[yz125rider]

I just bought a 1986 R and put it together, it has an FTZ cylinder that was re-sleeved to the stock bore and a 38mm Carb off my '88 CR. We had set it up to run on pump gas (like 93 octane) and I believe compression was somewhere around 180 PSI. Well first time we took it out to tune on it, it blew up. My buddy who built the engine had been riding it and running it pretty hard and I got on it and took it for a spin and ripped it for a few seconds then let off to let it cool down and it died. The guy who we took the cylinder to to get bored after it blew said that the carb probably didn't have the right needle in it so it wasn't getting the right amount of fuel in the mid range and thats what blew it up. I was telling another guy about it who used to run banshee's and he said that it had way too much compression and 180 is almost needing alcohol. I was just wanting to get some more opinions on it. What kind of compression should I be running to be able to run the quad on 93 octane? I'm a poor young kid and I can't afford to be blowing the thing up everytime I ride it so I just want to make sure we get the problem solved before it grenades again. Thanks.

 

[8dave3]

93 octane is more than enough for 180psi.Im with the guy that bored it.when you get it going again,be sure it is broke in properly(you never mentioned if it was the first time).After you heat cycle the engine to break it in,do a plug check on it.That will tell you if it is jetted right.Then you can set the needle to the correct height.Just my opinion on this,stay far away from the guy that told you that you were close to needing alcohol.

 

[rook]

and just to be safe, put a main jet(about 2 sizes bigger) in it.

 

[BLACKeR]

banshees run substantially lower compression than 250R's, so thats why he was confused. as said if you didnt do a brake in before, do it this time. and check your jetting before getting on it. also always make sure the bike has had plenty of warm up before you ride it. the cylinder should be hot to the touch before you take off.

 

[yz125rider]

We had broken it in somewhat at the guy who built the engine's house and then I took it home and we took it out to tune on it like a week later. We let it warm up quite a bit and then putted around for a while before we raged on it. We had put bigger jets in it once and decided to go another step bigger and then my buddy took it out and did a plug test, after he did that I rode it and she blew. He said the plug was looking fine and when we took it apart the piston was covered in oil. I talked to him today about it and he said that it was because we weren't running good enough gas and so he's going to put a bigger spacer plate underneath the cylinder and clean the dome up because it's pretty gnarly from the last time it blew up. Do you think that will fix it? I'm also curious, I'll be doing a lot of trail riding and I want this quad to be somewhat reliable. Do you guys have problems with your R's alot? I will definately take care of it and maintain it, but I don't want to have to be replacing a top end every other ride.

 

[8dave3]

At 180psi,93 octane would be more than adequate.Im thinking it was either a break-in issue or jetting.Did the engine seize?Was the piston scarred up?Im not following your friends plan with the spacer plate?????Do a thorough break-in,I know its tempting to let it run a little then call it good,but if you want it to last,you got to be patient.I always fire mine up and just get where it will idle on its own and let it run for about 15-20min and then kill it.Let it cool off completely on its own(no fans blowing on it).I do this again after it has cooled.After the second cool down,I start it and let it run for 5-10 mins then ride it,actually lug it around in 3rd gear ocassionally blipping the throttle up to half.Ill do that for about 15-20mins then park it and let it cool again.After that its ready to ride,after it has warmed up good,I always let mine run for at least 5 mins before I get on it.

 

[BLACKeR]

some of us have no issues at all with engines some seem to have problem after problem. this leads me to believe that some of us take the time to brake it in properly, jet it properly, and warm ip up properly. it seems the others just kick it over and hammer down the street. warm up is very important on the 250R's because they have a steel sleeve. your aluminum piston expands differently and faster than the steel. if you dont let it come up to temperature slowly the piston will expand too quickly and lock into the bore.

i dont understand the whole spacer plate thing either. did the engine seize? did it just lose compression? or did it just stop running? if it didnt seize and you still have compression then you have a problem other than your top end.

