Coil relay mod question

[Ola]

Hi, I've been enjoying this forum for a few weeks (eagerly awaiting the official greeter!) and I guess it's finally time to start posting newbie questions.

I have an 86 GSX750ES. (euro version and I know the front page said < '85 but they were identical since '83! )

I've been having a slight stumble when hot at 2-3K rpm that disappears at 4-5K rpm. I've been trying a few things, have done my valves, some cleaning gas additive, new plugs and many hours of postponing the pulling of carbs. All of them seemed to make it better only for the stumbles and pops to return. But it's not there all the time and the past two days has been completely gone even when the engine was fully warmed up after riding for 40+ mins.

I read about the coil relay mod and it seemed like a good thing to try as intermittent problems like these does make me think electrical. I measured the coils today and got the following results:

1-4: 4.5 ohm primary, 32K ohm secondary, 11.68 volts on the input wire.
2-3: 4.5 ohm primary, 25K ohm secondary, 11.60 volts on the input wire.

Battery was 12.6 volts with ignition off, 12.2 with ignition on, so there's about 0.6 volts or 5% lost from battery to coil.

Is that enough to justify the relay mod?

And is 25K on the 2-3 coil too low? Should I think about replacing them?

The plugs on #1 and #2 look perfectly tanned, #3 and #4 look a tad lean. Not whitened, but no brownish deposits to speak of. I have about 800 miles on them all in all, cut them at about 2.5K revs when pulling up home. The slightly lean plugs might be indicative of dirty carbs but the bike really does run well, starts perfectly and I think I'll postpone carb cleaning until the season ends.

Also, one more question, but I searched the forum while typing and basically found the answer. My front wheel is slightly misaligned. The fork seems perfectly straight though. It's just to loosen the triple tree clamps, secure the wheel somehow and bend it straight right?

Thanks for your patience. Since it runs really well after all I leave you with some pics of my recent destinations on the west coast of Norway.

[Ola]

double post for the max image rule







Close up of the aftermarket headlight I suppose you've noticed. I didn't drop and trash the fairing, I just prefer the naked look and I think it looks ok. You can also see the cigarette lighter socket (cheap and great!) that steals 12v off an unused running light and feeds my gps. Setup works perfectly.

[PAULYBOY]

If you speak english as well as you write, it's very impressive. The countryside in your pictures is truly beautiful.Hope you have a safe season riding. I'm glad you found the answers you were searching for regarding the coil modification. When I did it to my 81 GS850, the bike acted like it was truly alive, even before I had my carbs sorted out all the way.

[salty_monk]

You'll get a stronger spark with 12volts at the coil. Less than 12 is similar to adding a ballast resistor & will restrict the spark.

My 1000G ran well with about 10v at the coil, I did the MOD & don't notice any real difference even though I now get exactly the battery voltage at the coil but it should help the bike make complete combustion & keep the valves & pistons from collecting carbon deposits.

Dan

[Ola]

Thanks for the replies. Can anyone report good results when the voltage diff is as little as 0.6?

[psyguy]

welcome to the forum

beautiful pics man!

that twisty hill road... i could camp there for a week just to go up that road then down the road and up and down...

...
i wouldnt attribute your probs to the small voltage drop at the coils

rather, make sure the intake system has no air-leaks and turn the mixture screws out a bit (1/2 turn at a time)

[don_gibb6512]

Well, getting back up to 12v at the coils will help but not sure .6 will make that big of a difference. I'm more concerned with the low resistance on the secondary windings of that one coil. I had a set of coils once that were breaking down under load in the secondary windings and the bike would stumble and fouls plugs a lot until I replaced them. The odd part of your experience is the difference in effect of plugs 2 and 3. Hmmm. I work with some electrical engineers and I'll ask around today to get some opinions and let you know what they say. If you do change coils, go with Dyna.

You've got a really nice bike there and Norway is amazing. You're a lucky man.

