4.3l lx help . runs slow

[Sprintracing82]

I have a 95 4.3l lx mercruiser runs slow . top speed is about 30 32 mph at about 3800 rpm. Thats is WOT. Carb was rebuilt last year. Didnt help. Runs rich. Diesels when shuts off sometimes. 14.25 21p prop . its also a truck block. Previous only f***ed me over.
Why am I only getting 30mph. Me wife 2 kids. Less then half tank of gas. 35gal tank. 1995 bayliner 2050ss. Any other i fo needed to better diagnose let me know. Sprintracing82@gmail.

 

[Rick Stephens]

Nothing wrong with a truck block, unless they didn't change to brass core plugs and a marine water pump. That's all Mercruiser purchases - right off the Chevy line.

Places to start... well first is redo the carburetor or even replace it it. Running rich causes dieseling and dieseling will ruin your motor when it kicks backwards and sucks water in through the exhaust. When you do the carb recheck and carefully set float height.

Next is do a compression check. Find out what the baseline quality of the motor is that you are starting with.

Follow the Mercruiser manual for that motor and set the timing appropriately and perfectly. You can read the online version at www.boatinfo.no

You may end up having to change the prop to one with less pitch. You need to make your engine run right, then check maximum RPMs at wide open throttle. Reprop to get the max with just you in the boat to 4800 RPMs.

 

[harringtondav]

Dieseling and rich operation are likely a carb float and idle mixture issue. Address these later. Low power diagnosis would start tart with checking out your fuel system from the tank to the carb. Remove your fuel filter water separator and dump the contents into a container and inspect. If there is crud, there is crud in your gas tank. You may be able to clean it up with several filter changes, but if it is severe you may need to drain and flush your tank.

Rebuilt carb. Lots of possibilities if you didn't do it yourself.

Other possibilities:Super dirty spark arrestor restricting air intake. Incorrect engine spark timing - too retarded. Stuck or malfunctioning choke on the carb causing a rich mixture and low air intake.

 

[Rick Stephens]

That's a good one to check!

 

[Sprintracing82]

I clean the arrestor all the time. I rebuilt the carb to try and get rid of the rich fuel smell and poor running. Helped some but still no top speed or rpm . I'm going to check timing make sure it is before tdc and not after. Maybe a new cap and wires ?? Taking it on a trip and want it to run better.. Its a big lake

 

[Rick Stephens]

Has it ever run properly for you? If you lug the motor through improper propping you are liable to break things. If you are running rich you need to do the carb again. Don’t feel bad, needing to repeat carb rebuilds is not uncommon.

You may very well need to drop to a 17 inch prop.

 

[nola mike]

Compression check, load test and or timing light on each cylinder. Dead or missing cylinder can cause those symptoms as well

 

[harringtondav]

I clean the arrestor all the time. I rebuilt the carb to try and get rid of the rich fuel smell and poor running. Helped some but still no top speed or rpm .

Two carb rebuilt issues that could cause rich operation are float height set too high. Don't know which carb you have but getting primary and secondary jets misplaced may also cause this. Check the linkage to the secondary to be sure it's not opening too soon, and is closing + needles dropping.

Your engine probably has Thunderbolt IV ignition. Base timing = 8 deg BTDC.

 

[Sprintracing82]

I timed it and it has no lines but one solid line across. It was a little to the left of the line. Meaninf the line on balancer is to the right of dash mark on engine. I also noticed valve cover saves for tech but it has a.c. mc43rt. Short ones. Could that be an issue? Pkugs not long enough? I am not sure what heads it is. I would not want to put one to ling and damge piston or valves. But if they are bot long enough it wont get good spark. How do I tell what i need? And is the timing the right direction? Seems to purr at idle.
Carb is a weber 4 barrel. Purchase kit from Dr carb .ot aomething. Had gauges to set bowls at and videos on his website.. Im not a newbie to engines.

 

[Sprintracing82]

I looked on theae forums and found stock prop was a 13.5 23p. So dont know if my prop is to big and not enough pitch? It is a thunderbolt iv. Thinking of getting new spark plugs and wires for shits and giggles but if they are to short it wont help. Would leaking intake be an issue. How to check that? Weather is rainy so cant take on water but have on muffs. Any help . willing to do anyrhing and hopfully find sonething that goes. " ah ha".

