Hello guys,

Past winter I got a 1992 502 465HP blue engine with GIL stainless thru hull headers, Holley 800 and 170 hours on it.

I run it with VR1 + Wix filter for the season and the engine was very reliable. Always starts at first turn, good temp, good oil pressure, nice power and better sound!

I’m planing the winter check outs and maintenance; this is the compression test results:

1.- 162
2.- 154
3.- 160
4.- 160
5.- 161
6.- 160
7.- 154.5
8.- 159

The spark plugs aren’t new, and they look totally black with black fouling (all look quite simillar)



I have to say, last couple of months I was running only cruise (3.000RPM) and no WOT, due to dirty hull.

I’m thinking in:

1.- weak spark (cold plug): It’s a TB4. I’m planing to update the ignition system with all OEM parts: coil, plug cables, rotor, cap and sensor.

2.- Rich fuel: carb hadn’t been rebuild, (we use an equivalent to PON 89 gas). I have to check also how is set the advance.

3.- Driving like my grandma!


The stern gets dirty quite fast with carbon residue during the summer.

The engine has almost non oil consumption and the colour of the oil is still clean after the season.

The only small issue of the engine I could appreciate is; when on neutral, If you accelerate fast to make the engine roar, you can listen an small gap, then roars.

Also I notice a bit of gas condensation on the spark arrestor.

Thank you guys, I want to read your thoughts please!

Your carb looks like it is dumping way too much fuel. Check float levels, look down carb while sitting at idle and see if it has excess fuel dripping into the intake.

Rich condition for sure. I'm surprised it even idled. Did you idle for a long time at the end of your last run?? Might have a bad powervalve and/or need floats adjusted. What do you mean by gas condensation on the arrestor?? Actual liquid fuel inside on the arrestor??
The hesitation when doing a quick rev may be a bad accelerator pump.

Whats up with all the blow by on #7 was it tight ?.....might be to much carb for the engine

465hp from a NA 502 is a relatively mild engine.

NGK plugs: lower number is hotter plug.

Cruising at 3000 in anything but a boat is considered heavy duty use. Like driving your fully loaded Motorhome up a steep hill for hours. A well tuned BBC will not fowl up like that marine cruising.

Your compression is nice and tight.

How is your oil consumption? Guides getting oil into chamber?

465hp is the at prop HP rating or crank?

800 DP carb is on the higher end of the carb sizing for the HP, but it should still be within the tuning spectrum. Dual plane vs Single plane manifolds have different carb needs. Dual planes suffer from low plenum volume. Why the lower deeper Dual plenum side feeds better power.

Best COA is put a carb kit in it. Check what jets you have, and Power valve size.

You don't want your secondaries open while cruising.

Overfuelling will wash down the cylinder walls, and likely show up in an oil sample of the VR1.

Your neutral throttle blips: you're saying there is a hesitation before it jumps up in rpm?

As Griff mentioned, your accelerator pump shot may not be giving a rich enough mixture with a massive lean condition of instantly opened throttle.

Bog is either too much fuel or not enough.

An engine won't accelerate well if it's lean. A well setup engine at peak RPM will be a tad rich, for the gear change RPM drop, maintains a rich environment for when you get on the throttle again.

A lean bog, mixture is so lean that it takes a lot of time for it to burn...so very little combustion pressure in the optimal piston location. Most is burned when the exh valve is open. Can't generate power even to accelerate the rpm in neutral. Not to mention, there is not enough fuel energy to make the acceleration.

I doubt its weak ignition or you'd be telling us about how you had to get towed in with those plugs or it would run fine cruising and break up at high speed. Do all your normal tuneup stuff you had planned and rebuild the carb. I would bet you have a partially or completely fouled close bleed in carb causing signal to booster to be too high and pull too much fuel. IF its not bleeds it could be a number of other things in carb that will make issue disappear once its all cleaned spotless and new gaskets. I think I also remember a hp 500 having a vent line from back side of diaphragm on fuel pump that goes to the flame arrestor. make sure theres no uncontrolled fuel going thru that clear hose if its still there and while your at it hook your hand vacuum pump to that fitting and see IF it holds vacuum, if not rebuild/replace the fuel pump, Smitty

Thank you guys for all your answers.

I just ordered a Holley full rebuild Kit following your recommendations.

The engine is all original “blue series”. It was produced from 1992 to 1994, then substituted by HP500.

the 465 is flat tappet and the HP500 is roller.

number 1 and 3 plugs are black due a burned exhaust gasket (replaced)

number 7 black oil came probably from a little bit of burnt oil from when I changed the oil filter (dirty job)

Engine runs very nice, better gas mileage than expected and no oil consumption.

I will give you news at this thread when carb will be redone and ignition updated.

Thank you again, Doc ✌️

Secondaries open at 3.200 aprox. I do cruise under that, till I get rich!

WOT, only at the begging of the summer and just 30-40 seconds. I don’t want to blow it

next summer I will try to drive strong at least once every time I go out. It seems the engine need it