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1979 OMC 260 carb and/or idle issues

floattest

New member
Problem: poor off idle performance after running in water in gear at temp for 20 minutes or more. Boat seems to run rougher and rougher over time unitl it dies and then is dinfficult to start, can get it started in nuetral but then when dropped to idle speed and dropped into gear it dies. To get back last night I had to adjust the idle high and have friend hold off the ESA while I dropped into gear, stumble coughed and got going was able to run for about 25 minutes at 2K then it died again. Luckily within a hundred yards of the launch. I stretched the no wake rule in the channel just to get back to the launch.

I just rebuilt my carb and I went through all the carb adjustments in the book before installing. There is quite a few and some of them even by going to the manual seem tricky. Like the choke linkage adjustments have you measre betwee the choke valve and the carb body, but the choke linkage is in a slot so it depends on if you hold the valve to the edge of the body or away from it to measure. Anyway. I made several bends in the stops and links to get what I think are the correct adjustments. I have an OMC factory manual from 1982. My motor is 1979 but I made sure the carb sections were dealing with the same carb model. This is a 4MV, 17059286. By the way the factory OMC manual is awesome. Anybody with an OMC needs one.

Installed the carb and set the idle screws 3.5 out.

After installing the carb and a new spin/on fuel water seperator I fired her up and it came to life immediatey. This is in the drivway marina. It seemed to idle nice and I let it come up to temperature. Made sure of no fuel leaks and every thing was normal.

Went to the launch to adjust the idle screws and the idle stop screw as the last adustments.

With it in the water let it run up to temperature and tried to do the lean drop process by turning in one screw at a time until idle drops and then back off 1/2 turn. This doesn't seem to go by the book. The idle screws don't seem to have much affect.

I ended up at about 2.5 turns out on the idle screws. I think I had the idle stop set a little high, maybe 650 rpms, but I decided to set out and see how she ran.

That's what led me to this article on quadra-jets when I struggled with this before. This is some good stuff and the guy has written lots of articles about q-jets. ALthough they are not neccessarily marine apps I think a lot of it could be useful for us here at iboats.

http://www.florida4x4.com/tech/quadrajet/qjtshoot.pdf

So anyway to continue, off idle perf was good I was able to shift in and out of fwd and reverse and navigate the launch and channel and idled out to the sound. It takes about 20 minutes to get past the no wake zone and then I ran here for about 20 minutes at 3.5K and all seemed pretty good.

When I slowed down to 1.5K it started running rough and died and then the problem stated starts up.

Had the same problem last year with a different carb rebuilt by a marine shop.

That is another reason I think my problem is listed in the linked document.

I know I may need to go through some tests to diagnose it, I'll do my best as you suggest. I'm not a marine mechanic but I'll work through this.

Just a quick history: I have gone through cooling issues and ended up putting on a new set of center riser style manifolds and a new water pump. Cooling is steady at all conditions but it does seem to run a bit high, guessing by the dash gage I would say 175 or 180. The old manifolds were crudded/rusted up and it overheated frequently. Oddly It ran pretty good, relating to idle and off idle, other than the overheating. When I went through this last year is when I had the other carb rebuilt and I installed it. I had a few stalls around the launches and wanted to make some preventative steps. So I did a normal tune-up and put the rebuilt carb on. Used a new carb gasket, the 1/4 inch thick one.

If you are still reading, thanks. If you have an idea or maybe a place to start I appreciate any advice I can get, thanks.
 
Re: 79 OMC 260 carb and/or idle issues

" idle screw had no effect ", classic idle circuit clogged up. No and ,if or buts thats the problem.

But also do a psi test on your fuel pump.

And when you have the carb apart, pay attention how the accelerator pump operates , is it too loose in the " Holder " You did put in a new one ? And the ball bearing under the accel. pump, you replace that also.
 
Re: 79 OMC 260 carb and/or idle issues

First answer, Dwell 30 degrees, timing 10 degrees.

My timing is a little jumpy at 600 rpms. The engine seems to be running smmothly but the light pops all over from 7 to 13 or so degrees. The Distro shaft is not loose, but I may have another issue here.

Chief, I respect your opinions and have read lots of your stuff. You helped me with changing the water pump in this thing. But I think my issue is different. I'm on the second carb here both rebuilt and clean and I have the same issue.

