'78 70hp dies when shifting into gear

[superwooter]

hi,

i've recently had the carbs cleaned on my 1978 mercury 700 70hp thunderbolt engine. it now runs smoothly from idle to high rpm and back again, something that it hasn't done since i acquired the boat a couple months ago. now the problem is that it dies when i put it in gear if i move the MerControl slowly or at a normal movement. however, if i very quickly shove the control through the engagement portion of the movement of the lever, it does fine. it just seems to bog down while the prop is engaging if i move it normally. if you're not familiar with the old MerControls, it has nearly 90 degrees of motion just to engage the prop before the throttle actually starts to be increased.

after cleaning the carbs, we had to turn out the idle screw from the factory 1 1/2 turns out to 3 full turns. it idles at about 1100 rpm. should i try turning out the idle screw another half turn or so to prevent the engine from dieing while the prop is being engaged or should i just live with what i've got and shove the lever forward quickly to get it past the engagement and on to the throttle before it dies? like i said, that works but it seems like it should work better than that.

here's the link to my thread about getting the engine to idle correctly...
http://forums.iboats.com/showthread.php?t=360910

thanks for your input,

wooter

 

[intrepidvoyager]

Re: '78 70hp dies when shifting into gear

have a look at this ......


http://forums.iboats.com/showthread.php?t=262145


when you say you got it to idle properly ...well sounds like at 1100 you are way to high .......could be when you shift into forward the initial speed of the prop is stalling the engine ..... if you slap a paddle down on the water it will bounce off versus a slower entry and your momentum will keep going ...maybe !!

If you can't get the idle speed down to the 850 range .....( that sounds right to me but i am 3000 miles from home with no manuals ) I think you still have a carb problem or timing or carb sync ...... you see I wonder if at 1100 you are still on the idle circuit.

If you don't get some answers to this thread I would start another and ask what the idle speed should be .... my experience is you got to get the basics right first .....in other words the engine should idle smoothly at the correct idle speed which I am sure is way below 1100 for your engine.

Also if you are going to keep the engine a good idea is to invest the $40 or so in a manual ...... it will save you countless hours trying to figure things out and $$$$ in service.

 

[intrepidvoyager]

Re: '78 70hp dies when shifting into gear

or as BADLY BENT OR BROKE stated a fuel pump problem

 

[superwooter]

Re: '78 70hp dies when shifting into gear

i'll be running the boat again this weekend. i plan to replace the inline fuel filter and reduce the idle speed a little at a time to see if i can get it closer to the 850 rpm mark. i was under the impression that the fuel filter had been changed not long before i got the boat, but its so cheap that there is no reason for me not to change it to be sure.
i've inspected every other part of the fuel delivery system (tanks, line, pump, etc.) and everything else seems fine.
you're definitely right about getting the basics right. without that foundation, there's less chance of properly diagnosing the problems.

 

[superwooter]

Re: '78 70hp dies when shifting into gear

i copied this out of another thread...

"Adjust idle speed to achieve the lowest stable idle in gear in the water. This will minimize the shock to the gears when you shift. BTW always shift in and out of gear with a sharp, quick motion. These are not designed to 'ease' into gear."

that came from the following thread related to idle speed on a '77 85hp merc. similar enough i'm sure...

http://forums.iboats.com/showthread.php?t=363155&highlight=correct+idle+speed

 

[intrepidvoyager]

Re: '78 70hp dies when shifting into gear

firm movement into gear makes sense with a dog clutch.

here is something else that may be of interest re timing issues

http://forums.iboats.com/showthread.php?t=168855

good luck

 

[superwooter]

Re: '78 70hp dies when shifting into gear

ha. yeah, i found that a thread through the search funtion a little while ago, copied the text and emailed it to myself.
thanks,
wooter

 

[college_guy]

Re: '78 70hp dies when shifting into gear

any update for the cause

 

[superwooter]

Re: '78 70hp dies when shifting into gear

yes. i'm glad you asked. it's been several months since i started this thread and i've just recently dealt with this issue again. since writing the first few posts here i've had plenty of opportunities to familiarize myself with the way the MerControl wants to be operated. also, 2 weeks ago, i replaced my fuel tanks, fuel line, filter and bulb... basically everything fuel related outside of the engine. i had discovered that the fuel pickup in my "good tank" had a crack in it about half way down and was sucking air. the other tank pickup was badly clogged with old oily gas, so i just replaced them both.
i was able to get the idle down to below 1000 by a little. later in the summer, i plan to take the carbs off and clean them again and use a carb rebuild kit this time. it's not bad enough at all at the moment to warrant tearing it down again.

 

[ajgraz]

Re: '78 70hp dies when shifting into gear

superwooter,

Funny, there's at least four guys (myself and you included) fighting these devil 77-79 3cyl 70hp's right now. We oughta start a club! I've got a 1978.

