Air/Fuel mixture screw question. - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 16 (permalink) Old 05-20-2020, 10:51 AM Thread Starter
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Air/Fuel mixture screw question.

Hey All,
Yesterday was slow in terms of work so I decided to do some work on the bike. My goal was to marble it and also do some troubleshooting for my temp gauge. Turns out my temp gauge itself is bad so that’s now a job for another day. With the tank off it was real easy to get the marble in on the right side of the bike, popped off the side cover on the left to get leverage on the reed valve tube and got the other one in as well. Bike still pops, primarily on the LEFT side, and since my mufflers cracked, I currently have only exhaust tips on the bike the popping is starting to annoy me, especially when I’m riding through the neighborhoods. So next thing I want to try is the Air/Fuel mixture screws. I was able to get the plug out on the RIGHT side of the carb so far. I’ve done a lot of reading on the forum as to how to set it but I still have a couple questions.
1st question, Is the screw on the RIGHT side of the carb for the rear cylinder?? I ask because if it is I’ll leave the other plugged for now, no sense in messing with something else if it’s good.
2nd question, What are the consequences of turning the screw out too much? I only plan to turn it out between 2-3 turns but just want to understand what taking out too far will do so I know what to look for.
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post #2 of 16 (permalink) Old 05-20-2020, 11:35 AM
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I believe the left carb is for the rear, but my engine is apart so someone will have to double check.
Look carefully at the short tubes below the carbs. It should be fairly easy to see if the tube slants down towards the front or the rear on a given carb.

Going too rich will cause stumbles and mis-fires at idle and poor response coming off idle. It will hurt your cruising MPG, and may cause excessive carbon build up.
Since you're running without a muffler, that carbon may find it's way onto your rim (as it did with my bike when I had an oversize main jet).

Have you done an ear shave?
I found my bike did NOT need any tuning adjustments between mufflers/no mufflers when I had stock exhaust.
However, after an earshave, putting the mufflers back on caused a rich mis-fire when accelerating.
If you have done an earshave, I'd expect your bike to require different tunings from front to rear, since you're running one with a muffler and one without.

When I tuned my idle screws, I turned one in until the idle started to drop and noted where that was at (2 turns out).
Then I turned that same one out until the idle started to drop and noted where that was at (3.5 turns out).
That gave me my acceptable range (2 to 3.5 turns).
I then chose a spot on the lean side of the middle (2.5 turns).
Do the same with the other carb (when you are able).
Anything in the range that you find should be fine to run with.

Mine pops on decel (which I actually like). Going out 0.5 turns got rid of the popping. I can't say the MPG difference between 2.5 and 3 turns. I didn't run long at 3 turns.


'89 VN750 Bought stock 4/12/18 @ 17805.9 miles $800. Wrecked once. 27454.5+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB. 110/90 Front Tire. Pick-up coil mod. Shindengen MOSFET FH020AA r/r. USB Outlet + Voltmeter. Repainted Metallic Blue w/ new tank bages. Ear Shave (wo/ coasters) 40/140 NO shims 45MPG. MCCT conversion.

Planned modifications:
Custom LED turn signals. Windshield. Highway Pegs. (Possible) ignition adv. Tuxedo Mod. Ignition coil mod.
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post #3 of 16 (permalink) Old 05-20-2020, 12:42 PM Thread Starter
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I have NOT done the ear shave, and to clarify I have no mufflers at the moment. The clamps cracked off of both of them. So they’re sitting in my basement at the moment waiting to be repaired. I have read a few times through my searches that the GB provides enough back pressure. Not sure if that’s accurate but the bike feels good (At least to me anyway). I did adjust the one screw I uncovered but still pops so maybe you’re right and it’s the other.
When you say “tubes” I’m assuming you mean where the carb mounts to the cylinders, and because that screw is closest to the rear tube is why I asked if the screws were opposite of the exhaust
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post #4 of 16 (permalink) Old 05-20-2020, 03:05 PM
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If the right carb tube where the carb mounts to the cylinders is towards the rear, than that's tuning the rear cylinder.
The rear cylinder does have it's exhaust on the left side of the bike.

The GB does provide some back pressure. Enough that running straight pipe after the GB doesn't require retuning.
However the ear shave reduces the intake restriction considerably. I saw a tuning difference between mufflers and no mufflers.
In your case with no earshave and both mufflers missing, your tuning should be close to stock (+/- your preferences).

Keep in mind, that the GB does connect both pipes, so a pop in the back cylinder will come out both pipes, just louder on the left side.
Same with the front, it will be out both pipes, only louder on the right.


'89 VN750 Bought stock 4/12/18 @ 17805.9 miles $800. Wrecked once. 27454.5+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB. 110/90 Front Tire. Pick-up coil mod. Shindengen MOSFET FH020AA r/r. USB Outlet + Voltmeter. Repainted Metallic Blue w/ new tank bages. Ear Shave (wo/ coasters) 40/140 NO shims 45MPG. MCCT conversion.

