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Left side spark plug keeps dying

5673 Views 15 Replies 6 Participants Last post by  Shep
I have an 81 GL500 that sat idle for 15 years. With the help of this forum I have it running at 80/90% after about 2 weeks of riding/tuning.

The problem I have now is that the left side spark plug keeps dying on me. I have replaced it 4 times in two weeks. Each time I put in a new one it runs like a charm for a day or two and then it starts to spark intermittently and then not at all. I can't find anything on this forum that will point me in the right direction so I am starting a new post in the hopes that someone has encountered this before and can tell me what the heck is going on.

I was using the ngk spark plugs and tried a different brand from a different store, just to make sure it wasn't a bad batch I was buying from, but the problem continues.

Has anyone encountered this? Tips/hints/suggestions?


Vancouver, BC Canada
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It could be that the resistor in the plug wire cap has gone bad and is causing your problem. There is a thread about the spark plug caps and how to replace the resistor with a section of copper or brass rod on the forum that has been posted in the new forum sometime in the past year. It can help show you how to take apart the cap's guts to replace the resistor section. I'm guessing you have a problem with the cap having an open or too much resistance causing that cylinder to drop out and foul.

The plugs you are pulling...are they carbon fouled (black) or what do they look like?

The caps unscrew from the wires. A long screwdriver with the blade of the correct width will allow you to unscrew the guts of the cap to disassemble it...

You can still get new caps from Honda but they are pricey. No good aftermarket replacement is available because of the length. There's a place in the UK that carries a replacement.

I'll see if I can dig up the post and I'll post a link to it in here if I can find it.

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Here's a link to the "how to" from the old forums on the Spark Plug Cap surgery I was talking about...


Hope this helps you out.

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Thanks David,

I'll look at that link. The plugs are coming out carbon fouled.

Sounds like an oil fouling problem to me. It could be a loose valve guide or damaged piston ring or cylinder wall. Airplane guys have a scope that pokes through the spark plug hole and allows you to see the cylinder wall but its easy to remove the head if you have a gasket. If its a ring it will show on the wall. Try wiggling the valves at the top end to find a loose valve guide. Mielage info would help this diagnosis. Higher meilage engines have more valve problems. The last time I saw this problem the oil control ring had a problem and one new valve guide was larger than spec ID. The two worked together to use oil and foul the plug.

Capt Frank
Oil fouled wet black plug or rich mixture dry black plug?

Can you post some photos of the plug(s) you've pulled?

Capt Frank - I hope that's not the case as we have a short riding season in Vancouver! If I can get through the next three months, I'll be happy to work on the engine during the fall and winter.

David - the plugs are dry & black. They were wet before I worked on the carburetor last week, but coming out dry and jet black after just a day of riding right now. I'll post some pictures tomorrow as I would like to get to the bottom of this. I did the aluminum rod replacement for the resistors tonight. I'll throw a new plug in that left side tomorrow and see what happens. I have the mixture screws set to 2.5 turns out. I'm sure this will sound like a stupid question, but do I turn them in or out to get it leaner?

Capt Frank - the odometer reads 20,000 miles, but I have no idea if that is accurate. I have had the bike for a year, but just on the road for a few weeks. One owner before me and he had it parked for the past 15 years. It looks like he took great care of it.

Ignore reference to Electronic Tacho.Not needed.

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I originally posted the problem I am having (spark plug issues after just a day of riding) was always in the left cylinder, but it has happened in the right as well.

I'm having trouble getting a clear image of a spark plug with my cellphone cam, but the shot below gives you a pretty good idea. After just one ride, my plugs come out dry and black. I checked my mixture screw and set it to 2 turns out. I checked my air filter and it looks good. I'm not riding at low rpms so that doesn't explain the black plugs and even if I was, it shouldn't happen in less than an hour.

I put in a new set of plugs and the bike runs great - smooth idle, power right through from idle to 9000rpm. Took it out on the hwy and got it up to 75mph, rode it for about 30 minutes with a mix of hwy and city and then it started giving me trouble again. On the way home I had to rev the engine at stop lights to keep from stalling.

The manual says carbon fouled plugs are from too cold a plug (running the NGK DR8ES-L in 65 degree weather in Vancouver Canada), weak ignition (I recently did the brass rod fix and tested my ignition with a multimeter - everything appears to be to spec), dirty air cleaner (I'm not sure where to get a new one, but the one I have looks pretty good), too rich a fuel mixture (I'm set at 2 turns out), or excessive idling (not a chance).

When I checked the spark plugs after last night's ride, one was sparking from everywhere but the tip due to the excessive buildup of carbon. A quick clean and it was working fine again, so the problem would seem to be caused by excessive carbon buildup.

Has anyone experienced carbon buildup this quick and this bad?

I don't have a history on the bike, but the PO clearly took great care of it, then parked it for 15 years. I got it running easily after cleaning the carbs and changing the oil. The exhaust coming out of the tailpipe looks good, no blue and very little black (shouldn't I see a lot of black with how quick the plugs get coated?).

I'm lost and appreciate any more advice. The bike is a pleasure to ride but I can't afford to change the spark plugs daily!!

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Is there a chance that the choke plates are sticking on?

Seems weird to soot up so fast.If this were a CDI engine I'd say next step renew coils.You could also try Iridium plugs.Much more robust and forgiving.
Check for closed choke plate (as suggested) and attach clear tubing to the carb drain nipples. Loop the tubes up and open the drains to compare the gas level in the float bowls to see if the problem side is noticeably higher. It seems like a "too rich" condition, as poor ignition would likely give poor performance from the get go, not only after the plug carbons up. Then if no problem is found, disassemble the carb to check for incorrect placement of jets or jet unscrewed.
Just to add.A spare set of carbs I serviced had both sticky Throttle butterflies and choke plates.The alloy corrosion of being left unused had caused the small holes where their axles meet the bodies inside the intakes stopped them operating correctly.I worked them loose using Rust release and operating them for several minutes and then cleaning with Carb/Brake cleaner.I also of course full cleaned and lubricated the linkages.This is best done with the carbs off the bike so I suggest a full carb service.

Full section covering carbs here,
The choke and throttle butterflies check out ok. I just pulled out the carburetor and the choke is full open and the throttle butterflies open when I crack the throttle.

I'm curious now though about the throttle pistons. They don't appear to be connected the actual throttle in any way, so how do they work - on vacuum or air pressure?? I would have thought that the throttle would lift this piston, but all I can see that the throttle does is control the butterfly.

Shep, I went through carb sections after following your link, and I realized that I didn't remove the black banana cover and clean down the air jets. I'm not sure how these might affect the lean/rich mixture, but I'm going to clean them and see what happens.
Shep, I went through carb sections after following your link, and I realized that I didn't remove the black banana cover and clean down the air jets. I'm not sure how these might affect the lean/rich mixture, but I'm going to clean them and see what happens.

They are,"The" most problematic area of the carbs IMHO.They are the tiny venturi part of the low speed idle circuit.If not cleaned properly will cause all sorts of problems.The Carb pistons/slides are,as you correctly assume,raised by the partial vacuum created at the intakes of the carbs.

I use copious amounts of rust release down them,then carb-brake cleaner and whilst doing so also poke a thin guitar string down them.Then high pressure air blow them through.

On refitting carbs the most common problems are not re-checking and setting the float heights and introducing new faults via carb boot air leaks.

Note:I never poke any metal implement down soft brass main jets.
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