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Motorcycle Repair/1980 cm200t fouled NGK C7HSA Plugs


QUESTION: After sitting for years, took engine apart and installed new gasket kit, new rings, valve guides.  Compression 160/160.  Owned it for 30 years. Bought a new aftermarket carb, fouled plugs in a few miles, bought a different aftermarket carb, fouled plugs in a few miles.  Timing looks perfect, timing light shows right on, runs perfectly through all gears, good power.  R/R is putting out 6.3 V at battery (new battery), should be around 7.5, could that cause fouling?  Any ideas?  Thanks

ANSWER: Alan, you gave me some good information, but not enough about the rest of the setup and your problem...

"Fouling plugs" can mean either oil fouling or fuel fouling.. which one is it?

Oil fouling could be rings not seated yet. Fuel fouling is generally incorrect jetting or high float level.

What size aftermarket carburetor did you install? What was it jetted for? Stock jetting is #42/110 idle/main jets for a fairly small bore carburetor. I know some versions of these bikes have a heat insulator plate that is part of the carb insulator which keeps heat off of the float bowl. If that was discarded, it could be adding to the problems.  #5

I took a moment to check eBay for carburetors and see them listed from $20 to $120 for replacements. I don't know if your old carb was beyond saving, but I would stick with OEM parts if possible.

Did you change the exhaust system or just the intake side? Any changes on either end usually require a jetting change and perhaps a needle height adjustment.

The always ON headlight tends to suck a lot of voltage when the engine is running. Be sure you don't have a stuck tail light switch in ON position, which aggravates the battery and charging system, too. Don't add a larger wattage headlight bulb either. The coil should be relatively happy with 6+volts at the points. DO make sure that you are getting close to battery voltage at the points when they are open.

You might want to check the spark plug caps to see that they are not higher than 5k ohms each, disconnected from the plug wires.

It would seem that whatever carburetor choice you made isn't compatible with the engine's normal configuration of cam timing and exhaust/intake tract lengths or the ones made in China are not accurately manufactured or jetted for this application.

Bill Silver

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Fouled plug
Fouled plug  
QUESTION: The plugs look to be a dusty black, not an oily shiny black. They are both fouled the same also.  The bike air cleaner (cleaned it out), intake, exhaust is all original, except the new carb.  The new carb is an "OEMSTD" brand and claimed to be specifically for the bike, with decent customer reviews.  The 2 jets do not have a jet number on them but are clearly Keihin jets.  The heat shield is original and installed.

I have the original Keihin "PD" carb.  It was more to rehab than buy a new one so I did.  The rebuild kits were aftermarket also.  It is in good condition.  Any suggestions for a rebuild kit brand would help.  I could try that but I am leaning toward a consistent spark issue.  Points are relatively new and look new.

There are no stuck lights or anything like that.  The voltage across the points is 5.3V and the battery is 6.2V with the switch on.

I replaced the old corroded 15 MOhm spark plug caps with two new NGK 5 kOhm caps as part of the refurbishment.  Total secondary coil resistant is 21 kOhm plug to plug.  Primary is 1.5 Ohm.

It is hard to get it to idle also.  At 1200 rpm, it wants to stall.  Tried adjusting idle fuel mixture screw to no avail.  It almost sounds like its missing at lower rpm's.  Not sure of a good way to test consistency of spark.  Does all the fuel supplied at idle come from the pilot screw hole?

Thanks for help.

ANSWER: I would see if you can swap the OEM jets into the A/M carburetor or find a way to measure each of them accurately. Laying the two carbs down side by side  all apart might give some clues. Check the needle taper and height in the slides when installed. If the little butterfly clip isn't seating down properly, the needle will jump up and down inside the slide giving poor metering results.

Keyster kits can sometimes be poorly engineered, but stuff like float valves and gaskets are usually okay to use.

Idle mixture screws/jets control the idle mixtures exclusively.

Look for a high float level, perhaps, unless it has plastic non-adjustable floats.

Make sure your valves haven't tightened up after the rebuild. Clearances are only .002" but need to be at least that all the time.

Check o-ring on the insulator side of the intake tract for flattening out. Spraying brake cleaner, WD40 or even water around the intake area while idling can reveal intake manifold leaks.

Spark advancers can hang up and not advance/return properly which gives uneven idling symptoms.

I am assuming that you got the camshaft installed properly so that cam timing is not an issue. Your compression readings seem close to spec though, so it is probably okay. Stock compression is rated at 171 psi, however. When compression is even but low on both sides, it can mean either a stretched camchain or late cam timing. You should be able to turn the engine over at the crankshaft to the T mark and "feel" if the camshaft/rockers are stopped on one cylinder or the (overlap when both intake and exhaust valves are just slightly open equally).

I don't think that the ignition is to blame unless the timing has shifted due to advancer issues. Idling problems are generally related to compression/air leaks/metering inaccuracies/ignition timing errors. Spark requirements are less than when the throttle is cracked open and under mid-full throttle conditions.

Bill Silver

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QUESTION: I'm back at it and still fouling plugs in a few miles.

The carbs look almost identical except the new ones (I have tried 2 brands now) have a screw in idle jet and the original one has a press in jet.  I put the original fast jet on the new carb and it didn't do anything.

The floats are set to spec and are about level with carb.  No fuel is overflowing or anything like that.

Valves are still exactly .002", i set these when the engine was still on the bench.

I tried wiring advance springs closed and ran it low rpm's for a while and it still fouled plugs.

Sprayed carb cleaner around intake and it did not change rpm's noticeably.

The cam timing is text book, align to T on advance and make sure cam notch is parallel with top of cylinder.  Looks perfect.  When at T, cylinder one tappets are all at .002" (full closed valves).  Rotate another 360 degrees, cylinder 2 is the same.

Didn't quite understand your "feel" of the compression around "T", but seems good.

I also tried checking spark jump, FSM says 0.25" jump is needed.  The spark did jump 0.25" but it looked kind of inconsistent and small.  When trying to kick start, the spark would occasionally not jump at all for a moment.  bad points? (they are new).

I also tried hotter C6 plugs that fouled about as fast as the c7's.

Maybe the plugs are oil fouled.  Is there a definitive test?  oil vs fuel.  There is no smoke at the exhaust either, which is weird, considering the fouled plugs.


I am not sure what to try next.  Have tested all I can think of.

Your inconsistent spark comment is worrying. If the plugs are fuel-fouling, then the spark can short down the side of the electrode instead of jumping the gap. Check spark with fresh plugs. IF still irregular, then clean the points make sure gap is about .014" and timing set to F mark alignment.

If the points are already pitted or are < < cratered, then the condenser is failing. If they look like they are making good contact and nor arcing heavily, then coil could be breaking down when it gets hot or you have some kind of iffy wiring connection in the ignition circuit.

I still don't have a lot of faith in the Chinese copy carbs being offered out there. OEM is usually the best bet.

Mixture screw should be out about 2 turns to begin with. With fresh plugs turn mixture screw in as far as it will go and keep idling. If it is nearly closed, then you are still getting excess fuel from an intermediate circuit or you are not getting enough air from the idle air bleed passages.


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Bill Silver


Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.


I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

VJMC (Vintage Japanese Motorcycle Club) of North America

VJMC newsletter, as editor for two years and as contributing editor currently.

3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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