Sudden revs drop while cruising on highway

Hi xj community. I'm sorry if this question has been addressed before but here it is.
My otherwise fine '82 Seca 750 is making my heart stop when the revs suddenly drop only to pick up again.
It will then run fine as I'm turning around and heading back home thinking it will leave me stranded on the side of the road. Is it electrical ( bad coils acting up?-the battery is brand new) or bad gas or clogged fuel
filter or something else. I love my bike but it does this once almost every ride.
Thanks
Juniorglide

 

Can you define revs drop a little better? Does it act like someone shut the key off or is more like it is starving for fuel? My 82 Max 650 would die and come back when the kickstand switch was acting up, scared the crap outta me the first time as it felt like you were about to go over the bars, probably because it was so unexpected.

 

Hi. It's a drop from 6000 to 3-4000 . If I would happen to ride with one hand, the sudden jerkiness could maybe make me lose control.
It usually happens when I've been riding at 60-70 for a little while so the engine is warmed up.

 

Let's start by narrowing down the possible causes.

When was the last time you checked the valve clearances?

 

sounds like a fuel issue to me.

 

The valves were done early January 2016, about 4000 miles ago. I hear them ticking like a sewing machine so I think
it's alright. A friend mentionned the gas filter could be clogged or using regular gas as possible causes but I'm not sure. I did use Lucas injector cleaner to try to solve the issue.

 

Was the petcock rebuilt?
Is the petcock vacuum line actual vacuum line, and is it new(ish)?

 

As far as I can tell, it's the original petcock with a vacuum actuated valve, not rebuilt by me. When the drop happened, the petcock was in the reserve position
becuse I had put a double ( or triple!) dose of injector cleaner and wanted to run that till the last drop. But I've had other rides with rev drops in the on position as well.

 

I forgot to mention that each drop in revs is accompanied by a backfire- sign of sudden lean mixture?

 

Regular gas shouldn't be an issue. A plugged filter, failing petcock or vac line(it will collapse if soft), or faulty fuel cap (vent needs to allow air in) could be to blame while running higher RPMs, all of these will eventually cause the engine to starve for fuel. When the revs drop, the fuel consumption does as well and allows the engine to "catch up" again. If you don't know the history of these items I'd recommend a new proper filter and at least pulling the petcock apart for inspection. Chacal has complete rebuild kits if you want to do it once and be done.

 

how to clean gas cap
Your Gas Cap and You

does this problem occur when running on prime?

to test open gas cap to see if it fixes problem
replace the vacuum hose with vacuum hose 4mm it is cheap fix and should be done if you do not know how old line is

 

Thanks everyone, much appreciated help. I will check the petcock, gas cap, fuel filter and change the vacuum line, all items I would not have considered.
I thought the coils were not firing momentarily and skipping a cycle.
Thanks again, I hope this will cure the problem.

 

XJ550H hit the troubleshooting on the head. Run on prime and if it goes away, its probably isolated to the vacuum line or petcock. Try running with the cap loose and if it goes away you probably have a vent issue.

 

Will do. Thanks for the easy -to -do tests. Will report back soon.
Juniorglide

 

Hi community. Allow me to make another hypothesis about the rev drops before I test for petcock and gas cap:
pick-up coils. I read it was always a problem for Yamaha even on the Star 650 until 2008. What is your opinion on this?
Thanks

 

Steer flop. Pickup coils rarely go bad, and if the pickups on Yamahas were failing at a high rate we'd see failures on other manufacturer's motorcycles as well.

These sorts of rumors seem to be started by people who don't understand how the Japanese industrial system works. Yamaha doesn't make any of their electrical parts (the music division might though). They use parts that are made by other Japanese companies, just like all the other manufacturers do. It's a (smart) holdover from the need to rebuild their industarial base following WW2. That's why you will still see Mitsubishi made electrical parts on a Toyota, and on a Honda, and on a Mazda (and Yamaha heads on Toyota engines, and Subaru engines in a particularly sexy model of Toyota).

Having said that: it is not impossible that you have that sort of failure, but it is far more likely that you have a fuel delivery issue as rubber parts fail far more often that solid-state electrical parts do. Cas cap vents also get plugged over time from sitting outside, or from bugs making nests.

Bottom line: check the easy and inexpensive things first.

 

I would also check your fuses, especially if you have the original fuse box with the old style glass fuses.

I had my ignition fuse work loose a while back and it caused intermittent contact which I at first thought was a fuel issue. I really need to install that new fuse box I got sitting on my workbench.

 

Same thing happened to me. Check fuse box in addition to other ings above.

 

another easy check would be to mist the coils with water before a ride

 

I just came back from a short ride in the rain to test some of the tests (!). First, I do have a new fuse box, a new agm battery, I checked the stator for resistance. The battery posts screws were loose so I tightened that. I removed the petcock this morning and removed the screws to all 3 areas- lever, gas and vacuum. The thin black gasket of the vacuum hose had no holes in it and looked good. Then I went out and had to turn back after 1 mile cause the rev thing happened again. It's like sudden engine braking. I turned the petcock to prime on the way back but it happened again. I couldn't test the gas cap thing because of the rain. Right now, my bike is totally unreliable and it sucks!
So I understand that pick-up coils can't stop sending signals for a fraction of a second and cause the drops?

 

Happening that soon and the engine braking description make it sound like it may be electrical. Try hitting your kill switch while running at a good clip down a straight non busy section of road and see if the symptoms are the same. I'd be looking at switches and loose/corroded connections before coils.

 

Hi Jayrodoh. It was always my feeling that the problem was electrical. I guess my only option is to remove all old plastic connectors ( their shape changes with heat and time) and solder the wires together. Sounds like a winter project!
Thanks to all

 

Don't go soldering all the wires together. If it's not the problem, now you'll have to unde stuff when you DO need to replace things.

Sounds like a TCI Issue to me.

 

me too, did this start when the weather started getting cooler?

 

Definitely not your only option, there are plenty of 30+ year old bikes running thousands of miles a year with the original connections. A tedious cleaning of them is recommended though.

Have you done any electrical trouleshooting on the problem?

 

Not really. Don't go doing that. Waste of your time.

Diagnose the problem before jumping to a conclusion.

 

I'm going through all the connectors and cleaning them up. Funny, I disconnected one ( three-wire near the battery) and the engine kept running, could restart and all the lights and flashers were functionning! The only wires I hard wired this summer was the one coming from the stator. The connector had melted. I think I found the one from the pick-up coil, it's behind the solenoid? Next I'm going to remove the tank and clean all those connectors. Thanks for the tip.

 

Three-wire clip near the back of the battery? Probably the harness to the rear lights

 

Hi. Last post about my problem. I will clean the connectors with de-oxit. Should I also do the ones in the front lamp housing, meaning are there connectors there whose state could cause the revs drops?
Thanks