Intermittent Full Throttle-'73 65hp Johnson

[little mfg]

Hi everyone. I've been working on this motor all summer long trying to get it to run at full throttle. So far, I've rebuilt the fuel pump--no change. I've replaced the hoses/clamps and fuel line/primer bulb and gas tank--no change. Lastly, I pulled the bottom carb, cleaned it and reinstalled it--no change. I've had some minor events of brief success the last couple of sea trials, using the towel over the carb throats to see if any junk would spew out. The bottom carb was, I thought, the culprit. I especially suspected it because it had the least amount of vacuum pull on it when covered--hardly any. Now it seems on par with the other two carbs. The gaskets looked OK. The needle valve had just a slight partial score, but not enough IMO to cause a major problem. The jets were all clear. The float is level. It does have a surface crack but I floated it in gas overnight and no fuel inside while shaking it.

Today it ran at full throttle for about a minute when I realized the choke butterflies were hanging partially closed. I manually open the with the choke knob full in at the motor and vroom, off she went. Stopped the motor, put the hood on it and it reverted right back to 8mph top speed. It is also guzzling a ton of gas! Oh yeah, I also did the seafoam in gas treatment early in the summer. What should I do next? Any suggestions would be greatly appreciated. Thanks in advance! Doug in Ohio (Lake Erie, central basin).

 

[oldboat1]

Check/repair-as-needed the choke linkage (check fastener between links). If satisfied the manual choke operates correctly, I would disconnect the solenoid and operate manually at least temporarily.

 

[little mfg]

Thanks for the reply oldboat1. What's the best way to disconnect the solenoid. Ground wire?

 

[oldboat1]

That would do it. (Wrap the loose ground.)

 

[little mfg]

Great. Thanks!

 

[Tim Frank]

Have you done the basics?
Compression check....spark jump a 7/16" open-air gap....?

 

[little mfg]

Hi Tim, Thanks for your help! I have healthy spark on all three, although I have not checked air gap. I do not have a tester to check air gap at the moment, plus the plugs for it don't have the standard electrode because they're the anti-fouling type. I will scrounge an old standard plug and do that! I haven't checked the compression yet, only because it runs so smoothly and doesn't foul the plugs putting through the long no-wake zone to the lake. Maybe a poor assumption on my part. I will do that, too! Thanks for the suggestions.

 

[racerone]

You do not use sparkplugs to test "maximum ignition voltage " output.----You force the system to jump a gap of 7/16" using a test device or you make one yourself with a piece of plywood and some wires..----You could have water leaking into the cylinders via leaking exhaust cover gaskets.

 

[little mfg]

I bought a spark gap tester and tested all three at 7/16". They all looked healthy to me. I also checked the compression dry. 125, 130, 125 on 1,2,3 respectively. Inspecting the spark plugs was a bit troubling. 1 and 2 were fouled, which for my no-wake river traveling to and from the lake seems normal, BUT the bottom plug was not fouled at all. I'm puzzled!

 

[Bosunsmate]

do a drop test on the cylinders. Post a photo of your plugs, the bottom may be getting water ingress
Headgasket, exhaust gaskewt, block, lower seal

 

[little mfg]

Hi Bosunmate, thanks for jumping in! Never did a drop test before. Let the cylinders leak down? How do I go about that? Photos coming shortly.

 

[little mfg]

Here's a photos of the plugs: 1,2,3 left to right.

 

[oldboat1]

Think I would replace the surface gap plugs -- believe Champion L77J4 is the OEM plug, gapped at .030. Get the sea foam out of the tank if that is the way you are using it, and run the oem plugs for a while, then check.

To do the drop test, "drop" one cylinder at a time by pulling off the boot. See if there is a change -- if no change, that plug may not be firing. I repurpose a timing gun for this when underway -- no flash, no spark.

Pulling and resurfacing the head is a DIY job. Doing that and replacing the head gasket might give you some improvement.

 

[little mfg]

Thanks, old boat! I believe you're correct on the original plug type. The sea foam is gone by now. I used about a gallon of fuel in a small container with a can of seafoam per their website. With the drop test, if I'm showing hot spark on all three, wouldn't that mean it's firing on all three? What am I missing? Thanks for the info!

 

[oldboat1]

When ignition parts heat up, as at high throttle, spark can drop out -- need to check for spark when having operation issues. Try the timing gun test.

 

[jimmbo]

Heat and Compression drive up the Voltage needed to get the Spark to jump the gap, which is why the 7/16" - 3/8" test gap is specified.
The pic shows what look like 3 different brands of plugs
Myself I like the Surface gap plugs, but then I am never one to idle an Engine much.
Did it always run better with the Cover off the engine?

 

[little mfg]

OK! So just hook the timing sensor up at the suspect spark wire?

 

[racerone]

You need 4 things on EACH ( and #3 ) cylinder for the motor to run.-----Fuel from the # 3 carburetor.----Spark for #3 at the right time .----Crankcase compression in #3 crankcase.----Compression in #3 cylinder.

 

[little mfg]

Understood. We'll see how the lake's layin' tomorrow. I'd love to get this motor running in time for fall perch. We'll have to see. Thanks !

 

[Bosunsmate]

Thanks, old boat! I believe you're correct on the original plug type. The sea foam is gone by now. I used about a gallon of fuel in a small container with a can of seafoam per their website. With the drop test, if I'm showing hot spark on all three, wouldn't that mean it's firing on all three? What am I missing? Thanks for the info!

Nope, as its not just spark that shows a cylinder is working. So by using insulated pliers to pull off the boot you are checking whether everything is working for that cylinder