Low Revs

P

Private Nemo

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Aug 8, 2012
RO Number
32985
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5
Hi,
I have twin Mercruiser 5.7, 1985. 4bbl, Thunderbolt IV ignition. Over the winter I install rebuilt heads on both motors. Now one engine maxs out at 4600 rpm and the other maxs out at 4000 rpm. Check carbs , both open all the way. New plugs , caps and rotors. Same drives, same props. Cant figure it out. Open to suggestions.

thanks

Drew
 
Do comp check and vacuum check.

Q-jet secondaries shouldn't have to open all the way. It's capable of flowing 750 CFM. Your 5.7 isn't flowing over 400 CFM.

Try closing the secondaries slightly with your finger as someone else gives it full throttle.
 
What Charlie said . ^

Aren't those engines just rated for 4200-4400 WOT RPM? Just asking.

What WOT RPM did you observe prior to the head replacement issue?

Add'l suggestions for the slow engine:

-Check timing , base and advance , making sure any Timing Mode is re-configured back to Run Mode after. Is the timing advance of the T-IV ignition all electronic or is there by any chance still mechanical advance in the distributor?

- Put an inductive timing light pickup on each plug cable to confirm they are at least all firing at high RPM under load, even though this doesn't confirm actual combustion in all cylinders .

- Re-confirm all plug cables are connected in the correct firing order.

-In spite of apparently observing carb plates fully open (possibly chokes?), try disconnecting the control cable at the engine and holding the carb throttle lever to full while underway to confirm the low WOT RPM.

- If separate tanks , try running off the strong engine's tank, or a separate portable tank with new fresh gas.

- Have both props scanned to see what kind of shape they are really in compared to original specs and how well matched they are to each other.

- If still mystified, it might help to check high-rpm fuel pressure vs spec. .
 
How are you measuring RPMs? If you have Tiara gauges did you give them a rap with you knuckles to make sure they were reading correctly?

Mike
 
Yes recheck firing order and timing on the lazy engine. Then check compression and inspect plugs.
 
quote:
Originally posted by Michael Clemensen

With all the above I’ll add “Lifter Adjustments/Valve Lash.”

Click to expand...


^^^^This^^^^

How did you set the adjust the valves? These are hydraulic lifters. There is no lash. Did you adjust them running, or static before installing intake manifold? You may have some too tight.

TB IV does not have timing mode like TB V. I run my TB IV engines up to 3500 rpm, check timing to make sure it's all in, then let them drop to idle to make sure the electronic advance works and is at the correct advance for the idle rpm I run. If you are going to time at idle, make sure your engine is running at the stated rpm for the timing advance curve of your module.

Pm me with your email address and I can send a pdf of the Thunderbolt manual which covers not only the TB IV, but other versions as well.
 
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