Would Losing The Vacuum Port Plug Cause Cylinder To Not Fire?

[syscrusher]

1993 VR with stock TCI, I switched to Coil Over Plugs (COP). A few days ago I noticed a lot of decel popping on the right muffler. My "Mark's" collector sends the right front and left rear cylinder exhaust to the right muffler. Feeling the exhaust headers I noticed that the left rear one was remaining cool until heat from the right rear one conducted over through their common shield and the where the manifold was joined. I decided to switch the front and rear COP coils on the left so that I could verify whether the problem moved with the coil or stayed where it was. I sat on a short stool while pulling the coils which gave me a view of the vacuum ports (carb sync) and noticed that the end of the port cover for that rear one was laying next to the twinkie leaving the port open on that cylinder and so I replaced it. I did the coil switch as well and now all of the headers get hot pretty quickly after starting the bike.

 

So, no coils were actually found to be bad, my wiring is as it should be, etc. Could having an open vacuum port on a cylinder cause it to not fire? I assume that the slide would not rise in carb for the cylinder that didn't have vacuum but the idle circuit should have been working and heating the pipe. Tell me your theories, or if you really know how this should work, please share the knowledge with me? On my way home last night before fixing this the engine was actually shaking the bike while sitting at a stop light, sort of like a Brand H bike would. I hadn't felt that on this motorbike before yet.

 

A side question, is it a good thing to have the left front exhaust and right rear exhaust routed together through a common muffler and vice versa or not? It is not the way that Mamma Yamahama did it originally and so I have to wonder. It seems like it would be easy to add a crossover pipe between the two rears if it would make things fabulous.

[cowpuc]

1993 VR with stock TCI, I switched to Coil Over Plugs (COP). A few days ago I noticed a lot of decel popping on the right muffler. My "Mark's" collector sends the right front and left rear cylinder exhaust to the right muffler. Feeling the exhaust headers I noticed that the left rear one was remaining cool until heat from the right rear one conducted over through their common shield and the where the manifold was joined. I decided to switch the front and rear COP coils on the left so that I could verify whether the problem moved with the coil or stayed where it was. I sat on a short stool while pulling the coils which gave me a view of the vacuum ports (carb sync) and noticed that the end of the port cover for that rear one was laying next to the twinkie leaving the port open on that cylinder and so I replaced it. I did the coil switch as well and now all of the headers get hot pretty quickly after starting the bike.

 

So, no coils were actually found to be bad, my wiring is as it should be, etc. Could having an open vacuum port on a cylinder cause it to not fire - yep, loss of vacuum that is sucking air thru the venturi of the carb and thru the low speed jet circuit will cause no (or very little) fuel/air mixture movement into the combustion chamber and therefore no boom boom? I assume that the slide would not rise in carb for the cylinder that didn't have vacuum but the idle circuit should have been working and heating the pipe - see above. Tell me your theories, or if you really know how this should work, please share the knowledge with me? On my way home last night before fixing this the engine was actually shaking the bike while sitting at a stop light, sort of like a Brand H bike would - sort of like your scoot had a MAJOR out of sync problem. I hadn't felt that on this motorbike before yet -that is kind of a mystery though because I have ridden lots of miles on these v-4's on 3 cylinders and found them remarkably smooth while running on 3,,, hmmmmm,, wonder if you may have gotten into a vacuum system that screwed up the timing curve for the TCI,,,,, cant wait to hear from someone who actually knows what they are talking about in response to your thread here:crackup:.

 

A side question, is it a good thing to have the left front exhaust and right rear exhaust routed together through a common muffler and vice versa or not? It is not the way that Mamma Yamahama did it originally and so I have to wonder. It seems like it would be easy to add a crossover pipe between the two rears if it would make things fabulous.

My new scoot has one of those Marks collectors on it too Sys - pretty interesting contraption I tell ya!

[syscrusher]

Yeah, I thought I read that Mark originally was providing a performance collector replacement for the VMAX and had a good following with those riders. As I remember the story, VMAX has a different firing order than VR and so a performance pipe on VMAX may no be so great on a Venture. I just wonder if I have this right or if Mark tuned the collectors for Ventures differently?

 

Thanks for your explanations on the vacuum issue. So my popping was from firing on an extra lean mixture? I suppose some damage could have been done running it like that too much.

[Marcarl]

If you had EFI then there might not be a large issue, but carbs depend on vacuum for them to work. Loosing the cap would decrease the available vacuum in the carb and so screw things up. With reduced vacuum there would be reduced variance of pressure on the fuel bowl and so then nothing pushing fuel through the jets into the mixture process.

When you felt it shake, you were probably running on just 2 cylinders.

[yamagrl]

Yeah, I thought I read that Mark originally was providing a performance collector replacement for the VMAX and had a good following with those riders. As I remember the story, VMAX has a different firing order than VR and so a performance pipe on VMAX may no be so great on a Venture. I just wonder if I have this right or if Mark tuned the collectors for Ventures differently?

 

Thanks for your explanations on the vacuum issue. So my popping was from firing on an extra lean mixture? I suppose some damage could have been done running it like that too much.

 

I'm pretty sure the firing order is the same. If the firing order were different then the valve timing would be different also, thus requiring different cams tat would not interchange.

[syscrusher]

I'm pretty sure the firing order is the same. If the firing order were different then the valve timing would be different also, thus requiring different cams tat would not interchange.

 

Thanks for the correction there. I wasn't sure about that when I put that in there. There is a difference with the stock collectors due to our mono shock positioning, length of front pipes, etc. I really don't know where I got the idea that the firing order was different on the VMAX though. Some people restrict their hours for posting on forums to those when they are normally awake, maybe I should consider something like that?

 

:doh:

[syscrusher]

If you had EFI then there might not be a large issue, but carbs depend on vacuum for them to work. Loosing the cap would decrease the available vacuum in the carb and so screw things up. With reduced vacuum there would be reduced variance of pressure on the fuel bowl and so then nothing pushing fuel through the jets into the mixture process.

When you felt it shake, you were probably running on just 2 cylinders.

 

Do you have a technical explanation for why it may have been running on two cylinders?

Do I need to look into more possible problems? All of the headers heat up now at least....