Explosions at/near carbs or airbox - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 29 (permalink) Old 04-19-2013, 10:32 PM Thread Starter
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Explosions at/near carbs or airbox

Hi all, I'm getting some small explosions (kinda like backfires) around the carb or air box area.

What happens is, I lose power and a split second after I do I get a pop at the air box or carbs and I get the power back.

This is getting dangerous, that second when I lose power can get pretty intense... especially like today, when I was starting a sharp, slow turn and lost power for that second... had to kick the bike back up to get through the turn.

It usually seems to happen when I back off the throttle and get back on it again. Like when I shift or when I come to a stop.

1985 VN700 unknown mileage (over 33K)

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post #2 of 29 (permalink) Old 04-19-2013, 10:51 PM
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Ceal, is it a lean pop? Can you count on this behavior or it random? Always at low rpm?
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post #3 of 29 (permalink) Old 04-20-2013, 12:24 AM
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Question

Yep, could be a lean pop or lean sneeze.

An intake valve sticking momentarily would do that, but the throttle and rpm description sounds more like lean pop.

Carb boots tight and not leaking?
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post #4 of 29 (permalink) Old 04-20-2013, 01:11 AM Thread Starter
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I get a pop at the exhaust all the time... this one is new, I can hear and feel it down near the carbs or air box.

I did tighten the carb boots last time I took the carbs off, but I guess I'll check again.

I know I had a very small leak at the carb float chamber... I can't see any leaking at all now though. It was very small, only a little drop would form and drip down over time.

The behavior doesn't happen EVERY time I let off the throttle, but I think that when it does happen it's always when I roll off the throttle and then get back into it.

I think that yes, it is at low rpm.

Recently I took out the spark plugs and posted some pictures on here and was told they looked alright, no major lean condition going on.

So what should I do here?

1985 VN700 unknown mileage (over 33K)

AGM Battery
Iridium spark plugs
NGK wires and caps
Relocated R/R
Marbled
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post #5 of 29 (permalink) Old 04-20-2013, 01:29 AM
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Can you tell which cylinder, or is it both taking turns?

Read the plugs again.

Check the boots.

Have you ever adjusted the air mixture screws?

Exhaust popping on decel or while cruising too?

A lot of popping in all conditions would lead more to sticky/leaky valves or valve timing. Could include ignition timing but that's pretty much fixed or unchanging on the 750.

Seafoam, Riselone, or Marvel Mystery Oil in the oil is good for cleaning valve stems on sticky valves.
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post #6 of 29 (permalink) Old 04-20-2013, 01:41 AM Thread Starter
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I set the idle screws 2.5 turns out.

I can't tell which cylinder, I feel and hear the pop coming from the area under the tank but can't even tell if it's at the air box or at the carbs or where exactly.

My exhaust has always popped on decel, on idle and while cruising. Only time it doesn't pop is on acceleration.

I'm gonna try to get some seafoam, but it will have to wait until next weekend, cause my gf already said she's not going to the U.S. this weekend :-/

I'll check the plugs tomorrow and see if they have changed at all. Although this pop was already happening when I had them out last time.

1985 VN700 unknown mileage (over 33K)

AGM Battery
Iridium spark plugs
NGK wires and caps
Relocated R/R
Marbled
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post #7 of 29 (permalink) Old 04-20-2013, 02:07 AM
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Ceal, I've noticed that you've mentioned decel popping on several threads. Have you checked your carb coast enrichers? If they aren't working you would get a lot of decel popping from an extremly lean mixture. I don't think they would cauce your sudden lack of power and intake backfire like you are describing on this thread. Your current problem sounds like a spark or valve timing issue.
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post #8 of 29 (permalink) Old 04-20-2013, 02:39 AM
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Cell, I had the same problem but I could hear it with the bike at idle also... I even took a video of it and you could actually see the Carb twitch.... someone on here noticed from the video that my choke was very partially stuck open. Check that... and if you want, search for my post and watch the video to see if it is the same.

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post #9 of 29 (permalink) Old 04-20-2013, 02:41 AM Thread Starter
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Quote:
Originally Posted by FoolAmI View Post
Ceal, I've noticed that you've mentioned decel popping on several threads. Have you checked your carb coast enrichers? If they aren't working you would get a lot of decel popping from an extremly lean mixture. I don't think they would cauce your sudden lack of power and intake backfire like you are describing on this thread. Your current problem sounds like a spark or valve timing issue.
Indeed, one of the coasting enricheners isn't in good shape at all. Just haven't had $35 to replace it. I just got the float chamber gaskets recently, but haven't installed them. I was thinking of waiting to get the coasting enrichener to put it all in at the same time.

I have noticed that the bike doesn't start quite as quickly as it used to... maybe the spark or valve timing issue could be tied to this as well?

Don't really know how to check for those things. I did the coil relay mod so I'm supposed to be getting great voltage to the coils... gonna measure it again tomorrow to see if nothing's changed.

1985 VN700 unknown mileage (over 33K)

AGM Battery
Iridium spark plugs
NGK wires and caps
Relocated R/R
Marbled
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post #10 of 29 (permalink) Old 04-20-2013, 02:43 AM Thread Starter
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Quote:
Originally Posted by Chris32250 View Post
Cell, I had the same problem but I could hear it with the bike at idle also... I even took a video of it and you could actually see the Carb twitch.... someone on here noticed from the video that my choke was very partially stuck open. Check that... and if you want, search for my post and watch the video to see if it is the same.
We've had some very dusty days here, a stuck choke is very possible. I'll add this to the checklist for tomorrow, thanks!

Many thanks to everyone else who has responded as well!

1985 VN700 unknown mileage (over 33K)

AGM Battery
Iridium spark plugs
NGK wires and caps
Relocated R/R
Marbled
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