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Feels like detonation at 1/8th throttle.

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1986 XR200

Been all through the carb. Clean as a whistle. Clean air filter with no snorkle. A few extra holes drilled into muffler end cap. Good new gas. But still some small what sounds like pre-detonation just off idle in the 1/8 to 1/4 throttle opening. Very annoying when negotiating tight trails. Once I throttle past that it revs up nice.

Any ideas?

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cam chain tension is the first thing that comes to mind aswell as ignition timing too far advanced maby weak springs in the timing advance allowing it to advance too soon

Edited by steve_1987

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With a stock engine it would be too much advance. But my 218 w/ 11:1 also make noises at light throttle/low rpm that i haven't completely solved. My suggestion is to tension the timing chain and then check the timing. A static check of timing will get you close enough since these engines were designed to run on regular pump gas, but I always run premium.

 

The split collar lock (#10) on the cam chain tensioner often stay locked on the adjuster shaft (#6) preventing adjustment.  I remove the 10mm bolt (#15) and then loosen the 14mm bolt (#11) about 1/2 turn,  Then insert a small Phillips screwdriver into #11 and push down, this will push on adjuster shaft #6.  If the collars have unlocked you will feel the tensioner spring pressure push back as you push down, if not give the screw driver a sharp rap to loosen the collars.  Tensioning is automatic after the collars unlock so retighten the 14mm bolt, but not too much, and the chain should be quiet, if not repeat.  I often need to perform the adjustment several times to get a quiet chain.

 

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When checking ignition timing always turn the crank forward towards the marks (CCW e.g. engine running direction) to replicate engine running condition.  Also adjust the cam chain tension before checking the timing.

 

To check ignition timing; pull the spark plug, CDI cover, and the two plugs on the left side cover. Carefully turn the engine forward (CCW) with a socket on the flywheel bolt via the lower hole on the left side cover until the "F" mark on the flywheel aligns to the mark on the case (top of the upper access hole).  Adjust the sensor plate so that the line on the Advancer aligns with the line on the Sensor.  This will set the initial timing. While you're there you can check total advance by moving the flywheel CW back past the double marks, and then rotate CCW to the double marks; the left mark (first) is 32 degrees and the right mark is 28 degrees.  Twist the advancer CCW and check the alignment between the advancer and the sensor.  For a new engine I think you should limit total advance to 28 degrees, which is the right mark.  This will slightly reduce initial timing with a very slight loss of bottom end but you can go back to factory timing after break in, which is total advance between the two marks (30 degrees).  I have run stock engines on pump premium with up to 34 degrees total, but not a modded engines.

As a side note Honda rates these engine for regular grade gas but I've always run premium.  A while back I bought a 90 XR200R and I really liked how it ran, nice and crisp with good throttle response.  When I checked the timing I found it was running almost 34 total, so there is some tolerance if you run premium gas.  But my 218 has 11:1 plus the Torque cam with mild timing so that tolerance probably isn't there.

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