1982 Seca XJ750, ZRX1200 has company now.

[Bonz]

DiverRay had mentioned a turbo back on the first page there could have been some confusion there.

[bikerphil]

Sorry Bonz, I thought it was 650, thank you for the explanation.

[Bonz]

Fwiw, it is not an XJ650 Seca Turbo... even though that wouldn't make a difference one way or the other. It is an XJ750 Seca as per the thread title.

Exhaust spec is .006" to .008". .004" clearance is 33% below MIN spec, and 50% below MAX spec. It's not the size of the number, it's the relative amount that it is out of spec.

Think of it like other adjustments on the bike such as fuel level/float height, jet sizes, oil level, compression in cylinders, cam chain position on sprokets, engine timing, etc. Most bikes wouldn't run or cause damage if those adjustments/numbers were 33%-50% different than what they should be.

Whether turbo or a non turbo, overly tight valve(s) would have lead to looking deeper at causes and the same remedy once all other systems check out.

Valves fully seating each firing cycle remove heat from the valve into the head. If the valve is tight, the valve never completely closes and cannot transfer heat to the head through contact. As well, if the engine doesn't make sufficient compression due to an overly tight valve, it will not fire, particularly at idle, At that point, hot exhaust gases when at higher engine speed from what combustion does take place rush by the partially open valve and burn the exhaust valve.

[bikerphil]

I'm glad you got the bike running properly. I never would have never seen it coming, a valve out only a few thou causing such an issue. Is there a clearance difference because it is a turbo?

[Bonz]

Will stop updating this thread, seems interest was lost when the problem was solved and problem was the tight #2 exhaust valve vs carburetors or ignition. Hopefully it was helpful for someone down the line that reads it and they investigate a tight valve.

[Bonz]

167 mile ride today, bike is flawless. 50 mpg. Hot as the devil's crotch coming back through Colorado Springs, low 90s and stop and go on the interstate for the short section we can't avoid. Didn't miss a beat.

[Bonz]

For the collective, this XJ750 Seca has a unique gearing combination apparently?

The ultimate question is has anyone had experience or any credible information on an FD swap on this bike or the XJ750 Maxim as they are the same except for rear tire size (18" Seca, 16" Maxim). Like the Special and Standard XS1100 and different rear tire sizes accounting for the difference in rpm.

5th gear at 60 MPH shows 3,900 RPM taking a real close look at it. Road tests back in the day are consistently reporting 4,500 RPM at 60 MPH in 5th gear.

The bike feels like it's turning 3900 RPM at 60 miles per hour, double checked by installing a simple auxiliary tachometer I got from Amazon. Auxiliary tach shows 4060 rpm at 60 mph. Anyway you slice it something's been done to the gearing in my opinion. 60 mph checked out plus or minus with a GPS, seat of the pants said I was going 60 mph and it was accurate.

Anyway I can't find any substantive information or write-ups of a final drive swap on the XJ750 Seca. What I do find shows nothing of a combination that would give that RPM even if someone had done it.

Part of the issue with the hanging /slow return to idle was when the bike tachometer said 1050-1100 rpm, it it was turning 1350 rpm by the auxiliary tach. Adjusted idle speed using auxiliary tach and it returns much better now, although still a tad on the slow side to return. As a note, the higher the RPM indicated, the closer it agrees with the auxiliary tachometer. At 7,000 RPM it's less than 100 RPM difference.

Going to do another synchronization now that everything's buttoned up, hopefully takes care of that last little bit.

Edit: Sync took care of that last bit of hanging idle. For grins, I checked before using the YICS tool, sync was pretty good. To be expected with sharing common vacuum. YICS tool installed showed the true colors and needed some adjustment. Dialed in with the tool now, idle smooth as can be, throttle is like a rheostat in any gear or rpm.

[Bonz]

She fired right up after the the valve adjustment and it runs like a beautiful 44-year-old Yamaha should.

#2 exhaust valve was the culprit. I left the cheap eBay coil on from the last time when #2 wouldn't fire at idle. The exhaust valve adjustment was the only change made from the last ride when #2 was misbehaving.

