Low Power on Rebuilt Merc 120

[GaJeff]

Hey guys,

Just finished up a 100% rebuild on my 81 120. It's running great at idle and does a pretty good job when under a load but it does seem to be low on power. I ran it on the test stand for 2hrs with no problems. Broke in the cam at 1500 rpm for 30mins of that 2hrs and varied the rpm the rest of the time, no load of course.

I put it in the boat and it was doing good just a little slow towards the top end and kind of stumbles when I slow down from plane around 2400 rpm and below. I double, triple checked everything. Timing is correct and advances like it should. Dwell is within spec. I checked the carb while underway and it seemed to be a little rich. I adjusted the float and checked needle and seat. I ended up swapping from #52 jets to #60 jets. Its not stumbling anymore and is running fine all the way through the rpm just seems low on power.

Before the rebuild I had accelerator pump problems so i rebuild the carb then and it ran great with plenty of power and 4500 rpm. A hole in my riser is what caused the rebuild. So fresh engine now but low power. I'm only running 38-4000 rpm now.

Do I need to break this engine in more before I will see more power or is something else possibly wrong? I have 4.7hrs on it so far.

Thanks Jeff

 

[alldodge]

Re: Low Power on Rebuilt Merc 120

Hey guys,

Just finished up a 100% rebuild on my 81 120. It's running great at idle and does a pretty good job when under a load but it does seem to be low on power. I ran it on the test stand for 2hrs with no problems. Broke in the cam at 1500 rpm for 30mins of that 2hrs and varied the rpm the rest of the time, no load of course.

I put it in the boat and it was doing good just a little slow towards the top end and kind of stumbles when I slow down from plane around 2400 rpm and below. I double, triple checked everything. Timing is correct and advances like it should. Dwell is within spec. I checked the carb while underway and it seemed to be a little rich. I adjusted the float and checked needle and seat. I ended up swapping from #52 jets to #60 jets. Its not stumbling anymore and is running fine all the way through the rpm just seems low on power.

Before the rebuild I had accelerator pump problems so i rebuild the carb then and it ran great with plenty of power and 4500 rpm. A hole in my riser is what caused the rebuild. So fresh engine now but low power. I'm only running 38-4000 rpm now.

Do I need to break this engine in more before I will see more power or is something else possibly wrong? I have 4.7hrs on it so far.

Thanks Jeff

Did you bore the cylinders and use over size pistons?

Numbers for your engine from Manual 3 are:
Compression 140 PSI
WOT 3900 to 4300 RPM
Plugs MR43T gap .035
Point gap .022, dwell 28 to 34 degrees
Timing 8 BTDC

Did you replace the fuel pump? Have you measured your fuel pump pressure?

Check that your anti siphon valve is not hanging

 

[GaJeff]

Re: Low Power on Rebuilt Merc 120

Yes it did get bored... 20 over and 10/10 for rod and mains.

no I did not check fuel pressure or replace the pump. Anti siphon valve is new. II just got done with transom, stringers, and deck. I installed a 28gal tank under the floor. It was used but it did a pressure check before install and replaced the valve. It originally had a 19gal above deck on port side of transom.

compression is spot on compared to the 65 across before rebuild.

i just got back from a run and and i did get up to 4300 today. Only 4300 and with the current 4000 against so i guess it's doing better . I will check the pump pressure this weekend because it is circa 1981.

thanks Jeff

 

[GaJeff]

Re: Low Power on Rebuilt Merc 120

Just rechecked the compression today and with 8hrs on the engine i feel the break in has changed my numbers...
cyl.1 158avg
cyl.2 145avg
cyl.3 155avg
cyl.4 144avg

i used a compound vacuum gauge last night and the needle jumps rapidly between 17-19hg. I done some reading on how to read these meters and it says the valves in one of the cylinders may be too tight or worn guides. Possibly ignition problem. I'm not the expert on carb engines as i work primarily on diesel engines.

The ENTIRE engine has been rebuilt so i think i can eliminate worn guides. But my lash could be wrong and my ignition could be wrong as well. My dwell is set to 31* but that brings my point gap to .030-.031". Is there a problem with my dizzy cam or should i worry about dwell not point gap?

I'm going to open the valve cover and run the rack again to be sure.

