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vacuum readings - carb adjustment

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bobct

Advanced Contributor
Both of my carbs are rebuilt and I've been tuning them on the hard. Timing is 10 degrees at idle.

Vacuum readings on both engines are in the 17-19 range but I can't get it to stay rock solid, there's a fluctuation of at least 1/2 on the scale. I would say it's more of a needle shake within that range, not really moving up and down.

http://www.secondchancegarage.com/public/186.cfm

It looks most closely like the example which references "high performance" engines with lumpy cams. Scenario #3 but readings aren't low but similar movement.

Do you guys find the same or can you get the needle to stay dead on at idle?

Also, did I read somewhere NOT to use the felt gaskets under the flame arrestors? The carb rebuild kit came with them, boat did not have them when I bought it but could have been long gone.
 
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I get what you could call a needle shake on mine, as expected, because it is an undamped gauge. Is your fluctuation a 1/2" of HG or a half of a 'tick mark' on your gauge?

the other trick I've learned is to wait until the engine is at operating temperature before making the final mixture adjustment and final idle speed settings.
 
Mine looks about like Scenario #3 on your link. I would say it is a quarter of an inch Hg. I have never seen one steady like Scenario #1. Mine typically runs at 18.5 to 19 inches at 800 RPM.

Chuck Hanson
 
ok, I think I'm close then based upon what you guys are saying. I did wait until the engines were fully warmed up. How warm? I have selection of temps I can plug in right now :)

I would say it's 1/2" of HG, I'll have to do it again. I think my idle might have been a little low though, I'm waiting for my digital tach to arrive and then I'll do them both again. Plus I'll have the chokes fully operational.


Bob
 
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On outboards, we make DAMN sure the motor is fully warmed up AND in gear before making idle adjustments. I usually do this with the boat on the trailer or tied to the dock so as to not trick myself--I'm listening to idle speed in making the adjustment.

One other caution: Don't be a dope like me and use the PCV valve hose as a "convenient" vacuum source. Why? It's a controlled vacuum leak, so plugging it up makes the motor run rich soon as the vacuum gage is, ah, removed. Doh!

Jeff

PS: I had to remove the choke pull off hose to find another vacuum source, there being none other on my engines (unlike car setups with multiple vacuum points one can use).
 
After getting a digital tach I was very surprised to see the idle fluxuation that my green ears could not hear and the dash was not showing.

I'm also in process of tuning.
Back to port engine this weekend. Never finished to my liking.

Mark, what is a damper for the vacuum gauge? I bought a cheap vacuum tester from sears that I was using it last month. It came with hose and a tee the same size as choke pull off, so I tap in there. I do have a plug in manifold and was thinking it might be better place to hook into.

Last question is what is the procedure to tune carb at idle with a vacumn tester. All I really know is ilde screws between the two extreems and make sure I don't see dripping from primarys.

Thanks
 
Scott:

The typical "damper" in a gauge is fluid, usually with a glycerin base. Some Fluctuation at idle is to be expected due to normal valve activity.

Tapping the choke pull off is fine; what is needed is a source of manifold vacuum, not one of the ported vacuum points. with the standard marine q-jet, the choke pull off is the readily available source (though not always easy to access).

In terms of the idle adjustments, I usually do the following:

Complete warmup - typically leave the slip, run it up on plane a few miles then back to the slip. Make sure the choke is fully open. Set the idle speed screw low enough to make sure NO fuel is running from the primaries (adding idle bypass air usually eliminates this set and not all carbs need this.) Hook up gauge and optical tach. adjust mixture srews, one at a time, IN unitl engine rpm drops, note position, then back the screw out until rpm drops again (should increase as the screw is back out then drop when the mixture richens up). Set screw to the midpoint in between the two rpm drop points. Then I tweak the screws using the vacuum gauge, trying to get max vacuum (lowest pressure). It usually occurs pretty close to the "midpoint position.

After the mixture is set, adjust the idle RPM. if in the slip, double check the lines and put the gear in FWD. Adjust idle screw to get spec speed. Return gear to idle and make sure the idle speed doesn't exceed max "shift" rpm (1000 rpm for most gears); if too high, lower idle speed setting.
 
The food afterwards may be cheaper than the fuel consumed during......

Yes, same procedure would apply to those 1409's.

How long is the drive.....I can reach Atlantic City in about 3 hours w/o rush hour???
 
Yea Mark. I'm planning to take boat down to DC. Maybe even this summar. I hope you are around because I know I'll enjoy a nice big dinner bill. You have helped me bond with my rig! Same goes to DD and jeff and al, and the whole lot of ya...
 
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