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Author Topic:   Rebuilt 1996 Johnson 175: Low RPM Vibration

hoppinjohn posted 10-29-2007 07:00 PM ET (US)
[I] bought a rebuilt 1996 Johnson 175-HP for a 1978 [unrecognized acronym]. It has a bad vibration at idle and low RPM from 4- to 8-MPH. After that, smooth as silk. Is this common for this motor?
jimh posted 10-29-2007 07:12 PM ET (US)
Do you just see the vibration in the motor's cowling, or is the vibration transmitted to and felt throughout the whole boat?
hoppinjohn posted 10-30-2007 04:41 PM ET (US)
thank you for replying. The vibration is in the entire boat.
I have the utmost trust in the gentleman's business and all my dealings with him, but when I cannot focus on my mapping gps due to the vibration, it has me wondering.
jimh posted 10-31-2007 07:45 AM ET (US)
There are three possibilities for explaining the vibration:
--Everything is normal. The vibration is just inherent in the 90-degree V4 engine design.
--The engine is normal. The propeller has an odd resonance. This is imparted to the boat, which also has a resonance at this same frequency. Some combinations of boat, motor, and propeller produce vibrations at certain speeds.
--The engine is not running smoothly. This could be due to several problems. There could be a cylinder misfiring. The engine mounts could be damage. The propeller could be damaged.
If the boat. motor, and propeller previously all worked well together and did not produce this vibration, something probably has malfunctioned to cause the vibration.
The most likely problem is a misfire of one cylinder.
george nagy posted 10-31-2007 09:04 AM ET (US)
That is a 60 degree v6 engine. my 1998 150 does have a rather annoying vibration at around the same speed although not as bad as not being able to read the gps.
Jerry Townsend posted 10-31-2007 03:02 PM ET (US)
hoppinJohn - you mention the engine was rebuilt - was the rebuilder knowlegable and experienced in rebuilding two cycle engines? Were any parts replaced - if so, was everything balanced?
Who is the individual you refer to in your "... I have the utmost trust in the gentleman's business and all my dealings with him, but when I cannot focus on my mapping gps due to the vibration, it has me wondering. ..."? Is he the rebuilder?
Now, regarding the vibration. All engines will vibrate to some extent. That is the reason for balancing means (weights or drilling holes) are used to balance the entire engine assembly. Your engine was other than 60 degrees (Johnson/Evinrude only made their 115 and 90 in a 60 degree block - and those two block were virtually identical) - but that does not make all that much difference.
With your entire boat vibrating, what is happening is that the source of the vibration (obviously the engine/prop) - at idle or slow wpeeds - is producing a driving frequency that is at or close to one of the "natural" frequencies of the boat and components. As the engine speed increases, the vibration ceases - because the driving frequency increases to that above the natural frequency of the boat and components.
Jimh's mentioning an unbalanced prop or a misfiring cylinder(s) are possibilities - but you can learn which by running a simple test - with the engine in neutral, increase the speed of the engine to 1000 - 2000 rpm. If the vibration is not there, the problem is the prop. If the vibration is still there - the problem is in the engine.
A misfiring engine is typically caused by one or more bad spark-plug wires. A seemingly small "break" may still conduct electricity - but will "throw" an engine out of balance, resulting in significant vibration. At higher speeds, the ignition voltage is higher, eliminating the problem. Now - I am not sure of your ignition system type.
The engine mounts are, in part, intended to isolate the engine vibration. If those mounts are bad - there is little or no isolation, allowing any vibration from the engine to be "more directly" transmitted to the boat.
Regarding your gps - isolate your mount by putting as soft as possible rubber between your boat surface (top of console et.al.) and the gps mount. The "soft" rubber will than prevent higher frequency vibration from being transmitted through the mount - and your gps will be "rock steady". --------- Jerry/Idaho
hoppinjohn posted 10-31-2007 06:06 PM ET (US)
Thank you all for replying.
Yes the individual I bought the motor from rebuilt it. Been in the business for over 30 years. It is the second rebuilt engine I have bought, the first a 1996 Johnson 90 on a 66 or 68 ( can't remember) Nauset hull.
