Inline six help! please

[Georgia Boy]

I have been trying to get my old boat ready to ride this summer, I took it out today and it started and ran smooth, except when i opened it up, it would respond normally and then after say ten seconds or so I started loosing Rpms. it would back down say 500-1000 rpm, as if it were loaded, but if you eased up on the throttle and punched it again it would jump back up and ten seconds or so later it would start dropping back down again, can anyone sugeest where to look to solve my problem. this motor is a merc inline 1400 72 model. it has always run good, has new plugs, new diaphragms in the fuel pumps, and new fuel filter it will start in less than a full rotation,and idles smooth. I just can't seem to figure out why I am loosing RPM's. any help is greatly appreciated.

[Bill Rogers]

Fuel pump? Sqeeze the bulb when it starts to lose rpm and see if it picks back up

[Rickracer]

....and I'm sure you know this: If you have portable fuel tanks, make sure the vent valve is open. They will run with it closed, but, not well, nor at WOT.. Don't ask me how I found this out, :D It definitely sounds like a fuel related problem though.

[Raceman]

Sounds fuelish to me too. Usually when it's a vent problem it'll suck the bulb partially to fully closed. I've had some bulb problems in the past with old fuel lines too. I assume the gas is decent???? You know, the 72 model engines had the higher dome pistons as all the inline 1500's did. I don't like regular gas in these, but doubt it's your problem. Of course the octane requirements are greater under full load.

[XSRaveau]

93 pump premium? Or did you cut that back to 1/3 rg to 2/3 pp?

I just gassed up with 50/50 Sunoco 110 and Exxon 93 and was thinking about going to 1/3 to 2/3.

BTW, running 32:1 Penzoil Synthetic.

[Mark75H]

Another trick I used to play on myself once in a while that did the same thing: Fuel line between the tank and motor pinched...tank or battery sitting on it, duh.......

[sosmerc]

If all else checks ok....then maybe you have an inner water jacket leak. Run at wide open, then shut down immediately after problem occurs and inspect plugs. If it looks like a chocolate milkshake on any of the plugs, well, then you have found your problem. Pain in the ass to fix, hope this isn't it.

[Georgia Boy]

this is the first post I have done in quite a while and good to see everyone is still as helpfull as they used to be. The fuel pumps are new so don't think that could be it, but I will recheck them, I just pust new 3/8"fuel line on todayfrom tank to engine, Could the screens in my carbs be dirty or clogged by chance, it appears to me that it would have to be a fuel supply problem, just not able to run this one down for some reason. SoS I have checked the plugs when it happend the first time and they are as clean as can be. thanks guys for all the help.

[Dave S]

It could be the switch box, I had that problem one time.

[sosmerc]

You may want to check your main jets. One of them may be partially plugged. Get someone else to drive so you can use your fingers to partially choke each carb when the problem occurs. If it picks up speed when you choke it you have found your problem...or at least isolated it to that particular carb. If the engine has been laying around for awhile without use it is not uncommon for the remaining gas in the carbs to turn to varnish and leave a residue that can mess up the main jets.

[Mark75H]

I don't think it is a main jet problem. If it was it would never "respond normally and then after say ten seconds or so I started loosing Rpms" A main jet problem shows right away it coughs and dies when opened up, you never get that initial normal response to die off from after a few seconds.

[Raceman]

Eric, I'm still doing it, but it's overkill in most of the stuff I've got it in. No more than I run any of this stuff it's just easier to keep up with as far as being consistent in everything.

[sosmerc]

Sorry, but I have seen PARTIALLY plugged main jets exhibit what was described...especially on single carb motors such as a 15hp evinrude. Would accelerate and run great for a few seconds, and then lose power. And just blowing out the jets with compressed air is sometimes not enough.....the varnish-like build-up can sometimes require a drill bit run through it...or jet replacement.

[Capt.Insane-o]

or at least pull off the float bowl covers and have a look.There is a brass plg on the front of the carb that the main jet is behind. Take a peek in there with a pen light. They are pretty cheap and very easy to clean/rebuild. And it may be time to look under that distributor cap:eek: . sos has a good point also

[WillyT]

Do you have one of those little anti-syphon ball valves at the gas tank/fuel line interface (most tanks do, per Coast Guard requirements). That is a VERY common culprit and causes the very problem you describe (and they go bad with no warning and at the strangest times!). My mechanic buddy used to say to give them the "float test." Throw it in the water, and if it sinks, you didn't need it! Check that next, if you haven't already. I'll leave it up to you as to whether or not you want to replace it with a new one.