 

[86 Quad R]

technically the break-in time is dependent on how tight the tolerances were set between the piston and bore. that has a lot to do with whether or not its a cast or forged piston but i usually break them all in the same which is very similar as mentioned. i'll do at least 3 heat cycles at a rather fast idle, letting the cylinder heat up to the touch and allowing to cool each time. after the heat cycles i ride it through the trails, never getting into the pipe.

i'm thinking that the only thing your buddy may be getting at with the spacer plate is to get the compression down a bit but that can be done easily with head gaskets. as mentioned, i dont think its actually high enough to be a problem. did anyone check the head and base nuts after the engine had a lil run time? was a leak test done. i'm also curious as to what damage was done when it quit.

 

[yz125rider]

You're right on about the spacer plate, he wanted to put it on there to lower the compression and give it some more piston to head clearance, we already had like 2 or 3 head gaskets on there so he didn't want to stack a bunch more on. We never checked the base or head nuts after it ran. One of the head nuts was leaking, but apparently that head has always leaked (he knows some other guys who had that head on their R before me). Once we get it back together we're going to goop that up to seal it. When it died I kicked it over and it had lost all compression. We took it apart and the piston was pretty scarred up and the cylinder wall was a little bit too but it hadn't totally grenaded the piston or anything, the damage wasn't really THAT bad. I'm sure we let it warm up enough before we rode it but the initial break in wasn't done the way you guys have suggested, I'll definately remember that for when we get it back together. The top of the piston was also showing some signs of either detonation or hitting the head. We also had no coolant in it at the time, it was just water because it would leak out the head nut. I think the jetting was pretty close to being spot on, my buddy is big into tuning his machines perfect. I really appreciate all the help you guys have given me, it's nice to get a second opinion. I trust my friend and he definately knows his shit (he is a car mechanic) but he also thinks he knows all, so I like hearing something from some other people who actually know what their talking about and not just some school dip shits.

 

[BLACKeR]

you shouldnt run an engine thats leaking at the head. your probably getting an air leak from that. thats going to make it run lean, get too hot, and lock up. you said this was an FTZ cylinder sleeved down to stock bore. is it an aftermarket pro X cylinder or a stock cylinder ported by FTZ? if the head is a leaky head, its possible its warped and either needs to be machined down or replaced.

 

[yz125rider]

I believe it is a stock cylinder ported by FTZ but I'm not 100% sure on that. A couple other guys have ran this same head before and haven't had problems with it, apparently my friend has some kind of sealant that fixed the problem for his other friend. If I were to put that on there, do you think I would still have issues with the head?

 

[bushwesl]

I believe it is a stock cylinder ported by FTZ but I'm not 100% sure on that. A couple other guys have ran this same head before and haven't had problems with it, apparently my friend has some kind of sealant that fixed the problem for his other friend. If I were to put that on there, do you think I would still have issues with the head?

Using sealant on a head isn't necessarily fixing the problem. It's only a band-aid if anything. You know that this head has been problematic in the past with others, and it's being problematic to you now. I think that it would be a wise investment to buy a new or used head that will work without any band-aids.

 

[yz125rider]

If I run like a used head with the same dome will it still have the same amount of piston to head clearance?

 

[8dave3]

If you are using an aluminum coolhead style head then yes,it should stay really close,provided noone has jacked with the head you have now.If its a stock head,then it will depend if it has been milled or not.The clearance you are talking about is known as the squish band.I think on the R,most guys look for about .035" clearance on high compression builds.Heres the thing with you build,with a leaking head,you never knew what your compression really was.Your best bet would be to find a stock untouched head and a stock 3 piece gasket and go from there.There is no way i would invest in new piston and rings and run a known defective head.Im guessing your buddy is using 3 bond,Honda bond,or some variation of the Three bond sealant.Its great in places,just not to,as HondaR stated,"band aid " it.