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Adjusting Idle on a 600SC

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Old 04-18-2011, 10:43 AM
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Default Adjusting Idle on a 600SC

Im not a big carb guru so I thought I would post up in here. I have searched the forum as well, but I guess I'm looking for a solid answer. From what I've read, to adjust the idle on the carb, you use a vacuum gauge to adjust the 4 fuel delivery screws (2x on each holley 700). They are set right at the highest vacuum reading, correct? Then you adjust each idle screw (1x on each holley) to set the idle speed. I did some minor adjustments, out of water, and got the engine to idle at 1000rpm, but plan to adjust in water with a vacuum gauge. Before, it would idle around 1200 to 1300 (previous owner had both carbs rebuilt) rpms, but from what I read, is a bit too high. Is 1000rpm, assuming the vacuum readings are right, an okay neutral idle speed to not damage the drive? I know I have to adjust more because if you rev the engine a bit, the idle drops low for a split second, but not to the point of stalling. Im thinking this would cause a stall in water though.. Any help, to adjust these correctly, to a carb newb is appreciated...

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Old 04-18-2011, 10:51 AM
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Im not a big carb guru so I thought I would post up in here. I have searched the forum as well, but I guess I'm looking for a solid answer. From what I've read, to adjust the idle on the carb, you use a vacuum gauge to adjust the 4 fuel delivery screws (2x on each holley 700). They are set right at the highest vacuum reading, correct? Then you adjust each idle screw (1x on each holley) to set the idle speed. I did some minor adjustments, out of water, and got the engine to idle at 1000rpm, but plan to adjust in water with a vacuum gauge. Before, it would idle around 1200 to 1300 rpms, but from what I read, is a bit too high. Is 1000rpm, assuming the vacuum readings are right, an okay neutral idle speed to not damage the drive? I know I have to adjust more because if you rev the engine a bit, the idle drops low for a split second, but not to the point of stalling. Im thinking this would cause a stall in water though.. Any help to a carb newb is appreciated...
1000 is still high IMO. You should be able to idle down to 800 and lower without stall issues. If you need a high idle to keep from stalling you have air screw issues or a vacuum leak.

Idle is set by the stop screw, jetting is set by the air screws. A good holley guy can do it by ear
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Old 04-18-2011, 10:57 AM
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1000 is still high IMO. You should be able to idle down to 800 and lower without stall issues. If you need a high idle to keep from stalling you have air screw issues or a vacuum leak.

Idle is set by the stop screw, jetting is set by the air screws. A good holley guy can do it by ear
Since the last owner wasn't a carb guy and neither am I, I think the idle is high because the air screws are not adjusted properly, so the stop screws were adjusted to bring the idle up and not stall.. Is there any trick to setting the air screws or just get them to where the vacuum reading is at its highest point? Also, where is a good spot to hook up a vacuum gauge to get a good reading?

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Old 04-18-2011, 03:47 PM
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you could have a leaking power valve causing it not want to idle down smooth enough and they just idled it up to make up for the issue...if the carbs have not been apart in a while with the B/S gas we have now....seen all kinds of crap in so many carbs lost count..good luck..Rob
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Old 04-18-2011, 03:54 PM
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I had the same problem. Idle was 1050rpm. Give it the gas and drop it down hard. The idle would go to about 700 for just a second. Shift in gear. I did find the problem. I took off my exhaust tip mufflers. Now it idles at 800rpm and I can tell I have a lot more Horse Power.
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Old 04-18-2011, 04:40 PM
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Assuming the carbs are physically in good shape. If you are unable to do this find a good carb guy.

Start by making sure the power valves are correct and not leaking before you start trying to adjust idle mixture and speed. If it is not the carb will be drawing off the main metering circuit and you will need a fast idle speed to compensate. Only after you have the correct power valves in place can you set mixture and idle speed. Lots of other tricks that can be done with air bleeds and such but that is best left to someone who really knows their chit. You should be able to get them to 700-800 rpm. At 1200 rpm your going to do some drive damage when shifting in to gear.
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Old 04-18-2011, 06:52 PM
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you could have a leaking power valve causing it not want to idle down smooth enough and they just idled it up to make up for the issue...if the carbs have not been apart in a while with the B/S gas we have now....seen all kinds of crap in so many carbs lost count..good luck..Rob
Both carbs were rebuilt last season. One in the beginning of the season, one at the end. I just dont think the mixture was ever done correctly. When I got it down to 1000 the other day, it would shift into gear without a drop (again, this was out of water with just running the hose to it). Yesterday it was cooler out and was a little more choppy, thats why i was thinking maybe the mixture needed to be adjusted with a vacuum gauge so it was correct then maybe it would sit around 800 with no problem.. If it ends up that it wont hold the lower idle without stalling (hasnt stalled yet), id have a carb guy look at the power valve.
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Old 04-19-2011, 07:07 AM
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you really cannot set them up put of the water, when you drop in gear the prop drag will change the in gear idle & RPM, can get close but needs to be fine tuned in the water...
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Old 04-19-2011, 07:10 AM
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you really cannot set them up put of the water, when you drop in gear the prop drag will change the in gear idle & RPM, can get close but needs to be fine tuned in the water...
What about adjusting the mixture screws? Can that be done out of water since you are looking for the vacuum reading or does the drag affect that as well? I dont want to waste time, so if it all should be adjusted in water, I'll just wait until I actually get a decent day when no one is at the marina so I can mess with it in water.
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Old 04-19-2011, 09:39 AM
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If you want to check vac there is a Port on the front of carb that's not used use that one. You can turn screws in all the way and go quarter turns out for adjustment to front first than back. I have a carb guy do mine. I know how to do it and still suck at it he jumps n and done in 10 mins. You need it to idle around 800 above a grand and you will kill your drive in short time.
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