vacuum gauge reading 12

[Sashap]

mercruiser 3.0.
I hooked up a vacuum gauge to the intake manifold today. It reads about 12. While it was warming, it did jump up and down about 4-5 either way. When it warmed up, the jumping levellled off to the 12 reading. I attempted to set the idle screw but it was really hard to turn. So I decided to take it out. Apparently, the previous owner put quite a bend in it so it wobbled quite a bit as I was turning it out. The bend was about 1/3 into the thread. This explains why I couldn't turn it in to set it. I have to order another screw so I can't do much about adjustment for a couple of days.
My question is regarding the timing/fuel mixture relationship.

Why is it that the timing at base even though it reads 1degree BTDC and at 2500 RPM with the base jumper taken off, it reads about 12 degrees BTDC. So basically, if I set the idle mixture better (clearly it needs it as it could run much better and stronger), will the timing improve?

Is the timing wonky? Is it just the idle mixture that is wonky? Symptoms are so so accelerations, gets up on plane a bit slow but middle performance. I just don't think it would easily pull up a skier at this rate. Also, top rpm at 75% throttle is about 3300 RPM. It planes out and goes smoothly, just doesn't feel as fast as it should. When I give it the extra bit of throttle to the top, I can hear a slight increase in engine power but there is just no extra speed increase or even RPM increase.

I read that with a properly set idle mixture it would give me the kick it needs to bring it out of the water fast and give me full throttle all the way to the end of the lever. Right now, I don't have either. I don't think it's the prop because it is a stock 19" and the prop wouldn't have any effect on the top 25% of the throttle.

Ideas Gentlemen? And Ladies of course!

 

[Sashap]

Forgot to mention that the carb has been rebuilt, cleaned, checked etc. Air and fuel is flowing properly and there is no stumble in acceleration to indicate fuel delivery issue. I am confident that the issue is mixture related or some instance of timing. Not sure which. The timing also does not seem to sound too different in base mode from regular, indicating to me that the idle mixture is wrong because the idle is just ok and it sounds slightly off in that it isn't 100% smooth, but a bit rumbly.

 

[achris]

Done a compression test? Have to ensure the engine is healthy before we start looking for other reasons for poor performance. Do compression test and verify the engine is good, then get the timing right, THEN work on the fuel system. Seems you are coming at this backwards. Once you have those in order, then look at what the maximum revs are at wide open throttle. If they are not in spec (and to know that we need to know EXACTLY what your engine is, so how about a serial number?), we play with prop pitch....

Chris.........

(what a lot of people do is put all the engine details in your post signature, like I have... That way, you don't have to remember to add it to the first post, and it's there for us to reference on each and every post, meaning on long threads we don't have to scroll back to the first post... )

 

[Sashap]

first thing i did was compression. it was even and within a few pounds of each other. cant remember the exact number but based on looking it up it was normal. i just spent a week dealing with all aspects of the ignition. all new parts except distributor body which was in great shape and wires/plugs which i will do, but i did gap the plugs. wires are in great shape with no leakage. timing was set at 1 degree btdc as per manual. serial 0D706324 good idea on the signature! will do!

 

[achris]

Looking at vacuum gauge interpretation charts, it's pointing at late valve timing... These engines have a steel gear on the crankshaft and an aluminium gear on the camshaft... Unless it have stripped a tooth and jumped I don't see it. But, do you know if the engine has been apart recently?

Chris..........

 

[Scott Danforth]

what is the problem you have exactly. read your post 3 times, and I could guess that you want more power.

its a 3 liter. if you have a 16' boat, you can get away with a 19p prop. you may need to drop down in pitch. so what is this 3.0 in?

timing is fine. set base timing at 1 BTDC and then when you remove the jumper, the ignition module controls it from there.

what was your compression numbers?

 

[Sashap]

Looking at vacuum gauge interpretation charts, it's pointing at late valve timing... These engines have a steel gear on the crankshaft and an aluminium gear on the camshaft... Unless it have stripped a tooth and jumped I don't see it. But, do you know if the engine has been apart recently?

Chris..........

As far as I can see the engine has never been touched. Idle mixture screw was bent and could not be adjusted. As I backed it out it didn’t really get much worse running though.

also, I couldn’t find how to edit my signature on my profile...

