Fuel Mixture / Exhaust / Hesitates to idle.. some questions - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 6 (permalink) Old 06-14-2014, 03:54 PM Thread Starter
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Fuel Mixture / Exhaust / Hesitates to idle.. some questions

Have a couple things here, not sure if they're all related but I don't want to make separate posts. I've noticed some issues with performance when riding; occasionally after riding for 30+ minutes, the bike stalls out while idling at a light. Also, it hesitates when going back down to idle when the clutch is engaged. For example, if I'm riding at 4k RPM and come to a light, I'll pull in the clutch and downshift - holding the clutch the entire time.. but my idle speed doesn't go down to 1k RPM unless I let the clutch catch a little. My idle knob is set correctly, and as far as I can tell nothing is catching the throttle. It also only happens when the engine is hot. Here's a video (slow motion as well, really highlights the issue):

https://www.youtube.com/watch?v=wHp-1MJQZOM

In the video I revved up to about 4k RPM and quickly let off the throttle. You'll notice it goes down to about 1.5k RPM for a second, then down to 1k RPM. This is after a cold start. If I let her warm up a bit (or go for a ride) the problem gets worse and worse as time goes on, to the point where revving up to 4k RPM it will go down to 2.5-3k RPM and sit there, never going down to 1k RPM unless clutch is released a bit to put a workload on the engine.

Second issue.. noticing my exhaust (painted with exhaust paint) is wearing on my cylinders in a very uneven way. The paint on the front cylinder is cracking and has lost it's gloss.. on the rear cylinder, the paint looks much better (closer to what it looked like when it was first applied). Now, of course I could have f'ed up somewhere when applying the paint, but I don't think that's the case.

Here are pictures of the front and rear cylinders: http://imgur.com/a/v5GbX

This leads me to think that the front cylinder is running hotter than the rear, which leads into the next "issue".

Third issue: Might be related to the exhaust. I installed new plugs two months ago, this is the first time I've pulled them: http://imgur.com/a/zCJ9x

The front plugs don't have as much build up compared to the rear. I also noticed the threads on some of the plugs have some deposits on them - not sure if that matters? They were all hand tightened and then tightened up an additional 1/4 turn with a socket wrench. Is one cylinder running rich/lean? Not sure how to tell.

Any advice would be appreciated!
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post #2 of 6 (permalink) Old 06-14-2014, 07:53 PM
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Plugs look pretty normal to me.

Lean condition can cause a slow return to idle. Does it help if you choke it partially and adjust your idle for running like that? It's actually an enrichment circuit... doesn't choke, it adds extra fuel instead. Have you cleaned the carbs lately? And I mean complete disassembling, not just spraying them while on the bike. Check for vacuum/intake leaks too.

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post #3 of 6 (permalink) Old 06-14-2014, 09:42 PM
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Quote:
Originally Posted by Hexadecimus View Post
Plugs look pretty normal to me.

Lean condition can cause a slow return to idle. Does it help if you choke it partially and adjust your idle for running like that? It's actually an enrichment circuit... doesn't choke, it adds extra fuel instead. Have you cleaned the carbs lately? And I mean complete disassembling, not just spraying them while on the bike. Check for vacuum/intake leaks too.
When I had the slow to return to idle problem, I discovered a vacuum leak around the carb boot where boot mounted on the carb.

hth

Seafoam is your friend. Use as directed in tank once every 2 - 4 fill-ups.


'86 VN750 13,700 mi -Stock
-Purchased 2008 w/8800mi
Replaced ALL Cables . Kuryakyn LED Voltmeter
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After TOC MCCT's..... so amazingly quiet I discovered a rattling heat shield on my stock exhaust !
...have a vulcan good day!
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post #4 of 6 (permalink) Old 06-14-2014, 11:06 PM Thread Starter
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Quote:
Originally Posted by Hexadecimus View Post
Plugs look pretty normal to me.

Lean condition can cause a slow return to idle. Does it help if you choke it partially and adjust your idle for running like that? It's actually an enrichment circuit... doesn't choke, it adds extra fuel instead. Have you cleaned the carbs lately? And I mean complete disassembling, not just spraying them while on the bike. Check for vacuum/intake leaks too.
I haven't tried choking it, will give that a shot and see if it helps. I had the carbs completely disassembled and cleaned everything out pretty good (bike barely ran before that when I bought it). What's the best way to check for vacuum / intake leaks?

Quote:
Originally Posted by WilliamTech View Post
When I had the slow to return to idle problem, I discovered a vacuum leak around the carb boot where boot mounted on the carb.

hth

Seafoam is your friend. Use as directed in tank once every 2 - 4 fill-ups.

I'm going to remove/remount the carbs next weekend. I've heard there are alignment arrows on the boots that I didn't notice when installing them, so that could be the culprit.
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post #5 of 6 (permalink) Old 06-14-2014, 11:31 PM
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Also make sure your carb synch screw isnt too far counter clockwise. If im remembering correctly. When you take off your carbs just eye ball that the flaps are closed the same amount and open the same. Out kf synch carbs wil have you playing with the idle and at a poknt the kdle no longer makes a diference if that synch screw is too far out or in cant remember. Its explained in a youtjbe. Search vn750 carb synch. Found it

https://www.youtube.com/watch?v=Zc59BThqRa4&app=desktop

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post #6 of 6 (permalink) Old 06-30-2014, 11:24 AM Thread Starter
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Well, "next weekend" turned into the weekend after next. I pulled the carbs and remounted. As I suspected, I didn't have the carb boots aligned at all. I had them in the correct position (front/rear), but the front boot was turned 90 degrees, and the rear boot was upside down. I remounted the boots, popped the carbs back in and tightened the hose clamps much tighter than before (really hard to get at these without stripping the screw heads). I also cleaned/oiled my air filters. Once I put her back together it's like a brand new bike! The high idle issue is mostly gone (it still hesitates when coming down from a high RPM, but it doesn't stay at 3-4k RPM when letting off the throttle). I can pull the clutch in while coasting to a stop light and the engine RPM dropped down to 1k RPM almost immediately.

Lesson learned: mount your carbs correctly.
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