NO WOT - 1975 70hp Evinrude

[jhetrick55]

I have a 1975 70hp Evinrude that was new to me just under two years ago. I have replaced head gasket, exhaust gasket, changed lower unit oil, cleaned/rebuilt carbs and done the impeller. Recently I had it out and it was running fine - 5400rpms and with four people in my boat, about 25mph. All pretty standard in my experience. Then all of the sudden at full throttle the RPM's dropped to 2300-2400 and speed to a max of 16mph. Since then I have buttoned up some possibly lose gas line fittings and checked compression; compression for each cylinder is between 110-115. I don't know what else to check here. It seems to idle fine and start fine, but I am wondering if anyone else has experience something similar where you lose RPMs/speed at WOT. Thanks!

 

[jimmbo]

One cylinder is not firing. Does it have spark to all three? Plugs good? Spraying gas-oil mix into the carbs, one at a one, make it better or worse? If it runs fine at low speed, it might be a plugged main jet-circuit, or a wire, in the ignition system grounding out or opening as the timing plate moves.
An engine needs fuel, spark, and compression to run. You just have to figure out why one or more are lacking

 

[jhetrick55]

Jimmbo - will check spark this evening and get back to you. Confident the compression is ok. If there is spark I will be fairly certain there is a fuel issue. Thanks!

 

[oldboat1]

Don't be tempted to run a solvent through it to cure a suspected carb issue. Only way is to get up close and personal, and clean the jets.

 

[jhetrick55]

After testing compression again, spark - all three cylinders had good spark - and checking the timing advance, I am fairly confident I will need to clean the carbs, at the very least the high speed jets. Seems like Oldboat1 is on top of it. So, that is my next step!

 

[oldboat1]

Time consuming, but not difficult. I recommend disassembly and soaking in some kind of solvent (I use lacquer thinner), then using soft wire or monofilament in all openings, followed by carb spray in each opening using the plastic nozzle. I leave the jets in place because of the risk of damaging them in removal. Also recommend removing the domed fitting on top before soaking, to access the idle passages. (You will need kits for replacement plugs.)

Couple of things you might check before digging into the carbs:

If there is an inline filter somewhere, check that first — might be the culprit for problems at high rpm. I don’t think it would be original equipment, but somebody may have added one.

Additionally, check the throttle linkage at the motor, and check the operation of the remote cable. Make sure it’s throttling up properly.

(Good luck with it. Hope it gets you back up and running.)

 

[jhetrick55]

Oldboat1 - thanks for the input. I have cleaned the carbs on this once and it wasn't too bad but a more experienced person like yourself is great at giving efficiency tips. BTW, I have checked the linkages and they're in good shape. There is no inline filter, but I have checked the fuel pump and its self-contained filter and they both seem to be in good shape. Will report back after I get those carbs cleaned and get it in the water again. Thanks!

 

[racerone]

Those compression numbers if taken with a good gauge are not encouraging !

 

[jhetrick55]

The compression may be low, but it has been consistent since I've had the motor. My inexperienced opinion is that they may be low but have been consistent and so I don't know that I think they are contributing to this new problem. At some point I may choose to do a complete rebuild and then that could be addressed.

 

[jhetrick55]

One other thing I am going to do is a decarb of the engine. Don't know if one has ever been done on this engine. I bought two cans of Seaform Deep Creep and will spray that in the cylinders, turn over the engine and repeat and then let sit overnight before I blow a big puff of white smoke out the exhaust! Hopefully that combined with cleaning the carbs really helps the old girl run well!

 

[jhetrick55]

Did the decarb and I got a lot of carbon out of the engine. The carbs themselves were clean, couldn't see anything, but did clean everything like I should. All installed, back on the boat and for some reason now when I fire it up it wants to rev up over 2000rpm right away,. Everything is the same as it was so I don't know why this is. Any ideas?

 

[jhetrick55]

OK, well, no luck on the decarb and cleaning of the carbs. Still not running right. However, I replaced the spark plugs with new ones and after a 15 minute run yesterday I pulled them this morning to check them and the plug on the left which was in the top cylinder doesn't appear to have fired. Going to do some spark testing, but ultimately my coils were in rough shape anyway so I will likely just replace them all.

Any other ideas?

 

[jhetrick55]

I replaced all three ignition coils, two were cracked, and put it on the muffs and it purred like a kitten at 1020rpms. I will try to water test this weekend and report back.

 

[jhetrick55]

Update here - The new coils I did were much needed and I did notice a difference on the water, but a very small difference. Ultimately, the attached pics tell the story. I am confident I need to replace the exhaust gaskets. Can anyone else chime in on the substance that made its way out through the spark plug holes?