oldnslo Posted December 27, 2003 Report Share Posted December 27, 2003 I've been mixing pump gas with race gas to prevent spark knock. Would it be possible to place a second head gasket or base gasket on to reduce compression so I may run pump gas? What, if any, effect would this have on performance?. I ride in areas where there is no way to get race gas or carry enough with you. Thanks, Bill. 315R Quote Link to comment Share on other sites More sharing options...
charliechitlins Posted December 27, 2003 Report Share Posted December 27, 2003 Head "gasket" is probably an O-ring, so there's nothing to do there. Extra base gasket would work...but it's hard to be sure how much. You're probably better off working with the timing. Quote Link to comment Share on other sites More sharing options...
clarkp Posted December 27, 2003 Report Share Posted December 27, 2003 There was a fairly exhaustive post about this subject on the trials.co.uk forums. I think the upshot was what charlie suggested - play with the timing if you can - rotate the stator in the direction of crank rotation slightly to retard the timing (isn't it mounted inside the side-case?). Adding base gaskets does the same thing as adding head gaskets, BUT, it changes the port timing slightly. Quote Link to comment Share on other sites More sharing options...
rineholio Posted December 28, 2003 Report Share Posted December 28, 2003 You have a couple of options. You can probably find a machine shop that specializes in bikes that can cut enough material out of your head to lower the compression ratio enough to allow the use of pump gas. About $40. You need someone who knows how much to cut and how it will affect your bike afterwards before you let them do it. Some bikes run better, but with a trials bike, you might lose lowend snap and response. Timing is probably your best bet. It's easy to change and if you don't like it you can put it back. I wouldn't mess with base gasket or head gasket. It won't reduce your compression enough to help, IMHO, and things might leak later on. Good luck! Quote Link to comment Share on other sites More sharing options...
mark christopher Posted December 28, 2003 Report Share Posted December 28, 2003 can not understand why you can not run it on pump gas? mine is fine, get the carb jetted! Quote Link to comment Share on other sites More sharing options...
rineholio Posted December 28, 2003 Report Share Posted December 28, 2003 Mark brings up a good point. You might be able to jet out the ping by richening it up. By the way, I'm assuming your engine is in good shape and you have the correct plug in it. 1. If it knocks only at full throttle, go up a size on the main jet. If that makes it better and the plug doesn't foul, try two sizes. If that fixes the knock, and the plug still doesn't foul, and your knock is fixed, you may now have a slower responding midrange because it's too rich at half throttle. You could then try dropping the needle a notch to lean your midrange and get your throttle response back, without affecting the wide-open throttle richness. 2. If it only knocks at partial, say half-throttle, try raising the needle a notch. If this fixes it and plugs don't foul, you're set. There are lots of combinations you can try, for example richening the carb, and slightly retarding the timing. You're just going to have to experiment. IMHO, the quality of gas varies from place to place, and I believe octane levels have fallen over the past years, regardless of what it says on the pump, at least here in the People's Republic of California. Several years ago, all of my bikes ran fine on pump gas, but recently they've all become octane sensitive. It's not carbon buildup, because it happened to both bikes, fairly suddenly, and the CR has a new top end. I've already played with jetting, and can't eliminate it without going way too rich. I'm going to play with the timing the next time I head out. If I don't get satisfactory results, I'll start buying race gas, which is plentiful here. Oh yeah, you asked about how performance will be affected. This is my rough observation for the bikes I've owned in the past. Retarding the timing from stock- a little less throttle response down low-the engine revs less quickly, however you can still snap in full throttle and get power. Mine lugged a little better, and was a little smoother down low. A little less top end (for me). Advancing the timing from stock-Better throttle response and midrange power, but I believe I lost some rpm on top. I ran my timing retarded for slippery, muddy conditions, and stock for everything else. I have yet to mess with the timing on my newer bikes. Quote Link to comment Share on other sites More sharing options...
gaff Posted December 29, 2003 Report Share Posted December 29, 2003 I added another head gasket to my 315 with no problems to report, it lowered compression and stopped that pinking/knocking noise i used to get whilst giving the bike a hand-full of throttle. Gaff. Quote Link to comment Share on other sites More sharing options...
marky g Posted December 29, 2003 Report Share Posted December 29, 2003 Different bike I know but.....I bought my 250 Sherco with 3 base gaskets fitted to it and retarded ingnition!!. The bike wouldn't rev out and didn't feel very responsive, I put it back to std and wow...what a difference, it's a dream to ride now If your bike is 'knocking' on full chat, buy some fuel additive...If your altering things to make it feel tamer...buy a smaller cc bike BTW, Cool Avatar Rineholio... Quote Link to comment Share on other sites More sharing options...
mark christopher Posted December 30, 2003 Report Share Posted December 30, 2003 im sure in the uk aditives are banned by the acu? Quote Link to comment Share on other sites More sharing options...
mattylad Posted January 5, 2004 Report Share Posted January 5, 2004 I have put a second base gasket on mine, the effect is good as it softens the motor slightly and is not as agressive of the pilot jet. It also gets rid of the knock at high temperatures common to most monts. There is also no increase in loading up as the ports are raised. ( :santawink: I also use Boyesen reeds, a lightened flywheel and a longer tailpipe) Quote Link to comment Share on other sites More sharing options...
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