4.3L w/ holley 600, help with tuning.

[garrett113]

hey guys and gals,

I have a 1990 omc cobra with a 4.3l. I've been rebuilding the boat for three years now,everything from transom, stringer, deck, engine mounts, engine, carb, and paint.I bought a re-built long block and swapped everything over. The carb was rebuilt by a mechanic friend of mine and the rebuild kit was for the proper application as I checked the part number. I did not convert the engine to fit the holley 4 barrel it was already that way when I bought the boat. But i have seen that people do this conversion and I also noticed that the high output version comes with a holley 4 barrel. I am not sure if there are any other differences besides the ignition system and the carb but anyway.

So after messing around for quite some time I have finally gotten the thing to idle but it's fairly rough.
my timing is set to 6 degrees before top dead center at about 900-100 rpm with 89 octane (this was spec from the manual )
my fuel pressure was hard to read because my gauge goes well over 100 psi but it seemed to be around 5 or so (fuel pump is new as well as filter)
I set my idle mixture screw 3/4 turns out. this is the spec for the h/o 4.3 with 4brl.
i had to take off the spring for the idle speed screw in order to get it in far enough to idle at 900-100 rpm
it is in so far that is has started to press the accelerator pump.
Breaker points ignition with good spark using spark tester
choke fully open

Adjusting the idle mixture did not seem to have much of an affect on the idle but I plan to test that more tomorrow.

Engine is running rough, I already know it is because of a rich air fuel mixture. Smell of gas, carbon suet on plugs, plugs smell like gas. I was going to cut the spring on the accelerator pump screw so that I could adjust it a bit more, which i plan to do tomorrow.

I feel like I am missing something. Any of you running a 4.3 with holley 600?
I'm concerned that the carb could have not been rebuilt properly, but he is a pretty well qualified tech for Lexus so I'm going to assume for now it is right.Do you guys have any advice for what I should check?

Thank you in advance for any help you can offer.

-Garrett

 

[jerryjerry05]

Don't cut the spring.
​Sounds like it's sucking air or the springs in the dist. are stuck??
Try spraying starting fluid around the base of the carb and the intake manifold.
Not all rebuilds are successful????

You rebuild the distributor?
Take it apart and check the inside for rust??

 

[Scott Danforth]

going to start with the ignition first. did you set the dwell correctly?

you cant go with the specs in the manual anymore unless the carb is the exact same carb that OMC used

first off, 900-1000 is way too high. if it wont idle below that, there is still dirt in the idle passages. you need to run a wire into each passage to make sure its clear.

next, if your carb is a 1990 vintage and you have had a few backfires, you could have ruptured your power valve. later carbs (after 1992) have the check valve to prevent this. you should either have a #25 or a #35 power valve. the wrong power valve and the motor will stumble off idle. a ruptured power valve and it will be running really rich.

what are you running for primary jets? secondary?

no need to cut the spring on the acceleration pump. that statement right there is a red flag that you do not understand a holley carburetor or how to tune one.

here is some reading https://www.amazon.com/Holly-Carbure.../dp/0895860473 and https://www.amazon.com/Holley-Carbur.../dp/1884089283

 

[alldodge]

The carb was rebuilt by a mechanic friend

had to take off the spring for the idle speed screw in order to get it in far enough to idle at 900-100 rpm

Engine is running rough, I already know it is because of a rich air fuel mixture. Smell of gas, carbon suet on plugs, plugs smell like gas.

No offence to your friend but your flooding

 

[Svshaw82]

I agree, sounds like flooding. The power valve is another thing to check as stated. Check those things and be sure to check the jet sizes. The pump is only there for when you throttle the engine. That shouldn't effect the idle.

I'm interested in how your engine is running after its good to go. I have a 89 OMC with the cast iron intake and rochester 2 barrel. I just rebuilt my carb, but I'm still interested in upgrading to a 4 barrel.let us know your findings.

 

[jimmbo]

Have you ever hooked up a Vacuum gauge to the engine?
Has this setup always run crappy, or only since the rebuilt block was installed? If it has never run properly you may have something like a crankshaft-camshaft timing error(timing chain off by a tooth, or a poorly made camshaft sprocket). But that's getting a little ahead a this point.

