Engine detonation

[scottieh]

I recently bought a rebuilt 1986 XT600 from a friend who had bought it from a reputable shop in California called Brown's Cycles who specialize in vintage Yamahas, secifically dirt bikes and dual sports. They're actually one of the secrets for parts in the US I've just discovered. I'm talking rafters full of parts. Shhh...

Anyways this bike is pretty solid. Some minor things here and there: sticky throttle, rebuild brakes, welding up some minor frame cracks. Anyways I've noticed this bike does have a unique pinging sound when put under load for attempting to launch off of curbs and pop a front wheel off the ground. Only when i flick the throttle and ask it for a lot of torque. I've been warned about something called detonation when using low octane fuel, which is what i started to put into this thing since that's what I've put in every one of my mid-80's xt600's and the Tenere. Never had a problem with pinging. Now i have this new bike and i called the shop in California to ask about it and they suggested detonation as the cause of the sound.

What can be said about this engine phenomenon that i know very little about? The shop said everything about this engine is pretty much stock. Most of my bikes have been stock too save the overbored ones, and I've never had this problem before.

[ActionManMelb]

Run some 98 octane or higher (with no ethanol, not even 5 %) and see if that fixes it.

It may have had a high comp piston fitted.

Someone may have played with the ignition timing or pickup wires.

Is it hard to kick, with the occasional kickback?

[scottieh]

Run some 98 octane or higher (with no ethanol, not even 5 %) and see if that fixes it.

It may have had a high comp piston fitted.

Someone may have played with the ignition timing or pickup wires.

Is it hard to kick, with the occasional kickback?

I was worried it may have very high compression. It is not hard to kick at all. Decomp cable takes care of a lot of that issue i guess. It's currently running a little lean so I'll fiddle with the air screw. l was gonna try the high octane next.

I was planning to travel with this bike next week, hitting up some small towns through the american southwest. Ethanol free gas isnt exactly easy to find and 92 octane can be a hope and a prayer in most of new mexico. I have a copper head gasket and stock piston but not the rings yet to do a top end. May have to place an order and take some time mid trip to make this right.

High compression would explain why this bike runs like a bat out of hell. One of the fastest xt's I've ever ridden.

[scottieh]

Ran a compression test and got some pretty average numbers. 120 psi and 130 psi cold.



Plug looked a little chalky, like it had been running lean, which it was



I had disassembled the carb yesterday to see if i could figure out why i was getting a sticky spot in the throttle twist. Oddly enough it wasnt consistently in the same place. Sometimes it was at the bottom, sometimes in the middle but only when it was at speed. I did discover a small amount of veneer on the butterfly which i cleaned up and then oiled the springs, the cables and the throttle housing. I also checked the jets and everything seemed good, except that the air screw was set to four and a quarter turns out from bottom! I reinstalled it as i found it but yesterday's ride was full of popping and a hot engine so today I adjusted the air screw in about a turn or so and got it to mostly stop popping. Runs much cooler too. But there's still a fair amount of pinging especially under acceleration around 50mph.

I may venture to take a look under the head if the higher octane doesn't help. I'd much prefer to run low octane fuel and lose some performance. I've heard that short of shaving the piston to reduce compression you can stack base gaskets to increase the piston clearance, thereby reducing compression. Not sure if my 120psi cold reading is high enough to justify such a bodge.

[steveloomis]

Your compression numbers are low, try squirting some oil in the cylinder then do the compression check again. If the pressure goes up then the rings are leaking the pressure. If no change then your valves are leaking. Simple test.

[scottieh]

Higher octane fuel (93 octane) worked to almost entirely cure the knocking. Now, since that's not what mister and missus Yamaha ordered way back in 1986, should I anticipate rebuilding my top end to solve this issue? I'm not keen on spending $2.90/ gallon for the rest of this bike's life considering that the cost of a rebuild would only take about 200 gallons (50 fill ups) of fuel to tip the scale, plus who knows when gas prices in the states will go back up to +$4.00/gallon for premium. Yes, for gas we are spoiled here in the states.

I'll crack open the head this week and find out if the piston is stock. Are there usually marks on the top of a piston that indicate original size vs. oversize?

[ActionManMelb]

Instead of pulling the head off, why not put a pencil camera down the plug hole and look at the piston.
Std comp. piston is flat top.
Wiseco pistons will have and id number engraved.

The reason I said to use non-ethanol high octane is because although ethanol is an octane enhancer it will make your engine run LEAN in a stock carb unless you put bigger jets in.Modern cars get around this because the EFI system can richen or lean the mixture at will by change the pulse width on the injectors.

Now we know hi-octane fixes the pinging then you could use E10 with some jet changes.

Looking at your plug, you are running way too lean. Start by raising the needle jet one groove. Then maybe a bigger main jet.

A few simple things will ultimately fix your pinging, just slow down and try things one at a time. Do not tear into the engine for no good reason.

Regarding the extra cost of hi-octane...it's about 50c a gallon dearer. 100 gallons would add $50 to your fuel bill, 1000 gall is $500 more.
Assuming you are getting around 50mpg, that 1000 gallons will take you 50,000 miles. I doubt you will ever do 50,000 miles on this bike.Can you really rebuild your engine for less than $500 or even $250.

[steveloomis]

Jets hard to come by, USA models needle jet are not adjustable, at least easily. No extra grooves. Can large carbon buildup cause detonation due to hot spots? With compression as low as it is, If it were mine, I'd pull it down and have a look after doing the oil in the cylinder test.

[scottieh]

Instead of pulling the head off, why not put a pencil camera down the plug hole and look at the piston.
Std comp. piston is flat top.
Wiseco pistons will have and id number engraved.

Regarding the extra cost of hi-octane...it's about 50c a gallon dearer. 100 gallons would add $50 to your fuel bill, 1000 gall is $500 more.
Assuming you are getting around 50mpg, that 1000 gallons will take you 50,000 miles. I doubt you will ever do 50,000 miles on this bike.Can you really rebuild your engine for less than $500 or even $250.

Last year I put 30,000 miles on my 1985 XT in about 6 months. This year I'm looking at doing another 10-15,000 miles in three months heading up to Canada again. Additionally I intend to take this bike down to south america next year, so cost will add up with the miles pretty quickly. Plus finding 93 octane gas in countries other than the US and Canada is very unlikely. I already have a spare stock standard and second oversize piston and rings, plus the base and head gaskets. The only thing I'm short of are the wrist pin clips and the time to break in a freshly rebuilt engine before scooting off on Sunday. Cost is minimal. Luckily my second bike is waiting for me in salt lake city so I'll pay the octane price to get up to Utah and then rebuild the engine up there and then continue onward with the 85 XT.

I'll have to find someone with a pencil camera. I'm curious to know what's underneath. Going to order some extra jets and play around with those after the rebuild if stock settings don't cut it.

[steveloomis]

The very best to you on your trip.... Sounds like fun....

Yeah, from what I've read on RR's, fuel can get pretty scarce in some countries and you have to resort to buying from people along the road out of glass jugs etc. No telling what you get. Better be able to run on very poor quality fuel and a good running stock XT should do fine.