Issues with timing, carb or both

[Sprintracing82]

Boat has a mercruiser thunderbolt 4. 4barl carb. It was a 1995, but has a truck block from previous owners. Plastic cover with notch. I think timing is set wrong, since it has no marks. Might be 8* atdc. And issues with stumble when taking off. First from dock, in no wake zone for a while. Then try to take off and it coughs a few times, then fine. Once warm it won't take off pulling tubers or skiers. I don't slam it to WOT from a stop I gradually feed it but i get to about 1/4 or 1/3 throttle and it seems like it's bogging down, like rpms drop, then will rev like normal past the 1/4 throttle. And diesels a lot when shutting off. Will run for 10-15 secs. Smells of fuel after wards.
any help. Any other questions to ask me to narrow it down.

 

[Scott Danforth]

welcome aboard.

need to know what you have

regarding the motor, sounds like carb issue, specifically accelerator pump is not working properly

 

[Sprintracing82]

1995 mercruiser 4.3lx 4 barl carb . Has a truck block with marine stuff. Thunderbolt iv ign.

 

[porscheguy]

Rochester or Weber carb? TBI truck block, or vortec?

The date of boat manufacture suggests it should be a Weber. IIRC the Weber/edelbrock carb is known to stumble if the accelerator pump isn’t working or adjusted properly.

TBI truck engine’s made up through ‘95 had full timing marks. Vortec engine’s from ‘96 onward had the single timing mark that you’ve described. The easiest way to determine this is counting the bolts on the intake manifold. Vortec blocks have two bolts at each corner, TBI blocks have bolts in the center as well.

the dieseling could be caused by:
1. Low grade gasoline.
2. Lean running.
3. Incorrect timing.
4. A combination of some or all factors above.

someone with more experience can chime in. If the trucks used the same timing cover as the mercruiser engines, then you can check the timing with an advance timing light set at 8 degrees. If you’ve got the timing light, you can rule out timing as a culprit in a matter of minutes and focus on the carb.

 

[porscheguy]

Edit: I stand corrected. In ‘95, the 4.3 appears to have updated to the newer single mark timing cover.

 

[Sprintracing82]

Yes it's a Weber I believe. It has a notch for timing mark and only one solid line on balancer. I will try to check timing, but with no marks hard to do. Just time at idle. So solid line should be to the right of notch for 8* btdc?
would changing position of accl pump help, or how do i check if it is working correctly?
it's almost 4th of July so not pulling carb til after that. But looking up stuff, seems like accl pump might be issue with hesitation.
but what about dieseling? I can check timing, and put higher octane gas in before next trip.

 

[Sprintracing82]

I have a advancing timing light. Do i put it on 0* during idle, or at 8* on the gun? And that's on left or right of balancer line? Just want to double check before I try

 

[porscheguy]

You have to put the boat in base timing mode. On top of the engine there should be a purple wire with a white stripe that hangs loose near the carb. Attach a jumper wire from the loose end to a suitable ground. That puts it in base timing mode. Set the timing light at 8 and see if the mark on the balancer lines up with the mark on the timing cover. If it does, you’re good to go. If not, make the needed adjustment until they line up. The only caveat here, and those more experienced will hopefully weigh in, is whether or not the mercruiser timing cover is the same as that on the trucks.

As for the carb, it could be time for a thorough cleaning or rebuild. This is where you need to determine if you’ve had a vortec block installed or a pre vortec as carb settings may differ. It sounds like an issue with the accelerator pump. Beyond that, my knowledge is limited.

One other thing, pull a spark plug or two and note their condition/color.

 

[Sprintracing82]

What would a vortex block be different? It's just the lower half. The, heads , intake and etc are mercruiser. I understand they did different intake or heads for vortex engines, but not sure how blocks are different. I don't know for sure.. I know this truck block has the balancer in it , just like the marine engine.
I thought I read on other post that grounding purple is only for thunderbolt 5.
so when timing put the gun to 0* or 8* and check.?And will it be to the left or right of line on balancer? One way is atbc other is btdc? Which one?

 

[porscheguy]

The blocks are essentially the same whether they’re vortec or not. The heads are what makes it a vortec, and the intake manifold is whatever was designed by GM because they’re identical whether they’re on a Volvo Penta or a mercruiser (only difference is color.)

Vortec heads are more efficient and flow much better than the prior non vortec heads. The reason I’m asking if the current engine has these heads is because I know the original engine did not. If the carb was set for non vortec and is now on a vortec engine, it could explain some of your running issues. Make sense?

and you’re right. TB4 has no base timing mode, just point and shoot. I think you’ll do it with the gun set at zero. I can’t remember which is before and which way is after. I just rotate the distributor one way or the other until the marks line up.

 

[Sprintracing82]

Checked timing today. With out other li es but the one on balancer. It was a few degrees retarded. Seemed to be close to 8*. Spark arresting was black and nasty from fuel backfire. Wet, black on inside of it, smelled like fuel. I cleaned it before timing. Was just cleaned at beginning of last season. I sprayed some cleaner in carb to clean it.
accelerator pump is on the highest hole. Should I move it down and see if it improves or worst.

 

[Rick Stephens]

With an advance timing light set it at 8º BTDC and the marks should line up at idle. Then set your timing light to 24º BTDC and run the engine gently up to 2000 RPM and it should also be close as the module advances the timing. Don't run but for a few seconds up over 1500 RPM.

The Weber carb is pretty easy to calibrate and dead simple to rebuild. The Mercruiser Service manual for that year engine has excellent directions with pictures on how to rebuild the carb. I would take about 2 hours to do it and the rebuild kit is often available from your local auto parts store. Chase every port, the Weber doesn't have all that many. Run a wire or heavy fishing line through them, use compressed air and carburetor cleaner to get them perfect. Replace the check balls and needle seat. Double and triple check float height. If you are running rich the usual culprit is float height or a dirty check ball in the accelerator pump.

There is no way I would expect to run through the fourth pulling hard with a carb crapped up.

Rick

 

[Sprintracing82]

I got it out for a little while over the weekend. It was late cause if the 110* heat. I couldn't find gas that didn't have a contain10% ethanol sticker. I.put some Lucas oil cleaner in fuel tank . It fired up great, and didn't diesel once. Didn't drive it much to get hot. But now there was throttle lag just past 1/2 throttle. Even if I was on plane running at half, soon as I give it more it would bog down for a few seconds then come out of it. Didn't matter if I hammered it or eased into it.
timing gun only worked in other direction. I turned nob but make moved farther away.

 

[Rick Stephens]

But now there was throttle lag just past 1/2 throttle.

Mainjet or needle plungers are crudded up.

 

[Sprintracing82]

What is best rebuild kit? I see a bunch of cheap kits on eBay, some don't have both gaskets some have accelerator pump, other don't. Is walker a good set? Can order through oriellys. Or get edelbrock 1477 from jegs and accelerator pump from edelbrock, not sure of part # on that yet

 

[porscheguy]

What are both gaskets? One for the float bowl and one for the base to the manifold?

sierra 18-7749 appears to have both plus the accelerator pump.

if you’re new to rebuilding carbs, a few words of advice.

1. Take pictures of the carb up close while it’s still on the engine. Pay attention to the linkages.
2. Take more pictures of the linkages while the carb on the work bench.
3. Before you remove any adjustment screws, screw in all the way until they’re lightly seated. Count the number of turns until they seat. Write it down if necessary. This way, when you get it back together, at your discretion you can install the screws to the previous settings or to whatever base settings in the service manual.
4. If the carb is painted, most aerosol carb cleaners will strip any paint right off. The dip cleaners aren’t as harsh.