Any advice would be appreciated

Jeff

*all of these values are cold engine, WOT, and plugs removed. No oil added to any cylinders

 

[alldodge]

Re: Low Power on Rebuilt Merc 120

I think your good with points, the vacuum gauge is telling the truth. You probably have some rockers to tight.

To adjust rockers you back them off with the engine running. Back them off one at a time until they clatter and stay that way, then tighten until the clatter stops. When clatter stops turn very slowly until you have one additional turn. Valves are set

 

[GaJeff]

Re: Low Power on Rebuilt Merc 120

Ok... reset valves, still have a rapid gauge. It evens out when i raise the rpm and vac goes to around 20-21hg. When i was building everything i installed a new exhaust flapper. I hasn't had one since I've had the boat so who knows how long that's been. If I'm having low power and the rapid gauge at idle could the flapper have something to do with this. I say this because it evens out at higher rpm... not steady but not rapid.

What other problems cause a rapid gauge at idle?

Jeff

*also I found that my accelerator pump has quit working. I rebuilt this carb at the end of last summer and ran it on the engine before the rebuild and it responded quickly. I thought I had a rich condition so i put those other jets in. I disconnected the throttle from the accelerator pump and tried to pump it but it just dribbles until about 5 quick pumps. So I guess I need to check that out too.

 

[alldodge]

Re: Low Power on Rebuilt Merc 120

Ok... reset valves, still have a rapid gauge. It evens out when i raise the rpm and vac goes to around 20-21hg. When i was building everything i installed a new exhaust flapper. I hasn't had one since I've had the boat so who knows how long that's been. If I'm having low power and the rapid gauge at idle could the flapper have something to do with this. I say this because it evens out at higher rpm... not steady but not rapid.

What other problems cause a rapid gauge at idle?

Jeff

*also I found that my accelerator pump has quit working. I rebuilt this carb at the end of last summer and ran it on the engine before the rebuild and it responded quickly. I thought I had a rich condition so i put those other jets in. I disconnected the throttle from the accelerator pump and tried to pump it but it just dribbles until about 5 quick pumps. So I guess I need to check that out too.

The exhaust flapper has no effect on the gauge. Fresh rebuild and this kind of reaction I would ask who rebuilt the heads and are they any good? Steady vacuum is created by a constant and smooth air being taken in by the carb. If any of the valves do not fully close air/exhaust is pushed by into the intake. This is caused by worn guides or valves

 

[GaJeff]

Re: Low Power on Rebuilt Merc 120

I know that 2 valves were replaced. I saw those. I was told the guides were done also. I deal with the guys on the regular for my diesel heads and have never had a proble. That doesn't mean that a mistake wasn't made here.

So I should start investigating the valves and guides? Should I do a cylinder leak down test to confirm the problem? What other possibilities should i consider as problems to check for?

Jeff

 

[alldodge]

Re: Low Power on Rebuilt Merc 120

I leak down test would be in order prior to disassembly. I PM'ed another here hoping he can put some fresh eyes on the issue.

 

[Walt T]

Re: Low Power on Rebuilt Merc 120

Bouncing gauge is always a pulse in the incoming air caused by something not closing or opening. Ignition misfire, plug wires not correct, cam lifter, rockers, valve both intake and exhaust.

Does the needle jump up or jump down? what is the reading that it jumps from? Did you try adjusting valves while it is running and the gauge attached? That can really point you in the right direction.

Sorry for the late response I am not good at checking my notifications.

 

[GaJeff]

Re: Low Power on Rebuilt Merc 120

It jumps between 16 and 18-19. Not sure if it's going up or down I would assume down. I did adjust the valves but not running. I just talked to the guy that did my machine work and he said to possibly check the intake manifold for cracks. Being that the intake and exhaust manifolds are the same piece I guess it's possible.

The only part of the ignitionignition that hasn't been replaced is the cap and that's because I can't find one locally. I'm going to order one, although I have my doubts it will fix it.

Once above idle... around 900-1200 rpm it evens out at around 21. Not steady but not rapid... more like vibrating

Jeff

 

[alldodge]

Re: Low Power on Rebuilt Merc 120

It jumps between 16 and 18-19. Not sure if it's going up or down I would assume down. I did adjust the valves but not running. I just talked to the guy that did my machine work and he said to possibly check the intake manifold for cracks. Being that the intake and exhaust manifolds are the same piece I guess it's possible.

