7.5 hp merc missing

[MercruiserMan]

I have a 78-82 Merc 7.5 hp that is missing at idle..At WOT its fine. It put a new carb and carb kit on it with no luck.

It has new plugs, carb, carb kit.

Do you have any idea what could be doing this?

 

[freddyray21]

Re: 7.5 hp merc missing

sounds like it might be a lean sneeze. There are several threads on adjusting carbs on here. What I do it so start out at about 1 turn from bottom out. Then adjust up or down until the motor runs smooth at idle. I think the proper way is to turn it down until it starts to run rough or lean sneeze then open it up a 1/4 of a turn, but the more learned will let you know. I think Joe Reeves has an excellent thread in the faq about carb adjustment.

 

[freddyray21]

Re: 7.5 hp merc missing

http://forums.iboats.com/showthread.php?t=167352 here it is

 

[dajohnson53]

Re: 7.5 hp merc missing

My next guess would be one of the coils or the switch box (if that's the sort of set up it is, my experience is with a '74 9.8).

I would check the spark on each cylinder with an actual spark tester. YOu might find one is weak causing it to run on one cylinder intermittently. If so, then switch the coils and see if the weak spark follows the coil.

On my engine I got similar symptoms from a bad switch box. The weak spark did not follow the coils, but was on the same cylinder regardless. I put in a new switch box and now it runs like a champ. Bad news is that while the coils are reasonable price , the switch boxes can be rather dear.

 

[freddyray21]

Re: 7.5 hp merc missing

I would think though a bad coil or switch box would cause it run poorly at all ranges. He indicates it runs fine at wot. I still think lean sneeze.

 

[dajohnson53]

Re: 7.5 hp merc missing

I would think though a bad coil or switch box would cause it run poorly at all ranges. He indicates it runs fine at wot. I still think lean sneeze.

I've had coil and/or switch box issues which only showed up at idle, revved up, it worked fine. Don't ask me why though.

 

[MercruiserMan]

Re: 7.5 hp merc missing

Sorry I haven't replied but I was away fishing!!!!I talked to a mechanic about the issue today and he suggested adjusting screw as well and also check the thermostat to see if it was stuck or even there. If it was stuck or missing he said the engine would not warm up and cause it to miss. I checked the t-stat housing and it was MIA..So I picked one up and will try it in the next few days as well as adjusting the screw on the carb...Hopefully it is a easy fix...Fingers crossed!!

Thx for the replys

Cheers

 

[MercruiserMan]

Re: 7.5 hp merc missing

Well I get installing some new parts yesterday and cleaned out the water jackets etc... Put I all back together with new gaskets and now I am pumping no h2o at all. I remove the t-stat again so its an open system and no water is coming up from the pump at all. I remove the leg and look at the pump and a fin is badly damaged but I just put a new one in a few weeks back. So I have to get a new one today. I removed everything again and no passages are blocked at all and I reassemble. I shoot garden hose water up the leg and it comes out the t-stat housing ok

Where the water pump housing attaches to the lower leg, the water pump does not sit even in its housing. Its like there is more space on one side than the other so the fins of the pump are pushed to one side more than the other. Is this normal? What else would cause my pump not being able to get water up into the engine? Is there something else I should be looking at here.

BTW..I adjusted that screw out approx 4-5 turns and it runs better with no missing at all...so at least something went ok.

Help

 

[MercruiserMan]

Re: 7.5 hp merc missing

update..the missing did not go away but I did solve the water pump issue.

After I warmed it up more, with the throttle advanced a little, the missing comes back. So it happens at more than just idle.

Any ideas..coil-switchbox...???

cheers

 

[MercruiserMan]

Re: 7.5 hp merc missing

Ttt

 

[dajohnson53]

Re: 7.5 hp merc missing

Ttt

I'm no expert, but have dealt with a similar problem on my '74 9.8 hp. Maybe theres' enough similiarities to help you.

Here's excerpts from my story from a previous post I made:

'74 merc 9.8

...this ignition has parts under the fly wheel with wires running to a switch box. The switch box has + and - wires running to each of two ignition coils which are also grounded to the mounting bracket. Each coil fires a cylinder.

