'78 thunderbolt 700 falling on its face

[superwooter]

i started posting about this problem in another thread, but the original topic of that post ended up not being the problem. here's the link to that thread for reference...
http://forums.iboats.com/showthread.php?t=398522

i've been dealing with this problem for nearly 2 months now. i get about 2 or 3 days a week to mess around with it. oh, this is going to be a bit long, but please read...

my engine had been idling rough, so my dad and i cleaned the carbs. it tested out great on the water muffs in the drive way so we took it to the lake a couple days later. once in the water, it would idle great, had a great hole shot, but once it got up to 4000 rpm, it would pulsate and not run up to 5000 rpm like it should. you could squeeze the bulb and it would push on up to 5000 rpm. my first thought was we did something wrong in the carbs or that maybe we moved some crud around in the carbs and now the high speed jets are blocked. however, the symptoms sounded more like a bad fuel pump diaphragm. so, i ordered the fuel pump kit that included the check valves at the recommendation of ajgraz from here on iboats. this had zero effect on the performance and the problem persisted. next, i changed out all of the fuel line inside the engine case. about 2 months prior to the problem, i had changed out my fuel tanks, fuel line, bulb, filter and all connectors (metal connectors), so i figured i could rule those out since it had functioned perfectly for some time before my dad and i cleaned the carbs. the new fuel lines also made no improvement. at this point, i was pretty frustrated, so i took it to my buddy who is a boat mechanic. he said on the muffs everything checked out great (fuel pump, compression, spark). i asked him to go ahead and clean the carbs again just to make sure i did everything right and that some of the old fuel line hadn't been pushed into the carbs. after the cleaning, he said the carbs looked great and that he really wasn't sure what the problem could be. i tested it in the lake and the problem was still there.

at this point i noticed that the molded plastic air tubes (not sure their real name) that go into the airbox had been in backwards. it has 6 tubes; 3 longer and 3 shorter. the shorter tubes should have been facing away from the engine to give clearance to the throttle linkage stop screw adjustment (see video and pics below). i don't blame my mechanic too much though because he put the part back in the way he found it. i put it in wrong before i sent it to him. i suppose i was on crack that day.

i had a fishing trip planned that i didn't want to miss, so we took the boat regardless of the problem and we just took turns pumping the bulb to get from fishing spot to fishing spot. between the lake test before the fishing trip through the end of the fishing trip, it got worse and worse. it was to the point that it would not run above 2000 rpm without pumping the bulb. as we were loading up the boat at the end of the fishing trip, we noticed that the fuel connector on one tank seemed loose. it turns out that 3 out of the 4 connectors i had, had gone bad. i suppose that all that bulb pumping could have over-pressured them and caused them to lose their seals. yesterday i replaced the connectors and filter again and tested it in the lake. it is still falling on its face but i could get it up near 3000 rpm without pumping the bulb. it still idles great and has a good hole shot up to 3000 rpm.

my guesses (at this point, that's all i have) are:
1. maybe the airtube piece somehow knocked it out of time and i need to perform link and sync.
2. maybe there's still something wrong with the fuel pump but i have no idea what.
3. maybe there is still something blocking my high speed jets in the carbs.

i'm about to the point where i want to put a few rounds of .45 acp in the thing and call it quits, so please help me to NOT do that!

here's a few pics and a video...

video link (click it)...


throttle in neutral (notice the idle stop screw never bottoms out)...

throttle at full...

throttle in neutral...

throttle at full...

 

[superwooter]

Re: '78 thunderbolt 700 falling on its face

and here's a couple pics of the fuel pump with all new fuel line...

