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Mercruiser 4.3 V6 engine idling fast

D

dash48

Member
I just rebuilt my 4.3 V6 engine I have been building motors for several years and for some reason I cannot adjust the idle it's idling at about 2,000 RPM top dead center the idle screw on the carburetor is already turned out there are no Jets to adjust on the carburetor the engine is on a engine stand so I can run it from there. I have adjusted the valve lash on three different occasions and thinking that maybe my valves we're still staying slightly open I am open to any suggestions on why the RPMs are still advanced I have done everything possible including set the timing with the advanced timing gun and the engine still idles faster than recommended. I'm 67 years old don't want to put it in the boat hook all the wires and cables fuel line and drive Stern without making sure the engine is 100% ready to be installed. The only thing I can figure is the value adjustment is still not correct
 
dash48 said:
I just rebuilt my 4.3 V6 engine I have been building motors for several years and for some reason I cannot adjust the idle it's idling at about 2,000 RPM top dead center the idle screw on the carburetor is already turned out there are no Jets to adjust on the carburetor the engine is on a engine stand so I can run it from there. I have adjusted the valve lash on three different occasions and thinking that maybe my valves we're still staying slightly open I am open to any suggestions on why the RPMs are still advanced I have done everything possible including set the timing with the advanced timing gun and the engine still idles faster than recommended. I'm 67 years old don't want to put it in the boat hook all the wires and cables fuel line and drive Stern without making sure the engine is 100% ready to be installed. The only thing I can figure is the value adjustment is still not correct
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would look for a vacuum leak on carb base or intake manifold. There should be idle mixture screw(s) to adjust the fuel mix at idle as well as the idle speed screw.

Are you sure the throttle plates are returning to full closed? If speed screw is backed out and throttle blades are fully shut you are getting air in from somewhere else otherwise it wouldn't run.
 
Thanks for your suggestions I rebuilt the carburetor and it's got new gaskets but I'll check for any kind of vacuum leak the intake manifold was put on and I don't detect any type of air leak I've looked all over the carburetor for mixture screws and I cannot find them it's a two barrel but I would have to look again to see what kind of carburetor is on it I'm surprised also that I can't find the mixture screws it runs great after the rebuilt just idles too fast I will check the throttle blades again tomorrow to make sure they close completely
 
dash48 said:
Thanks for your suggestions I rebuilt the carburetor and it's got new gaskets but I'll check for any kind of vacuum leak the intake manifold was put on and I don't detect any type of air leak I've looked all over the carburetor for mixture screws and I cannot find them it's a two barrel but I would have to look again to see what kind of carburetor is on it I'm surprised also that I can't find the mixture screws it runs great after the rebuilt just idles too fast I will check the throttle blades again tomorrow to make sure they close completely
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check the intake to carb gasket is the correct one. Often they put multiple gaskets in the kit. Last mercarb rebuild I did wrong base gasket was used didnt idle for chit... Can spray starter fluid at base of carb/gasket surfaces it will change the idle if you have a leak

You will for sure have idle mix screws. I'm assuming it is a mercarb or rochester 2 jet, which are very similar carbs. They will be down on the throttle plate for the carb on front of engine. Mercarb has a single mixture screw, Rochester will have two. Post a pic of front of carb. These would have needed to be removed to clean for a rebuild.

On the off chance it was replaced with a holley they will be on sides of metering plate down low.
 
Okay I took a quick look at the carburetor earlier today and I didn't see any idle screws but I will definitely look again with a strong flashlight I am 100% sure that it's the right gasket for the carburetor everything bolted right up four screw bolts on each corner. Facing the front of engine on the left side is a mechanical vacuum Valve it's definitely a two barrel it came with the motor when I got it with the boat I'll try to find a name I am familiar with Rochester's I used to run them on dirt track cars back in the day pretty sure it's not a Rochester I'll try to post a pic of the carburetor tomorrow I have a YouTube channel old school mechanic James you can see the complete rebuild of the engine I put in new main bearings new cam bearings Pistons rods crankshaft cam it's probably my seventh build mostly I've done V8 I think you're on target with a possible air leak or Idle mixture srews
 
dash48 said:
Okay I took a quick look at the carburetor earlier today and I didn't see any idle screws but I will definitely look again with a strong flashlight I am 100% sure that it's the right gasket for the carburetor everything bolted right up four screw bolts on each corner. Facing the front of engine on the left side is a mechanical vacuum Valve it's definitely a two barrel it came with the motor when I got it with the boat I'll try to find a name I am familiar with Rochester's I used to run them on dirt track cars back in the day pretty sure it's not a Rochester I'll try to post a pic of the carburetor tomorrow I have a YouTube channel old school mechanic James you can see the complete rebuild of the engine I put in new main bearings new cam bearings Pistons rods crankshaft cam it's probably my seventh build mostly I've done V8 I think you're on target with a possible air leak or Idle mixture srews
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Does it look like this

