81 550 cuts out above 5000rpm

[Wallygator]

I have been rebuilding my 81 550L for the last year and heva finally got it running. The problem is it falls on it's face at 4-5000 rpm and misses and pops. Pulling the choke out will get another 500 rpm and it smoothes out a bit. I have worked on bikes for 35 years (Suzuki 2-stroke singles and triples) Miss my GT380 bad.
What I have done so far.
1. Complete rebuild including new rings valve guide seals and gaskets, compression 142,145,144,143
2. Carbs have been cleaned and completely rebuilt (All jets, o-rings, gaskets replaced)
3. New manifold O-rings (Rubber is still fair to good) Checked for leaks with WD40 and engine is steady. Will idle at 1100rpm fairly steady occasional 100rpm up n down
4. Battery is new and charged, 13.1volts not running 14.7 running at 3000rpm
5. Checked voltage at coils 11.8 to frame ground, 11.9 grounded at the battery
6. Air screws were at 2 turns out per Clymer manual. Opened them to 3.5 turns per, better idle but same miss at 5000
7. Airbox is connected but filter is out.
8. Mufflers were drilled out by some previous owner (loud)
Spark at all plugs is strong and fat, plugs are clean and gapped right, don't see any crossfiring at night. I am thinking about doing the coil relay tonite after I finish putting the carbs back on for the (lost count) time.
Help!
Rick Longfellow
In sunny Ocala Florida

[Steve]

First suggestion...put the air filter back in.

These systems are designed to operate within certain airflow parameters, and depend on a bit of restriction for the vacuum slides to operate properly.

If that does not fix your problem, I would question the main jets. You say they were all replaced, but were they replaced with the correct jet? If you are going to run without a filter and you have less restriction on the exhaust system, you might need jets that are one or two sizes larger to compensate for the additional air flow.

.

[Wallygator]

Steve,
I finished putting the carbs back together last night. Main jets are 94.5 stock size is 92.5 so that should be close. I also installed the coil relay mod last night. Tonite I will get the carbs back in the bike and might get it ready for another run. Still have to find new foam for the filter as mine was turned to powder, none of the dealers have any so I will probbably have to order from MAC.
Rick

[Wallygator]

\\/
Finished the relay mod and carbs Friday night. Engine now revs to 9000 before starting a slight miss. Put in a temporary foam filter and that seemed to help a little. The idle is now smoother and less hunting after tewaking the idle air screws. I think most of the improvement came from the relay mod as I didn't put in the filter untill later so I could judge which action actually worked. Plugs are showing slightly rich and that's fine with me. Now just need to finish painting the tank and side covers and ride it to get comfortable with the reliability.

Rick