The block is a 60 degree block. I just had an Tach installed, same guy. Should be test riding it within a day or so. I will let you know.
hoppinjohn posted 10-31-2007 06:10 PM ET (US)
Back again. Please explain "bad engine mounts." How can I tell. Last motor had no issue.
jimh posted 10-31-2007 07:26 PM ET (US)
I am sorry I confused the engine cylinder block design. I don't know where I got that notion. A V6 engine with a 60-degree block is a more balanced design. I do not think you would notice a severe vibration from it. They tend to run quite smoothly.
The stiffness of the engine mounts determines how much isolation there is between the engine and the boat. Some manufacturers use soft mounts. A soft mount allows the engine to vibrate but keeps the vibration isolated from the boat. If you looked at the cowling, you might see the cowling shaking, but the vibration stays isolated to the engine. If the engine mounts are stiff, the vibration of the engine is coupled to the boat. The boat often acts as a dampener, so the engine cowling might not shake as much, but more of the vibration energy is coupled to the boat and is felt there.
Jerry Townsend posted 10-31-2007 07:56 PM ET (US)
And in this same light - what I should have said regarding the engine design is - I'm not sure of your 175 HP engine, but at least the 115 and 90 engines are 60 degrees. -- Jerry/Idaho
makoman310 posted 11-18-2007 02:41 PM ET (US)
upper motormount will cause this have the same thing on my outrage 225 oceanpro nothing serious but anoying at low rpms and speed
seahorse posted 11-18-2007 03:14 PM ET (US)
There is a rough area in almost all carbureted outboards that is the transition phase between the low and mid range jet circuits in the carbs. Most of the time it is between 1200 and 2000 rpm where the boat is plowing water and struggling to get up on plane.
There is a series of linkage and timing adjustments called in the trade, "sync and linc" which is the relationship between the spark advance and throttle opening. If not adjusted perfectly, the motor will run rougher than normal at the above idle rpm range.
Carburetors set too rich at idle and plugged recirculation valves, hoses, or fittings can also do the same.
If the throttle roller is broken or or the sleeve fallen off, that will also change the linkage geometry.
Follow the service manual or have a QUALIFIED tech adjust it for you. Also make sure your motor has Champion QL78YC plugs gapped to .030" and that the thermostats are getting each cylinder head temp up to about 140°F at idle.

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Author	Topic: Rebuilt 1996 Johnson 175: Low RPM Vibration
hoppinjohn	posted 10-29-2007 07:00 PM ET (US) [I] bought a rebuilt 1996 Johnson 175-HP for a 1978 [unrecognized acronym]. It has a bad vibration at idle and low RPM from 4- to 8-MPH. After that, smooth as silk. Is this common for this motor?
jimh	posted 10-29-2007 07:12 PM ET (US) Do you just see the vibration in the motor's cowling, or is the vibration transmitted to and felt throughout the whole boat?
hoppinjohn	posted 10-30-2007 04:41 PM ET (US) thank you for replying. The vibration is in the entire boat. I have the utmost trust in the gentleman's business and all my dealings with him, but when I cannot focus on my mapping gps due to the vibration, it has me wondering.
jimh	posted 10-31-2007 07:45 AM ET (US) There are three possibilities for explaining the vibration: --Everything is normal. The vibration is just inherent in the 90-degree V4 engine design. --The engine is normal. The propeller has an odd resonance. This is imparted to the boat, which also has a resonance at this same frequency. Some combinations of boat, motor, and propeller produce vibrations at certain speeds. --The engine is not running smoothly. This could be due to several problems. There could be a cylinder misfiring. The engine mounts could be damage. The propeller could be damaged. If the boat. motor, and propeller previously all worked well together and did not produce this vibration, something probably has malfunctioned to cause the vibration. The most likely problem is a misfire of one cylinder.
george nagy	posted 10-31-2007 09:04 AM ET (US) That is a 60 degree v6 engine. my 1998 150 does have a rather annoying vibration at around the same speed although not as bad as not being able to read the gps.
