Please help me solve an issue with a 1984 Vt500 Honda Shadow

I am seeking the wisdom of this group to help me solve an issue with a 1984 Vt500 Honda Shadow.. Mileage 45 k ( Kilometers). Sometimes known as the Ascot it?s a Twin cylinder 500 CC.. Two CV carburetors, dual coils, Dual spark (CDI) ignition (two sparkplugs per cylinder) 6 speed, shaft drive
Symptoms: Surging or loss of power in the upper RPM range (4.5 to 5K RPM equating to 50 -55 MPH ish in 6th gear. The issue is consistent in 6th gear and intermittent in 5th gear at similar RPM?s. In 6th gear when applying additional past 4.5 to 5K the throttle has no effect and the bike will actually drop in road speed ( RPM drops by about 2 K before throttle is again responsive. if you shift to 5th then throttle is immediately responsive .) There is no perceived similar loss of power in the lower gears at any revs. Issue started at beginning of the season last year and seems to have gotten progressively worse.

Things tried so far
Ignition: Swapped both coils and CDI units with ebay sourced units ( Used) No change using any combination of original and the ebay units. New Spark plugs. No perceived arcing of dry or wet plug wires to any ground source (Dark Garage) Wires show good with ohm meter.New Battery this season ( Bike always starts and idles fine)

Fuel:
Used ancillary fuel tank to bypass the vacuum actuated petcock in original fuel tank, I used an ultrasonic bath/ cleaner on the Carburetors (Twice) No perceived holes in CV rubber slides. Float heights appear to be within spec. ( I don?t have the special gauge required.) No evidence of anything missing or damaged and no air/vacuum leaks found..

Ideas ??

Re: Please.. Please help me solve an issue with a 1984 Vt500 Honda Shadow

for RPM specific power loss I would have guessed ignition first, but it sounds like you've covered that option with the used coils. Still possible the used units are not perfect, but probably unlikely they'd have the same exact symptom as the originals unless they all have a common failure mode. But did you use the same wires and plug caps with either set of coils? Your problem could be in the wires/caps, not the coils themselves. The fact that its worse in 6th gear than 5th (and you say its OK in gears 1-4) could be due to the higher load on the engine changing the cylinder pressure. Ignition problems are often cylinder pressure dependent because it takes a higher voltage to spark across a higher pressure cylinder.

have you checked the valve clearances and compression? Perhaps there's some valve clearance issue that effects compression at that RPM range.

You may have a partially clogged jet, I'd pull the bowls and run a fine wire like a guitar string into the jet passage, but others recommend a non-metallic equivalent to the guitar string to avoid any possibility of scraping the I.D. of the jet in the process. Not sure how you determined the float height but I think you can check it with clear tubing, never had to do it myself. A quick check is to drain both bowls into a graduated cylinder (one at a time) and see if they're at least consistent. If one of your float needles is failing or dirty it would show up as substantially more fuel in one of the bowls.

Will the bike run on one cylinder? If you can test it on the road with each cylinder individually it might narrow down the cause. Make sure you do that on a low speed untraveled section of road like a residential street (oops, just realized you need to be @55MPH or so to be in the trouble spot, so maybe try somewhere else).

Re: Please.. Please help me solve an issue with a 1984 Vt500 Honda Shadow

Give bike a dose of fuel conditioner. SeaFoam is good. Run it for 15 minutes and let it set for a few days. This has cleaned up a lot of bikes.

Jim

Re: Please.. Please help me solve an issue with a 1984 Vt500 Honda Shadow

I'm guessing fuel..... maybe lean a bit. If you haven't drained the tank and put fresh fuel in do that first thing. Try running at your trouble spot with a bit of choke, and see if it clears up. Yes to the Seafoam, go for a half hour run, then let it sit for a few days.
I see you've cleaned the carbs, good idea. Mostly that should have fixed it, but still may be some gum in the jets. A good boil with some water and Simple Green might be the next move if the Seafoam doesn't clear it up.

