1976 Evinrude 70 hp hesitates upon full throttle

Hi everyone,

We have a 1976 70 hp Evinrude that is experiencing an intermittent hesitation when going to full throttle.

The engine starts and idles beautifully, usually on the first turn of the key hot or cold. It will troll all day long and runs beautiful wide open. The problem only manifests itself after the engine has been idling or running at low speed for a while. I haven't nailed it down, but if wide open throttle isnt achieved withing roughly half a minute of startup, the engine will hesitate and bog. It'll eventually reach plane and smarten up and then run beautifully.

If I raise the idle with the warm up lever and "clear it out" just before going to full throttle, it seems to behave relatively normally.

If it idles or trolls for an extended period, beyond say half a minute, jumping to full throttle usually stalls the engine. A quick restart and a bit of warm up lever clears it out and away it goes.

If I'm cruising at full throttle, I can back out of it and bring the boat to a stand still and it'll accelerate normally again if it doesn't sit too long.

The carbs were cleaned and rebuilt last summer with Sierra kits. I used a piece of mechanics wire on each jet to make sure they were clean and blew each one out with compressed air. They are very clean.

Spark jumps a 7/16" gap easily and is nice and blue and plugs are new champions at .030"

Compression is around 100-105 for each cylinder cold.

All fuel lines are new urethane 3/8" line and the fuel pump and filter are new. The tank vent is open and the primer bulb never goes soft.

I have previously checked the timing using Joe Reeves method and found it to be slightly over advanced (we did replace a timer base last year due to a broken part which could explain that). Since backing it off a few degrees the motor generally seems happier in all RPM ranges but still has this hesitation. Spec is 17 degrees, so using Joes method I set it as close to 13 as I could get it.

I checked the link/sync of the carbs and everything looks dead on.

I'm otherwise happy with the performance of this little motor, except this hesitation is a pain when trying to pull a skier out of the water.

Any thoughts would be appreciated.

Thanks

-Aaron

I apologize for the multiple posts... was having internet problems... page errors when clicking post, did not know post went through more than twice. Mods please delete the unnecessary duplicates.

Bad news is either your compression meter is terribly out of whack, or you have very low compression..<br>low compression is not good if you want to ski with the motor.. what size prop are you trying with ? 13 pitch ???<br>Have you had the motor long ?    if so has it always been like this.<br>what size are the jets in the carbs ? <br>did you let carbs sit in carb cleaner for  few hours or so ? <br>did you remove the welch plugs in the carbs then blow out with compressed air ?<br><br>lots of questions, but need to narrow it down a bit more<br><br>is he motor running too lean on wide open throttle ? have you checked if its too lean on high side.<br><br>one way to see if too lean, take spark plugs out and clean them , then go for ride at FULL THROTTLE for about 3 minutes or so then switch engine off W<strong></strong>ithout<br>moving the throttle lever, then look at the plugs. if very light tan or grey its too lean ...<br>This will give the best indication as to how the mixture is on the high speed side .<br><br>put a can of mercury powertune in the plug holes to free up some carbon in the ring grooves and it might bring back a touch of compression<br>do exactly what it says on the tin.<br type="_moz">

Phillnjack,

Thanks for the reply. I can narrow this down a bit for you AND make it worse at the same time

I've had the motor for two years. Last summer I bought it with the boat in non-running condition. The boat had sat for probably 8 years without running because the powerpack was toast.

I replaced the powerpack, timer base (was broken), coils (for peace of mind), plugs and rebuilt the carbs because they were REALLY gross. The jets sat in carb cleaner overnight and were cleaned with a piece of solid wire per Joe Reeves recommendation.

After this work, the motor ran great, but exhibited the hesitation right off the bat. This spring I put some seafoam in the fuel for the exact reason you mentioned, thinking it may be carboned up a bit from sitting so long. However, we dont have enough hours on the boat so far to get what I'd call enough through the motor to make a difference.

Also, as I mentioned, if I come off of a full throttle cruise, hit neutral and then accelerate again, the motor runs perfectly. If it idles too long and I give it a bit of warm up lever, it will puff a bit of extra blue out the exhaust and then behave almost normal. Sometimes it dies, sometimes it sputters and recovers on its way.

I've never looked at the plugs in the water after a full throttle cruise like you mentioned, but the fact that it runs so well at WOT once it hits its stride and the fact that I can accelerate so well after coming OFF of full throttle lead me to believe the mixture is ok?