I'm going to do some testing today to try to isolate a possible vapor lock issue.

I had to replace my manifolds last year and I think my engine runs about 175 to 180. And I only have the hard hot starting problem after running for a while and then when I slow down it starts to idle rough and stall or start hard.

I use the blower at low speeds but after thinking about this I'm going to insulate my steel fuel line where it passes near the intak manifold and the temp sender.

Thanks for the input.
 
Re: 79 OMC 260 carb and/or idle issues

Chief, second answers. The pump assembly is a little loose but the rubber seal is in good shape and in pumps fuel in the bores by a visual test. I re-used th old one because the new one in the kit was about 1/4 inch longer and had a different spring rate. I think it was the wrong part for my carb.

I don't remember a ball bearing under the pump. There was one under a screw next to the jets. Did I forget something?

Thanks.
 
Re: 79 OMC 260 carb and/or idle issues

Me i would first make sure the filter in the fitting that enters the carb is clean. You have a filter in the fuel pump? If yes is it clean ? Then i would hook a external tank of fresh fuel straight into the fuel pump bypassing the sep, and onboard tank.

Run it problem solved we know its back from the fuel pump the problem, not fixed then its forward of the fuel pump. And about the fuel pump you test it to see what psi it's giving you?

A outboard 6 gallon type tank is good make sure you open the vent.

Post back we will go from there.
 
Re: 79 OMC 260 carb and/or idle issues

filter in carb brand new, fule sep brand new. I didn't pressure test the fuel pump. I did a test from the seloc manual. hook a hose to the end of th fuel line and pump into a bucket with the starter for about 5 secs. It seemed good from that but I know it's not a great measure of the pump's health.

One reason I'm thinking the fuel pump is good is becasue one of my recent runs where the original post symptoms appeared this thing ran great at over 4K rpms and over 40 mph for about 30 minutes. didn't miss a beat. Wouldn't a bad fuel pump be exposed under those conditions. On that same run when we slowed down to run up the channel at idle the symptoms reappeared about 15 minutes later.

I know I may be speculating with the vapor lock issue, but that and questionable gas are two areas on my target list right now.

The gas is old in this thing, maybe 9 months. I used a gas conditioner but I'm afraid maybe I should drain and replace.

thanks for the help.
 
Re: 79 OMC 260 carb and/or idle issues

You had the fuel pickup tube out of the tank ? There is a screen on the bottom maybe, clogged.

Anti-syphin valve could be clogged.
 
Re: 79 OMC 260 carb and/or idle issues

The gas is old in this thing, maybe 9 months. I used a gas conditioner but I'm afraid maybe I should drain and replace.

thanks for the help.

Probably not the main cause here but I don't run ethanol blended fuel in mine. It turns to varnish the longer it sits and you can imagine the problems that causes. Seldom used machinery and ethanol blended fuel typically don't play well together.
 
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Re: 79 OMC 260 carb and/or idle issues

Floater, I wish I could buy non ethanol gas. Don't think anybody sells it here in Washington.

So I checked some things yesterday. Reset the idle screws at the nominal 3 turns out. checked the dwell it was at 34 so I reset it to 30. Put the timing at 8 degrees. Factory is at 10. A local mechanic says that 7 degrees works better considering the crappy gas we have around here.

Yes, I had the pickup tube out last year when I siphoned out a bunch of gunk from the tank. What I did was take off the sender unit and tilt the boat up in the front and siphoned about 4 gallons out of the aft lower corner of the tank. It was pretty dirty. I was actually only trying to check the pickup tube screen. Turned out it didn't even have a screen on it. Then I rebuilt the pickup tube because it was kinda broke loose from the fitting where it goes into the tank. It was a plastic tube epoxied into a brass fitting in a galvanized reducer, cripes, almost broke my tank trying to get that galvy fitting out of the aluminum tank fitting. why people!. Now it has a brass tube and fittings with no screen. I replace the fuel/water separator at least once a year if not more.

I still have about 30 gallons in the tank from a fillup in October. Do you think I can burn it through the engine if I put "heet" in it?

I also installed a rubber fuel line insulator on the part of the fuel line that passes close to the intake and the temp sender.

Anyways it seems to run pretty good now at the driveway marina.

