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jjjb

Any 650L/600 Engine experts out there

14 posts in this topic

I have modified my poor XR650L to the point that it no longer can operate on pump gas. It has a Edelbrock pumper carb, JE 10.5:1 piston, Barnums cam w/ hardwelded rockers and FMF exhaust. Problem is I need to feed it CAM-2 or equivolent or it detonates/pings under load. I'm looking for advice on the easiest solution to return to 93 octane pump gas. Any help would be greatly appreciated.

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Well, make sure you're jetted properly first. The only way to know for sure is make it richer until you know its TOO rich, then lean it a little.

Jetting it a little rich may help as well.

Dave

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Unfortunately using two HGs kills quench and while it lowers compression, it can also possibly do nothing or make pinging worse. If I had to pull the head off, I'd just go the extra step and pull the barrel and install a 9:1 piston. Another option is to get a cam with a grind that bleeds off some compression or perhaps adjusting the one you have. What rpm range is the Barnums cam for? If its a low/mid, that isnt helping because they tend to make alot of static compression whereas top end cams focus more on dynamic compression.

On a side note, I would have figured that most pistons under 11:1 compression would allow for 91-93 pump gas. Air these aircooled XRs that demanding of octane?

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If you have not richin the carb that maybe the problem. The stock jetting in a edelbrock carb is not for your setup. You may want to look at smaller metering rod. Another item is the plug. One or two ranges colder may help. A cheap octane booster is xylene paint thinner from a paint store. Mix it 3 parts gas to one part thinner. I run a buick grand national and it works in it.

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2 base gaskets the go if you must run that piston. This would drop your comp ratio back a fair bit. Also you could go back to stock one which isn't that bad (lighter). Otherwise there's no other option than to run higher octane fuel.

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Thanks for all of your help. I'll play with the edelbrock and spark plug when I run all the Cam-2 out of her. The carb is already setup rich, my rear fender looks like a chimney and there is no lean pop on deceleration. For the cam, I asked for maximum bottom/mid. I did try off the shelf octane booster and that did nothing. I am driving this bike cross country this summer and so I would probably go through the hassle of a piston swap as a last resort. I was also thinking of a mild timing adjustment by messing with the pulse generator timing but since no one suggested that I think I'll try the two headgasket trick/ plug/ misture and go from there.

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I was also thinking of a mild timing adjustment by messing with the pulse generator timing but since no one suggested that I think I'll try the two headgasket trick/ plug/ misture and go from there.

Is the ignition trigger/pulse pickup/whatever even adjustable? Too bad the ignition isnt externally programmable so you can back off the advance curve.

By the way, when exactly did your bike start pinging? Did it happen after installing a part/parts, or did it develop?

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Unfortunately using two HGs kills quench and while it lowers compression, it can also possibly do nothing or make pinging worse. If I had to pull the head off, I'd just go the extra step and pull the barrel and install a 9:1 piston. Another option is to get a cam with a grind that bleeds off some compression or perhaps adjusting the one you have. What rpm range is the Barnums cam for? If its a low/mid, that isnt helping because they tend to make alot of static compression whereas top end cams focus more on dynamic compression.

On a side note, I would have figured that most pistons under 11:1 compression would allow for 91-93 pump gas. Air these aircooled XRs that demanding of octane?

***HIGHJACK***

sounds interesting...what is dynamic and static compression, does it have to do with valve overlap and scavenging?

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The bike ran fine with an edelbrock pumper and the fmf pipe and at the time a stock piston with hot cams camshaft. It was when I added the 10.5:1 piston and Barnums cam that it started hating pump gas. The pulse generator is not adjustable per se however since it uses a bracket held in place by two bolts, a little careful dremeling makes just about anything adjustable. I could think of no easier way to retard the timing just a tad.

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What temperature is the oil running at with the high comp piston? Is it higher than with the stock piston?

I would add an oil cooler to try and get the head temps down, that will help reduce pinging. Other than that, retarding the spark is a good idea as well.

MGS

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***HIGHJACK***

sounds interesting...what is dynamic and static compression, does it have to do with valve overlap and scavenging?

Not exactly a hijack if someone else wants to know. :cry:

I'm sorry, I left out a few things. Static compression is your compression ratio as calculated by measuring the chamber, bore, and stroke. Dynamic compression is the actual compression ratio as it would be while the engine is running. Cams are usually the biggest influence on this as one with lots of overlap will blow some of the intake charge out the exhaust before it can be squeezed by the piston. The manufacturers usually state that these cams require higher than stock compression for this reason. These cams kill your bottom end torque until the RPM comes up high enough for a "ram air" effect to happen in the head. This is when the port veocity gets high enough that the air entering the cylinder piles up like a train and as a result, more air gets into the cylinder than could by vacuum alone. Its actually possible for very well designed normally aspirated race engines to exceed 100% volumetric efficiency.

Low RPM cams do the opposite. They make full use of the available compression. This is why a high comp piston and a grunt cam open the possibility of pinging. So yes, they both involve overlap and scavenging. :cry:

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