Hesitation problem Merc 7.4l

[jeremyp111]

Guys,
I've been around here reading for several years now and I'm finally posting on a problem I can't seem to find a solution to.

I have a 1995 Regal 322 with twin Merc 7.4L Bravo 3's with Thunderbolt V ignition systems and Weber 4bbl carbs. I have a hesitation / stumble around 1300-1600 rpms with low to moderate thottle on the starboard engine and as I keep throttling up it lags behind the port engine until around 2300-2500 rpms and then after that it's fine. I put in brand new caps, rotors, plug wires and I'm running NGK Iridium plugs (0.45 gap). The carbs were rebuilt 4 yrs ago and the starboard one was gone over by the service dept at my marina last year while trying to track down this ghost.

If I hammer the throttles I don't get the problem at all so I usually don't notice it. This past weekend I was out at night with a full load of people and gear and I was going pretty slow and kept the rpms around 2000. I was getting the hesitation / stumble out of the starboard engine and I also got a backfire as well. This has caused me to want to finally get this licked once and for all.

Any ideas of where to start??

Thanks,

Jeremy

 

[Bondo]

Re: Hesitation problem Merc 7.4l

I was getting the hesitation / stumble out of the starboard engine and I also got a backfire as well. This has caused me to want to finally get this licked once and for all.

Ayuh,... That's a sign of a Lean condition,...

I'd start with the Carb,+ fuel filters..

 

[Fun Times]

Re: Hesitation problem Merc 7.4l

Guys,
I've been around here reading for several years now and I'm finally posting on a problem I can't seem to find a solution to.

I have a 1995 Regal 322 with twin Merc 7.4L Bravo 3's with Thunderbolt V ignition systems and Weber 4bbl carbs. I have a hesitation / stumble around 1300-1600 rpms with low to moderate thottle on the starboard engine and as I keep throttling up it lags behind the port engine until around 2300-2500 rpms and then after that it's fine. I put in brand new caps, rotors, plug wires and I'm running NGK Iridium plugs (0.45 gap). The carbs were rebuilt 4 yrs ago and the starboard one was gone over by the service dept at my marina last year while trying to track down this ghost.

If I hammer the throttles I don't get the problem at all so I usually don't notice it. This past weekend I was out at night with a full load of people and gear and I was going pretty slow and kept the rpms around 2000. I was getting the hesitation / stumble out of the starboard engine and I also got a backfire as well. This has caused me to want to finally get this licked once and for all.

Any ideas of where to start??

Thanks,

Jeremy

Also pull the plugs and look at them really good for what they might look like a lean plug and also look to make 100% sure none of the plugs are cracked or broken on the out side by lightly bending or shaking them a round a little looking and lisening for a crakling sound. You can do that test while they are still installed in the head befor you pull them out. Also try pulling a few of the other enging plugs to compair them to the bad engine so you know what you are looking for. Has any one ever aujusted the engine timing it might be off some compaired to the other engine. Make sure the "dist" bolt is tight and their is no paint between the manifold and the "dist" it could loose it's ground and make sure all off them are tight and clean also. If all that checks out ok try spraying some carb on the intake looking for a vacuum leak. If the engine reves up while spraying then it has a leak. I am assuming this engine did this before you did any type of work or did it start after one of or all of the job where done? The Quickest way to test the carb is to just switch them over and see if the other engine starts to do it. I hope this gives you a starting point let us know what you find out.

 

[jeremyp111]

Re: Hesitation problem Merc 7.4l

Thanks for the replies. Here's some additional information that may throw another wrench into the situation...My friend (auto mechanic) took a quick look at it last night for me and just gave me this...
While holding the throttle in the 1300-1600 rpm range where the problem starts to show he noticed that the rpms sometimes climbs a bit but at the same time the voltage gauge drops to 11. He then put a meter on the alternator and it's giving 15.3v constant even when the gauge drops to 11 when the engine climbs a bit. Checked all of the gauges for good ground and clean connections and they are all super clean and tight. Also checked the ignition switch and all connections were clean and tight. Final thing that was checked was the big round Merc plug by the distributor for any corrosion or loose connections. Clean there as well.
Could this still be a 'too lean' condition or would this new info lead more to an ignition problem??

