Rocket Ship!....Almost (low RPM WOT)

[jumpjets]

I just installed a 19 pitch 4 blade prop, and a new 1.47 Alpha one outdrive on my 21ft wellcraft. My old drive had a 21 pitch 3 blade with a bent hub and a bent prop shaft. See my sig for more details on the boat/engine

The new drive/prop combo is like night and day! The boat comes up on plane like a rocket ship. The acceleration is amazing! It is about 3-4 mph faster at all rpm. I am really impressed with it's performance....almost.

At WOT, it only reaches about 3800rpm, and about 42mph. I think this boat should run over 50mph. In order to optimize the engine, and get the last 800rpm out of the engine, I'm thinking I'll try the following:

New quality NGK spark plugs properly gapped at .035
new 8mm race wires
new mallory ignition coil
New cap and rotor
new fuel filter

I'll also check the compression to ensure the engine isn't tired. Is this a good place to start trying to optimize my engine, or am I just throwing money at a problem? Do you have any other suggestions?

 

[Bondo]

Re: Rocket Ship!....Almost (low RPM WOT)

New quality NGK spark plugs properly gapped at .035
new 8mm race wires
new mallory ignition coil
New cap and rotor
new fuel filter

Ayuh,... I'd go with Stock, over the 1st 3 things listed...
AC Delco MR43T sparkplugs are my choice,...
A general tune up, 'n compression test oughta give ya a baseline to work from...

Is the tach readin' Right,..??

 

[greg82255]

Re: Rocket Ship!....Almost (low RPM WOT)

Agreed on the AC Delco MR43T's - your engine will run NGK BR6FS, but Delco's are the way to go IMHO.
The race wires aren't going to give you a noticeable difference, and aren't really worth the price increase. I tried it once with my old Checkmate and didn't see any difference.
Lots of people on this forum (Don S especially) do NOT recommend Mallory parts - they say they are junk. I'd go with an MSD Blaster II or just the stock coil.
When you replace the water separator fuel filter, pour the contents into a clear container to see if any water is in your fuel. If you have an in-line filter, replace it too.
I believe there is also a thread in the Adults Only section that deals with low WOT rpm. Goodluck

 

[Silverbullet555]

Re: Rocket Ship!....Almost (low RPM WOT)

Your comparing a beat up prop and driver to a drive that is fixed and a new prop. The fact that it is fast is not surprising.

Now that you are not wasting a ton of power on a bent set up you can really find the right prop. Might be as simple is changing props.

 

[jumpjets]

Re: Rocket Ship!....Almost (low RPM WOT)

Ayuh,... I'd go with Stock, over the 1st 3 things listed...
AC Delco MR43T sparkplugs are my choice,...
A general tune up, 'n compression test oughta give ya a baseline to work from...

Is the tach readin' Right,..??

I'll def do the comp test. It had good comp when I tested it in april though.
I'm sure the stock plugs, wires and coil will be cheaper than the race stuff, so good advice there.
Is the tach working right? Good question, it very well might not be.

Agreed on the AC Delco MR43T's - your engine will run NGK BR6FS, but Delco's are the way to go IMHO.
The race wires aren't going to give you a noticeable difference, and aren't really worth the price increase. I tried it once with my old Checkmate and didn't see any difference.
Lots of people on this forum (Don S especially) do NOT recommend Mallory parts - they say they are junk. I'd go with an MSD Blaster II or just the stock coil.
When you replace the water separator fuel filter, pour the contents into a clear container to see if any water is in your fuel. If you have an in-line filter, replace it too.
I believe there is also a thread in the Adults Only section that deals with low WOT rpm. Goodluck

I just found that adults only thread on WOT low RPM. I had always assumed the "Adults only thread" was for something entirely different!!! Giggity.....

Your comparing a beat up prop and driver to a drive that is fixed and a new prop. The fact that it is fast is not surprising.

Now that you are not wasting a ton of power on a bent set up you can really find the right prop. Might be as simple is changing props.

The bent prop wasn't the best baseline, but the previous owner had said it was a bit overpropped. I selected a new prop 2 pitches lower, assuming that the 4 blade prop would negate 1" decrease in pitch. I will consider getting a new prop, but only after the engine is optimized.

I checked the simple stuff first tonight: I took apart the throttle linkage on the carb just to ensure that going WOT on the throttle lever meant the carb was actually opening all the way up. That was good.

Next I checked the air cleaner. It was covered in an oily substance that looked sorta like laundry lint. I cleaned it real well, and reinstalled it. Hopefully that will give me a couple hundred extra RPM.

Quick question: the giant black plastic "Hat" on the flame arrestor...... does that need to be there? It seems like it would inhibit airflow a little. I took it off, and I'm gonna try running without it unless someone tells me it's a safety hazard.

