Rpms stop at 5k when accelerating under load

[IanDMacDonald]

Hey Gabe- If you'd like, you can do some trouble-shooting one step at a time with some of my bike. Carbs first? My Mac exhaust is sitting unused in my girlfriend's trunk at the moment. Exhaust second? The third option would be my airbox? Fourth option is all three for you to envy how well my bike runs at the moment, lol.

[Schming]

I do agree though that there might be some sort of vacuum leak happening somewhere because my idle is a bit erratic. I seem to have to fiddle with the idle a lot because either it will be too high after I blip the throttle or after I make a good run it is too low and will stall. I just don't know where it could be coming from since the carb boots are in great condition.

This is a good indication of why you need to vacuum synch your carbs.

[D0wn5h1ft]

I tried taking the pods off an running it and putting the slip ons back on and all I got was that coughing out of the back of the carbs from before when I ran without the pods on.

With the open pipes and stock jetting it is actually running about 90%. And I assume the remaining issues are from non-synced carbs and possibly a small vacuum leak. I guess I just have to go out and find an exhaust that doesn't destroy my performance, any suggestions? I think Ian still has a set of dual mac slip ons (that aren't broken like mine) he might be willing to part with.

Ian, I might take you up on that offer since you have a set of vacuum gauges too right?

[D0wn5h1ft]

Also, I've been toying around with the idea in my head all day today but BikerPhil got me thinking. I think the issue here is coming from the pod filters. With the dual mac slip ons on the bike it isn't pulling enough air through the carbs to match the amount of fuel being introduced by the carbs at WOT. This is because, as he suggested, the bike cannot exhale properly leading to poor airflow through the carbs. So when I open up the throttle more gas is being introduced into the system but it does not have enough air flow to match it. This shouldn't normally be a problem because the diaphragm operated slides should match the correct fuel mixture based on the vacuum pressure inside of the carb but I think these Pod filters are messing with the pressure differentials on my setup causing the slides to not open up the way they should.

I'm not sure if that reasoning is sound but from what I've read on here there are a number of theories as to why problems like mine happen and it all comes down to ideas surrounding how pod filters affect CV carb operation.

Just a thought, I am still sort of a newbie when it comes to carburetor operation. I think putting some velocity stacks on might clear up a lot of these problems because it will remedy this pressure differential issue.

I'm going to go searching for the thread on how to make velocity stacks but if anyone has the link handy I'd appreciate a point in the right direction.

[WMarshy]

Not trying to rehash your carb cleaning again but didnt read any confirmation, did you remove the emulsion tubes most recently? Have you checked your diaphragms for holes?

Easy way to check without taking them all apart again to perform a light check, is to reach in the bell mouth of the carb with one finger and lift the slide all the way to the top then block the large port on the face of the bell mouth at 12 o'clock (this vents one side of the diaphragm to atmosphere). Let go of the slide and see if it hangs. If it drops quickly then you have a hole in a diaphragm... JAT

This is why Phil and others asked about the pods you are using because a particular kind has a lip on the inside that can partially block that port and the other air holes in the face...

[D0wn5h1ft]

Not trying to rehash your carb cleaning again but didnt read any confirmation, did you remove the emulsion tubes most recently? Have you checked your diaphragms for holes?

Easy way to check without taking them all apart again to perform a light check, is to reach in the bell mouth of the carb with one finger and lift the slide all the way to the top then block the large port on the face of the bell mouth at 12 o'clock (this vents one side of the diaphragm to atmosphere). Let go of the slide and see if it hangs. If it drops quickly then you have a hole in a diaphragm... JAT

This is why Phil and others asked about the pods you are using because a particular kind has a lip on the inside that can partially block that port and the other air holes in the face...

I will go out and check using that method of block off those ports and seeing what it does after I release the slides. The only check I did of the diaphragms was a visual inspection for holes and after the carbs were assembled I pushed up on them and made sure the decent had a good response on each, which they did.

Also, I did drop all of the emulsion tubes out and they were squeaky clean when I took them out but I did clean them anyways just to make sure.