Don

[don_gibb6512]

One more thing, you can't really avoid a good carb cleaning for too long if it hasn't been done before. It's well worth the time to do it right and thorough.

[Ola]

It really is a lot of spectacular riding roads up here. And going between cities there's always the "low road" to take somewhere, too narrow and windy for the easily annoyed four wheelers.

I think I will hold off on the relay mod. Particularly now as I haven't heard the stumble for several days. Right now it's really hot outside, going for a ride to the shopping mall and will see if it is indeed gone.

Later today I'll get to straightening that front wheel...

[Matchless]

Ola,
I like those pictures and it must be nice riding in such an area!

On the coil mod, just some comments. A voltage loss even of only 0,6 volt can point to something not right. It also may be a dynamic fault, meaning it may change as conditions vary, thus worsen of get better. If you do not plan on doing the relay modification I would suggest that try to you prove where the voltage drop is. It may be accumalative or it may be just a corroded connection or the ignition switch or fuse block and if you can fix it the modification is really not necessary.

[Zook]

The stumble goes away when the temps are high, right? If the stumble comes back when the temp drops you are indeed too lean. Warm air cannot hold as much oxygen as cold, so when the temps climb it has the effect of making the carbs richer.

BTW-have you been riding at higher than normal altitudes? That would explain the stumble going away.

[Ola]

No, the stumble comes when it's hot. It's smooth and pulls from low revs when cold. As it heats, it starts vibrating a bit and the stumble comes. If I'm at 2K revs and pull mid throttle it will bog down slightly, maybe backfire a bit and then spring back to life at about 4K revs.

On my long trips it behaved worse at altitude (about 3000 feet) but would regain a lot of power in the cold, dense air in the tunnels.

However, during my ride today the stumble was completely gone. It's a hot summer day (sea level) and I rode about 40 minutes. Pulling into the driveway I went in a too high gear on purpose to provoke it and while it struggled it did not stumble or backfire.

So...it could've been a gunked carb that cleared or it's those intermittent electrical gremlins having their day off. There's still a tad of vibration that makes me think the cylinders are not making equal power, but all in all it's running very well.

[Skievryday]

I might just not be picky...

But most of the time my bike doesn't like the first 20 minutes of riding at much less than 4k rpm. I wouldn't expect roll-on acceleration at anything less than that...
I am no expert at the technical, but on these older bikes doesn't vacuum in the carbs have to catch-up? I thought basically you are dumping air into the carbs, and the jets catch-up.
I learned to use the mid-high range of rpms in order to get the most of your bike. Riding less than 4k shouldn't really be happening even if just cruising. 4k would be a minimum... couple of threads about this.

As for coil relay mod- does it hurt? Take about 20 minutes. Your stumble might go away with better combustion, as this seems a very slight problem.

[tkent02]

As for coil relay mod- does it hurt? Take about 20 minutes. Your stumble might go away with better combustion, as this seems a very slight problem.

Takes no time to see if it will help, just hot-wire the coils and see if it will run better.
If it helps do the relay mod.

I do it to all my bikes just for the added reliability.

[Ola]

Figured I'd update this since I solved the problem and it might help out others with a similar problem.

The problem turned out to be intake related. I was missing the airbox snorkel, no. 7 in this pic:



The stumble is completely gone, I got it for next to nothing from a breaker's which was quite pleasing compared to ordering a new set of coils and not removing the problem.

I'm guessing the snorkel helps some pressure pulse voodoo work in favour of the engine at that rev range, but it did take some of oomph out of the 4-7K range which I assume is the reason some previous owner removed it. I prefer a smooth low range.

If you enjoyed the pics in the first post, there are loads more here in addition to a narrative of the two long trips I did this summer.

http://www.advrider.com/forums/showthread.php?t=407819

Next summer I'm planning to take the Geisha beyond our borders to England, Scotland, Wales, France, Switzerland, Germany and Denmark...budget permitting of course. She'll get new tires, chain, sprockets, brake pads and fork seals this winter.