 

[TilliamWe]

Re: Rocket Ship!....Almost (low RPM WOT)

I

The bent prop wasn't the best baseline, but the previous owner had said it was a bit overpropped. I selected a new prop 2 pitches lower, assuming that the 4 blade prop would negate 1" decrease in pitch. I will consider getting a new prop, but only after the engine is optimized.

I checked the simple stuff first tonight: I took apart the throttle linkage on the carb just to ensure that going WOT on the throttle lever meant the carb was actually opening all the way up. That was good.

Next I checked the air cleaner. It was covered in an oily substance that looked sorta like laundry lint. I cleaned it real well, and reinstalled it. Hopefully that will give me a couple hundred extra RPM.

Quick question: the giant black plastic "Hat" on the flame arrestor...... does that need to be there? It seems like it would inhibit airflow a little. I took it off, and I'm gonna try running without it unless someone tells me it's a safety hazard.

The 21 pitch was "overpropped"? So right there, you needed a 19p 3 blade. Which meant you should have gotten a 17 or 18p 4 blade. Assuming that the TACH is reading correctly, and the motor is not damaged.
Cleaning the flame arrestor is good, but probably won't make much difference.
The black cover is just there to look pretty. As long as your flame arrestor is bolted down, you're fine. But it's not going to make ANY difference. Sorry.

Confirm tachometer and compression. then see what you got.

 

[jumpjets]

Re: Rocket Ship!....Almost (low RPM WOT)

The 21 pitch was "overpropped"? So right there, you needed a 19p 3 blade. Which meant you should have gotten a 17 or 18p 4 blade. Assuming that the TACH is reading correctly, and the motor is not damaged.
Cleaning the flame arrestor is good, but probably won't make much difference.
The black cover is just there to look pretty. As long as your flame arrestor is bolted down, you're fine. But it's not going to make ANY difference. Sorry.

Confirm tachometer and compression. then see what you got.

Whoops. My logic was little off with the prop. I figured the 19p 4 blade was enough of a step down.
I can do a compression check easily.
How would you go about checking the tach though? I usually just plug a scan tool like diablosport or hypertech into my cars to check stuff like that, but obviously this boat isn't computer controlled. Is there a tool I need to buy to cross check the tach?

 

[WILSONS88]

Re: Rocket Ship!....Almost (low RPM WOT)

I have the same size boat and motor not sure on the ratio on my alpha. I have a 5 blade stainless 19p prop. I get 52 mph (gps) with 5 in the boat and run around 4600 to 4800 at WOT. Alone I get 55 to 56 mph at 5000. I dont do that very often though I like my engine. You should have gotten way more out of that 19p at WOT.
They have digital multimeters that read rpms. I know Harbor Freight has one not sure about Northern Tool.

 

[jumpjets]

Re: Rocket Ship!....Almost (low RPM WOT)

I reattacked the issue today. I believe that my tach is close, but not 100% accurate. I was averaging 42mph on an iPhone GPS, and my tach was around 3800ish rpm. On one run, I touched 45mph, but still averaged 42mph. On a separate run, my tach read 4200rpm, but the average of multiple runs was 3800rpm. It very well might still be overpropped. I'm gonna do a tune up and ops check on it regardless though.

 

[DFW Miles]

Re: Rocket Ship!....Almost (low RPM WOT)

Might want to rebuild the carb and check the timing. Both easy to do while you are at it. I just did a tune up on my 4.3L including carb rebuild and timing. I thought I was overpropped before (17P) but when I got done with the tune up I had a TON more power. Turned out my carb was all gummed up and the timing was off by 12 DEGREES!!!! I was able to change to a 21 pitch prop and got an extra 7MPH! Good luck

 

[TilliamWe]

Re: Rocket Ship!....Almost (low RPM WOT)

Something's up. 42 mph at 3800 rpm is 11% propslip. That slip number is not bad at all. But if the speed was still 42mph at the 4200 rpm, it's 18%, which is too high for your prop.
Check out the motor's compression and timing.

 

[jumpjets]

Re: Rocket Ship!....Almost (low RPM WOT)

Might want to rebuild the carb and check the timing. Both easy to do while you are at it. I just did a tune up on my 4.3L including carb rebuild and timing. I thought I was overpropped before (17P) but when I got done with the tune up I had a TON more power. Turned out my carb was all gummed up and the timing was off by 12 DEGREES!!!! I was able to change to a 21 pitch prop and got an extra 7MPH! Good luck

Something's up. 42 mph at 3800 rpm is 11% propslip. That slip number is not bad at all. But if the speed was still 42mph at the 4200 rpm, it's 18%, which is too high for your prop.
Check out the motor's compression and timing.

