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post #1 of 40 (permalink) Old 05-07-2013, 12:16 PM Thread Starter
bug
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Top speed problems

Hello, i have a 1992 vn750a model, i own it for 3 years now and i fixed some problems on it already but i need some opinion to solve this one im posting.
recently i noticed that my top speed lowered i could reach top speed before and now not anymore, so when i push the trottle she goes fine but when it reaches arround 4000 rpm it starts to struggle to accelerate more, it never did this before and now it's constant i can only reach now 120km/h wich is arround 60 mph if im not misstaken, it feals like it looses compression on one of the cylinders. The stator, rectifier, battery are new. i can start the bike easy cold or hot, spark plugs are new also. another point, when warming the bike some times i get white smoke.

Any solutions or sugestions please reply

If this post is not on the right spot sorry but since i dont know where the problem comes from, engine or electric i didn't know where to post this

sorry for writing misstakes english is not my main language
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post #2 of 40 (permalink) Old 05-07-2013, 05:10 PM
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First off check or replace the vaccuum hose from the carb to petcock.

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post #3 of 40 (permalink) Old 05-07-2013, 05:25 PM
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Quote:
Originally Posted by bug View Post
Hello, i have a 1992 vn750a model, i own it for 3 years now and i fixed some problems on it already but i need some opinion to solve this one im posting.
recently i noticed that my top speed lowered i could reach top speed before and now not anymore, so when i push the trottle she goes fine but when it reaches arround 4000 rpm it starts to struggle to accelerate more, it never did this before and now it's constant i can only reach now 120km/h wich is arround 60 mph if im not misstaken, it feals like it looses compression on one of the cylinders. The stator, rectifier, battery are new. i can start the bike easy cold or hot, spark plugs are new also. another point, when warming the bike some times i get white smoke.

Any solutions or sugestions please reply

If this post is not on the right spot sorry but since i dont know where the problem comes from, engine or electric i didn't know where to post this

sorry for writing misstakes english is not my main language
What symptoms are you experiencing when you reach the bikes new max speed? Does it surge, load up / bog, etc. Initial thought is a fuel delivery issue. What condition is the inside of the gas tank? If there is noticeable rust, could be the petcock pre-screen filter is getting clogged as the fuel demand increases

As for the white smoke, it could be a couple of things. One, your burning coolant. A tell-tale indicator is a sweet smell coming from exhaust pipe. Two, condensation burning from inside muffler. This is normal. Or three, might be unburned fuel slowly burning off as bike warms up.

Hope this helps.

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post #4 of 40 (permalink) Old 05-08-2013, 09:45 PM Thread Starter
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Hey, at this point i still havent done any test's, first i removed the rpm indicator form the bike due to my customization process i need to put it back again to check at how many rpm's it starts to struggle 3k or 4k or 5k ... the tank is in good condition inside it was cleaned some month's ago, the tap assy is new also vaccun working fine fuel flow is fine also, carbs are clean. The white smoke, im sure it is not condensation i ride my bike everyday and also the white smoke is constant when the bike is cold and i pull the choke and leave it for a while, i say arround 45 sec to 1minute after this time it starts to send out a constant white smoke it only stops when i turn off the choke maybe i will do a video and post it soo you guys can see it. It can be cooland mixed and maybe this means i have to take out the engine and check the gasket's?. maybe this weekend i will start to do some test's on it, i will go whit the easy stuff first checking the air/fuel mix etc... i will post news has soon has possible