Holley 0-80402-2 Holley Marine Carburetor - Aluminum

All Holley Marine Carburetors are designed to meet Coast Guard specifications. A marine carburetors fuel metering system is calibrated to compensate for unique engine loads found on boats. The J-style vent tubes direct fuel/fumes back into the carburetor for safe operation while on rough...
www.holley.com www.holley.com
 
No it looks nothing like that and nowhere on it doesn't say Holly I ran 4412 on my race cars it's raining right now here in Houston Texas I'll check it out tomorrow I really appreciate your help I'm hoping to get this resolved so I can get this thing engine back in the boat before I get too old LOL
 
Okay I finally found the mixture screws you were correct they're on the front and there's two of them it's a quadrajet I'm not sure if it's a Rochester I believe and it looks like a four-barrel it's got two large blades and two smaller blades they close all the way but I did find the screws in the front here are some pictures well I tried to post some pictures not sure where they were it's my first day on this forum one of the pictures that said quadrajet on the carburetor was declined the server said it was too large it was just a photo
 
initial timing would probably be better @ 8 deg BTDC - depends upon the specific model you have.

quadrajets are great carbs...but can take a while to optimize if you need to go down that path...Check to make sure that vacuum hoses are not split and fully seated (choke pull off)...also, they usually work better with 'thicker' base gaskets...especially if their throttle plate has warped from time and heat. Carb cleaner sprayed in the area will work just like Dieter suggested...

finally, connecting a vacuum gauge to the intake with the engine running can give you additional clues...
 
dash48 said:
Okay I finally found the mixture screws you were correct they're on the front and there's two of them it's a quadrajet I'm not sure if it's a Rochester I believe and it looks like a four-barrel it's got two large blades and two smaller blades they close all the way but I did find the screws in the front here are some pictures well I tried to post some pictures not sure where they were it's my first day on this forum one of the pictures that said quadrajet on the carburetor was declined the server said it was too large it was just a photo
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the photo was just to help ID what carb you have. The mix screws are kind of hidden on a Q jet... They should have been removed during the cleaning. When it is running if you screw the mix screws in or out does it change idle speed and or quality. Normally 1.5-2.0 turns out is a good starting point. If they have to be way out to idle that is indicative of a vacuum leak

Q jet is as good as any carb ever made. Spray for leaks
 
Will do thanks Dieter and makonark we'll turn the screws in all the way and then back them out one and a half to two turns and start the motor and see if there's a difference I will also set the timing to 8° below top dead center I will double check the vacuum line I do have a vacuum gauge where would I connect it on the intake I haven't looked to see if there is a outlet or do you recommend coming off of the carb lastly I will spray starter fluid around the base of the carburetor to see if there's any air leaks appreciate all your help I've been fighting this battle for 3 months now
 
There should be NO vacuum port on a Rochester 4bbl Marine Carb. If there are it is an automotive carb.

Timing, Its 8* BEFORE top dead center. Not Below....lol

If there is a barbed fitting at or near the choke housing at top of carb or possibly at or near the bowl. This is for a plastic yellow hose which connects to the mechanical fuel pump. Marine mechanical fuel pumps have a barbed fitting where an auto pump is just a hole for fuel to leak if diaphram ruptures. Marine pumps, the fuel is diverted to the carb.
 
Also, the right gasket for the QJet is that thick one, not the thin ones you see sometimes, make sure to use the right gasket. The QJet is a fairly complex carb and it helps to watch videos on the rebuild process before you start pulling it apart. That said, if it is an original un-molested Marine unit, in good shape inside with no corrosion, it is an excellent carb. I had mine apart twice before I had it the way I wanted it. Now the engine starts in a half turn of the crank.
 
If there is a vacuum leak OR if the throttle valves are open too much, the idle mixture screws won't have any effect....
 
kghost said:
There should be NO vacuum port on a Rochester 4bbl Marine Carb. If there are it is an automotive carb.

