Hey what's up Guy's, I'm currently working in a 1984 Yamaha Venture. I'm also a member of the Venture forum, and those guys have tried to help but we're all stumped. What's happening is it is running ok until I put my Airbox Cap on, when I do it instantly Revs to around 5500 rpm and holds, without the Carb Butterfly's moving at all. I have completely Cleaned and Synced the Carbs and set Pilot Screws to Factory Spec, Re-seated the Carb Boots, Bypassed YICS System, Disconnected Cruise Control.. The Spark Plugs are NOT indicating a Lean Mixture. Since the Venture Runs Virtually the same V4 the Vmax does I was hoping maybe someone here had experience with my issues.. Thanks
Need Help!
- Thread starter JerrodR
- Start date
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Hey, so without the butterfly's moving(weird) about all I could think of is maybe somewhere your causing an air leak sufficient for 5+k as you work the lid down. Should have asked first if the revs begin as the box lid throws its shadow or not, as in it revs just getting close or after it seals.....and you have a good enough look down into that venture box to be sure the butterflys aren't opening?
I think I'd give the carburetor balance mechanism a good going-over. No stripped screws, things move when the balance screws are adjusted. Adjustment springs have tension and are not coil-bound. Same going-over for the carburetor enrichment mechanisms. Are the pistons all freely-moving fully-open and closed with the lever?
Use starting fluid to check for air leaks airbox to carbs, carbs to VBoost manifold, and VBoost manifold to cyl heads (O-rings). Check for proper slack in the throttle cables.
Use starting fluid to check for air leaks airbox to carbs, carbs to VBoost manifold, and VBoost manifold to cyl heads (O-rings). Check for proper slack in the throttle cables.
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Problem I fixed on my bike was pretty strange: bracket, holding idle speed adjustment bolt was bent, pushing throttles open. I could remove bolt completely but RPM was still very high. Solution was to bend it back.
Another mistake I made was trying to adjust idle by turning synchro bolt
RPM cannot be 5000 if throttles are closed!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Hold on a second! I had same problem on some bike and there was some metal pipe sticking out of a carb, sucking air, I plugged it. Use medical type stethoscope, listen for sucking sound.
Remove air box, file nail thin and check clearance of throttles (bench synchronizing)
Another mistake I made was trying to adjust idle by turning synchro bolt
RPM cannot be 5000 if throttles are closed!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Hold on a second! I had same problem on some bike and there was some metal pipe sticking out of a carb, sucking air, I plugged it. Use medical type stethoscope, listen for sucking sound.
Remove air box, file nail thin and check clearance of throttles (bench synchronizing)
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Carbs have been Cleaned and Synced, with the Pilot Screw set to Factory Spec.. And that's what I thought, but I have the idle Screw backed all the way off and I will hold my Finger on the Throttle Linkage when I Put the Cap on, and the Throttle doesn't budge at all.
Yeah, gone through the Carbs 3 Times now and everything is as it should be. Throttle Cable tension is good as well. Been working on this thing for going on 2 Months now lol.
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With just Velocity Stacks it Idles at 25, when I put on the Filter it jumps to around 35, and then when I put the Cap on the Vaccum Sucks it down, and it instantly Revs to 5500 and holds.
What happens id you put the lid on without the air filter?
Parminio
Well-Known Member
Back up a moment here:
Did you set the screws and sync the carbs without all the filters and caps on?
Because if you did, that's at least part of the problem. You need to have that all on when you set the idle and sync the carbs. It's like syncing the carbs on a Vmax without having the air box cover sealed - it doesn't give the engine its true state of operation and can cause it to change wildly after you put the cover back on.
Your bike should be "roll out" ready when you sync your carbs and set your idle and mixture screws. Everything should be on it ready to go.
Edit to add: Also, did you make sure to cap off the cruise control when you disconnected it? Make sure that's not a factor.
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Hmm, may have found something. The Crankcase Vent Tube is all torn up. Now my XJ's
Yeah, I removed the entire Cruise System completely including the Switch.. Plan on Re-installing in the future.. And I have not Synced them with a Vaccum Gauge yet, I bench Synced them to the number 2 Carb by eye (I'm pretty sure it was Carb 2).. But I did have them installed with the Throttle Cable on and adjusted when I set them..
And a Guy on the Venture Forum told me to Set the Pilot Screw's to 2 turns out from Closed. So I just gently snugged them up, and then backed them out 2 turns. He said I would have to Fine Tune them with a Vaccum Gauge eventually, But 2 Turns is the Original Factory Spec.. It won't be Perfect, but safe to Run so I can Fine Tune with the Instrument..