Time to floattest!
 
Re: 79 OMC 260 carb and/or idle issues

I once ran about 15 gallons of year old gas in my old truck and it stumbled here and there but I made it. Had to change fuel filters of course and I pulled the carb, took it apart as best I could, blew things out with compressed air and put back together. Fresh fuel and it ran better after that.

I have used Seafoam in my gas for every internal combustion engine I have (seasonal use) and it seems to work good. Non-oxygenated fuel is kind of sparse here in MN I have to drive out of my way to get it, but its worth the effort imo. I have't used Heet so I'm not sure. I have seen Sta-bil commercials recently they market a product specifically for ethanol tainted fuel in marine engines.

You probably already have checked but in addition to your main fuel filter, make sure you have a good filter in the fuel inlet housing going right into the carb itself.
 
Re: 79 OMC 260 carb and/or idle issues

Check Shell Gas. Here in Ontario Canada the high grade dose not have ethanol in it.
 
Re: 79 OMC 260 carb and/or idle issues

Well you ain't got obama the terrorist telling you that you got to run the crap gas thru your motors.
 
Re: 79 OMC 260 carb and/or idle issues

Hey Chief,
I noticed you put the big kybosh on this thread when you brought up politics. Try to refrain, eh? :)

So I'm still in the same boat, (makes me laugh whenever I say that).

I tried this: Put some heet in the gas. I insulated about a foot of the fuel line near the temp sender and at the end of the line near the carb.

Also put a product in my heat eschanger called water wetter. A gearhead friend said it can help keep an engine 5 or 10 degrees cooler.

This was all in an attempt to eliminate a possible vapor lock issue. No luck.

took the boat out. Starts good, idles good. All good, ran for almost an hour at 4K, real smooth. Was hoping that if it was gas or a vapor lock issue that my dime store fixes had cured it, was letting it get a good heat soak to test my theory.

So then I slow down (perfectly located about 500 yards straight upwind from the boat launch :))

It stalls. Can get started in nuetral, but when I try to shift into gear it stalls.

I was able to get it in reverse without stalling. Used that to help dock it. But it was idling rough.

I've had two different carbs that were clean and rebuilt on this thing with the same symptoms.

I thought maybe the ESA is my problem.

I tested the switches. They both work. I have tested and adjusted the overstrok switche. The ramp blocks were off slightly. I am going to test the ESA per the book tonight, but honestly it doesn't seem to stumble the motor when the switch is depressed. I'll go through the procedure though.

Does anyone have any other idea what is wrong with my boat?

I'l try anything at this point.

I'm about to start throwing parts at it, but I hate to waste money if I don't need to.
 
Re: 79 OMC 260 carb and/or idle issues

Did you run it on a external tank of gas?
i have seen junk in a tank how it got it there i don't know,clog the fuel pickup tube.

You slow down the piece of paper or tin foil clogs the pickup.

Shut off the motor and then falls off the pickup tub end.

Understand ?

Also the hose it good, see the inside can peel this new fuel is eating old hose like crazy.

So what happens the inside peeled and is blocking the hose, shut off and it flaps back till the next time you slow down.

Understand ?

Take fresh hose run a external outboard tank straight to the fuel pump with absolutely clean fresh fuel.

Make sure one more time the fuel filter at the carb inlet is clean.

No fuel water sep. Straight from tank to fuel pump to carb all fresh hose. Make sure the tank vent is open when you run it.

Run it on the water. We gonna fix this or die trying.
 
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Re: 79 OMC 260 carb and/or idle issues

Thanks Chief, "die trying" good one. Let's hope it doesn't come to that.

Yes, I understand.

Well one thing I did last year was pumped out some of the gas.

Raised the boat way up in front, took the fuel sender out and pumped about 5 gallons of varnishy gas from the aft lower corner of the tank. This is a 55 gallon aluminum tank. Rebuilt the fuel pickup tube. There is no filter on the end of it. Just a straight piece of copper tube into the tank about 1 inch from the bottom with a slight angle cut on the end of it.

The old pickup tube was plastic and epoxied into the fitting but it wasn't secure and I figured while I had the thing out I should fix it.

I also replaced the rubber fuel line from the tank to the new fuel water seperator I installed last year.

