Newbie needs help; stripped sync screws

[Okie Bill]

Howdy from Oklahoma! I'm ashamed to have been so ignorant for the 20 years I've had my '96 VMAX. Mechanic who rebuilt carbs a few years ago advised the sync screws were stripped so he could not properly sync the carbs. I presume that means the part that the screw goes into is stripped or he'd have simply replaced the screw(s). And I'm not sure if this means all 3 sync screws or just one or two. I was too ignorant to ask when work was done but I believe I can find the mechanic for discussion once I have better understanding. The bike runs terrifically. Idles smoothly and screams at WOT. But a bit rough when underway at steady part throttle, at least at certain RPM (I need to gather more info on that if y'all think critical to diagnosis). Anyhows, three questions please: 1. Does my description sound like a sync problem? 2. Am I damaging engine or carb diaphragms etc by being a bit out of sync? 3. Is this a part I should be able to find? I guess I should say 4 questions cuz if you don't think it's sync prob, what else? Thanks for any help. Cheers, Bill

 

[Fire-medic]

You could try re-threading the hole, I've done that before. Can you feel or see the tip of the brass screw in the throat of the carb, sticking out of the wall? When it's all the way-bottomed out in the hole? If you don't have the screw portruding into the carb throat, something's keeping the screw from its full extension.

 

[sdt354]

For me, its always been the threads on the screw itself. You want the ultra fine thread to get fine adjustment. Your carbs are most likely due for a shotgun cleaning and a sync. I don't see damage from being out of sync a bit. They go out of sync with riding time. I think it was Dave, that used to put fingernail polish on his sync screw threads to keep them from moving as much. That's my story and I'm sticking to it.

 

[Okie Bill]

Thanks FireMed and SDT (dang near said STD!). I'm gathering your advice and I copied photos and discussion from another gent's post on how to sync the carbs. I don't have the syncing tool but next I'll at least pull the covers off and find out for sure which screw(s) is/are the problem. Just what are the sync screws balancing within the intake system? What is being restricted or opened up? The intake air AFTER it's been through the carbs and entered the manifold? Is the process driven by "engine pulses"? Does out of sync mean one or more cylinders is slightly too rich or lean? Inquiring minds are like weak minds in may case, eh? Again, thanks for teaching an old geezer what he should have learned long ago.

 

[sdt354]

What you are doing is equalizing the vacuum between cylinders by moving the throttle plates to be open all in unison.

 

[Okie Bill]

Thank you Mr. SDT. Please hold my hand just a bit longer. You're helping more than you might realize. Is it fair to say, then, that the mixture of gasoline and air going to each cylinder is the same ratio but the cylinders will not be getting the same volume of that mixture at a given throttle setting? Thus one or more cylinders will be firing a tad harder (making more or less power) than one or more of the other cylinders, correct? And that's why we can say the imbalance should not damage the engine because if the air/fuel mixture was properly set we're not going to burn valves or load up with carbon under normal fun driving conditions of widely varying engine speeds and throttle openings. I apologize again for being so slow on the uptake. I simply don't want to get smoke blown up my hiney by dealership service folks. I need to find a good VMAX mechanic in a smaller shop, eh? Know anybody within 500-700 miles of Oklahoma? With help from you folks I'll be able to make good decisions. I'm grateful.

 

[maleko89]

Are you talking the three sync screws or the mixture screws? Subject says sync but some are replying like it's mixture?

Mark

 

[Okie Bill]

Hi Mark. Yes, one or more of the 3 sync screws will not adjust (spin in place, presume stripped, per Yamaha mechanic). And yes, one reply appeared to be thinking mixture. As my learning curve ramps up thanks to other posts and replies here I intend to pull the "H" covers so I'll know more of what I speak before I seek a qualified mechanic. Hey, you're nearly a neighbor in Nebraska, close enough if you have (or are) trustworthy mechanic who is not typical dealership guy. I'm grateful for any ideas.

 

[sdt354]

Mixture screws are in a 1/4" tunnel at the lower part of the carb. They come plugged with brass plugs at the factory, but quickly removed by the new buyer. You are right about sync screws, they do not change mixture. Just adjust the throttle plates. You'll need a sync tool to proceed changing out the sync screws. Though counting the turns out until they are free will get you somewhat close.
Steve-o

 

[maleko89]

Hi Mark. Yes, one or more of the 3 sync screws will not adjust (spin in place, presume stripped, per Yamaha mechanic). And yes, one reply appeared to be thinking mixture. As my learning curve ramps up thanks to other posts and replies here I intend to pull the "H" covers so I'll know more of what I speak before I seek a qualified mechanic. Hey, you're nearly a neighbor in Nebraska, close enough if you have (or are) trustworthy mechanic who is not typical dealership guy. I'm grateful for any ideas.

To fix those, I've epoxied and re-drilled. Or, just get new linkages.

As for folks to work on your carbs, if you are comfortable removing/shipping (there are links in carb section showing how), you can send them to Sean Morley, Danny, Kyle...any of them and others will make 'em all spic 'n span. I used to but hardly have time anymore do to my kid's sports and other commitments.