Timing, Its 8* BEFORE top dead center. Not Below....lol

If there is a barbed fitting at or near the choke housing at top of carb or possibly at or near the bowl. This is for a plastic yellow hose which connects to the mechanical fuel pump. Marine mechanical fuel pumps have a barbed fitting where an auto pump is just a hole for fuel to leak if diaphram ruptures. Marine pumps, the fuel is diverted to the carb.
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Thanks kghost it's a quadrant quarter barrel yes 8° before top dead center that was a voice recorder error. It has a barbed fitting and yes there is a rubber hose connected and it is diverted back into the carb
 
Well guys here's the update and I appreciate everyone's help I put the engine outside and checked where the mixture screws were it was impossible to get to it with any kind ordinary tool I'm sure they have a special tool that has an elbow to it to reach down into that closed spot so I pulled the carburetor and I was told by the previous owner the carburetor was rebuilt but obviously it wasn't the mixture screws I had to spray with WD-40 just to loosen them up I turned them in and out turned them out one and a half turns out if anyone recommend more like they should go out two terms let me know. I put the carburetor on a second time without checking the gaskets they looked to be okay and I'll explain why I said gaskets. I fired the engine back up set the timing again to eight degrees before top dead center ran 2000 RPMs again however sounded like a brand new motor. Now here's where I discovered air was getting in through the gaskets when I took dirtier suggestion and sprayed starter fluid around the base and sure enough it lower the RPM. Took the carburetor back off check the gaskets and to my amaze there was actually two, one that goes with the quadrajet and then another one that was just a square four barrel gasket took them off cleaned the surface area for a new gasket and that's where I'm at right now. Anyone have a part number or recommend what carburetor gasket I should purchase for my quadrajet 4 barrel I'm thinking that is going to solve the problem
 
dash48 said:
Well guys here's the update and I appreciate everyone's help I put the engine outside and checked where the mixture screws were it was impossible to get to it with any kind ordinary tool I'm sure they have a special tool that has an elbow to it to reach down into that closed spot so I pulled the carburetor and I was told by the previous owner the carburetor was rebuilt but obviously it wasn't the mixture screws I had to spray with WD-40 just to loosen them up I turned them in and out turned them out one and a half turns out if anyone recommend more like they should go out two terms let me know. I put the carburetor on a second time without checking the gaskets they looked to be okay and I'll explain why I said gaskets. I fired the engine back up set the timing again to eight degrees before top dead center ran 2000 RPMs again however sounded like a brand new motor. Now here's where I discovered air was getting in through the gaskets when I took dirtier suggestion and sprayed starter fluid around the base and sure enough it lower the RPM. Took the carburetor back off check the gaskets and to my amaze there was actually two, one that goes with the quadrajet and then another one that was just a square four barrel gasket took them off cleaned the surface area for a new gasket and that's where I'm at right now. Anyone have a part number or recommend what carburetor gasket I should purchase for my quadrajet 4 barrel I'm thinking that is going to solve the problem
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I like it when its the simple ****. Kghost has you on right path for flex drive tool for screws. if they were frozen you probably need to rebuild the carb. Good thing is Q-jet is a great carb Cliffs high perf is the go to site for parts. Thick base gasket is below


Cliffs rebuild kit if you need it

 
kghost said:
Here is the tool you need see link below. I have this one and a snap on expensive mechanical tool.

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Thanks kg just ordered it $12.84 should arrive on Monday really appreciate you guys help I was a blind man until I found this forum I called several Builders and they couldn't help me
 
Cliffs is great for parts and he has a forum where you can ask questions about rebuilds and tuning problems. With the QJet getting the idle air tubes clean is important for getting a smooth idle. I would usually set my idle needles at 3-3.5 turns out. That flex tool with the attachments is a must for setting the idle mixture.
One last thing is if you have to use E10 gas Cliffs sells accelerator pumps that are made to stand up to ethanol unlike the standard ones which will swell and leak after a while….
 
louc said:
Cliffs is great for parts and he has a forum where you can ask questions about rebuilds and tuning problems. With the QJet getting the idle air tubes clean is important for getting a smooth idle. I would usually set my idle needles at 3-3.5 turns out. That flex tool with the attachments is a must for setting the idle mixture.
One last thing is if you have to use E10 gas Cliffs sells accelerator pumps that are made to stand up to ethanol unlike the standard ones which will swell and leak after a while….
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Thanks I'll check into it I got my gasket in waiting on my tool on Monday slap this thing back together and see what happened I'm hoping the gasket will fix the problem and then I'll set the mixture screws and readjust the timing thanks for everyone's help
 
Hey guys great news put on the new gasket and the engine purred like a kitten I set the RPMs at 900 anyone else recommend different? I'll wait on my tool to adjust my air mixture and then check the timing quick question on the timing should I have the RPMs at 2000 with 8°BTDC
 
If you have an Alpha drive, Idle speed MUST be 650-750 rpm in gear under load. (In the water)

700-750 out of the water is a start.

Anything higher and you will break gears in outdrive
 
on the initial timing, you want the 8 deg BTDC at 700 RPM.

FWIW, manual #07 recommended doing the idle mixture adjustment in gear and at 550-600 RPM...I think the manual has a typo when they refer to neutral...
 
Its funny you say that,
I was going to suggest to set idle mixture screws at a idle rpm of 600.

If all is good, setting idle mixture at a low rpm typically works very well. If it can be done at that low rpm, it is a, One and Done!!
 
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