That is also when I put the first rebuilt carb on and filled it up with fuel and went boating. worked pretty good for several times and then somewere last year I started having these issues of poor idling. Figured carb for sure. so rebuilt my other q-jet and tried this just recently. Same symptoms.

Am I wrong to think that the Fuel/water seperator will filter anything the fuel pickup tube throws at it? That what I was thinking when I installed the new tube and didn't put a filter screen on it.

Anyway, I will get the tank and the lines to do the test you are suggesting. At least then we will know if it is a gas issue.

Thanks.
 
Re: 79 OMC 260 carb and/or idle issues

At the risk of getting scolded for not following directions I will post anyway. Have not acquired the tank and fittings to do the external tank test. Still plan to. I did spend Thursday evening trying a few more things. I wanted to check timing and dwell one more time and run it in the driveway marina long enough to repeat the symptoms out on the water. I know there is no load on it so I wasn't sure if it would repeat. Then my plan was to once again remove the separator and look at the contents. (more in a moment).

Ran great at first dwell right on 30. Timing at 8 degrees and it was steady. no missing. That was at about 650 rpms which was also steady. After up to temp I spent about 20 minutes adjusting the idle mixture screws both in and out of gear. starting at about 3 turns out and turning in one at a time until the rpms drop and then back off 1.2 turn. Still running smooth. set the idle stop screw so it was idling about 650 to 700. Ran through a bunch of series of fwd and rev shiftings to see if it would stumble or die. nope. not yet. I had gone through the overstroke adjustment so I wanted to see if it had any affect. none noted yet. still haven't been on the water since then. used the dwell meter to note on the voltage selection it say 7 to 7.5 volts. Was told that is correct, but I really don't know.

Put the timing back at 10 degrees where it say it is supposed to be. By this time the engine had been running almost 45 miuntes except for when I was setting the timing.

Now at this time it was getting difficult to keep it running at any rpm. moving the timing back to 8 where I had it at the beginning of this made it worse. It would smooth out if I turned it all the way to 12-16 degrees btdc. But it would refuse to idle at any rpm below 1000 when I put the timing at 8-10 and tried to lower the idle speed to less than 800. It would die at neutral or in gear. It was interesting how the idle had deteriorated so much. Also the misses could be easily seen on the timing light.

Anyways, shut down this process and then took out the fuel filter and dumped it in a glass container.

look at this picture. bad gas on right (I'm assuming) and new gas on left.

So I decided to simply pump out the tank.

Now I have 22 gallons of that sh!t on the right for my lawnmower :)

And the second picture is of the tank fittings. I was going to inspect the anti siphon valve, but I don't see it on this tank. I blew out the contents of the vent hose with compressed air. the fitting on the tank where the vent hose goes was quite dirty and corroded but couldn't tell if it was blocked. Some water and gas came out when I blew the air through it, but not very much.

I believe on top of replacing the gas that I should still do a fuel pump pressure test and the external tank test because of the information I could gather.

But I'm going to see what new gas does for her right now.

Thanks,
FT
 

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Re: 79 OMC 260 carb and/or idle issues

Carb now got to be rebuilt. The filter in thefitting that enters the carb needs to be cleaned.

I would not run the motor except on a external tank of gas.

Any marina out here by me would lends you a tank. Just tell them what you want to do.

I have 2 extra ones and all my friends have one.
 
Re: 79 OMC 260 carb and/or idle issues

Sorry for the Long delay. Thanks for the help Chief. It has been an adventure. I AM REPORTING SUCCESS. ENGINE FIXED!

It wasn't a fuel issue afterall. 2 rebuilt carbs, I've pumped the tank twice now, All filters have been replaced more than once, many iterations of idle screw adjustment, also cleared the tank vent (more on that later) and blew out the antisiphon valve with carb cleaner, it's in good working condition too. Anyway, Who cares now, it's fixed. Just a lot of time lost, but the good news is a lot of new knowledge that I can share.

After replacing the fuel the last time and no success I was convinced I may have an iginition and/or ESA problem. I tested the two switches on the shift converter and made a slight adjustment to the overstroke switch. I found some corrosion and questionable condition on the old ESA so I put a new one on it. Also replaced the coil and wires. The plug wires were pretty old so I figured no reason not to eliminate them as the cause.

