Misfire when hot

[Piercems3]

I have a strange issue I have been having trouble resolving, and I am hoping for help from the experts here. I have a 2005 Vmax with about 8500 miles. It was well cared for, always garaged. It has developed a strange misfire over the last year or so, and it only shows up when it is over 80 degrees F or higher.

When the weather is cool, it runs like a scalded cat. It always starts easily. No other issues at all, just a slight misfire (mostly at lower rpm), when throttle is first increased. Once the misfire happens (like a hiccup), it takes off and runs strong.

What I have tried: replaced fuel lines, cleaned fuel tank, replaced fuel filter, pulled carburetors apart and cleaned them / adjusted fuel level in floats, balanced carbs, checked all electrical connections (and eliminated many of the quick disconnects), installed COPS (although I am running without resistor because when I installed them I didn't know it was advised). I have also put in fresh spark plugs. Voltage appears good when I check it, usually over 13.7v even at idle.

It is hard to describe, but it almost feels like it is getting too much fuel or not quite enough spark at lower rpm when I first hit the throttle. It is a quick hiccup/misfire and then it takes off and runs strong to redline. This hiccup can happen at higher rpm also, I have noticed it even at 4,000 rpm when cruising along and then jabbing the gas.

 

[Vinmax]

Here's a few things I'll mention to check;
-many times when engines run worse when warm, is indicative of rich running. So idle mixture, needles, dirty air filter, sticking slides, etc. can all cause the mixture to run richer than it should when warm. You didn't mention the air filter. All clean?
-Very good that you have COPS installed. The stock coil casings have been known to crack and then can cause issues. I had Mitsubishi COPS installed when I first got rid of my stock ignition coils and wires, and when my bike was warm, I'd have this strange misfire. Turned out to be one of the Mitsu COPS was sparking through the casing right to the engine block. I could see it when I'd be parked in the dark garage with the engine idling. It was crazy to finally see and I knew I had finally figured out the issue. I did some research and found that the Misubishi COPS were recalled because of this issue. I had then changed to Denso COPS from a GSXR and never had a problem again. This was years ago. Check that.

Vinnie

 

[Piercems3]

Here's a few things I'll mention to check;
-many times when engines run worse when warm, is indicative of rich running. So idle mixture, needles, dirty air filter, sticking slides, etc. can all cause the mixture to run richer than it should when warm. You didn't mention the air filter. All clean?
-Very good that you have COPS installed. The stock coil casings have been known to crack and then can cause issues. I had Denso COPS installed when I first got rid of my stock ignition coils and wires, and when my bike was warm, I'd have this strange misfire. Turned out to be one of the Denso COPS was sparking through the casing right to the engine block. I could see it when I'd be parked in the dark garage with the engine idling. It was crazy to finally see and I knew I had finally figured out the issue. I did some research and found that the Denso's were recalled because of this issue. I had then changed to the Mitsubishi's COPS from a GSXR and never had a problem again. This was years ago. Check that.

Vinnie

Thanks for the advise Vinnie. I will check on the COPS first. The air filter was cleaned about 1000 miles ago (K&N). It still looks clean.

 

[[email protected]]

What exhaust do you have? Hard to tell from your avatar but it doesn't look stock.
What is your carb setup? You may just need to do a little tuning.

 

[Piercems3]

It has UFO 4-2 exhaust. The original owner stated he did use a jet kit but said he lost the info for sizes. I did have the carbs apart to clean everything out about a year ago when I adjusted the float levels but I didn't check what was installed.

 

[Fire-medic]

Vinnie, not-sayin' you didn't fix your problem, but the majority of posts on here in the past about COPs' problems are that the Denso ones are better on our bikes than the Mitsubishi's. Maybe Gannon wants to weigh-in on this. He developed and sold the Splicker Designs COP kits for our bikes, he's since sold his inventory to Sean Morley.

That's a good piece of advice about giving a good inspection to your bike's (or any internal-combustion system's) primary ignition in the dark. I've used that method many times over the years on bikes, cars, trucks, and even small gas engines on yard products. When your engine sparks like Nikolai Tesla, sitting in a Faraday Cage, while his Tesla coils shoot lightning bolts out you know you have issues there!

Your coils should-not be acting like van de graff generators.

BTW, the VMax coils are well-known to develop splits over-time, but in many cases, those are cosmetic defects, and do-not affect the proper function of the primary coils. Using that 'in-the-dark' method of checking for current bleed-off if you discover coil cracks should show you if you need to-replace your ignition coils, with OEM, DYNA, or COP's.


Some corroboration, pros/cons, about Mitsubishi vs. Denso COP's:
https://www.vmaxforum.net/threads/cop-problems.16368/#post-181507

 

[Piercems3]

I'd love to provide an update saying I fixed the problem but ever since i posted about it I could not duplicate it. I have ridden about 500 miles now in 80-90 degree weather and it has ran crisp and strong. Sometimes it is easier when things fail big to diagnose!

Edit: I have Denso 129700-4400 COPS.

 

[Vinmax]

Vinnie, not-sayin' you didn't fix your problem, but the majority of posts on here in the past about COPs' problems are that the Denso ones are better on our bikes than the Mitsubishi's. Maybe Gannon wants to weigh-in on this. He developed and sold the Splicker Designs COP kits for our bikes, he's since sold his inventory to Sean Morley.

That's a good piece of advice about giving a good inspection to your bike's (or any internal-combustion system's) primary ignition in the dark. I've used that method many times over the years on bikes, cars, trucks, and even small gas engines on yard products. When your engine sparks like Nikolai Tesla, sitting in a Faraday Cage, while his Tesla coils shoot lightning bolts out you know you have issues there!

Your coils should-not be acting like van de graff generators.

BTW, the VMax coils are well-known to develop splits over-time, but in many cases, those are cosmetic defects, and do-not affect the proper function of the primary coils. Using that 'in-the-dark' method of checking for current bleed-off if you discover coil cracks should show you if you need to-replace your ignition coils, with OEM, DYNA, or COP's.
View attachment 72064

Some corroboration, pros/cons, about Mitsubishi vs. Denso COP's:
COP Problems

You are so right! I stated that incorrectly and edited my original posting. I had Mitsubishi COPS FIRST, and then switch to the Denso's and have been with them since.
In fact, years ago, I detailed my issue with blowing up the Dyna and did mention that I switched to the Denso's. Thanks for clearing that up. Here's the old post from years ago (5th post down. I even included a picture of the blown up Dyna);
Mitsubishi cops with dyna 3000