'78 model 700 not idling right

[superwooter]

i have a 1978 mercury thunderbolt 700 on an old boat i just got that had sat up for about a year. at first, the boat would crank and idle fine, but when i tried to take off, the engine would barely accelerate at all. the tach was also not working. i took that to mean i had a rectifier problem. i am still working out all the kinks, so don't think poorly of me when i tell you that 2 of the nuts were missing that hold the wires onto the rectifier! also, the 2 yellow wires that go the the stator were corroded and the insulation had almost completely deteriorated. i replaced the bad wire and the missing nuts, and i tightened all of the other nuts as almost all of the nuts were loose on the panel.

i put the boat back in the water. it cranked and idled ok, but a little rough, but would run like a scalded dog at once the throttle was applied. i ran through the gas in the tank that was connected and switched over to the second tank. after the switch, it would not stay running at idle. i am quite sure that the fuel in the second tank was a little oil-rich and i've been told that that can cause the fuel to be too lean and cause idle troubles. tank #1 was mixed at 50:1 and tank #2 was probably closer to 30:1.

my concern is that even while running on tank #1 the idle was rough and i had to ease on the throttle up to WOT to prevent it from bogging down. is it possible that the fuel mixture screw on the carbs is set too lean? what else could be causing the rough idle and the tendency to bog while accelerating when the fuel is mixed properly?

thanks in advance for your help and comments. sorry for the length, but i wanted to properly paint the picture.

wooter

 

[Chris1956]

Re: '78 model 700 not idling right

Incorrect gas-oil mix, dirty carbs and incorrect carb-advance synch will cause all your symptoms. You might check compression and spark as well.

I would recommend cleaning the carbs and rebuilding them as well.

 

[superwooter]

Re: '78 model 700 not idling right

when you say "carb advance sync", are you talking about the sync and link thing i keep reading about? i've read on here that if you do a carb re-build, you pretty much have to do the sync and link.

i don't know how to check compression. is there a gauge or something i need to buy?

it looks like the spark is good. i feel like it's most likely something with the carbs.

 

[Chris1956]

Re: '78 model 700 not idling right

The Link and synch is the procedure to set the carbs to open at the correct timing spec. You also set max spark advance, throttle stop and idle stop settings.

A compression gauge is used to check the compression of each cylinder. You want them to be 100+PSI and even.

A factoryMerc Service Manual would be a good investment, as it will tell you how to link and synch, and how to clean and rebuild carbs.

 

[superwooter]

Re: '78 model 700 not idling right

i found where to get the proper service manual here on iboats.com but was not able to locate a compression gauge. can you post a link where one is available?
thanks,
wooter

 

[Chris1956]

Re: '78 model 700 not idling right

Compression gauges are available at most/any auto parts stores. Checking the compression on that lowest cylinder is difficult with a standard compression gauge. i found a gauge that I could add a brass elbow to make the gauge turn 90 degrees. That works pretty good.

 

[superwooter]

Re: '78 model 700 not idling right

mechanics question: what do you do if you find out the compression is inadequate? i'm good with the basic maintenance stuff, and i have a decent aptitude for mechanics. i'm just ignorant to some of these issues. i learned most of what i know about mechanics from working on my 65 ford falcon the past 10 years. so far i've not encountered a problem i can't tackle after reading about it.

 

[Chris1956]

Re: '78 model 700 not idling right

If compression is low, generally you need to tear down the motor, bore out the low cylinder and install oversize piston and ring set.

 

[superwooter]

Re: '78 model 700 not idling right

wow. probably not worth it on a motor that old. could buy a used motor in good shape for less than that i'm sure. so basically, i just need to keep my fingers crossed and hope the compression is good. it runs terrifically as long as throttle is applied though, so maybe it's just the fuel mix and the carbs needing a cleaning. i'll be taking it out again this saturday so we'll see.

 

[intrepidvoyager]

Re: '78 model 700 not idling right

For every engine that won't run right due to low compression there are 100 that won't run right due to a dirty carb. try cleaning the carb ....pull the carb apart ..use spray can with the little red tube in every hole and tube .... make sure the carb cleaner comes out somewhere ....check there is no crud in the tank ... lines or fuel pump .... disconnect the pump line from the carb and crank the engine ..let the fuel run over a peper towel and look for specs of debris. If the engine runs better but not 100% clean the carb again ....I have cleaned so many carbs twice that now I spend 30 mins and use a whole can of carb cleaner. if the engine has been sitting for a long time sometimes you gotta soak the carb in cleaner.

