help needed, dont know what to do

[draggin-accord]

you can see what i started w/ in my sig. i recently bought & installed a cylinder overbored to 69mm, a head, new piston & cometic gaskets (they came w/ kit) off a shop on ebay. head was mx porting that looked really good when i recieved it, not hacked up. i've been in process of breaking it & i finally took it out. seemed strong, but not that much stronger than my stock jug 20 over w/ no porting.

when i took it out it made a rattle when i hit powererband. so i did some research here & figured it to be detonation. so i do a compression check... 238psi w/ 2 gauges. wow, i'm shocked. keep in mind in purchasing, this was supposed to be pump gas motor. "premium pump gas will be fine" was what i was told. i just fun ride, just want something to run w/ buddys and spank the 4strokes.

we start looking at things more closely, and it appears that the new head is .075" shorter than the head i had on.

so i contact the seller & he tells me 238psi is fine, we need to fatten up the midrange is all, detonation can be cooled off by richening up the mid... leave the main & change needle settings. leave the br9es.

current jetting (all @ shops recomendation):
pj36 (38mm on the way)
225 main
cgj needle 2nd clip (dgj on the way)
a/s 2 turns out
55 pilot
br9es plug

i really try to do all my research before asking questions, but i cant seem to find anything that helps me. everything i read makes it seem like i shouldnt listen to this place. they have been very helpful and answered all my questions, but i don't know enough to know if the answers are legit... i guess i'm looking for a direction to go.. :ermm:

would a trx head gasket drop my comp down enough into an acceptable range? should i put my orig head back on?

p.s. if this helps. i checked the distance from the edge of piston to the top of the cylinder w/ feeler gauges & came up w/ .040..

[C-LEIGH RACING]

Originally posted by draggin-accord
you can see what i started w/ in my sig. i recently bought & installed a cylinder overbored to 69mm, a head, new piston & cometic gaskets (they came w/ kit) off a shop on ebay. head was mx porting that looked really good when i recieved it, not hacked up. i've been in process of breaking it & i finally took it out. seemed strong, but not that much stronger than my stock jug 20 over w/ no porting.

when i took it out it made a rattle when i hit powererband. so i did some research here & figured it to be detonation. so i do a compression check... 238psi w/ 2 gauges. wow, i'm shocked. keep in mind in purchasing, this was supposed to be pump gas motor. "premium pump gas will be fine" was what i was told. i just fun ride, just want something to run w/ buddys and spank the 4strokes.

we start looking at things more closely, and it appears that the new head is .075" shorter than the head i had on.

so i contact the seller & he tells me 238psi is fine, we need to fatten up the midrange is all, detonation can be cooled off by richening up the mid... leave the main & change needle settings. leave the br9es.

current jetting (all @ shops recomendation):
pj36 (38mm on the way)
225 main
cgj needle 2nd clip (dgj on the way)
a/s 2 turns out
55 pilot
br9es plug

i really try to do all my research before asking questions, but i cant seem to find anything that helps me. everything i read makes it seem like i shouldnt listen to this place. they have been very helpful and answered all my questions, but i don't know enough to know if the answers are legit... i guess i'm looking for a direction to go.. :ermm:

would a trx head gasket drop my comp down enough into an acceptable range? should i put my orig head back on?

p.s. if this helps. i checked the distance from the edge of piston to the top of the cylinder w/ feeler gauges & came up w/ .040..

Are you saying the piston is 0.040 down in the cylinder bore at TDC.
What head gasket you have on the engine now.
If you have the thin head gasket, then yes, useing the stock thick one would lower the compression some.
You might even need to go to that other head you have to get the compression lowered.
Around 180 compression is high as you can go & still run pump gas.
If you have a 225 main jet in that 36mm carb & strong as the signal is going through the venturi, its pulling enough fuel up through the jet nozzle to flood most other engines.
A CEL needle would supply more fuel up through the jet.

I dought with that much compression if any amount of pump gas fuel would stop the spark knock.
Buy you some race gas & see what it will do, just as a test.

What about the ignition ??? has it got an adjustable timming plate to addvance or retard the timming & do you know if the flywheel has the key in place or is not sheared.
Neil

[TheFontMaster]

Nothin you do to the carb will get your motor to be able to run on pump gas, the guy you talked is full of **** on that part. Using a TRX head gasket will lower the compression. But I highly doubt it will be enough to get even close to running pump gas. If you can buy bigger domes for the head you have I would look into that. That or like Neil said, use the head you were running before.

[draggin-accord]

neil,

yes @ tdc the edge of piston is .040 down from the cylinder (where the head gasket would sit). it has a cometic gasket in there now. i dont know if that is a "useful" measurement. i took it when i was assembling more for myself just wondering. glad i did now however.

imo the gasket doesnt look overly thin. i had the factory atc gasket in there before.

should i get the compression down before i even try to run it again? is there anything i should worry about if i'm going to put my orig head back on a cylinder w/ larger bore?

i have been comparing needles & to my untrained eye, the CEL would be leaner both 1/2-3/4 & 1/8-1/4...

i have been referencing this.

http://www.carbparts.com/keihin/needles_tuning/pj_jet.jpg

[draggin-accord]

Originally posted by TheFontMaster
Nothin you do to the carb will get your motor to be able to run on pump gas, the guy you talked is full of **** on that part. Using a TRX head gasket will lower the compression. But I highly doubt it will be enough to get even close to running pump gas. If you can buy bigger domes for the head you have I would look into that. That or like Neil said, use the head you were running before.

both heads are a factory head, however it APPEARS that the newer head has had .075 milled off the bottom. i havent taken the new head off, but using calipers & some calculated guestimating you can see the head is thinner top to bottom.

[GPracer2500]

Mine pumps 235psi. I tired a 50/50 mix or Phoenix area 91 pump and VP110. Didn't work well. It would still knock and overheat when run for any period on the needle circuit. As long as I stayed on the throttle hard it would do OK but as soon as I dropped down the pace and started using the pilot and needle circuits more I'd have problems. I figure two stokes' sensitivity to the latent heat of vaporization is responsible for that.

Runs great on straight VP110.

Rich jetting can get you over the hump if you're right on the edge of a deto issue, but it's a patch not a solution.

FWIW:
--I'm a couple sizes over the stock bore (can't remember exactly)
--unknown porting (it's got some porting but I don't know who did it)
--CR head gasket
--PT high rev
--K&N with modded airbox lid
--PJ38
--DGK needle on the 3rd clip
--178 main
--58 pilot.
--It's a duner so I normally only run near sea level

[draggin-accord]

how much more wear & tear are on things w/ that much compression? 220psi? 190psi? i've read about stress on the wrist pin & crank seals... any more thoughts on that or other issues?

i'm just wondering if i can get the compression down to a more "normal pressure" is it worth it to run the race fuel? will it be that great a difference?

[C-LEIGH RACING]

If you want a 250R engine to pull good down low rpm where most riders can handle the bike, you got to have big compression like that to make the engine respond.

On the other hand if you want to go out & rev it to the moon every time you ride then the compression needs to be lower, at least if you want the engine to last any amount of time.

Some drag guys will run big compression so it will pull good down low & then rev them as well, but its very few of those out there. Those that do dont mind spending the big money to keep them running like that.

Race gas might be what you need to think about & leave the engine as it is.
Neil

[draggin-accord]

i'm leaning toward lowering the compression to a more reasonable level, whether it be race gas or on pump gas. from all the reading i've done thats one the higher #'s i've heard. like i said i want a fun quad, not something i have to tear down every 3 rides. i guess tommorrow will be more testing & tuning than i had planned on.