Same thing. It would run great for about 30 minutes in the driveway at 1K or on the water at 3K. didn't matter. The idle quality loss and stalling when going into gear was repeatable every time.

Finally dug into the distributor and completely dissasemble it. (lesson learned, should have looked at distributor long ago, after first carb rebuild) Well, one of the weights had lost it's advance plate guide pin. Was rattling around inside the dizzy. Not the pivot pin but the other one that goes in the slot in the advance plate. The shaft bearings were good except for too much end play. Was going to replace the weights but decided to make the plunge and upgrade to breakerless. Total cost about $410 including the CDI ESA.

I bought the Mallory breakerless, coil and wires kit. This coil requires the ballast resistor, part 700. Then you run a 12V wire from ignition switch to the ballast resistor/coil. The new ESA is the CDI unit and does not require a diode fix. I like the plug wires from mallory a little better than the sierra wires. They both seem like good quality but the boots and the terminals seem like better quality on the mallory wires and the mallory wires are trimmed to different lengths. They fit better. Who knows, they are both better than way old wires!

Mallory kit to replace the old YM624JV: M9-26000 SS
It includes YLM624AV and coil 9-29716 and wires set, then add ballast resistor Mallort part 700.
The ESA is CDI 123-9898P.
Also needed 25 feet of 16 gage wire and a few connectors.

Idle is steady now, shifting is perfect. It will stumble slightly as it is designed to it seems both into and out of gear. When at idle around the docks shifting from fwd to revers it is smooth and easy, idle drops down to as low as 500-600 and no stalling! In nuetral I have the idle at about 700. Maybe a bit high.

The one weird thing left to figure is that My cheapo direct hook up timing light doesn't work right. Can't figure this out. It fires at about 30 degrees btdc of #1. Been all through this finding tdc 1 and setting the dizzy multiple times. Had to time it by ear and then test run it to eliminate ping. Not happy about this but it has proved successful as we were pulling hard on her yesterday through some tough wind chop on Puget Sound and going 27 mph. Daughter thought we caught air a couple times, but it was just the big 5 feet deep holes between the chop, fun ride, dry too, this hull is a tough girl.

So my plan is to find a better timing light, inductive, to perhaps weed out this strange problem. I really want to be able to get the timing perfect by the light, but I probably have near perfect the way it is.

So the last hing as I mentioned earlier is the tank vent. I noticed while doing some wiring that it has a loop. Kinda like you do with a dishwasher hose. I'm sure that the idea is to keep water from getting into the tank from the vent, and maybe it works becasue the last time I filled it with gas it was spitting water/air OUT of the tank vent while filling indicating that there was water in the bottom of the loop. Anybody have this loop in the tank vent hose? My concern is that if the lower portion of the loop get's water or gas in it that the vent won't pull air and it will stall. The last two cruises kinda disprove that though. I'm thinking that if on a long cruise in rough water a lot of water could get pulled into the tank through the vent and the loop.

Anyway, all minor stuff now.

Took my buddy crabbing on Thursday. Took 5 dungeness. On Friday Wife, Daughter and captain went for a long cruise by Hat Island and Whidbey. Great Fun! Run most of the time around 1.5K and about 7-8 mph on the gps. I like that speed best for low stress cruising.

Thanks again for all the help and happy cruising to all.
everettWA.jpg
 
Re: 79 OMC 260 carb and/or idle issues

I want to add this for anyone wondering how I set the timing. The dizzy has 20 degrees of full adavance I'm told by the mallory tech (very helpful folks too) I set the timing by ear to the fastest smoothest idle and then lowered the idle to about 650-700 in nuetral on the hose in the driveway. Went to the sound to test. The motor would ping (detonation) when a good shot of dino juice was dumped in. Moved the dizzy about 3 to 4 degrees retarded. Tried again and lot less ping but still when opening the big secondaries a slight ping when getting up to 35 mph or more. So another adjustment of 3-4 degrees retarted and then a second adjustment of idle stop screw has it where no noticable ping at acceleartion, idle is around 650-700 in nuetral and when shifting at idle it drops to around 400 for second then stablaizes around 550-600. NO STALLING.
 
