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My motorcycle (1982 Yamaha Seca 650) has had a strange problem with the idle for a few years now. I have not been able to diagnose it and I'm hoping that someone here might be able to help. It starts fine and will run fine while driving. seems to have lots of power and very responsive. it will idle just great at 2000rpm, but once the revs drop below 1500rpm at some point it's like one cylinder stops firing completely, the engine will drop from 1200rpm to 500 instantly, the sound changes noticeably and if I don't hit the gas it'll stall out after running like that for 5 to 10 seconds. Also while warming the bike up before rides the exhaust pipe on cylinder 4 would take much longer then the others to heat up (even when the bike has been idling at 2000+ rpm the whole time, which makes me wonder if that cylinder is ever getting a proper burn)

I've tried cleaning out the carbs a couple times, this winter I went through closely and them made sure to clean every little passage and jet. I've tried switching the ignition coils and spark plugs, same cylinder stayed cold.

really starting to run out of things to try.

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    Have you tried to flip your coils around so the 1 and 4 coil is being used for 2 and 3 to see if your cold cylinder changes?
    – DucatiKiller
    Commented Mar 27, 2016 at 18:24
  • @DucatiKiller yup I did, cold cylinder stayed the same.
    – tyler mackenzie
    Commented Mar 27, 2016 at 22:27
  • So as it warms up there is no cold cylinder. Once it heats up then it cools? Also, 1 and 4 work on a dead spark system on that bike. Every top dead center the spark fires whether it's on the compression stroke or the exhaust stroke. That means you can take the lead from #1 and put it on #4 and vica-versa. Have you tried that yet? If not, please do and we can see if #1 becomes the cold cylinder. If #4 stays cold then it's not a electrical/ignition issue and could revolve around valve adjustment or carburetor fuel delivery.
    – DucatiKiller
    Commented Mar 28, 2016 at 20:40
  • nope, the cylinder stays cold when I start the engine and eventually heats up hot. Hard to say if it gets as hot as the others by hand though. Also, as I had stated in my question, and in my comment. yes, I tried switching the coils and leads. the cold cylinder did not change, it stayed at cylinder 4.
    – tyler mackenzie
    Commented Mar 28, 2016 at 23:42
  • Can we start a dialogue in chat. I've owned two of these things and I'm a former mechanic. I think we can figure it out together. chat.stackexchange.com/rooms/340/the-pitstop
    – DucatiKiller
    Commented Mar 29, 2016 at 16:14

1 Answer 1

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More than likely, you have an air leak

If you haven't cleaned out and rebuilt a bunch of 4 carburetor banks in your life there is a good chance you have accidentally created an air leak in the system. Unmetered air will create the symptoms you describe.

The giveaway on most lean conditions is the falling idle. It idles high and is getting enough fuel but the cylinders with the lean condition begin to misfire, the idle rapidly drops and eventually the engine stalls.

Places to check

  • Manifolds - you have four rubber manifolds that are 34 years old. The manifolds get dry rotted. The rubber goes through too many heat cycles and too much UV light exposure over that period of time and the manifolds can easily crack and allow unmetered air into the system. Unmetered air enters the engine between the head and the slide or butterfly of the carburetor or throttle body. This changes the anticipated Air Fuel Ratio and creates a lean condition.

Here is some information regarding dry rot of a tire.

Here is a QA on identifying lean or rich conditions on a motorcycle.

Troubleshooting Techniques

Here are some methods to troubleshoot and discover if you really do have a lean condition.

After you have completed your troubleshooting and resolved the issue, you will want to balance your carburetors. Here is some instruction on that procedure.

It's frustrating issue to troubleshoot, patience is the key as well as tenacity.

Other thoughts

I was thinking this may be a valve adjustment issue but your post doesn't have all the symptoms. If your clearance is too tight and you need to put in thinner shims you will get similar symptoms but the bike won't start when it gets hot after a ride. You would need to wait for it to cool, the valve stems shrink just enough to allow the valve to close all the way and then you can start and it acts normal until it gets warm/hot. So I believe that focusing on a lean condition, checking all of the vacuum lines and ensuring they are connected to your vacuum petcock, your vacuum fuel pump (yes, you have one of those on that model) as well as the YICS bolts on the sides of your cylinder head along with the manifolds is going to be key to success.

Check those links in this post. The information you need is in there.

For good measure, here's a link to instruction on adjusting the valves on a very similar bike to yours. In fact, it's the same exact cylinder head. Good luck.

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  • Good Idea, I have actually noticed the plastic tubes leading from the carbs to the intake are all cracked. never thought they might be leaking. I'll try your test this weekend. I had the engine mostly apart 2-3 years ago and checked the valves then. So they should be good, plus I have no trouble starting when it's hot. So I don't suspect that.
    – tyler mackenzie
    Commented Apr 7, 2016 at 22:26
  • That's good. Doing the shim dance can be a pain in the butt if your not in the mood for it. Those manifolds should be pretty cheap if they are failing. Good luck and hit me up if you need anything. As well, you can squirt carb cleaner on where you have suspected cracks and smell he exhaust for the NOx odor which is really sharp and completely different than normal exhaust odor. OR even spray water on suspected cracks with a squirter to see if idle normalizes while the cracks are temp blocked by the fluid. Good luck buddy! :-)
    – DucatiKiller
    Commented Apr 7, 2016 at 22:59
  • I think you were right. The propane test didn't seem to do anything. but we tried spraying WD-40 on the outside of the rubber manifold and the bike idled perfectly at a low RPM. brought it all the way down to 800RPM or so and still wouldn't stall. It never would have done that before.
    – tyler mackenzie
    Commented Apr 13, 2016 at 1:33

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