 

[Sashap]

what is the problem you have exactly. read your post 3 times, and I could guess that you want more power.

its a 3 liter. if you have a 16' boat, you can get away with a 19p prop. you may need to drop down in pitch. so what is this 3.0 in?

timing is fine. set base timing at 1 BTDC and then when you remove the jumper, the ignition module controls it from there.

what was your compression numbers?

Yes, lack of power but more concerned that low rpm on wot. Bayliner classic 1950. Yes, it’s a bigger boat but all info I read indicates from other same boat/motors max rpm 4400+ and higher speed using same prop. Can’t remember exact compression but I think 140ish? I can check again today.

The carb was a mess and clearly someone before tried to adjust mixture but stripped the screw head because the screw was bent and couldn’t be adjusted in. Will try to find a new one today.

 

[TurtleTamer]

My guess is you have a carb with an unsuccessful rebuild. Sounds to me like it's running rich and lugging. Icing on the cake is a damaged idle screw. How does one even do that? I'd also be concerned that the carb body is stripped along with the screw. I'd go back through the carb and go from there.

 

[Sashap]

My guess is you have a carb with an unsuccessful rebuild. Sounds to me like it's running rich and lugging. Icing on the cake is a damaged idle screw. How does one even do that? I'd also be concerned that the carb body is stripped along with the screw. I'd go back through the carb and go from there.

I think so too at this point. The internal rebuild is fine. I checked it FOUR times afer the rebuild just in case! The damaged screw body is bent at maybe 20 degrees! I think the previous owner must have dropped it. The screw itself wasn't stripped, just the slot for the screwdriver was so bent that I had to use a wrench to take it out as it was jammed in from the bend just at the beginning of the hole in the carb. The threads look unscathed on both the screw and carb. Just the bend prevented further screwing in to adjust it properly.

Also, another question is that when I use the trim the tach jumps up. What do I look for to fix that? Or just leave it?

 

[achris]

Idle mixture won't have any effect on wot. But the tacho jump piqued my attention. If there's a power problem, then that could have an effect on wot....

Go through all the main power connections.. positive and negative. Remove them, clean to shiny and refit. Don't forget the clean the engine block where the cable attaches too. ..

Chris. ...

 

[Sashap]

Idle mixture won't have any effect on wot. But the tacho jump piqued my attention. If there's a power problem, then that could have an effect on wot....

Go through all the main power connections.. positive and negative. Remove them, clean to shiny and refit. Don't forget the clean the engine block where the cable attaches too. ..

Chris. ...

I read the other day that idle mix will definitely improve how the engine responds from out of the hole.
Also, I agree about the power connections. Boat has sat over 7 years from previous owner with no use. That is why the carb needed a rebuild. Amazingly, the gas in the tank ran my car perfectly! I thought I would try it in my lawnmower and it ran great so I poured the remaining 20 gallons into the truck. The carb, on the other hand stank something awful. That's where the whole rebuild started. I also verified every gas connection, line, fitting etc and all was perfect condition. The pump is strong and when I took it apart, the diaphragm was strong and like new. Did not do a pressure test on fuel, but now I can use my vacuum gauge for that as it is dual purpose.
I did notice that the anti-siphon valve required a bit of extra "puffing pressure" to push down the ball on the spring. But it was clean and shiny and did move freely. I think the spring is stronger than my "puffer" could push!

 

[Rick Stephens]

Carburetor idle circuits do not have any effect once the throttle plates are opened. They are designed to inject fuel into the tiny stream of air going by at the edge of the plate. Once the throttle plates start to open the idle ports stop injecting fuel and the midrange ports and main jets take over. Idle adjustments are just for idle.

Carburetor adjustments do not effect timing. All you can do is set using base timing mode, then check that the timing is around 10-12º BTDC when base is removed. If you want to make sure the module is working then check that the timing is advancing to somewhere around 26º BTDC when the engine gets up around 2000-2200 RPM. You need an electronic advance timing light to check the amount of advance, but can certainly see the advance without knowing what it is exactly with a standard timing light.