[Georgia Boy]

I took the caps off the top of the carbs that cover the screens. and from what I could tell I did not see any signs of gum or gelled gas. How ever I have not thought of the distributor cap, and to be honest I am not sure that it has been placed since I have had the boat about ten years, I have always been pretty thorough as far as keeping it up, but the cap for some reason i have not thought of, How long do the caps on these motors last? also not sure if this tells you anything but appearantly the load has nothing to do with it, because I put it on the hose today and it did it there also. just playing around with it letting it run abit. How bad a job are the carbs as far as tearing down and cleaning and rebuilding. I am sure I am capable of rebuilding them but I have been told it is tough to get them adjusted. so I am thinking of just having someone else do it. I would rather do it myself but not sure on how to adjust them once I get them rebuilt.

[crazy horse]

The inline carbs are pretty easy to rebuild. My cap lasted about 20 years but it was cleaned every year and the center carbon was changed about four times I think. Just for fun, Check and see what good old Merc get for a inline cap. It'll make your day. :D

[Capt.Insane-o]

Sunsabitches.

[Georgia Boy]

ok, So now that I am kicked in the @$$ on the price of the cap, what about the wires? I know they have to be high as hell or is it better to make them yourself?

[Capt.Insane-o]

:eek: Wires were'nt too bad. I'd replace the carbon for sure. The trigger housing is pretty crazy too. It's a good thing they are reliable. cuz they is spendy. CDI or Rapair are the best for availiability and cost. Make damn sure it's not in the fuel systen though. I'd check to see if the check valves in the fuel pump are tight also. the carbs are a piece of cake to rebuild. i don't think there is a much more simplistic carb out there.Float is easy to set, jets are very easy to get at. Adjusting them just takes a little patience. But count the number of turns each slow speed jet comes out. And put them back in the same and start from there. Pull a float bowl cover off and look in the bottom of the bowl, that's where the goo hangs out at.

[JOHNNYGUN]

I just read through all the posts, and went into my thinking room, to sit on my throne, and contimplate the ways of the world, when suddenly, there it was!
I remember fighting a problem just like the one you have, many years ago.(funny how that stuff comes to ya in the strangest places!)
Two things;
one - who did the fuel pumps?
I remember having my fair share to drink, and installing the check valve in backwards:eek: she'll run, but starve for fuel above a high idle. All the pumping in the world wont make fuel pass fast enough.
two - exaust tuner
I had one that I chased for like a month! One of my own, so a customer didn't have to pay for all the parts I changed;)
I went through the carbs. , fuel pumps, coil, box, cap, wires, even tried a new trigger:mad: and changing that on those aint no fun!
Sorry to drag this out, but I got another donut left, before I gotta go to work.
What I ended up with, was deciding that I must have had something internal, like rings, or reed cage seals, or something.........so, I pulled the powerhead, ready to tear it apart, and found that the exhaust splitter plate had broken, just above the bolts, and half of it was covering the exhaust tuner!
Can you say, Restriction!!
So after all the time and money I spent, I eliminated the rest of the plate, and she run great ever since:cool:
Well.......................
until that time I came out a da water, and she reved a little too high, and broke the crank:eek:
Just some ideas from the other side of things.
Hope it may help!

[WillyT]

Did you check the anti-syphon check valve yet (see earllier post)? Your symptoms are EXACTLY the same as two other people I know, and their anti-syphon valve was the culprit in each case.

[Mark75H]

Oh, yeah! I remember a mechanic that worked for my dad's dealership had that broke a crank on his personal 110hp inline. Darned crank broke just below the top bearing.....worked fine at idle, wouldn't rev up because the distributor and flywheel weren't going as fast as the rest of the crank. It only got up to 1500-1800, a fast idle speed.


Fuel pump problems can be bypassed by pumping the primer bulb underway (you become the fuel pumps). If the antisiphon check valve in the tank is stuck the primer bulb sucks itself flat....pretty easy to see.

I had an ignition module get wet (sunk) and sort of did what you are describing, but it ran maybe 40 seconds fast and then the engine definitly "broke up" as far as smoothness and lost 1500-2000rpm........a few seconds later it would go up to full speed again for another 40 or 50 seconds...........it was definitly longer than 10-15 seconds that it ran OK.

Goop that funks up the main jets is not usually visable at the inlet screens at the top of the carbs; it forms in the lower part of the carb basically right at the jets.

