454 wont idle below 1500 RPMs
 

1993 454 that will start up and run, but only at 1500 rpms or higher. The motor stalls at anything below that. I have tried adjusting the electric choke and mixture screws with no luck, and the float levels appear to be ok. The carburetor is a holley. Does this sound like a rich/lean problem with the carb or do I have something else going on?

Sounds like a vac leak

Would this be from a way to lean mixture? I usually associate a vacuum leak to a high idle with the throttle in the nuetral position. I need throttle to get the motor running and I cant bring my throttle to the neutral position without it stalling.

If the carb is clean inside and idle jets are not clogged, then No
It would be from things like the intake gasket, carb gasket, or other place on the intake. Take some WD40 and spray around the gaskets while motor is running. If the spray changes the motor rpm, then you found the place

I think I mispoke, I didnt mean was the vac leak caused by a lean mixture. What I meant to say is, would the vacuum leak cause a serious lean condition enough to make the motor not idle?

I will be pulling a plug tonight to get a reading and then I will check for vacuum leaks.

A vacuum leak does lean out the fuel mix.
Again if the idle circuits in the carb are clogged then there is not enough fuel being supplied so the motor idle at lower rpm. Increasing the idle speed gets it past the dile circuit (so to speak) and lets it use the main jets to get fuel

Either can cause the issue

Is this a new to you boat, or just a new issue?
Has the fuel filter contents been looked at?

This is a new to me boat. Motor supposedly has 40 hours on a rebuild, and this summer all new ignition components were put on and carb was rebuilt.
I have not checked the fuel filter yet, but I can hook up my fuel pressure gauge. In my head though if fuel pressure was an issue I would be having problems in the higher RPM range, not idle issues.

Have you tried to adjust timing, it might be way off? Or try adjusting timing to see if it can be idled down

Checking the fuel filter contents was just a thought that if it had water and crud in it, then it may be clogging the crab idle circuits. Carb being rebuilt is good if it was done correctly

Timing is in my list of things to check but I wanted to see if I could get it idling in more like the 1000 RPM range before doing that. Probably not a bad idea though to adjust it slightly and see if it makes a difference or not.

Did it do this on the sea trial ?

Though yesterday was pretty nice up in here the great north, I have not had this boat on the water and do not plan on it before it gets put away for the winter.

have you disconnected the throttle cable to see if its binding

I have not. The throttle is pretty stiff though and can be tough to get into nuetral/reverse.

Is there supposed the be idle mixture screws in both the front and back of the carburetor? I have 1 on each side in the front and none in the back.
https://cimg3.ibsrv.net/gimg/www.off...7d9ae72ea0.jpg
https://cimg7.ibsrv.net/gimg/www.off...ec8a2a8a4e.jpg

Idle is only on the front 2 barrels

BTW, those other larger screws are for float level measurement

Tried again tonight with another choke adjustment and to check for vacuum leaks. 3 pumps of the throttle and a little gas and it fired up and was actually idling around 1000 rpms. Tried to quick check timing but couldn't see the mark on the balancer..
So the I checked for vacuum leaks and nothing obvious popped out at me. But the warmer it got the more it would stall, and by the time I was done checking for vacuum leaks it was closer to 2000 rpms it had to run at without stalling

How did you check for a vacuum leak?

Remove the flame arrestor and with it running, look down the carb to see if fuel is dripping down the throat of the carb

So it doesn’t run at 1500rpm on its own, you have to hold it there to keep it from stalling?
Picture of carb looks like mixture screw is way out to me. Turn them in all the way(gently) then back out 1 and 1/4 turn. It should run there and adjust later.
I would remove flame arrest or, run engine to warm up, and when you bring the throttle slowly down to idle when it starts to die out give it a short burst of carb cleaner in the top of the carb. If you can keep it running that way you know you have a lean problem. Be careful, carb cleaner is flammable.
Did you check around carb base plate for vac leak? Does it have a PCV system? That’s another possibility for vac leak.

WD40 around the carb base gasket and all around the intake.
There is fuel going down the throat of the carb when it was running at 1500 rpms, it dies immediately when I back off the throttle so I cant get back there quick enough to see what's happening at lower rpm.

