Too much compression?

[scalejockey]

Been driving me nuts! My "new" (98YZ)bike has been acting like it's lean,,hot pipe,detonating,pinging etc. but at the same time will foul plugs at least two a ride. Finally checked the compression again,first check(few weeks ago) was after the ALL new top end with no break in,it was 210. This time,after 5 rides it's now 240. psi. with stock squish of 1.3mm or about .055" I gotta tear it down to see if someone had cut the head or the cylinder base:banghead:

Does anyone else been able to run this high of comp on 92 oct swill?

Jetting is basicaly stock.with a new FMF gnarly pipe. (Second one in three rides) Geeeez i hate rocks!!!

[hardhitwarrior]

i think your running a bit too low of an octane.

[Isa Yamaha]

I have my squish set at 0.9mm and it pings if I run anything lower than 98 octane. Try a higher octane or race fuel. Another thing i would check before tearing it down would be the ignition timing.

[BDubb106]

premium pump and race fuel mixed 50/50 will likely fix your issue. Also the reason your comp. likely went even higher was because the rings seated.

[adam728]

Have you checked to see if someone advanced the timing? That would be first on my list to check.

I know comp gauges vary, but my bike typically tests 245-250 psi on my gauge (lots of engine work). Other stock YZ250's I've tested have been around 220 psi in good shape, a tired/weak one was down in the high 180's before being rebuilt. My squish is 0.048" and I get away with pump 91 octane for most my riding. When I was out west I'd boost the octane up for deep sand riding, either going 50/50 with race gas, or adding about 20-25% toulene. And looking at my maintance sheet I've seen 8-12 psi picked up after running an hour or more after a new top end.

Also, what elevation are you riding at?

[scalejockey]

Riding at 3000 ft. Iv'e checked the timing with a timing light,dial indicator also 'by the marks'. I adjusted to 0 BTDC and to 3 degrees BTDC,didn't help.. I looked close at the head and cylinder,before putting it together. I didn't see any evidence of being machined,the only thing iv'e changed is the power valve,the old one had been "ported" for more top end.(that didn't change the way it pings)but at least it has power now.

..Ya,some people should not be allowed to own tools like that!!

FMF jetting sugestion for my pipe is 165 at sea level i run a 168,down from a 170, 5mm float level 3 notch on the needle. I normaly use a ethnol free gas 92 octane.

Race gas can not be an option,i simply trail ride too much.

[BDubb106]

If race fuel is not an option it sounds like it may be time to start measuring to see where there have been mods made. Are you sure the cylinder/ head wernt machined for race fuel prior to you owning it? Race fuel is expensive, but so is a new cylinder or head if yours have been worked to run with race fuel. Puts you in a tough spot.

[Toddwr250r]

gp 100 avgas from your local airport then perhaps?

[adam728]

Another thing to check is that the silencer core isn't broken, end cap smashed, or the stinger on the pipe isn't carboned up. I had a buddy who's pipe stinger was very full of carbon, bike ran hot, pinged, was rich, burned up 2 top ends, was just a POS. Replaced the pipe (could have hot-tanked it) and the thing never ran better. Not enough bleed, too much back pressure.

If you want to see if higher octane will do it for you you can try some toluene for fairly cheap. It's ~114 octane, costs around $15, and you should be able to get it at any paint or hardware store.

1 gallon toluene to 3 gallons of 92 octane ~97.5 octane

1 gallon toluene to 4 gallons of 92 octane ~96.4 octane

If that cures your pinging, then you may need to have the head machined to open the bowl back up.

[scalejockey]

Thanks for everyones input.

I rebuilt the silencer with the rest of the bike when i bought it. It's a fmf turbine core 2.

I took a close look at the cylinder and the head when i rebuilt it,having done a lot of cylinder and head maching myself,,it really didn't look modded. But,anywhos...it's coming back apart today . I'll report back in two days or so,when it's running again.

[bobbyward]

my 04 had done everything it can to keep me from riding it, my timing turned out being its main problem of fouling plugs, the other things by my gauge and how close it is i don't know but i have 265 lbs compression and run pump gas with no problems ? maybe i just finally got lucky on jetting and oil mixture and timing that i found something the stupid thing likes, pulled the plug after the race and it was about perfect with a nice tan/brown burn. couple weeks ago i was gong to have a bon fire and it was the main coarse, now i think i will keep it ! 2 rides in a row with no issues...........................good luck !

[scalejockey]

Pulled the head off today,very little carbon build-up on the piston and none on the head even after 8 (hard running)tanks of gas,,humm.

