XJ750 SECA coil and jetting question (Possibly Solved!)

Synopsis: Fouling plugs at idle and no power below 3k rpm's until after the bike and been driven 1-2 miles. Found what looks to be a bad coil and possible too small pilot jet?*

If that interested you, read on!

Being gone for a significant amount of the summer, I have only recently returned to trouble shooting a problem from last year, which I discussed on this board (and you can read here: http://xjbikes.com/Forums/viewtopic/t=20226.html go to last page).

My current problem is this: Bike starts up nice, idles well, but fouls the plugs fast. To get the bike moving, I have to keep the RPM's above 3k or the bike will die. It WILL idle GREAT, but can't take any load. After I've driven about 1 mile, the bike runs great. Good power even at low idle and the aforementioned problem gone.

Carbs: Last year I completely tore the carbs apart and cleaned/served them throughly, including replacing throttle shaft seals and idle screw o-rings and whatever else was needed. (also shimmed the valves and balanced the carbs)Today, I pulled the carbs apart again to verify they were still clean and to check the enrichment circuit. Everything operates as it should and is very clean. There is no way the enrichment plungers are sticking, being held open or any other enrichment related problem. Idle screws are installed correctly and are currently at 2.5 turns out, though I've experimented between 2 and 3 turns which seems to have no affect on the problem. My main jets are 124 and my pilot jets are 40 (all other jets are installed correctly and are clean, as are the emulsion tubes). I have a 4-1 exhaust and currently still have that K&N Uni Filter. I feel 124 on the main is probably ok, but i'm wondering if I should bump the pilot up to 41 or 42? There is hesitation at times between 2-3k even when it's warm and running nice and it seems like I read that the pilot should be increased a little if the main is bumped up significantly.

Coil's: My feeling is that my problem, unless it is related to the pilot jet somehow, is not the fault of dirty carbs or the enrichment circuit. Only this week have I turned my attention to the coils. Seeing spark (though not super strong) on all plugs made me not look at the coils in the past. I pulled out my ohm meter and found the 1/2 coil to be way out of spec. The Hi Tension is in spec but checking across the leads I get more like 4.5m ohms (caps on or off, which seems strange) whereas on the 2/3 coil I get 22k ohms...however, when I first started checking, I couldn't get an ohm reading on the 2/3 coil till I removed the caps and clipped a 1/4" off or so and re-installed the caps. All caps are 5k resistor caps. Additionally, when I bought my last two sets of plugs, I didn't bother checking the service manual for the part number and relied on my parts guy's info...he gave me NGK BPR6ES plugs. Oh the 1/4 coil, there is a significant crack on the bottom of the case.

All plugs always are uniform in their color (both after a ride when they are burned clean with a light tan, or after an idle or too short a ride where they are sooted up)

Could the coil issue and/or the pilot size be the cause of my problem. I would love to point at the coil (and possible bad cap contacts with 2/3) but doesn't the coil usually get worse as it heats up?
Before anyone asks, float levels are all perfect and the carbs pass the clunk test perfectly as well...the battery is new and charged, fuses and connections are good and tight.

Thank you for reading through this long post and for any feedback. Just trying to answer all the obvious questions and give background.

 

Re: XJ750 SECA coil and jetting question (fouled plugs)

Well, cracked coils can never be a good thing, particularly if you already have bad readings on a meter. A good tip I read recently is running a bike in the dark to see spark leakage from the coils/wires/caps.

Seems the electrics are always the weakest link in the gas-spark-compression chain of events. However, I'd do a compression check on all cylinders just to find out where they are.

 

Re: XJ750 SECA coil and jetting question (fouled plugs)

Well, I have a replacement coil on it's way and I got non "R" plugs. I've pretty much verified the carbs are right, though I'm thinking about bumping the pilot jet up one step. I found my compression gauge so hopefully tonight or tomorrow I'll get a new compression check...Seems like when I got the bike after it had been sitting for a long time, compression was in the low 120's. Not great, but I don't think the cause of my issue.

What about the "Starter?" air jets? The ones in the intake horn area...(there are two) It looks like from my manual they should be size 40 but the ones I have in are 36...could this be a cause of my problem as well? At 36 they are still pretty huge holes, would a step up to 40 make a difference (or more appropriately a step down to 36 cause a problem)?

Thanks!

 

Re: XJ750 SECA coil and jetting question (fouled plugs)

I'm no expert on these carbs. The Hit 32 and 33 drive me crazy, but I read very little in XJB about folks changing the air jets. However, sounds like your bike is in need of air.

The 'clunk test' proves that the slides drops freely, but they have to rise under vacuum in order to let air through the throats. I'd pursue that a little. Maybe take the airbox boots off so that you can see the sliders while the bike's running. Goose it, see if they rise.

