Carburator Issues

[maresca]

I am the first owner of a 1988 350 King Cobra 260 hp with 500 hours on it. Before first use the engine was stored for several years. After its first start I have never been able to get the engine to the operating range 4200-4600 rpm. I have tried several propellers 15X16 pitch getting 4000 RPM and 15X17 getting 3800 rpm but 2 more mph, if I reduce pitch even more like 15X15 I get a slight increase in RPM but loosing many MPH. The engine and drive are working fine, but the iddle has always been a bit unstable, specially when cold and I have almost closed the iddle needles to reduce raw unburned fuel, but it seems the way my carb is, these needles dont provide much chance of adjustment to correct the real situation. Because of the iddling condition I took the carb to a shop and they installed a new KIT (probably automotive), but the was no change or improvement. After reading the different coments on this forum Im starting to think that both the iddle and the RPM issues have to do with the righ adjustement and the right components, probably I have to change the power valve spring, metering rods and jets or a combination. I need some advice to verify if my carburator has the right pieces and if purchasing an original carb kit for this specific engine will include those parts or do they have to be purchased separatelly.
Thanks for the advice!

[stonebreaker]

You may also have a valve spring problem. Has the motor ever been over-revved? Does the sound of the engine change when you get to your new max rpm?

[maresca]

Hello stonebreaker
So far the max RPM I can get on this engine are 4000. The engine has never been over-revved. The engine has a rochester quadrajet 4 barrel carburator. I can go up to about 3500 rpm 32-33 MPH on 2 barrels, then going full throtle there is a change in sound and I will get 500 aditional RPM to 4000 and 6-7 aditional MPH up to 39-40. As you can see not very much gain at full throtle.
Im wondering if my carburator has the right metering rods, jets and power valve spring for my engine, but im not able to verify this in the engine parts manual since this detail is not indicated. On the other hand I might have the wrong power valve spring and/or carburator adjustement as you mentioned.

[alex fletcher]

Hi Maresca
Have you checked the basics Like Timing, Compression, Condition of Exhaust Manifolds, Riser, Elbows and Spark condition.
More info would be nice like the size and type of boat?

[stonebreaker]

When you said the shop installed a kit, do you mean they rebuilt the carb?

[Frosty]

The different noise you mention is the back barrels opening. Are you trimming the boat.

[maresca]

Hi Maresca
Have you checked the basics Like Timing, Compression, Condition of Exhaust Manifolds, Riser, Elbows and Spark condition.
More info would be nice like the size and type of boat?

Alex, thanks for the input
I have had this rpm issue since the engine was brand new. Over the last 500 hours I have on the engine now I have done preventive maintenance, timing was checked, spark condition is fine, plugs tend to become dark probably due to a rich air-fuel mixture, with 300 hours I changed plug wires and switched from points to a petronix electronic ignition module that fits in the distributor instead of the points, this change did not modify engine condition I just did away with the points ignition. Exhaust risers and manifold are still ok but Im planning soon to change manifolds and risers because they are getting close to their usefull life. The engine is installed on a 24 feet cigarrette sportboat (deep vee) Basically I think that Im not getting the maximun full throtle performance for that engine and that the iddle speed is not even

[maresca]

When you said the shop installed a kit, do you mean they rebuilt the carb?

Hello stonebreaker
Thats correct, I took the carb to a shop to have it checked to see If I would get any improvement, they probably installed new gaskets and cleaned the carb, I dont know exactly what parts were changed and which parts are included in the maintenance kit, but after this job was done there was no change in engine performance.

[maresca]

The different noise you mention is the back barrels opening. Are you trimming the boat.

Hello jack
Thats correct its the transition noise from 2 barrels to 4 barrels when you go full throtle. Im trimming the boat to maximize performance

[stonebreaker]

Have you looked at your ignition system at all? Coil, ignition module, distributor, coil and plug wires? Try running the engine at night and look for arcing. Try spraying water on the plug and coil wires with a spray bottle and see if you see any arcing. Have you had the timing adjusted on the motor?

You might also try replacing your fuel filter and check fuel pressure at both idle and WOT.

[Frosty]

Do you have advance wieghts in the distributor. As the boat has stood for a long time they might be stuck wich will result in no advance at all.