Did a complete elect ohms check cold and heated. ..Nothing too strange.

Although the secondary on the upper coil was hit and miss,,,took the plug cap apart and found a little corrosion in the resistor & spring,cleaned it all up and put it back together,re tested,all ok.

I'm going to re-shape the head a little on the lathe tomorrow to tone down that compression,, just cus i can!! LOL!!

More fun latter...

[bobbyward]

i smoothed mine up some with just some sand paper since i don't have those kind of toys at hand, took off the rough spots around the port opening too, didn't port just cleaned them up, might make sure there are no real rough spots too.

[YZRob]

oil rich and fuel rich are different. you could run a richer oil mix and be lean on the fuel /air mix which will make it ping under load. I run 178 main jet with 40 to 1 oil. 50/50 c110vp and 93 pump gas. so if I go 32/1 on oil, i would be running lean on the fuel/air mix because i have more oil and less gas. Also running oil rich will not allow your engine to cool properly. the fuel that comes in will cool the engine better. hot engine = ping cool engine = HP

Make sense?

[adam728]

oil rich and fuel rich are different. you could run a richer oil mix and be lean on the fuel /air mix which will make it ping under load. I run 178 main jet with 40 to 1 oil. 50/50 c110vp and 93 pump gas. so if I go 32/1 on oil, i would be running lean on the fuel/air mix because i have more oil and less gas. Also running oil rich will not allow your engine to cool properly. the fuel that comes in will cool the engine better. hot engine = ping cool engine = HP

Make sense?

But more oil AND proper jetting will cool more, as more heat can be transfered from the piston skirt to the cylinder wall, as well as from the piston to the rings, and from the rings to the cylinder wall.

[bmf725]

running race fuel will make your engine run cooler. Dont believe me take a cup of premium pump fuel and pour it on your hand and let it evaporate. then take some quality 110 race fuel and do the same.

[scalejockey]

oil rich and fuel rich are different. you could run a richer oil mix and be lean on the fuel /air mix which will make it ping under load. I run 178 main jet with 40 to 1 oil. 50/50 c110vp and 93 pump gas. so if I go 32/1 on oil, i would be running lean on the fuel/air mix because i have more oil and less gas. Also running oil rich will not allow your engine to cool properly. the fuel that comes in will cool the engine better. hot engine = ping cool engine = HP

Make sense?

Yes i'm aware of that 'theory',,but iv'e never understood how 128 oz container of fuel is greatly effecting the jetting by a change of .5 oz of oil.

Maybe the burn rate,but not 'jetting'..

I run 3.5 oz per gallon of Maxima Super M.

Have done this for 10 years with no problems.

[scalejockey]

running race fuel will make your engine run cooler. Dont believe me take a cup of premium pump fuel and pour it on your hand and let it evaporate. then take some quality 110 race fuel and do the same.

That is evaporation rate you feel. Your motor does not 'feel' evaporation.

But good point, if you can't remember what can had race gas in it .

Anywho...

once again,, ..Race fuel is not an option!!

[hallsy]

I'm going to re-shape the head a little on the lathe tomorrow to tone down that compression,, just cus i can!! LOL!!

More fun latter...

Is the pressure you get on the compression gauge a reflection of the compression ratio, or a reflection of the rings ability to seal?

[adam728]

Yes i'm aware of that 'theory',,but iv'e never understood how 128 oz container of fuel is greatly effecting the jetting by a change of .5 oz of oil.

I think people really overblow the premix ratio effect on jetting thing. If you go from 20:1 to 50:1 that's one thing, but guys changing from 32:1 to 40:1 is another (and is often met with "No! Your jetting will change!")

Lets look at this.

32:1 = 4 oz oil

40:1 = 3.2 oz oil

Calculating it out, a 40:1 mix is 97.56% gasoline, a 32:1 mix is 96.97% gasoline. Ratio wise 40:1 is 0.61% more fuel passed to the engine for a given amount coming though a jet. That's right, less than 1% difference in fuel flow.

You'll see a wider variation than that between two jets of the same size, let alone need to change a jet size. Depending on the jet size the flowed range allowed is typically 1-3%. So a oil mixture ratio change might effect your jetting as a 1% fuel flow change, but changing between two different 170 jets might have a 2% fuel flow change.

Going back to 20:1 vs 50:1, you get 2.9% more fuel in the 50:1 mix. So now you are changing 1 jet size, say from a 175 needing to go to a 172 so it's not rich. I'd guess that most bikes on this forum are already a jet size or more from "perfect" anyway. If you truely are dialled in exactly spot on, then you'll be swaping brass with about any weather change.