jeepwm69 7,783 Posted December 2, 2020 As many carbs as I've cleaned, I've never understood the function of the valve on the side of the carb. Should I be able to blow through one of these things when it's off the carb? A buddy brought me a 350 Rancher carb that needed work. It honestly looked pretty good when I tore it down, so I'm wondering if this might have something to do with it. https://www.partzilla.com/product/honda/16510-MBW-601?ref=7b7898ee4ecac4f54d262bab4ec9f114c0c82399 Share this post Link to post Share on other sites
retro 4,074 Posted December 2, 2020 The cutoff valve's function is to enable & disable the idle fuel enrichment circuit depending on the signal strength of intake manifold vacuum. While the throttle is closed (motor is idling or decelerating) there is high intake manifold vacuum present in the hose port which pulls on the diaphragm and opens the cutoff valve, which turns on the idle enrichment circuit. While the throttle is open under acceleration or while running along at cruising speeds the intake manifold vacuum signal goes low, which releases the diaphragm to return under light spring pressure to the closed cutoff valve position, which disables the idle circuit entirely. The valve diaphragm must be able to hold a vacuum when a vacuum is applied to the hose port. If you can suck air through the valve by drawing on the hose port then the cutoff valve diaphragm is leaking air past it, the valve is open when it should be closed.... so it needs to be replaced. You can tell if you have a failed cutoff valve if the motor will not idle, or is idling extremely lean and the pilot screw adjustment is unresponsive... if you hear popping/afterburning out of the exhaust during deceleration (lean condition) then its possible that your cutoff valve has failed too. A vacuum check confirms this. 2 Share this post Link to post Share on other sites
jeepwm69 7,783 Posted December 2, 2020 Thanks, so not being able to blow through it is a good thing.... I've soaked it, blown out all the passages, and cleaned it up. Just wanted to make sure I got everything before I give it back to him. 1 Share this post Link to post Share on other sites
retro 4,074 Posted December 2, 2020 Yup the diaphragm must not leak air pressure. The valve itself gotta work too though, so check that air does not flow through the ports when vacuum is applied and that air flows through them when vacuum is not applied. You can apply enough vacuum to make the valve work by sucking on the port using your mouth & lungs. I check them all that way myself, its the quickest & easiest way. 🙂 EDIT: I squirt WD-40 through the valve ports while sucking on the diaphragm port to test that the valve is opening & closing completely. WD-40 just stings a bit when ya get it in your eyes.... and doesn't poison ya all that bad if you shoot it all over your lips. Haha! 1 Share this post Link to post Share on other sites
jeepwm69 7,783 Posted December 2, 2020 I've sipped on WD40 my whole life. Keeps me regular! (for stupid people who have no common sense, that is a joke) 5 Share this post Link to post Share on other sites
TBRider 1,411 Posted December 2, 2020 1 hour ago, jeepwm69 said: I've sipped on WD40 my whole life. Keeps me regular! (for stupid people who have no common sense, that is a joke) I knew a pipeliner who would rub wd on his elbows and wrists, said it helped with arthritis...🤔 1 2 Share this post Link to post Share on other sites
_Wilson_™ 6,603 Posted December 3, 2020 i bet AEROKROIL would work better, but you'd smell like Lysol..... 1 Share this post Link to post Share on other sites
shadetree 6,036 Posted December 3, 2020 17 hours ago, TBRider said: I knew a pipeliner who would rub wd on his elbows and wrists, said it helped with arthritis...🤔 and it does work !!. 1 Share this post Link to post Share on other sites
87Iroc 344 Posted December 11, 2020 (edited) On 12/2/2020 at 3:18 PM, retro said: You can tell if you have a failed cutoff valve if the motor will not idle, or is idling extremely lean and the pilot screw adjustment is unresponsive... if you hear popping/afterburning out of the exhaust during deceleration (lean condition) then its possible that your cutoff valve has failed too. A vacuum check confirms this. Hrmmmm, I have this problem on mine. On decel's it popps. I'll pull it apart sometime. I loosely checked it and it at rebuild and it seemed OK(no airflow in my hands)...but I will double check. Things ain't cheap! Edited December 11, 2020 by 87Iroc 1 Share this post Link to post Share on other sites
Misterclean 420 Posted December 11, 2020 (edited) I thought it just added air on deceleration or at least that's what is does on street bikes. It stopped the popping from unburned fuel exploding in the exhaust system. Edited December 11, 2020 by Misterclean 1 Share this post Link to post Share on other sites