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502 crusaders power / engine RPM loss

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Propdr

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Hello Marine Engine Guru's

i have a pair of 502 Crusaders that won't come up over 3000 rpm. / significant loss of power.

I have done about everything.
new plugs, wires, caps, rotors, been through the carbs and put on a Dyno to verify fuel flow prior to reinstalling carbs.
pulled distributors cleaned and reassembled .
Time ing right at 8 deg.
compression check both engines and all holes are 150 psi.
propellers are excellent and have been scanned for accuracy.

the issue is the sudden loss of RPM at WOT. We took a boat trip about 50 miles one way. Both engines ran great. On the way home it was like we were running with 60% less power.
the above checks and total tune up was done and still lack of rpm. No change.

the boat is a 1990 Carver 380 with 500 hrs on both engines. Uses no oil and does not leak any as well.

Any ideas?

Thanks,
Gary
 
fastjeff said:
Holy cow! I'm stumped. This mystery needs a competent, on-site mechanic to solve.

Jeff
Click to expand...
Hi Jeff, Thanks for your reply...How ever, I have been a certified marine mechanic for over 35 years. Seen a lot of stumpers but just reaching out like the Dr's. do looking for that area I may have missed. I will keep you posted as to what I find. Regards, Gary
 
I would suspect the anti-siphon valve on the gas tank. Is it a single gas tank? Either way, try running the same but remove the gas cap(s). If ok, you found your problem
 
Hello, Thanks for your reply . I been down that road but failed to mention that on my header! Twin 150 gal. tanks. Both valves replaced, actually removed them and tested with out valves! I suspect fuel quality... so I am going to temporary two test fuel cells for short line delivery to the fuel pump. And run, since fuel was purchased on this trip it's possible I may have gotten a bad load. However, the engines start and run so smooth, they just don't have the power it's like running with a 500 lb anchor tied to the back of the boat with 100 feet of line.
 
could also be residual collecting on the pickup's screen(s) as well as a bad batch of fuel...likely if both engines are underperforming...

would also be prudent to make sure the ignition timing is advancing with rpm on both engines.
 
Thanks, I am going back to the man cave tonight ( Engine Bilge ) This time with fuel cells to by pass total fuel system. I might even try 100LL Av. fuel. ! ! :) will keep all posted. Thanks
 
We're waitng breathlessly! Seriously.

Was about to suggest what you're going to try. Have to eliminate things, one at a time.

Good luck.

Jeff

PS: Have you tried the Fastjef Dyno Test? To wit: Find a comfortable cruising speed (say 2,500 rpms) and set both engines at that speed. Then open one throttle all the way and note the rpms achieved (and how quickly the motor reved up). Return to the 2,500 rpms and repeat on the other side, then compare.

If ONE engine is dogging it, that pulls down the performance of the other one as well, limiting the rpms BOTH can achieve if all is well. My little dyno test reveals which one is laying down on you.
 
Did that! The problem seems to be in the starboard engine. I totally isolated the onboard fuel system with fuel cell and new gasoline 91 octane ( Premium ). No change at all so now I am on the ignition track! I removed distributor and going to replace it. Will keep you posted. Thanks for your input.
 
I also noted the fuel scan flow when we were running last night. Bothe engines using about the same amount at 1400 RPM 2.7 gal. ph
 
After rereading your posts, I believe you are saying that the issue is not intermittent (your 50 mile trip example was a little confusing at first, but I guess that was when the problem started). So it is always there above 3000 rpm and it is in one engine only. You have beat the fuel flow possibility to death and you have good compression and I will presume that you know that your timing does advance to specs at high rpm. Looking at the remaining possibilities about all that fits with the major power loss is that you have several holes that go dead at high rpm. The only logical cause would be a weak spark and you have already tackled plugs, wires and distributor so they are not the problem. If you concur you might have a weak spark that does not get it done in several cylinders at high rpm I would look next for a bad coil or low voltage to the ignition system. Others may have some additional suggestions.
CaboJohn
 
never saw mention of timing advance being verified...no mention of which distributor is used - could be the coil getting hot as well as the pickup in the distributor...

it's a major pain on some carb setups but it would also be prudent to verify the fuel pump makes adequate adequate pressure at the higher rpms...
 
makomark said:
never saw mention of timing advance being verified...no mention of which distributor is used - could be the coil getting hot as well as the pickup in the distributor...

it's a major pain on some carb setups but it would also be prudent to verify the fuel pump makes adequate adequate pressure at the higher rpms...
Click to expand...
Mark, I did check fuel pump pressure and verified flow with a flow scan system. Both sides pulling the same amount of fuel. Yes max timing advance is working as per dock test in neutral. At momentary 3200 RPM no load the advance is 29 Degrees as per Crusader. The distributor is a factory OEM Distributor. I previously changed the ignition coil..... I did a OHM's test on the pick up coil as well as the module and everything checks out. I did this with the engine cold as well as hot. I spent time removing the distributor again and dis assembling. I just keep going back to how rusted the internal components were when I first cleaned it up and did my OHMS tests cold. The root of the problem is the small points and magnetic sections were rusted away to the point where it wouldn't make or break the magnetic field. I installed a new complete distributor and the boat never ran so good. We did a test run last night and both engines now pull 4400 RPM. @ 28.5 gal. per hour on both engines. Our speed came up 2.3 KTS. As per Crusader the part number has changed several times 38065 changed to #RA107039E to 22752K which is a complete ignition kit. You get the distributor, coil, and wiring harnesses as well as cap, rotor and complete set of spark plug wires.
502 distributor 01.jpg
IMG_0003.jpg
IMG_0004.JPG
Notice how magnet areas are rusted away. Look at the points most gone as well. Moral of the story throw the distributor away and replace with new and don't rely on OHM's test for electrical components. I am replacing the other distributor as well. Marysville Marine told me they buy this distributor kit by the pallet and there's 80 to a pallet...... What does that tell you? Thanks to all who gave ideas and are watching this thread I hope someone else can benefit without going through the headaches I have.
 
Glad you figured that one out...yes, that amount of deterioration will cause irratic spark...makes you wonder why they dont passivate the pole piece BEFORE it gets riveted on....probably cause its not cost effective for GM...

Where is the boat located - wonder if the local environment contributes to the accelerated deterioration...???
 
Thanks for your reply. The boat was located on the Great Lakes just North of Chicago but is now on the Mississippi River at Dubuque, IA. So the salt water isn't something we can be concerned about as for the excelled corrosion factors. One would certainly expect to see this amount of deterioration in the salt water areas. But the boat was in North Chicago since it was new. Thanks again. If you ever have any propeller issues we can certainly help with those. Linden Propeller Co., designed, patented and build the Prop Press 360 and now the new 500 series for those larger yacht propellers. you can check us out at www.lindenpropeller.com Thanks, Gary
 
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