Maybe pull the hats off, and re-seat the diaphragms. Or, pull them out and check for holes. You can do that w/o pulling the rack.

Keep it simple.

 

Re: XJ750 SECA coil and jetting question (fouled plugs)

Yeah, I don't have any problem with my slides and no holes in my diaphragms. I actualy have the whole carb rack off the bike right now just going over all this stuff to make sure I wasn't missing something. So far, everything checks out. I was thinking about the jets due to the fact that they have been messed with, there is a 4-1 exhaust on it as well as a K&N Uni Filter. I'll be very happy if the bulk of the problem is my coil issue...we will see. I'll keep you posted.

 

Re: XJ750 SECA coil and jetting question (fouled plugs)

cold engines like a little richness or they do things like yours,then after a mile the engine is warmed up and it's happy
when it's acting bad does a little choke help?
my two 750's seem to like about 3.5 to 4 turns out, real close to 3.75
using the right plugs ? BP7ES
the fowling might be from the coil problems, see how the new ones work

 

Re: XJ750 SECA coil and jetting question (fouled plugs)

I have a 1982 Seca 750 (runs great!)
I cleaned my carbs, they are factory original.
My air jets two per carb (at sides) #36
The size #40 in manual is for fuel starter jet.

The spark plug spec is NGK BP7ES (These are what I use)
There are posts of others using resistor plugs causing issues. From reading the posts, it convinced me resistor plugs will produce a weak spark.

I sealed cracks in one of my coils by smearing with black hi-temp RTV. Make sure the coil is dry before sealing. I used a hair dryer and engine heat during dry weather.

 

Re: XJ750 SECA coil and jetting question (fouled plugs)

mdee, thank you for that info on those air jets...I expected them to be stock, but after talking with another mechanic friend it got me wondering. The "starter fuel jet" is that referring to the pilot then? If so, my carbs are setup to stock specs with the exception of the 124 main jets ... due to the exhaust on it. I was considering moving to 41 pilot to help the pilot>main transition as there is a light stumble right now, though not too bad. On my coil with the crack, it tests bad, so I am not going to try to fix that one. On the other coil, I need to pull it and look it over for cracks...it currently tests out good, so I'm hoping I can keep it. I just bought the BP7ES plugs today, so when everything goes back together, that is what I will put in it. Hopefully I will have found my main problem. Or, if nothing else, solved one of them.

I did a compression test just now. Now my gauge is ancient so maybe it shows low (need to calibrate it), but I was basically getting 115psi each cylinder. Carbs were off, coils disconnected, battery jumped. Putting some oil in the cyl jumped the compression....very hi. The bike is bone cold, hasn't been ridden much at all, just a few 5 mile rides in the past month and nothing much beyond that for the previous year. Is that crazy low...enough to cause problems, or just "low" and I should consider new rings at my "leisure"

Thank you!

 

Re: XJ750 SECA coil and jetting question (fouled plugs)

Your welcome Geej

My Yamaha Service Manual lists in same box of specs:
Main jet #120
Pilot Jet #40
Starter jet #40
Air jet, Main #80
Air jet, Pilot # 225

(So there are two different #40 jets)
My notes of carb cleaning I list fuel pilot jet #40, and no other #40, so now makes me think I missed cleaning a piece.
My notes on the others match the listed specs.

Interesting results on your compression test.
I got same when I did mine. About 110 – 140 psi cold and not very consistent when re-measuring same cylinder. Then lower readings with engine warm. Adding oil spiked compression crazy high 185 – 235 psi so I stopped my nonsense before a busted something.
My compression tester is also ancient. I dis-assembled, found all in order, simple curled air chamber that uncurls with pressure, found no calibration mechanism. Seems simple enough. Since tester will go as high as 235 psi during test, I assume it is working and not leaking before entering tester and somewhat accurate. It holds the pressure result till air valve is depressed and will hold same reading overnight. I found my test results bizarre. My bike runs good and strong so I ignored my compression test results.

Compression method:
Unplug the TCI
All spark plugs out
Strong battery
Wide open throttle

BTW, you should unplug the TCI so as not to damage it during compression test because you have the spark plug wires unplugged. I did not see that listed in your description of how you tested.

 

Re: XJ750 SECA coil and jetting question (fouled plugs)

the starter jet is in the float bowl, unremoveable

 

Re: XJ750 SECA coil and jetting question (fouled plugs)

Polock, I missed your earlier post, sorry about that...The bike usualy needs about half...sometimes full choke but once started I can lean it out to half or slightly less pretty quick and it doesn't take too long to be off choke all together. When it's acting bad in that first mile, choke usually makes it worse. It almost seems to start making it a tad better, but my feeling is it's fouling the plugs so fast it becomes a negative. I have to keep the revs over 3 to prevent the bike from dying...once past that 1 mile ish mark, it acts as it should.

