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Carbs and Fuel System
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post #1 of 32 (permalink) Old 12-29-2013, 05:48 PM Thread Starter
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Still not running right

Looking for some ideas 'cause I've about run out! The problem: at constant throttle, usually around 4k rpm, the engine will begin missing. Open the throttle or add choke and it smooths out. Runs good for a while then does it again. Here's what I've done: carbs have been off twice and cleaned thoroughly. Only thing I didn't do was soak the jets but I ran a fine wire through them and I think they're ok. Found the coasting enricher
diaphragm on the rear carb was rotten so its been replaced. Replaced rear carb boot since the original showed signs of damage. Replaced carb vaccuum lines. Lastly, carbs have been synced and are perfect at fast idle. They do show a slight difference with engine running about 4k with bike on center stand and transmission in neutral.
On another note, for online parts ordering Partzilla has great pricing and quick shipping. If you have lots of time, SME Bike has even better pricing. What I didn't realize is SME is in New Zealand so it took over three weeks to get the parts.
If any of you guys can shed some light on what I can do to fix this problem I'd certainly be grateful. I enjoy working on the bike and the local shop gets $96 per hour labor so I'd like to fix it myself.
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post #2 of 32 (permalink) Old 12-29-2013, 05:54 PM
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how does the main diaphragms look

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post #3 of 32 (permalink) Old 12-29-2013, 05:56 PM
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Quote:
Originally Posted by rsavant View Post
Looking for some ideas 'cause I've about run out! The problem: at constant throttle, usually around 4k rpm, the engine will begin missing. Open the throttle or add choke and it smooths out. Runs good for a while then does it again. Here's what I've done: carbs have been off twice and cleaned thoroughly. Only thing I didn't do was soak the jets but I ran a fine wire through them and I think they're ok. Found the coasting enricher
diaphragm on the rear carb was rotten so its been replaced. Replaced rear carb boot since the original showed signs of damage. Replaced carb vaccuum lines. Lastly, carbs have been synced and are perfect at fast idle. They do show a slight difference with engine running about 4k with bike on center stand and transmission in neutral.
On another note, for online parts ordering Partzilla has great pricing and quick shipping. If you have lots of time, SME Bike has even better pricing. What I didn't realize is SME is in New Zealand so it took over three weeks to get the parts.
If any of you guys can shed some light on what I can do to fix this problem I'd certainly be grateful. I enjoy working on the bike and the local shop gets $96 per hour labor so I'd like to fix it myself.

Sounds like fuel starvation.


...more info requested to your situation.
Have you alltered carb jets to prior to this problem ?
Are u running aftermarket exhaust ? Earshaved ?


'86 VN750 13,700 mi -Stock
-Purchased 2008 w/8800mi
Replaced ALL Cables . Kuryakyn LED Voltmeter
SilverStar Ultra 9003/HB2 H4 Headlamp, Jardine Fwd Controls,
Iridiums DPR7EIX-9 & Wires, Tuxedo Mod, Coil Mod, P/U Sensor Mod, Fork Seals,
Splines Lubed - 11/4/2012 - MF AGM Battery
Additional Flashing LED Brake Light on Trunk
Dampers went out @ 13+K !
After TOC MCCT's..... so amazingly quiet I discovered a rattling heat shield on my stock exhaust !
...have a vulcan good day!

Last edited by WilliamTech; 12-29-2013 at 05:59 PM.
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post #4 of 32 (permalink) Old 12-29-2013, 06:16 PM Thread Starter
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Still not running right

The bike is completely stock. I'm the second owner but I'm fairly certain the jets are the factory originals.
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post #5 of 32 (permalink) Old 12-29-2013, 06:46 PM
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Quote:
Originally Posted by rsavant View Post
The bike is completely stock. I'm the second owner but I'm fairly certain the jets are the factory originals.
....you pullled/cleaned carbs...... As previousy stated...sounds like fuel stavation.... unk !

Suggest you wait for Slim, WMsonta or Vulcan2000 wade-in on this problem, or other qualified 'opinions' (shootin' in the dark here') by other members.

good luck


'86 VN750 13,700 mi -Stock
-Purchased 2008 w/8800mi
Replaced ALL Cables . Kuryakyn LED Voltmeter
SilverStar Ultra 9003/HB2 H4 Headlamp, Jardine Fwd Controls,
Iridiums DPR7EIX-9 & Wires, Tuxedo Mod, Coil Mod, P/U Sensor Mod, Fork Seals,
Splines Lubed - 11/4/2012 - MF AGM Battery
Additional Flashing LED Brake Light on Trunk
Dampers went out @ 13+K !
After TOC MCCT's..... so amazingly quiet I discovered a rattling heat shield on my stock exhaust !
...have a vulcan good day!