Cheers,

Mark

 

[sdt354]

Mixture screws are in a 1/4" tunnel at the lower part of the carb. They come plugged with brass plugs at the factory, but quickly removed by the new buyer. You are right about sync screws, they do not change mixture. Just adjust the throttle plates. You'll need a sync tool to proceed changing out the sync screws. Though counting the turns out until they are free will get you somewhat close.
Steve-o

But, that's all under normal circumstances. Stripped screws or threads won't matter how many turns are counted.

 

[Okie Bill]

Thanks Steve. I reckon next step is for me to pull the covers and ensure I understand which screws are not adjusting...not screwing in or out I presume. I'll do it gently and measure my turns. If there is genuinely no adjustment possible I'll try to determine whether the problem is in the body or the screw. I'll report back here when I know more. I appreciate the advice!

 

[Okie Bill]

To fix those, I've epoxied and re-drilled. Or, just get new linkages.

As for folks to work on your carbs, if you are comfortable removing/shipping (there are links in carb section showing how), you can send them to Sean Morley, Danny, Kyle...any of them and others will make 'em all spic 'n span. I used to but hardly have time anymore do to my kid's sports and other commitments.

Cheers,

Mark

That's the problem with raising good and active kiddos....they require even more time, eh? Bravo! I complimented my neighbor just last eve for teaching his 8-yr-old how to properly mow the yard. My mower was my income at that age. Mark, are you saying that if the female part where the sync screw(s) go in is stripped, that part can reasonably be replaced (or epoxied and rethreaded as cheaper faster alternative IF I'm capable)? Will the "linkages" you reference show in exploded view of carbs? I hope you don't mind if I check back with you after I've gathered more info from pulling covers and trying to adjust the sync screws (yes, carefully!). Thanks again!

 

[dannymax]

If the tab (female threads) is stripped you can drill out what's left of the threads and braze a nut of the same threads on the backside of the tab. I'm not a big fan of this but it will work.

 

[maleko89]

That's the problem with raising good and active kiddos....they require even more time, eh? Bravo! I complimented my neighbor just last eve for teaching his 8-yr-old how to properly mow the yard. My mower was my income at that age. Mark, are you saying that if the female part where the sync screw(s) go in is stripped, that part can reasonably be replaced (or epoxied and rethreaded as cheaper faster alternative IF I'm capable)? Will the "linkages" you reference show in exploded view of carbs? I hope you don't mind if I check back with you after I've gathered more info from pulling covers and trying to adjust the sync screws (yes, carefully!). Thanks again!

I'd epoxy and re-drill if you can. I can't recall the thread pitch though? IMO, forget about switching linkages out. If I recall, you'd have to remove the throttle butterflies which is a bit of a pain and would require donor carb bodies.

Cheers!

Mark

 

[Edward]

If the tab (female threads) is stripped you can drill out what's left of the threads and braze a nut of the same threads on the backside of the tab. I'm not a big fan of this but it will work.

I haven't done this but couldn't you drill the holes out to the next size and thread them with the same pitch tap?

 

[Okie Bill]

You guys are terrific. I've learned more in the last few days about my VMAX than in the 20 years I've owned it. Yes, that's pathetic. I'm high centered for a day or three, busy with unimportant parts of life. But I'll get back to this important part ASAP. ONE MORE QUESTION PLEASE: Am I correct that the sync screws simply push a tab that then opens a butterfly that is within the intake system (thus the butterflies, NOT the tab or sync screws, are exposed to vacuum). THEREFORE, epoxying (that's a verb, right?) and rethreading the sync screw "holes" is all done outside the intake system so "cuttings" can't be ingested by engine, correct? I'm hoping to examine sync screws by Friday. AND make a small donation to this website by then too! I'm grateful.

 

[maleko89]

You guys are terrific. I've learned more in the last few days about my VMAX than in the 20 years I've owned it. Yes, that's pathetic. I'm high centered for a day or three, busy with unimportant parts of life. But I'll get back to this important part ASAP. ONE MORE QUESTION PLEASE: Am I correct that the sync screws simply push a tab that then opens a butterfly that is within the intake system (thus the butterflies, NOT the tab or sync screws, are exposed to vacuum). THEREFORE, epoxying (that's a verb, right?) and rethreading the sync screw "holes" is all done outside the intake system so "cuttings" can't be ingested by engine, correct? I'm hoping to examine sync screws by Friday. AND make a small donation to this website by then too! I'm grateful.

Correct, it pushes a tab which is directly tied to the throttle butterflies. The only one not adjusted by a sync screw is #2 (front left). The idle thumb screw adjusts the butterfly for this carb.

Mark

 

[dannymax]

I haven't done this but couldn't you drill the holes out to the next size and thread them with the same pitch tap?

I don't see why not, may not even have to match the thread just using the finest thread available for that size would probably work ok.
I have run across them where someone re-tapped with a coarse thread and there is no hope of syncing those, just looking at the sync screw changes the setting!

 

[dannymax]

I'd epoxy and re-drill if you can. I can't recall the thread pitch though? IMO, forget about switching linkages out. If I recall, you'd have to remove the throttle butterflies which is a bit of a pain and would require donor carb bodies.

Cheers!

Mark

I have quite a few of these throttle plates with rods & attached linkage for each of the 4 carbs, including new screws but you're right Mark, it's a pain. Staking the new screws can be very tricky.