1982 XJ750 Seca - Ongoing

Also if the plug caps are older, there are resistors in them that can fail from heat, they are cheap I'd replace them too.
Otherwise I'd think you are starving for fuel, the bikes air cooled so the engine "can't" overheat even if your sacking it.
Drain each bowl into a container, see if they are all full and check for contaminants.

 

Well, if the oil gets low....or if the oil gets thinned out by gas leaking thr0ugh into the crankcase, they CAN overheat.

 

Actually I'm not, I was before I changed the petcock last year but I was having a hard time finding one I felt was big enough that would also tuck under the tank. The PO was running a metal housed one that looked like a window screen when I removed it so don't think it was doing much.

When carbs went to Church I also replaced the petcock as the old one (original) had flooded once and also had some cracks in the screen. The new one looked like the screens were very fine and of course intact so I skipped the add-on filter for now. I will reverse flush / blow the petcock while I have it off...

 

Moving this issue over to my bike thread to keep it together and unclog the board. Thanks for everyone's suggestions... I was panicking if I would make it home or do more damage.

https://www.xjbikes.com/forums/threads/1982-xj750-seca-ongoing.118087/page-2#post-601899

 

Pick up from thread
https://www.xjbikes.com/forums/threads/where-would-you-look-first.120946/#post-608001
Sputtering and low power when fully hot...

 

If it happens after you have gunned it then you could be fouling your plugs. Riding for a while at a different (lower) RPM would then burn off the carbon.

 

Merged.

 

Sooo….
Drained oil:

Changed plugs: 1-4 top to bottom


Checked crankcase breather - OK

Checked valve clearances: No changes - the .21 was borderline and will tighten.


Found valve cover was weeping a little, trying shim method of tightening rubber donuts, we'll see:
Brass washer inserted above rubber donut.


Checked ignition coils and that's as far as I got... and getting screwy readings. Both primaries were 2.5 ohm as spec. Secondary on one was 30K, on the other was 150K+ and rose slowly like a capacitor. Checked the pick-ups on the crank and got 700 ohm to check my meter and seems OK. I'm going to borrow another meter at work tomorrow and confirm if my readings are correct. But I don't get why the bike would seem OK and then get weird after a 1/2 hour running if the coil(s) are bad. Wouldn't it run badly right away?

 

.21 isn't borderline, it's out of spec. It opens too late, and closes too soon.

 

I mean it was borderline by I had to force it a little to get that one...
Any comment on the coil issue?

 

if your coils are acting up from heat, it's going to be in traffic, stop n go, heat comming up off the engine type stuff. out on the road they would stay cool from the breeze. if it it coils your going to lose two cylinders at once
what's up with #1 plug?

 

That first pic is of the oil drain plug.

 

It was carbon... wiped right off

Colortune will take care of it when I'm done

 

Was this done with the spark plug caps still attached? If so, you could have a problem with the caps or the connection with the caps to the plug wire. Will see what you get with the other meter but I suspect it will be the same. If so, remove the caps and check individually and check the coils with the caps off. You may just need to trim a 1/4 inch or so from the plug wires and reattach the caps if the parts ohm out OK individually.

Those plugs do look a bit old and tired. It also looks like you are running the BPR version (resistive). That probably wouldn't matter much but Yamaha calls for BP7ES (non-resistive) for the 750 Seca.

 

A coil changes low voltage to high voltage, like a transformer and they do produce heat.
They can get worse with heat and run time, the wires inside separate, I'd replace the one coil that is out of spec.
Consider doing both as the have the same hours on them, also, unless they are new change the caps with the new coil(s)
The resistors in them can break down and cause coil damage if they overdraw (resistor shorted) too much juice thru coil, or under draw (resistor too high) not enough juice, it builds up in coil.
Side note you can test the caps 5K ohm resistance +\- 10%
Hope this answers your question.

 

As per resistor plugs, they suppress RF interference, they may foul easier, but won't hurt if it calls for non-resistor.
Never! Run non-resistor if it calls for resistive, it can damage coils or worse CDI...

