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TOPIC: can't get over 3500 rpms

can't get over 3500 rpms 17 Jun 2009 04:03 #1

  • mtbiker9000
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To start this a buddies boat that I am helping him with so please do not look at my profile for tech info. My buddy has a 1993 33 ft carver with 2 x 7.4l carbed mercs. He tells me that he has been having this problem for some time now. His starboard engine will rev up to 3500 rpms and just sit there, I drove the boat with him today and found nothing out of the ordinary. It seems to get to 3500 rpms with out any issues but just kinda stays there, there isn't much left to the throttle when it gets to 3500. Meaning its not like he gets to 3/4 throttle and then gives it more and nothing happens I mean to say the throttle almost maxes out just about when it hits 3500rpms, ther eis some left in the throttle maybe a half an inch of helm throttle handle travel where by no more power occurs.......?

Any suggestions, I know this is a vague one but I am at a loss here

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1995 Maxum 2700 SCR
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can't get over 3500 rpms 17 Jun 2009 04:53 #2

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Make sure the timing is advancing properly and their is enough fuel at that rpm. I dont know much more about carbed engines, sorry.

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can't get over 3500 rpms 17 Jun 2009 13:47 #3

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Yes, check timing.... not only BASE advance, but also what the ignition advance is doing at 3K rpm or so. TA or TAT.... (Total Advance Timing)
Base advance means absolutely nothing if the ignition distributor is not offering the proper progressive spark advance.
Both egines need to be receiving the exact (or very close) same ignition advance throughout the rpm range up to the limit rpm.

He likely has Mercs..... so these may be the Thunderbolt ignition systems..... of which use a module for the advancing (I don't care for this system)..... he may have a bad module.
See his OEM manul for trouble shooting info, timing specs and on how to set timing for these.
If he should want to move away from the TB ignition, there is a man who is selling the Mallory YLM series distributors at an excellent price. $150 including shipping!
He has both standard and counter rotation units .

The beauty of having twins is that with the cost of a few gaskets, he can switch caburetors from one engine to the other and then do some testing.

However, I'd sure start with some basics such as fuel flow, tank pick up tube, anti-siphon valves, filters and ignition first.

.





.

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Rick E. Portland, Oregon
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can't get over 3500 rpms 17 Jun 2009 15:02 #4

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Has he changed cap, rotor, plugs and wires lately?

I do the cap / rotor annually and keep a close eye on the wires and plugs.

These motors are drawing incredible current at these loads and any small degradation / corrosion will significantly impact performance.

I have to assume (since he has twins) that they were both setup the same (prop, timing, etc) so short of this motor never running right, I'd start looking at things that have changed on this motor alone.

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can't get over 3500 rpms 17 Jun 2009 16:02 #5

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Did you check to make sure the throttle plates are fully open in the WOT positition?

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can't get over 3500 rpms 17 Jun 2009 16:24 #6

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Wingman;311032 wrote: Did you check to make sure the throttle plates are fully open in the WOT positition?



I was thinking the same thing...first thing is to advance the throttles with the engine off...then go look at the carb..see if the throttle is all the way open....that will elliminate carb problems ..then I'd start looking at carb mounting...make sure it's torqued properly...about 120 in/lbs....
then look at the timing as suggested......

:arr arrr

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can't get over 3500 rpms 17 Jun 2009 20:11 #7

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I hope I don't have to point out to check the flame arrestor, those things get dirty/greasy and limit the air flow, thus hindering the performance of the engine.

Lou

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can't get over 3500 rpms 17 Jun 2009 20:29 #8

  • Ifallsguy
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Is anything hanging the throttle cable up on that engine? Any binding or out of adjustment can cause that type of problem.

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can't get over 3500 rpms 18 Jun 2009 01:04 #9

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thanks for all of the great advice guys, I will be performing a full tune-up and check everything you all just stated. On the other hand I would love to check the timing, however even though I am very technical I have no idea how to do this procedure or even where to start I have done some research and haven't gotten anywhere. My buddy has twin mercruiser 7.4l thunderbolt 4 ignition engines. Can anyone direc t me on where to find the corrct procedure fo rdoing this I have never checked timing on an engine before. Thanks everyone

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1995 Maxum 2700 SCR
Twin 4.3L LX mercruiser engines
Tenacity
Salem Ma.

can't get over 3500 rpms 18 Jun 2009 05:22 #10

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Start by looking at the harmonic balancers....... see if by chance they are marked off up to let's say 35 degrees or so. This would be clockwise of TDC on one engine/counter clockwise of TDC on the other.