Last edited by WilliamTech; 12-30-2013 at 03:48 AM.
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post #6 of 32 (permalink) Old 12-29-2013, 09:45 PM
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Quote:
Originally Posted by WilliamTech View Post
....you pullled/cleaned carbs...... As previousy stated...sounds like fuel stavation.... unk !

Suggest you wait for Slim, WMsonta or Vulcan2000 wage-in on this problem, or other qualified 'opinions' (shootin' in the dark here') by other members.

good luck

Well.... the rest of us would like to thank you for narrowing down the field as to who has done these things in real life and putting the rest of us in the "shootin' in the dark" members in our place, nicely done there,WT.



BTW at least I know the different between wage and wade.

Good luck to you as well. You're gonna need it when you ask us wannabes for help. Nicely played,Nicely played.




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Last edited by denny6006; 12-29-2013 at 09:53 PM.
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post #7 of 32 (permalink) Old 12-29-2013, 09:50 PM
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Higher RPM fuel starvation can be caused by a small vacume leak. With low vacume the needles don't pull up well. Not saying that is your problem but worth considering.

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post #8 of 32 (permalink) Old 12-30-2013, 12:02 AM
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Quote:
Originally Posted by rsavant View Post
at constant throttle, usually around 4k rpm, the engine will begin missing. Open the throttle or add choke and it smooths out. Runs good for a while then does it again.

Found the coasting enricher
diaphragm on the rear carb was rotten so its been replaced.

Lastly, carbs have been synced and are perfect at fast idle. They do show a slight difference with engine running about 4k with bike on center stand and transmission in neutral.
I am kinda hesitant to post here. First at least two of the previous posters either have made or do make their living as a mechanic. I have not sold mechanic work since '87.

Based on the Original Post-the bike lays down @ about 4k rpm's using A SMALL AMOUNT OF THROTTLE OPENING? If you open the throttle on up at that time the bike will accelerate away normally? Are these two questions correct?

Last edited by wmsonta; 12-30-2013 at 12:55 AM.
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post #9 of 32 (permalink) Old 12-30-2013, 03:55 AM
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Quote:
Originally Posted by denny6006 View Post
Well.... the rest of us would like to thank you for narrowing down the field as to who has done these things in real life and putting the rest of us in the "shootin' in the dark" members in our place, nicely done there,WT.



BTW at least I know the different between wage and wade.

Good luck to you as well. You're gonna need it when you ask us wannabes for help. Nicely played,Nicely played.
So sorry Denny...... with your experience, you are a valuable source of info/knowledge. ...My oops... My duh..... My brain blew it !
(was NOT intentional)

You have proven Top's in the carb experience and are certainly knowledgeable of mechanic's.

Again, sorry.

WilliamTech

(I fixed the wage / wade wording)

'86 VN750 13,700 mi -Stock
-Purchased 2008 w/8800mi
Replaced ALL Cables . Kuryakyn LED Voltmeter
SilverStar Ultra 9003/HB2 H4 Headlamp, Jardine Fwd Controls,
Iridiums DPR7EIX-9 & Wires, Tuxedo Mod, Coil Mod, P/U Sensor Mod, Fork Seals,
Splines Lubed - 11/4/2012 - MF AGM Battery
Additional Flashing LED Brake Light on Trunk
Dampers went out @ 13+K !
After TOC MCCT's..... so amazingly quiet I discovered a rattling heat shield on my stock exhaust !
...have a vulcan good day!

Last edited by WilliamTech; 12-30-2013 at 04:06 AM.
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post #10 of 32 (permalink) Old 12-30-2013, 07:00 AM Thread Starter
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Still not running right

Main diaphragms inspected during first tear down and they appeared ok. Increasing throttle or adding choke usually smooths out the stumbling. The puzzling thing is the engine may run perfectly for miles, the stumbling will start then it smooths out again. I still suspect a vacuum leak somewhere. The problem is most evident in 5th gear, right around the 55 to 60 mph speed. Higher rpm seems to be good whether I just increase speed or downshift and maintain speed.
Thanks, guys. I want to eliminate all possibilities before pulling the carbs again.
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