Bog @ 3400 rpm

[Rick Stephens]

Is it normal that one motor would act differently than the other..? There is definitely a difference between the two....
Are you thinking that I am lean, or rich?

If identical motors with identical wear, then no. Same calibration should work for both. Hence the suggestions you check float levels. I would also double check the numbers on the jets, make sure they are really what the box said they were.

 

[gtfireftr]

Motors are both new... Carbs are both new... twins in every aspect..... jets are the same, I checked upon installation ....the motor never backfired, just bogged down...
Cruising at about 3000 rpm's as per instructions on break in... then went up a bit to try to come up on plane... port motor revved up, and
starboard motor bogged down....(I was watching the tach's) ... once that happened I cut back on the throttle, and then pushed up on it again,
eventually it picked up and revved with the port motor....never popped, or backfired that I could hear.....
Not sure how to accurately explain, but that is kind of what happened.....
Je

 

[alldodge]

Is it normal that one motor would act differently than the other..? There is definitely a difference between the two....
Are you thinking that I am lean, or rich?

As I mentioned before

It won't be the first time the carb straight out of the box was not adjusted correctly

Could also be that the carb was bounced around in shipping a bit harder then the other one

 

[Rick Stephens]

Another place to check is the intake gasket. Spray a little carb cleaner around it at idle and see if the RPMs change.

 

[gtfireftr]

Ok so I got another 1409 (brand new)... put the jets, step up springs and metering rods from the thread Rick had posted.... Same as the port motor as well.....
Port motor revs up as expected..... I back it down once I approach 4k..... the Stbd motor gets to 3200 or so and never revs higher..... plenty of throttle left....
Only parts I did not replace are fuel pump, and antisiphon valve..... I am hoping my problem lies within those 2 parts...
Any thoughts....??

 

[Rick Stephens]

So, no intake leaks.
No valve issues.
Throttle linkage opens carb all the way?
New motor, so might check compression if you got suspicions.
Timing rechecked?
What ignition does it have? - Any chance a screwed up governor?

You know it is propped right?
(or is this happening on the hard and not on the water? - Revving a Mercruiser up over 15-1800 on muffs can collapse the water supply hoses.)

 

[gtfireftr]

Compression is good.... so valves are good.....
Timing is 8 BTDC
Thunderbolt 4
Props are the same on both motors, so I think we are good there...
This is happening on the water coming up on plane... NOT ON MUFFS.... NEVER OVER REV ON MUFFS....

 

[alldodge]

Check to see if your getting 14 degrees of advance at 2400 rpm, to total advance of 22 degrees (8+14)

 

[Rick Stephens]

Only because it' can do this - correct gear ratio on the outdrive? Is the motor bogging or is it running out of power?

I would not be thinking fuel delivery. It wouldn't run up to 3400 without displaying problems.

Have you taken the flame arrestor off and while running, opened the throttle and verified primaries are opening all the way? You can hear secondaries, so those are kind of moot. And unneeded to get over 3400.

 

[gtfireftr]

Same outdrives as previous engines.....I can't hear secondaries at all.... on either motor... I had 2 brl on previous engines, so not sure what to listen for....
It runs up to about 3200 with no sign of issues, but then when I come up on plane it does not rev past 3400-3500 at all... I am about 3/4 throttle, and push the throttle all the way down with no increase in rpm.....
I will double check timing tomorrow.... could a stiffer step up spring have an effect??
Pretty frustrated.... spent a lot of $$ and time and can't even go out..... ugh....

Thanks

 

[Rick Stephens]

Damn forum software doesn't load my answers. Hate typing it twice.



You will have to run the motor at 3400 for a little, then shutdown and pull a plug before you know what needs recalibration on the carb.

You should have someone run the boat while you look in the carb with the arrestor off to make sure everything is opening correctly.

Check timing, verify tach reading.

Check advance curve. You'll need a digital light. Easy to compare what you get with the other motor. Should get around 24-26? at 2000 RPM, if I remember right.

 

[gtfireftr]

So today I decided to get back to basics and verify everything I could....
Went to marina... fired her up... runs perfect.... got to operating temp... put my timing light on......
I put the #1 probe near the distributor because I could not get to the plug wire under manifold....
timing seemed very jumpy... really could not get a handle on the line....
Pulled all the plugs... they looked light tan, even maybe light tan... certainly not fouled with gas or anything....
Compression check is 180 plus or minus a couple on every cyl ... perfect in my eyes.....
Put a new set of plugs in... fired her up... still runs perfect...... put my timing light on again..
I put the #1 probe near the plug wire under manifold....this time the line was steady, perfect 8 degrees BTDC at idle,
Then revved up to about 2400 and got 22-24 degrees.....
I checked the antisiphon valve... it seemed fine....
I guess I will take her out again and see if I fixed it by doing pretty much nothing..... The only good thing is I did verify the engine is strong....
So I have either a fuel delivery problem, an ignition problem, or some other unknown problem....
Any suggestions??
Thanks

 

[Rick Stephens]

How close is dash tachometer to timing light tach?

Things I was worried about that seem resolved are internal on the motor... like timing chain off or valves incorrectly adjusted. No way it runs great up over 3k.

I'm running out of ideas.

 

[gtfireftr]

Tach and light are very close.....
Not good..... Should I go to a stronger step up spring? Perhaps this motor pulls more vacuum than the other one??
It is evident that it is either fuel, or ignition.... would you agree??

 

[gtfireftr]

I am gonna give it another shot tomorrow, but if that does not remedy the issue I may change to a Delco ignition....
I have one, and I was just concerned about the shift interrupt circuit.... Not sure how to wire that....
So that would eliminate ignition....

 

[Rick Stephens]

No way I would try and guess carb calibration without pulling a plug after quickly shutting down from a run at the speed you desire information about. I also don't believe it is fuel delivery. Swapping carbs already told you that. That leaves linkage opening all the way, which you have to eyeball.

 

[gtfireftr]

I agree with you... BUT,... I swapped carbs and used the same jets, metering rods, and springs as the other one....So it's like the same carb....

 

[alldodge]

Time to check fuel pressure

 

[gtfireftr]

Time to check fuel pressure

Where does that go, and where can I get one??

 

[alldodge]

Where does that go, and where can I get one??

It is Merc part number 91-18078 and can be found by searching the part number. Might also find one at the local parts store don't know.

The fuel line is removed from the carb and this is installed between line and carb. Then connect a vacuum fuel pump tester, find at auot store, HF and the like
https://www.amazon.com/Lisle-20300-Vacuum-Gauge-Tester/dp/B000CODRFO