Cant get WOT above 3400

Walleye4Days said:

Ok... So, I LITERALLY just went through this same issue and have worked on many others who've ran into the same issue. So, bare within me as I go through the list and problems I've encountered tracking down loss of top end.
First things first: when did this start happening? OR, is this a "new to you" boat, and you're just trying to work the kinks out now?
I noticed you said "replaced with hei"... Important: if you replaced it with standard cheaper HEI (like the default $200 one that is sold on ebay, Amazon, and by tons of others at a relatively cheap price around $180 to $250 or so - they are NOTORIOUS for the 4-pin modules crapping out in them and not allowing spark to keep up/advance correctly with the higher rpms. Which, results in a heavy loss of top end performance. Along with their coils crapping out sometimes and internally arcing a lot in the coils windings, causing other issues in top end Performance Similar to yours. So, take out a multimeter and test the components of your hei's if the cheaper ones OR if a little older now. If that's not it, go on to fuel.

Check out pressure at end of your fuel lines/system, and run a pressure test on it and inspect all fuel lines thoroughly. If you have the slightest seepage of a hole letting air draw into the line under load (because the pressure raises and the outside air draws into the lines causing a very lean mixture that ends up not being able to give proper power at higher rpms/loss of top end.) If all is well with the fuel and all spark, then check out your advance curve under load out on yve water.

Take a timing light and mark down where the timing is at idle, 1000rpm, 1500, 2000, 2500, etc. Make a little graph of the results and see where your advance curve is at. Sometimes the springs or flyweight combo on the HEIs are just a little out of whack, causing improper advance firing while under load or at higher rpms. If the advance curve isn't matching up what it should via VP specs per your engine, then you've found the problem and can swap the springs to appropriate springs that will give you a better curve and fix the issue.

Now, if BOTH engines aren't taching out above the threshold individually, I would doubt its the hei's. I would put it more towards an early fuel issue somewhere before it splits (causing a leaner mixture to hit both engines, or a faulty/clogged pickup tube, or etc.

But, if it's just one engine that's performing a little less than the other (causing overall top of end loss for the twin screw system as a whole) then all of the issues l posted above applies and should be easier to figure out. Just spitballing possibilities due to you not having any answers yet.

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Indy1K said:

Looking for ideas here. 1986 Sun Runner 31ft cruiser. Weighing approx. 10,000lbs. Two new chevy 350 small blocks from Advanced Marine engine. Two rebuilt AQ290a SP drives in 2015. Props are 15.5 x 17p counter-rotating. Added HEI 65,000 volt Marine distributors, timed at 8 deg BTDC @800rpm., and new Edelbrock 1409 carburetors. New fuel filters and separators. Engines start up and run fine once warmed up and chokes drop off.

With 2 adults aboard, I can accelerate to Wide open throttle with the Bennetts down and the boat will plane out at about 24-26 mph. Tacks are reading 3200 to 3400 and I cannot get anymore rpms out of her. If I raise the bennetts up, she will nose up and drop off plane. If I throttle back, she will drop off plane.
I can plow water at 2200 rpm at about 8-10 mph.
First off...... stay out of this RPM range for any duration.
Your GM engines are fitted with the GM Full Dished pistons, of which makes these engines very prone to Marine Load and Ignition Induced DETONATION!


My understanding is this boat should cruise at about 3500 to 3800rpm and WOT to 4500rpm.
That sounds about right!
Keep in mind that the OEM WOT RPM is a test RPM only.

I had 19 pitch props on it, but it seemed worse getting up on plane.

I am running Quicksilver Aluminum props. I have also installed the Hardin Marine Edlebrock Throttle cable mounting plates and linkage.

One idea I had was... What if the lower gear sets were not 1.61:1 for V8 engines. What would be the result?
Your AQ series s/p drives should be the 1.61:1 reduction. If not, you have the wrong reduction lower gear units.

Pull the props.
Pull the ring anodes.
Wire brush the AFT most area of the prop shaft bearing carrier to expose the numbers.
The reduction will be stamped into the carrier, and should read 1.61
No other s/p drive reduction will work for these engines........... period!


Any ideas?

Yes..... you mention ignition advance at 8 deg BTDC @800rpm of which describes BASE advance only.
Our engines fire up on BASE, and they idle on BASE.... and that's it. More importantly will be the progressive advance and the total advance and at what RPM the system is Full-In!
If the progressive and total are not correct, these engines will not produce the power that you would expect them to.


Suggestion:

If not already marked off, mark off your harmonic balancers in degrees BTDC up to approx 35*, and in increments of 5*.
Connect a standard style strobe light to # 1 spark plug cable.
Start engine and watch the advance as you increase RPM up to 3,500.
Plot this out in graph form for each 5* of advance.
Post the results here.



Thanks,

Kevin

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Indy1K said:

Walleye,
Thanks for all the great info.
The engines are new to the boat and were changed as the original 1985 engines had other issues with compression and oil leaks.
The HEIs are new and were around 450 each. That being said, I tried them on the old engines with same issue.
I am wondering if it could be the fuel lines from the tank to the filter/separators and to the pumps. I will check the curve too. You say to do this under load? While under way?
Unless the ignition system receives engine data from the ECU/ECM including knock-sensor data, there is no need to look at the advance while under load.

According to the decal on the spark arrestor, I should be 30 deg @ 4500rpm

30* @ 4,500 RPM is listed as the full in advance.
More importantly will be what it's doing at around 3,200 RPM, in which case it would be bordering on excessive at 30* for a 1985 GM SBC Marine engine with the GM Full Dished pistons.

Thanks,
Kev

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