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Carb Tuning Help - Pilot Screw Seized?

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Carb Tuning Help - Pilot Screw Seized?

PostAuthor: TahoeVirago » Fri Sep 06, 2019 3:07 am

Hey Virago Tech!

Been creeping, searching, and learning for a while now. I've been excited about tearing into my carburetors and finally got down to it tonight, I ran into a little problem i couldn't find a solid answer to -

My issue - I cant get the pilot screws to actually spin. In or out. It feels like they are stuck on the o-rings, they will turn slightly one way or the other but 'bounce' back to the starting alignment/position.

*See Image Below - I am referring to the two brass screws/jets on the bottom side of both carbs, my manual identifies them as #16 "Pilot Screw" - if this isnt a pilot screw, or has another name - please correct me, I'm new to carburetors.

Question - I searched and read that these screws have o-rings that can deteriorate over time - So to those of you with experience, Am I stuck on the o-ring? Am I safe to put a little effort into getting these to back out? I wanted to err or the side of caution and ask for help before I went ahead and cranked them out of there.

Thanks!
Mike

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Last edited by TahoeVirago on Sun Sep 08, 2019 2:46 pm, edited 2 times in total.
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Re: Pilot Screw Seized or Old O-Ring?

PostAuthor: Flyingdog » Fri Sep 06, 2019 3:47 am

:VTF:
TahoeVirago wrote: I cant get the pilot screws to actually spin. In or out. It feels like they are stuck on the o-rings, they will turn slightly one way or the other but 'bounce' back to the starting alignment/position.


You're correct..those 'are' the pilot screws...and yes...you're most likely hung up on a seized/stick'n 0-ring. Those pilots have a groove in the upper portion of the screw the 0-ring sits in. You'll just have to twist the screw beyond the 0-ring's seized point. At least, unscrew to the point the screws threads are disengaged from the bore's threads. Maybe you can grab the screw at this point and pull. Most likely tearing the 0-ring. No biggie as it's probly old as dirt, anyway. Important though, you use the correct size of screw driver blade that contacts the brass screw's slot perfectly. Even if you have to sacrifice one and grind it to correct size. Trying to eliminate as much of "any" taper as possible. That shorty flat blade has too much taper, as you can see the screw slots starting to cam out at the ends. Those screws are of the softest brass known to man and will strip the slot in a heart-beat. IE: No slop between blade and slot. Silicone/WD/Liquid wrench/what ever..even alittle heat should help the un-sticking. Naturally, go slow as to avoid the dreaded "cross-thread". Pretty damn easy with those fine threads. And..make sure there are "no" pieces left in the bore/tower, after screw is out. :bg:

This is what you're pilot looks like:
750 carb kit1.PNG


Grinding a blade to fit square and tight:
Jet Screwdriver Tip...PNG


This is what "I" consider "not" fitting right.. (too much taper):
Jet Screwdriver ground.PNG




You're lucky it's turning..many times, if in bad shape..people will crank and crank, till there's no slot left. End up having to drill a hole and thread in a tiny extractor bit. Above all..be cautious crank'n to the right. Don't wanna ruin the tip or the seat area within the bore. Either way, I'd surely proceed with care. Kinda though make'n long distant procedures.
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Re: Pilot Screw Seized or Old O-Ring?

PostAuthor: TahoeVirago » Sun Sep 08, 2019 2:46 pm

Flyingdog wrote::VTF:

You're correct..those 'are' the pilot screws...and yes...you're most likely hung up on a seized/stick'n 0-ring. Those pilots have a groove in the upper portion of the screw the 0-ring sits in. You'll just have to twist the screw beyond the 0-ring's seized point..


Thanks Flying Dog! I left them sit in PB overnight and then unscrewed the next morning thanks thank your confidence boost! Had to grab em with tweezers once i was free of the threads.

Got everything cleaned and rebuilt, she fired up first try!

I am having issues getting the pilot/idle circuit dialed now that shes running. I've tried been tuning one cylinder at time (plug grounded to engine) as well as together by RPM & ear but shes still not comfortable, i cant find the sweet spot. Its somewhere around 1 turn out, maybe a little less - which tells me I need a smaller pilot jet at 6200 feet.

*I am at 6200 feet elevation and I'm thinking all the gunk and goo i just cleaned up might have helped lean out the mixture just enough. The old pilot needles were dirty and bent - Before cleaning she would only start/cold idle on choke, after cleaning she wants no choke at all to start. So clearly my cleaning added fuel to the mixture.

Should I be swapping pilot/main jets for leaner mixture? I found some information online but uncertain if this metric or calculation are correct -

Using .05mm per 3000 ft, my pilot jet should be .1mm less than factory so a #40

Using 6% per 1000m (3280ft), my main jet should be 11% less than factory, so a #108 or #110 main

Want to get out riding ASAP! Thanks!
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Re: Carb Tuning Help - Pilot Screw Seized?

PostAuthor: bstig60 » Mon Sep 09, 2019 9:43 am

Probably a good place to start, but rejetting is a trial and error thingy at best. Try https://www.google.com/search?client=fi ... =jets+r+us
for your aftermarket jets.
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Re: Carb Tuning Help - Pilot Screw Seized?

