83 RZ350 hits a wall at 7000rpm

[Warnzie]

So have finished restoring my RZ350K - new carb rebuild kits installed, cleaned, new rubber boots ect ect. 98% all new in terms of fuel and intake.

She starts first kick, choke or no choke, idles fine, runs fine, but seems to fall flat on its face when she hits 6500 -7000rpm. Doesn’t matter how much more you twist the throttle she seems to barely want to climb in revs. At times, it will for a second or two start to open up but bogs again. I had this problem on my rg250 which turned out to be a rubber bung on the airbox had to be pulled off and presto sorted it straight away so was a case of not getting enough air.

Have checked PV function and all working ok. Air filter is new so decided to take the snorkel out to see if it made much of a difference but alas nothing. Am I running lean or is it a case I’m not getting enough air? All stock jetting and carb settings and bike is totally Stock. Look forward to your thoughts

[Redline Junkie]

Did you set the float heights when you rebuilt the carbs? What the main jet sizes?

[Warnzie]

Yes set the mm to 21mm as per manual and yes all new jets / rebuilt kit from yambits which are 240mains from memory

[tacky1]

Yambits jet kit might be a problem. I hope they are better than any of their seals.
Did you use crank seals from yambits
Have you done a leak down test.

[silverstrom]

Take the top off of the airbox and go for a ride. If it was rich you'll know right away.

I have zero faith in any generic carb kit from Yambits.

I built an RZ350engine for a guy several years ago and he insisted on Yambits crank seals so he could save a few dollars. I protested, but he insisted and it was his engine, so I did what he asked. 100 km later the crank seals were leaking and it wouldn't hold more than 3 PSI.

As a rule here I no longer use any Yambits part unless it is the last option available. Been down that road already. Crap crank seals, cheap bearings, etc.

[MK]

The rpm where the effect occurs is suspicious.
It matches the effect of having mistaken the two bowden cables of the PV motor and ending up in a PV that closes at high rpm instead of opening.

Unmount the left PV cover.
When starting the engine and revving it up from idle to above 6k: Does the pulley turn counter-clockwise (opening) or clockwise (closing)?

Plus I agree on that cheap Yambits stuff being crap, especially everything that is carb related.

[Warnzie]

MK, the PV on ignition when doing its self clean turns “clockwise” then back to align itself with the hole, and when running / reved to 7k rotates counter clockwise to align / move towards the hole

[Warnzie]

Have triple checked the cables and they both match up to the no’s on the pulley and also the servo motor

[Warnzie]

The yambits carb kit was a key star kit from memory which I thought were a quality Japanese kit I’m really thinking of the simple stuff first, would blocked headers cause this? The stock pipes I have are, let’s say oiled up quite well and wonder if there is carbon build up in them? I did pull the baffles out and all the holes in the perforated section were blocked, scrubbed them up and all holes free now, took for a ride and was better, would for a few seconds rev out but would then hold back...I wonder

[MK]

In that case you properly checked the PV and it's not to be blamed.

BTW: The company is named Keyster and I'd not use anything in my carbs that does not carry the genuine Mikuni stamp.

Regarding the pipes: are we talking about aftermarket or stock ones?
The aftermarket stuff can't really get clogged as there's nothing inside the belly. So getting the cans off and having a look inside should unveil excessive carbon in the stinger. Removing it from there is rather easy.
Stock pipes have lots of parts inside that can build up carbon. I once had a PW50 that had low performance and even stopped working after a while - after a proper heat treatment the performance was back again.

But to create an effect THAT big, you need A LOT of carbon or loose pieces that block whole passages.
I'd put most of my eggs in the basket with the "carb repair kit" label.

[Warnzie]

Anyone got experience with standard bike with standard jetting here in Australia?

I think as I replaced the air filter with a new one, it might be just me but it does seem a little more denser than the sad excuse of foam the previous owner had hacked up to fit, I might try running without the filter and see what she does...at least this will give me a answer of either yep way to rich or yep still to lean

[Warnzie]

OK so some success

I took for a ride without my baffles in the chance that maybe they were restricting things a bit...still no go...ran cleanly and fine under 6500 rpm with a hint of the usual RD/RZ flats-or between 4-5k rpm...pulled over to the side of the road and thought i would try running it with the fuel tap set to "PRIME"...well that instantly sorted it

Now it revs right through the rev range without any stumbling. Now, its obviously not getting enough fuel at higher revs due to vacuum im guessing when on the "ON" position. Fuel cock vent tube installed to right hand carb, fuel t piece clear, slightt hairline crack on of the fuel hoses up top but not leaking (have new fuel line coming) What would this indicate going forward?

[85RZwade]

Apply vacuum to the fuel valve in the on and reserve positions; it should flow fuel then. In the prime position, it should flow (and it sounds like it does) regardless of vacuum. If you can't get flow by sucking on that vacuum hose, you've got a problem in the fuel valve. Rebuild, replace, modify as preferred.

[silverstrom]

OK so some success

I Fuel cock vent tube installed to right hand carb?

Not sure what you mean with fuel cock vent connected to right hand carb.The hose that goes to the right carb is the vacuum supply to operate the petcock diaphragm. That vacuum fitting is connected to the rear cover of the petcock.

You can rebuild the petcock, but my experience has shown that a new petcock tends to be a better long term investment. You can also use a petcock that needs no vacuum.

[Warnzie]

Ok so a update - I took her out for another 15min test ride and left the fuel position to “prime” and it fell flat again at 6.5-7k rpm...scratched my head for a bit and remembered I tested
the on and prime position the day before without the exhaust baffles. Running on the “on” position without the baffles still gave me the fall on its face symptom, but on “prime” without the baffles it ran fine. So I took the baffles out again and ran it on “prime” and she pulled like a skid that no laundry detergent would get get off

So it would seem without my baffles it runs perfectly but with them in it doesn’t - along with my fuel tap vacuum problem. I have cleaned them and left them in kerosene overnight (whilst doing my restoration) so a little...
”baffled” see what I just did there!!!