Lost power- HP500 carb motor
 

Help!

This past weekend, during a poker run, my buddy’s Sonic developed an engine problem that I’m helping to troubleshoot… he’s running carb HP500’s.

Running at 4500rpm or so, suddenly the starboard engine develops a huge miss, loses rpm and power. He shuts down, and checks the obvious: plug wires look ok, no smoke or oil in the engine compartment. Restarts, good oil pressure and temp. Engine still missing, won’t pull a load at all.

Take her home, and pull the outboard plugs (NGK’s). Reading is as follows:

First plug: good
Second plug :dirty but not wet. Thought we found the culprit!
Third plug: looks slightly lean, but ok
Fourth plug: good

Ran a compression check on outboard cylinders. ALL GOOD! 180 lbs each. We’re baffled, thought for sure we had a burned valve on the second cylinder but the compression check is A-OK. We pulled the first plug on the inboard side, and it looked good. The others are a b*tch to get to, so we’ll pull them tonight.

Any ideas? Fuel issue? Ignition? It runs fine on the hose, (no load.) Could a plug just have died?

Thanks!!

Jim

Re: Lost power- HP500 carb motor
 

Yes, how old are the plugs? Sound like an ignition problem. When was the last tune up? ever?

Re: Lost power- HP500 carb motor
 

Tune up? You're supposed to tune these things? :rolleyes:

He he!! He just brought this boat down from the great white north a couple of months ago, so not sure when the plugs were changed last. That's the first thing that'll be done. Then wires.

Re: Lost power- HP500 carb motor
 

missing? or popping back thru the carb? If popping, check worn cam lobes or broken valve springs.

If true misfire with no surging, then continue to look towards ignition related stuff.

Re: Lost power- HP500 carb motor
 

No popping thru carb. Just a miss....

Re: Lost power- HP500 carb motor
 

fuel filter?

Re: Lost power- HP500 carb motor
 

If it was sudden, then could be a broken spring or worse (broken roller lifter). Been there with my HP500's. A lifter broke while running at 5000 rpm and the motor suddenly started popping through the carb. Tore down the motor and found several of the valve springs had broken inner springs and dampeners and the entire motor was filled with metal shavings. I had purchased the boat used and was not aware of this developing problem. Once the springs become weak, the motor can still run ok, but it eventually causes the lifters to pound out the roller axles and bad things happen. Also, the metal shavings are taking their toll on the bearings.
Pull the valve covers and look for pieces of spring and metal shavings lying in the head. Also check the oil filter for metal.

Re: Lost power- HP500 carb motor
 

I had the same problem as 35/25 had.

I just tore a friend of mines down, and found
the roller missing on the lifter that runs # 5 intake
valve.

This has happened to a few others on OSO too.

Pull off the valve covers, see if there's any loose
push rods, You'll need to crank the engine over
a few times to check them all.


What is the year, and what are the hours ?

jt

Re: Lost power- HP500 carb motor
 

HP 500's have inherent valve spring problems.

Re: Lost power- HP500 carb motor
 

"This has happened to a few others on OSO too. "


Yes it has :rolleyes: Please check the valve trane

Re: Lost power- HP500 carb motor
 

Thanks guys!!

Compression checked out on all 8 cylinders, so must be ignition or fuel. Going for full tune-up now.

Thanks!

Re: Lost power- HP500 carb motor
 

Compresion check doesn't tell you schit about the condition of the valve springs, lifters, any of the valve train really. It will alert you to a ring, warn valve, or head gasket problem only.

How many hours!!

Do a full service, fuel filter, and all ignition components. Not worth the risk of missing something.

Re: Lost power- HP500 carb motor
 

Even with a roller missing off a lifter,
( or a weak spring)you'll get enough valve movement for
a good compression test #.

I would still pull the valve covers and
take a lookie.


I did have a coil fail this spring, but the symptoms
you describe don't match mine.


jt

Re: Lost power- HP500 carb motor
 

Check timing. I had a distributor holddown bolt come loose once and the dist moved enough that the engine ran good up to 2500 and then was missing and had no power.

Re: Lost power- HP500 carb motor
 

150 hrs.

Will check it all. Thanks!

Re: Lost power- HP500 carb motor
 

You have a piece of crap stuck in the main jets. remove the fuel bowl and clean.

Roby

Re: Lost power- HP500 carb motor
 

or this :o

Re: Lost power- HP500 carb motor
 

Have you checked the power valves in the carb?

Re: Lost power- HP500 carb motor
 

bglz- based on how you describe the problem in your original post, I'd take a hard look at the ignition timing on that engine. Aside from a clogged fuel filter/marginal mechanical fuel pump/water separator,my experience with engines that rev freely,but don't pull is that you lost advance or the timing chain has jumped. Again, based on you're words, it appears that something happened abruptly. The valve spring/lifter suggestions are also consistent with the symptoms you describe.--Lou

Re: Lost power- HP500 carb motor
 

Thanks guys! It was ignition...

The owner replaced all plugs/cap/rotors and fuel filters. Runs like a champ now!

Jim