Carburetor rebuild - Pilot screw turns question

tech_dog

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I'm rebuilding my 1995 Yamaha V6 2 stroke carbs and have been verifying the Pilot Screw turns from all in before i remove them. Each carb has two of these adjustable screws.

The first carb was 1-3/8ths turns on both sides. The 2nd carb was 1-3/8 on one side and 2 on the other. I now know the spec on this 175T engine is 1-3/8.

Should I return the adjustment to spec, or could there be a legitimate reason this was adjusted differently? I only see one spec in the Yamaha manual, so I'm assuming it applies to all six adjustments on all three carbs.

Thanks for any feedback,

T.
 
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TMANN

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I'd set them all the same, get it fired up and adjust all as needed to get the correct idle RPM.

I also always suggest getting a factory service manual.
 

seasick

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Did you find anything different on the 'out of spec' adjustment jet? If not and you also verified that the threaded hole that the adjustment screw goes into is clear and you correctly adjusted the float height, I would start with the 1 3/8 turns and see how it goes. Sounds simple but if you disassembled the throttle linkage that connects the carbs together, the issue may be that when you reassemble, the throttle plates may be out of sync.
Synching the carbs can be very difficult without a multi (6) port manometer.
Depending on the motor model, two cylinders may not fire below about 1200 revs. That makes verifying that all cylinders are firing correctly somewhat tricky.
 

seasick

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For some strange reason and it has happened more frequently lately, the message I posted yesterday did not get sent. hmmmm
 

seasick

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I'd set them all the same, get it fired up and adjust all as needed to get the correct idle RPM.

I also always suggest getting a factory service manual.
The problem is that setting the idle using the master idle screw is a no brainer but in the case where the carb to carb to carb linkage adjustment was changed, just setting the master idle screw does not insure that each carb is sucking the same amount of air.
 

TMANN

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The problem is that setting the idle using the master idle screw is a no brainer but in the case where the carb to carb to carb linkage adjustment was changed, just setting the master idle screw does not insure that each carb is sucking the same amount of air.
That's part of the reason I mentioned getting a service manual and following the steps for link/sync. Admittingly I'm much more familiar with Mercs. My understading is the pilot screws are just metering the air to the idle circuits prior to the blades opening. Inital ignition timing should be verifed also.

The one set two turns out could have just been an error, or compensating for a dirty jet or another issue such as a float like you mentioned.
 

tech_dog

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Did you find anything different on the 'out of spec' adjustment jet? If not and you also verified that the threaded hole that the adjustment screw goes into is clear and you correctly adjusted the float height, I would start with the 1 3/8 turns and see how it goes. Sounds simple but if you disassembled the throttle linkage that connects the carbs together, the issue may be that when you reassemble, the throttle plates may be out of sync.
Synching the carbs can be very difficult without a multi (6) port manometer.
Depending on the motor model, two cylinders may not fire below about 1200 revs. That makes verifying that all cylinders are firing correctly somewhat tricky.
I'll reinstall the carbs this weekend, and my starting point will be the 1-3/8 spec. I did carb rebuilds, with kits, so everything is clean.

I reused the floats, and as far as I can tell the float heights are the same as before. It seems a difficult measurement and an inexact adjustment, so I'm a bit uncertain here. I'm hoping these are not finicky and a simple reinstall with new needles sets it to spec.

I'm planning on checking the timing, not expecting anything to have changed. I've seen people claim this rarely drifts.

I plan to install the linkage as-was, and will walk through the process to adjust if necessary. This seems more straightforward. ( I did just buy the Yamaha service manual.)

I plan on being very delicate with the pilot adjustments, adjusting by ear.

This is my first carb R&R, so I appreciate the info and any potential gotchas. What tends to be finicky, what isn't, and what is likely to go wrong?

Thanks,

T.
 

seasick

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The screw you are referring adjust the amount of fuel at idle. The throttle plate on the other hand adjust the amount of air. If you kept the linkage intact and the motor ran OK before the rebuild, just reinstall and leave alone.
The master idle adjustment for the motor should be obvious since it makes all carb throttle plate shafts turn. The other adjustments also will have a dab of colored lock tite sort of stuff that was applied at the factory.
Also verify in your service manual if two cylinders are deactivate at low revs. If so, adjusting the idle mixture is tricky for those cylinders.
 
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