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Rough idle and slight miss at higher revs

A

aquakenbush

Contributing Member
"My starboard engine runs a li

"My starboard engine runs a little rough at idle and when I get the revs up when trying to plane it misses randomly at around 3500-4000 RPM. (Sounds like a puff)The port engine runs notably smoother. I changed plugs,wires,cap,rotor and it didnt seem to make a lot of difference (at idle). One thing I wonder about was if a vacum leak could have a similar affect. I notice that the intake manifold has some silicone around it which would make me think it has been off, I checked all the bolts and was able to tighten 3 of them a little and the water neck bolts a little as well. I fired it up and I think it runs better at idle, I was not able to take it out today but I will take it for a sea trial at plane in the next few days. My other thoughts were perhaps changing the coil or the EGR valve (Does it have one?). I also wondered if it might be related to old gas, the boat had been sitting since last Sept. and only ran a few short times. This I think is unlikely though as the port engine runs like a top.

Any suggestions?"
 
"First, what power plant model

"First, what power plant model (and year) do you have? Is it twins AQ175A? Or...?

If you think you have a vacuum leak, connect a vacuum gauge to the intake manifold and see what happens at idle. There is usually a plug around the back of the intake that you can remove so that you can attach a fitting for the vacuum gauge. If the vacuum at idle is below 15" you may have a leak. However, the rough idle could be due to something as simple as the idle mixture not being well adjusted or the electric choke acting up.

There is no EGR valve on conventional marine engines, but it seems they will installed in newer engines in the not too distant future."
 
Twin 4.3 L (model is calle

Twin 4.3 L (model is called a 432A SP I believe) Chevy's with VP 290's. They are 1994 models I believe.
 
"Hi Andrew, you read my post a

"Hi Andrew, you read my post as it was similar to yours, I changed the fuel filter as well as the electrical parts you did, and it seemed to have water in it along with a load of horible looking bits! the boat now runs fine. Hope this helps!"
 
"Thanks David, I will have to

"Thanks David, I will have to have a look at the fuel filter tonight and see if it has any water in it etc.

Thanks,

-Andrew"
 
"one other thing, I also chang

"one other thing, I also changed the battery as the old one had been in for a while.

Good Luck."
 
"Ok the work I did the other d

"Ok the work I did the other day definately didnt help the random WOT miss, I couldnt change the fuel filters as they gave me the wrong ones, I picked up the right ones this morning so hopefully I can change them in the next few days. The motor still shakes noticibly more then the other but doesnt seem to die at idle in gear anymore.

The engines are 432A and the drives are 290 sp-c.

Thanks,

-Andrew"
 
"One thing to note is it only

"One thing to note is it only misses while under load, if I just rev it up to 3800 with throttle only and let it sit there it never misses."
 
"Have you adjusted the idle mi

"Have you adjusted the idle mixture and checked the ignition timing? Also, have you checked that the choke is opening after the engine has been running for awhile?"
 
"I havent messed with the idle

"I havent messed with the idle mixture yet, I did bump up the idle speed a little bit.

The choke appears to be opening as when I looked under the air filter they seemed to be wide open.

I have not checked the timing yet but I would assume if it was off it would miss randomly all the time and not just under load?

Here is what I am thinking so far:

1. Bad gas or water in the gas/filter - Going to change the filters tonight.
2. Intake has a vacuum leak - Going to get my hands on a vacuum gauge and check that.
3. One of the heads has a sticky exhaust valve that isnt seating properly. - Need to do a compression test.
4. Fuel mixture setting on carb is not right.
5. Timing is out a bit. - Have to check the timing.
6. Weak coil. - Might have to replace
7. Head gasket is blown or leaking. - Compression test

Any other suggestions?"
 
"Any more thoughts El?

Goin

"Any more thoughts El?

Going back out to the boat in like an hour


Thanks,

-Andrew"
 
I think you have covered prett

I think you have covered pretty much it. See if you can bring with you a timing light and a vacuum gauge to help in the diagnostic.
 
"Ok I changed the fuel filters

"Ok I changed the fuel filters and I think its running better, seems to be shaking less at idle. I wont be able to tell for sure until I get it back up on plane again. (I was alone tonight)If that doesnt work then it will be on to compression test, then timing, then vacuum check, then new coil, then new distributor.

Oh and the gas I took out of the old filters turned cloudy when it sat in a bowl. Is that an indication that there is water in the gas?

Thanks,

-Andrew"
 
"Yes, but the water probably n

"Yes, but the water probably never made it to the carb. To me, this is an indication that the filter was doing its job by trapping/removing small quantitites of water from the gas."
 