If your engine is running correctly, and you are prop'ed correctly, then your RPMs at WOT throttle should be between 4400 and 4800 RPMs. That prop may be too tall for that size boat, and it certainly sounds like the engine is not putting out proper amount of power. Something else to consider, the boat may also be waterlogged and heavier than the motor/prop combination was designed to push. Depending on how the boat was stored these last years of disuse, if it got a lot of rain water in it, it may have soaked a bunch up in the foam.

 

[Sashap]

When I was doing the base mode timing it looked correct as far as I could tell. However, the last time I was checking it the I accidentally pulled the jumper off the white wires and the engine stalled. I then reconnected it and restarted but honestly, I couldn't hear the difference in idle when it was hooked up. It does read 1 BTDC in base and out of base when I rev to 2500 I can see the timing mark has moved up past the timing numbers where I could barely see it. It does not seem to change when I unjumper it using the exact method as in manual. This is the third module I tried and it definitely works compared to the other 2 as I couldn't even get past 2000 rpm with those first ones.

The waterlogging could be an issue as it was just sitting out there. That being said, under the carpeting it is fiberglassed plywood and is very stable as well, the drain plug was always out so no water was sitting at all. I will find a hidden spot and drill an inspection hole just in case. It does feel heavy when I trailer the boat but it is naturally twice as heavy as my other boat so I do feel the difference. I might go weigh it too.

But the weight doesn't account for the throttle issue. When I throttle up on the water up to about 75% it keeps increasing. But when I use the last bit of throttle, I can hear the engine work a bit harder but no increase in speed or rpm. If it were weight, then it would be a constant ramping up an not go flat at that point.
I did at one point pour extra fuel in the carb when on the water and it didn't have any effect, but that was during the bad module issue. I will try that again for sure now because this might give me some answers. Could the float bowl be too low at the top of the throttle? If so, do I lift the float a bit? It's adjusted to measurements right now. How do I know if it is set too high?

Which further raises the question that if the fuel delivery at WOT is not enough, would this cause an issue regarding not enough rpm?

Would I gain a better hole shot if float is adjusted better too?

I will change the cables and plugs today as that needs to be done anyway as a point of maintenance.

 

[TurtleTamer]

All you're really hearing in that flat spot at the top of the throttle is the bores being fully opened. It makes it a bit louder, kind of "growling" more but not necessarily doing more work. I would think you're overfueled vice running lean because if lean, you'd most likely hear the motor pinging. Kind of sounds like loud "snapping" and it would concern you.

You should be propped about right for that boat. It'd probably be a bit slow out of the hole but shouldn't drop your WOT that much. You should be up to at least 4200-4400 even if you're a bit overpropped. A 17p may be better but again, the 19 should work.

Some things I'd check in the carb: gasket(s) in the venturi area. A leaking cluster gasket will bypass the venturis and flood the carb. There's also a small gasket on the middle screw but that shouldn't let a crazy amount of fuel in. The top gasket could also leak into the bores. To check these areas, removal of the top of the carb would be best but you can use a flashlight to peer down into the carb and see if it's wet around the venturis. While you're at it, run the thing on muffs and watch for dripping fuel while someone else shuts the engine down. That's a dead giveaway something is wrong. Last thing would be to check the jet size. You have to remove the jets to be able to read the numbers but most 3.0s use 1.55mm jets. If the previous owner ever swapped carbs, he/she could have put the wrong one on there. Not really that uncommon for someone to send theirs in as a core and get one back set up the wrong way. That could even explain the bend in the idle screw: shipping damage. Maybe that's why the boat sat so long. Previous owner replaced the carb and "could never get it to run right" so it sat.

I think you have your timing issue corrected at this point.

On idle mixture affecting hole shot: I've read something similar in that running the idle a tad richer will help in the transition to the mid circuit. That's not how these work though. Idle mixture is to be set for idle only and not anticipation of other circuits.

I also have forgotten since your other thread, have you checked compression?

 

[Scott Danforth]

A 3 L motor and a nearly 20 foot boat that maybe waterlogged and a 19 pitch prop will not turn 4400 RPM

3 liter in my 16 foot boat and a 19 pitch prop was all I could do to turn 4600 RPM

 

[Sashap]

could be.
I have to check it.