I was losing a cylinder intermittently only at lowest rpms. Sometimes it would get really rough, running on one cyllinder when I was trolling - but only intermittently. It would reun consistently at higher rpms. ...when running in the barrel, it starts fine, but when idled down as low as it will go, it would be fine for a while then lose the cylinder and die unless I revved it up. The cylinder would kick back on if I gave it a little gas to higher idle.

I exchanged the plugs - no change in cylinder affected.

Exchanged coils and the same cylinder still went dead (lower). Got out the spark tester. One coil would give strong, blue 3/8 inch spark, the other nothing most of the time, occasionally a good looking spark.

Switched coils - good and bad spark did NOT follow coils.

Switched plug wires - spark did NOT follow plug wire.

In other words, the good spark stayed with the switch box leads that fire the top cylinder regardless of coil or plug wire. So I'm thinking switch box or the parts under the fly wheel, neither of which I can really analyze..

... I disconnected, cleaned up and reconnected every ignition wire I could find, testing the spark as above each time I cleaned up something. None of them were obviously badly corroded. None of this helped.

Finally I brought it to a shop and they checked the parts under the fly wheel and thought every thing was OK. They replaced the switch box and the engine ran great.

I recommend trying everything you can before the switch box because they can be very expensive ($2-300 for mine!!). The coils aren't cheap ,but not real bad - plus it's easy to see if the dead spark (if that's what it is) follows a coil or not.

 

[MercruiserMan]

Re: 7.5 hp merc missing

Hey thx for the reply..The only issue with the troubleshooting of mine is that the miss is so random, I would not be able to find a weak or dead spark.

I am looking for another 7.5/9.8 that I can swap parts with for troubleshooting, then I will purchase what ever parts I need new.

I am just having trouble finding anoter engine.

Cheers

 

[MercruiserMan]

Re: 7.5 hp merc missing

Hey guys..Would a broken reed cause my issue?

 

[dajohnson53]

Re: 7.5 hp merc missing

Hey thx for the reply..The only issue with the troubleshooting of mine is that the miss is so random, I would not be able to find a weak or dead spark.

I am looking for another 7.5/9.8 that I can swap parts with for troubleshooting, then I will purchase what ever parts I need new.

I am just having trouble finding anoter engine.

Cheers

You might try the gapped spark tester anyway. They are cheap and the test is easy. I found that even though my issue was also intermittent, I could see it if I pulled the chord enough with the spark tester in place. If I recall, the spark was observably weaker than the other. It wouldn't follow the coil when I switched them, leading to switchbox.

 

[MercruiserMan]

Re: 7.5 hp merc missing

Hey all...I was talking to a friend of mine thats a small engine repair mechanic about my issue and he suggested spraying a WD40 kinda substance around the flywheel to see if maybe I had a leaky crank seal.

He said if the seal is bad, the engine will bog down and then recover while I spray the WD40.

Sure enough, I did this and it eliminated the missing completely and then it bogged down and recovered as advertised.

So now I have to change the crank seals. Is there any thing else I should look at while I am in there ( Doing the rings anyway)??

Is there anything I should keep in mind while doing this or anything I should know before I do this job?

Thx again

Chad

 

[wbeaton]

Re: 7.5 hp merc missing

Funny, I just read this post (I don't come to this forum much) and the upper crankcase seal is exactly what I was thinking. I'm in the process of repairing a motor with the same problem.

Good luck getting the seal out. I have not yet found the tool to remove it. I understand Tecumseh might make one. OMC used to, but its no longer available. You can however drill some holes in the seal, thread some small screws in them and pop the seal out that way. Its a serious pain. That's why I'm still trying to locate the proper tool.

Let us know how it turns out. I'll be watching this time.

 

[MercruiserMan]

Re: 7.5 hp merc missing

Hey thx for the reply.. I looked into the re and re yesterday and the upper seal on my motor anyway is installed in a top cap.(of the engine)..This one requires no spliting of the cases. The lower seal requires the spliting of the cases. Bummer

 

[MercruiserMan]

Re: 7.5 hp merc missing

I replaced the upper crank seal and my missing stopped completely once I re-adjusted the carb again.

But when I spray the contact cleaner on the r/h side of the powerhead, it died again. It seems like it is sucking in the cleaner around where the cases meet. Are these cases mated with a gasket or just some kinda sealer?

 

[Laddies]

Re: 7.5 hp merc missing

The cases are sealed with 518 loctite