They had lots of crud in them

They had lots of crud in them too and I checked the log and it said they hadnt been changed since April 2005. I wonder if even that is correct as the filters had manufacture dates of 2000. There were also mercruiser filters and smaller in height.

I put some Red Line fuel system cleaner in too.

I was also wondering if I should put a litre of alcohol in the gas tank just incase?

One thing I forgot to mention is I also sprayed the distributor with WD40 as it seemed kinda rusty in there the other day when I changed the cap and rotor. My brother mentioned something about a 'pick up' that might not be moving properly.
 
"If the distributor was rusty

"If the distributor was rusty inside, you should consider checking the condition of the springs and centrifugal weights. If the timing advance is incorrect as a consequence of seized weights or broken/worn springs, the engine performance will be far from brilliant. In fact, you may want to consider taking the distributor to a shop where they can check that the advance curve is what it should be.

The pick-up your brother refers to is probably the electronic ignition module, and those modules seldom cause problems: they either work or they don't."
 
"Funny my brother asked about

"Funny my brother asked about the springs too but I didnt really see any like I would see in like a chevy 350 with HEI. There just seemed to be a large O ring thing at the base and some other thing underneath it that looked kinda like a cog. I didnt see any springs or counterweights.

However when I first changed the cap and rotor it was very rusty on the shaft and there was oxidization on the aluminum. It seemed rusty below as well.

I agree about the module its either on or off but that isnt what my brother was refering to.

Well I will try to get it out for a run tonight and if nothing is solved I might move on to the distributor next, well after a compression check."
 
"That is definately what it lo

"That is definately what it looks like and it looked plenty corroded as far down as I could see...

500$ US for a distributor, ouchy. I am betting I can use one for a 1994 Chevy 1500 4.3l as that is what I used for cap/rotor/plugs/wires

That should be ok right?"
 
"Andrew, [b]<u><font color=""f

"Andrew, <u><font color=""ff0000"">never ever use an automotive distributor on a marine engine</font></u>. The advance curves are different and there is no vacuum advance on marine distributors. Besides, the marine distributors must be coast guard approved and meet the SAE-J1171 standard to prevent "KABOOMS!!!". Yours should not be so bad that it cannot be overhauled. As I already suggested, consider taking it to a shop where they can test it and install new springs if necessary."
 
"See that is why I ask these s

"See that is why I ask these silly questions
happy.gif


I will give it a go tonight and if I am still having issues I will give her a compression test then pull out the distributor if the compression is ok.

Thanks!!"
 
"Ok gave it a good run tonight

"Ok gave it a good run tonight, I thought it was better and seemed a bit more random and the misses were sometimes fewer and more time in between. That being said it did seem to slowly get worse. What gets me is just how random they are. It will run for a good 5-10 seconds and then miss or it will miss 3 or 4 times in 5- 10 seconds, one time it ran just fine for almost 30 seconds. I cant figure out what would be this random... Its starting to drive me nutz and I havent even owned the boat a week..."
 
Have you tried swapping coils?

Have you tried swapping coils? A bad coil might not show until it heats up or is under a load.

The beauty of having two engines is diagnostics can be simple and easy if one engine runs good and the other doesn't. Just swap parts one by one until you find the culprit.

I doubt if it's something to do with water in your fuel if one engine runs good and the other is having trouble. Unless you have two seperate gas tanks you will have trouble with both engines running smooth.
 
I am kinda leary of swapping t

I am kinda leary of swapping too many parts back and forth as I know some parts are particular to the rotation of each motor. Also seeing as I dont really know all of the parts this may affect I am being cautious about that.

BUT GET THIS!

Went to the local VP dealer and my distibutor is discontinued and to get one costs over 9000$! The actual VP guy over the phone with the dealer guy said to go to a napa and find one....?

Oh and VP coil was 99$ Cdn.
 
"Andrew, somehow I suspect tha

"Andrew, somehow I suspect that both your engines turn the same way and the reverse rotation is done in one of the outdrives.

There should be more than one type of distributor available that works on your engine. You may want to check with The Anchorage in Nanaimo, they deal in VP and they also have a good boneyard. As for Sue or Rick at 250-754-0945.

Regarding coils, you should be able to obtain much cheaper versions than the one sold by VP. "
 
"Wow that looks like the perfe

"Wow that looks like the perfect solution, plus I can take all the stuff I already replaced and put it on the port engine.

Thanks EL!!

I had another thought, what if the filter in the carborator is all fugged up? Was thinking of just yanking it out and trying another run, well I guess it depends what it looks like inside"
 
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