[sho305]

When I put fuel pump diaphrams in my '73 1500, I found rubber chunks stuck in the valves & in filters. Turned out to be the bulb valves/hose going away. New hose/bulb was cheap. I had another motor that had a bad valve in the bulb restricting fuel also.

The dealer told me $100 for a cap, and $300 for the rotor/shaft. I cleaned them and sanded contacts well...works great now. Cleaned wires and gooped all ends with dialectric, no problems. They had no visible damage. I flushed the tank, all lines, and the carbs w/ new filters. It would run a little ways and loose power before. Took carbs off and cleaned good.

I've had more bad fuel problems in OBs that anything else. Carbs have to be perfect clean. I shoot some carb clean in the jet plugs every spring/fall and had no problems; to flush the bowl out just in case. Only let it sit 6 months max with premium, and drain the floats for storage. Well, the gas goes into something else in the fall anyway. Float drains are pretty easy to get at on this inline.

Gas likes to react with the carb metal in many small engines I've found, and they fire up great if you drain the carbs only. Except automotive Holleys, where the gaskets shrink right into the carb if you dry it out. I think I have done this fuel system clean job with every OB I have bought or not run the year before. I run premium 40:1 and don't happen to run wot extended here much due to limited lake.

You should be able to take a carb line off, and when you spin it over (just a couple revolutions) the fuel will come out; a lot of fuel. Of course, disable the ignition and run the fuel into a container or something. It burns real good;) be careful. Also pumping the bulb will deliver lots of fuel to a carb line. I think I had each line off the pumps to make sure each was delivering due to the common fuel problems with these I was warned about here on S&F. Many run electric pumps on these.

I also ran a prop when trying some, that had a JohnyRude larger diffuser. It made it die/missing on the top end bad! Turned way rich at speed. Never heard of this, but it happened and that diffuser was not much larger. Beware.

[Raceman]

"Darned crank broke just below the top bearing.....worked fine at idle, wouldn't rev up because the distributor and flywheel weren't going as fast as the rest of the crank. It only got up to 1500-1800, a fast idle speed."


Sam, say what??????? How 'bout 'splainin' that in detail. The distributor's gotta turn at the exact speed of the powerhead or the motor ain't gonna run. I recon a crank broken at the top could sure let the timing bounce around as long as it stayed pretty much in one place. You wuz just pokin' to see if I wuz payin' attention.................right?

[Mark75H]

The break in the crank was at a slight angle and there was enough friction between the pieces to crank, start and idle. Above idle speed the two pieces slipped and the motor didn't run, you are correct......all kinds of imaginable pop bang snort going on.......the slower you brought up speed the higher it went before the bad stuff started, but cranking and idle it stayed synched, ran smooth and started easily. Before it was taken apart it was about blowing our minds........the spark was good, the timing was set perfect, the carbs were perfect, but it wouldn't rev up. I think maybe they were thinking somekind of reed problem or something that they had never seen, so they opened her up.

I don't think this is what is going on with the problem motor here.

[JOHNNYGUN]

I seen the same thing on an old one a guy brought in!
He broke da crank, right under the top bearing, and drove it 4 miles home. He could run at idle, or a little above, but the engine would die at more RPM. Strangest thang you ever saw!
We actually ran it in the test tank, just to see if it was true!

[Georgia Boy]

Rechecked the fuel pumps, Everything looks good, no trash in the lines, the check valves were in right, the lines are new, replaced last year, bulb also, took distributor cap off, cleaned/polished everything and put back together, but as fast and smooth as the engine idles and starts , I think the electrical is ok, but I sprayed the carbs with carb cleaner really well, and will try it again this weekend and let you all know what I find. If that fails I will pull the carbs down and go thru them, How ever the exhaust tuner episode mentioned above, Did remind me that I pulled the exhaust liner and relieved the exhaust over the winter. Could this be causing that? maybe not enough back pressure? I still have the liner but hate to pull the power head if it is not the cause of it.

[Raceman]

Exhaust relief, although not helpful much except to make for a better sound wouldn't cause the problems you're having.

[Mark75H]

Please explain exactly what you did. Did you only remove the tube in the leg?

[Raceman]

Sam, I don't think what he did matters much. When I was runnin' a 1500XS on a 15 Allison, I cut 2" holes right behind the lower motormounts with the liner in, then later pulled it. A friend cut em up higher. We never saw any difference anywhere, except it made the motors sound cool. An inline is a great soundin' engine with a little exhaust relief and it's nothing like the V6's.

[Georgia Boy]

Ok I was only looking for the sound Raceman. That boat is not that fast, lol However just for the record I pulled the liner out, and milled a one inch by two inch oval in the mid on the both sides as far down as I could on the midsection . nothing else changed.

