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idle mixture set up


Tahoe Tom

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I have a sunsetter which has a new-ish (about 4 year old) engine which has a Mercruiser MC-50 (Carter/Weber 7613) 4-bbl carb on a new high rise manifold. The engine is a Chevy 5.7L high performance 4-bolt main motor which is a marine set up engine (I checked; all items are set up for marine service and the engine was sold as a crate motor for marine racing application)

Enough for the back ground. I have an issue with fairly poor idle and need to adjust the idle mixture and idle speed. Both idle screws are at different turns (roughly 1-1/2 each turns out, but one is about 1/2 turn more, one is 1/2 turn less). No idea how they got this way, but that is where they are.

I want to raise up the idle also as it is too low (about 500 RPM) and it stalls at idle occasionally when I am in high altitude applications.

I think about 800 RPM would be better.

Can someone tell me what the correct approx number of turns the idle mixture screws should be? Doing this by feel/sound is not really working for me.

Otherwise, the boat runs great.

One note: I run her in lakes at about 1000 feet elevation, 3500 feet and 6300 feet. Do I need to make any adjustments to the carb at either extreme (I live at about 250 feet so garage tuning is at this low altitude).

I run different props when low versus high (was a huge issue till I learned that I needed to do this!). 13x11 vs 13x13. The 13x11 is fine at all elevations, but it rev's too quickly in the low elevation lake so I switch back so I don't have to worry about my wife or step daughter over-reving. Probably not a huge concern, but why worry if I don't have to.

I know enough to be dangerous, but am also not afraid to try to fix issues, not a mechanic by any means.

Thanks for the help in advance,

Tom

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I have a sunsetter which has a new-ish (about 4 year old) engine which has a Mercruiser MC-50 (Carter/Weber 7613) 4-bbl carb on a new high rise manifold. The engine is a Chevy 5.7L high performance 4-bolt main motor which is a marine set up engine (I checked; all items are set up for marine service and the engine was sold as a crate motor for marine racing application)

Enough for the back ground. I have an issue with fairly poor idle and need to adjust the idle mixture and idle speed. Both idle screws are at different turns (roughly 1-1/2 each turns out, but one is about 1/2 turn more, one is 1/2 turn less). No idea how they got this way, but that is where they are.

I want to raise up the idle also as it is too low (about 500 RPM) and it stalls at idle occasionally when I am in high altitude applications.

I think about 800 RPM would be better.

Can someone tell me what the correct approx number of turns the idle mixture screws should be? Doing this by feel/sound is not really working for me.

Otherwise, the boat runs great.

One note: I run her in lakes at about 1000 feet elevation, 3500 feet and 6300 feet. Do I need to make any adjustments to the carb at either extreme (I live at about 250 feet so garage tuning is at this low altitude).

I run different props when low versus high (was a huge issue till I learned that I needed to do this!). 13x11 vs 13x13. The 13x11 is fine at all elevations, but it rev's too quickly in the low elevation lake so I switch back so I don't have to worry about my wife or step daughter over-reving. Probably not a huge concern, but why worry if I don't have to.

I know enough to be dangerous, but am also not afraid to try to fix issues, not a mechanic by any means.

Thanks for the help in advance,

Tom

I did my Edelbrock idle mixture screws by watching the RPM's which ever gives you the highest RPM's is where you leave them. If your trying to change your idle, you need to use the fast idle adjustment screw. Here is a video that shows how Edelbrock Adjust Idle Also, here is the site for all the videos for working with a carbeurator.Edelbrock Carb Videos
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not sure about that carb, but on holley carbs you turn the idle mix screw OUT to get more RPM.

so turn the one that is in more, out to match the other and it should increase the idle

800 is likely too high, shoot for 600-700, so the 1/2 turn should up it above 600

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Using a vacuum gauge is the best way to adjust your carbs idle mixture. Instead of typing it all out I just cut and paste for you :)

Before you fire up the engine, slowly turn each idle-mixture screw in until it gently bottoms out and count the number of turns it takes. Do this for each idle-mixture screw and then return them to their original positions. If all the idle-mixture screws are not adjusted the same, do that now. Remember to always adjust both screws the same amount. This will help balance the idle mixture and allow the engine to run as smoothly as possible. If the carburetor is new to the engine, start at 1½-turns out (counterclockwise) from full in.

Now start the engine and set the idle speed to around 850 rpm with the idle-speed screw. If your engine has a big cam and must idle at a higher speed, that’s OK. Note the reading on the vacuum gauge. Next, turn one idle-mixture screw in ½-turn and note the change on the vacuum gauge. If the vacuum reading increases—say, from 14 inches to 14-½ inches—move around to the other side of the carburetor and turn that mixture screw in ½-turn as well. Again, note the vacuum gauge reading; if the gauge continues to climb, then adjust each idle-mixture screw in another ½-turn. On most carburetors, turning the mixture screw in (clockwise) leans the mixture, while counterclockwise (out) enriches the mixture.

Initially, if the engine stumbles or the vacuum drops when turning the mixture screw in, turn both screws out about a ½-turn and evaluate the results. The goal of adjusting the idle-mixture screws is to achieve the highest possible idle vacuum at a set idle speed. If the idle speed increases after you adjust the idle-mixture screws (which is likely), be sure to adjust the idle speed back to the base speed. This is important because a higher idle speed will increase the vacuum reading. Maintaining a standard idle speed will make it easier to evaluate changes to the idle-mixture screws.

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