 

It was with the caps attached as they are the original and not removable and the hayes says should be 11K ohm with caps, so if that's wrong I would expect 21K, or 11+ caps...

I remember asking for regular plugs, must not have noticed the guy at the store gave me the "R".

 

You can't even buy non-resistor plugs here anymore, unless you can find some old stock. I have to order them online.

 

Well, Haynes got that from the Yamaha FSM, so both are incorrect. You should get as you suggested, 11K plus plug caps.

The caps are removable (turn CCW and pull lightly) and should be 5K each on the Seca. Once again the FSM gets this wrong saying the 2,3 caps are 10K, Len gets it right as usual. For whatever reason it seems the 1, 4 caps are most prone to failing by going above spec. The longer 2, 3 caps can actually be dis-assembled and cleaned.

https://www.xjbikes.com/forums/threads/the-ultimate-relay-switch-sensor-and-diodes-guide.27543/

XJ750 air-cooled models:

Pick-up coils:
650 ohms +/- 20% = 520 ohms to 780 ohms acceptable range


Ignition Coils:

Primary side (input from main wiring harness):
2.5 ohms +/- 10% = 2.25 ohms - 2.75 ohms acceptable range

Secondary side (spark plug wires, without their end caps):
11K ohms +/- 20% = 8,800 ohms - 13,200 ohms acceptable range


Spark plug caps:
1981-83 models: 5K +/- 20% = 4,000 to 6,000 ohms per cap acceptable range
1984 RL models: 10K +/- 20% = 8,000 to 12,000 ohms per cap acceptable range

Spark plugs:
0 ohms per plug

 

Caps are removable from end of wire?? I had tried... didn't seem to be.

 

Yes, and some aftermarket and OEM coils
are interchangeable, and have removable wires too
(GPz900R Kawasaki springs to mind, only 'cos I had one of those, eons ago, and succesfully used a set of those coils/wires on my XJ's.......)

 

They screw into the copper strands.

 

Well, I went to the shop and got non-resistive plugs... will save the others as emergency back-ups.

 

Thanks a bunch gang... believe I found it.

Now I see how the plug caps are attached (Not a fan I must say). Removed caps from suspect coil and clipped back leads to where I got solid copper with tight insulation. Ohmed out at a perfect 11K. Put caps back on... nothing. Found one cap had no resistor that I can find. Really looked and don't think I dropped it? Anyone think I could have been riding around missing a resistor completely? The spring was broken in half and the bottom of the brass plug was black instead of shiny like the other so I think so...

Anywho… took another cap off my parts bike that guts looked really good and now perfect 21K through caps and coil. Will do the other one tomorrow as it's reading 30K. Looking forward to Colortune and sync since I have it apart anyway and weather is supposed to be awesome later this week.

 

So the bike seemed great for a couple days... then the same thing started up again. On doing the other coil / caps mentioned above, I never got the assembly down to the 22K ohms required. It was 28K.

I'm going to pool all the parts with the other bike I have and see if I can get the real 22K I'm looking for through each coil.
Then if she acts up again start looking at the TCI. Or at least take the time to measure when it's hot and actively exhibiting the problem. Maybe I will only get wonky readings then.

The correct plugs are brand new and gapped so shouldn't be that... Oh, and found that resistor I thought was missing and it was open resistance and 1/2 brown / burnt, so thought that was most of the original problem but I guess not.

 

Done dealing with these old plug caps and resistors...

Suggestions on inline splice plug cables?

 

If I had to do it I would either go with new Dyna Coils or the Honda coils that are a good match. Splicing just seems half a##.

https://www.xjbikes.com/forums/threads/dana-coils-a-must-upgrade.118318/#post-592895

https://www.xjbikes.com/forums/threads/yes-dynatek-coils-make-a-difference.117797/#post-589572

 

Agreed, it "seems" so, but my $1,300 bike is now a $2,000 bike, and when it becomes a $3,000 bike I'll be pissed enough to just buy a $7,000 bike and not have to fix it every two months.
I spent $32 connectors, $30 caps, and $15 wire. With coils the connectors turn into at least another $80 for those coils. I have four original coils off two bikes I have, each of which ring out to in spec at 2.5 ohm and 11K ohm so I have to believe two of them are good.