Re: Please.. Please help me solve an issue with a 1984 Vt500 Honda Shadow

I had a similar problem, with 85 Chevy Blazer.
The work done, does leaves out, one thing. If your bike
has one.
Fuel filter - if it is starting to clog, then you would be starving
the engine at higher rpm's, with a heavier load on the engine.

My 2 cent

Re: Please.. Please help me solve an issue with a 1984 Vt500 Honda Shadow

As others have stated it sure sounds like you have some old fuel which has caused you some issues, pull the carburetors and clean out jets, pull the tank and make sure there is no crud in the tank. Good luck.

Re: Please.. Please help me solve an issue with a 1984 Vt500 Honda Shadow

Sounds like starving for AIR or FUEL or both.

Re: Please.. Please help me solve an issue with a 1984 Vt500 Honda Shadow

+1 on the fuel filter as something to check. Sometimes it's the simple things.

Re: Please.. Please help me solve an issue with a 1984 Vt500 Honda Shadow

Does it have the stock air cleaner and airbox? The CV carbs setup for a stock airbox and its restriction do not work well with less restrictive replacement filters even with larger main jets and pilot jets. A really bad midrange hesitation can occur even with a stock airbox if the airbox boots between the airbox and carbs become hardened from age and do not seal well. Also pay close attention to the slide needles - make sure the plastic retainer is on top of the needle holding the needle down properly and make sure that the vacuum hole in the bottom of the slide is not being restricted by the plastic retainer. If this retainer is not positioned correctly and the vacuum hole is restricted the slide opening will lag far behind the throttle position when opening and cause this kind of hesitation.

Dan

Re: Please.. Please help me solve an issue with a 1984 Vt500 Honda Shadow

Here is perhaps some relevant reading........
https://www.st-owners.com/forums/showthread.php?137241-Vacuum-Petcock-Question-tell-me-I-m-getting-it-or-not!

Re: Please.. Please help me solve an issue with a 1984 Vt500 Honda Shadow

Hello Thank you all for the excellent suggestions.
Bdalameda : Yes the bike has the OEM Stock Air Filter.. After I saw your suggestion and I looked at a parts Fiche I noticed that mine was missing its sealing ring.. (I have cobbled one together from weatherstripping and now have a good seal) I will let the group know if it makes any difference. The needle retainer doesn't appear to be blocking the vacuum hole in the bottom of the slide. The clip is a compress and twist to lock it into place and there are standoffs on its underside to keep it from blocking the slider hole.. I will see if its possible to have it incorrectly installed.

Ray: I have tried sea foam multiple times ( Great stuff by the way) I will try running with a bit of choke to see if it makes a difference. I used boiling water in my ultrasonic cleaner, but will try some simple green this time..
dwalby: Again excellent suggestions . I do know that the bike will run on one cylinder ( At least I know it will run on the front as I forgot to connect the rear coil one time, before I took it for a spin <Grin> )) I did check valve clearance last year, but will check it again.. ( I just found my dad's old compression tester so I will be comparing front to back also..) I have been using compressed air to blow out the carb passages after cleaning, but will see if I can get something suitable to poke through the holes..
Regarding fuel: The gas tank petcock has two screens and there are additional screens located directly under each needle valve, all are clean. Gas is fresh
Stuart

Re: Please.. Please help me solve an issue with a 1984 Vt500 Honda Shadow

I would check vacuum hoses

Re: Please.. Please help me solve an issue with a 1984 Vt500 Honda Shadow

Success.. I went for a 60 mile loop last evening and not a hiccup or loss of power was evident. Bdalameda's post prompted me to consult an online parts Fiche to review the various pieces involving the air-box and Air path and I realized that a gasket was missing where the Air filter assembly mounts to the main air-box . I used a piece of weatherstripping as a replacement for the missing gasket. ( A big THANK YOU goes out to you all. )

Re: Please.. Please help me solve an issue with a 1984 Vt500 Honda Shadow

congrats, glad it turned out to be an easy and cheap fix