[Brian]

Did you look to see if you have a check valve on the fuel tank? If so, remove it. If its a portable, do you have the vent open? If its a built in, make sure the vent hose is not kinked or plugged with dirt or a spider nest. Also check the pickup in the tank and make sure it's clean. You say you replaced the lines with 3/8 line, do you have any air leaks? Another thing I see happening on the older inlines is that people don't get the lid on the carb where the screen is on correctly and they leak gas and suck air there also.

[Mark75H]

Brain could be right, maybe air is getting in. But if it is after the primer, fuel will come out when you pump it. The tops of the carbs that cover the filters will leak like crazy if not on correctly, I don't think anyone would go far with one on crooked and I don't think much air would get in compared to the fuel flying out. An air leak is more likely between the bulb and fuel pump inlet.........maybe the connector area. You didn't change the connectors did you? There are 2 sizes on the old Mercs if I remember correctly, one for smaller motors and one for the 35 or 50hp and bigger.


I will repeat: if there is a blockage in the fuel line, tank (siphon valve, etc) or tank vent before the bulb, it is quite obvious......the primer bulb gets sucked flat.


Dick Davis, with the pretty, blue Scott 3 pointer tricked himself with his new rig (see it under the "Hey Sam" topic in the history forum). He cleaned, painted and set up fresh 6 gallon tanks with new pickup lines in them. Ran great for a few minutes, until the pickup line (which was 1/4" or so too long) sucked itself tight against the bottom of the tank. His new tanks only worked when completely full and the weight of the fuel in the tank bowed the bottom away from the pickup. When he used a little fuel it sucked tight against the bottom of the tank........also sucking the primer bulb flat.

[sho305]

Take a fuel line loose near the carbs/filters. Pump the bulb and fuel should pour out quite a bit easily pumped. If your vent is good, then you should not have a flow problem to the carbs. You can run it with the cap loose to check that, or keep pumping and it will not want to come out(then loosen the gas cap and listen for it to suck air). Mine will dump the gas out with the bulb. You should even be able to take the float bowl drains out, and when the carb is empty pump fuel in to see if the float is restricted. Should get a little stream there. There is only carbs after that.

If it runs 10 seconds and dies out, then comes back it would be a fuel restriction problem like that most likely; as everyone thinks, it is using up the float full of fuel. A jet will be bad all the time or when ever you use that part of the throttle; like a main jet works only above idle and would die as soon as/any time you throttle up or get progressively worse with throttle. Sometimes you can hit the choke and see what happens, like it will happen faster as the choke uses up the fuel if a restriction, but may help a clogged jet. Once fuel is gone from carb it will die choke or not. On mine the bottom two cylinders are not choked

Hold some pressure on the bulb and look closely for leaks, you might spot an air/fuel leak maybe. I would inspect the pumps again if none of that shows up, as if flow is good maybe there is no pumping. Mine has the two pumps. You could rig one line safely away and see if it pumps on the starter then the other(or both), it should pump on the starter quick. Be very careful of fuel/fire, I did this just to check my new diaphrams. Could be one pump is bad. Your new bulb would gradually loose pressure if shot diaphram(or bad float). Blow back into the pump, they are one way and should not go back. Could even be a chunk of gunk in a pump valve sticking it open. My carbs are piped together, so one bad pump would loose fuel supply to all carbs but the test would show it.

I have heard of alternator slow speed windings doing funny stuff, but I am guessing like my '73 it has one winding only and not high and low. Mine would run fine with a bad rectifier and no charging on a good battery. Low batt would make it cut out when you hit gas fast, and at high rpm. I have heard of triggers getting hot but that takes a while normally. Always start with new plugs first. I would recommend cleaning the carbs well as a lean condition could cause a problem with this motor. Yeah, they are a pain to get out of there too.

With my big diffuser prop, it would run fine up to about 4500rpm, and would not go above that.

[Mark75H]

The ignition on the 72 140 will work well down to about 9.5volts, it is not finnicky. I think he would have known if the voltage was low: the motor would never restart.

[sho305]

I had a cell go bad. It would charge and start fine about twice, then go dead after an hour running. Thought I had a drain as it was fine for weeks of weekend running before that....duh...2 years old and a delco marine.:confused: Had good luck with them otherwise. I guess it got me to fix the rectifier faster. Good old Thunderbolt! What happened to the cool names for stuff?