The only part of the ignitionignition that hasn't been replaced is the cap and that's because I can't find one locally. I'm going to order one, although I have my doubts it will fix it.

Once above idle... around 900-1200 rpm it evens out at around 21. Not steady but not rapid... more like vibrating

Jeff

Your machinist may be on to something with the intake. If there is a crack that would cause a vacuum issue. During high idle the vacuum gauge is moving less due to the higher rpm, and an the responce time of the gauge.

Don't think it's the cap

Your first post mentioned it had plenty of power after you rebuilt the carb, and you were turning 4500 rpm (bit to high). While need to figure out the gauge bounce, do you thing the carb may be part of the issue still. How long did it sit with fuel in it before the rebuild started?

 

[Road Rash Dave]

Re: Low Power on Rebuilt Merc 120

A bouncing vac reading is not a vacuum leak its a valve issue vac leaks just cause low vac and high and or surging idle..there is a valve not seating properly ...cam timing will kill performance and vacuum.. but not make the gauge flutter

 

[GaJeff]

Re: Low Power on Rebuilt Merc 120

That's what I was afraid of. Since 2 valves were replaced, it's probably one of those...i really don't want to pull this head again. It WILL be warranty work if something isn't right in the head.

Should I do a leak down test to find the leak? Also if a valve was leaking wouldn't I have low compression numbers?

Jeff

 

[alldodge]

Re: Low Power on Rebuilt Merc 120

That's what I was afraid of. Since 2 valves were replaced, it's probably one of those...i really don't want to pull this head again. It WILL be warranty work if something isn't right in the head.

Should I do a leak down test to find the leak? Also if a valve was leaking wouldn't I have low compression numbers?

Jeff

See Post 9

No, compression is done much slower then with the engine running (no bounce)

 

[GaJeff]

Re: Low Power on Rebuilt Merc 120

Thanks....i missed post 9... I've been going back and forth with myself on whether to do it or not. So if I have time I'll get the tool, if not I'll use the compression test hose and shop air

Jeff

 

[888]

Re: Low Power on Rebuilt Merc 120

Hey guys,

I ran it on the test stand for 2hrs with no problems. Broke in the cam at 1500 rpm for 30mins of that 2hrs and varied the rpm the rest of the time, no load of course.

Did u leave rpms at 1500 while breaking in the cam or did u vary the rpms during the 30min period?
If your didnt vary rpms during break in you prolly wiped the cam
Did you use break in lube? or oil real high in zinc?
BTW accel pump has nothing to do with it,you can slowly work into throttle to avoid the need of accel pump for now

 

[GaJeff]

I was having such a hard time figuring out how to post anything since the upgrade. I logged in 5x before i was able to get it... GEEZ... Anyways, I have an update on the engine. I found a leak a the manifold to head gasket. I sprayed some ether to the gasket area and found the engine would speed up. What has me stumped is it only does it when i spray in the center behind the choke oven. So i guess i could have a gap or something big enough to make to the intake ports. I pulled the manifold and checked for leaks with acetone. I'm good there. I checked for flange flatness and I'm good there too. I done the cylinder leak test and this is where I think i found my problem.i bought a new gauge set from matco. It doesn't read like a traditional one where as the gauge has percent readings. You have 2 gauges that read 100psi and you read the drop and figure the percentage. 90psi out of 100 is 10% 60psi is 40% and so on... These are my numbers Cyl. 1 92psi 8% crank case leakage Cyl. 2 75psi 25% exhaust valve leak and crank case leakage Cyl. 3 85psi 15% exhaust valve leak and crank case leakage Cyl. 4 70psi 30% exhaust valve leak and crank case leakage Now I'm not sure if my valves weren't lapped or if my springs are weak. If I lift up on the spring the psi starts to come up. I done this test before i installed my manifold so i could actually feel and hear the leak. The gauge says to get the engine to temp first but I doubt it would make that much of a diffenence. So ill be calling the machine shop in the morning because this is something they should have caught or repaired in the first place. I was clear about wanting it done right... it's to much of a hassle to keep.pulling parts to have them "repaired". Is it possible that my springs ARE weak or should I just take it down and replace the valves? The new valves they put in look to be intakes as they are a different color than the rest. long post... i know...thanks Jeff

 

[GaJeff]

It took away all my paragraphs... WTF sorry