Problem: poor off idle performance after running in water in gear at temp for 20 minutes or more. Boat seems to run rougher and rougher over time unitl it dies and then is dinfficult to start, can get it started in nuetral but then when dropped to idle speed and dropped into gear it dies. To get back last night I had to adjust the idle high and have friend hold off the ESA while I dropped into gear, stumble coughed and got going was able to run for about 25 minutes at 2K then it died again. Luckily within a hundred yards of the launch. I stretched the no wake rule in the channel just to get back to the launch.

I just rebuilt my carb and I went through all the carb adjustments in the book before installing. There is quite a few and some of them even by going to the manual seem tricky. Like the choke linkage adjustments have you measre betwee the choke valve and the carb body, but the choke linkage is in a slot so it depends on if you hold the valve to the edge of the body or away from it to measure. Anyway. I made several bends in the stops and links to get what I think are the correct adjustments. I have an OMC factory manual from 1982. My motor is 1979 but I made sure the carb sections were dealing with the same carb model. This is a 4MV, 17059286. By the way the factory OMC manual is awesome. Anybody with an OMC needs one.

Installed the carb and set the idle screws 3.5 out.

After installing the carb and a new spin/on fuel water seperator I fired her up and it came to life immediatey. This is in the drivway marina. It seemed to idle nice and I let it come up to temperature. Made sure of no fuel leaks and every thing was normal.

Went to the launch to adjust the idle screws and the idle stop screw as the last adustments.

With it in the water let it run up to temperature and tried to do the lean drop process by turning in one screw at a time until idle drops and then back off 1/2 turn. This doesn't seem to go by the book. The idle screws don't seem to have much affect.

I ended up at about 2.5 turns out on the idle screws. I think I had the idle stop set a little high, maybe 650 rpms, but I decided to set out and see how she ran.

That's what led me to this article on quadra-jets when I struggled with this before. This is some good stuff and the guy has written lots of articles about q-jets. ALthough they are not neccessarily marine apps I think a lot of it could be useful for us here at iboats.

http://www.florida4x4.com/tech/quadrajet/qjtshoot.pdf

So anyway to continue, off idle perf was good I was able to shift in and out of fwd and reverse and navigate the launch and channel and idled out to the sound. It takes about 20 minutes to get past the no wake zone and then I ran here for about 20 minutes at 3.5K and all seemed pretty good.

When I slowed down to 1.5K it started running rough and died and then the problem stated starts up.

Had the same problem last year with a different carb rebuilt by a marine shop.

That is another reason I think my problem is listed in the linked document.

I know I may need to go through some tests to diagnose it, I'll do my best as you suggest. I'm not a marine mechanic but I'll work through this.

Just a quick history: I have gone through cooling issues and ended up putting on a new set of center riser style manifolds and a new water pump. Cooling is steady at all conditions but it does seem to run a bit high, guessing by the dash gage I would say 175 or 180. The old manifolds were crudded/rusted up and it overheated frequently. Oddly It ran pretty good, relating to idle and off idle, other than the overheating. When I went through this last year is when I had the other carb rebuilt and I installed it. I had a few stalls around the launches and wanted to make some preventative steps. So I did a normal tune-up and put the rebuilt carb on. Used a new carb gasket, the 1/4 inch thick one.

If you are still reading, thanks. If you have an idea or maybe a place to start I appreciate any advice I can get, thanks.

1969 Thunderbird 233, 210hp OMC 305 w/Rochester 4 Brl Quadra Jet.

YES, the OMC Shop manuals are awesome. Been having similar problems as you for a couple of years. Will start and run beautifully to wot,( usually doesn’t go there until choke closes and engine warmed). after running for atime when going to idle it surges and quiets with no start until it cools down leaving wife and I stranded (oh, fun!) thank you BoatUS for tow. Last shop said it was overheating causing fuel to vaporize (vapor lock), starving the engine.

they said needs impeller and it was (hopefully) replaced, (at least that is what I paid for). Did not get it back soon enough to try out so went into winter storage. Did not come out until this spring. Now I have a surging at warmed idle and again stall.

Cleaned out carb, even substituted another quad and ran it, same result. So, now we are looking at ignition.

Also another shop suggested that the fuel line (mine is steel) maybe to close to the exhaust manifold, causing the vapor lock, which sort of makes sense. We will try insulating the fuel line, next.

Anybody with ideas? They will be appreciated!
 
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