Exhaust coming through the pod filters?

I recently installed pod filters on my KZ440, cleaned out the jets, and adjusted the float needle. To my surprise it actually runs pretty well without any carb adjustments, and idles better than it ever has before. However, when I shut it off, I noticed exhaust coming out of the pod filters. Is this normal, or does this mean there is some sort of back pressure built up, or that there's a valve issue, or something else?

I don't think exhaust should be making its way all the way back through the carbs and out the air filters.

What do you think?

Might have an intake valve not seating all the way. Have you checked your valve clearances lately? Might run a compression or leakdown test to see if everything looks right.

I checked the compression a week or so ago, and it was within 10 psi of the standard 156 psi, which seems pretty good for a 31 year old bike. Isn't there a way to diagnose if low compression is due to a valve clearance issue? Let me know if you have any insight into this.

Del_Herring wrote: Might have an intake valve not seating all the way. Have you checked your valve clearances lately? Might run a compression or leakdown test to see if everything looks right.

Well, I did some research and answered my own question from before about diagnosing using compression tests. I ran the compression test again; my main reason for running it again was that after I reassembled the carbs and turned on the bike, there was so much more fuel in the mixture because of the clean jets, more air because of the POD filters, and more mixture in general because of me forgetting to re-adjust the throttle cable, that the rpms skyrocketed almost to redline right from the get-go.

This was a completely cold start so I was afraid I may have knocked a valve out of sitting properly; however, this was clearly not the case when I ran the test. The test showed 140 PSI in the left cylinder, which was a drop from the upper 140's a week ago (Target is 156 PSI). I added oil after the first test and it brought it back close to 150. The right cylinder showed 120 PSI with no oil, down from the upper 140's a week ago; I added oil and it brought it close to 150 PSI. So I think I nearly blew out the right piston ring with such a hard start.

Do you think the back-exhaust coming out of the airfilters could still be a valve issue even though the valves don't seem to be the source of the low compression?

Maybe I'll make that my Spring project; new piston rings, and maybe check the valves too. For now I need to get it running good enough to drive it to the shed to winterize it.

Diagnosing low compression is typically best done with a leak down test.

Have you synchronized the carbs since you rebuilt them? The 120 is kinda low, and a little far off from the other. Was that done with the engine warm? It's not terribly uncommon for them to be off by more when cold, and that's not such a big deal, though it's beter if they're the same.

Air coming back out of the carbs would be a symptom of an intake valve closing late, or not closing all the way. I'm struggling to think of what else would cause that. Maybe someone more experienced will chime in.

When you shut the engine off, but before the exhaust blows through the carbs, do you notice that the engine fires a couple more times with the key off (as in detonation)? If so, the detonation may occur early enough before the pistons reach TDC to actually make the engine spin backwards for a couple rotations. The engine will not actually be firing during this backward rotation. This backward rotation will suck exhaust gases from the exhaust system and push them out though the carbs. While I understand that this sounds somewhat impossible, it actually can happen. At least I know it can on a 1965 Austin Healey 3000 Bj8.

I'm not familiar with a leak down test, can you explain?

The compression tests were done with the engine cold; not ideal conditions but it can't account for the 120 PSI in the one cylinder, whereas it was in the upper 140's before. The battery was on its last leg, but I don't think that would show low compression; as long as the piston rises, the air should compress, it doesn't matter how fast or slow it compresses.

I ran the bike again, and this time the exhaust came out of the right pod filter, as opposed to the left one last time (I'm certain the bike was facing the same direction).

I'm more inclined to think that the exhaust is a valve issue more so than the engine rotating backwards, because I would think there would be exhaust coming out of both pod filters instead of just one or the other if backwards rotation was the case. Interesting idea though 650ed; I'll have to pay attention and listen for any detonation after the ignition is off.

Austin440 wrote: ...

I'm more inclined to think that the exhaust is a valve issue more so than the engine rotating backwards, because I would think there would be exhaust coming out of both pod filters instead of just one or the other if backwards rotation was the case.

That is an excellent point! Both carbs on my Healey would blow exhaust when it cycled backwards. Ed

Every bike that I have changed over to pod filters (mostly K&N filters) has shown what looks like exhaust coming from the pods when shut down. I think it's just unburned fuel vapors backing out of the intake. If your engine runs good and does not use oil, you should be fine. My Yamaha SR500 did this with a pod filter, and did it alot more when I installed an after market carb. Keep an eye on things, but ride it and have fun.

893cv - thanks, that's what I was hoping to hear from someone. I guess it's possible that it does the same thing with the stock air box, we just don't notice it because it's well hidden and it's a much more constraining filter.

Just for reference. A leakdown test has it's own tool, but you basically pressurize a cylinder, typically to 100psi. And then see how much it's actually holding. I've never actually done one, but have seen them done by others. It'll have two pressure gauges, I believe one shows the pressure in the line (which would be the 100), and the other shows the pressure in the actual cylinder. The ncie thing on a leakdown is you can start listening to figure out where the air is coming out of, because you have a constant supply. Patton has a post on how to do a poor man's leakdown. Basically just stick a compressed air line into the spark plug hole And plug the hole up and listen for the escaping.

The eninge isn't turning during the leakdown, especially not if you're doing the poor man's version.

I have one of the Motion Pro leakdown test setups and I try to test my bike engines every year. A fresh engine will hold 95% or more. Most of the air cooled engines I test don't do too well. If an air cooled engine gets hot even one time, it will start to loose %. My FJ1200 got to 85% before I had to get into it. Leakdown testing was used during WW2 to keep track of Navy aircraft engines. It can give accurate info on hot or cold engines, and it does alow the tester to locate the problem areas. Its a much better test then a simple compression test.