Merc 5.7 Skipping, cutting out at high RPM's?

[HogWildOne]

I've done my share of rebuilding and modifying small blocks so I've troubleshooted a million problems. I don't want to waste money so I thought I'd get some input before I did. I have an 88' Merc 5.7 4 barrel. Since I've had her she most always (but sometimes not) cuts out when I open up the 4 barrel and try to get up to top speed. This happens at apprx 4k rpm. The motor is smooth and has plenty of power under full throttle from a start or going along under 4k rpm. It feels and seems like its misfiring but I have done the plugs cap rotor, wires..etc. I'm wondering if anyone has seen this before? I'm thinking it's the carb? (i checked the fuel filter its fine) Or it's the ignition coil itself, which perhaps is failing under high rpms? It will rev up to it's 4800 rpms but while its getting there it is skipping and it does take it longer when thats happening. Occasionally, it doesn't do it at all but thats rare. I've tried the timing, it's irrelvent, premium gas vs not, dry gas.. Would appreciate some input before I spent lots of $$ going through everything. Thanks, Steven

 

[HogWildOne]

Re: Merc 5.7 Skipping, cutting out at high RPM's?

Anyone??

 

[RandyJ]

Re: Merc 5.7 Skipping, cutting out at high RPM's?

Crank it up in the dark and look for fire jumping anywhere along the plug wires. Is your throttle cable rubbing against anything when it opens up? Any possible vacuum leak? Distributor cap clean & no cracks? Rotor button? Have you pulled the valve covers and checked the lift of the cam (I just found that one on my 140). Have you hooked up another fuel tank (like a 6 gal plastic one) in place of the one installed in the boat? SURE you dont have a fuel line leak allowing it to suck air? Just thought I'd throw some suggestions.

 

[Don S]

Re: Merc 5.7 Skipping, cutting out at high RPM's?

Check the distributor shaft bushings. I have replaced a lot of distributors in that era of Thunderbolt distributors for worn bushings and shaft.

 

[HogWildOne]

Re: Merc 5.7 Skipping, cutting out at high RPM's?

Randy & Don, I'll check out those things. I am really hoping it's not worn cam lobes robbing me of power. I'd like to think it's a fuel or ignition problem. Since it isn't 100% consistent i'm optomistic it's not that serious and i'll figure it out. Thanks

 

[rbezdon]

Re: Merc 5.7 Skipping, cutting out at high RPM's?

I doubt it is cam lobes. There is no way that is inconsistant and the loss of power is premanent, you'd never get the full output. Any chance its the timing chain getting sloppy causing the timing to jump around?? When I was a kid some of my old small blocks had "issues" with timing chains getting stretched and sloppy.

 

[HogWildOne]

Re: Merc 5.7 Skipping, cutting out at high RPM's?

Yeah Ron I had some loose chains that you could hear hitting the cover but it didn't quite make it run like this..This feels more like something is getting cut off like fuel or spark. I'm off work today so i'm going to dig in and start checking things one by one. Thanks

 

[jmccall001]

Re: Merc 5.7 Skipping, cutting out at high RPM's?

Two Cs coil and carb cured my similar problem. Aslo replaced plugs

 

[HogWildOne]

Re: Merc 5.7 Skipping, cutting out at high RPM's?

Well here's an update, <br />I took a look at the fuel and it's fine, no water. <br /><br />Then I bypassed the electrical system to eliminate that, it was fine as well. <br /><br />Plugs are new already and look fine. <br /><br />Compression checked out great, approx 100psi. I took of the carb and cleaned it well.<br /><br />And after all that, nothing. So lastly I played with the two mixture adjustments on the front of the carb. They were way way out. For the hell of it I turned them in little by little and kept testing. And bingo I have it running almost perfect. Bit of a rough idle but it runs strong. If it doesn't stay like this i'll replace the carb. Thanks

 

[AdamB]

Re: Merc 5.7 Skipping, cutting out at high RPM's?

Are you saying you adjusted the idle mixture screws? They only affect idle, that's why you are getting a rough idle.<br /><br />I had a similiar problem, and it was the spark plug wires arching, which you can see when it is dark.

 

[HogWildOne]

Re: Merc 5.7 Skipping, cutting out at high RPM's?

Yeah I thought it was only for idle but it seemed to do something.. But today I re-check by backing out the screws and your right it idles better and it seems to be running pretty good.?? So I really did nothing in the end.. confusing.. So I still have the occasional sound that it's misfiring.. The wires are new Merc Suppression, but i'm going to take a look tonight in the dark. It only takes one i suppose. Thanks

 

[HogWildOne]

Re: Merc 5.7 Skipping, cutting out at high RPM's?

Ok someone correct me here if I'm wrong or crazy. I just came in from the dark. I don't think I should see sparks jumping from the distributor to the metal side of the coil and I also can see the actual sparks flowing through the wires?? Now I've never seen this in any of my older cars so unless someone tells me this can normally occur I'm changing the wires tomorrow.

 

[Don S]

Re: Merc 5.7 Skipping, cutting out at high RPM's?

I would sure check those plug wires. Were they a good set or just the cheapest set you could find?<br />Plugs could also cause problems. If you chose Champion, you might want to try a set of NGK, AC, or Bosch. Had way to many non firing NEW Champions to ever put one in a boat again.<br />The coil tower could also be cracked, and could also crack from a bad wire or plug.<br />Did you check the dist. shaft bushings? The sensor hitting the rotor can also cause serious problems.

 

[lowshovel1]

Re: Merc 5.7 Skipping, cutting out at high RPM's?

Conventional Idle Mixture Screws that provide a leaner A/F when turned clockwise and richer A/F when turned counter clockwise. The idle air flow is controlled by a conventional screw that opens the Primary Throttles. The following procedure should be used to set the idle mixture and speeds.<br /><br />1. Fully warm engine and ensure choke is fully open. <br /><br />2. Air cleaner in place. <br /><br />3. Set desired speed with the air screw. <br /><br />4. Adjust the Idle Mixture Screws on ONE side to get the maximum possible RPM. Do not go rich beyond the maximum speed point. <br /><br />5. If the above changed the idle speed more than 40 RPM, then readjust the speed. <br /><br />6. Adjust the side OPPOSITE of that in Step 4 to get maximum RPM. <br /><br />7. Reset the speed. <br /><br />8. Carefully trim each Idle Mixture Screws to again get the maximum idle RPM. <br /><br />9. Go leaner just enough to get a 20 RPM drop in speed. <br /><br />10. Reset the speed to the desired RPM. <br /><br />11. This is a Lean-Best Idle Set. Setting richer than this will not improve idle quality or performance, but could tend to foul plugs.

 

[Sooner44]

Re: Merc 5.7 Skipping, cutting out at high RPM's?

I had same problem, it was the accelerator pump on the carb. May want to check it out.