MerCruiser inboard RPM fluctuation problem

[express cruiser 1]

I have twin MerCruiser 1990 7.4 liter (340hp)freshwater cooled inboards (carbs, V-drive, Borg-Warner transmissions, rack-and-pinion steering)<br /><br />At the end of last season the port engine developed a habit of changing its RPM setting to higher or lower speeds at random, on its own. This occurred at higher RPM only, never at headway speeds. I could hear the change in the engine and feel the performance difference.<br /><br />I would adjust the starboard throttle (not slaved) and in a few seconds or minutes the port engine would adjust itself again. <br /><br />The engine has about 650 hours on it and has been well taken care of. It runs great except for this problem.<br /><br />I know there can be a number of issues causing this such as the condenser, a wire, plug, fuel line, carb bowl debris, fuel pump, etc. Has anyone experienced a similar problem and have an idea where I might start to look for a cause?

 

[paulie0735]

Re: MerCruiser inboard RPM fluctuation problem

Well I’m thinking that if the engine is running well but than suddenly 'increases' rpm than its not electrical. Leaves only air and fuel. Your lucky that you have 2 engines, makes it easier to find the problem. I would start by swapping the carbs over. If that makes no difference than it sounds like you have a vacuum problem, which can lead to a leaner mixture or vacuum advance fluctuations, both of which could cause the problems you describe.

 

[jamesr1]

Re: MerCruiser inboard RPM fluctuation problem

Maybe the prop slipping also.

 

[QC]

Re: MerCruiser inboard RPM fluctuation problem

Yeah, I am thinking more on the load side than the engine side too. Do they perform similarly when you are pushing her onto plane? If they seem to go about the same then I would discount the prop slipping. Is there a possibilty that the port prop is getting some turbulence from somwhere that the starboard is not seeing?

 

[Haut Medoc]

Re: MerCruiser inboard RPM fluctuation problem

James has a good point & would be the easiest place to start, switch props & observe......I'm not aware of any vacuum advance on what I assume is a TBI ignition system....Switching the carbs as suggested would be an easy way to see if it is indeed a carb problem, but I would check the fuel pumps psi first.....I doubt that a vacuum leak would be intermittant. What kind of carb are we talking about? Is the choke working properly? ......JK

 

[express cruiser 1]

Re: MerCruiser inboard RPM fluctuation problem

Thanks for your replies. I will get back to you tomorrow (I'm on my company computer and I have to run)<br /><br />Best Regards

 

[paulie0735]

Re: MerCruiser inboard RPM fluctuation problem

Not totally sure of the setup you have but many most petrol engines of this vintage had various vacuum related sensors controlling warm up settings, fast idle, smog control systems not to mention vacuum secondary setups in the carbs. Vacuum can be very intermittent if it is being controlled by heat sensitive switches. Worth checking!! The prop is an interesting idea but the boat would actually slow as the rpm increased, if that’s indeed what’s happening than yeah, check that prop.

 

[express cruiser 1]

Re: MerCruiser inboard RPM fluctuation problem

Good Morning,<br /><br />To answer a couple of your questions, I have Rochester 4bbl carbs, getting onto plane was no problem with good throttle advance results, seemingly no loss of speed with RPM increases, I have tunnel drive with no problems of water flow and my yard is going to inspect my props cutlasses, struts and bushings. <br /><br />You all have good points and I appreciate your responses. Keeping an eye on expenses and believing in looking at the simplest solutions first, please let me know what you think of my proposed courses of action (come spring as my boat, the Marion Aleda, is up on jackstands for the winter):<br /><br />1. Sand and paint the engines and accessories (this should hi-lite any corrossion issues there)<br />2. Engine tune-ups including new condensers caps, plugs and wires with attention to dwell, timing and advance, as well as clean the flame arresters (should take care of any arcing over, ignition leakage or other electrical issues)<br />3. Inspect the throttle cables (maybe there is slippage without the throttle actually moving at the helm)<br />4. Check the intake manifolds and head gaskets with soapy water (air leak issues-but I'm thinking an air leak could cause erratic performance but would normally be constant rather than intermittent as I've experienced)<br /><br />If these actions doen't work I would next look at the entire fuel line including the anti-siphon filter and in-line filters at the carbs. <br /><br />Then I would finally go ahead and break the seal on the port carb and have it rebuilt (if I'm going to start having the carb pulled apart, I might as well go all the way)

 

[Bondo]

Re: MerCruiser inboard RPM fluctuation problem

That Sounds like a Good Solid Plan..........

 

[Buttanic]

Re: MerCruiser inboard RPM fluctuation problem

Possible clutch slipping issue.

 

[express cruiser 1]

Re: MerCruiser inboard RPM fluctuation problem

Thanks again for your help. I'll let you know how the problem shakes out in the Spring.

 

[paulie0735]

Re: MerCruiser inboard RPM fluctuation problem

On re-reading your original post something reminded me of a similar problem I had a few years ago, the engine (351 with volvo drive)in my old ski boat was actually increasing and decreasing in rpm, but as you describe it only happened at WOT or near WOT, it turned out to be the thermostat! This boat at the time had over transom exhaust and the waste water from the engine cooling system would exit via a fitting towards the end of the pipes, also the extractor runners were double walled and each runner had its own water fitting to help keep them cool and would also act as a noise suppressor. So, as the thermostat opened and closed it would restrict the supply of water to the exhaust system and what was actually happening was the engine ‘noise’ would increase quite a bit giving the impression that the speed was increasing and decreasing with the freeing and restricting of the water flow out of the engine and through the exhaust. The problem with the thermostat wasn’t hard to diagnose, it was simply opening and closing to quickly and would not settle properly, only happened when the engine was under a fair amount of load and generating more than the normal amount of heat. A replacement was fitted and ‘no more problem’ I now fit a new thermostat as part of the yearly maintenance program.

 

[express cruiser 1]

Re: MerCruiser inboard RPM fluctuation problem

The thermostat may be a good place to look for a solution; like I said, the simplest things are best looked at first!<br /><br />In my situation though there was a noticeable loss of performance (or the reverse) during RPM lessening or advance and the tach would register the change in addition to the noise level difference.