Yes, the problem has been there prior to all the new parts and work. I could certainly swap carbs when I get back to the boat on Friday night to see if the problem moves to the other engine.

any other ideas in light of the new information?? Certainly stumped my friend...

Thanks,

Jeremy

 

[jeremyp111]

Re: Hesitation problem Merc 7.4l

OK, here's an update to this problem.
I swapped carbs - problem still the same
Swapped ignition module - problem still the same
Swapped coils - problem still the same
Put a jumper wire from bat positive to coil positive - This fixed the erratic volt meter, but the runability problem is still the same
Checked timing - at 8 degrees
Sprayed base of carb - no change in rpm at all
Pulled plugs - some looked good, some looked too black. One looked white, too clean maybe?
Put in new NGK BR6FS plugs gapped to .035 and the problem didn't go away
Rechecked all ignition wires were connected properly and in the correct order - all good
Put timing light on coil wire to see if I could pick up a skip - Steady stream of light at idle up until I get to the hesitation / stumble rpm range. This caused the light to flicker some indicating some type of ignition skip?

At idle or when I am taking off to get on plane the hesitation / stumble starts at around 1300-1600 rpms and continues to about 2200-2500 rpms and then it takes off and runs great. I just can't figure out what's causing this issue.

Any help is appreciated. I'm not sure what to do next here.

Thanks,

 

[Fun Times]

Re: Hesitation problem Merc 7.4l

A few things to try one I know you have checked it to see if it is cleaned but this time try wiggeling the big round merc plug while the engine is running and watch all the gauges to see if they go out and lisen to the engine to see if it's trying to die at all, Also it time to start wiggeling every wire you can find to look and see for any changes with the engine on. Try switching the coil wire it's self from the coil to the dist cap on both engines, Then try unhooking the gray wire at the back of the rpm gauge or at the coil if you can find what one to unhook A shot in side a tack gauge will do this to your engine. Also look for any sparks at the coil wire to ground on the coil. Then coming out of the dist, it's self there should be too white with one red and one green wires and some times there is a black little ground wire going to the bottom of the dist housing look on both engines to see if your has the ground or not for a quick test. Make sure the white and red/green are pluged in tight. Next the sensor in side the dist hooked to the white and red/green wires could be going bad that is a weak link to the engines sometimes and will cause a no spark to the coil. Then pull the dist cap and try wiggeling the dist shafts/Rotor to see if it is real loose, Because the rotor could be vibrating around so the metel finns of the rotor are hitting the dist sensor as it passes thur shorting out the system. Also make sure the rotor is pushed all the way down, And try to lift it up and down the rotor should not move on the shaft, If it dose then some one did not putt any blue lock tite on the bottom side of the rotor and the rotor might be floting in side under the cap. Make sure the cap is not cracked any where and leaking sparks, Some times they crack at the mounting screws it's self. And if you get a chance to run the engine at night try looking at all the spark plug wires at each end and look for sparkes going to ground. ??? Does it only do this while under load in gear or dose it do it while not in gear and just reving the engine at the dock??? I do not think your system will have this but you can look and try any ways, look at the shifter cable assy mounted on the engine is if there is a wire plug going to it try unpluging it and run it while you are out there. If all this don't work then the next test you would need to do is to seprate the boat from the engine by huhooking the 10 pin wire connector, [The big round merc plug] and tap in to the right three wires to start the engine and keep it running, this way you could see if the engine is at falt or if the helm is at falt some place, So you know what to try to fix the boat or the engine. Also you need to make sure all the grounding points are clean of any paint like the wire to the module if it had one, paint between dist and the manifold [ect] Also do all the testing with with the light hooked up to the coil wire so you can see if any thing changes. Well I hope some of this helps you out good luck with it.