The carb jets are stock sized. The prop is also the stock 13.25/17 OMC prop.

Thanks

I would re-check the compression with a different gauge. You can also try re-doing the link and sync and leave the throttle cam off of the roller about a 1/4-1/8". Those motors like a little timing advance before the carbs move.

Flyingscott,

I think the link and sync is pretty spot on, at least to all the methods I checked out. I don't have an OMC book for the motor though, so who REALLY knows how well its set. I can say for sure though that there is maybe only 1/16" between the roller and the cam the way its set now.

I've also heard that these motors usually run ok, even IF the compression is in the low 100's.

I will reset the roller to 1/4" from the cam and see what happens, as well as use a can of the Mercury Powertune as suggested by Phillinjack.

Thanks for the insight, anything else is appreciated.

-Aaron

people only say 100 and low 100's is ok is those trying sell one. compression realy needs to be atleast 120 psi per hole.
I would say take the carbs back off an clean them correctly, for this you will be needing new freeze plugs as the ones in the carb now need to come out.
place the whole carb body in carb cleaner overnight, then re-assemble with new carb kits for each ca.
do check float heights too, this is important.
Have you done a check to make 100% sure the TDC mark on flywheel is actually bang n TDC ? do not just trust the marks on the flywheel as there
has been plenty that are out by a few degree's !!!.
make a simple tdc finder from old spark plug if you dont have one.

what plugs are in there exactly ? what size prop.
and put the roller ON the cam as close as possible. 1 /16th of an inch is too far away. it was designed to be touching on the 2nd line.

The roller SHOULD NOT be touching the cam. You want the roller to open the carbs when the mark on the cam meets the middle of the roller. I learned the cam gap trick from a guy who actually worked on those motors when they were new. The bog is a common problem to those motors. You may have to play with the link and sync.

Conflicting information on thr roller/cam distance here...

Plugs are champions at 030" as mentioned above. Prop is a 13.25x17 as mentioned above.

Checking tdc is actually something i havn't done... Is it set off cylinder 1 or 3?

Carbs were rebuilt completely with frost plugs removed and replaced with new Sierra kits. Float heighrs were set according to the instructions in the Sierra kits.

Will get a compression gauge on it now thats its run for a few hours and see where i stand in comparison after some combustion chamber cleaner.

Thabks guys

Get a manual before you do anything!

Flyingscott,

I have a seloc book, which is just ok, however its a bit generic.

Checking tdc is something im ok with, i fix bmws all day long so the concept isnt new to me

Best guess is its checked against the top cylinder. If thats the case, then ill double check the position of the tdc marker and reset timing accordingly.

A co-worker of mine suggested that using regular 87 fuel could be part of my issue.... He figures his dads old 70hp had the same problems until they started running a good quality 91. Come to think of it, ive never bothered to check the fuel requirements for these engines.

Thoughts?

The 1976 70hp model was prone to hesitation and a three page bulletin was issued by OMC to modify the carburetors. I have since then emailed (attached) the bulletin to many boaters that own this model, all of which cured the hesitation problem. Perhaps yours were never modified. PM me with your email address and I'll get the bulletin to you.

Also, check this measurement as anything else interferes with the idle timing setup.

Joe,

I checked the length of the rod two weeks ago at the lake.... 100% spot on.

I will pm you this evening. Thanks for the help.

Email address received... bulletin sent. (Joe)

Hi Joe,

I feel like a bone head here, but here goes

I was digging through my pictures of the carb rebuild last summer to see if I could find any indication of wether or not they had been updated or replaced based on part numbers. Unfortunately, I cannot see any part numbers but I can see that the top of the carbs are NOT marked with an "X".

However, while looking through said images, I found a picture of the timing advance sticker above the carbs... 1975 Spark advance, 17degrees.

Blast!

Whats the chance the 1975 70 HP was affected the same as the '76 units? I suppose I'll never know unless I find out which carbs are actually installed on this motor.

Moreover, whats the chance that NOT running the stock silencer box assembly (either of the 2 pieces) is contributing to a lean condition as set forth in the bulletin?

Regardless, thanks for the info, I'll double check when i'm back out there. Sorry if this causes any head banging

-Aaron

I'm not familiar with the bulletin you mention, but the silencer is part of the pulse tuning. It has to be there if you want it to run right.

AJ... Your last post, you state "However, while looking through said images, I found a picture of the timing advance sticker above the carbs... 1975 Spark advance, 17degrees. Blast!"

