high speed miss

[picker523]

I've got a 87 3.0 mercruiser and I've replaced all the electronics(points , plugs .condensor) . rebuillt the carb.. valve job , new fuel pump, and set the valves. I didn't gap the plugs. It's running the best it's run since I've had it (1 year) but it still misses at high speeds ( about 35oo rpms) and I don't know if it;s a fuel or firing problem. Anyone got any ideas? Could it be plug wires?
'

 

[Don S]

Re: high speed miss

Sure it could be plug wires. Is it a constant miss, or just random times?
If constant miss, Have you done a compression test? Did it do it before you put the new plugs in? Why don't you just pull the plugs and check the gap. It's just a 4 cylinder and they are easy to get to.

 

[protozo1]

Re: high speed miss

It could be the wires. If you pulled on the wires to take them off you could have seperated them a bit internally or open up the connectors enough to cause a problem. I'd replaced them too.

 

[180shabah]

Re: high speed miss

I don't know if it;s a fuel or firing problem. Anyone got any ideas? Could it be plug wires?
'

Could be fuel or ignition related, you will have to determine that.

Start with the easy stuff, like setting the proper gap on your new plugs.
Compression has been mentioned.....
When you replaced points did you set point gap, dwell and timing?
Do you have a water seperator/ when was the last time it was changed?

 

[picker523]

Re: high speed miss

Yes... I did check the compression and it was fine. I was in a hurry when I put the plugs in but I will go back and set the gap.Isn't the dwell and point gap the same thing? And it is a intermittent miss so I know it's not one cylinder in particular. THe only reason I thought it might be fuel related is I adjusted the needle valve a little to see if it was running rich and it seemed to miss a little more but it did that after it had warmed up so I wondered if the coil could be breaking down after warming up. Could that happen? Also I did just put a new fuel ppump on it so it has a new fuel filter.

 

[Don S]

Re: high speed miss

Isn't the dwell and point gap the same thing?

No, points and spark plugs are two different items.
You also have to set the point dwell, then the timing and check the advance timing if you want it to work correctly.

Your idle adjustment as nothing to do with the high speed operation of the carb.

 

[Don S]

Re: high speed miss

If you click here you can download a service manual for your engine
You want manual #10 for the engine, #6 for your outdrive.

 

[Fishermark]

Re: high speed miss

I think DonS misread your question. He suggested gapping the plugs - someone else suggested you check your point gap and dwell.

Yes - point gap and dwell is the same thing. Well, kinda. You need the proper point gap - the best way to achieve that is with a dwell meter - and then you set the timing. You can also set the point gap with feeler gauges - but it is generally not considered to be as accurate as setting them by dwell.

 

[Don S]

Re: high speed miss

I put the plugs in but I will go back and set the gap.Isn't the dwell and point gap the same thing?

Yea, I think I read that and turned it into plug and point gap being the same..... duh.

The point gap is usually set as a starting point. Once the engine is running, the dwell angle basically measures that gap in degrees and is much more accurate, and you can do a final adjustment if your gap is wrong.
But gap (Dwell) does affect timing. Any change in dwell, changes the timing.

 

[picker523]

Re: high speed miss

Thanks for clearing that up. I had it on the water yesterday and it sure was good getting to enjoy the fruits of my labor. I've been working on it for over a year. I noticed that as long as I stayed around 3000 rpms it didn;t miss but when I got up to 4000 it started missing. I also noticed when I hit the trim button at those high rpms it really made it miss. Could the electrical current from the trim be affecting the plug wires?

 

[180shabah]

Re: high speed miss

Could the electrical current from the trim be affecting the plug wires?

Yup, if you have a weak battery AND/OR dirty, corroded cables and wires, you will not have the power to run both. That trim pump is a huge load and is basically "stealing" power from your ignition.

I would start at the battery, and disassemble, clean and reassemble every connection. Also, when you are reconnecting them, use a dab of dielectric grease, to help prevent future corrosion.

 

[Haut Medoc]

Re: high speed miss

I would clean up the connections......
Did you do anything wierd when ya put the points in, ie bend anything?....
You might want to check the spring tension on the points, if it is wrong it can create a high speed miss.......​

 

[picker523]

Re: high speed miss

I seem to recall the spring coming off the points but I think I reinstalled it right. As far as the battery is concerned it seems to take a full charge but it is kind of old. All the connections are clean. Does anyone think the plug wires could contribute to the problem?

 

[picker523]

Re: high speed miss

well I took it out again today and it started out running farely well and as time went on it got worse. It started out running up to 4000 rpms with no problem but later would only get up to 3000 because of the bad miss. It is a intermittent miss that gets worse when I hit the trim button.The battery seems to have a good charge/I've got new plug wires on the way and I hope it helps/

 

[fireicyhott]

Re: high speed miss

Definitley check the points again. I had left the silver spring tensioner out and caused the same problem. The only thing providing tension when I did that was the copper strap. Ran fine at idle and low rpm and like crap at higher rpms. Duh. I learned on that one.

Jeff

 

[Don S]

Re: high speed miss

well I took it out again today and it started out running farely well and as time went on it got worse. It started out running up to 4000 rpms with no problem but later would only get up to 3000 because of the bad miss. It is a intermittent miss that gets worse when I hit the trim button.The battery seems to have a good charge/I've got new plug wires on the way and I hope it helps/

Like is said in my first reply, plug wires CAN cause a high speed miss. But with the added information you have given, I doubt that is your problem.
Are the battery cables in good shape, I have found lots of cables that were tight on the battery, but the cable ends were loose or corroded so bad the engine would barely start. Look at both the Positive and Negative cables, throw away the wing nuts on the battery and use regular nuts and tighten them down with a wrench.
Get the battery load tested, and make sure the alternator is working.
Also watch around the coil tower for arching. May have some carbon tracking or crack that will allow it to fire to the smaller terminals on the coil. A plug wire with high resistance or an open will cause the arching on the coil.

 

[180shabah]

Re: high speed miss

It is a intermittent miss that gets worse when I hit the trim button.

Check/replace your battery cables

The battery seems to have a good charge

How do you know, has it been tested?

I've got new plug wires on the way and I hope it helps/

Doubt it.

 

[picker523]

Re: high speed miss

No I have not load tested the battery but I'm going to try it with one of my newer batteries and see if it helps. If the alternator is good wouldn't it support the engine at high speeds? the battery cables are not corroded .

 

[picker523]

Re: high speed miss

I took it out yesterday and it ran much better. New battery cables and plug wires helped.Got it up to 40mph at about 4200 rpms. Still has a slight miss intermittent but it's a whole lot better. I'll tweek the points a little more. THanks for all your advice.

 

[Don S]

Re: high speed miss

I'll tweek the points a little more.

There's no tweaking to it.
You set the DWELL, then the timing, and you are done. Might check that the condenser is tight.
Are you sure the coil tower isn't cracked from the bad wires?
Are you getting proper advance?