Power Loss & MPG Drop

[Nightflight99]

My standard Valk has a history of having deplorable gas mileage (average 24 mpg), even compared to my Interstate (average 30 mpg, same riding style), but I've always blamed this on the open Cobra pipes (in spite of rejetting).

Last weekend, while on a quick 300-mile ride, I suddenly noticed a significant power loss. It was significant enough to where I had to pin the throttle in order to respond to relatively minor increases in pace around 75-90 mph, which has never been the case before. On that same ride, the gas mileage dropped into the realm of dark comedy: at one gas stop, she had consumed 4.65 gallons in 64.1 miles (13.78 mpg)!!

She did not lug or produce any other obvious symptoms, just showed a significant drop in power and the gas mileage of a Toyota Land Cruiser.

Other background info on the bike:
- Carbs were done in 2023 and the bike never really gets a chance to sit more than a couple of weeks
- Spark plugs were replaced and gapped about 8 months ago
- Petcock was replaced a few months ago

I have some time this weekend, and am about to organize my plan of attack. So far, I plan on

1) Checking spark plugs and & drain holes
2) Checking air filter
3) Checking fuel lines   
4) Checking fuel filter

David mentioned the possibility of one or more bad air cut valves. The thought of removing the carbs again is making me cringe, but I don't suppose there would be any way to test them without removing both carb banks.

Is there anything else I should be adding to the list of items to check?

[Pluggy]

Nightflight, Have you checked he enricher function, commonly called the choke?  It has to be 100% off or the bike gobbles fuel. Easy to check.

[F6Dave]

My Interstate also gets better mileage than my Tourer. It also is more picky about fuel and will ping badly in hot weather without premium. The Tourer runs on anything, even the 85 octane crappy gas they sell at high altitude.  I've occasionally seen mileage in the teens on both, when running at 80 MPH with a strong (think Western Kansas) headwind.

With mileage that bad I'd pull the plugs first and have a look. Maybe one will be worse than the others to give you a clue. Seems like they'd have to be black and sooty.

[scooperhsd]

My standard has gradually gone done in MPG - from 35 down to sub 20. I'm not sure how far I want to trust it....

[Nightflight99]

Pulled the plugs and they looked...crusty. Especially when you consider that they were brand new just a few months ago. Spark plug tester confirmed that all but a couple of them were producing spark much less consistently compared to brand new ones.

After that, I did a compression test on each cylinder. All had similar compression (190-200), but of the rear cylinders on each side built up compression more slowly.

Going to check valve clearances tomorrow, then run a leak-down test for peace of mind.

I have a bad feeling that this is going to be carburetor-related. To be fair, that feeling could also just be the lengua tacos I had earlier...

Nightflight, Have you checked he enricher function, commonly called the choke?  It has to be 100% off or the bike gobbles fuel. Easy to check.

Hmm. Good call. I'm 99% certain that it is not engaged, but its a good idea to check that off. Thank you for that!

[Mooskee]

Air cut valves seldom need replacing. In addition, you have to break the carbs apart and remove the ACVs to test them. Most of the ones I have found bad had very likely had carb cleaner sprayed into them.

I looked at the number on the spark plugs. The DPR8EA-9 are for extended high speed riding, but it sounds like that is what you do. The DPR7EA-9 is the standard plug. Page 3-7 of the manual.

[Nightflight99]

Air cut valves seldom need replacing. In addition, you have to break the carbs apart and remove the ACVs to test them. Most of the ones I have found bad had very likely had carb cleaner sprayed into them.

I looked at the number on the spark plugs. The DPR8EA-9 are for extended high speed riding, but it sounds like that is what you do. The DPR7EA-9 is the standard plug. Page 3-7 of the manual.

Yeah, I was hoping to avoid having to battle that airbox again, but I'm slowly making peace with it, if it ends up leading that way.

When I replaced the spark plugs, I opted for the DPR8EA-9 due to my riding style. I never really noticed any difference compared to the regular plugs I ran before.

[Mooskee]

Air cut valves seldom need replacing. In addition, you have to break the carbs apart and remove the ACVs to test them. Most of the ones I have found bad had very likely had carb cleaner sprayed into them.

I looked at the number on the spark plugs. The DPR8EA-9 are for extended high speed riding, but it sounds like that is what you do. The DPR7EA-9 is the standard plug. Page 3-7 of the manual.

Yeah, I was hoping to avoid having to battle that airbox again, but I'm slowly making peace with it, if it ends up leading that way.

When I replaced the spark plugs, I opted for the DPR8EA-9 due to my riding style. I never really noticed any difference compared to the regular plugs I ran before.

That is why my son and I invented the Valkyrie Easy Install Airbox Mod Kit. You can check it out on our website. https://valkyriecarbsandcustom.com/ols/products/valkyrie-easy-install-airbox-mod-kit

[Nightflight99]

That is why my son and I invented the Valkyrie Easy Install Airbox Mod Kit. You can check it out on our website. https://valkyriecarbsandcustom.com/ols/products/valkyrie-easy-install-airbox-mod-kit

That could come in handy. I'll be back at the shop on Friday to see where this journey is about to take me.

[h13man]

From the looks of the plugs, definitely a issue to all cylinders equally. I'd replace them as they look like the one's I had on two at different cylinders (#2 & #5) and change intervals quit just right after 10,000 mi. on the nose. OEM manual say's to change @ 8,000. Now if one rides mainly open road probably go further on change intervals but standard plugs are cheap enough to replace. I replaced mine with  iridium's and as of now they have 16,500 on them. Cold starts are phenomenal. I 1st. used them on the VT1100 Spirit and they were vast improvement at cold starts and anybody who has owned a VT1100 know how contrary they were at 1st. of the season start up. Lately my mpg. has dropped 32 mpg. thus a plug replacement may be needed. I guess I need to pull a plug and see whats going on. The plugs have been reset .028 recently under a experimentation but the plugs do have considerable mileage. Next set of plugs will be standard @ .028 as I'm in the process of selling. IMO iridium's are worth the money spent. This season start up required two push of the starter button instead one push over the years before.

[Nightflight99]

Looks like I might have found the culprit. After verifying the valve clearances and opening the bowls to the rear-most carburetors did not yield anything, I shifted to a different direction. While checking the ignition coils, I realized that the connections to the spark plugs were bad, and their poor shape correlated with the condition of the spark plugs (the rear-most ones were the worst). Fixed them and replaced the plugs. Didn't have enough time to ride, so I just let it idle for a while, then checked the plugs and they looked great.

I'm gonna do a proper test run later this week and then check the plugs and gas mileage again. Fingers crossed!

[Hook#3287]

" I realized that the connections to the spark plugs were bad,"

What does this mean?

Wires were compromised, end connection faulty or caps damaged?

That's not a common issue that I'm aware of.

[Nightflight99]

" I realized that the connections to the spark plugs were bad,"

What does this mean?

Wires were compromised, end connection faulty or caps damaged?

That's not a common issue that I'm aware of.

The wires looked compromised at the point of the end connection. I did not think to take some pics of it, but we ended up cutting and redoing them. I won't have time to test the bike until this weekend, but it certainly would explain the symptoms.