Misfire

Hi everyone I have an xj750r seca and I'm stumped. I have replaced the could on my bike due to faulty coils. The bike stumbles occasionally and runs on three cylinders. I have new wires coils and plug caps. The coils read 2.8 ohms the caps are 5000 ohms and the wire read correctly. The ignition pickups are also reading right within the ohm spec. I'm lost for ideas. I have spark on all four cylinders its seems to happen after an hour or so ride. Any ideas will help I'm thinking its the tci box as their was a very trace amount of corrosion not bad in the slightest.

 

Are the coils brand new or new to you? But if that is not the problem ,it must be fuel related as 1-4 , 2-3 fire together .
Or one bad plug. Have you swapped plugs to see if problem follows plug?

 

^^^ that ^^

Swap the plug from the dead cylinder to another cylinder.

 

^^^ replace all 4 plugs first

 

I'm gonna replace the plugs tomorrow gonna start simple and the coils are brand new came out of bubble wrap and everything.

 

No luck its better but not great as well what size are the fuses for the xj750 the P.O. Did a new fuse box and it seems to have the wrong sizefuses

 

20 amp main, all the rest are 10 amp.

Pics of the fusebox will help us help you.

 

its just a bunch of blade holders currently.
It has a 30 main and a 5 for the computer

 

The circuits are:
Main 20 Amp
Head 10 Amp
Signal 10 Amp
Ignition 10 Amp

The systems monitor (computer) is fed by the main (20 amp) fuse.

Having said that (and looking at the nice work by the PO; a rarity), the stumbling issue is not related to the fuses.

Replace that 5 amp fuse with a 10, and lable the darned things so you know what is what.

Before we had to the TCI (which I think is the place to head), Have you tried a new set of spark plugs?

 

And get that 30 amp fuse outta there. That's 10 more amps you gotta load up on a system that was designed to kick the bucket at 20 amps before it'll blow.

Doubtful fuses are the issue. Good on ya for checking all the components already. While electrical corrosion is always a concern, there's not a ton of connections between the pickup coils, TCI, and coils.

When you say feels like running on three, that's odd because there are two coils firing a pair of cylinders. In my experience, loosing a coil results in a drastic reduction of power and it will not idle. I've had some fuel delivery problems before that would cause a momentary reduction of butt-dyno power and some stuttering but it would recover. How's the vac lines for the petcock etc? You could try running on prime to take the petcock out of the equation.

 

I tried running on prime and doesn't seem to help. Ill set out to reliable the fuses correctly as well as change the fuses. I also have a new set of plugs in now it stumbles under around 2k rpm and clears up at higher rpm. Its running on all four cylinders when I run it in the driveway. One thing I noticed is the ignition pickups are not flat they have a gouge in them not sure if that makes a difference?

 

could be the pilot jets are clogged they are running the bike under 2k rpm

as long as the pick ups ohm out properly should be good. can you post a photo of them?

 

Fuses that I have and specified in the Haynes manual and the FSM for XJ750R Seca US models:

Main - 30 amp
Head - 10 amp
Signal - 10 amp
Ignition - 10 amp
Tail, License, Meter lights - 5 amp, located in headlight bucket

The 650 USA versions and 750 UK versions utilize the 20 amp main fuse as they have either lower wattage headlights, no auxiliary light, or only one brake/tail bulb, or a combination of that to lower the size of the main fuse.

Photos would be good as XJ550H suggested. After that hour long drive, does the bike continue to run poorly? If so, testing the pick-up coils hot would be a good idea.

Is it safe to assume the valves have been checked and put into spec?

 

Update: valves are done have been for a while. The ohms on one pickup when hot is at 800 ohms and drops as it cools the other was right at the top of the spec at 765and drops when it cools I have my meter set to 2000 when testing. As I ride it stays about the same. It now misfires at low rpm and slightly better at high rpm but still stutters

 

Apologies for not caching my own typo. The main fuse is indeed 30 amps.

 

Verified today pretty much normal. Cold 667 went to 805 hot, and the other 693 cold went to 852 hot. Open to ground both hot and cold as they should be.

Is this still true or do you have the low speed miss and slightly better at higher rpm all the time now?

Have you gotten to the point of ordering a TCI? Anyone local to your area to test it for you in a known good running bike?

 

I do not have anyone with a spare tci and it's a low speed miss then gets better

 

If you are willing, I would be happy to test your TCI in my Seca 750 just to be sure that's the issue before you go spend money on a replacement or repair.

 

I really appreciate that k-moe let me eliminate carbs as a possible suspect first. I had them off this winter to make sure they were all clean. Anything can happen though

 

I'm trying to follow this and I am a bit confused. When you say it runs on 3 cylinders do you mean one cylinder is consistently not firing? If that is the case then why would the TCI be suspect? I don't see how a TCI failure could target a specific cylinder.

 

Right, with wasted spark, the TCI can't be at fault for a single cylinder failure. Should check plug, cap, wire for that cylinder. If those are good, fuel is the next thing. (and vacuum leaks can be per-cylinder)

 

I think this is the reason for suspecting the TCI, but symptoms have changed. What was an issue after an hour of riding is now a low speed miss and better at higher rpm.

Also this:

I interpret that as just missing, not a cold pipe isolated to one particular cylinder. So yes, possibly a weak spark caused by many reasons including the TCI.

So, this still leaves a fueling issue or vacuum leak that was suggested for the current low speed miss and better at higher rpm's, or perhaps an issue with weak spark as this will cause engines to stumble or miss when under load. Seems there could be more than one problem since the symptoms have changed.

And, for the current symptoms (low speed miss) does a little choke make the problem worse or better?

Sometimes, it is just not easy to find the problem particularly since the TCI does not lend itself to easy diagnostics other that to test in a known good bike or install a known good TCI in the defective bike.

 

^^^yes^^

Eliminating (or confiming) the TCI as the issue will simplify diagnosis and repair. Normally I would not head in that direction, but the naure of this particualr problem lends itself to cutting directly to testing the TCI.

 

TCI tested today. No sign of a low-speed miss, or any miss.
That's not to say that the TCI might not have an intemittant problem, but I'm fairly confident that it does not.

 

My last guess would be the pickups or something severe like it jumped timing