 

[superwooter]

Re: '78 model 700 not idling right

well i have an update on the motor, and i still have issues. i checked the compression. all 3 cylinders are nearly exactly 130 psi. the fuel pump diaphragm appears to be in good condition. my fuel mixture is correct. we flushed the carbs and found no debris. the spark plugs looked a little dirty, so i scraped them off and wiped them down. backed the boat into the water and it cranked and idled beautifully... for about 20 seconds. then it started running rough again. it seems as though it misses at random. there's no rhythm to the missing.
on the way home from the lake, i stopped by the parts store and picked up some new plugs. once i got home, i inspected the wiring. i had previously replaced most of the bad wiring but the 2 yellow wires running from the rectifier to the stator i hadn't been able to replace all the way back to the stator. i cut open that wire harness all the way back to where it goes under the fly wheel. the 2 yellow wires were missing most of their insulation but the wire itself seemed ok. i wrapped the yellow wires individually in electrical tape to keep the bare wire from touching each other. i haven't had a chance to check it again since leaving the lake though.
my dad was helping me. he has been a mechanic his whole life and currently works on oil well pump engines, however, he has never worked on outboards. he suggested that, even though the purpose of the yellow wires is only to charge the battery, that since they were bare and crossing each other, that they could be drawing a current and preventing the proper current from reaching the ignition, thus causing a sporadic miss fire.
does that sound feasible?
i know that i really need to pull the flywheel and entirely replace the 2 rotten yellow wires, but i have no idea about how to do that and i understand it requires a specialty tool.
i REALLY need this boat to be functional by friday. what do i need to do?

thanks again,

wooter

 

[superwooter]

Re: '78 model 700 not idling right

here's a link to the wiring diagram for my engine for reference...
http://www.maxrules.com/oldmercs/Wiring/1966ona/55.jpg

 

[superwooter]

Re: '78 model 700 not idling right

wow!
my brother just informed me that an old high school buddy has a local boat mechanic shop. i called him up and he's going to walk me through getting this thing right! he said from my description, the low speed jets are clogged on the carb. i'm meeting with him either tuesday or thursday evening and will give an update after that.
thanks,
wooter

 

[intrepidvoyager]

Re: '78 model 700 not idling right

hi there ...... have a look at my previous post ..."if the engine runs better but not 100% ..clean the carb again" ...sounds like you got it to run fine for a while ...... before you do that if you can start the engine again just when it starts to die spray some fuel ...not starter fluid .... into the carb .... if the engine picks up you know for sure you got a carb issue, however this will not tell you if you have an idle air issue ...re cleaning... you gotta spray carb cleaner with the little red tube in every hole and tube ...... follow the drilled chanels and the cleaner should come out somewhere ....make sure the float needle is not sticking ...... use a whole can of carb cleaner ... I have cleaned carbs 3 times to finally get them running right . ... good luck ....ps .once you have done the carb to the point where there is no further improvement then look for other issues.

 

[superwooter]

Re: '78 model 700 not idling right

i'll be meeting up with my boat mechanic buddy tomorrow night. i sure am looking forward to this thing running right. i'll post a summary of tomorrow night's events maybe with some pictures when we get it done.
thanks,
wooter

 

[intrepidvoyager]

Re: '78 model 700 not idling right

just read your post again ...... 130 in all is great !!!! ...."low speed jets clogged" .... sounds right to me ......IF THE ENGINE WILL RUN AT HIGHER RPMS BUT NOT IDLE .... 95 percent its the idle circuits ...HERE IS HOW TO CLEAN ... there are as you may know 2 parts to this ...fuel delivery and air delivery ..got to be close to 14 to one or won't work .. the idle fuel port is a tiny hole in the venturi ... it is fed by a tube going into the float bowl ...so skuirt LOTS OF carb cleaner in the idle fuel tube and make sure it comes out in the venturi. there may be two or 3 tubes ..just find the right one .... you got to make sure idle fuel is getting to the venturi ......NOW screw out the idle mixture screw half way and repeat the process ......carb cleaner should come out both the fuel port and the AIR supply somewhere ....if not put your finger on the venturi fuel port and skuirt more carb cleaner into fuel tube ... if no carb cleaner comes out somewhere the idle air delivery is blocked ....once its clear put your finger on the fuel port and spray LOTS OF cleaner both ways . its not only dirt you are looking for .... old fuel glaze ..really bad if you use gas with ethenol in it. some carbs have a removable mixer jet .... if you can't get it out just repeat the cleaning process ten times. By now you should be soaked in carb cleaner ...if not you have not used enough .. .... now if your idle screws have o rings replace them ...sometimes i find they are missing ..... coat the o ring with vasoline when you instal so the o ring does not get hung up on the hole ...good luck.

