Missing Powerjet Tubes - Does It Matter?

[Crass]

So, I was reading the very interesting article on the Slingshot powerjet circuit and remembered seeing the rubber tube connecting the float bowl to the top of the carb on my 750L - easily visible on carb #1.  Then yesterday I'm doing a deep clean of the bike as part of winter storage and whilst poking about in the bank of carbs I notice that the plastic elbow at the float bowl on carb #2 has the vertical part broken off and is missing the rubber tube.  But wait - elbows all present and correct on carbs #3 and 4 but no tubes either.  Hmm!

I've had the bike almost two years now and did a pretty comprehensive stripdown resto on it.  I know little to fuck all of the bike's previous history.  What I do know is that it came with a K&N filter in the stock airbox and when I'd got it back on the road I had it dynoed to check the fuelling was set up ok - it was pretty much, just needed leaning a touch on the pilot circuit with the pilot fuel screw.  Bike starts and runs fine all the way to the limiter, no running problems at all.

So my question is, with three of the pipes missing what, if any, difference would this make to the running of the bike?  My understanding is that it makes no odds at the float bowl end as the jets there are blanked off but at the top of the carb there should now be a outlet in the bellmouth open to atmosphere, right?  Would this alter running noticeably, or is it just a case that it will be admitting a small jet of unfiltered air into the carb?

It's a bit odd that one elbow is bust and three tubes are missing.  Why would you piss about with these?  This may suggest that someone has deliberately removed them - and plugged the bellmouth outlet? - I dunno.  Like I say, I've no idea what, if any, work was done to the bike in connection with fitting the K&N (and it has an aftermarket exhaust) but it may have had some re-jetting done and the powerjet bits taken away.  All I know is however they are set up the fuelling is correct.  But if unplugged would give noticeable running issues then I guess that means they must be plugged.  But if they are not plugged, should I?  Not a problem to connect up a short length of plugged tube to the top of the carb if necessary.

Thoughts?

Edited November 22, 2015 by Crass
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[Phill106]

I know fuck all about these powerjet systems but if it's running ok, riding ok and your fueling is ok, then I'd just leave it. 

 

[Captain Chaos]

yes, if it aint broke don't fix it.

BUT if it keeps you awake at night, make sure that the bellmouth-ends are really plugged. Just connect a piece of vacuum pipe to them and try to blow/suck a bit. I would also think that if they are open it would run like shit.

[Crass]

yes, if it aint broke don't fix it.

 

Never thought I'd hear that on OSS

Good idea about trying to give it a blowjob .  Now why didn't I think of that?  Will try on #1 with the pipe that already exists - you would think that if that one was plugged then it would follow that ones without pipes deffo would be.

[Captain Chaos]

Never thought I'd hear that on OSS

 

you were not going to install an 1100 engine anyway so it's not worth messing about with it.

[Crass]

I've just tried blowing down the tube on #1 and it isn't plugged at the bellmouth end, so I doubt the others are either.  I can see with a mirror that the metal pipes at the top of carbs 2-4 aren't plugged on the outside.  I'll seal them up anyway, it's an easy job.  What difference, if any, it will make to the carburation remains to be seen but the fact is that there isn't supposed to be a jet of air allowed to be pulled into the bellmouth via this route so I'll stop it and see.

[Crass]

Well that's that done and I'll report back after the winter layup whether it has made any difference blocking them up.  Bloody fiddly job as there's no way you can get pipes onto 2-4 with the carbs on the bike, you can't get in from either the top or bottom to fit them.  So I bent a piece of wire at the tip, loaded it with silicone jointing compound and went in from the top, poking it down betwen the carb bodies to get under the open end of the brass pipes, then brought it up to fill the open ends.

My way of thinking is that if air is getting in through these pipes it will be akin to having a slightly leaky airbox.  Which woiuldn't be a good thing.

If it makes no difference it has cost me nothing more than a couple of hours of time.

[Crass]

Right, reporting back on this.  No, it's made absolutely no difference whatsoever, bike is just as before.  So, it would appear that if the tubes are disconnected / missing and not plugged you can just forget about it.

Edited April 18, 2016 by Crass

[Jelly]

It only does matter in really high revs. Also refer to the article on the frontpage: http://oldskoolsuzuki.info/archives/531

[Crass]

Yeah, I've read the article, very good it is too.  But that only applies in cases where the powerjets are not blanked off at the floatbowl, which in UK spec bikes they were.  So it doesn't matter at any revs if the jets are blanked.  What I'm getting at is that if the tubes are missing, so that there's four unplugged holes on the airbox side of the carb mouths, it makes no difference whether they are plugged up or not.  Well, maybe it's better they are plugged to keep anything unfiltered from getting into the carbs, but performance-wise it makes no noticeabble difference.