But I actually found an issue last night, my Crankcase Vent Tube that Runs to my Airbox is Completely Shot. So I pulled a Tube of my old CB-750 and Replaced it. My little Boy was in Bed, so I haven't had a Chance to Start it yet. Going to when I get Home. Moment of Truth part 64 lol.
Parminio
Well-Known Member
Thank's
I meant to add about the idle adjustment setting, the stop, the screw, and the spring, but DreamV4 said it before I got back to the thread.
I'm not optimistic about the crankcase vent tube having anything to do with the issue, but fix all known problems, and proceed.
Here's the factory manual if you don't have a copy. I suggest printing one out and keeping it in a loose-leaf binder for reference.
VMX12- Service-Manual.pdf (vmoa.net)
Take time to read the layout, at the beginning there are multiple years' worth of yearly updates. The area at the back-appendix has things like cable routing, torque values, oil and coolant circulation, and a lot more.
I'm not optimistic about the crankcase vent tube having anything to do with the issue, but fix all known problems, and proceed.
Here's the factory manual if you don't have a copy. I suggest printing one out and keeping it in a loose-leaf binder for reference.
VMX12- Service-Manual.pdf (vmoa.net)
Take time to read the layout, at the beginning there are multiple years' worth of yearly updates. The area at the back-appendix has things like cable routing, torque values, oil and coolant circulation, and a lot more.
I'm not sure that is correct - as far as I'm aware there is no factory specification for idle mixture screw setting. Each carb would be set individually at the factory to get the correct mixture. Two turns or 2 1/2 turns on a Max would only be a nominal position. It will run OK at that.
I can see how running without the airbox may affect the idle speed but less sure how it would alter the balance as you would not have changed the relative position of the throttle butterflies.
I also am unsure the vent tube would be the causal factor but as Mr Medic suggests it does need attentions..
Sorry if I'm being more thick that usual but could you explain what you mean by 'it idles at 25, when I put on the Filter it jumps to around 35' - what does 25 and 35 refer to?
I am also sceptical that trying to synchronise by eye will be particularly successful. If I was doing it off bike I suggest you use a small drill bit as a gauge to adjust the butterflies to the same relative position. Use vacuum gauges to fine tune when on the bike.
One final point for clarification; on the Max the top of the air filter is open so would not, IMO, impede air flow. From what I can establish that is not the case on the Venture and it covers the intake?
That being so and given that revs rise as soon as you put the air filter on then I would at least see what happens when you put the lid on sans filter; better still replace the filter with a new OE one.
The worst that will happen is that you still have the problem but at least you have a spare filter.
Another thing to consider is that the carb slides are not operated by the throttle cable of course, only the butterflies. Perhaps when you assemble the airbox sufficient vacuum is created above the slides that the slides rise and although there may not be much air getting in, the extra fuel through the jets is causing the rpm to increase? Intake vacuum is transmitted to the space above the slide diaphragm by an internal drilling in the carbs. I know that this is not a complete answer to your problem because of course why would a correctly assembled intake system create too much vacuum? - but it may help you identify the underlying cause.
Parminio
Well-Known Member
Yep. That's been the "go to" setting for nearly every carburetor ever built. Even the old Rochester Quadrajets on old Chevy engines started there.
I have a Tonawanda casting 327 V8 (single-hump heads, block and heads date code 1968) with a cast-iron Tonawanda intake manifold for a Rochester Products Quadrajet (I pulled the carburetor for rebuilding) sitting on an engine stand right now, in the garage. One of my brothers worked at Rochester Products/GM when he was off from college, in the 1960's.
I start at 3 turns-out. I've rebuilt a few Quadrajets on Trans Am's and Z28's I've owned. Some people hate the Rochester Products Quadrajet, but I never found them difficult to rebuild or to tune properly. I am not a professional mechanic.
I start at 3 turns-out. I've rebuilt a few Quadrajets on Trans Am's and Z28's I've owned. Some people hate the Rochester Products Quadrajet, but I never found them difficult to rebuild or to tune properly. I am not a professional mechanic.
Blocked or collapsed air filter? It's OK until the filter goes on.
I know on some carburetors the throttle butterfly has a 'pull down' facility i.e. the vacuum will pull the butterflies open a little to allow some air to by-pass.
I don't recall the Mikunis having this facility but it would cause a higher vacuum if it was required and not functioning.
