RPMs wont go above 4k...not overpropped.

[Railroader12]

I posted earlier, realized it was in the wrong section, hoping I get some help over here.

New to me last week 2006 Tahoe 228 22.5' deck boat, 5.0 Merc TKS Im nearly positive its thunderbolt V. Engine bounces off limiter in neutral will NOT rev past 4k boating. Went from a 15.25x21" prop, to a 14.5x19" to a 14.5x17 yesterday and GPS speed doesent change a single MPH (31 max) nor does the Tach show much difference in RPMs between the 3(maybe 50RPM). The only difference is it becomes easier to plane on the 19 and 17". Engine runs Great and very smooth, I cant hear any misses or hicups, only thing to note is it smells pretty rich at idle..It just has no top end whatsoever. Tahoe claims the boat has a top speed of 46? with a 14.5x19" prop, Im not even in the same galaxy...

I removed plugs and few were BLACK and they were all gapped to .060, book calls for .045 so I changed them and gapped accordingly, base timing was 4 I set it to 10 as book states in base mode with the purp/wht wire grounded, I then re-set the idle speed and idle mixture screw to specs and replaced the cap, rotor and coil since there was some oil leaking in the cap and to rule any of that out. I ran a cold compression test and all 8 show 160-165 so were good there. Block and heads were replaced recently and they swapped everything else over from the old block. Think the old block had cracked due to sitting in a marina unwinterized for a cpl years. Has a new fuel filter/water seperator and fuel lines and the tank is clean with good fuel. All plug wires go to the right plug.

I read about merc's "Guardian" or limp mode, thought maybe that could be it.....after HOURS of digging I cant find a single thing relating to either for the TKS, TBV ignition. Alarm doesent sound when key is turned so there is obviously something not hooked up correctly there or the alarm is bad, but, I dont think this engine had the limp mode anyway so there went that. All gauges work correctly on the dash, slightly high oil psi but Im not concerned with it, temp stays where it needs to be. This engine has no knock sensor to retard timing either.

So? What do you guys think? Rather not unload gobs of money to figure this out, but not sure which direction to head next, I was sure the new ignition items were going to solve the issue....not so lucky.

 

[Railroader12]

Thunderbolt V Ignition

Spark Control Features
IDLE SPEED SPARK CONTROL
The ignition module controls ignition timing to maintain a calibrated idle speed by making
small spark advance adjustments. This feature is only active between 400-700 rpm.

ACCELERATION SPARK ADVANCE
When accelerating, the ignition module may add more spark advance to the Base Spark
Timing Curve. The amount of spark advance added depends on how fast rpm increases.
This feature is also active between 1200-4000 rpm. Within this range, the module can add
approximately 10 degrees of spark advance to the base spark timing curve.

MEAN-BEST-TIMING (MBT) SPARK ADVANCE
During light load cruising, the ignition module maintains optimal ignition timing by making
small spark advance adjustments. At a given rpm, the module will add a small amount of
advance and wait to see if there is an rpm change. If rpm increases, it will increase timing
more. The module will continue to advance timing until it no longer gets an increase in rpm.
Conversely, if it senses an rpm drop, it will start to retard some of the spark timing. Between
1200-4000 rpm the ignition module can add approximately 10-15 degrees of spark advance
to the base spark timing curve.
NOTE: The Audio Warning System is also connected into the ignition module circuit. If the
audio warning system becomes activated by the closing of one of the audio warning system
switches, the MBT feature is deactivated.

HHmmmmm.......didnt see this before. Guess Ill look into this more, sounds like it could possibly be my issue. The ICM on my engine riser is a 861459-1 5.0 LG+ ALPHA which I beleive is the TB V ICM.

When I got the boat there was no warning alarm with the key turned, I pulled the inst/gauge panel out to clean some things up and realized the alarm was unplugged from the ground so I re-connected it but it still did not function and planned to come back to it after I got the RPM issue fixed. Ill investigate this and take a timing light with me to the lake today and see if I am actually getting any spark advance or not.

 

[HT32BSX115]

Engine bounces off limiter in neutral will NOT rev past 4k boating.

Howdy,

Welcome aboard!

PLEASE DON'T DO THAT! High RPM "reving" in neutral runs the risk of damaging the engine AND drive!! And it tells you nothing except that the rev-limiter functions!

After verifying the TACH (many are way off at higher RPM), drive ratio and speed, have a look at the following section for things that you can check.

It's worth perusing all the threads here but focus particularly on #15 which refers to possible reasons for not making rated max RPM.

https://forums.iboats.com/forum/engi...rch-version-of

Let me also add that item #12 in the "low RPM" thread talks about "overloaded". In a deck boat, (AKA a "Pontoon Boat") 1 or both pontoons may have enough water in the pontoons to make the boat heavy.........and slow.


Cheers,

Rick

 

[Railroader12]

Horn/alarm was dead....replaced it and I get a buzzer on key turn which then shuts off after starting when oil psi comes up. There goes that....this is really starting to **** me off ....