 

[RaScLeS]

Re: '78 thunderbolt 700 falling on its face

I pretty much gave up on my 70 they are no good.

for you it seems you need a stonger fuel pump kit or a rebuild.....id be laughing if i could hit 4k and squeeze it to 5k. these things are ALL ignition trouble and when the ignition isnt screwed its something else

yamaha 70hp

 

[ajgraz]

Re: '78 thunderbolt 700 falling on its face

Referencing back to this thread of yours:

http://forums.iboats.com/showthread.php?t=365346

I still strongly suspect a problem (corrosion, ground-out) in the wiring from either the trigger or stator way up under the flywheel where you can't see or reach without pulling the flywheel.

My first guess would be trigger bias (white/black wire), since as I said in that thread I had all of the exact same issues you had until I replaced my trigger...including: not being able to reliably go over 3500-4000rpm WOT, hesitation in the 2000-2500 rpm range, and not being able to idle under 1100 rpm (in which case idle stop can't even bottom out, like in your pic). From my understanding, a bad/broken bias wire throws timing way off so that you cannot set the timing right at any speed, no matter how carefully you link n' sync. Spark can still be strong, just not at the right time

I strongly urge you to get the special Merc flywheel puller, pull off the flywheel, and check out the hidden wiring on those ignition parts.

 

[superwooter]

Re: '78 thunderbolt 700 falling on its face

thanks aj. what i'll do first is peel back all of the electrical tape from where i attached the new wires and see if any of that is coming loose. i know when i originally changed the wiring back several months ago i only changed what i could reach and that just out of reach under the flywheel, some of the wires were bare and wearing thin.
i guess i've just been hung up on how everything was working fine at the top end of the throttle until after i cleaned the carbs. i probably wiggled the wires loose when i was breaking it down to get to the carbs.
you guys will be proud of me though. i finally ordered a repair manual from iboats today!

 

[superwooter]

Re: '78 thunderbolt 700 falling on its face

here's a link to an ebay ad for the puller. can you tell me if this is the right one for my engine??
http://cgi.ebay.com/ebaymotors/Flyw...6748765QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear

thanks,
superwooter

 

[ajgraz]

Re: '78 thunderbolt 700 falling on its face

Yep, that'll work. For $5 more you can get the one with the lifting ring, in case you ever need to pull off the motor to replace the steering cable, repower, throw out this motor when you get sick of it or whatever:

http://cgi.ebay.com/ebaymotors/Merc...at_Parts_Accessories_Gear&hash=item483af524b3

I have that very one, and it works very well.

 

[OldMercsRule]

Re: '78 thunderbolt 700 falling on its face

Sorry I'm and inline 6 guy, (n' a few inline 4s). Never had a 3. Good Luck. JR

 

[RaScLeS]

Re: '78 thunderbolt 700 falling on its face

^ they are built similar to the 6's of their year.

i had replaced the trigger on mine, woke up for a few hours, cooked the stator, then it would only work at wot, now its dying out i dont use it or mess with it anymore

 

[superwooter]

Re: '78 thunderbolt 700 falling on its face

my boat mech buddy told me he'd pop the flywheel for me at no charge so i went with that option. the trigger wires are actually in pretty good condition. but the 2 yellow wires coming off the stator were horrible. they were grounding out when tested with the volt meter. we repaired the wire with solder and heat shrink and now they are not giving a ground-out reading. going to the lake this evening to test it out. i had previously repaired a small piece of one of the trigger wires but just used crimps for the repair. we went ahead and soldered that section for good measure and heat shrunk it while we were at it.

 

[SeaKaye12]

Re: '78 thunderbolt 700 falling on its face

Does anyone know the story about the 70's Mercs wiring?

What company did they use that supplied such crappy wires?

What caused them to choose such inferior wiring? I have other OMC outboards from that time period and even older and they show no signs of deterioration at all in the wires used ....

Must be a story there ....

Chuck

 

[starcraft1982]

Re: '78 thunderbolt 700 falling on its face

not all to do w/it,but,being all metal housing in the cowl area causes alot of condensation in there.eventually it breaks down the wires.

