i think i got some knock/preignition...

L

Little-J

New Member
Nov 3, 2009
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stuck at work :( designing engines : )
so...

had my cylinder re sleeved after some port work, put in a vertex piston, 66.25mm with 0.05mm clearance. Re cut the head chamber to be 16cc's and my corrected compression ratio is just about 7.5:1..

im running 4% Shell racing M oil in a mix of half new and half old pump gas. I have added about 1.5 maybe 2 degrees of spark advance..

climbing a hill towards the end of the long break in period in 4th, ive got mid range engine speed and i whack the throttle open.

quad starts to accelerate beautifully and then gives me a loud "click"..
it went click..click..............click
at which point i layed off the throttle and made my way home..

i was in a rush to get it back to gether, so i didnt check the squish clearance but ill do that tomorrow..
any ideas?

EDIT:

i just changed to my spare spark plug as the one i had in was fouled. I was running a BR8EG..
 
use fresh fuel for starters, did you do your sums properly when you did the head?? who did it for you? you need to calculate that all PROPERLY if you had anyone other than KOC or Flotek or any other REPUTABLE builder do it.

whats yous jetting at and elevation
 
yeah, my sums are ok.. i checked them.. the peak of my exhaust port is 32mm from the deck and I have a deck height of 0mm

that gives 110 cc's of trapped mixture and with a head volume of 16 cc's, that gives a ratio of 6.9:1

if i say its 35mm from my deck (more realistic as the port top is rounded) then i get a corrected ratio of 7.54:1..

and the safety factor is that it doesnt take into account the head gasket thickness which if 0.7mm for a squished gasket.. if i take that into account, then the corrected ratio goes to about 6.7:1...

oh and jetting, im running a PJ34 with a 155 main, a 45 pilot and the needle in the middle position (sorry, dont remember which needle) and my elevation is about 1000 feet
 
What was your target squish clearance, what's your exhaust port duration, and what's your squish area diameter?

The first thing to do is to check your squish clearance using soft solder and confirm it's not too tight. Also, now that you've broke the engine in, running a cranking compression test on it and see what that number is. Either squish clearance being too tight or compression being too high (or as more often the case both!) can cause the predetonation.
 
i dont remember what my exhaust duration is, and i dont have my laptop with software. can someone run the numbers; ur a techie, u must have some software?

stock blaster stroke and rod, exhaust port opens 33mm below the deck
deck height is 0mm
piston pin offset is also 0mm

i was aiming for a chunky .9mm of squish clearance at least but ill check that tomorrow


ill try fresh fuel as a first because it doesnt do it at high engine speeds, only at midrange when it starts to come on pipe. that should mean its not a squish velocity issue..?..
 
Little-J said:
i dont remember what my exhaust duration is, and i dont have my laptop with software. can someone run the numbers; ur a techie, u must have some software?

stock blaster stroke and rod, exhaust port opens 33mm below the deck
deck height is 0mm
piston pin offset is also 0mm

i was aiming for a chunky .9mm of squish clearance at least but ill check that tomorrow


ill try fresh fuel as a first because it doesnt do it at high engine speeds, only at midrange when it starts to come on pipe. that should mean its not a squish velocity issue..?..
Click to expand...

the exhaust duration would depend on your porting job in the exhaust, sure we have the deck to top of exhaust but but we do you have the bottom of exhaust to end of stroke?

best way to check this is to use a degree wheel and check yourself.

perhaps fatten up that main jet with a 34mm carb 155 may be a bit lean with all that extra air going in
 
Lets tackle the pre-ignition first.
AIR LEAK - did you do a leak down after the build? Air leaks are very very easy to come across and can cause pre ignition. It makes the engine run lean - can very the leanness depending on situation. leaner is meaner but it's hot. do a search on making a home made leak down tester - or i can tell you how for cheap.

Head cutting - After cutting the head down, there is often a hard edge around the area cut. This create hot spots on heads and need to be cut down. This can cause pre-ignition if it gets hot enough(high rpm)

Ports are sharp - If your ports are not chamfered, your rings can catch and they can get hot(specialy the exhaust). Say good by if they catch - youll break rings, can mes up your bore, piston, ect.

Fuel - You man need a higher octane fuel. Octane is the resistance to detonation - higher it is, less likely to detonate - higher comp you can run. If your not already running 91, give that a shot. If that doesn't do it, try adding a little octane boost(do your math). Blasters have to keep the comp low due to the lack of cooling. 160psi is pushing the limits of gasoline. After that, you'll need ethonal/alcohol.

Piston clearance - 0.05mm this is kind of tight, .06mm-.07m is closer to the manual - but your piston manufacture should tell you for this situation(they will know more than me)


My 2 cents
Your exhaust duration is 171.7, that is at 33mm from the deck and if your on the stock stroke.