 

[Silvertip]

I don't know how you can bend an idle mixture screw 20 degrees and even get it in the hole. If it was bent that badly, the metering hole is like damaged as well (such as partially plugged or not metering at all.)

 

[Sashap]

All you're really hearing in that flat spot at the top of the throttle is the bores being fully opened. It makes it a bit louder, kind of "growling" more but not necessarily doing more work. I would think you're overfueled vice running lean because if lean, you'd most likely hear the motor pinging. Kind of sounds like loud "snapping" and it would concern you.

You should be propped about right for that boat. It'd probably be a bit slow out of the hole but shouldn't drop your WOT that much. You should be up to at least 4200-4400 even if you're a bit overpropped. A 17p may be better but again, the 19 should work.

Some things I'd check in the carb: gasket(s) in the venturi area. A leaking cluster gasket will bypass the venturis and flood the carb. There's also a small gasket on the middle screw but that shouldn't let a crazy amount of fuel in. The top gasket could also leak into the bores. To check these areas, removal of the top of the carb would be best but you can use a flashlight to peer down into the carb and see if it's wet around the venturis. While you're at it, run the thing on muffs and watch for dripping fuel while someone else shuts the engine down. That's a dead giveaway something is wrong. Last thing would be to check the jet size. You have to remove the jets to be able to read the numbers but most 3.0s use 1.55mm jets. If the previous owner ever swapped carbs, he/she could have put the wrong one on there. Not really that uncommon for someone to send theirs in as a core and get one back set up the wrong way. That could even explain the bend in the idle screw: shipping damage. Maybe that's why the boat sat so long. Previous owner replaced the carb and "could never get it to run right" so it sat.

I think you have your timing issue corrected at this point.

On idle mixture affecting hole shot: I've read something similar in that running the idle a tad richer will help in the transition to the mid circuit. That's not how these work though. Idle mixture is to be set for idle only and not anticipation of other circuits.

I also have forgotten since your other thread, have you checked compression?

I just finished taking off the carb and dealing with that idle screw. Put two new gaskets on the bottom half and bottom. I turned the key and in one crank it started right away. I tried adjusting the idle but it didn't make much difference either way. Ran ok I guess. Hooked up the vacuum gauge and it shows better than before. just a hair under 15. Before I was lucky to sit at 12. The needle only jumped 1 while it was warming up but stopped when warm. I could hear some hissing but wasn't sure if it was through the carb or where and really it could have been some other small noise. I did have the flame arrestor off and when i cupped my hand over the carb it still sounded like it was elsewhere. Not sure what it is at this point. I went to get some wd40 and from the shop I could hear a change in the engine sound. Got back in time to see it stall out. Turns out it was the vacuum hose came off with the tester. Put it back on and restarted. It started for a minute and just died. I could not start it no matter what. I am hoping that the gas is out because that would be better than who knows what. I will try wd40 when I get a chance later. At least I have 15 on vacuum. I know it's not ideal, but it's a step in the positive direction.

Last time I checked, there was no dripping fuel in or out of the carb. I had that issue before and I changed the gaskets and put on new screws and it was tight as a drum later. There was major leaking after shut down at that time right in the barrels. If this doesn't improve much I will look at getting another carb. There is a guy that sells used parts and is very reputable and stands behind his products. He has one for me if I need it. This one has just too many little things that have been issues since I started this project and honestly, I'm tired of it. Seems like I am constantly finding something else that I need to "patch". At least I will have spare parts!

The accelerator pump works great though, so I am not concerned about acceleration. The water log issue I will look into later this afternoon. Drill a hole here and there. (maybe from underneath!) Just kidding.

 

[Sashap]

I don't know how you can bend an idle mixture screw 20 degrees and even get it in the hole. If it was bent that badly, the metering hole is like damaged as well (such as partially plugged or not metering at all.)

it wasn't bent at the tip. it was bent half way up the screw. Couldn't tighten it further or far enough. I just spent a couple of hours in my shop and made a new one on my lathe. Put it on and it runs. Stupid thing is that it doesn't make any difference! It idled ok as it was. Regardless, this time was also spent putting on new gaskets and ensuring everything was tight etc. Put the carb on and it started right away. Vacuum improved to just a hair under 15. Not great but a positive step in the right directly. Still have to spray wd40 around the base to see if it leaks.