 

[Rick Stephens]

'95 was typically the last year BEFORE Vortec heads were installed. Easy to tell though, count the intake bolts. If there are 12 total, 6 per side, it is pre-Vortec and requires the plugs you have already. If there are only 8 total bolts, 4 per side, then you have Vortec and require the longer plugs.

Not really anything like a given for stock prop size. Prop is determined by RPMs at Wide Open Throttle. You want, no, you need to hit right near 4800 RPMs at WOT for the motor to be propped properly. Lots of other things can effect WHY it isn't reaching the correct RPMs - like engine isn't running right, water logged foam, hull covered with growth and so on. But no matter what, even if you are fighting with a heavier boat than when it was propped at the factory, it still need to be propped to reach 4800. And as a motor gets tired, they all need to be propped down to compensate for the lowering torque.

 

[Rick Stephens]

I timed it and it has no lines but one solid line across. It was a little to the left of the line. Meaninf the line on balancer is to the right of dash mark on engine. I also noticed valve cover saves for tech but it has a.c. mc43rt. Short ones. Could that be an issue? Pkugs not long enough? I am not sure what heads it is. I would not want to put one to ling and damge piston or valves. But if they are bot long enough it wont get good spark. How do I tell what i need? And is the timing the right direction? Seems to purr at idle.
Carb is a weber 4 barrel. Purchase kit from Dr carb .ot aomething. Had gauges to set bowls at and videos on his website.. Im not a newbie to engines.

The only way to check timing is with an electronic adjustable timing light. They are cheap. I bought mine well under $100. Then yuou also must follow the correct Merc manual on how to set timing properly.

 

[Sprintracing82]

No growth on Hull. It stays out of water and I was it yearly. Prop was purchased on Facebook just to have one. As boat came with a plastic prop.
Is it timed right direction then? Could cap and wires not let me get full rpm and speed. ? What am I missing ?

 

[Sprintracing82]

I got a timing light. Wouldnt a 17 pitch make it slower? Higher pitch is more top speed. Smaller size prop would be more rpm

 

[Rick Stephens]

I got a timing light. Wouldnt a 17 pitch make it slower? Higher pitch is more top speed. Smaller size prop would be more rpm

An electronic timing light lets you dial in what setting for a given RPM you are aiming for. The Merc Manual tells you both the procedure with Thunderbolt IV and where to set it.

So a pitch change also changes RPM. There are tools, one is one the Merc site, that help you understand how efficient your prop is. We usually see between 7 and 10 or so slippage with a good setup. My boat, which is pretty light for a 4.3L motor, comes out like this:



What that comes out to with the numbers you gave at the first post, is this for your boat. Notice how high the slippage is, that is prop turns that do not equate to 21 inches of travel,. You are losing 22 percent, which is quite a bit, not to mention it is lugging your motor:



What I think would happen with a 17 inch prop on your boat, all other things being equal, is about this:

 

[Sprintracing82]

I notice a small nipple for atube off distributor? Does this need a vaccum line for advancing timing. And if so whwre does it get vaccum.

 

[Rick Stephens]

I notice a small nipple for atube off distributor? Does this need a vaccum line for advancing timing. And if so whwre does it get vaccum.

That's just venting. It is 90 degree so sparks can't go through it. No tube attached. Advance is electronic from Thunderbolt module.

 

[Sprintracing82]

I noticed a tube off distributor . is a vaccum line supposed to be on it to advance timing with vaccum? If so where do i get the vaccum from?

 

[Sprintracing82]

Ok. I can borrow a 19p prop from a friend a give it a shot. Just thinking its more motor issue. I pulled out #4 plug. Looked great.

 

[Bondo]

I noticed a tube off distributor . is a vaccum line supposed to be on it to advance timing with vaccum? If so where do i get the vaccum from?

Nope,..... Marine distributors don't have vacuum tubes,....... Donno what ya got there,.....