Your carb is to big for your port job, The 34mm carb should allow for peak power at 9500rpm. Your exhaust is going to choke around 7000rpm. I'm not sure about the pipe, what rpm it likes. I'd recommend raising that exhaust port to atleast 185 degrees. This is 31.71mm from the deck. This will put you closer to a target RPM of 9000.
 
rubbersalt,

the top of the port is actually 32mm from the deck (i had figured 33 from the deck to more accurately work out my trapped volume given the way my exhaust port roof is curved to preserve ring life), and the port floor was lowered to the lowest position the piston will go to (BDC) by the guy that had this quad before me.
The exhaust is a Paul Turner with a Fat Boy silencer.

I know maybe the 34 is a tad big, but it runs really well despite its size, low end power wasnt hurt when i put it on. i love it actually, because i can whack the throttle open at any speed and the engine take it an doesnt bog.

piston clearance: manufacturer recommends .05 so i stuck to that, and its generally been fine for previous pistons. just gotta warm it up to avoid a four corner. I run my CR with .04 and thats fine according to the honda manual. depends on your engine..

yeah, did a leak down, n it was fine, and spent about 40 minutes carefully chamfering and deburring my ports.
after the head was cut, it was scrubbed with brillo and then wet sanded with a 1200 grit paper till it was nice n smooth.. i radiused the chamber to avoid hot spotting.

so if anything really, it only did it once, im gonna get some 100 octane pump gas n try again.

it didnt do it at high rpm. only when i whacked the throttle open it did it just as it came on pipe and then cleared.. ive lost a lot of the designs id done when my laptop crashed so i dont actually remember my port timings etc. my blaster is my "learning" engine.. I have another two heads and another cylinder im gonna spend time porting over the next few months. any software you recommend??
 
ok seems like you have been cautios so id look at fattening the main up a bit!! do plug chops etc. ONLY ONCE YOU ARE SURE they squish is nothing less than 0.9. i did mine at 1.0 to be safe ran a 330 in the stock carb and have had No preignition
 
I do have some software to crunch that data through but my home computer is so crappy it won't run it right. I can do it on Monday but just from the sound of things, .9mm squish is a LITTLE tight. It's been my experience the optimal squish for anything less than a drag bike using alcohol ends up a little north of 1mm. You start getting under 1mm and you're trying to move the charge too fast and it'll just explode instead of moving that fast.
 
Blaner said:
ok seems like you have been cautios so id look at fattening the main up a bit!! do plug chops etc. ONLY ONCE YOU ARE SURE they squish is nothing less than 0.9. i did mine at 1.0 to be safe ran a 330 in the stock carb and have had No preignition
Click to expand...

im running a keihin carb and the numbering on the jets is different..

im running a 160 main, if i run a 165 the top end is flat and it bogs. if i run a 155 its spot on, never bogs and rips! so its already a bit on the fat side..

my minimum squish clearance was 1.4 mm but im afraid that the issue is the angle at which ive cut the squish band.. i should have cut it at 9 or 10 degrees, not 12.. i think i have detonation due to the end gas in the rather wide squish band igniting due to radiant heat before the flame front arrives. i mean, i think a wide and too large squish band defeats the point of a squish band by increasing the amount of end gas..
 
sicivicdude said:
I do have some software to crunch that data through but my home computer is so crappy it won't run it right. I can do it on Monday but just from the sound of things, .9mm squish is a LITTLE tight. It's been my experience the optimal squish for anything less than a drag bike using alcohol ends up a little north of 1mm. You start getting under 1mm and you're trying to move the charge too fast and it'll just explode instead of moving that fast.
Click to expand...

you mean you have one of those little programs that calculates msv?

i need to get me one of those..
 
i did mine at 10degrees if i remember correclty. piston done is 9degrees. so as long as the angle is just a bit greater than the dome angle, you fine. what is your deck height? measured it yet?
 
LIL-J IS RUNNING THE SAME CARB AND JETS I AM, WITH MY KOR PORTING/HEAD MOD
IT RIPS !!!!!!!!!!!!!
I'D TRY NEW FUEL, THEN REVERSE THE TIMING ADVANCE AND CALL IT A DAY !!!!!

sorry for the caps, i'm not retyping all that
 
what octane gas you useing, cause it makes a difference. how many kicks it takes to start, how long it needs to warm up, if you have pre ignition,if you fry your piston or your rings if you happen to use race gas however unlikely. or if your jetting is off causing insufficent compression. All of these problems can be highly suspect....
 
Ive been reading a lot of different stuff about other bikes (like some dirtbikes, and a few other 2 stroke quads), and it seems like a few degrees of timing retard actually seems to be the ticket for some of those guys. You can actually look at your ground strap on your spark plug and see where the color change is. If its about halfway on the bend and up, youre supposed to be OK. If its just the you should advance, and if its the entire strap, you should retard.

As for your jetting, It sounds like you knocked as you were coming onto the main. Maybe you should try not even changing your main jet, but raising your needle clip up one at a time (You should only have 2 slots left if youre in the middle) to see if the det goes away.
 
it almost sounds like you have piston slap, what is the clearance between the piston and cylinder? when mine did that it would only knock when it was going on the pipe and it was a a quick burst of knocks that sounded like detonation until i found out i had .015 clearance
 
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