Your On-Line Port'o'Call

Previous Thread
Next Thread
Print Thread
Rate This Thread
#572610 - 07/22/19 06:38 PM High speed miss  
Joined: Jul 2019
Posts: 1
Clogee Offline
Warrant Officer
Clogee  Offline
Warrant Officer

Joined: Jul 2019
Posts: 1
Granby, CT
I have a new problem with my boat. After about an hour of use, the boat develops a high rpm miss/ breakup. Seems to be most notable between 3500 and 4000 rpm. Boat starts beautifully and runs great at lower rpms. Problem is repeatable day after day. Has occurred the last 3 times I have used the boat. This is a new problem, this boating season.

The boat is a 1990 Chaparral 2300sx with a mercruiser 5.7 liter V8 with the Thunderbolt IV ignition. Approximately 4 years ago, a new Mercruiser factory rebuilt motor was installed. At the same time, new exhaust manifolds, new risers, new edlebrock marine intake manifold, new edlebrock 750 cfm marine carb, and new mercury distributor installed. At the same time, the motor was converted to the mercury electric fuel pump setup.

So far I have replaced the cap, rotor, plug wires, spark plugs, and fuel filter, added seafoam to the tank, added 35 gallons of fresh 93 octane fuel, and verified solid fuel flow at the carb. I have tested for a bad tach by disconnecting it from the coil. I have verified that timing at idle is at 8 degrees BTC and it seems to advance properly as the rpms increase. Nothing has fixed it.

I am considering replacing the ICM mounted on the exhaust manifold as it is the only part of the ignition system that hasn’t been replaced. My reservation is that I don’t want to spend the $500 on a new one and find out that wasn’t the issue.

I have read about the anti-siphon valve, but seams to show itself at lower rpms. I have also considered going into the carb, but I am religious about adding stable during fall boating to get it ready for winter storage.

Any thoughts/ help would be appreciated

A Word from Our Sponsors
#572611 - 07/22/19 07:46 PM Re: High speed miss [Re: Clogee]  
Joined: Jan 2003
Posts: 3,962
Jack T Offline
Admiral
Jack T  Offline
Admiral

Joined: Jan 2003
Posts: 3,962
Southern California
Several possibilities. Not all problems of this sort are caused by ignition. If I were to begin, I would go after fuel delivery. In carburetors, accelerator pump diaphragm somtime can not keep up wth demand and give the same symptom as you see. My experience has been fuel delivery that needs to be addressed first.
You are correct that it could be the check Valve. Could also be a small break in the fuel. Line between the fuel tank and the pump. As more fuel is demanded, air gets sucked into the fuel pump.
Possibly the pump itself.
If you can duplicate the system, you might try rigging up a clear fuel line to a portable tank and attach the line to the input of the fuel pump. Run the engine at higher rpm while looking at the tube. Should be no air.
Good luck. Please keep us updated as you work this out.


Have a great day of boating
2012 Monterey 224 FS,
300 HP Volvo Penta with catalytic converters

#572614 - 07/23/19 02:41 AM Re: High speed miss [Re: Clogee]  
Joined: Feb 2003
Posts: 11,857
Frantically Relaxing Offline
Admiral
Frantically Relaxing  Offline
Admiral

Joined: Feb 2003
Posts: 11,857
The electric fuel pump-- is it a 'thumper' diaphragm type pump, or a direct drive gear or vane type pump? If it's a thumper, could be the diaphragm is getting past it's prime. Well used electric pump diaphragms are notorious for losing pumping ability as they heat up. You're good to go for an hour, then you start losing power, so it's a reasonable possibility... Electric pumps are cheap and easy to replace, and after 4 years you've got your money's worth out of the first one. If it proves to be the solution, great, if not, at least you shouldn't have to worry about the fuel pump for awhile! (and keep the other as a spare)...


[Linked Image]
#572619 - 07/24/19 07:10 AM Re: High speed miss [Re: Clogee]  
Joined: Jul 2003
Posts: 6,645
Dave R Offline
Admiral
Dave R  Offline
Admiral

Joined: Jul 2003
Posts: 6,645
Raymond NH
I suspect it's fuel delivery related. You could easily Tee in a portable fuel tank (with a shut off valve) between the fuel pump and the carburetor. When the engine starts to miss, have someone hold the tank a couple of feet or more higher than the carburetor and open the valve. If the miss goes away, it's a fuel delivery issue. If it still runs the same, it's likely either the carburetor or ignition. Sometime a coil can cause this issue, especially if the ballast resistor was left out of the circuit. I have a couple of portable fuel tanks and you can have one. They are from a garden tractor and a snow blower. I live in between CT and Winnipesaukee.


"Mischief Managed II"
1992 Tollycraft 44 Cockpit Motor Yacht
Twin CAT 3208TA inboards
#572622 - 07/25/19 05:00 PM Re: High speed miss [Re: Clogee]  
Joined: Aug 2005
Posts: 2,427
WayWeGo Offline
Admiral
WayWeGo  Offline
Admiral

Joined: Aug 2005
Posts: 2,427
Oakton, VA
It sounds like you have done everything I would have to find the problem.