 

[superwooter]

Re: '78 model 700 not idling right

so i didn't get to work on the carbs last night. bummer. my boat-mech buddy had to go out of town at the last minute.

this may be a ridiculous question, but is there a way to clean the carbs at least partially without actually removing them? i'd like to try something if there's something to try, but i don't feel comfortable removing the carbs without the first-hand guidance of someone who has done it before. it will be next week before my buddy will have a free minute to help me out, and i would love to use the boat before then if i can get her running.

please excuse my ignorance. i won't be ignorant for long!

thanks,

wooter

 

[intrepidvoyager]

Re: '78 model 700 not idling right

OH ....thought you had them off once already .....OK ....try this ...take the float bowels off if you can .....maybe tricky without detaching the carbs and do as much of this as you can .. (if you can't get the float bowels off easily try last para below) ... while you are doing this tilt the engine slightly forward so as not to totally flood the engine.

...fuel delivery and air delivery ..got to be close to 14 to one or won't work .. the idle fuel port is a tiny hole in the venturi ... it is fed by a tube going into the float bowl ...so squirt LOTS OF carb cleaner in the idle fuel tube and make sure it comes out in the venturi. there may be two or 3 tubes ..just find the right one .... you got to make sure idle fuel is getting to the venturi ......NOW screw out the idle mixture screw half way and repeat the process ......carb cleaner should come out both the fuel port and the AIR supply somewhere ....if not put your finger on the venturi fuel port and squirt more carb cleaner into fuel tube ... if no carb cleaner comes out somewhere the idle air delivery is blocked ....once its clear put your finger on the fuel port and squirt LOTS OF cleaner both ways . its not only dirt you are looking for .... old fuel glaze ..really bad if you use gas with ethenol in it. some carbs have a removable mixer jet .... if you can't get it out just repeat the cleaning process a few times. By now you should be soaked in carb cleaner ...if not you have not used enough .. .... now if your idle screws have o rings replace them ...sometimes i find they are missing ..... coat the o ring with vasoline when you instal so the o ring does not get hung up on the hole ...good luck. I have not gone into mid range or high speed jet ( emulsion/mixer ) as that is not your problem.

assemble using new float bowel gaskets ( don't use sealer ) and seat the idle screws and back them off 2 turns ............ let the engine sit overnight and then try starting .... it won't start right away as there is no fuel in the float bowels. ....if it runs fine then drop the bowels and use sealer unless you got o ring profile rubber gaskets which don't need sealer.

If you get some improvement you'll know you are on the right track !!

PS ...if you can't get the float bowels off open the drain screws pull the idle screws and using the little red tube poked thru a rubber washer squirt carb cleaner in the hole ...the idea is to hold the washer against the opening surround with a long 6 or 8mm socket so most of the carb cleaner goes in and does not spray back at you ..... This worked once on a boat that had to leave asap and i only had half an hour ...good luck !!!

 

[intrepidvoyager]

Re: '78 model 700 not idling right

OOPs ...as you probably have not done this kind of stuff before ....you should pull the mixture screws but do not mix them up ...set them aside ...pull the drain screws ...set them aside ....... do one carb at a time with a 3 second burst in the idle screw hole as in previous post ...then let the carbs sit for 5 minutes then do them all again .... then let them sit for 5 mins .... then do them all again ..... TEN TIMES ...... its not only dirt you are looking for but also old fuel glaze which needs repeated soakings to get rid of.

 

[superwooter]

Re: '78 model 700 not idling right

i'm finally getting a chance to work on the carbs tonight (probably). i emailed myself the text from your last 2 posts so i'll have them handy on the ole blackberry. thanks for all the response. i'll post my results later.