 

[Railroader12]

Ive only bounced off the limiter one time just to verify that it would, and that the ICM was working at least in that way. I dont plan on doing it again.

I guess next would be a fuel issue so thats where Ill head now.

 

[HT32BSX115]

Ive only bounced off the limiter one time just to verify that it would, and that the ICM was working at least in that way. I dont plan on doing it again.

I guess next would be a fuel issue so thats where Ill head now.

If you haven't verified your tach by changing it for a known tach, you're wasting your time checking anything else until you do.

 

[Railroader12]

Ive got a tach on order, the reason Im continuing my troubleshooting in the meantime is because a new tach is Not going to give me 15mph wether its reading more precise than this one or not. There Has to be another issue going on here. 3 seperate props, same exact GPS speed and nearly identical RPMs. Boat should have a top speed around 45mph, Im getting 31....

 

[Railroader12]

To add to that, changing from a 15.25x21 prop to a 14.5x17 should have made a big difference up top, possibly even maxxing out on the rev limiter but SURELY changed the speed to a lower max right?? This whole thing is just very odd to me.

 

[HT32BSX115]

Ive got a tach on order, the reason Im continuing my troubleshooting in the meantime is because a new tach is Not going to give me 15mph wether its reading more precise than this one or not. There Has to be another issue going on here. 3 seperate props, same exact GPS speed and nearly identical RPMs. Boat should have a top speed around 45mph, Im getting 31....

True. But if the tach is way off, you may be over propped and not know it. The list covers about everything that could go wrong but to know for sure, you must know RPM, speed accurately and prop pitch + drive ratio.

Also for example, put a 1.36:1 or a 1.47:1 drive on a big heavy boat, (or do a backyard rebuild using those gear ratios) and don't tell the new owner, he'll chase his tail trying to figure out why the boat doesn't run right because the ratio printed on the side of the drive (and it's wrong) .

With "new-to-me" boats, that are not brand new, it's possible that it has the wrong drive ratio and by default the wrong prop,

Sometimes people sell boats where the drive was damaged and then rebuilt/replaced prior to the sale. Since they were hot to sell it quick, they grab the first drive that comes along (that may be the wrong ratio) on the other hand, put a way low ratio drive(or gears) (like 1.68/2.00:1) on it, with the same defective or mis-calibrated tach and you may be up against the rev limiter all the time.....and with a tach that doesn't read above 4000rpm, it's double trouble!

The easy stuff to verify first is the drive ratio, tach and prop pitch etc......

Looks like you're on the right track!

Cheers,

Rick

 

[Railroader12]

Okay, hooked up the vacuum gauge and something isnt right. The vacuum gauge is swinging from 11-14hg at idle fluctuating from 500-750. The engine was not at temp yet, as I was working on it in my driveway and trying to not **** off my neighbors so I have investigate this more out on the lake this afternoon. Air/fuel mixture is set to the book as is the idle speed.

Im beginning to think its possible that they literally swapped over the block and didnt even check the valve lash at the marina that did the work before I bought it. That in combination with some junk in the carb somehwere or a faulty fuel pump. I pulled the carb filter and it looked brand spanking new as does the water seperator so they probably had these problems and tried replacing the low cost items to remedy it, then just sold the boat for cheap rather than getting to the bottom of it.

 

[HT32BSX115]

its possible that they literally swapped over the block and didnt even check the valve lash at the marina that did the work before I bought it. That in combination with some junk in the carb

I made an assumption that your engine was stock and EFI. If it's Carbed, you'll clearly have another "fly in the ointment"!

Also, if it's an older engine that they "swapped", was used and has a flat tappet cam, It may have one or more flat lobes. (That happened to me with my previously installed OMG 460.......It tested 150-160 psi compression but had several flat lobes and would only turn about 4000rpm when it previously did about 5100rpm at WOT)

 

[Railroader12]

I made an assumption that your engine was stock and EFI. If it's Carbed, you'll clearly have another "fly in the ointment"!

Also, if it's an older engine that they "swapped", was used and has a flat tappet cam, It may have one or more flat lobes. (That happened to me with my previously installed OMG 460.......It tested 150-160 psi compression but had several flat lobes and would only turn about 4000rpm when it previously did about 5100rpm at WOT)

Im getting no pops, backfires, missing or any other issues aside from low RPM. Really hope I dont have to pull this engine that would SUCK but I wouldnt rule it out just yet either. Block and heads are both from an engine they "SAID" had 5 hours on it. I pulled the valve covers and everything looks brand new though.

 

[tank1949]

Ive got a tach on order, the reason Im continuing my troubleshooting in the meantime is because a new tach is Not going to give me 15mph wether its reading more precise than this one or not. There Has to be another issue going on here. 3 seperate props, same exact GPS speed and nearly identical RPMs. Boat should have a top speed around 45mph, Im getting 31....

