Author Topic: Vm34'S tons of white smoke not idling  (Read 1920 times)

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Offline Ryan4455

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Vm34'S tons of white smoke not idling
« on: September 02, 2016, 03:55:19 AM »
Hey so I need help please !!!

I have a stock 1974 cb750, it has duel carb set up Vm34'S.  The bike was running perfect, then one day went out to ride and boom would fire up . After a lot of kicks it did how ever start up; gas was coming from the carbs and it just didn't sound right . Changed the sparks , they were all overly rich . Changed the coils and now there's white smoke coming from the exhaust . And it's still running supppppper rich

I have tried everything I can think of and searched the forms if anyone can push me to the right answer I'd would be appreciated

Ohhh ps. It's not holding an idle also fires up with the choke on and stays on after choke is off however gotta hold the throttle and then it still shuts off anyway

Offline Airborne 82nd

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Re: Vm34'S tons of white smoke not idling
« Reply #1 on: September 02, 2016, 07:50:20 AM »
Hey so I need help please !!!

I have a stock 1974 cb750, it has duel carb set up Vm34'SThe bike was running perfect, then one day went out to ride and boom would fire up . After a lot of kicks it did how ever start up; gas was coming from the carbs and it just didn't sound right . Changed the sparks , they were all overly rich . Changed the coils and now there's white smoke coming from the exhaust . And it's still running supppppper rich

I have tried everything I can think of and searched the forms if anyone can push me to the right answer I'd would be appreciated

Ohhh ps. It's not holding an idle also fires up with the choke on and stays on after choke is off however gotta hold the throttle and then it still shuts off anyway

Are you sure about that?
« Last Edit: September 02, 2016, 07:53:03 AM by Airborne 82nd »

Offline Ryan4455

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Re: Vm34'S tons of white smoke not idling
« Reply #2 on: September 02, 2016, 12:07:50 PM »
Sure about what ?

Offline Scott S

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Re: Vm34'S tons of white smoke not idling
« Reply #3 on: September 02, 2016, 12:18:44 PM »
 I think he's saying twin VM carbs are not stock on any 750.
 I'm assuming you meant stock engine? Any other changes, like exhaust?
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Offline Ryan4455

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Re: Vm34'S tons of white smoke not idling
« Reply #4 on: September 02, 2016, 12:37:26 PM »
Got ya , yes I ment stock bore / Engine ..... yes 4 into 1 performance exhaust with carpy horn

Offline PeWe

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Re: Vm34'S tons of white smoke not idling
« Reply #5 on: September 02, 2016, 01:19:05 PM »
I have seen white smoke before...it was when I blew a hole in a piston.....
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline pampadori

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Re: Vm34'S tons of white smoke not idling
« Reply #6 on: September 02, 2016, 01:23:20 PM »
I'm confused.  Dual VM34 carbs indicates that he has two carbs on his 4 cylinder motorcycle.  Maybe a cycleX manifolds or something?  And the part about running rich...how do you know this?  by the smell of the exhaust or did you measure the air to fuel ratio with a wide band o2 sensor/uego?  I'm wearing my kid gloves now when i say, you might not know what the heck you are saying at this point. :)  (keep reading though as i do offer some advice that i think will get your bike running just fine below)
"Gas was coming from the carbs and it didn't sound right".  Gas was running out of the overflow tubes on the carbs onto the engine or did you mean something else? DId it sound like maybe it was  not running on all cylinders? This statement doesn't precisely tell me what the heck your noticing with your motorcycle.  The more precision and articulation you use explaining a motorcycle issue, the better and more accurate the advice will be. So just remember that for future reference and read just a little bit more below....

Since you have 3 posts and mentioned you bought parts from Carpy, i'm going to assume that the bike is fairly new to you.  I'm also going to wager that your mechanical abilities and motorcycle experience is limited and try to give you a lesson in how to keep old bikes running.  A little "old bike 101" if you will.

To run, your engine needs to take in fuel and air when the piston is being drawn down the cylinder and then ignite that same fuel and air when it comes back up the cylinder and compresses it.  That is to say, your engine needs fuel and spark. 

If you are able to get it to run, even very poorly, you can narrow it down to which cylinder(s) might not be getting fuel or spark.  The exhaust pipes will be cooler to touch on the cylinders that are having trouble.  That hopefully narrows you to 1 cyl. 
If you remove the spark plug from that cyl while keeping the plug wire connected, you can rest the end of the plug against the metal engine and attempt to start the engine again.  You should see a nice spark in the plug tip if the coil, points, and plug are functioning.  If you do not, it might be safe to assume you don't have spark.  Focus your energy there.  It might be the plug, it might be the points.  I doubt it would be something else but could be a coil or wiring too i suppose.

If the above tests good and the cold cyl does indeed have a spark, move on to fuel.  Get the engine running again and spray some starting fluid in the mouth of the carb feeding the cylinder that is cold.  Does this change the sound and does that exhaust pipe begin to warm up now?  If yes, you need to take the carbs off he engine and go thru the process of cleaning them.  The bottom bowls will need to be removed and you can use that nice little plastic straw that comes with the can of carb cleaner to force cleaner thru all of the narrow passages and the jets that are likely clogged with varnish.  The idle or low speed jet is likely to clog before the main jet since it has a much smaller diameter metering hole in it. 

Watch a few videos on cleaning motorcycle carbs on youtube and you'll figure it out in no time.  All of this is very basic mechanics and 9 times out of 10 is all you need to know to get an old Honda motorcycle to run. 

Now, the next lesson.  Never let the gas in your bike sit for longer than a month or so.  I mean, unless you like cleaning carbs often.  Over winter, drain it and run the bike until it stalls and then remove the carb bowls and empty them.  If you are lazy, like i am, you can skip this and just by some stay-bil fuel treatment and usually that will get you thru winter.  But this is why people stop having a running bike in their possession.  The let gas get old and it varnishes and clogs the carb's jets. 

I hope none of that sounded too snooty.  It really is 99% of what you need to know with old bikes though.  And the last bit of sage advice, leave them stock and they run way better, way more often.  :)
Good luck to you RYan!  I'm sure you'll get it fixed before you can say "you meet the nicest people on a..."
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