I've been poking back through your posts, and all your symptoms are identical to mine: first it kinda lost a few hundred rpm at high end, then it wouldn't go unless I floored it quick, then it wouldn't idle below 1100rpm and not without idle screws out 3 to 3.5 turns, now it won't go over 2500rpm at all without bogging and conking out.

First I fixed everything in the fuel system (bulb, pump diaphragm, filter, carb kits); didn't fix it.

Now I'm looking closer at harness/ignition parts issues. My main internal harness is corroded to hell, and the yellow wires on my stator and the white/black bias trigger wire have crumbly insulation and are almost corroded away to nothing. Somewhere along the line you said you did some rewiring on the engine and it helped somewhat??? Can you elaborate on that?

Thanks.

 

[superwooter]

Re: '78 70hp dies when shifting into gear

yeah, the first time i successfully got it to run while on the lake, it would only rev to about 2000 rpm if even that. we took the cover off and realized that the wiring was shot, almost all the nuts were loose that hold the wires to the ignition, etc, and a couple of the nuts were missing altogether leaving the wire just dangling over the post. we ended up replacing all of the bad wiring as close to where it goes to the stator under the flywheel as we could reach. after that and after replacing the missing nuts and tightening down the existing nuts, we got back to the lake as quickly as possible and ran it wide open for the first time. according to my tach, it gets just over 4900 rpm now. but according to some responses i've had over in the propeller section, my tach might be reading low.

side note: would any one of you guys with this engine be able to confirm that the gear ratio is 2.3:1? i have yet to follow intrepidvoyager's good advise and buy a repair manual and i can't seem to find any conclusive results from a google search.

thanks,
michael

 

[superwooter]

Re: '78 70hp dies when shifting into gear

i can snap a couple pics this evening when i get home if you'd like, but i don't have any 'before' pics of the bad wiring.

 

[college_guy]

Re: '78 70hp dies when shifting into gear

my gas tank is made of metal and i cant seem to find the air vent, how can you tell if its a air vent? i tried leaving that gas tank unclosed but it seems to make no difference.

 

[ajgraz]

Re: '78 70hp dies when shifting into gear

side note: would any one of you guys with this engine be able to confirm that the gear ratio is 2.3:1? i have yet to follow intrepidvoyager's good advise and buy a repair manual and i can't seem to find any conclusive results from a google search.

thanks,
michael

My SELOC manual says, for 70hp 3cyl models 1977-79 and 1980-83, gear ratio is 1.64:1
(It says the 1987-89 70hp 3cyl is 2.3:1)

 

[superwooter]

Re: '78 70hp dies when shifting into gear

thanks ajgraz. i quoted you over in the propeller section.

here's a few pics of my re-wiring. notice that i didn't use the factor wire colors but it's the right gauge though (10 if my memory serves me).

 

[superwooter]

Re: '78 70hp dies when shifting into gear

college guy,
here's a couple pics of the gas tanks i replaced. they were the original metal tanks. in the second picture, i'm pointing at the vent on the lid. you just twist it counter clockwise to open the vent...

hope this helps.

 

[charliehines4126]

Re: '78 70hp dies when shifting into gear

Oh Man I'd Buy New Tanks weather I thougt thet were good or not looks like you're asking for problems here.Invest!!!save in the long run just my opion

 

[superwooter]

Re: '78 70hp dies when shifting into gear

Those are my old tanks. I have shiny new plastic tanks now. Just showing college guy where the vents are located.

 

[college_guy]

Re: '78 70hp dies when shifting into gear

mine dont have a air vent that big, there is a screw that is located at a corner but i dont know if its a vent or not. maybe ill take a photo to see if you guys know where the vent it.

 

[ajgraz]

Re: '78 70hp dies when shifting into gear

It can't be coincidence, there must have been something wrong with certain insulation formulations used by Mercury way back when, or some kind of design flaw(s) in the circuitry. Every wire you've replaced are exactly the same wires I've had trouble with:

Gray choke solenoid wire (your replacement yellow; well, all your replacements are yellow ):

White/black bias wire from trigger (white arrow added to pic), and two yellow wires from stator (yellow arrow):

Obviously you've replaced the wire segments coming off the trigger and stator that are accessible; problem is, these wires (for me anyway) had their insulation crumbled away and the wires corroded to near nothing all the way up to the parts themselves, way up under the flywheel (red arrow, purple arrow).

Do you have the puller to pull off the flywheel and check out the wiring all the way up to the stator and trigger?

I'm working on a theory that it's the bias wire coming off the trigger that's the issue for the symptoms we're having in common. I suspect that if that wire is shorting to the powerhead ground due to lack of insulation or is not completing circuit due to corrosion, it would throw off timing badly...perhaps VERY badly.