Vacuum Leak Test - How Different Should Revs Be ?

Hi, this one's probably a dumb question. I'm finally getting back onto the project of trying to get the Z650 on the road, after work totally soaked upt most of the last few months.

If I'm checking for air leaks around the intake by squirting a bit of Easy Start around the inlet rubbers, how much should I expect the revs to jump by if there is a leak ?

The bike is a Z650B1 with the kind of Mikuni carbs with an air screw at the back. It's got pods rather than an airbox, but they're decent ones that don't block the pilot jet airway. I've rebuilt and cleaned the carbs more times than I care to think about, so I'm reasonably sure they're clean; they've been bench-synched, and had a quick blanace on the vacuum guages but they're by no means fully dialed in yet.

The symptoms I'm getting are those of a vacuum leak, based on what people tell me. The engine starts easily and ticks over well, but if I give the throttle a sharp blip it fluffs and dies, and if I try to ride the bike it dies when I open the throttle. It's been doing this no matter what the state of the carbs or how recently they were cleaned. I've recently replaced the inlet rubbers, vacuum caps and banjo clips, just to be sure.

I'm struggling to work out if any change in revs is thanks to Easy Start being pulled into the engine, or if it's just within the normal variability in the tickover of a set of carbs that aren't fully dialled in yet. Any change isn't small enough that it doesn't show on the rev counter, so I'm listening to the sound of the engine and going bonkers trying to work out if any minor changes in tickover sped I'm hearing are just me, or there really is a leak.

If it's not that, I'll have to look yet again at timing, and wiring.... I thought I'd already sorted all that out ! The one thing I couldn't change were the springs on the timing advancer because the crank end bolt appears to be seized in. They advancer definitely works (I can see it opening up), but I wonder if the springs are so weakened that they open up too easily. Any other suggestions for what might be causing the issue gratefully appreciated !

 

This method isn’t full proof and any change in revs will depend what you spray and how big the leak is, normally an increase in revs is “audible” rather than a jump on the tachometer.
If you are running pods you will likely need “air correctors” from zed1015 and possibly bigger main jets (I think your problem is the pods if the idle is steady)

Thanks for your reply !

The air correctors are definitely on the menu, but only once the actual problem is nailed down and fixed. My understanding of the air correctors is that they help sort out the flat spots that come with pods, but probably won't actually sort out deeper issues.

The carbs were up-jetted slightly to compensate fothe pods, and at the time when I was regularly using the bike, the setup worked fine. That's what is telling me that there's something else going on rather than just final fine-tuning.

I think the only answer is to go right back to basics and just work through re-checking and re-doing everything until I find out what the problem actually is.

Have you set the “float heights” 
Have you tried a independent fuel supply 

Yes, I've set the float heights using the clear tube method.

But it occurs to me that when the airbox is fitted, the carbs sit so the mating surface is level, but without the airbox the carbs naturally want to sit with the back end slightly higher. It's a very slight difference, maybe 1mm or so between the fuel level at the front and rear of the carbs, so I took the level at the centre (so effectively inline with where the jets are inside the carbs).

I've used an external fuel tank whenever I synch the carbs, and I used it when testing for air leaks just in case the Easy Start took the paint of the tank, plus it made it easier to get at everything and see what was going on. The fuel filter I've fitted is clear and I can see fuel flowing through it. Plus I drain the float bowls after each use to prevent varnish, so I'd have spotted if one was totally dry. I keep meaning to get something fabricated so I can drain the bowls with just one tap rather than having to unscrew all four drain plugs each time !

 

Did you remove the pilot jets and hold them up to a light, to verify the orifice is open?

When cleaning the carbs, did you make sure the path is clear from the choke pickup tube to the hole on the bottom of the choke plunger?  

Is the choke pickup tube passage inside the float bowl clear?  

What jets are you running?  How long are you allowing the engine to warm up before applying throttle?  

Did you clean and set the points for clearance?  Make sure the centrifugal spark advance moves freely.

Are the valve clearances in spec?  Is your battery fully charged?  

Thanks for your reply !

I think I've covered all those things off; when I cleaned the carbs, I put them through my ultrasonic cleaner (one at a time, to avoid mixing the parts up, and with all rubber bits removed), then sprayed carb cleaner through each passageway to check it emerged at the other end, then blew each one out with compressed air. I included the choke pickups, and took the pilots out to clean them, checking I could see through them and that carb cleaner would blow through and also come out of the side holes if I covered the end.

The jets are 102.5 mains and 15 pilots, which is what worked when the bike was last running reliably. It's a slight up-jet to compensate for K&N air filter pods. I experimented with 17.5 pilots and it seemed to be too rich.

In terms of warm-up time, the Z has always been a cold-blooded beast ! I typically find it takes 5-10 minutes on gradually reducing choke before it's happy without choke. It hasn't got points any more, it runs Dyna ignition and coils, and most of that setup has been recently replaced, all connectors and some of the wiring also replaced. The only bit I haven't changed (yet...) are the HT leads, but the packet arrived with those a couple of days ago, so fitting those will be the next job.

The timing advance definitely works, I can see it opening up when the engine speed increases, but until I can find a way to get the seized crank end bolt out without doing damage I can't measure the springs to find out if they are too weak making it open up too fast (although that's a real long shot when it comes to causes of the problem).

The battery is fairly recent and I've checked it's fully charged. I give it a little trickle charge periodically if I haven't started the engine. I checked the valve clearances very recently, and they are all in spec. One is at the upper limit, but none are too tight. It's hard to check compression because the starter motor doesn't work. Testing it using the kickstarter isn't quite such a good method, but the result seems reasonable - a little low, but all four cylinders are about the same and are within the spec given in the manual.

But all of these are things I'm going to work through and re-check, just in case I missed something first time around ! Right now, I'm thinking the most likely causes are either something electrical meaning the spark isn't strong enough under load, or something really wrong with the carbs that I haven't found yet.

Has the bike been standing for any length of time? A blocked exhaust will give similar symptoms (critters build nests in exhausts)

Tbh I just skimmed your posts but I read you were testing for vacuum leaks using a solvent spray. The way you do this is with a lot of solvent spray; not just a squirt here or there. You deliver a constant stream of aerosol spray for half a minute or so at the suspect area or until you hear the revs change. Also, most solvents will cause engine revs to fall or even cut out the engine, as the engine is already idling using the mixture the carbs are delivering, and you're massively enriching the charge that enters the engine. That said, you've got a Z650, so as it's a 4-cylinder bike it may not cut out but if there's a vacuum leak it will change the engine revs considerably. In the case of commercially available carb cleaner or brake cleaner, the revs will drop. 

Just as an update, I think I found the problem.

First of all, I checked for mice in the exhaust. It's a Motad, so poking a stick and gently twirling it didn't go all the way in. But the exhaust appears to flow freely with no sign of bits of nest or fried mice being blown out. The only squeaking comes from the front brake. I haven't yet been brave enough to pull a 90 degree wheelie and see if anything falls out......

So having ruled out mice, I started through my checklist of possible issues. When I looked at the spark, it was there but a little weak. I changed the HT leads for a new set I had lying around, and was amazed at how much stronger the spark looked. So I'm pretty sure that what was happening was that the old leads were strong enough to carry a tickover spark in neutral, but failed under load.

I also discovered that the clutch was dragging somewhat and needed to be adjusted again.

With both of those things done, the bike now runs at least as far as down the lane and back up the hill again, which is massively better than before. I know I still need to do a vacuum balance on the carbs and fine adjustment on the air screws, but at least I know I'm headed in the right direction now....

Thank you everyone for your ideas and support !