And if I do end up getting coils eventually, I still need the wire and caps and the connectors go in the toolbox for the next bike . The caps are what is falling apart on my bike... that last post was after mixing and matching for an hour and not being able to get stable cap and resistor readings.

 

FWIW, new caps from Yamaha aren't bad. I think I did my bike for $25-ish. They are really simple to replace. Unscrew the old ones from the wires, snip 1/4" off the wire to get fresh copper and screw the new ones on. Job done!

Edit: I say from "Yamaha", but from a dealer's parts counter, I got the NKG caps. 2-90 degree, 2-60 degree.

 

Exactly the four caps I just bought

 

And that is what I did also, only I never used the 2,3 caps as the original Yamaha caps are still in spec. Hard to imagine 35 year old plug wires hanging in there, but they look good and since I ride very little I am not likely to get caught in a rain storm. The OEM coils are not cracked, and if I did need new plug wires I guess I would be tempted to use the splice rather than try the detailed coil surgery to replace the wires.

 

Frustrating Update!!!
So my plug wire was taking forever to get here, and I just shortened the existing leads and added the new caps / plugs. All coils and caps ohm out exactly in spec.
Took the bike out for a 20 mile ride... no issue, running well. Even floored it in the area it usually dies on the way home.
Took the bike to work next day (23 miles) running well.
Took the bike around town to a few appointments after work, fine...
Then went home and 1/2 way there, same sputtering and uneven combustion as above, but 2-3 miles sooner than usual. It then dawned on me that this is happening at the same level of gas tank after filling... about 2/3 empty every time. I switched to reserve, and even though it made no difference the last couple times, because I was further out from home it had sufficient time to even out and start running normal again... for 2-3 miles, then started sputtering again a 1/2 mile from home.
When I got into the garage immediately popped the plug caps off and measured the coils / caps resistance. Still perfect.

So I'm guessing clogged petcock screen(s)? Might be able to check it tonight....

 

In line fuel filter...??
i was having trouble with one of mine, switched it to a more bike specific one.....no more problems

 

The aftermarket petcock had really fine screens (Maybe the problem), and I haven't found one I felt was big enough that also fit under the Seca 750 tank... so no filter right now...

 

Len/Chacal sells one that fits, and works.....

 

Gas cap has an internal atmospheric vent passage and an anti-leak "valve" (basically a ball bearing within the passage) that can get stuck/clogged and create a "vacuum lock" within the fuel tank, thus fuel will not flow. This situation manifests itself as stalling/sputtering (i.e. lack of fuel) after a few miles of riding as fuel is drained from the tank, and finally the vacuum "lock" effect takes places and prevents any more fuel from flowing. Try opening the cap the next time this situation occurs and see of the situation resolves; if so then it's time to clean/rebuild the cap:

http://www.xj4ever.com/gas cap.pdf

The vent hole in the cap and the "ball valve" can be seen on the top of page 7.

 

Thanks,
I opened the cap, let sit 3 minutes or so last time and it seemed to make no difference when restarted.

Going out to the garage shortly...

 

Darn!

 

OK, I would have thought that this is a ridiculous question... but has anyone ever had it such that the gas stops flowing from one side of the tank to the other??

I just drained the tank, zero sediment or crud. As I was draining, when flow diminished went to reserve... NO CHANGE. Popped the gas cap, no change. Looked inside, there was still 3"+ of gas on that side. And when I slowly drained most of the rest, I took the petcock out, no clogging, and tipped the tank to get the gas out of the other side and had to REALLY tip it over and work at it to get the gas to come to the petcock side.

