99 1500 29,000 miles, Rough engine at 65+

[Markesmi]

I'm looking for help troubleshooting a rough engine at higher speeds, 65+ and heavy acceleration.
It acts like one cylinder is cutting out.
The roughness continues sometimes even at idle.
Let it sit for a couple weeks or a month and it will run again, until high speed...
This started over a year ago and gradually got worse.

I have changed the sub filter to the vacuum system to the carbs. Before that I overhauled the vacuum cut off valve. Fuel flows is good, and has a new fuel filter. Sea Foam has made no changes or improvements. I have inspected the spark plugs, they are clean and appear to not to be burnt and show little wear. I didn't see any cracks, but wasn't looking for them.

Any suggestions? I'd like to get back on the road with full trust again.

[MikeB]

I'd suggest changing the spark plugs. The NGK DPR7EA-9 plugs are quite inexpensive really.

[Markesmi]

Sounds good to me. I'll probably get that done tomorrow.

[John K]

Wonder if your fuel petcock has a vacuum leak ?
25 year old thin rubber
Difficult to see cracks or hole best to run vacuum test per manual
Best of luck

[Markesmi]

I have overhauled fuel petcock. It should be in good condition now.

[Markesmi]

Now which plug option?

DPR7EA-9 is factory
DPR6EA-9 is hotter, for cooler climate.
DPR8EA-9 is cooler, for longer highspeed

[MikeB]

NGK DPR7EA-9 plugs have worked for me for the last 300,000 miles or so on two different GL1500's.

[John K]

Per Honda Service Manual

Trying to help

A whole list of things to check out

Best of luck

[John K]

Always plugged into Battery Tender at house
Back 20 years ago I added too many lights for stock alternator to handle
I found out while on trips as bike would start missing while riding it, sometimes won't start after stopping for gas, lunch or whatever
Replaced battery on several trips
Should have put a voltage gauge on it
Replaced stock alternator with a Compu-Fire unit and no more Battery replacements unless they are 7 years old or older
Have replaced lights blubs with LEDs so even less draw
As Rambozo and WingAdmin have said several times the 1500 requires a Battery at it's best

Best of luck

[FM-USA]

I'm looking for help troubleshooting a rough engine at higher speeds, 65+ and heavy acceleration.
It acts like one cylinder is cutting out.
The roughness continues sometimes even at idle. (Vac leak?)
Let it sit for a couple weeks or a month and it will run again, until high speed...
This started over a year ago and gradually got worse. (Ethanol suxs, eventually leaves a GOO when sitting)

I have changed the sub filter to the vacuum system to the carbs. Before that I overhauled the vacuum cut off valve. Fuel flows is good, and has a new fuel filter. Sea Foam has made no changes or improvements. I have inspected the spark plugs, they are clean and appear to not to be burnt and show little wear. I didn't see any cracks, but wasn't looking for them.

Any suggestions? I'd like to get back on the road with full trust again.

I know your dilemma (sorta).
1991-I Seemingly 1 cyl cutting out I get once or thrice a year. So I let it be.
Once I got several cyls cut out in a row and THAT prompted me to dig deep.
Never been able to diagnose it due to randomness, but because of the several misses it's down to an electrical issue. A fuel miss acts a bit sluggish vs electrical's decisiveness.

Sorry for the long wordy post but it's my hands on (hopefully) fix.

Of mine it's been happening for roughly and randomly for +200K miles. Still on OEM coils & wires, BTW.
Speaking of wires, are yours solid or carbon cores? If carbon the ends might be burned out a bit. Can't trim much, custom cut to fit.
Farmers trick, stick a 'straight pin' in the ends.

When pressure washing, the engine will not start until dry enough. Since then I leave it running & spray normally until I hear the miss. It occurs only from the right side (helps diagnose). I check the spark plugs wells for tell-tail water puddles, none.
By just reaching in with very long-nose pliers, I was able to partly wrap the 3 coils with parchment paper. Seemed to help during washings.

Once I entered "The Coil Zone" deeply, I properly wrapped the coils leaving the rear & bottom open for cooling. Easiest way to get to the whole area, remove carbs & coolant overflow bottle. While in there, hows your thermostat & vacuum hoses?
While in that area I pulled each wire & looked for arcing, none. Ohm'd the plugs & wires to the coils, all fine.
I brake cleaned the coils top & looked for carbon tracks, none but was layered with aged dirt that was well adhered, electrical loves swimming in water. Cleaned to a Like-New shiny finish. (fingers crossed)
While pressure washing, those miss's not occurred "easily" as before but if I direct the spray, it wants to die. Doc says, 'don't directly direct the spray'.
While riding, I've not yet had that 1 cyl. miss. Related? Unsure but it's a possibility.

SO... could your issue be like mine? Better than 50% I'd say yes.

OH, nearby neighbor had your exact issue. Gave him suggestions to check but he moved. So no feedback.

.