If the balancers are marked off, you just go through the BASE timing procedure, but go back into Run Mode, and then increase the RPM and watch the strobe on the markings.
I do not believe he'll have an ECM or knock sensor on those engines that would affect a standard timing reading!
Check the OEM procedure!

I'm not a fan of digitally advancing strobe lights for this critical setting........ I'd prefer to see this system advancing in real time/real degrees.

Your call on that!

.

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can't get over 3500 rpms 18 Jun 2009 08:02 #11

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Problem is, I didn't understand half of what you just said. To start I believe I know what the harmonic balancer is. However do I need to just check timing at idle or at a range of rpms. Also to check timing do I just put a strobe light on spark plug wire 1 and check to see if the image being strobed on the balncer is reading the right advance/retardation. I thought I had to use some sort of tool on the 7.4 liter to put it into some limp or base mode, see this is where I start getting confused, any full description of this procedure would certainly help.

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1995 Maxum 2700 SCR
Twin 4.3L LX mercruiser engines
Tenacity
Salem Ma.

can't get over 3500 rpms 18 Jun 2009 09:19 #12

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okay so I have gotten some insight in perforimg the timing on this engine from the following link http://www.boatfix.com/merc/Techbk/95/95HGB4.PDF however I am still confused this just shows initial timing how do I do this tat timing or total advance timing? Do i just rev the engine up to like 3000 rpms and do timing again what of needs to be adjusted wouldn't that mean that the idle timing will now be off, som insight would be great thanks everyone.

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1995 Maxum 2700 SCR
Twin 4.3L LX mercruiser engines
Tenacity
Salem Ma.

can't get over 3500 rpms 18 Jun 2009 13:10 #13

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mtbiker9000;311451 wrote: okay so I have gotten some insight in perforimg the timing on this engine from the following link....................... however I am still confused......... this just shows initial timing....... how do I do this tat timing or total advance timing? ............ Do i just rev the engine up to like 3000 rpms and do timing again what of needs to be adjusted .............. wouldn't that mean that the idle timing will now be off?

That is usually the problem, mtbiker9000!
When/if they DO NOT include this in the timing instructions, it can infer that the OEM system will deliver the correct TA at all times. So how does average Joe know or check this?
Not all mechanics will bother to check TA.

However, upon reading that section 4B-7, it does mention a TA of 27*.
Here it is, (an example ONLY at this time)....... the first one on 4B-7 (the EST)!
Apparently you would check this when the jumper wire has been removed.

With timing light still connected, and engine at
idle, verify that timing did advance to 12 degrees
BTDC, (± 2 degrees). At 2400-2800 RPM maximum
(total) advance is obtained and should be 27
degrees BTDC (± 2 degrees).


NOTE: without knowing what his 1993 7.4L carbed mercs would have for the ignition system, I'd research this further.


And yes...... you are correct, you would rev the engine up to X,XXX rpm and view the timing marks with the strobe light. (be careful..... pulleys/belts are spinning fast)

DO NOT use the above data until you verify what engine model he has.

Look at page 4B-13 and you'll see listed the advance curve for the MCM 7.4L
Then look at page 4B-14 and you'll see yet another curve.
Again, you will need to use the correct info for his engine model.

Note that the curves shown are: TOTAL SPARK ADVANCE MINUS INITIAL TIMING (or what many call "Base" timing)

It sounds like you are understanding it..... just use the correct specs, and you will be fine!

Hopefully, some Mercruiser guys that know this will chime in here.

.

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Rick E. Portland, Oregon
2850 Bounty Sedan Flybridge model
Twin 270 HP 5.7's w/ Closed Cooling
Volvo Penta DuoProp Drives
Kohler 4 CZ Gen Set
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