Planned modifications:
Custom LED turn signals. Windshield. Highway Pegs. (Possible) ignition adv. Tuxedo Mod. Ignition coil mod.
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post #5 of 16 (permalink) Old 05-20-2020, 05:45 PM
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Quote:
Originally Posted by mmart View Post
Hey All,
Yesterday was slow in terms of work so I decided to do some work on the bike. My goal was to marble it and also do some troubleshooting for my temp gauge. Turns out my temp gauge itself is bad so that’s now a job for another day. With the tank off it was real easy to get the marble in on the right side of the bike, popped off the side cover on the left to get leverage on the reed valve tube and got the other one in as well. Bike still pops, primarily on the LEFT side, and since my mufflers cracked, I currently have only exhaust tips on the bike the popping is starting to annoy me, especially when I’m riding through the neighborhoods. So next thing I want to try is the Air/Fuel mixture screws. I was able to get the plug out on the RIGHT side of the carb so far. I’ve done a lot of reading on the forum as to how to set it but I still have a couple questions.
1st question, Is the screw on the RIGHT side of the carb for the rear cylinder?? I ask because if it is I’ll leave the other plugged for now, no sense in messing with something else if it’s good.
2nd question, What are the consequences of turning the screw out too much? I only plan to turn it out between 2-3 turns but just want to understand what taking out too far will do so I know what to look for.
The right side carb is for the rear cylinder.

Turning the air mixture screw to the left, looser, will richen the idle mix, which should help reduce popping.

This idle mixture adjustment won't affect running over ... about 2000 rpm.

Have you been through the carbs? Stuck coast enrichers will cause popping also, and straight exhaust usually has the tendency to pop.

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
Repair Manual: http://www.mediafire.com/file/mj7z81..._Catalogue.pdf

Owner's Manual: http://www.mediafire.com/file/nscb5f...ers+Manual.pdf

Last edited by Spockster; 05-20-2020 at 05:48 PM.
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post #6 of 16 (permalink) Old 05-20-2020, 05:52 PM Thread Starter
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I took the other lead plug out, I’m going to take the bike out and mess with the tuning to see if I can get it to at least pop less. I don’t mind a few pops here and there but when it sounds like a gun fight it needs to be looked at. If I can’t reduce it then I guess I’ll start hunting down a possible exhaust leak. The only other thing I can think of is if it’s at all possible the marbles didn’t completely seal up the hose. It felt snug but that would be my luck lol
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post #7 of 16 (permalink) Old 05-20-2020, 08:48 PM
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Could stick a bolt in the hose and put a clamp on it, but I don't think that's the problem causing the pops.

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
Repair Manual: http://www.mediafire.com/file/mj7z81..._Catalogue.pdf

Owner's Manual: http://www.mediafire.com/file/nscb5f...ers+Manual.pdf
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post #8 of 16 (permalink) Old 05-20-2020, 11:28 PM Thread Starter
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Quote:
Originally Posted by Spockster View Post
Quote:
Originally Posted by mmart View Post
Hey All,
Yesterday was slow in terms of work so I decided to do some work on the bike. My goal was to marble it and also do some troubleshooting for my temp gauge. Turns out my temp gauge itself is bad so that’s now a job for another day. With the tank off it was real easy to get the marble in on the right side of the bike, popped off the side cover on the left to get leverage on the reed valve tube and got the other one in as well. Bike still pops, primarily on the LEFT side, and since my mufflers cracked, I currently have only exhaust tips on the bike the popping is starting to annoy me, especially when I’m riding through the neighborhoods. So next thing I want to try is the Air/Fuel mixture screws. I was able to get the plug out on the RIGHT side of the carb so far. I’ve done a lot of reading on the forum as to how to set it but I still have a couple questions.
1st question, Is the screw on the RIGHT side of the carb for the rear cylinder?? I ask because if it is I’ll leave the other plugged for now, no sense in messing with something else if it’s good.
2nd question, What are the consequences of turning the screw out too much? I only plan to turn it out between 2-3 turns but just want to understand what taking out too far will do so I know what to look for.
The right side carb is for the rear cylinder.

Turning the air mixture screw to the left, looser, will richen the idle mix, which should help reduce popping.

This idle mixture adjustment won't affect running over ... about 2000 rpm.

Have you been through the carbs? Stuck coast enrichers will cause popping also, and straight exhaust usually has the tendency to pop.
I have not been through them, I don’t have a lot of experience with carbs. I’m learning slowly from this forum but not quite there yet. What normally causes that to stick? Gunk? Or is that a sign of wear?
One more question on those A/F screws, most of what I read says “turn them all the way in first but be sure not to force them”. What is all the way in? Is it when you first start to feel the spring tension push back? That’s what I did and I want to be sure I’m doing it right.
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post #9 of 16 (permalink) Old 05-20-2020, 11:38 PM
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All the way in is where it bottoms out in the hole, with light finger pressure. If you torque it tight, you can bend the tip, which is like the tip of a dart. Then good luck finding one.

Shellac, gunk, sticky corn goo ... Can stick the 'piston'. Or the rubber in the diaphragm can crack, rot, or rear.

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
Repair Manual: http://www.mediafire.com/file/mj7z81..._Catalogue.pdf

Owner's Manual: http://www.mediafire.com/file/nscb5f...ers+Manual.pdf
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post #10 of 16 (permalink) Old 05-21-2020, 08:45 AM Thread Starter
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https://www.partzilla.com/catalog/ka...rburetor-parts

Are the A/F screws called "SCREW-PILOT AIR 16014-1054" ? if so they're $19 for the screw alone, not to mention the springs etc. I'd rather just be gentle and do it the right way and go at it with a light touch lol.

But back to what you were saying earlier, should i double my sea foam use?. I heard of guys spraying it right into the carbs, where do you do that? do you remove the fuel line coming from the petcock? I took the tank off for the first time the other day and finally got a good look at the tubes so I have a little better idea of what goes where.
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