thanks for the answers
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post #5 of 40 (permalink) Old 05-08-2013, 09:56 PM
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If the choke is on, that enriches your fuel mixture. In some cases I have heard that on a Motorcycle like ours the white smoke may just be from raw unburnt gas. Most cases though. White smoke is usually coolant being burned though. If it is coolant there will be a sweet smell.
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post #6 of 40 (permalink) Old 05-08-2013, 10:15 PM Thread Starter
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Originally Posted by ExpectationLESS View Post
If the choke is on, that enriches your fuel mixture. In some cases I have heard that on a Motorcycle like ours the white smoke may just be from raw unburnt gas. Most cases though. White smoke is usually coolant being burned though. If it is coolant there will be a sweet smell.
Yea if there is a sweet smell it can be coolant
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post #7 of 40 (permalink) Old 05-09-2013, 01:16 AM
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Quote:
Originally Posted by bug View Post
Hello, i have a 1992 vn750a model, i own it for 3 years now and i fixed some problems on it already but i need some opinion to solve this one im posting.
recently i noticed that my top speed lowered i could reach top speed before and now not anymore, so when i push the trottle she goes fine but when it reaches arround 4000 rpm it starts to struggle to accelerate more, it never did this before and now it's constant i can only reach now 120km/h wich is arround 60 mph if im not misstaken, it feals like it looses compression on one of the cylinders. The stator, rectifier, battery are new. i can start the bike easy cold or hot, spark plugs are new also. another point, when warming the bike some times i get white smoke.

Any solutions or sugestions please reply

If this post is not on the right spot sorry but since i dont know where the problem comes from, engine or electric i didn't know where to post this

sorry for writing misstakes english is not my main language
FYI 100 km/hr = 61 mph.

American vn750 gearing will give you about 14.5 mph per thousand rpms in 5th gear.
European models have one more tooth on one gear, and I do not recall if it makes the engine speed slightly higher or lower at a given road speed.

Therefore an American bike at 4,000 rpm will be travelling 58 mph (~95 km/hr)
At 5K rpms it will be ~ 72.5 mph (~119 km/hr)

I hope these numbers help you identify your engine speed from your road speed.

Gordon

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Purchased May 16, 2008
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post #8 of 40 (permalink) Old 05-09-2013, 02:15 AM
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You say youre feeling like you are losing compression, also getting white smoke....you say the smoke smells sweet....

How is the temperature gauge reading...high ?....if so, it can be the water pump gone bad....a bad seal on the water pump can push coolant into the engine....has your coolant level gone down ?

Did you hear noises in the lower front of the engine ?...if yes, then Im guessing the water pump is bad. You dont have to remove the engine to replace by the way. I might need to do mine soon. Its not hard.



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post #9 of 40 (permalink) Old 05-10-2013, 10:34 AM Thread Starter
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Quote:
Originally Posted by OlHossCanada View Post
FYI 100 km/hr = 61 mph.

American vn750 gearing will give you about 14.5 mph per thousand rpms in 5th gear.
European models have one more tooth on one gear, and I do not recall if it makes the engine speed slightly higher or lower at a given road speed.

Therefore an American bike at 4,000 rpm will be travelling 58 mph (~95 km/hr)
At 5K rpms it will be ~ 72.5 mph (~119 km/hr)

I hope these numbers help you identify your engine speed from your road speed.
Hmm nice information thanks, my vn750 is an american model from 1992 so it should be an "A8" i will check the water pump, cooland level, easy to see and no need to remove the engine thanks for the tips.
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post #10 of 40 (permalink) Old 05-10-2013, 10:45 AM
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Quote:
Originally Posted by OlHossCanada View Post
FYI 100 km/hr = 61 mph.

American vn750 gearing will give you about 14.5 mph per thousand rpms in 5th gear.
European models have one more tooth on one gear, and I do not recall if it makes the engine speed slightly higher or lower at a given road speed.

Therefore an American bike at 4,000 rpm will be travelling 58 mph (~95 km/hr)
At 5K rpms it will be ~ 72.5 mph (~119 km/hr)


I hope these numbers help you identify your engine speed from your road speed.
Don't know about all American bikes, but my Sportster @ 3,000 rpm is running 64 mph in 5th gear, about 21.33 mph per 1,000 rpm...

PS-Sorry, just figured out you were speaking of the American mod. of the VN750......
Have a good one...Old Dog...

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