float bowls running dry under high load RZ350

General forum on engines, transmissions, gearing and modifications to each

Moderator: rztom

Post Reply
Message
Author
577nitro
Posts: 5
Joined: Thu Sep 21, 2017 4:02 pm

float bowls running dry under high load RZ350

#1 Post by 577nitro » Fri Sep 22, 2017 1:54 am

info:
stage III engine build.
30mm mikuni's
jetting is perfect, runs great.

problem is under high load, low rpm and a lot of throttle the engine quits...both cylinders at the same time. roll the throttle off and the engines sputters then catches. I thought I had fixed this by rebuilding the petcock....but it happened again the other day when I was bucking high head winds. Hi load, low rpm, and lots of throttle. either the vacuum drops to the point that the petcock valve does not open and the bowls drain, or the gravity feed system can not keep up with the fuel needs. Turning to prime seems to neither help nor worsen.

My gut says the fuel system can't keep up, I've already increased the tube sizes.

Anybody have this issue? Bigger petcock mods? I've also opened the breather up as well.

577nitro-

User avatar
Smoker
Posts: 4777
Joined: Sun Mar 23, 2008 8:08 pm

Re: float bowls running dry under high load RZ350

#2 Post by Smoker » Fri Sep 22, 2017 4:35 am

Fairly common problem.

There are larger aftermarket bowls available for many carbs. Sounds like you need them.

Lots of owners also run an in-line fuel pump.

User avatar
kpke
-----
-----
Posts: 2531
Joined: Sun Nov 23, 2008 2:22 pm
Location: Livermore, CA

Re: float bowls running dry under high load RZ350

#3 Post by kpke » Fri Sep 22, 2017 10:36 am

Can't help much other than to say my thrasher bike is ported (stage???), vm30s, pods, toomeys, and it runs fantastic. It gets ridden pretty hard every weekend. I run on prime as there is no vacuum nipple on either carb. The vacuum on the (stock) petcock is capped off. Probably pushing 7 -8K miles with this top end.

Yours sure does sound like fueling. No float height adjustment on these carbs either. But sticking floats??? Or bad floats??? Hard to imagine both would go bad at the same time though.

Good luck.

Bare
Posts: 1196
Joined: Sun Jan 29, 2006 2:18 am
Location: vancouver

Re: float bowls running dry under high load RZ350

#4 Post by Bare » Fri Sep 22, 2017 1:25 pm

Your 'modded' motor would be bottlenecked by a Healthy Fuel tap.
Yours is Not. Most are sketchy unless rebuilt.
Suggest you fit a moderne, even higher capacity NEW one

User avatar
JanBros
Posts: 197
Joined: Tue Jan 20, 2015 8:18 pm
Location: The Land of Francorchamps

Re: float bowls running dry under high load RZ350

#5 Post by JanBros » Fri Sep 22, 2017 8:29 pm

how can a carb run out of fuel on low rpm and not on high rpm when a lot more fuel is consumed at high rpm ? no better flowing tap or bigger bowl can cure TS 's problem.
if it runs, you can race it !

577nitro
Posts: 5
Joined: Thu Sep 21, 2017 4:02 pm

Re: float bowls running dry under high load RZ350

#6 Post by 577nitro » Sat Sep 23, 2017 6:32 am

JanBros wrote:how can a carb run out of fuel on low rpm and not on high rpm when a lot more fuel is consumed at high rpm ? no better flowing tap or bigger bowl can cure TS 's problem.
It does, don't why, but I think it all has to do with intake velocity/pressure. If I back off the throttle, it fills the bowls and it picks back up. BUT, I've run WFO for extended periods without running into this. It only happens with this particular condition. rpm ~3500 to 6000 some form of heavy load, going up a steep hill or bucking a head wind. So the throttle is open 1/2 or more. The throttle position means the carbs are running on needle and mains mostly. With slow intake velocity but I don't know if there is high vacuum or low. If its low, then the petcock valve may not have enough vacuum to open and the fuel is "turn off". Or, it's high vacuum and just sucking up all the fuel faster than the tank can supply.

When I say the engine is a stage III, its the stage III mod level from Spec II, full race port, milled head, modded reed block, Boysen reeds or TDR(tried both), 30mm carbs, FMF boost, reed block spacers, pipes, K&N Y filter intake, welded crank.

577nitro
Posts: 5
Joined: Thu Sep 21, 2017 4:02 pm

Re: float bowls running dry under high load RZ350

#7 Post by 577nitro » Sat Sep 23, 2017 6:37 am

Bare wrote:Your 'modded' motor would be bottlenecked by a Healthy Fuel tap.
Yours is Not. Most are sketchy unless rebuilt.
Suggest you fit a moderne, even higher capacity NEW one
e

I fully stripped, cleaned and rebuilt the petcock with all new parts...but I'm thinking your right...just does not flow enough fuel.

User avatar
JanBros
Posts: 197
Joined: Tue Jan 20, 2015 8:18 pm
Location: The Land of Francorchamps

Re: float bowls running dry under high load RZ350

#8 Post by JanBros » Sat Sep 23, 2017 7:04 am

577nitro wrote: It does, don't why,
as soon as it happens, stop the engine and drain the bowls. you'll know exactly if it's that or not.

at 2/3rd of rpm of max torque, expansion pipes do more wrong than good. take any powergraph from any 2-stroke with pipes and you'll see a dip at that point. I'm quite sure that dip on your bike will be somewhere arround the 3500-6000 region. the more you feed the engine/pipe at that point the more it'll do wrong. partialy cures are playing with your exhaust valve, more ignition advance for those rpm's or slightly richer jetting.
if it runs, you can race it !

RD400Fiend
Posts: 30
Joined: Sat Mar 05, 2016 2:34 am

Re: float bowls running dry under high load RZ350

#9 Post by RD400Fiend » Sat Sep 30, 2017 2:30 pm

Having similar issue with my Daytona. Only dying on left side though. Gonna pull plugs when it happens to make sure not electrical issue.

paul baritakis
Posts: 922
Joined: Thu Jul 23, 2009 10:41 am
Location: australia
Contact:

Re: float bowls running dry under high load RZ350

#10 Post by paul baritakis » Tue Oct 03, 2017 8:10 am

Fit a pingel dual outlet petcock,,and make sure your fuel cap is vented
Your only alive at redline. 1983 RZ cheetah Trinity485 Tss 5oo gp power valved custom.

Post Reply