That puts me at somewhat of a loss. Are you saying that your engine is a 1975... not a 1976?

Joe, thats what im saying yea... Sorry for the confusion, the boat is a 1976 as verified by the tag on tge transom. Ive always just thought it was the same year as the motor.

Serves me right. Ive worked on it enough that i should really have paid attention to the year written on the timing sticker.

So... Where does that leave me, beyond just double checking for tdc and proper timing, de-carbon and verify link/sync?

Sorry again for the misinformation on the year

Should be a plate attached to the port transom bracket with the model and serial number on it. If so, what are they?

1 - Since full throttle is okay once you get there, one would assume the HS jets are okay... but check and clean them anyway.

2 - Check to see if the timer base is sticking as you apple throttle.

3 - Check throttle butterflies to make sure they "all" open and close at the same time... not 2 closed, 1 ever so slightly open, that sort of thing.

4 - Check to make sure that the throttle roller isn't damaged... should be perfectly round and approximately 3/8" in diameter.

5 - The butterflies should just start to open when the scribe mark of the metal cam is dead center with the roller... not before or after.

I really appreciate the help guys.

Joe,

1- my thoughts exactly... but at this point its coming apart again to verify regardless as you suggest
2- timer base moves freely through its entire range of motion
3- butterflies are dead accurate the whole range of motion
4- will measure and verify condition
5- We are talking about the first (lowest and shortest) scribe mark, correct? Is there any spec as to where the "start" mark should be at any point for reference?

Also, can you please verify for me Joe, that the silencer box/cover is/is not necessary for proper running as Fdrgator mentioned? Mine is currently off the motor and has been since I got it running. Made it easier for me to get the carbs and lines off as I replaced them throughout last summer and I never bothered to put it back on. Please advise.

I think I may drive out to the cabin this weekend and get this boat back to the city so I can get working on this instead of waiting until august.

Thanks so much,

-Aaron

The first plain scribe mark that comes along as one gives throttle is the one to line up with the roller. The "start" scribe mark is one I never bothered with.

I always leave the carburetor face plate units off until I get ready to final test run the engine.

That was Johnson service bulletin 1429, dated Sept 1976. Back in the day I used it quite a bit and it always cured the problem.

Boobie, as ive found out (im blind apparently haha) the motor is a 1975... Any chance the bulletin would be worth doing on these carbs as well, or were the '76 carbs that much different?

Joe,

I finally got a chance to grab the boat from the cabin and take a look at the what's going on.

In regards to the throttle roller and the first scribe mark, it was out... The throttle linkage was starting to move about half way to the "start" scribe.

I've adjusted everything so that the roller is DEAD center on the first shortest scribe at the point where the linkage begins to move.

I have a video I will try and post later so that you might be able to verify for me that it looks ok.

Will update later. Thanks for the info. Right so far

-A

Hi Joe,

Im sorry... Its a 70573E, which confirms its a 1975.

Here is a video I uploaded to youtube showing the linkage JUST starting to move as the scribe comes up to the center of the roller. Let me know what you think.

https://youtu.be/LJzl7zXmQDk

Again, thank for the help Joe (et all)

-Aaron

Hi everyone,

Had the boat on the water on friday before dragging it home and managed to run a whole can of Seafoam top end cleaner through the carbs. Smoked and smoked and smoked, but ended up running a little nicer at the top end or the throttle range. Still exhibited the hesitation though. Plugs were FOULED!!!

Rechecked compression tonight finally, 130psi on all three cylinders.

Pulled the carbs apart to check all the jets, all were clean. I poked some wire through them regardless. I checked the jet numbers against the parts listing for the 1975 units, just in case the carbs had been exchanged at some point. All jets match up, so the carbs are likely the original '75 units. Floats are still perfect as set according to the sierra instructions.

I DID find that the bowl gasket on all three carbs partially obscured the jet in the bottom on the carb body where it meets the bowl. I paid careful attention putting them together to make sure the rubber gasket didn't get squished into a shape that would inhibit any fuel flow.

I'm going to throw in a new set of plugs at .040" and see how it runs on the water this week (hopefully). I will update after that.

Till then, thanks for the help.

-Aaron

I've been working on my 76 70hp johnson trying to get it to plane better. New to boat mechanicing but I enjoy reading your advice and learning. Not sure if my issue is fuel or fire. Idles great. Replaced power pack to the spark plugs. I would love to see that carb upgrade file and try it