75hp Johnson Stinger low Compression PLEASE HELP

[camohunter]

I've got a 1980 75hp Johnson Stinger 20" club foot on a 1652 aluminum boat. I've ran this motor a good bit in the last year and it always runs really strong and fast, but will not idle very well. The carbs are clean and have new main jets installed (did not help idle problem). It spits and sputters a mist back out of the top carb and cuts off under low rmps.

I bought a compression tester and the compression checked out weak yesterday @ 79,80, & 85 lbs. Is this low compression possible on a motor that runs strong? Do you think I got a bad compression tester. I tested the motor warm with all the plugs out and at wide open throttle.

Thanks in advance for any advice.

Justin

[captbone]

Check and replace the reeds. The mist out the top carb is hint something maybe going on with reeds.

[camohunter]

Can the reed valves affect the compression readings?

[captbone]

No but your compression readings are all with 10% of each other. I would not be overly concerned. Replace the reeds and retest your engine. I think they are your issue and will fix idle and run quality.

Just my 2 cents.

[25XS]

Sounds to me like you need a THOROUGH carb cleaning, followed by a procedure in your service manual called "Sync & Link" where you synchronize (properly per the book) the carb butterfly opening to the timing advance mechanism... I have done lots of them and they make a sick engine run like new.

You also may have lost the clear, outer sleeve off the carb advance cam roller ("Roller & Sleeve ***'y" in the parts list) so the timing is advancing ahead of the throttle plates. That would cause the "Lean Spit" backfire through the carbs.

I'm in Georgia, too (Buford / Lake Lanier) Where are you located?

Tom

[camohunter]

Sounds to me like you need a THOROUGH carb cleaning, followed by a procedure in your service manual called "Sync & Link" where you synchronize (properly per the book) the carb butterfly opening to the timing advance mechanism... I have done lots of them and they make a sick engine run like new.

You also may have lost the clear, outer sleeve off the carb advance cam roller ("Roller & Sleeve ***'y" in the parts list) so the timing is advancing ahead of the throttle plates. That would cause the "Lean Spit" backfire through the carbs.

I'm in Georgia, too (Buford / Lake Lanier) Where are you located?

Tom

I live in Wayne County on the Altamaha River.

I've had the carbs off and adjusted the linkages and stuff when I installed the tiller handle, so that could be the problem. I really don't think it's trash in the carbs. I feel like I'm pretty good at cleaning the carbs. I would take it to the boat mechanic here, but I don't have a lot of confidence in them. Do you think my compression is too low? I have read that these stinger motors should be around 150psi.

Thanks for the help! I'll research the Sync and Link

[25XS]

Camohunter posted, "I would take it to the boat mechanic here, but I don't have a lot of confidence in them."

I understand... That is an understandable predicament.

When you cleaned the carbs, did you remove the round, aluminum discs (core plugs, welch plugs...) and remove all the orifice jets to expose the deepest inner workings of the carb when you cleaned it? You'll need a complete carb kit to do this.

The middle carburetor should have a "Cam Follower" plastic wheel on it's actuator lever and that cam follower rides on the timing advance cam. You have to synchronize that motion "PER THE BOOK" and that procedure starts with verifying the TopDeadCenter pointer is accurate.

Regarding your compression numbers... Yes; I think your gauge is bad. Clean the Schraeder valve (check valve) near the sparkplug end of the hose and try again.

Tom

[camohunter]

Camohunter posted, "I would take it to the boat mechanic here, but I don't have a lot of confidence in them."

I understand... That is an understandable predicament.

When you cleaned the carbs, did you remove the round, aluminum discs (core plugs, welch plugs...) and remove all the orifice jets to expose the deepest inner workings of the carb when you cleaned it? You'll need a complete carb kit to do this.

The middle carburetor should have a "Cam Follower" plastic wheel on it's actuator lever and that cam follower rides on the timing advance cam. You have to synchronize that motion "PER THE BOOK" and that procedure starts with verifying the TopDeadCenter pointer is accurate.

Regarding your compression numbers... Yes; I think your gauge is bad. Clean the Schraeder valve (check valve) near the sparkplug end of the hose and try again.

