Need a little help with a stumbling 4.3 merc

[silvert]

I have a 95 maxum 2000sr with a 4.3lx alpha 1. It has a 4 barrel Weber. Boat has a bad hesitation when you give it throttle. It's never been right since I've owned it for the last three years. I've rebuilt the carb, tried every accelerator pump adjustment possible, compression test on all cylinders which checked out OK, changed plugs, etc..
Boat will hit 45 on the GPS at 4500 rpm. Has a very hard time planning out and can barely pool a skier out of the water.
I was thinking maybe the engine is down on hp, but it has no issue going 45. Can it still be low on hp but still hit the top speed?
Checked timing on the thunderbolt 4 ignition. It is set at 9 degrees btdc.
Thinking of buying one of the edelbrock 1409 600 cfm carbs and the v6 kit for it.
Main concern is, could it be down on hp? Not pulling a skier out of the water really makes the boat no fun lol. Hoping it is a carb problem. Also has new wires and dist cap. Has original coil and fuel pump. New fuel filter every year, also has had at least 15 tanks of fuel through it the last couple years.

Any thoughts?
Thanks,
Kevin

 

[Tnstratofam]

Do you know the boats prior maintenance and storage history? When you say it has a hard time planing out do you mean it takes a long time to plane, or it only planes at or near wot? Could be something other than motor/carb related. Such as water logged foam, or Incorrect trim setting.


Edit: I just reread your post. You said the motor has bad hesitation. Have you replaced the essentials, fuel filter, water separator, anti siphon valve?

 

[silvert]

It had an engine replaced at some time.
If there is more than 4 on the boat. 2 have to go to the front for it to plane. If no one goes the front, it will not plane and seems to get stuck at 3k rpms or so. It will do this no matter where I set the trim it seems. Not sure what you mean by the foam comment

 

[silvert]

Water separator/fuel filter have been changed, twice. What is the anti syphon valve?

 

[silvert]

I can replicate the hesitation with the boat in neutral. Almost sounds like it is backfiring a little out of the carb. I've rebuilt the carb to spec, twice. Replaced accelerator pump, and no adjustment seems to help. Verified the choke opens and the secondaries open also

 

[Tnstratofam]

Ignore the foam comment I made above. I was thinking that if the boat had been stored outside uncovered there might have been water intrusion below the deck that had been retained by the flotation foam. That would cause the boat not to plane properly, and the extra water weight would kill it's performance. Any way it sounds like you have something more mechanical going on. Have you checked the timing?

The anti siphon valve is at the fuel tank it is usually threaded into the tank with the fuel line connecting directly to it.

 

[silvert]

Thanks for the replies so far.
Timing is set at 9 btdc on the thunderbolt 4.

 

[achris]

Sounds like you have 2 problems. First is the hesitation, the other is the poor performance.

For the poor performance I would be looking closely at the prop. What's on it right now?, and what's the drive ratio? (can usually be found stamped on the decal on the port side of the drive). for that era, some were 1.81:1 and some were 1.62:1

As for the hesitation. You say the block was replaced. With what? If someone has replace the original Gen II block (not to be confused with a Gen II drive :facepalm with a Gen+ block (vortec heads) then the carb jetting will be wrong.... and that could be the problem.

How to tell the difference between Gen II block and a Gen+ block+? Intake manifold bolts. Gen II block has 12 and they go in at an angle, Gen+ block has only 8 and they go in vertical....

Chris..........

 

[silvert]

Thanks Chris, it has a ss prop on it, forget the size but will report back after work. It has 12 bolts on the intake, so it is the non vortec engine although the valve covers say it lol

 

[silvert]

I'm pretty sure the prop is a 21p, but I can not recall the diameter

 

[Tnstratofam]

In doing some research I see that you've had this problem since you got the boat. In one of your posts to another thread you stated that you could almost eliminate the hesitation with two people in the boat by having one person in the bow. However with more than two people in the boat, or with allot of weight i n the boat the hesitation was much worse. This makes me wonder if you have trash in your fuel tank that is sloshing around and partially blocking the anti siphon valve thus cutting off your fuel supply until the boat becomes more level. Or perhaps the valve itself is bad cutting off fuel intermittently. Just trying to get the whole picture as you have asked this question a couple of different ways through your posts.

 

[silvert]

Yes it is the same as it was last year. Since then I've played around with the metering rods and step Springs but no real change. Rebuilt car again, hoping something was missed the first time. But there wasn't. Changed plugs, wires, cap, filters, compression check, timing check, cleaned spark arrestor. Few things I haven't done was check voltage at coil, which I'd like to do, replace coil, replace carb or just bite the bullet and send it to a place that works on boats. Scared that I will rack up a $1k bill for stuff I've already done/checked.

 

[achris]

Also need to know what the propeller family is.. Mirage, Venture, Raker, Vengance, etc.....
If the ratio is 1.62:1, then 21" is a lot of prop to be swinging for a 20' boat... (I know as I had a 1994 4.3LX previously in this boat VVV)...

 

[silvert]

What would a proper prop to run?

 

[achris]

My boat currently has a 4.3MPI with a 1.62:1 drive, and I run a Vengance 17". That's slightly under-propped, but I love the way that prop works. If I run a 19", I can hit 43 knots (50mph) at 4600rpm, but the accelerations is quite a bit slower and I can feel it hurting in the mid-range... Be careful of the Mirage props. They have great bow lift, but with a much larger diameter also tend to soak up lots of torque, so you need to run down as much as 4" to make them perform...

To find out if you have the 'right' prop. Load the boat 'normally' and take it for a WOT run. With that engine you should make 4400 to 4800rpm (which I think you are)... But if it's in the lower half, the engine will be working harder (think driving a car around one gear higher than normal, so cornering in 3rd, climbing hills in 5th etc).... Rule of thumb with prop sizes is each inch of pitch change (for the same family prop) will change the revs by about 200... So if you have a 21" and are getting 4500rpm at WOT, then changing down to a 19" will increase your WOT to about 4900... Which I would say is absolutely spot on...

Chris......

 

[keith2k455]

... Almost sounds like it is backfiring a little out of the carb...

Is it a backfire or a bog? Did someone happen to put a spacer plate under the carb? You may want to put a vacuum gage on it. It should read about 19 at a smooth idle. If you are below 15, there is either a vacuum leak, your carb is way out of adjustment or a worse problem.

I really think this sounds like a lean bog.

 

[Volphin]

Advance your timing a bit, and retest. Agree on the vac readings. My gut says take the primary jets up one size.

 

[achris]

DO NOT advance your timing. You're timing is already at 9BTDC. If you advance it beyond that you run the risk of detonation and popping holes in the pistons.

Chris.........

 

[Volphin]

3 degrees won't Chris... and is common for 4.3L setups. It's well within limits. I guess I should have specified... not like 10 degrees or such. 12 BTDC MAX and retest. If no improvement, go back to 9.

 

[Tail_Gunner]

If youve done your timing correct do a compression test followed by a vacum test at idle if that all pan's out move on to a new carb.Four barrel carbs can be tricky even for very experienced mech's and many times they just cant be rehased. there are a lot of circuts than can be clogged and you wouldnt know it.