle are you running? What clip setting?
Nice race qwads you have there
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You check that float height yet? That's the only tricky thing about them PWKs man. Betcha it's about 3- 5mm too high. It sounds exactly like mine did when I was tuning my 39mm. When u get it right, it pulls and pulls thru all the gears man. I went from a junk 34mm mikuni to my 39 PWK and once tuned right, it made a HUGE difference mid-top without losing any at all on the low end
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well just check the float height, it is 16mm from the edge to the carb to the bottom of the float with needle seated and spring plunger not pushed in also checked to the line on the float a 7.7 mm.
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