Tom

I didn't pull the welch plugs. I'm sure that's sound advice. I did pull all the brass orfices and made sure the passages were open. The "cam follower" is there and seems to be in good shape.

[75luva]

I live in Wayne County on the Altamaha River.

I've had the carbs off and adjusted the linkages and stuff when I installed the tiller handle, so that could be the problem. I really don't think it's trash in the carbs. I feel like I'm pretty good at cleaning the carbs. I would take it to the boat mechanic here, but I don't have a lot of confidence in them. Do you think my compression is too low? I have read that these stinger motors should be around 150psi.

Thanks for the help! I'll research the Sync and Link


The stinger engine you have is just a fishing engine nothing special
150psi you wont see in your engine as its a stocker ,they should be a bit higher than 80 tho
I have a 75stinger myself but its not stock
My ft-19s is a factory race motor with ported block and pistons and im hoping for 150-180psi on it

[camohunter]

The stinger engine you have is just a fishing engine nothing special
150psi you wont see in your engine as its a stocker ,they should be a bit higher than 80 tho
I have a 75stinger myself but its not stock
My ft-19s is a factory race motor with ported block and pistons and im hoping for 150-180psi on it

A guy on here who worked for OMC said all the 75hp models came from the factory with raised ports, cut heads and I think bigger carbs. I'm not sure about the psi.
My motor is SPECIAL :mad: LOL
Justin

[75luva]

if its a 20'leg they are everywhere thus y they are refured to as fishing engines common as a brick
15'legs are more rare n have the same powerhead as a 20'
I have several of both engines
Untill you pull the head and exhaust plate u wont know if anything has been done to the exhaust ports
I can show you some if you like
Whats ur email

[25XS]

I didn't pull the welch plugs. I'm sure that's sound advice. I did pull all the brass orfices and made sure the passages were open. The "cam follower" is there and seems to be in good shape.

The welch plug on top covers a chamber. That chamber is fed gasoline based on the idle air orifice... In turn, the chamber "drains" into the venturi above the butterfly plate through TINY!!! holes that need to be clean & clear. This affects your idle quality alot. Lean condition causes a crankcase backfire that you may be calling a "mist out of the carburetor".

You have to clean them right, then replace ALL the fuel lines between the fuel filter and the carbs prior to reassembling the carbs to the engine. Otherwise, debris in the ancient fuel lines that is falling off will clog your new carb rebuild job.

Tom

[camohunter]

if its a 20'leg they are everywhere thus y they are refured to as fishing engines common as a brick
15'legs are more rare n have the same powerhead as a 20'
I have several of both engines
Untill you pull the head and exhaust plate u wont know if anything has been done to the exhaust ports
I can show you some if you like
Whats ur email

I've got several of the 20"ers and the parts to build a short. I have a clean looking 49 powerhead for my short but I'm thinking about getting a 56 to go on it. 56s are $ harder to come by here. I'm going to put it on an aluminum duck boat. Id like to see what you have. Id really like to see the difference on the ports inside. camohunter1981 at gmail. Thanks

[camohunter]

The welch plug on top covers a chamber. That chamber is fed gasoline based on the idle air orifice... In turn, the chamber "drains" into the venturi above the butterfly plate through TINY!!! holes that need to be clean & clear. This affects your idle quality alot. Lean condition causes a crankcase backfire that you may be calling a "mist out of the carburetor".

You have to clean them right, then replace ALL the fuel lines between the fuel filter and the carbs prior to reassembling the carbs to the engine. Otherwise, debris in the ancient fuel lines that is falling off will clog your new carb rebuild job.

Tom

I didn't know it was common to pull the welch plugs but I'll try that because the crankcase backfire is what's happening

[sharpeye Mike]

I think you have more then one problem with your motor, for one thing your compression is way, way to low, having said that I would try another compression tester just to make sure, on stock heads with a fresh motor you should be around 130-135lbs., shave the head .050 in. and that should bring you to 150lbs.