Mdee: I unplugged both coils before doing my comp test...should have pulled the TCI instead. Anyway. My pressure readings were all within a couple PSI's of each other. My gauge does hold the pressure like you said, but it does bleed a little bit, still it's likely not far off.

And thanks for the clarification on that starter jet. I won't worry about those Air Jets being 36 and will just hope they are stock or not a big deal.

Waiting on my coil, got the proper plugs now.
Any feedback on my compression numbers? Low enough to FIX NOW or just put on the todo list?
Thanks!

 

Re: XJ750 SECA coil and jetting question (fouled plugs)

I think you should not worry about the compression #s. At least until you fix plug and coil issue. From what I’ve read, compression difference between cylinder’s is more important, should be small difference and that’s what you got. I would discount the accuracy of the absolute #. You will likely, like me, accept the compression as a non-issue once your bike is running better.

I have confidence your plug fouling is caused by wrong (resistor) spark plugs and bad coil.

I think maybe the way you are using the choke is confusing you to the issue. In my opinion you shut it off too soon. That’s why your bike runs better once warmed up, but you closed down the choke too soon while it’s warming.
My bike runs better by gradually closing choke over as much as 5 minutes. From stone cold, I let my bike idle for at least 2-3 minutes with choke, adjusting for a slow idle. Then ride and close fully after a mile or so. Once warm, engaging choke at slow rpm will make engine stumble. I think of the choke as just a small gas spigot. The engine will need more then less extra gas till fully warm. Although sometimes I have found myself forgetting to fully close the choke.

 

Re: XJ750 SECA coil and jetting question (fouled plugs)

i start mine and ride away with the choke to the first s sign (.5 mile) pull out with the choke then shut it off and do about 3 miles at 45 to 50 in 4th
catch a red light and have a slow (800) idle but it's ready to go
open those screws up to 3.5

 

Re: XJ750 SECA coil and jetting question (fouled plugs)

I agree as Polock suggests turning pilot fuel mixture screws out to 3.5 turns. You may think you don’t want more richness, but the plugs and coils are the likely issue.
My Seca 750 runs good with pilots 3.5 to 3.75 turns out. As a try it and see I have turned mine out to 5 turns and the engine runs great. Idles at 900rpm smooth, immediate response to throttle blip, never hesitates, strong acceleration.

Polock – How do you set your pilots? Do you use colortune?

 

Re: XJ750 SECA coil and jetting question (fouled plugs)

Those Compression Numbers are going to have an effect on the Tuning.
You are going to have to bring-in each Cylinder with a little bit different Pilot Mixture to Compensate for varying Ignition situations.

To "SEE" how the Cylinders with the lower Comprpession Values are going to Ignite under less Compression a ColorTune Plug would be a very valuable asset.

 

Re: XJ750 SECA coil and jetting question (fouled plugs)

I wasn't too worried about the compression at this point seeing that they were all even (almost 100 percent identical in fact). I know/knew that does have an affect on tuning, but I wanted to be open to the possibility that it was more of an issue than I believed.
As to how I run the choke, it is similar to how you stated, but as fast as my plugs fouled, the sooner I could get it off and get the bike moving (and keeping rpms hi) the better luck I would have keeping the plugs from getting so fouled that they were no good (happened a couple times).

I'm happy to hear you guys think my coil issues coupled with risistor plugs is the main culprit, that proving to be the case would make me a very happy man.

Rick, I do have a color tune plug, but these other issues was making it's use not so useful. Now that I have completely gone through and re-verified my carbs, I'm looking forward to going back together with the correct plugs and good coils. If the bike runs right then, I will be able to go through the tuning steps of sync'ing and colortuning and bringing the bike into that happy land of ... fine tuned. It was already running pretty good once warm and had a beautiful idle....looking forward to no cold issue and even better tune.
Any other thoughts, I am very grateful to everyone.

 

I finally got the replacement coil in and a chance to put the bike back together and try everything out.

Bike started up pretty easily and idled well. Used a little choke and left it on about 1/4 to start out. Good power starting out and none of the usual spitting and lack of power issues when cold. None of the light mild backfire that I sometimes got at constant 5k running and just overall very pleasant riding. After my short ride I pulled the plugs and they looked pretty good! To be fair, these are fresh plugs, so it will take a few more start ups and rides to feel 100% confident this solved the problem, but so far indications are positive...if nothing else, a definite improvement.

So, if you are experiencing my problem, check to see if you have resistor plugs and if your coils are out of spec!

Now to dig out my color tune and get the mixtures dialed back in.

Thanks for all the help