high compression using 86 pistons

[muddymess]

I just put an 86 style jug and piston on my 87 bottom end and even with using a vary thick gasket for one the spacer plate gaskets I'm still showing almost 240 PSI. This cant be normal can it?

[86 Quad R]

seems mighty dayum high.

about how far does the piston fall below the deck at TDC? how thick of a gasket you using? what head are you using, what cc chamber and how deep is the squish band?

[C-LEIGH RACING]

Dont do anything else to it, except make sure the carb is jetted rich & dont try to turn the engine high rpm.

You got you a grunt N go engine & dont know it & all you need to learn is where to shift gears.
Neil

[jcs003]

get some hi octane fuel too.

[All250R]

You might want to find out what your squish clearance is if you're not sure where things got tight. You might have an edge on your head that the piston will kiss. Custom engines should be planned, not just thrown together and see what you have, ideally. Check the shape of the solder when you do a squish check and make sure it's flat.

[muddymess]

the piston almost looks like it falls even with the top of the cylinder when i cranked it over.

the gasket is probably 3 times thicker then a normal base casket. I don't remember the actual thickness measurement.

im using the stock head off my 87

I did get some cam2 110 to run since the compression was so high.

I didn't do a squish test but i did put everything on without gaskets and spun the motor to make sure there wasn't anything hitting.

I did jet it out and it was running fine. Had the color good and everything but it seemed to stop building power a lot earlier in the rpm rang

One of the reasons I'm asking about the compression is that I was having running issues at a race and I was just wondering if this might have something to do with it.

This is what happened. About 5 min into a race the bike acted as if i fouled a plug. I took the plug out and it was light brown and dry... didn't look fouled at all. Put a new plug in and it started right up so i took off again. About another 5 min in it did it again. Kicked and kicked but no good. Changed the plug again and it also looked fine. I took this time to crank the air screw out using a stick to try and get back to the finish line. Now it ran for probably a good 20 min on this one but every once in a while i would hear it start sputtering and stuff like it was trying to foul again so i would try and rev on it and clean it and eventually it would take off. Eventually it did die on me again. Now this is the weird part. I only had 2 extra plugs so on this 3rd time i stuck one of the "fouled" plugs back in and it started and ran like it was new plug. It would still stutter every once in a while but i was able to do 2 laps on that plug and it didn't give me any real issues until after the race when i was riding back to the pits it was spitting and sputtering bad.

[jcs003]

is it an ignition issue. almost sounds like your plug is loose...

[muddymess]

well i cant image me putting the plug in loos 4 different times but I asked a few old 250r racers that where at the race and they told me to check my cdi and coil.

[86 Quad R]

i would certainly check the numbers of the stator, coil and trigger to the repair manual and see whats up and also try another CDI.

[machwon]

get a new plug, close the plug gap to .020 and then work on the jetting. You likely have too high of compression to survive, but gauges can vary alot too.

[All250R]

It's best to know the compression ratio more than the static cranking compression - even if your gauge is accurate, but like Machwon said, gauges can cary a lot.

The idea behind a squish clearance isn't just to be set so the piston doesn't hit the head while the engine is on the bench. A 250cc engine should have conservatively .040" clearance because of flex and stretch in the engine parts of a running engine. Jogs and irregularities can cause detonation as they heat unevenly from the rest of the material exposed to combustion.

I'm surprised at Neil's advice to just run it. You absolutely should check some basic things before assembling various engine parts, like an entire cylinder - more than just static compression. Did you check for an air leak?


Originally posted by muddymess

im using the stock head off my 87

What is the bore size of the new cylinder?

[muddymess]

I'm going to check again for air leaks but just by the way it was running it didn't feel like that.

I didn't do a squish check or anything for the simple fact that every "can you use an 86 piston" type question was answered with a simple "you just have to run a spacer plate" answer. So I assumed there would be no issues.

I have no intention of running this bike at this compression because A. 30 dollars in fuel every other week will kill me. and B. It has to wear stuff out faster. I do a lot of muddy XC racing and I hone on that motor a lot. I would feel safer just running normal compression. I only started this thread to see if anyone else has ended up with high compression from running a spacer plate setup.

[All250R]

Originally posted by muddymess
I didn't do a squish check or anything for the simple fact that every "can you use an 86 piston" type question was answered with a simple "you just have to run a spacer plate" answer. So I assumed there would be no issues.

Generically speaking that's true. But you didn't just swap the piston. You swapped the entire cylinder. The reason I asked what your bore size is, is if it's much larger than 66mm, the stock head is going to reveal the edge of the head inside the combustion chamber, some amount below the squish band, potentially the head gasket too if that's what you're using. You get an idea what things can go into a custom engine... The fact that you're having performance issues is telling you this also most likely.

[muddymess]

the bore is a 68

[All250R]

You have more issues to look at than just this one, but here are a couple pictures of what the a head made for a 66mm bore looks like at 67 and 69mm bores. If you're not sure why this is something to think about, ask and I'll see if I can help you.

http://all250r.com/scraps/crheadapplication/crhead67mm.jpg
http://all250r.com/scraps/crheadapplication/crhead69mm.jpg

Here's the overall thread if you're curious about that:
Heads on the TRX cyl (http://www.all250r.com/all250rforum/default.aspx?g=posts&t=100)

[265 sleeper]

I would take the spacer plate out and get the head decked.

[oachs83]

Went through the same problem on mine. I bought my R blown up. It was bored to a 265 and when I put it all back together I was getting around 260lbs of compression. Turns out I accidently bought the thinner head gasket and my cylinder head was a little shaved. I basically had to buy a OEM headgasket OEM base asket and OEM head to get the compression down to around 205-210 lbs.

[muddymess]

removing the spacer plate and then decking the head would only make a lovely bang once then nothing ever again.

Im using a moose racing gasket and its a lot thicker then the cometic gaskets that i usually use.

[265 sleeper]

I forgot to mention you can only do this If the rings don't come out the cylinder when you remove the spacer plate. So if the piston rings are below the cylinder deck height (tdc),measure the height that is sticking out the cylinder and
have a motor builder that knows how to calculate CC and have him mmachine your head to the right CC you can run ether the cr style head gasket which is a thin metal gasket or you can use the pancake gasket which is most common for the trx 250r. This is the setup that I'm running a 4mm crank with the 86 piston with a custom spacer plate that's about or more than 2mm the base gaskets are .20 thick and a cr head gasket. With my head being decked I will be running 180 psi with this setup and will gain 10-15 percent horse power increase running my motor like this

[muddymess]

or i could save my money and just buy a coolhead with the correct size dome.

[rsss396]

you need to do a squish test first, quit pissing around and get that done.

When you do that, the best way is to remove the head and lay a piece of solder from one side of the cylinder to the other inline with the pin.

people will tell you only need 1 side of the combustion chamber and while that will give you something close it does not account for rocking of the piston do to piston to cylinder wall clearance.

250r's have been modded since the day they rolled out the honda dealer so you do not have any idea if the top or bottom of the cylinder has been decked creating a tight clearance.

Honda does have very thick 3 piece head gaskets, we use to peel the 2 outer thin gaskets off and use just one of them as agasket back in the day. I think they were .010-.015 thick

[muddymess]

the head gasket was leaking and the coil was going bad. its working great now.

[265 sleeper]

I will make a thread that shows the power gains from running piston deck height above and even with cylinder deck height. Ill post some videos to show the differance. I'm building a friend the exact same setup other than pipe,carb,reeds and porting. ill show with my bike the differance it runs the 2 ways against my friends bike

I have the 5mm spacer you have and ran my bike this way before as well. It was fast but its nothing like the other way I ran my r