[zul8tr]

Low compression will cause starting and idle problems as well as reeds having issues but I have run engines with low compression (couldn't afford rebuild in the young years and made due) and they ran great at high rpm but never went slow so it didn't matter :) To test rings/bore you could add some outboard oil in the cylinders spin over a dozen times and recheck the compression, if a substantial increase in psi the rings/bore are suspect. As for the compression gage just use it as a relative psi measure and always use the same gage (important to see all cylinders near the same as well as not to low). As stated somebody could have raised (and squared?) the exhaust ports and that will lower the rope over compression. Could also have carbon build up and sticky rings so try some carbon remover like Seafoam, Merc #2 Quickleen, etc. I would do these simple things before a tear down and unnecessary $$$.:eek:

[camohunter]

The welch plug on top covers a chamber. That chamber is fed gasoline based on the idle air orifice... In turn, the chamber "drains" into the venturi above the butterfly plate through TINY!!! holes that need to be clean & clear. This affects your idle quality alot. Lean condition causes a crankcase backfire that you may be calling a "mist out of the carburetor".

You have to clean them right, then replace ALL the fuel lines between the fuel filter and the carbs prior to reassembling the carbs to the engine. Otherwise, debris in the ancient fuel lines that is falling off will clog your new carb rebuild job.

Tom

Ok I'll pull the welch plugs on the carbs if the compression checks out right. I've got to find another tester to verify my readings.

[camohunter]

Thanks everyone for the good advice. I'm going to try rechecking the compression first. My $15 eBay tester may not be accurate. I think pulling the welch plugs on the carbs is a must also. I did change the fuel lines, but I'm going to change them again and install a fuel filter. I've got a much cleaner 49ci powerhead in the shop, so if the compression fails I guess I'll be swapping powerheads.

What's the best way to pull the welch plugs on the carbs? Do I drill them out or is there a better way?

[25XS]

What's the best way to pull the welch plugs on the carbs? Do I drill them out or is there a better way?

Using a drill press on your first time is a very good idea. You have much better "depth control" with a drill press. Once the drill drops thru the welch plug, you don't want to plunge into the bottom of the chamber and damage the carburetor main body.

I use an 1/8th inch drill and pop a couple holes about 1/8th inch in from the edge. Then I use a very strong "scratch awl" (booger pick, o-ring pick, sharpened screwdriver, etc...) to pry out the welch plug.

A factory carburetor rebuild kit will include the new welch plugs. Installation does not require any sealant, you simply lay the new welch plug into the hole, support the carb body solidly and use a 1/4" drive socket extenstion and give it a sharp smack with a ball peen hammer to "set" the new welch plug in the hole. You'll get the feel after a couple of them. You don't SMASH IT... Just a small depression in the center will set the new plug.

Tom

[oledawg]

Using a drill press on your first time is a very good idea. You have much better "depth control" with a drill press. Once the drill drops thru the welch plug, you don't want to plunge into the bottom of the chamber and damage the carburetor main body.

I use an 1/8th inch drill and pop a couple holes about 1/8th inch in from the edge. Then I use a very strong "scratch awl" (booger pick, o-ring pick, sharpened screwdriver, etc...) to pry out the welch plug.

A factory carburetor rebuild kit will include the new welch plugs. Installation does not require any sealant, you simply lay the new welch plug into the hole, support the carb body solidly and use a 1/4" drive socket extenstion and give it a sharp smack with a ball peen hammer to "set" the new welch plug in the hole. You'll get the feel after a couple of them. You don't SMASH IT... Just a small depression in the center will set the new plug.

Tom
Did you check the reeds as suggested earlier? That will definitely cause poor idle and mist coming out the intake of the carb. The backfire is caused by a lean condition, which the reeds can create if chipped or broken. Also, contrary to other opinions, the bad reeds will reduce compression readings. On the piston down stroke, you create crankcase pressure which drives the intake charge into the cylinder. If reeds are bad, that pressure comes out the carb inlet, rather than going to the combustion chamber. The compression should be 125-135 psi. Also, the port timing is not different than the stock 70 hp. However, the factory did chamfer the upper lip of the exhaust ports to improve top end performance. All other ports are stock.

[Fast Fred]

never did find much under them plugs, sort out the compression issue, with out good compression there is no need to go to other stuff.