Low vacuum, CR29's

I have a set of CR29's on a GPz550 w/615 kit, cams, porting, pods, and Kerker(street baffle). It doesn't want to run on all four cyls below 3500-4000. It runs rough and then will cut out. On a test ride it will run on less than all four til 5000, then it pulls cleanly right past redline. The revs hang also.

-The carbs are clean (ultrasonic)
-There are no vacuum leaks at the manifolds (wd40 test)
-The plugs are new, and all four cylinders have spark


When hooked to a synch tool, it has very little vacuum compared to a stock motor, and it needs to be held at 4000rpm or more to show any at all. When the revs drop to less than about 4000, the vacuum reading drops to nothing, and then the motor basically stalls.

I know that the carbs are a bit big for a 550, but they were a common mod. I'm not expecting to be able to whack the throttle open from idle, but theres got to be something amiss for it to be this bad.

r u sure u are jetted properly?
just because the carbs were put in an ultrasonic cleaner does not make all passages clear. i would triple check if u haven’t already. mine had to go in the tank twice and then the passages still needed some work.
i dont see how u could drop to 0 vacuum below 4000rpm. at that rpm there must be vac. maybe the gauges are out ?

The carbs came from a complete bike that I bought, and they came with motor that I am using them on now. It had sat, but at some point they must have been jetted pretty well. I didnt verify what jets were in there. I've got two sets of these CR29's, both were bought second hand, and both sets were on big bore 550's. I did notice that the slides had different numbers on the two sets (20 vs 25).

I have a new Carbtune, so it should be reliable and accurate still. It showed between 6' and 10" of vacuum at 4000 rpm, but it really wasn't running smoothly on all four cylinders. The last stock 550 that I did had 24-26 @ 2000 rpm.

Next time I work on it I'll check all the passages and verify/set float level carefully. I also have a spare CDI box and coils that I can swap out to verify the function of those parts.

mark1122 wrote: .....i dont see how u could drop to 0 vacuum below 4000rpm. at that rpm there must be vac. maybe the gauges are out ?

After thinking about it for a while, my theory is that it's either getting too much fuel, or it's pulling unatomized fuel from somewhere that it shouldn't be because of something being clogged/restricted.

The vacuum must be dropping due to the fuel demand being met, and then exceeded, while the slide opening remains constant (at idle), kind of like when you drive a car that has a vacuum guage (or boost gauge), when you open the throttle the vacuum increases initially, but then decreases as the the engine speed catches up to the throttle opening.

I'm going to thoroughly check for restricted passages, and then carefully set the float height.

Cam timing sounds off to me.

Ok,

I checked and cleaned every passage and jet.

I checked the compression (165-170 all the way across),

I cleaned all the grounds

I swapped jets with my seconds set of CR's. 1st set had 60 pilots, 105 mains. Second set had 65 pilots and 162 main. Sudco says 65/125, and that I should install new pilots because the old ones get build-up in them, from sitting, thats impossible to fully remove (making the jet act much smaller). It did run better with the 65's installed, but it still revs high on start-up, won't idle below 3500, and then stalls out if you try to adjust the idle lower.

What size drill could I clean the 65's with? I know that the sizes usually correspond to the jets dimension, but I don't know their system for determining that. I don't mind buying some new jets, but I hate to wait.

Check your cam timing. I had similar situation, no vacuum, engine took air from exhaust side. I timed cams and vacuum was back.

Not sure who you spoke with at SUDCO but here, maybe this will help...

As Larry mentioned, insure timing is correct.

Possibly need to start from scratch thus Bench sync... Just a thought....

OEM CR Jet Sizes when shipped...


CR T-Shoot guide....


Just in case...

When I assembled the bike I carefully applied sealant to the intake manifolds, it didn't seal. They were all leaking, and the sealer was sprayed down the floor of my intake ports.I resealed them, and I'll be ordering new ones soon.

While I had it apart, I cleaned the jets with the proper sized drill bit, bench synched the carbs, and swapped the mains to 120's. The vacuum readings are now where they should be, they are actually higher than a stock motor, and it runs well. I still will have to synch it, and do some jetting, but It's very close.

Thanks for the suggestions.

not sure if you are using the stock 550 manifolds, but mine was leaking...hence the holed piston. Little bastards aren't cheap either!

I actually was using 750 manifolds, the 550 parts weren't big enough for the aftermarket carbs. The last 550 that I put together had leaks at the manifolds, so I was sure to seal these this time, but I used the wrong material.