No Spark -- Bum Ignition Coils? High resistance.

Hi all,

First time posting here. I've done some poking about... as far as I can tell, I should be getting spark at the plugs (battery has fine voltage; yes, I've gone ahead and went through the fuses, so let's please not focus on that).

I've got a 1982 XJ750RJ. I've got a Haynes manual that I know some others noted in the electrical systems FAQ that testing the secondary coils with the caps on is problematic. The Haynes manual doesn't instruct this. The resistance numbers are as follows with the multimeter leads on the inside of the plug caps:

Left side:
Primary - ~4.0 Ohms
Secondary - 29.7 kOhms

Right side:
Primary - 3.0 Ohms
Secondary - 23.5 kOhms

Specs:
Primary - 2.5 Ohms +/- 10%
Secondary - 11 kOhms +/- 20%

These numbers are clearly out of spec, perhaps due to corrosion? The bike used to run fantastic when I bought it 7 or so years ago... ran it for a few years great and then had it sitting around for a couple years while I was busy in grad school, and it's had problems ever since. Yeah, don't let these guys sit around. They should be ridden, and like to be.

First pass on getting spark would be to replace the ignition coils, correct?

Dyna dc1-1 is what I saw others pick up. I see some on ebay for $120. Then to pick up some NGK cables (7mm plug set) and I should be all right? Thanks for any input!

 

Be aware that all of the serice manuals have errors in them, most of which are documented on this site (though in various places).

The spec given is wothout the plug caps attached to the plug wires.

Remove the plug wire caps and clip back the wires about 1/4" to remove any corrosion. Test the caps separately (should be 5k ohms each) and the secondary side of the coils without the caps (should be 11K ohm +/- 20%). Make sure that the resistor holding screws on the caps are snug (you may wane to remove the resistors and check for corrosion).

Based on you testing with the plug caps in place your coils are in spec.

Please confirm that you are trying to start the bike when in neutral.
Also try to start it with the clutch lever pulled in.
There is a safety system that must be in working order for the TCI to get voltage, and I suspect that is where your problem lies.

 

Ah, I see what you mean now. Thanks for the notes.

Removing the plug caps...

Left side:
Secondary - 11.7 Ohms

Right side:
Secondary - 12.1 Ohms // definitely a bit of corrosion on the ends here

The plug caps were as follows:
1/3 - 9.0 Ohms
2/4 - 5.5 Ohms

I seem to have had two with one type of cap and two with another, slightly different. That certainly accounts for the high resistance.

So... get new plug caps, make sure the battery is charged up, and give it another go? It was in neutral and the clutch was pulled. I can hear the starter cranking--does power to the starter follow with power to TCI (with respect to safety features) or are these separate?

 

An alternative is 81 to 83 Honda CB750 coils they are cheap on ebay have replaceable wires. They are made by Hitachi same company as stock on the yamaha . They mount in the same location and measure same ohms my 82 seca 750 runs great with these .

 

You'll just need two plug caps, or beter yet two 5k ohm resistors for the caps if those caps hae screw-in retainers (which they should). Chacal sells the resistors seperately.
Also make sure that you have non-resistor plugs.

Power to the TCI is seperate from the starter circuit, but there is a series of conditions that must be met for both to functon.

You say that you checke the fuses, but did you check the fuse clips?

This may sound like a weird thing to ask, but bear with me. Does the headlight stay off the entire time, or on the entire time, or does it turn on after the engine turns over a few times?

 

The headlight is off the entire time. The auxiliary light won't turn on, either. It's been awhile since I've ridden this so I can't recall if that's normal...

Fuse clips seem fine. My main and headlight fuses are now in-line. I've got the pieces together to replace the ignition and other fuse (forgetting at the moment what it's for) clip setup for the in-line kind. Haven't done this yet but could.

I got spark from the raw wire ends (no caps), so that's a good sign? I'll have to replace those plug resistors/caps. Seems odd though, that I wasn't getting spark at the 2/4 plugs which had the 'normal' resistance at the secondary coil.

Should I be concerned about the heightened resistance at the primary coil, or not so much?

Thanks for the help!

 

Spark from the bare wire ends is a great sign.
The headlight and aux light behavior is normal. My question was to see if maybe a realy had been swapped around, which would make things not be sparky when they should be.
Sounds like you're going to have a runner soon.

The 4 ohms on the one primary side is high, but could be down to corrosion on the terminals. I'd see how the spark is once you get the caps replaced before getting a replacement coil. And if you do get a replacement coil, do both coils, and take Jetfixers advice if you have a tight budget.

 

Ok... I got antsy and checked them again after cleaning up the ends, and it appears the small amount of corrosion in the plug caps was enough. More like just posting on here was enough for the universe to give me a break of sorts. I got spark at all 4, even without replacing the resistors in two of the caps.

So I'll replace those caps but I'm going to go run and grab some fuel since it's always a grumpy runner without at least a gallon in there, and I drained some out when I took the tank off.

 

Ok! Got the bike to start--yet again, thanks for all the suggestions!

New (original) issue I had last year when trying to start it up: it'll start and run, but it won't idle. When it was running well, I could bring the choke all the way out and it would idle something like ~3k rpm. Now it just hangs ~1k and you can hear it slowly slowing down which takes about 10-20s before it dies.

I don't want to be going down the carburetor rabbit hole here... but perhaps it is inevitable. A family friend who used to have this same model took and cleaned them all out a few years back. Last year when I was trying to get it to run well I pulled the carbs out and checked them over--everything was spotless. They look great. No, I didn't check float levels or go very in depth, but when I got this motorcycle back from said friend it ran great (though with notably less fuel efficiency...). I trust him to good a solid job since he's a certified Yamaha tech and used to own one of these. In addition, I did fully inspect all the carb components when I did my check. Diaphragms looked great, float needles in good shape, all the jets were clear.

