Gremlin - power struggle at around 4.5K and up

[BA80]

I had that issue a while back and it turned out to be a exhaust restriction problem. It was fine sitting still but in gear under power it sucked.


http://www.xs11.com/forum/showthread...ght=power+loss

http://www.xs11.com/forum/showthread...usting+problem

[thumperjsa]

And now, the rest of the story

So we (Biker Phil has been assisting) pulled the carbs this weekend to check things. So I decided to do and try some things and hers my details. A long story, but I can't tell it too shortly.

First as to TopCats Dyna coil question, yes the bypass resistor was bypassed. I didn't use a timing light to check it. I do have spark to all cylinders and wires are to the correct cylinders. Reading back through the thread, TopCat was closest to the resolution. Not exact, but close.

The main jets were verified to be one size oversized to 140. Also, the needle was set in position 4 so they were moved back to the base position of three (in the middle) because of the thought it should be in the middle. That would also put the needle deeper in the 4th position and thinking is, that it was a lean condition issue.

Checking the floats, it was pretty dead on. Did a little adjustment but maybe off a millimeter on one carb so don't think that would be it.

Checked to make sure there were no issues with the rubber diaphragm and they all looked OK. Made sure mains were also clean and float bowls didn't have any debris. So then got it all back together and reinstalled the carbs. Got a little fuel into the air box. Forgot to tap on the bowls before I turned it to prime. Pulled the bottom of the air box off, cleaned and dried the filter and bottom cover. By the way, I did check the paper air filter previously and it seemed to be OK. After blowing some air to dry the fuel up, put the bottom cover back on and this time, tapped the bowls as they filled. No more drips and bike starts right up and idles smoothly.

Took it for a ride with Phil riding behind me. Tried hard take offs and it wasn't better. Phil didn't notice anything look or smell indicating too rich. And as I mentioned before, plugs looked OK. However I didn't yet get to run it up to as high I could get it in first and chop it to check plugs as Diver Ray suggested. And now the problem actually became a bit worse. Now it was acting up at a slightly lower RPM and sometimes even at 2500 RPM it was acting up.

On the ride I figured I would check if perhaps it was still lean and opened up the enrichment circuit for the test. And it got much worse when opening it on click, much worse at two. So now I'm thinking I'm wrong and fighting a to rich problem.

One other thought Phil brought up. Let's eliminate it's a carb issue by taking the carbs off his bike and replace it with mine. They are exact same and that while being a pain in the ass to do, would definitely remove it being a carb issue or not.

So I'm contemplating this on our ride and trying to think, if it's a rich problem, what's the easiest way to eliminate this issue because I really don't want to do a carb swap unless it's really the next test to see what the issue is.

When we get back to Phil's I think why it seems all of a sudden this was an issue and I remembered the fuel tee swap out. I don't think new fuel tees would be it and nothing fuel line wise is looking to be an issue. Fuel seems to flow nicely through the lines and filters fine. Verified this with filling the bowls after the carb reinstall. Then I think, maybe before the fuel tee swap out I had a bad habit of not tightening the air box bottom. I haven't been riding too often and that morning I gave it a shot of starter fluid. Maybe I had it open a bit and after the fuel tee swap out I know the air box was sealed well.

So my next thought, take off the bottom of the air box and filter and ride it down the street. With no air restriction to the carbs, that should lean the mixture a bit, right? So I fire it up and take it to the more major street I can get it to higher than 45 in first gear see if RPM will go higher than 4500. Well, I open it up not expecting too much because I've been at it for weeks and nothing so far minor or major has helped. Open the throttle and within a few seconds, I'm at 7k with barely a sputter. I go in the other direction just to make sure it isn't a fluke, and even better on the second run.

Now I'm about kicking myself that I didn't think of this weeks earlier. I borrowed a K&N air box filter from Phil and put the air box back together and take I think out on the street again. While not 100% efficiency I had with an open air box, it was still pretty close in opening the throttle and achieving the higher RPM. So all this work for what appears to be an issue of what looked like a good, and barely a year old paper air filter I installed must not have been allowing enough air to have a good mixture making the engine run too rich when I the throttle was opened.