Remaining niglet:
It's slow to return to idle from 2,000 rpm once warm and on the road. Need to sync again, as the way it ran before, no way the sync was accurate. I've checked for air leaks, so if the sync isn’t the issue, I'll open the pilot screws to add some fuel at idle.

Don't always think it's "the carbs", "ignition", and continue to go down rabbit holes when you know those holes are closed so to speak. Believe in your mechanical skills and knowledge when everything checks out and follow the clues till all things are eliminated.

Thanks for the input guys, nice to have the collective knowledge chiming in!

[Bonz]

Sorry for the confusion. I wasn't posting about carbs available because of your current problem. Way earlier in this thread you mentioned that one of your carbs is from a 650 so I posted thinking you might be interested in a complete set. Of course you also posted that you found a box with carb parts and bodies so my post may well be irrelevant anyway.

Thanks for the follow-up, I didn't see your post until now. Thank you for doing a bit of leg work, eBay is my source for carburetors and parts 99% of the time as well. It would be nice to have the same threads on the pilot screws on all four carbs, with 4 being the XJ650 carb based on the coarse fuel screw threads and the jets that were in it that match XJ650 jetting.

With that said, #2 is the confounding cylinder that is not firing at idle and comes online intermittently as RPM increases. The bike has fuel, air and spark on 2.

At this point, there's little to lose getting the very tight exhaust valve back to spec on 2. That's a classic issue that can cause the problem 2 is having. It's rarely, if at all, mentioned or talked about in this type of situation. Once the shims arrive and started up with a 3rd coil (Mikes XS) hopefully the story has a good ending.

[Bonz]

Agree on the Dyna coils. We shall see when the shims are installed to bring the clearances in spec and new Mike's XS coil installed on 2/3. Keep in mind the OEM coil when I swapped two and three, two kept misbehaving. When I installed the eBay coil, number two kept misbehaving. When I swapped the wires on two and three with eBay coil number two kept misbehaving. I don't see all of those combinations being the exact same issue with respect to a coil. As we both are saying to ourselves, we shall see what combination becomes the winning hand.

[bikerphil]

Hopefully the new coil does the trick. To me, it seems that once the ignition system is 100%, it cures many other problems. Good quality hot coils like Dynatech have become rather expensive but in the long run definitely worth it.

[cajun31]

The carburetors are not the issue. I've done enough of them to know when they've been cleaned thoroughly, with all candor. They have been taken apart and ultrasonically cleaned and put back together basically new. It's not them.

Sorry for the confusion. I wasn't posting about carbs available because of your current problem. Way earlier in this thread you mentioned that one of your carbs is from a 650 so I posted thinking you might be interested in a complete set. Of course you also posted that you found a box with carb parts and bodies so my post may well be irrelevant anyway.

[Bonz]

Spark is not the problem with the changes and swap outs I've done to rule that out. A tight valve is not talked about as being a cause of the issue the bike is having, but it absolutely can. Valves need adjusting anyway, .004 is an extremely tight exhaust valve on number two, IMO. When spec is .006 to .008.

[Radioguylogs]

Doesn't it seem that if #3 is has spark, then #2 has spark? (unless #2 plug is shorted)

[Bonz]

Good timing on asking that question... It is a wasted Spark system. I swapped the wires on #2 and #3 early on in the process as the first change besides new plugs which are a given. Made no difference. #2 was not igniting at idle and intermittenttly would run at higher RPM as has been shared.

Put in a new ebay coil on 2 and 3, the exact issue remained. After a couple rides with the intermittent firing, that coil gave up the ghost entirely. 2 and 3 went silent. A resistance check showed 75,000 Ohms on the secondary circuit, lol. Anyway, returned those pieces of lovable junk.

In the process of attempting to take the wires out of the 2/3 OEM coil to trim them back, thinking that might be part of the issue, I did not realize they don't do that easily. So the OEM 2/3 coil is not going to be used again, thus the ebay coil.

1/4 coil is OEM, however no issues.

Back to the good timing part from the beginning of the post... A new coil literally arrived from Mike's XS today. When the shims for the valves arrive later this week I'll get it buttoned up and the new coil installed.

As well, I have checked resistance on the TCI and the pickup, everything is right there in spec. Picked up a used TCI, swapped it in and the same issue remained before the 2/3 coil went out as described above.