Lost RPM & boost
 

Had problems with a bad impellor & overheating issues earlier this season. Now those issues are resolved the boat only revs to 4000 rpms & has no boost? It doesn't make any abnormal sounds like something is mechanically wrong , its just lazy? The blower is a Weiand 256 on a 509bbc. Can the blower to manifold seal be compromised causing no boost? I really don't know where or what to check?

Did it ever build boost? What was it before?

After an overheat, always do a compression test 1st.

Overheating can cause some issues. Did you make note what kind of temps you were getting and how long did you run hot. Comp test first as mentioned. Could be as simple as a head gasket to warped cylinders, exhaust valve and others. Assuming it’s running on all 8? Do a comp test and leak down of possible and let us know the results. Hope it’s something simple..... Also change your oil if you haven’t already. Does it look milky on the dipstick after you replaced the impeller?

I did do a compression check & the compression was lower than I expected but all cylinders were close to one another. A Holley tech today told me the head gaskets could be blown leaking into all cylinders. Any blown head gasket I've ever seen in the past is usually between two cylinders not all the the cylinders? I never had a boost gauge on it when it ran good so I don't know what it was producing before. Now that it has a boost guage it swings from 10" vacuum to zero & no boost. @ my elevation & pulleys I would expect to see 3.5lbs. No milky oil. The exhaust manifolds were very hot well into the 300's & the the motor was closer to 240.

I hate to replace the head gaskets before ruling everything else out, My thought is the blower is loosing boost & the drag of the blower is lowering the RPMs? If the intake manifold gaskets or blower to intake manifold gaskets failed would i hear the leak? The motor sounds fine it just has no power or boost.

btw: is this serp or cog belt ? if serp maybe slipping

16rib serp

Had the same thing happen a few times on my 525SC. Replaced blower belt and was good for another couple seasons.

do a leak down test,see if anything is compromised before throwing time and parts at this.

as griff said check the blower belt for slipage.

My vote is the belt. Cheap fix vs tearing everything apart.

What were the compression numbers?

While you're at it, Bring it to TDC and check if the timing pointer still aligns. It's not likely but I have seen broken or missing crank keys, just something extra to check.

Just an idea... maybe internal blower failure??

I’d think smell a slipping belt after running for any length of time but sure would be nice if that be the culprit. Leak down!!!!

We just dynoed my LS setup. and lost 1.8psi from first pull to second pull then held consistent. Had rubber dust behind pulley. Check your belt. Definite pissibility

The compressions numbers varied from 85 to 90 PSI per cylinder @ 6500' elevation. Before the blower installation I believe the compression numbers were closer to 110 per cylinder. The blower belt only has about 20hrs on it & no signs of belt dust anywhere. I did get a leak down tester but haven't had time to test yet.

when U say no boost is it 0 PSI? I know very little about blowers other than it's an air pump. Is there anything inside that can cause no boost such as seals or the drive to turn the rotors?

110 psi at 6500-7000' elev is around 135psi sea level. That's ballpark for a blower motor that's intentionally built with lowish compression.
90 psi is 110psi sea level which indicates a sick motor. You'll be lucky if it's just head gaskets.

Belt slippage can happen with no obvious signs, but you need to resolve any longblock issues before you go any farther.

I assume both compression readings were done with slightly warm motor and throttle wide open? If not, you could have test variations affecting the readings.
Leakdown. Do a good leakdown. Either have it done by somebody trustworthy or have them there to assist you.

M

I did the leak down after watching a utube university video. I pulled the plugs, inserted the hose into the cylinder & bumped the starter till I felt the compression build. Then I hooked up the the leak down tester & read the gauge. All the the cylinders were in the high 40's to mid 50% range. You could feel & hear the air coming from the valve cover port. Is this beyond just replacing the head gaskets?

Never a fan of the leak-down test. Either it holds or it doesn’t. I do a compression test to look for low cylinders. From there I pressurize that bad ones to hear where leaks are coming from... intake or exhaust... head/valves. Coming from the crankcase (valve covers) rings or a hole in a piston.

In your case ... sounds like the lower-end is tired sir. Rings or holed piston(s).

Wish it was better news.

This is when you start practicing the speech for the ol lady on why you need heads, stroker kit, etc. Ive got it down pat

Yup

Sounds like a bad case of Covid 19.

I've always done the leak-down with piston at TDC compression stroke.

That's the right way.. If you get it perfectly at TDC, the eng won't rotate but that can be difficult to do. Usually takes me a couple of tries. There's a replaceable orifice between the 2 gauges of the differential press tester The size of the orifice is determined by the eng size. If it's not the right size orifice, the differential readings will be off.

However, assuming the compression is a bit low due to bottom end leakage, I would think it would still produce some boost at 4k RPM. As mentioned prev, I'd try a new blower belt before tearing into the eng or blower..

I put a scope in the cylinders to see the pistons. Didn't see any holes but they did seem carboned up. Did the overheat cause the rings to stick?

Put a new blower belt on it and go run it and report back.
If pistons are have significant carbon build up, its from a rich condition. That rich condition could be from dumping in fuel at WOT and not making the rpm to use the fuel due to lack of boost.

Check or replace your temperature sensors
Did you replace the thermostat ?

1) Engine Coolant Temperature Gauge Sender

2) Water Temperature Gauge Sender

The overheat condition can cause the rings to lose their tension. If you are truly seeing 40-50% leakage past the rings, they have lost their seal. Pull it out and tear it down.

Any update OP?

So I was on vacation over the 4rth & playing catch up @ work last week. Today I did another compression check on a cold engine & installed new plugs. I squirted 3 shots of oil in each cylinder before testing. Most cylinders shot up at least 25psi higher than before except #8 which stayed the same @100PSI. After that I took the carb off & cranked the motor......the rotors aren't spinning? That's as far as I got since it was getting dark. Guess I'll pull the snout off next & see what it looks like?

Pulleys are turning but the rotors are not?? That doesn't sound good. #8 cyl compression not coming up after oil squirt sounds like a valve not sealing. Maybe #8 ingested some blower debris??

Removed the blower & the gear coupler bolts are sheared. The blower gears will have to be removed to remove the sheared bolts. Looks like the gears have grooves as well. This blower is going to have to be rebuilt. Bummer

see post #17

DMPE in MI has upgraded shafts for these blowers, is this a Merc blue 256?

Its not the merc version, I dropped it off @ a local shop the other day. The guy felt the overheat caused the rotors to swell making contact & shearing the bolts.