Will the engine free rev over 4000. I dont mean just open the throttle like you can a diesel but gently rev it up and over 4000. I am thinking there might be a rev limiting rotor arm that is not working correctly.

Are you sure the cable is getting full throttle on the carb?
Are you sure the tachometer is not sticking at 4000?
Slowly take it up to 4000 and then give it full throttle (if it will go more) listen- if the revs go up a bit but the tach does not then its sticking

[alex fletcher]

it a cigerette your porblem is stainless steel maniflod because SS is vry hard to manufacture SS mainfolds have a tendency to leak water into the the gas path of the minifold This requires a Very educate techincan to diagnose it, in our expireance it has taken at least 2 Runs in the vessel to identyify the problem it has been a problem with a fews of the in this Australia and their is not many here a good presurue Stainless welder and presure testing of the Exhaust Maniflod will fix it.
trust me Alex

[maresca]

Have you looked at your ignition system at all? Coil, ignition module, distributor, coil and plug wires? Try running the engine at night and look for arcing. Try spraying water on the plug and coil wires with a spray bottle and see if you see any arcing. Have you had the timing adjusted on the motor?

You might also try replacing your fuel filter and check fuel pressure at both idle and WOT.

hello stonebreaker
Yes, changed coil, went from points to electronic with no change, new plugs every 150 hours, new wires, no arcing, timing was adjusted, changed fuel filter both main and carb, but havent checked fuel pressure (I dont have the tool) since new I installed a antisiphon valve on the fuel tank outlet with the recomended size

[maresca]

Do you have advance wieghts in the distributor. As the boat has stood for a long time they might be stuck wich will result in no advance at all.

Will the engine free rev over 4000. I dont mean just open the throttle like you can a diesel but gently rev it up and over 4000. I am thinking there might be a rev limiting rotor arm that is not working correctly.

Are you sure the cable is getting full throttle on the carb?
Are you sure the tachometer is not sticking at 4000?
Slowly take it up to 4000 and then give it full throttle (if it will go more) listen- if the revs go up a bit but the tach does not then its sticking

Hello Jack
My distributor has weights and they are not stucked, by free rev you mean reving at neutral to 4000 rpm??? I dont think this engine has a rev limiting rotor, but it does have a electronic rev module. When I switched from points to electronic ignition the kit for this engine model included a new rev module that was installed but performance was not affected. Cable is getting full throttle on the carb. The engine will go smothly from iddle to about 4000 rpm at full throttle and the rev go up without stopping on the tach

[maresca]

it a cigerette your porblem is stainless steel maniflod because SS is vry hard to manufacture SS mainfolds have a tendency to leak water into the the gas path of the minifold This requires a Very educate techincan to diagnose it, in our expireance it has taken at least 2 Runs in the vessel to identyify the problem it has been a problem with a fews of the in this Australia and their is not many here a good presurue Stainless welder and presure testing of the Exhaust Maniflod will fix it.
trust me Alex

Hello Alex
this engine has standart cast iron manifolds not stainless, those manifolds have now 500 hours (about 6 years of use) on them, I already purchased new ones and will change soon. But actual performace has always been the same since day one, I havent lost performance but I think I can gain some

[alex fletcher]

Have Tryed disconecting the Tacho and then running over a known course and see if you get a speed increase on you log? Have you tryed checking The revs of the Motor with a workshop Tacho?

[Frosty]

I am just about getting to the end of any more ideas here. If it were my boat I would rev the balls of it and be shure that it will and wants to rev over 4000 It may be that it just bogs down at 4000. I have seen this on cars where they wont rev without the air filters on. Has it got air filters on? Is it mechanical secondaries or vacuum. can you look down the carb at 4000rpm make sure the secondaries are wide open and that the choke is not closing.
If its vacumm check the pipe. I remember some how converting vacuum into mechanical by using a small nut and bolt in the arm operating slide.

I am sure I do not need to point out that working on an engine at full throttle is dangerous beyond belief.

[stonebreaker]

Test a couple of valve springs. I know you said the engine has never been over-revved, but if nothing else works out, test the valve springs. If they're weak, they could be holding the engine back.