You say you had to screw the idle speed screw almost all the way in to get the engine to idle? Unless the screw and/or the its stop have been modified, that is opening the throttle way too much, upsetting the idle/off idle calibration. But if the carb is dumping too much fuel it wouldn't get enough air at normal settings. Check the floats for proper setting. A lot of Holley floats are hollow brass, check to see if they have fuel in them. As mentioned before the power valve diaphragm can crack/rupture. Where did you get this 3/4 turns out for the mixture screws? The only Holley carbs that speced an idle mix screw setting were the 'Reverse Idle' emission carbs, which are for cars and trucks not boats? Which then brings me to the next question. Is that a Marine Carb, or did the previous owner install a 'more affordable' auto carb?

 

[garrett113]

Hey guys I'm back with some more info. So after removing float bowls I found out that there is no power valve. The metering block is casted off where one would be, so we have ruled that out. The float level is adjusted properly to spec. I called Holley and they told me that when turned upside down the floats should be parallel with case, they are. With floats bowls off I ran some fuel line to the inlet then tried blowing in it as hard as I could and the valve seem to be seating based on that test. We then ran the carb off the engine with fuel pump line connect and no leaks occured.so we put carb on and ran it, it ran the same as before. I forgot to mention I cleaned the valve and valve seats in the fuel bowl which are new. We noticed a HUGE amount of fuel coming from venturies. I don't have any previous knowledge to base this on but at one point while ruling out fuel pressure we did a gravity feed setup and went through a small water bottle in a matter of minutes. I have checked the jets previously and confirmed they are the correct size for application. We tried everything from turning idle mixture in all the way to out 2.5 turns and still performance remains nearly the same. We have to open the throttle enough to give it enough air to comepensate for the fuel it is getting. The carb is definitely giving to much fuel! I'm thinking either the idle circuit is not working properly or well maybe something else that I can't comprehend. I have had a few friends come and take a look and at this point we are all equally baffled.

 

[garrett113]

I have pulled the carb back off and tomorrow I will bench tests it in yet another way to confirm it is not floats or valve seats by filling it with gas with some fuel line attached then run my compressor at 6 psi which is within spec for pressure if sealed I will run it with the fuel pump attached yet again to confirm it is not any one of those things

 

[alldodge]

We noticed a HUGE amount of fuel coming from venturi

With this happening the float level is not set correctly, the needle valve/seat is not shutting off the fuel or the float is sinking.

 

[jimmbo]

Hey guys I'm back with some more info. So after removing float bowls I found out that there is no power valve. The metering block is casted off where one would be, so we have ruled that out. The float level is adjusted properly to spec. I called Holley and they told me that when turned upside down the floats should be parallel with case, they are. With floats bowls off I ran some fuel line to the inlet then tried blowing in it as hard as I could and the valve seem to be seating based on that test. We then ran the carb off the engine with fuel pump line connect and no leaks occured.so we put carb on and ran it, it ran the same as before. I forgot to mention I cleaned the valve and valve seats in the fuel bowl which are new. We noticed a HUGE amount of fuel coming from venturies. I don't have any previous knowledge to base this on but at one point while ruling out fuel pressure we did a gravity feed setup and went through a small water bottle in a matter of minutes. I have checked the jets previously and confirmed they are the correct size for application. We tried everything from turning idle mixture in all the way to out 2.5 turns and still performance remains nearly the same. We have to open the throttle enough to give it enough air to comepensate for the fuel it is getting. The carb is definitely giving to much fuel! I'm thinking either the idle circuit is not working properly or well maybe something else that I can't comprehend. I have had a few friends come and take a look and at this point we are all equally baffled.

This the 4bbl? If so, there should be a power valve on the primary metering block. You most likely have a 4160 carb which has no secondary metering block but a metering plate containing the main jets and idle jets for the secondary side. Which venturis are dumping fuel, primaries, secondaries, or both?
Pictures of your carb, especially the metering block where you say you do not have a power valve.

 

[garrett113]

I will have to wait until I get home to send pics but yes it is a 4160 with a metering block on primary side and a metering plate on secondary. Fuel is only spraying out of primary side as it should be since it's not under any load. I took another look at the carb last night and noticed a puddle of fuel on the work bench. I figured out that fuel is leaking through a small hole in the base plate next to one of the screws. The whole it is dripping from is the vacume port for the power valve. So I pulled the metering block off and noticed some of the holes on the carb body are mushroomed out.aka not flat. The ting that confuses me is I could understand how this could happen to the holes where the mounting bolts are but there are two port holes that mushroomed out a bit. Might be hard to visualize I'll post pics tonight. Looking at the gasket though it looks to confirm that it's not getting sealed all the way around the power valve area.