I did a compression test today. All Cylinders were 150psi +/- 5psi. That's pretty good IMHO. The spark plugs were older than crap, but not fouled. I bought some AC delco rapidfire plugs, and I'll gap them at .035 and install them tomorrow. The plug wires are really new looking, and they are already 8mm hi performance wires.

I'll check the timing tomorrow. I would totally believe it could be way off. My distributor doesn't have the vacuum advance hose hooked up right now. Should it have vac advance, or is it supposed to be fixed timing?

The carb doesn't look too bad on the outside, but it could be gummed on the inside. Can I replace the carb with a 600cfm edelbrock hi perf carb off the shelf from an autoparts store? or.... do I need a special marine carb?

They had decently priced tachs at oreillys, so I might just get one of those as well. My current tach is getting less and less reliable.

 

[TilliamWe]

Re: Rocket Ship!....Almost (low RPM WOT)

You distributor should be a mechanical advance. And you have to use a marine carb. Edelbrock makes them.

 

[ziggy]

Re: Rocket Ship!....Almost (low RPM WOT)

My distributor doesn't have the vacuum advance hose hooked up right now. Should it have vac advance, or is it supposed to be fixed timing?

vac. adv. shouldn't be hooked up, shouldn't even be there. leads me to think ya got a auto distributor. as tw said, it's only mechanical advance. since ya might have a auto dist. maybe the advance curve isn't the same as marine. maybe look into that?

 

[jumpjets]

Re: Rocket Ship!....Almost (low RPM WOT)

vac. adv. shouldn't be hooked up, shouldn't even be there. leads me to think ya got a auto distributor. as tw said, it's only mechanical advance. since ya might have a auto dist. maybe the advance curve isn't the same as marine. maybe look into that?

I popped open my distributor today. Is this what the inside of a mechanical advance distributor is supposed to look like? It looks pretty simple to me. It's definatley not an automotive vacuum advance distributor.

 

[Bondo]

Re: Rocket Ship!....Almost (low RPM WOT)

I popped open my distributor today. Is this what the inside of a mechanical advance distributor is supposed to look like? It looks pretty simple to me. It's definatley not an automotive vacuum advance distributor.

Ayuh,.... That's a T-Bolt IV, or V....
It's Electronic... No vacuum, No mechanical....

 

[jumpjets]

Re: Rocket Ship!....Almost (low RPM WOT)

Ayuh,.... That's a T-Bolt IV, or V....
It's Electronic... No vacuum, No mechanical....

HA! Well.... That simplifies the problem. I guess I'll just hook a timing light up to it, and see if it has 22* timing at idle. If it does, I'm assuming that the timing portion is good.

On a lighter note, here on the trailer the AC delco rapidfire plugs seem to make a big difference. The engine starts up much easier now, and when I rev the engine in neutral, it whips up to it's desired RPM instantly. Before it would take its time winding up to RPM.

I also got a new Tach, and I'm installing it now. I'm taking my time and doing a super careful wiring job, since I just read about a guy in the stupid human tricks forum who had a wire fire in his boat. I don't want a fire in my boat!

On a quick test run, the new tach and old tach are already 200rpm off just at idle.

 

[Bondo]

Re: Rocket Ship!....Almost (low RPM WOT)

HA! Well.... That simplifies the problem. I guess I'll just hook a timing light up to it, and see if it has 22* timing at idle. If it does, I'm assuming that the timing portion is good.

Ayuh,.... Ya need to Identify it...
A T-Bolt IV, should be 'bout 6?, 8? at idle...
A T-Bolt V needs to be set to Base mode with a jump wire...

 

[jumpjets]

Re: Rocket Ship!....Almost (low RPM WOT)

I haven't checked the timing in depth yet. I did install a new Tach though, and it seems much more accurate and precise.

I took the boat out today, and on somewhat calm water, with 4 adults and half a tank of fuel, I was able to obtain a constant 4000RPM and approx 45mph. I'm going to go out tomorrow morning by myself, on fumes, and see if I can touch 50mph. I'm excited about it the opportunity to achieve that.

I have one or two more outings, and then I'm going to store the boat for the winter and head off to Afghanistan. I found a marine repair shop that will winterize the boat with new fluids and parts for ~$350, and shrink-wrap and store it for free for me since I'm active military.

WHen I bought this boat I didn't know which end was the front. I've learned alot this season, mostly from this forum, but I'm going to have the mechanic go over the boat with a fine tooth comb to identify and safety or mechanical issues it might be having that I haven't identified. I will communicate with the shop via email to authorize any repairs they deem necessary, then they'll store it for me for free over the winter and have it water ready when I come home next summer. I haven't shopped around for other mechanics prices here in town, but I got a really good vibe from "American Marine", the one shop I did visit.