I probably should have noticed a drop in engine efficiency when first installing the new filter, but didn't. Maybe it wasn't as restrictive when first purchased and while it never "looked dirty" or blocked, it apparently was the problem all along.

So there you have, the rest of the story. A new K&N air filter has been purchased and on its way. The eBay seller is on Miami so I should have it this week since I'm only about 25 miles north of there.

Anyway, thanks to all who chimed in. And especially Phil through his persistence and patience of working with me on the install of the new coils and pulling apart the carbs do check it all. Sometimes the answer is the simplest thing and easily bypassed because it "looked OK" meaning the cheapo paper air filter causing the apparent rich condition at higher RPMs and burning off any evidence of it once revving at lower RPMs.

I didn't get a harrumph from that guy.
-Governor Lepetomane

[BA80]

Once again I'll say I fought a similar problem with mine a couple years ago and it turned out to be exhaust. Much to my surprise.

Is your exhaust system stock or aftermarket?

[thumperjsa]

Aftermarket Mac 4-1

[BA80]

Here's a little video of how mine was. At the beginning I take off hard and it hits the stumble at about 4K but pushes through it in lower gear but at about 0:23 throttle it in high gear and it just lays down. You can hear it.

https://youtu.be/BLbZXaFoWlU

[BA80]

Mine was a MAC 4 into 2 with turnouts. I pulled the mufflers off and almost spun the bike out from under me. The restriction of those MAC baffles killed the power.

[thumperjsa]

I've heard of some people removing the baffles. But you can introduce all kinds of no issues if you don't have the back pressure as well. I've been running this exhaust pretty much since I got this bike around 14 years ago. Never had a problem with them but then again, I never really ran without this exhaust as I got it shortly after I bought the bike because the PO exhaust was in bad condition.

What keyed me in is setting the needle at the more rich made it worse. Then adding a little more fuel with the enrichment circuit made it real worse. But as soon as I removed the air intake restriction, it was a difference of night and day. And now with the new coils and wires and the K&N filter, it's time to hold on when I hit the throttle hard.

Probably would have been a good idea to take a video like you did when the restriction issue happened. I have some videos of rapid acceleration before the issue happened. Now I'm contemplating the stock airbox versus using some pod filters. If I'm still running a little rich, it may be nicer to go to pods. Or maybe do it a bit easier by drilling holes in the air box.

[BA80]

Yup. When I had my issue I swapped jets and the leaner I went the better it seemed to get until I got so lean it would hardly run.

I swapped jets, carbs, entire pick up coil/centrifugal advance units, coils, air filters (including going from stock airbox to pods), cams and finally someone at the shop I work at suggested removing the mufflers to test it. WOW!!! It cam alive.

I haven't completely removed the muffler system I simply got some MAC flare tip mufflers, exactly the same as the same configuration of the interior of your muffler on the 4 into 1, and built some straight through perforated baffles wrapped in stainless steel wool.

As for the "back pressure" myth, that's exactly what it is. Just do yourself a favor and TEST RIDE with the muffler removed. Not saying I'm positive that is your issue but it's one way to eliminate that possibility.

Here's a little reading on "back pressure".

http://www.xs11.com/forum/showthread...=back+pressure

[thumperjsa]

I did a bit more research after reading your link and now understand what you call the back pressure myth. Seems many people confuse this with proper tuning like thinking that the larger the exhaust, the more horsepower. Thanks for spurring me on to research. Anyway, in my case, the main issue was the intake restriction diagnosed by removing the lower air box.

Doesn't mean it isn't restricted a bit in the exhaust but there really is no way on a bike to test that without removing the muffler. It's not like we have an o2 sensor we can pull and test for it.

[BA80]

Simply a suggestion from experience.

It is what it is. Good luck.

[bikerphil]

Jonathan, That K&N I loaned you may need cleaning, I can't remember the last time it was so it may also be slightly restricted. The reason I say this is I took the K&N out of my Special and put in my Standard and it seems a bit quicker throttle response now. Your new K&N that you ordered should do the trick and fix your breathing issues (hopefully).