[Raceman]

Sam is dead right about the bulb sucking flat if there's a restriction from there to the tank, including vents and anti syphon valves. Sho, I've never seen an inline that only chokes on the top carb. Everything that I've owned has choke butterflies on the top 2 carbs.

As far as the lid on the carbs leaking and "sucking air", this isn't a vacuum situation, but rather on the pressure side. The bowls themselves are vented anyway, so if there was sucking it would be going there. If they're leaking, the only thing going on is a fuel leak.

I personally don't believe it's a partially clogged main main jet either. I have motors that sit around for years and haven't experienced the problem, although I think it can happen. I just don't think that an inline would run to full RPM, then back off if it was a jetting issue. As boarderline as they are on burning pistons anyway, I think a pair of cyl's lean enough to drop RPM would self destruct.

I think if I was foolin' with it I'd throw about 4 PSI of electric fuel pump at it and see if it goes away. If it does, I'd build the mechanical pump again and replace the hose, bulb and all. If it doesn't, I'd build the carbs. Electrical would be my last guess.

[heath brinkley]

I'd do like RM said, I'd certainly think it was in the fuel system. What he described would eliminate guesswork. a 4 psi electric pump only costs you $30 or less, for a cheap one to use as a test. If I read your posts right you still haven't rebuilt the carbs, correct? I'd exhaust every fuel delivery question before looking anywhere else.

[sho305]

Sorry for the wordy post, just hoping to explain something they might of missed. I know you regular guys knew it a long time ago.

My '73 1500 has three carbs, serving two cylinders each; the bottom carb is not choked so the bottom two cylinders are not choked. Can't use the choke to investigate a fuel problem there. I think if there happened to be a bad diaphram the fuel could get into one of those and make it rich? Though likely that would be apparent at idle I would think, not sure.

[JOHNNYGUN]

There is a test that you can perform, that will answer laot of questions about fuel restriction.
Tee into the fuel line, engine side of pump bulb, and connect a vacuum gauge. Normal reading, with the engine running, 2.5in. vac. or less.
Anything more, go after everything in front of the gauge.
anti- siphon valve
primer bulb
fittings
hoses
tank switches
pick up filter screens in tanks
ect..................

Another thought;
check to make sure that one carb. isn't flooding over.
I've had that, and it would flood out 2 cyls. and bring down rpm
float levels are kinda critical, also.

I would also double check your compression. That's the beginning.

[Mark75H]

Johnny there is no need for an elaborate vacuum test manifold you suggest. If there is a restriction on the tank side of the bulb it sucks flat, test done.

I suggest pumping the bulb manually while under way instead of the electric fuel pump......quicker to do, no set up required (other than having a second person in the boat, if you can't have a helper do this then the electric pump sustitute is a good second option). On most boats it only requires a functional primer, which, since this motor is not having trouble starting, would seem to be in place already.

You guys are making a Rube Goldberg situation out of something that is simple.

[Raceman]

What's a "Rube Goldberg"? Will it work on OMC's?

[heath brinkley]

to answer your questions.

1.Rube Goldberg is a situation (one you don't want to make)

and

2.NO, it won't! nothing works on a OMC (i thought you knew that one)

[Jeff_G]

Sam, the vacuum guage is the proper way to go. Although the way you suggest, just pumping the bulb will tell you it is a fuel problem it will not tell you where.
I have seen the check valves in bulbs come loose yet still pump fuel. Then while running block the flow of fuel. I have also see high vacuum that did not collapse the bulb!
A vacuum test will tell you of restrictions in the lines, tank, bulb etc.
A pressure test will tell you of proper pump pressures.

If the motor runs for 10 seconds OK it points to the fuel pump, in most cases. But if like Sam pointed out it continues to run if you pump the bulb it again points to the fuel pump.

[Mark75H]

I think the things you describe are are possible, but very rare. I have seen bulbs that would not suck flat too. They were so hard they were practically useless. The check valves can go bad in a primer bulb.

I suggest a simpler test if you suspect a bad check valve in the bulb: replace it.


We are talking about an old 140hp inline 6........probably does not have built in tanks, probably has fuel line that is completely removable. $30-$40 to replace.

Would you really rig up the vacuum gage rig when you could just replace the hose & bulb? Not me.

[Dave S]

I do not know if anyone mentioned this idea, but if you put the wrong diaphram gaskets from a newer style 3 bolt pump, the pump will not work properly.

[Smitty]

Be sure the welch (sp?) plugs are still in the port side of each carb. I had this happen once, had to hold my finger on it while my wife drove us to the ramp...