When I first got the boat the screws were like 3 turns out, first thing I did was bring them to 1 and a 1/2 turns out. This helped it start better but had little to no effect on idle.

And yes, it needs throttle to stay running. As soon as I try to bring it back to nuetral it stalls.

With throttle open you are off the idle circuit any way , but that will get give you a starting point after you find other problem.
Like Alldodge said look to see if fuel dripping out of venturies (flooding) if not try the carb cleaner to see if it’s to lean. Need a starting point for direction.

In some of my research last night I came across the idle transfer slots may have too much exposed, but the carb needs to be removed to check these. Going to look tonight for fuel dripping from the venturies and try once again to get a timing reading while it is cold and I can have it idling around 1000 rpms. If nothing comes from either of those things I think the next step is to take the front float bowl and metering block apart and see if the idle circuit has a clog. If still nothing after that, pull the carb and check transfer slots.

Grt the whole thing apart. These carbs are quite easy. Clean out all passages (don’t forget 8 air bleeds top of carb. ) with carb clean with red tube and compressed air. Rubber tip blow gun. :)

If needle and seats haven't been replaced ever or long time, good chance for those.

I’d start with a full carb rebuild and cleaning. Sounds like dirty carb issues to me. Also verify float levels. Too high and itll stall from excess fuel

Thanks guys, I did verify float levels and those were good. With the boat on level ground I could see fuel just below the sight holes.
Supposedly this carb was completely rebuilt this summer but I do not know if whoever did it knew what they were doing, and I do not know if idle issues were happening before/after the carb rebuild.

If you can't get the idle low enough to check timing you could mark the dist and just advance it to see if the idle will settle down any so you can get a light on it. Then you can put it back where marked and it will be clowe enough till you find the problem and set it correctly.
Is this a stock camshaft or performance?

I am unsure on the cam but my gut tells me its stock.

More research on lunch today and I see a lot of Holley carbs have 4 corner idle adjustments. Yesterday when I noticed that I had 2 idle adjustments in the front of the carb and not the back, I went to a chevy 350 I have with similar holley carb and it also only had 2 idle adjustment screws on the front metering block. The one on the 350 though the metering blocks in the front and back were clearly different. The Holley on this 454 the metering blocks front and back appear very similar, so maybe I do have the 4 corner idle adjustment and I need 2 more idle adjustments screws in the back of the carburetor?

I doubt that is it, but take a small wire and stick in the rear holes. I think you will find they are not drilled all the way.

Some Holley carbs have 4 corner adjustment and some only have 2. You should be able to adjust it with 2 just fine.

My 1/16" aluminum TIG rod goes in the same depth in the front passage as it does in the back.

Edit: finally was able to get my head in there to actually see the passage in the back and it is NOT drilled.

Had it idling at about 1000 rpms from a cold start and had 10-12 degrees of timing. Backed off the throttle just slightly to see if I could adjust the idle mixture screws and was able to make some change on the vacuum gauge with the throttle at that position , until rpms started to go back up. I backed off the throttle more and then it stumbled and quit.

It needed a good amount of throttle to start after that and I even had a backfire through the carb. Once running again it needed enough throttle to hold 2000 rpms, and would still run like this keeping the throttle there. As soon as you back off the throttle towards nuetral it stumbles and quits.

what ignition , distributor, coil are you running

With no vacuum leak, its a carb problem

You’re chasing your tail. Rebuild the carb. It’s cheap and doesn’t take that long. Only takes the smallest piece of crap to mess that up. Bring #1 to TDC and redrop the dizzy verifying firing order. Make your timing marks for 0°. Start from scratch and stop trusting other peoples work

If it idles down better cold than warm that would make me think it’s way rich. You should have a hose coming from each valve cover going up to breather. With engine running however fast it has to to stay running, put your thumbs over the end of those breather hoses. They should have a slight pressure build up. If they have vacuum on the hoses your intake gasket is leaking vacuum into the crank case.
Sounds like carb needs to come apart though.

Also disconnect choke and make sure it stays open while you’re doing all this

what is the list number on the carb?it is stamped on the front of the choke horn.

Thunderbolt 4 (the style that mounts to the distributor, not the manifold) distributor and coil appear to be stock mercruiser.

Only numbers I can find are on the side of the metering block.