1700 rpm max on Merc 350 mag??

[ironeel]

I have a 87 scarab with a built 350 mag, by built I mean high compression pistons, performance heads (2.02/1.60), RPM intake manifold, 800cfm Holley carb, kodiak exh manifolds and clay smith cam.

I got a good deal on the boat and had it all gone through. New head gaskets because of leak, new lifters and push tubes, had the heads checked and redone.

I had the timing set to 8 degree BTDC and the motor ran great out of the water with muffs. Checked compression and have 170-190psi cylinder pressure.

I put the boat in the water and it won't run over 1700 RPM? Acceleration is smooth up to that point but won't go any further?

1700 Rpm comes at quarter throttle and I can continue to give it more gas but it won't increase RPMs.

Boat also had a full tune up with new cap, rotor, wires, plugs, and ignition sensor inside the cap.

Any suggestion would be appreciated!!

 

[Uraijit]

Re: 1700 rpm max on Merc 350 mag??

How's your fuel mixture? Have you done a plug reading?

 

[John_S]

Re: 1700 rpm max on Merc 350 mag??

How big and heavy of a boat?

What drive, ratio, and prop specs?

Did it go above this before the "clean-up" work?

 

[MikDee]

Re: 1700 rpm max on Merc 350 mag??

Welcome to the forum!,,, Wow!, This engine was all gone over. I'd say a fuel delivery issue, but then I'd think it would die completely? A couple items to check, is your 4bbl opening completely? sometimes the choke linkage gets hung up, & prevents that, even though the carb linkage goes through it's range. One more thing, are you sure you don't have a crossed spark plug wire? I've seen these engines appear to run good with adjacent wires crossed. Finally, is your spark advance working? Off the top of my head, without being there, these are all I can think of now.

 

[hard-3]

Re: 1700 rpm max on Merc 350 mag??

check the throttle cable for correct travel. if it's not adjusted correct it may not be going to wot

 

[ironeel]

Re: 1700 rpm max on Merc 350 mag??

Thank you for all the valuable information!!

The boat is a 3100 pound boat with a Alpha (1.47 ratio) drive. The prop is a ballistic 23 pitch 3 blade stainless prop. I have checked the throttle linkage and it is working properly. I didn't have a chance to run the boat before the engine came apart because I bought it with several engine problems, such as a bent valve on the #8 cylinder and leaking head gasket. The engine came apart as soon as I got it home. Previous owner stated before the problems the 21 ft scarab would run 62 MPH with the current set up.

The boat with muffs on it out of the water will run RPM and the vacum secondaries on the Holley open. I can see fuel going into the intake. The timing is set to 8 degrees BTDC at 650 RPM's? I checked plug wires and they are correct. also checked gapping on the plugs and it is set to the recommendations on one of the valve covers (.035) I'm not sure where the timing should advance to but that will be the next thing I check.

Does anyone know how far it is suppost to advance?

Would the engine still run RPM out of the water if the timing wasn't advancing??

 

[MikDee]

Re: 1700 rpm max on Merc 350 mag??

With those specs, and that prop, I'd say it should hit 60+mph. One important item on a boat is the antisphon valve, & fuel filter on the fuel tank, it is a little spring loaded check valve in the fuel line, All boats have them, (Coast Guard regs) often they get clogged and block the fuel flow after a few years, and create havoc. If you haven't physically taken it off, & checked it out, now maybe the time.

By the way, a free running engine will usually do any rpm you want, even if there are issues with it, and it's not a good practice to test it this way.

It could very well be a spark advance problem? What ignition system is in it? The stock Mercury Thunderbolt, or an aftermarket?

 

[ironeel]

Re: 1700 rpm max on Merc 350 mag??

Thanks for the response Mikdee,

I had no idea about the fuel valve but I will check that out. I assume that it will be on the fuel tank? The ignition is stock Thunderbolt 4 ignition. I have a msd marine box for it but I want to get everything figured out before I start adding things to the boat. I just bought a timing light to check the advance.

Thanks for the info!

I'll post again once I check ignition and the fuel valve.

 

[ironeel]

Re: 1700 rpm max on Merc 350 mag??

OK, so the timing is good. It is set to the camshaft manufacturers spec which is 12 degrees BTDC. The ignition is advancing as I give the engine gas. I can see the ignition advancing as I hold a timing light on it while working the throttle. All plug wires are correctly routed, plugs are gapped correctly and I have fuel at idle. I think the next thing to do is replace the anti syphon valve? I pulled the old one out and replaced all fuel line associated while I was at at it. Might as well while I'm in there. I have a valve on order from a marina.

I had the boat out this afternoon and it is still only getting 1700-1900 RPM before dying out. If I back off the throttle a bit the engine will stay running but it must be leaning out when the secondaries start to open??

If anyone has other ideas I would appreciate it.

Thank you!!

 

[MikDee]

Re: 1700 rpm max on Merc 350 mag??

It just ocurred to me, maybe the power valve in the carb is not working right?
Do you know what I mean? Not the accelerator pump, although I'd check that also. But I heard, (and seem to remember) these power valves are a diaphram type, and were tricky to deal with on Holley carbs, either feeding too much gas, or not enough. I would go over that carb real good, I've never had much luck getting a Holley to work right!

 

[hondon]

Re: 1700 rpm max on Merc 350 mag??

With all that engine work and add ons have you considered misadjusted valves?

 

[ironeel]

Re: 1700 rpm max on Merc 350 mag??

Figured it out guys!! The ignition amplifier for the thunderbolt 4 ignition was shot! No ignition advance!! Unfortunitely Merc no longer makes the original style amplifier and the replacement requires a harness that is no longer made. I guess i am buying an aftermarket ignition set up?

Thanks again for all the great ideas!!

 

[MikDee]

Re: 1700 rpm max on Merc 350 mag??

Wow, Ok, Glad you got it worked out, but you did say you saw it advancing? so I'm confused But Whatever works, Thanks for the follow up. Mike

 

[ironeel]

Re: 1700 rpm max on Merc 350 mag??

Mikdee,

It was advancing off and on. After I took the thunderbolt box out of the boat and cleaned all my connections I placed it back in the boat and it was working with my muffs hooked up but I took the boat out and while attempting to run it I again had the same problem. I took my timing light with me and found that it would not advance all the time. I have since replaced needed parts and have not had a problem.
Sorry for the confussion and thanks for the help!!

 

[MikDee]

Re: 1700 rpm max on Merc 350 mag??

Glad to see it working for you! Now tell us how fast that sleek Bad Boy is?

 

[ironeel]

Re: 1700 rpm max on Merc 350 mag??

I got 66mph on the speedo with two in the boat?

I'm guessing actual speed is probably around 60? It moves pretty good I can't complain.

I just wish that gas was cheaper!!

Thanks again for your help!!