PostAuthor: Flyingdog » Tue Sep 10, 2019 3:40 am

Yea..those altitude/jetting charts are "all pretty much the same. And "sometimes" are a starting point, but not alot. The ones you quoted is the 'same' language for my chainsaws. :bg: Personally? I never pay attention to them. Going by reading the spark plugs is the best..IMO. Almost "nobody" is going to have an exhaust analyzer/dyno..to dial this in, so plugs will tell the story of what's going on. AKA: hand full of new plugs (cuz once a plug gets colored, it won't show "accurately" different stages)..a bin full of jets and do'n a proper "plug-chop" is a good starting point..IMO. Also..IMO....I sure wouldn't trust that formula for 6200'. That's awfully small jetting. 12 sizes?..that's quite a jump to "start" with. "JMO" Just incase, I'd also be prepared with jet sizes just atad towards stock, too. Fudge-Factor... :bg: But, I can't/or will not say for sure, cuz all factors of "your" bike comes into play. Intake/compression/valves/exhaust/shape & tune ..pretty much the tune of "all" parts. Just take's alotta experimenting, back and forth to get to that perfect spot. One point, "I" always make, is the combo of sizes (#1 vs #2). Many don't...I do and have "always" got good results. Specially on the Virago engines that come factory with the same size jetting for both carbs. Like the early 750. I "always" go for the (#1/rear carb) with the small jet size. Do to the air temp (density). Nother one of those "your assessment" things..unfortunately. One thing I might agree with you is bout the pilot. Having to turn the screw that small (1 turn out?).."would" indicate going a step smaller. Damn near shutting fuel completely off. Ya really wanna cushion for future adjustments. But then ya gotta factor the transition point from pilot (idle/off idle) to needles to mains (WOT). It's definitely a balancing act..takes time..long distant tuning (jetting) is a crap shoot, at best. Specially when ya start factoring in ambient temps/humidity/piston needles..etc etc etc. Just as an example..my 750's jetting has been changed a few times over the yrs. Cuz things change that you can't control..right now..my 750... at 5300', I'm sitting at #1@#120 and (believe it or not) #2@#2124. No change to pilot and a change to #1 carb needle. But I got lucky, only took three tries. The 1000..not so much. Meaning..it takes what it takes and no formula is going to work all the time. Even NASA as fudge-factors.... :bg: Once again, wanna qualify this as this is how "I' approach it. An altitude to jetting chart maybe just a thing for someone, then again, could be way off, for others. Anyway, good luck and many discover it's really quite fun tuning once ya get the hang of it.

One other tiny point..
your sentence: "just cleaned up might have helped lean out the mixture just enough. The old pilot needles were dirty and bent - Before cleaning she would only start/cold idle on choke, after cleaning she wants no choke at all to start. So clearly my cleaning added fuel to the mixture."
Seems to contradict between "lean'n out" and "adding fuel"..both after cleaning..just curious.
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Re: Carb Tuning Help - Pilot Screw Seized?

PostAuthor: TahoeVirago » Thu Sep 12, 2019 5:46 pm

Flyingdog wrote:right now..my 750... at 5300', I'm sitting at #1@#120 and (believe it or not) #2@#2124. No change to pilot and a change to #1 carb needle. But I got lucky, only took three tries. The 1000..not so much. Meaning..it takes what it takes and no formula is going to work all the time. Even NASA as fudge-factors.... :bg: Once again, wanna qualify this as this is how "I' approach it. An altitude to jetting chart maybe just a thing for someone, then again, could be way off, for others. Anyway, good luck and many discover it's really quite fun tuning once ya get the hang of it.


That sounds reasonable, I agree that dropping 12 sizes is huuuuge change, I wasnt going to jump that far on the first go. If you're at 5300' with a #120 main I shouldn't need much more than that, maybe down to a #118, we shall see! Thanks for confirming that this is a trial and error situation at best!

Also hacked up a shorty screwdriver for the pilot screw as suggested :thanks:

One other tiny point..
your sentence: "just cleaned up might have helped lean out the mixture just enough. The old pilot needles were dirty and bent - Before cleaning she would only start/cold idle on choke, after cleaning she wants no choke at all to start. So clearly my cleaning added fuel to the mixture."
Seems to contradict between "lean'n out" and "adding fuel"..both after cleaning..just curious.


I have a hunch the dirt/crud in the carb Before the cleaning may have restricted the fuel flow enough to lean the bike out just a little. Also the front choke linkage was not allowing the valve to open the whole way... It was a mess inside with dirty mains/pilots/emulsifier tubes (restricting fuel, leaner)
So now that the carbs are clean, passages & jets are allowing the proper fuel After the cleaning - the mixture is quite a bit richer. This combined with the altitude has left me with a very rich idle circuit. I rode up to 8200' yesterday and she was definitely not happy idling at that altitude, had to keep on the throttle to keep her running, not a huge deal but interesting to see/hear the difference.

I am fairly impressed with how the bike feels after the cleaning - actually pulls like a 750 now, I have power throughout the gears and RPM range. She is way more responsive and a lot more fun to ride. Before the cleaning 60mph felt like a struggle, now she purrs along at 75mph no problem.

...Heads over to JetsRUs for some smaller jets!
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Re: Carb Tuning Help - Pilot Screw Seized?

PostAuthor: Flyingdog » Sat Sep 14, 2019 12:26 pm

TahoeVirago wrote:I am fairly impressed with how the bike feels after the cleaning - actually pulls like a 750 now, I have power throughout the gears and RPM range. She is way more responsive and a lot more fun to ride. Before the cleaning 60mph felt like a struggle, now she purrs along at 75mph no problem.


Nice! Glad to hear you're zero'n on your final goal. Hell..sounds like you may not have to do much change'n of "anything". Coarse...unless ya just can leave it alone cuz of the "curiosity factor". :bg:
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