Fueling or Electrical - should I take the red or the blue pill?

[Guest]

Will try to make this concise, but there's a lot of info here, and I'm stumped as to which path to pursue first.

The setup:
1978 GS750E. Everything that matters is stock (exhaust, VM carbs, airbox, ignition system). I did a top-end rebuild including cleaning and polishing pistons, honed cylinders, polished and lapped valves, re-shimmed to bring into spec, all new gaskets, valve guides, etc. I installed new intake o-rings, and my boots are soft with no cracks. I soaked the carbs, even though they had no evidence of varnish or debris when I took them apart. Replaced all carb o-rings and reassembled with everything to factory spec as best possible. Installed a new air filter in original box setup. Adjusted timing using the non-timing light method. Spark plugs were clean, but I carefully polished them up even more, re-gapped them, and I don't see any extreme issues like burnt white or black wet. They are somewhere in the middle. The pilot screws are on the air side of my carbs and spec says 1 1/4 turns out. I have them right about 1 turn out, as that seems to be where it is working the best.
Hopefully I didn't forget anything.

The behavior:
Starts immediately with about 1/2 or less choke. As it warms and I drive, I can slowly drop the choke off over the first mile or so, and once good and hot, runs best with no choke at all. Up to 1/4 throttle, I'd give it a B+. It's not perfect, but acceptable. From 1/4 to 1/2ish throttle, it pulls hard, is smooth as the air-brushed skin of a swimsuit model, makes a beautiful sound and generally makes me grin like I just got away with something. At full throttle, it pulls very weak but still accelerates slowly, but the tone changes to mimic an exuberant high school Spanish teacher over-emphasizing how to roll your R's. It behaves the same in all gears.

Confusion and attempts at solving:
I read about fuel caps being clogged, so I tried running it WOT with the cap loose. No change in behavior, and felt lucky that no gas spilled.
I read about adding some choke during WOT, as that would mean you're lean. This had no affect either.
If I have everything involved in fuel delivery as stock from factory, would I still need a bigger main jet? I've read a lot about people needing bigger main jets to solve the ever-lean condition, but the choke test at WOT suggests it may not be a lean condition.
Thought about pursuing electrical/ignition issues, but my brain doesn't process those issues and solutions as easily. I read that the timing advance kicks in around 2500 RPM. It's still running butter smooth until at least 4500 or so, which makes me think it's not an issue with mis-adjusted advance.

Request:
Can someone help me narrow down which paths to pursue, with some explanation based on this description? I'd be happy to provide more detail and test results if I've missed something important.

Thanks so much for the expertise in this forum!

[Gorminrider]

I love your description! "....but the tone changes to mimic an exuberant high school Spanish teacher over-emphasizing how to roll your R's"

Not knowing much about your carbs and you've tried to richen the mix already...I don't suppose turning petcock to Prime would help and it's a delayed effect anyways but it's at least as useful as suspecting the gas cap!! idle mix "pilot screws" would be unlikely to cause a full throttle symptom, IMO. That's when they are the least important, so if it's a carb problem, fuel level jets and air leaks are the place to be really curious about.

You could try a "plug chop". take your plug wrench with you, get the bike to do bad things, and pull them immediately. Don't look at them after going home running the bike below 4500 because they will have washed theie faces and combed their hair by then.

...but seeing as you have points, a new condensor is always an easy thing...symptoms of a bad one would be sparks seen at points in a dark place. Also cracks at plug caps....or any sparks seen along leads. I'm thinking along the lines of since you are asking coils to spark faster, these things become more important....
by the way, It's hard to beat a strobe light if you have points because you can see advance "working"..I would not be without one, so if you "shop" to relieve stress, maybe pick one up on sale sometime

[John Park]

Each coil fires two cylinders; if one craps out early - and you do need more spark as the throttle is opened wider - it will 'roll it's Rs' . I'm guessing that it's point gap related, or a bad coil. Just because the timing is correct doesn't mean you have a proper, strong spark.

Some of those components like coil and wires may be almost forty years old. That doesn't mean they won't still work, but new plug caps are cheap, and cutting about a1/4" off the wires and installing new ones is a good idea regardless, unless they look pretty new.

If your points aren't looking new and gapped properly you can have this sort of thing; why they went to electronic triggers. The fixed set are easy to gap, but the 'wandering' set seems to change gap as you adjust it along the plate. While it's 'electrical', the setup and such are visible and mechanical. While you're there, make sure the mechanical advance mechanism is lubed and happy.

I had a perfect '78 750E and stupidly sold it; a really elegant and enchanting bike.

[JTGS850GL]

I'm betting points or condenser but doing a plug chop will verify if it's fuel or not. Run it in the range that's giving your problems and then do a complete shutdown and coast. Anotherwords, hold it in that range for a few seconds and then hit the kill switch and pull in the clutch. When you stop, pull the plugs and see what they look like. If they're black and wet, then that may indicate poor ignition. If they're white and dry then fuel is the problem.

[Gorminrider]

...and just in case, because Chilly, you say bike is not all that great below 4500, after a rebuild..(and though this might sound stoopid to those who know better but it occurs to me because I was just having to reset my own after a peculiar cam chain jump) but I was just wondering if a single link behind/ahead on the cam chain could cause this....ie: if the intake or exhaust valves were running a link "wrong"...?

[chilly_75]

...and just in case, because Chilly, you say bike is not all that great below 4500, after a rebuild..(and though this might sound stoopid to those who know better but it occurs to me because I was just having to reset my own after a peculiar cam chain jump) but I was just wondering if a single link behind/ahead on the cam chain could cause this....ie: if the intake or exhaust valves were running a link "wrong"...?

I should clarify that... It doesn't run perfect at the lowest throttle. So, between idle and 1500 or so, it's good but not great. It's the middle throttle that is spot on and sings a sweet Marvin Gaye melody.
What I get on the low end, is a very smooth idle, but when I snap the throttle, it hesitates or bogs a bit. It's not enough to kill the engine, unless I try to snap it to more than 1/2 throttle. Basically, I just have to twist the grip nice and intentionally. I don't have that snappy growl directly from idle. But once the RPMs get up over 1500 or so, I can snap it all I want and the bike just begs for more. But I'm not too concerned about the low end right now. My more immediate issue is resolving the high stuff... cuz who wants a bike that wants to go fast but can't???

[gustovh]

Check your cam chain and make sure you have 20 links not 21 between the cams. My bet is it is not your carbs. If it is the cam chain out of wack the bike will continue to accelerate to around 70 mph and then pretty much be flat out. It took me around 3 months to find this problem on a new to me 850. Runs like a scalded ape now.

V

[chilly_75]

Y'know... sometimes it's the simplest of things. Feeling a bit sheepish!!
John Park is the big winner!
When I did the timing and greased the point cams and oiled the felt... I skipped the step of checking the gaps. Over lunch, I measured, and the first was just a bit tight (around 0.25mm) while the second was ridiculously tight (around 0.10mm). So, adjusted both to 0.35mm and took it out for a test. Once I got to a county road (only a mile or so), I lit her up. She pulled hard and happy all the way to 8000 RPM before I decided that was enough. Can't describe how pumped I am that it's resolved, and how dumb I feel for not doing that in the first place.
Thank you all for your input and time. Much appreciated!