Jerry Townsend	posted 10-31-2007 03:02 PM ET (US) hoppinJohn - you mention the engine was rebuilt - was the rebuilder knowlegable and experienced in rebuilding two cycle engines? Were any parts replaced - if so, was everything balanced? Who is the individual you refer to in your "... I have the utmost trust in the gentleman's business and all my dealings with him, but when I cannot focus on my mapping gps due to the vibration, it has me wondering. ..."? Is he the rebuilder? Now, regarding the vibration. All engines will vibrate to some extent. That is the reason for balancing means (weights or drilling holes) are used to balance the entire engine assembly. Your engine was other than 60 degrees (Johnson/Evinrude only made their 115 and 90 in a 60 degree block - and those two block were virtually identical) - but that does not make all that much difference. With your entire boat vibrating, what is happening is that the source of the vibration (obviously the engine/prop) - at idle or slow wpeeds - is producing a driving frequency that is at or close to one of the "natural" frequencies of the boat and components. As the engine speed increases, the vibration ceases - because the driving frequency increases to that above the natural frequency of the boat and components. Jimh's mentioning an unbalanced prop or a misfiring cylinder(s) are possibilities - but you can learn which by running a simple test - with the engine in neutral, increase the speed of the engine to 1000 - 2000 rpm. If the vibration is not there, the problem is the prop. If the vibration is still there - the problem is in the engine. A misfiring engine is typically caused by one or more bad spark-plug wires. A seemingly small "break" may still conduct electricity - but will "throw" an engine out of balance, resulting in significant vibration. At higher speeds, the ignition voltage is higher, eliminating the problem. Now - I am not sure of your ignition system type. The engine mounts are, in part, intended to isolate the engine vibration. If those mounts are bad - there is little or no isolation, allowing any vibration from the engine to be "more directly" transmitted to the boat. Regarding your gps - isolate your mount by putting as soft as possible rubber between your boat surface (top of console et.al.) and the gps mount. The "soft" rubber will than prevent higher frequency vibration from being transmitted through the mount - and your gps will be "rock steady". --------- Jerry/Idaho
hoppinjohn	posted 10-31-2007 06:06 PM ET (US) Thank you all for replying. Yes the individual I bought the motor from rebuilt it. Been in the business for over 30 years. It is the second rebuilt engine I have bought, the first a 1996 Johnson 90 on a 66 or 68 ( can't remember) Nauset hull. The block is a 60 degree block. I just had an Tach installed, same guy. Should be test riding it within a day or so. I will let you know.
hoppinjohn	posted 10-31-2007 06:10 PM ET (US) Back again. Please explain "bad engine mounts." How can I tell. Last motor had no issue.
jimh	posted 10-31-2007 07:26 PM ET (US) I am sorry I confused the engine cylinder block design. I don't know where I got that notion. A V6 engine with a 60-degree block is a more balanced design. I do not think you would notice a severe vibration from it. They tend to run quite smoothly. The stiffness of the engine mounts determines how much isolation there is between the engine and the boat. Some manufacturers use soft mounts. A soft mount allows the engine to vibrate but keeps the vibration isolated from the boat. If you looked at the cowling, you might see the cowling shaking, but the vibration stays isolated to the engine. If the engine mounts are stiff, the vibration of the engine is coupled to the boat. The boat often acts as a dampener, so the engine cowling might not shake as much, but more of the vibration energy is coupled to the boat and is felt there.
Jerry Townsend	posted 10-31-2007 07:56 PM ET (US) And in this same light - what I should have said regarding the engine design is - I'm not sure of your 175 HP engine, but at least the 115 and 90 engines are 60 degrees. -- Jerry/Idaho
makoman310	posted 11-18-2007 02:41 PM ET (US) upper motormount will cause this have the same thing on my outrage 225 oceanpro nothing serious but anoying at low rpms and speed
seahorse	posted 11-18-2007 03:14 PM ET (US) There is a rough area in almost all carbureted outboards that is the transition phase between the low and mid range jet circuits in the carbs. Most of the time it is between 1200 and 2000 rpm where the boat is plowing water and struggling to get up on plane. There is a series of linkage and timing adjustments called in the trade, "sync and linc" which is the relationship between the spark advance and throttle opening. If not adjusted perfectly, the motor will run rougher than normal at the above idle rpm range. Carburetors set too rich at idle and plugged recirculation valves, hoses, or fittings can also do the same. If the throttle roller is broken or or the sleeve fallen off, that will also change the linkage geometry. Follow the service manual or have a QUALIFIED tech adjust it for you. Also make sure your motor has Champion QL78YC plugs gapped to .030" and that the thermostats are getting each cylinder head temp up to about 140°F at idle.