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Discussion Starter #1
Howdy Honda Twins Forums,

I’ve been banging my head on this problem for days now, and would really like some advice from those more experienced:
I’m working on my 1973 Honda CB200.
Currently, the right cylinder is firing like a dream (so far as I can tell with my hand over the pipe).
Meanwhile, my left cylinder is firing erratically and weakly. The left cylinder was running rich before the erratic firing was such a severe issue.
Until recently, I would only have trouble with the left cylinder from time to time, usually in the ¼-3/4 throttle range. I could usually regain consistency and power by fiddling with the choke and finding a sweet spot (the bike has never run well with the choke fully closed). After a few weeks sitting, the bike seemed to be running as usual, but once it fully heated up, the left cylinder became erratic and I lost power. At this point the left cylinder will not fire regularly at idle or any speed/temperature.

This is what I’ve done so far:
-Pulled plugs: left side black from running rich and right side normal (no improvement with new plug on left side).
-Checked fuel flow from petcock and replaced in-line filters (all good)
-Pulled carbs off and thoroughly cleaned all components. They were already shiny, but I did find one clogged jet hole.
-Swapped out needle jets with some aftermarket ones that sat a bit lower, hoping that might lean out the mixture. These made the bike run worse and fail to idle at all, so I put the OEM back in.
-Swapped spark plug caps to eliminate weak spark possibility (no change in either cylinder).
-Checked valve clearance on the poorly running cylinder for both intake and exhaust (TDC, compression cycle). Intake was set at roughly .005”, so I adjusted to a loose .002”. Exhaust was sitting between .002” and .003” so I didn’t fool with it (still no change).

I haven’t messed with the ignition timing as the right cylinder is running well. Please correct me if this is false logic.

I have tweaked the mixture screw on the bad side, but settled with the manufacturer’s recommendation of x1.25 turns.

I haven’t done a compression test, as I don’t have the equipment, but there is certainly high pressure on my finger over the plug hole while turning over.

I’ve never synchronized carbs, and I’m wondering if carbs out of sync could cause these problems.

Also noted, the O-rings between the head and the carb. manifold are pretty badly melted and misshapen. Looking to replace them, but I figure if anything, their failure would cause a lean mixture. Wondering though if an air leak here could compromise compression.

Thank you for your time,
Kyle :confused:
 

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You are making logical steps, don’t get dismayed!

Carb start (sync) could be out but as the problem got worse, it’s unlikely.
Plug gap?
Swap HT leads over, same issue?
I would check the spark on both sides, pull. Out plugs out, put a new plug into the cap on RHS and rest the electrode on the barrel and turn over, this is your reference for a good spark. Do it again on the LHS, how’s it look in comparison?

Simo recommend to another chap with poor running to remove any aftermarket inline filters, they can be too restrictive.
 

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Discussion Starter #3
Thank you Alex jb!
Spark seems the same on both left and right sides and switching the HT leads didn't yeild any change. I did find that the LHS plug gap was considerably smaller than recommended spec. I adjusted it, reinstalled, and there was definite improvement. Still thinking that i'm having a mixture issue.
Tweaking carb adjustment screws, I was able to get both cylinders firing consistently on idle, and more consistently upon revving. I've only been able to attain a high stable idle, and I got the bike a bit too hot messing around at a stand still. Letting it rest and doing a bit more research on carb adjustment and synchronization.
 

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If the filters are sintered bronze they can mess with the flow
I'd check and reset the timing for all you know the left could be spot on and the right out somewhere but if the timings off none of the baseline settings are gareenteed correct .
Post up some photos we like them and makes it easier to spot things obviously out

The settings for the carb screws are a start point not the finish line
1.25 turns out is where you start 1200rpm and a stready hum is where you stop
In between is a bit of back and forth
Look up Steve( 's aka 66sprint) carb setting for a cb175 here on HondaTwins and follow that after setting the timing
 

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If your head has seen a lot of plug in/out action or if it has had a helicoil it could be putting the plug in too far, I have this on my LHS, it was allowing the piston to tap the electrode and close the plug gap.
I solved this at the time I noticed with an extra plug crush washer and later with a copper washer.
 

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True, my points cam lobes seem to look worn differently though, could make one side fire a bit off.
Can’t adjust though, would need a new points cam
 
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