Low speed jet adjustment

I have recently rebuilt my carbs on a 1993 70hp Johnson.. last season I could not keep the motor running at idle. It ran beautifully at anything over idle. But at idle it would start to shake for a couple seconds do a small cough and stall.
Low speed jet adjustment is what I’m concluding.
My question is, when starting the sync procedure it says seat gently.. does this mean spring compressed till you feel a bit of resistance. The spring is not a very stiff spring so it doesn’t take much to compress it.
I have checked all gas lines for cracks, they all look like new. No backflow at the fuel bulb. Everything thing else with the motor is amazingly clean. Except the plugs, they get pretty black. I run premixed 50-1. VRO was deleted before I got the motor. I only run Penzzoil XLF 2 stroke oil..

This process starts with a compression test.----Post your actual values.----There have been issues with # 2 cylinder.-----And does the telltale hose come off the exhaust manifold ( side off the block ) , or moved to the top of the block?

racerone said:

This process starts with a compression test.----Post your actual values.----There have been issues with # 2 cylinder.-----And does the telltale hose come off the exhaust manifold ( side off the block ) , or moved to the top of the block?

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compression 120-119-120 telltale hose moved to the top. just need to know if you screw the low speed jet all the way in compressing the spring then bring it out 1 1/2 turns to start with. my low speed jet comes off the side of the carb(#15&23 in pic) not in the front,so spring is on the outside of the carb.

seat the needle all the way in counting turns as you shut it down... don'T worry about the spring

remove the needle and spring and blast the internal orifice (passage way) with carb cleaner and a blast of air do it twice just in case.... wear safety glasses while doing so

reinstall the needle and spring and reseat completely... as you did in step 1
unscrew the needle with the same amount of turns you had when you seated the needle closed.

see if idle improves and adjust

Have you balanced the carbs....so all three are opening at the same time. If you have one carb opening before or after the others you wont get a nice idle .

Jmurray65 said:

compression 120-119-120 telltale hose moved to the top. just need to know if you screw the low speed jet all the way in compressing the spring then bring it out 1 1/2 turns to start with. my low speed jet comes off the side of the carb(#15&23 in pic) not in the front,so spring is on the outside of the carb.

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As you screw the needle in the spring will compress. Screw it in only as far as necessary to very gently seat the needle. Then unscrew it 1½ turns.
If you screw it in too firmly you risk jamming the tip in the orifice and then breaking it off leading to a much bigger problem.

Have you removed the idle needles to see if they are damaged /

If the plugs are black then your running rich....could be weak spark, have you done a gap check for the spark ? Needs to be able to jump a 3/16" gap ! All carbs need to be synced together before making adjustments. Owned one of those motors, once properly setup and adjusted, it idled all day at 600 rpm and ran like a scared rabbit...great motor !!

Pilot screw settings can be tricky on these Mercs. You don’t always get instant results after making adjustments. It can take a few seconds for fuel burn to have an effect. Cleaning up whats in the crankcase.

Make an adjustment then wait a bit to see what effect it has. Rev the throttle up a bit once in a while to clean out the crankcase and cylinders. Let it idle down again. Where idle lands, you’ll likely hear the effects of the pilot screw adjustment that you made. This how I do it on my small twin-engine outboards, with one carb. Having 3 carbs to try and adjust must be challenging indeed.

Apologies for my post above. I thought I was in the Mercury thread here.

airshot said:

If the plugs are black then your running rich....could be weak spark, have you done a gap check for the spark ? Needs to be able to jump a 3/16" gap ! All carbs need to be synced together before making adjustments. Owned one of those motors, once properly setup and adjusted, it idled all day at 600 rpm and ran like a scared rabbit...great motor !!

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I checked the choke at full throttle and it wasn't open all the way. so I adjusted that now all are perfect open and closed. Starts up much nicer, have to take it to the lake to adjust the jet settings now.