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jaredc28

Loss of power and weird noise at full throttle?

36 posts in this topic

93' xr650l-big bore, supertrapp exhaust , daves mods, removed snorkel, de-smogged, just over 18,000 miles.

First off this problem only happens when on the highway in 4th or 5th gear, and only when the bike has been running for 10-15 minutes or more.

When I get on the throttle the bike starts to pull, then begins to loose power and I can hear kind of a pinging noise from the engine area (sounds like putting nuts and bolts in a metal pan and sliding them back and forth).

The bike does not make the noise if it is in neutral and I hit the gas, or when I am in the lower gears at high throttle.

HELP! :applause:

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Sure sounds like preignition, sometimes called detonation or "spark knock". The jetting might be too lean, spark plug heat range too hot, carbon buildup in the combustion chamber and piston crown or a combination of these.

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If it has just started this, it might be that you have a tank of bad gas as well. Try putting some octane booster into the tank, and also go to a known good gas station and top it off with premium fuel to see if that helps. If the problem continues, you are going to have to find it and fix it fast. If it is pre-ignition, and it is left unchecked, it will destroy the rod, piston, & possibly the head over time.

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If it has just started this, it might be that you have a tank of bad gas as well. Try putting some octane booster into the tank, and also go to a known good gas station and top it off with premium fuel to see if that helps. If the problem continues, you are going to have to find it and fix it fast. If it is pre-ignition, and it is left unchecked, it will destroy the rod, piston, & possibly the head over time.

Good point, low octane gas could very well be the culprit.

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Pre-ignition and detonation are two totally different phenomenon. Both are forms of abnormal combustion, but they are seperate and distinct.

jaredc28, what you're describing are classic symptoms of detonation (a.k.a. "pinging", "knocking", "spark knock"). What piston and cam are in there and what fuel are you using? Big bore air cooled engines are prone to deto when you start upping the compression. Also, make sure your not jetted lean. Lean a/f ratios are much more suseptable to deto.

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Pre-ignition and detonation are two totally different phenomenon. Both are forms of abnormal combustion, but they are seperate and distinct.

jaredc28, what you're describing are classic symptoms of detonation (a.k.a. "pinging", "knocking", "spark knock"). What piston and cam are in there and what fuel are you using? Big bore air cooled engines are prone to deto when you start upping the compression. Also, make sure your not jetted lean. Lean a/f ratios are much more suseptable to deto.

I have no idea what piston and cam I am running...I assume they are stock because the previous owner did not tell me otherwise.

I asked the previous owner if he ran high octane gas and he said he was told not to, so I have done the same since buying it in October 2006.

How do I make it "not lean"...bigger main or slow jets? When I did Dave's mods the jets were 52/158, should I change them?

Continued help please...

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Ahh. I read, "...xr650l-big bore..." and thought that meant engine work had been done.

Did this problem just crop up out of no where or ??

I can't help on jetting specs as I'm not that familiar with XR650Ls.

I'm sure a correctly jetted, internally stock XR650L can run deto-free on 87 with it's lowly 8.3:1 static compression ratio. Assuming the engine is stock I'd focus on the fuel and jetting. Bad or old fuel? Lean jetting? If both of those are elimintated as trouble spots then it'd be time to look at some of the more unlikely causes of a deto condition--like a sheared flywheel key causing too much ignition advance.

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When I first bought the bike I noticed it had the loss of power at high throttle, but I never heard the "spark knock", maybe it was doing it and I just didn't hear it.

I believe the noise and the loss of power have become worse after doing Dave's mods. If I were to focus on the jetting, would I go up or down from what I have now to potentially correct the problem?

THANK YOU!

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Leaner mixtures promote deto. Richer mixtures suppress deto.

Try blocking off some of the air to your airbox with duct tape. That will have the effect of richening your jetting for testing purposes without having to actually rejet. See if the deto condition and general performance improves any.

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I have a similar problem with my '95 600. It didn't do it for a year after I got it, but last spring/summer it started rattling at 3/4 throttle. It still does it even after the weather cooled down and I tried jetting up as high as 68/170. Supposedly had a recent XRs Only rebuild when I got it that I assume was stock. I only run premium(91) Arco or Flying J gas in it(cheapest around here, heh). I may try Chevron, Mobil, or Unocal to see if there is a difference. I'm not about to start buying race gas; the local bike and race shops want $8/gal! :applause:

Anyway, just getting on board to see what develops.

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Doing a compression check can be reveiling when chasing down a deto problem. Even though a rich a/f mixture in-and-of-itself is less prone to deto, if one is way too rich for long enough and carbon deposites build up on the piston, compression can start to rise. Too much compression without enough octane rating = deto.

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What does the spark plug look like? White residue (Lean), Brown Residue (Good), Black Residue to Black Soot (Rich). Are you running the correct spark plug?

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If it starts to pull good when you open the throttle and then starts pinging and losing power check your fuel supply to the carb. sounds to me like it may be a partially restricted line or petcock. Disconnect the fuel ine at the carb (bike shut off) and put the end in a jar or bottle, turn the petcock on and see if you have good flow. If so, not the problem, if not, loosen the cap and see if you have a vent problem. If still poor flow pull the hose off the petcock and try again. I always check the simple stuff first.

If the problem just started and flow is good I would dump the tank of gas and try a tank of good stuff, might be just bad gas.

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Ok, changed the plug (to a new honda plug) and the old one had a "brown" residue on it and it looked good overall, changed gas and upped the octane, checked my petcock flow and it was good, cleaned my k&n filter and re-oiled it...I am still having the spark knock at 3/4 to full throttle.

I assume this means that I am running lean and need to increase the jets? Any suggestions...as I said before, I am running 52/158 currently?

Thanks again, I really appreciate all the help and advice!:thumbsup:

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or maybe try just adding 1/2 gallon or so to a tank of gas of some race fuel, Maybe a buddy you know can sell you just a bit to see if it is a fuel thing. with a big bore they might have had a higher compression piston put in and IF you have some bad fuel it will be doing what it is just a thought

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Ok, changed the plug (to a new honda plug) and the old one had a "brown" residue on it and it looked good overall, changed gas and upped the octane, checked my petcock flow and it was good, cleaned my k&n filter and re-oiled it...I am still having the spark knock at 3/4 to full throttle.

I assume this means that I am running lean and need to increase the jets? Any suggestions...as I said before, I am running 52/158 currently?

Thanks again, I really appreciate all the help and advice!:thumbsup:

For dave-mods and stock exhaust , baseline jetting is 55/158.

With aftermarket exhaust the 158 may or may not be lean, the 52 is way too lean.

The slow jet should not affect WFO jetting, of course, but if its that lean, I'd fix that 1st.

If you look around, baseline for your setup is about 55/158 to 56/160, if I remember what I've read here correctly.

Since it has the stock slow jet, when you're in there make sure it has the slide drilled and the needle shimmed, if you bought it with the mods already done.

Dave

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Yeah, the carb may be dirty. Worth a shot. I have dissasembeled several different carbs in the past with engines that lacked power (some worse than others) and have found little pieces of dirt lodged in the main jet. However, the power loss was significant. yours sounds less severe as far as power loss in concernd. but may still be an issue. process of elimination.

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is "spark knock" more prone to happen at higher temps? it seems that my problem becomes worse when the bike has been running for more than 15 min or so.

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is "spark knock" more prone to happen at higher temps? it seems that my problem becomes worse when the bike has been running for more than 15 min or so.

Absolutely! High temperatures is one of the fundamental conditions that allows spark knock to occur.

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