All this and I can see zero loose rust / paint, etc. Also disassembled the petcock and found no issues. There is some rust in the tank like all of ours, but it does not look excessive at all or like it's breaking loose...

 

Sounds like something has plugged up the back end of the tank

 

RRRrrrr…. so in my frustration, went back out to the garage put the petcock back in and shook the heck out of the tank with the inch or so of gas still in it.
Low and behold... a 2" x 4" or so solid sheet of rust came forward into view, and must have been blocking the transition. Wonder what part of the tank it came from.

Going to take the tank to work tomorrow, put hot cleaning fluid in it and shake it to death. We have a boroscope so I can see if it's really thin somewhere.
Bummer because what you can see from the gas cap looks pretty darn good.

 

Cleaned out the tank best I could... got the pictured "stuff" out of it. Looks like the silver looking surfaces I can see from the gas cap are actually a botched liner job around the other parts of the tank.

The items pictured are all silver on one side and that solid rust cake on the other. The largest being about 4" long.

Also installed a decent size fuel filter, I probably have an extra couple miles fuel in the hose now from how roundabout I had to route it to get the filter under the tank.

 

Oooo, so you have a larger reserve! Lol

 

just throw a match in it

 

I may throw a match in it.... took it for a ride last night, started bogging down about 2 miles after filling...

Think there's too big an air bubble in the long fuel line I just ran, and since it started bogging so soon, thinking maybe the gas cap vent really is stuck too.

 

I am at a loss....
I have clear fuel hose on so I can see what's going on. There's a lot of air in the line between the carbs and the fuel filter which is at the outlet of the petcock (non vacuum, after market, been running fine for a year)

When I take the tank off and elevate it so the whole line is nice and straight up, both with the petcock on normal and reserve, the air will not flow back into the tank and allow the hose to fill. Took the fuel filter off and tried the same thing, still will not fill. Both tests were also with the gas cap open and closed and each way tapped the hose, petcock, etc. to try and break the air loose, nothing.

Took the tank off again and drained all the gas. While I did, gas cap open and closed, no difference in flow. Tank certainly drained in less time than would have at 85 mph so flow should be sufficient to keep carbs filled.

Opened the gas cap guts, no restrictions, appears to close sufficiently to allow air in. And when I eventually put gas back in, heard air escaping when sloshing around and cap closed...

Removed petcock and disassembled, found no debris or restrictions. Could easily enough blow air through tubes and ports in both directions. Only thing I could think of was flimsy screen over tube was collapsing in on itself. Gasket is thick enough and ports protrude enough that I don't think the gasket could possibly spin when opening valve, thus blocking port(s). And no wear or damage to that valve disk gasket.

Inspected inside of tank from cap and now removed petcock, shined a lot of bright light through and shook around. Can't find any debris.

Filled two feet of the clear hose attached to carbs with gas, steady level observed. Started bike and ran normally and smoothly and watched the fuel slowly empty out of hose, with all cylinders equally warming.

Reassembled tank and petcock meticulously but removed upper petcock screen to try out theory, and re-installed filter to cover for that screen. Petcock looks like this guy:


Filled completely with gas so no issue with flow between two sides of tank.

EXACT SAME SYMPTOMS... gas will flow to free air / gas can, but when hooked up will not allow air to bleed back to tank to fill fuel line. And again both with filter installed and not, and with gas cap closed or opened.

Going to get another type petcock but why this is happening is absolutely infuriating and a mystery....

 

did you take it for a ride? mine has a bubble in the filter/line every time it sets for a few days, when i get back the bubble is gone.
if you just want it gone right now, open the petcock to prime and open a float bowl drain into a jar.
the bubble should go right out to the empty bowl.
it can't go back into the tank because it would have to turn into wee little bubbles to pass through the screen filter in the tank

 

Was gonna try that maybe... but weird because I took the screen off the petcock (Aftermarket) and the air still won't purge or move out at all.

 

i think your fuel line's broke