 

[superwooter]

Re: '78 thunderbolt 700 falling on its face

well, repairing the stator wires made no improvement. BUT, thanks to my new repair manual and an ohm meter, i now know that both my trigger and stator are functioning properly. that's one more thing i can eliminate.

this time during our lake test, we tried something new. while the throttle was set just above where it starts to bog, i pressed the choke button. the motor almost instantly rev'd higher for a few seconds and then bogged again. my first thought was of the idle mixture screws. i thought it must be getting too much air and not enough fuel which would explain the surge in power when i choked it. we turned out the idle mixture screws a total of 3 and half turns out with no improvement, so we set them back to 2 turns out.

i was reading through the carb-rebuild section of the manual and had another thought. it mentioned that the float leveler (bendable metal arm that supports the float) had to be set exactly so that the measurement from the edge of the carb housing to the top of the float was 11/16". the last time i cleaned the carbs that metal arm got bent in the process, so we just eyeballed it when straightening it and put it back together. it was after that particular carb cleaning that it started having this problem.

so thursday this week, we will be going back into the carbs AGAIN (3rd time in 2 months) but with the repair manual close at hand. from what i could tell, we did everything else correctly. but if that arm is bent, it could be forcing the float to ... float wrong or something. i suppose that could cause it to stop sending the proper amount of fuel.

any thoughts?

superwooter

 

[79Merc80]

Re: '78 thunderbolt 700 falling on its face

This is in concern to your idle screw not bottoming out in neutral.

There is an adjustment to that. Follow the cable toward the front of the engine. You will see a hinge/locking mechanism that holds the cables from moving. Inside that is a barral shaped brass nut that can be adjusted back and forth.

Beore you do that adjustment though, take the cable off the throttle arm and (while running), adjust the idle stop where you want it. Then adjust the throttle cable to reach the throttle arm.

Hope this helps.

BTW, it's a LOT easier to do this on your engine than on the inline 6's with the non-removable cowling around it.

Criag

 

[superwooter]

Re: '78 thunderbolt 700 falling on its face

thanks for the info. though, it sounds like this would only adjust the position of the cable without actually lowering the rpm at idle. i can't figure out why my idle is at 1000 rpm and yet i have no capability of lowering it by the idle stop.
any thoughts?
thanks,
superwooter

 

[79Merc80]

Re: '78 thunderbolt 700 falling on its face

Like I said, disconnect the throttle cable from the throttle arm, get it to idle where you want it (Using the idle stop), then adjust the cable to reach the arm

 

[ajgraz]

Re: '78 thunderbolt 700 falling on its face

superwooter, when you checked out the wiring with the flywheel off...

On the trigger, did you strip the outer sheath all the way up to the plastic body of the trigger? The wiring that's way, way up inside that can have its insulation crumble and the wire corrode, and unless you strip off that sheath you won't see it.

Have you done any testing of ignition compnents with a DVA adapter on your meter? I've come to realize on this motor that a DVA adapter's the ONLY way to be 100% sure the stator and trigger are functioning properly.

 

[superwooter]

Re: '78 thunderbolt 700 falling on its face

ITS ALIVE!!!!
in one of my last posts here, i mentioned that the carb float leveler in one of the carbs got bent and that we just guessed at bending it back into place. so on my lunch break today, my cousin and i pulled carbs AGAIN and checked out the float height. according to the repair manual, they are supposed to be 11/16" from the housing to the highest point on the float. the one i bent was at 1 1/4" and the other one was also somehow set at over 1". we carefully bent them back so that the measurement was exactly 11/16", slapped it all back together and took off down the road to the nearest lake.
it finally ran at full throttle. you couldn't have wiped the stupid grins off our faces since we'd been waiting on that moment for about 2 months!
but...
it's back to not idling again. i think we're going to turn out the idle air mixture screw a bit more to fix that. at the moment, they are set at 2 full turns out.
just wanted to report the good news and to give an idea to anyone else who might ever come across this problem.
superwooter

 

[superwooter]

Re: '78 thunderbolt 700 falling on its face

oh yeah. pics!