Put motor in natural with timing light on aimed at timing tab and check as you advance throttle. If it doesn't advance as you increase rpms, you have a direction to go. Good luck!

 

[Railroader12]

Okay....just got off the lake with the wife and 3 little ones. VERY hard to troubleshoot engine problems when they all want to play so what I could do was pay very close attention to the engine while out and heres what I noticed.

1) SUPER rich at idle..smell of gas strong enough when idling I made a mental note to make dam sure I had a charged fire extinguisher closeby just incase. My wife mentioned TWICE, "Ummm why the hell do I smell gas?"

2) It only surges on idle randomly not every single time I start the engine or come off a run. A few times we came off plane and slowed to idle it sat perfectly at 700 RPM and looked flawless no surging whatsoever but eventually, when I restarted there was that rich smell and surge again.

...at this point Im pretty darn sure I have a carb issue somewhere or the TKS is sticking in enrichment mode regardless of temp. I plan to pull it off and do a full rebuild to, at the very least, remedy the rich idle situation. Hopefully that will take care of the low RPM issue as well but it needs to be addressed first so Ill do that over the next few days before I go out again and report back what I find.

 

[tank1949]

Okay....just got off the lake with the wife and 3 little ones. VERY hard to troubleshoot engine problems when they all want to play so what I could do was pay very close attention to the engine while out and heres what I noticed.

1) SUPER rich at idle..smell of gas strong enough when idling I made a mental note to make dam sure I had a charged fire extinguisher closeby just incase. My wife mentioned TWICE, "Ummm why the hell do I smell gas?"

2) It only surges on idle randomly not every single time I start the engine or come off a run. A few times we came off plane and slowed to idle it sat perfectly at 700 RPM and looked flawless no surging whatsoever but eventually, when I restarted there was that rich smell and surge again.

...at this point Im pretty darn sure I have a carb issue somewhere or the TKS is sticking in enrichment mode regardless of temp. I plan to pull it off and do a full rebuild to, at the very least, remedy the rich idle situation. Hopefully that will take care of the low RPM issue as well but it needs to be addressed first so Ill do that over the next few days before I go out again and report back what I find.

Mine would be kicking and screaming and complaining all FNNNN day. Better go with someone with some degree of mechanical skill next time .

 

[tank1949]

I made an assumption that your engine was stock and EFI. If it's Carbed, you'll clearly have another "fly in the ointment"!

Also, if it's an older engine that they "swapped", was used and has a flat tappet cam, It may have one or more flat lobes. (That happened to me with my previously installed OMG 460.......It tested 150-160 psi compression but had several flat lobes and would only turn about 4000rpm when it previously did about 5100rpm at WOT)

Take off the carb breather (flame arrester) and see if it is dumping fuel at idle or high RPMs. Look down inside carb throat while running. If it is dripping at idle, that is a problem. If my memory serves me correctly, around 1500 rps the idle jets don't matter anymore on carbs. . Then, check timing advance. If not advancing, u aint going to burn fuel efficiently. Good luck!

 

[Railroader12]

while down for my outdrive issue (I have a seperate thread for that) I decided to remove the carb and rebuild it....go figure when I disassembled it I found out it was JUST rebuilt. new gaskets, float, needle and seat the whole 9.... there goes more pointless money for a parts that wont help help.

So, just to re-cap, The plugs were fairly new and looked like crap, a few were tan, 2 were wet fouled, a couple looked good. New dizzy, coil, cap and rotor, carb had a fresh rebuild, timing set correctly to 10 BTDC in base mode, good compression check and it still wont pull over 32mph GPS or over 4k RPM....The only items left to check now that all that is done will be an exhaust restriction, fuel delivery, valve lash or wiped lobes.
......or what I fear the most...some toasted valves

 

[Rick Stephens]

Don't assume that whoever did the carb rebuild knew anything about what they were doing. Carb rebuilds are the most often job that needs redoing. Go through it, check the needle valve especially carefully and wet float height.

 

[hoowahfun]

I would second what Rick said. Verify the rebuild was done properly. Check every measurement and adjustment on every part, linkage, etc.

Maybe you've got a linkage out of adjustment and your carb never really opens up all the way when the control is at WOT.

 

[Railroader12]

UPDATE!
Just finished rebuilding the carb, it had been freshly rebuilt and I was bummed out the entire time until I went to check the float level while re-assembling it...whoever rebuilt it obviously had NO clue how to set float level. Book calls for 9mm float level and 24mm float drop....go figure he bent the tabs to snot to get the float to sit at 24mm AT LEVEL and it hit a whopping 40mm on drop. He set the float level to the drop height setting, essentially allowing maybe 1/3rd of the fuel into the bowl than shouldve been there. Im shocked I didnt have any lean back fires with it set this way but it could possibly be the culprit to my low power,.....or at least I hope! Im still waiting on a friend to loan me the alignment tool before I reinstall the outdrive and get back on the water but Ill let you all know if it helped!