I do have an airbox that came with the bike but I have no idea how to mount it since both XS11s I've owned have had pods. I'll take a look in the manual on how to mount them and give it a shot. If I can put it on and it improves performance then I think we've found the issue. And maybe later I can try to fab some sort of fancy intake manifold like I've seen others do that sort of emulates the operation of the airbox without all of the added plastic and crappy filter.

[IanDMacDonald]

Hey Gabe- Yeah, I'd be willing to give you my whole Mac system for $100. I can't be greedy, and they're just sitting in my gf's trunk anyways. As far as the pods go, I can't speak for the 1100's, but I have K&N's on my 750, and it runs wicked nice! Yes, I would definitely put your stock airbox back on. It mounts via four bolts on-top, and one on each side. It may be more complicated than that, as mine is modified. And yes, you can borrow my carb tune. And yes, I did ride to work this morning @ 34°F

[D0wn5h1ft]

Hey Gabe- Yeah, I'd be willing to give you my whole Mac system for $100. I can't be greedy, and they're just sitting in my gf's trunk anyways. As far as the pods go, I can't speak for the 1100's, but I have K&N's on my 750, and it runs wicked nice! Yes, I would definitely put your stock airbox back on. It mounts via four bolts on-top, and one on each side. It may be more complicated than that, as mine is modified. And yes, you can borrow my carb tune. And yes, I did ride to work this morning @ 34°F

Yeah, I'd like to try your slip ons and see if it behaves any different.

I "built" some velocity stacks today out of a crummy old xs11 airbox and gave a shot with my slip ons back on. Here are some pictures:





Unfortunately, these made it worse with the slip ons on. It made the mixture more rich(?) and instead of hitting a 5k wall it hit a 4.5k rpm wall. And yes I connected that petcock vacuum line back before I took it out for a spin and I set the petcock in the correct 6 o'clock position.

I'm completely out of ideas for this tuning nightmare. It almost seems to defy logic. How could stock jetting WITH open stacks be too rich ON an '80 set of carbs. Doesn't make the slightest bit of sense to me.

[DGXSER]

Just for S&Gs, take it out for a run and bring your spark plug wrench with you. Run it at that wall for a 1/2 mile or so, as far as you feel comfortable running it. Pull the plugs and see what the color looks like. That will confirm if it is truly a rich or lean condition.

[WMarshy]

Just for S&Gs, take it out for a run and bring your spark plug wrench with you. Run it at that wall for a 1/2 mile or so, as far as you feel comfortable running it. Pull the plugs and see what the color looks like. That will confirm if it is truly a rich or lean condition.

Yep, I agree. Post a pic of them too if its not too inconvenient.

[D0wn5h1ft]

Just for S&Gs, take it out for a run and bring your spark plug wrench with you. Run it at that wall for a 1/2 mile or so, as far as you feel comfortable running it. Pull the plugs and see what the color looks like. That will confirm if it is truly a rich or lean condition.

I'm not going to hurt anything doing this, will I? The whole bike shakes and chops pretty bad when I hit this point. Just trying to make sure I won't take another one of these great beasts out of this world.

[DGXSER]

If it runs that bad that quick it will not take long to show color. Heck, I know people who rode home on three cylinders cause number four had spit a valve into the exhaust pipe and holed the piston doing it. You should be fine for a 1/2 mile.

[IanDMacDonald]

It's interesting, as there are so many variables that could affect that wall. Is there ANY way possible that it is not electrical???

I'll be down in Fenton this weekend, and all of next week for a new restaurant opening. Hit me up Gabe, and I'll lend you my carbs and extra exhaust system.

[D0wn5h1ft]

Maybe I could have fouled the plugs at some point? That's the only thing electrical I can think of that would be the issue since the coils are brand spanking new dynateks. I removed the resistor and everything since they are 3ohm coils and I re-did all of the spark plug wiring when I moved them out of the '78 just to be sure.

I'll shoot you a text tonight or tomorrow once I know what I'm doing and we'll work something out.

[bikerphil]

That velocity stack with the foam will make it run leaner than with the pod type filters on I would think.