The idle mixture adjustment screw is only about one turn out from bottom. I see ~2.5 turns is recommended, and if I read correctly on other posts, clockwise -> higher rpm.

Any ideas? Or should I simply pull out the carbs and go to town on them a la Church of Clean?

I know I should re-check valve clearances... so that's also on my 'to do' list.

 

The 2.5 turn out baseline is correct for your carb. Your friend may be thinking of the earlier carbs with coarse-thread screws.

Clockwise leans the mixure, counterclockwise richens it. Idle speed is controlled by the idle knob that is below the carb rack, between the #3 and #2 carbs.

Bring the pilot screws out to 2.5 turns from a soft seat, and use a length of clear tubing to check the fuel levels. You can do that on the bike if you measure near the centerline of the carb body. The fuel level should be about 3mm below where the fuel bowl mates with the carb body.

 

Could you please clarify this point? I guess I'm a bit confused on where/how I should be checking this.

 

Setting the fuel levels

 

Finally got a couple days where I could start to look at this again.

The fuel levels seem to check out. The right-most bowl (sitting on the bike, so #4) was the only one with a bleeder that behaved appropriately and it was already set to spec (3mm on the nose).

EDIT: Correction, two floats were off by half a mm on their height relative to the top of the float bowl (when the carb is turned upside down). I'm making that adjustment. Do you think this would be enough to cause it to not idle? Seems unlikely, but I don't have first-hand experience with how precise these are.

The only thing to note was that some of the float drain lines did not bleed, or did so poorly, so I couldn't actually verify that all bowls are in spec for level. I'm looking to clean these bowls and try this again. They aren't dirty; I'll have to find a wire to push through there and make sure there's no obstruction.

Pointers on next steps? I appreciate all the help!

 

if your bowls are dirty so are your carbs.
remove the bowls and clean them

 

Bowls are pretty clean. I made the adjustments and all the floats are even now.

I just noticed that one of the jets was swapped... ... ... which could've been the issue? One of the smaller gauge jets was swapped with a larger gauge. Not sure when that occurred; must've been last time this was apart a few years back. That could certainly be the root of the issue. Bleh.

 

Hmmm.... the pilot and main jets don't swap on Hitachi carbs.
What jets do you have in there?

 

Interesting. I'm guessing they were replaced by the guy who went through the carbs a couple years back. Here's a picture.

Anyhow, I've gone back through and everything is about 4mm now.. going to try and bring that closer to 3mm. Fixed the bleed issue with the bowls so I can measure each individually.

The pilot has 40 stamped on it; the main jet is 120... which I think is spec for the XJ750.



Edit: wrong picture.

 

Those are the correct jets for the stock configuration.

 

You have 1mm of leeway on float height. 4mm is fine.

By swap, I mean that you can't accidentally fit a pilot into the hole for the main.

 

Yeah, I got what you mean. Not sure how these got the same thread?

I got it all put back together and it doesn't want to turn over. Not sure if it's a weak battery at this point (gets up to 14.8V by voltmeter but doesn't hold for long). Got a large backfire once.

I'll get it charged up or set up with a new battery and ensure the spark is happening properly again. I reset the idle to 2.5 turns out like mentioned before... bowls definitely getting fuel. Don't think I missed anything here.

 

Ok, got it started. It'll idle just fine, but a ton of white smoke, always. It required moving the idle screw out a couple turns. The choke has some effect but not a lot--not nearly what it used to. It was more effective before I moved the idle screw out.

When I rev it up a little bit I get some blacker/gray smoke, probably spitting out some crap from sitting around awhile. Main thing that concerns me is that it sounds rough. Rough enough that I'm afraid to take it above 5-6k rpm. It used to sound real smooth throughout (even before I looked at the carb) so I'm confused at how this could've changed so significantly. Throttle can be a bit sticky, perhaps it just needs to run for awhile. I only ran it for maybe a combined 5min.

Summary: now idling fine on it's own; white smoke; sounds rough at higher rpm (5-6k). Sometimes sticky throttle.

Ideas?

 

Bring the mix screws in the thickness of a dime. How does it idle?

Gary H.

 

Been awhile since I updated this... in the mean time, I drained the fuel tank (I was using some new stuff that a buddy had, but I don't think it was kept in an anhydrous vessel, to test the carbs--this could obviously be running rough once I'd thought about it), replaced the battery (clearly it was in poor shape), and did some brake maintenance/oil change while waiting about. It didn't want to start, turns out the plugs weren't sparking?

Replaced the plugs, fires up instantly. Runs great, although certainly a touch different from what I'm used to feeling. Hopefully this is a 'better' different, as I've gone and set the fuel levels. It was idling a bit high so I turned the idle screw out.. about all the way.

Think it's worth tiny adjustments of the mixture screw to see if it runs smoother yet, or am I probably set? I've ridden about 25 mi in the past couple days and it's been great.

Thanks for all the help!

 

I had a similar problem and all it was was my petcock was leaking which in turn was fouling the plugs. Take the petcock apart and stretch out the diaphragm spring in the back (towards the center of the bike when mounted on the tank) of the petcock. Basically you "refresh" the spring to get it to compress the diaphragm in its seat more, thus truly sealing off the flow of fuel.

 

If you're going to take the petcock apart, do it right and rebuild the thing.
Rebuilding the Yamaha Vacuum Fuel Valve w/pics

 

thats exactly what I had done previously but forgot to stretch the spring out a bit.

 

if your going to leave the sticky throttle till later, might as well take the brakes off and save some weight