As an option to the fuel delivery plan Dave suggested, you might put a fuel pressure gauge between the pump and the carb and see if you are actually getting the pressure specified. Playing with gasoline around engines makes me nervous, especially after one of my high school friends managed to put himself in the hospital with 3rd degree burns to his face and neck.

A high RPM miss that starts after an hour of running sounds like a heat related ignition problem to me. With the ignition box on the exhaust manifold, it could easily heat soak and start breaking down. Is there any chance you could find one to swap out for a test?

We had a '93 2500SX and our fuel tank was aluminum. I suspect yours is also. If venting was a problem, I don't think it would take an hour for it to show up if you are running at 3,000RPM. One thing to check is the tank vent. Bees and spiders make nests in there and that could cause a fuel flow problem, but I think it would show up sooner than an hour with a full tank of gas.

A 750cfm carb is pretty big for a small block engine that turns less than 5,000RPM -- we run 600cfm Eldelbrocks on our big block 440 Chryslers with a 4,000RPM redline.

The best old school automotive diagnostician I ever met used to say that if you think it is the carb, check the ignition. Once you are sure it is the carb, check the ignition again...



1975 Trojan F36 Convertible, Twin Chrysler 440's
2014 West Marine AL360 Inflatable, 1966 Mercury 6HP, 1992 Mercury 20HP
#572623 - 07/25/19 11:13 PM Re: High speed miss [Re: Clogee]  
Joined: Feb 2003
Posts: 11,857
Frantically Relaxing Offline
Admiral
Frantically Relaxing  Offline
Admiral

Joined: Feb 2003
Posts: 11,857
I'm thinking Clogee has come & gone, but this is still funner than work wink

Weirdest ignition problem I ever saw was with our old Sun Runner. The thing ran pretty good when we first got it, but second season or so at 7700' altitude, it started acting up, missing badly above 3000 rpm, and after warmup was worse. I started messing under the distributor cap when nothing else was working, and found the shaft was all kinds of sloppy loose.
I put these pics in my graphics program to measure the degrees-- that's not advance movement, that's pure slop...

[Linked Image]


The reason for the slop:

[Linked Image]


Yes, there was actually enough clearance to fit a paneling nail in there!

[Linked Image]

==========================
- the shaft was just all over the place. I guess someone- the broker's mechanics?- set the air gap to zero to get it to fire, and the trigger wheel eventually pushed the switch out of the way, or wore each other down..

The reason for the wear? Whomever installed that Prestolite marine distributor wasn't paying attention to the gasket- When I pulled the distributor out, fully half the rubber gasket was below and into the hole, the remainder up on top where it should be. So the whole distributor was tightened into place at an angle, putting lots of pressure on the the shaft bushings. The upper wasn't much better...

This took care of the miss : New Mallory Unilite distributor, Accel SuperCoil and an marine Edlebrock 650 to replace the Quadraflush...
That there engine never ever missed a beat after that smile

[Linked Image]

=============================






[Linked Image]
#572624 - 07/26/19 07:45 AM Re: High speed miss [Re: Clogee]  
Joined: Aug 2005
Posts: 2,370
tpenfield Offline
Admiral
tpenfield  Offline
Admiral

Joined: Aug 2005
Posts: 2,370
Cape Cod, MA
Perhaps @Clogee is a "1-and-done", but if not . . .

I'd check the fuel tank venting, as mentioned, then . . .

There is a 'sensor' beneath the rotor in the distributor that could be the source of the problem . . . it is around $75 - $90. Else you may be looking at the $400+ ignition amplifier (which is a total rip-off)

More caps, rotors, plug wires, spark plugs, and fuel filters get unnecessarily purchased & replaced because of issues like this. I made that mistake once too . . . (once) smile


Regards, Ted

Formula 330SS

My Boat Web Sites

Moderated by  Admin 

Sponsors
New Topics
If You Hate EPA/CARB Gas Cans - Watch This Video
by Dock Holiday. 10/13/19 07:03 PM
if your boat has a prop, YOU MUST WATCH THIS!
by Frantically Relaxing. 10/13/19 01:57 AM
Hi everyone!
by Jenny285. 10/12/19 10:05 PM
New Propeller Design - Sharrow Engineering
by Dock Holiday. 10/12/19 10:51 AM
Pulling the Boat
by On Holiday. 10/10/19 02:53 PM
A forum I always come back to
by Senoj. 10/08/19 05:57 PM
Tired of the SCAM phone calls
by Justification. 10/02/19 02:59 PM
unable to post photo links that work
by Dave R. 09/27/19 07:59 AM
My boats new house
by MarkHB. 09/15/19 09:20 PM
New Rocna Vulcan anchor
by Dave R. 09/05/19 07:54 AM
Who's Online Now
0 registered members (), 79 guests, and 4 spiders.
Key: Admin, Global Mod, Mod
Newest Members
genesis1, flemiis, Glaukos, LgfCoobalt, Jorge
5492 Registered Users
Forum Statistics
Forums23
Topics37,310
Posts564,185
Members5,493
Most Online252
Aug 5th, 2017
Powered by UBB.threads™ PHP Forum Software 7.6.0