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Periodic rattling/loss of power 13 Sep 2014 17:13 #647421

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Hi guys,

Recently my bike has started having issues, say for a minute or two every half hour or so I ride it, where it suddenly starts rattling at all revs, and is WAY down on power. After a little while, it will go away just as suddenly, and BOOM, it shoots forward.

I gave the carbs a good cleaning and sync'd them while I was at it, since I figured it might have been something getting clogged and then the obstruction is sucked through . . . but on the short test ride I took, it seemed to happen again.

Any advice for what to check? Apologies if this has been covered - I'm not sure how to express the sudden stop/start nature in a search query.
1980 KZ750 LTD
Edmonton, Alberta

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Periodic rattling/loss of power 13 Sep 2014 18:15 #647426

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Intermittent ignition failure, running on 2 cylinders? check/clean/replace coil connections and HT leads and caps would be a good start...

Can't think of a mechanical fault that would be so 'on and off' like that.
1980 Gpz550 D1, 1981 GPz550 D1. 1982 GPz750R1. 1983 z1000R R2. all four aces
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Periodic rattling/loss of power 13 Sep 2014 18:31 #647430

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Thanks, I'll give that a look as soon as I have a chance.
1980 KZ750 LTD
Edmonton, Alberta

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Periodic rattling/loss of power 14 Sep 2014 14:25 #647511

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I disconnected one coil's wires from the spark plugs and fired up the bike, and that's most definitely what it was doing.

So I cleaned all the connections near there and went for a test ride and it didn't happen. Fingers crossed that took care of the problem, but if it didn't, I know where to start!
1980 KZ750 LTD
Edmonton, Alberta

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Periodic rattling/loss of power 15 Sep 2014 06:31 #647591

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Grrr, nope. Problem reappeared on a longer test ride. Annoying because I won't know if it's gone until an hour of stop/start riding, and then it goes away before I can pull over and have a look.

So given that it seems to be running on two cylinders, it seems likely it's the connection to one of the coils or the coil itself, no? I'm just trying to narrow things down for ordering parts and/or diagnosing things if I can pull over while it's happening. On that note, it seems to happen at or near a stop and go away under acceleration, but that's just based on a couple observations.

Apart from replacing the leads, any other things I should look at?
1980 KZ750 LTD
Edmonton, Alberta

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Periodic rattling/loss of power 15 Sep 2014 09:25 #647612

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ok getting somewhere now, it is likely an ignition fault on one of the coils: this is transistorised ignition rather than points right? if so maybe a failing IC ignitor box or coil, but maybe your interconnecting wiring.

Intermittent faults are actually harder to fix than permanent faults - if it only happens when it is hot this could further point to a coil on it's way out.

Importantly, don't let it fire without plugs grounded as it can damage the ignition!

A common fault on these are the coil connections, they get hot and corroded over the years so I always crimp new female insulated spade connectors on and clean up the blade connectors. With ignition turned off clean up the two block connectors to the IC ignitor. Even if it isn't the problem I always do it on old bikes and you get nicer 'on the button' starting.

Ohm meter testing the coils can be unreliable - had a similar fault on my 750 but everything was within spec for coil primary and secondary resistance, but only replacing the coil finally fixed it.

Is it a cable driven tacho?
1980 Gpz550 D1, 1981 GPz550 D1. 1982 GPz750R1. 1983 z1000R R2. all four aces

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Periodic rattling/loss of power 15 Sep 2014 09:51 #647625

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Yup, transistorized ignition and cable-driven tach. I cleaned up the male coil connections but sounds like it's worth replacing the female connectors if they're especially problematic. If that doesn't solve it, looks like my next steps would be the HT leads and coils, huh? Thanks again.
1980 KZ750 LTD
Edmonton, Alberta

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Periodic rattling/loss of power 15 Sep 2014 10:11 #647630

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Easier for me to crimp new connectors on, but cleaning them up carefully and squeezing them in a bit with pliers to make a tight fit is ok too.

Probably only one coil, no need to change the pair. You might need to swap the new one over to find which coil is the problem.

New leads and plug caps is mandatory for me on an old bike - use 7mm wire.

No probs you're welcome! hopefully someone else will chip in as failing IC boxes can do this too I've read here...
1980 Gpz550 D1, 1981 GPz550 D1. 1982 GPz750R1. 1983 z1000R R2. all four aces

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Periodic rattling/loss of power 16 Sep 2014 10:08 #647759

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One of the plug caps was only loosely on the wire, so I cut off the bad bit of wire and screwed the cap on better. On the way in to work, the problem only appeared once for a second as opposed to a few times for a minute each, so that has me hopeful. Maybe I can keep using it until I get my hands on some new wires.
1980 KZ750 LTD
Edmonton, Alberta

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