Backfiring issue from exhaust, front cylinder - 1994 Kawasaki Vulcan 1500A

[Guest]

I've had this thing over a month and still haven't been able to enjoy a ride on it. Wasn't staying running when I first got it. Would start up strong, run for 10 seconds and then cut off like it ran out of fuel.

Pulled the carbs for a full teardown and cleaning. New O-rings, all jets back to shiny brass (they all were dirty and clogged) with Berryman's and welders tip cleaner, the works. Blew everything out with compressed air, made sure all passages were nice and clear. Reassembled, main diaphragms had a perfect whoosh sound when slid up and down. Replaced smaller diaphragms on the side of each carb as well, one had been torn. I forgot what those smaller ones were for. Set the pilot screws to 2.25 out per factory manual.

Replaced all the spark plugs with recommended ones via factory manual, gapped correctly.

Replaced battery with a nice sealed one.

It starts, it runs, but it backfires from the front cylinder. I tried spraying carb cleaner all around, no change in idle so I don't think I have a leak. Tried adjusting carbs using the highest idle method (not sure what it is called exactly but I think that's close) but still backfires at idle (just small popping here and there).

Any clues where I should look next?

It pops less now than it does in this video, but still pops nonetheless.

[steve murdoch]

Exhaust gasket leak?
Air leak up front instead?

Years ago a neighbour had a VN750 and it was notorious for back-firing. Something about throttle bodies?
The bike and him are long gone.

[Steve]

Personally, I would try going out a bit farther with the pilot screws.

Suzuki had set the GS screws on a machine to give the perfect mixture to meet the EPA standards, then said "Don't Touch". Most of the ones I have seen were right around 1 5/8 to 1 3/4. Funny thing is, ... they seem to run SO much better at about 2 1/2 turns. Part of that is simply richening the mixture so it runs better, another part is that there is less actual gasoline in what we get at the pump, so it needs to be richend up a bit to compensate for that, too.

I would try turning the screws out until it runs better. Continue turning them out until it starts to run a bit poorly because it is too rich, then turn them in slowly, listening for the best idle, which will be just before it starts running poorly because it's too lean. When it does start to run lean, back them out about 1/8 to 1/4 turn, leave them there. Pretty much like tuning a GS carb.

.

[growler]

I tried backing the pilots out more, they are both about a total of 3.5 turns now, but still backfires and pops - even with ACCELERATION.

[growler]

Exhaust gasket leak?
Air leak up front instead?

Years ago a neighbour had a VN750 and it was notorious for back-firing. Something about throttle bodies?
The bike and him are long gone.

I sprayed carb cleaner all over the usual suspects, no change in idle.

[steve murdoch]

I did see that you had sprayed the "usual suspects" but in my mind an exhaust gasket is not a "usual suspect".
There has to be a Vulcan site?

[jsandidge]

You won't see any change in idle by spraying carb cleaner around an exhaust leak. The carb cleaner won't enter the combustion chamber through the exhaust since the flow is going out, not sucking in. An exhaust leak can cause popping and backfiring though.

[growler]

I'm fouling plugs. All four. Smells like gas, not oil. Fun.

[gsrick]

I'm fouling plugs. All four. Smells like gas, not oil. Fun.

Growler, you've only had this bike for a month? Do you know if it sat for awhile before you bought it? You mentioned in your first post that it would only run for about ten seconds. That normally would lead me to think that the valves may be tight enough that your loosing compression once the exhaust valves warm up and everything is expanding. I've seen this a lot with single cylinder engines like on scooters. It will run for 5 to 10 seconds then nothing until it cools down. once it cools down it will start again, but only run for the 5-10 seconds. When was the last time the valves were adjusted? I'd try doing a compression check and a leak down test and see if your leaking out the exhaust valves. Also it's possible the cylinders and valves may have a lot of carbon build up. Some of it may be breaking loose allowing the valves to close a little more which would reduce the amount of combustion escaping causing less popping. if that is happening. some Seafoam in the fuel tank with the next few fill up might help clean that out. One other thing I'd look at would be to see if the carbs are synced together.

[growler]

Yes, it did sit for a while. It was my fathers and he hadn't rode it for a few months. The carbs were gross, jets clogged, so full tear down, cleaning, and rebuild with all new O-rings.

It ran after the carb rebuild, and I took it out for a 20 min ride. That's when the popping and backfiring was happening and has happened ever since.

I've never done any compression testing or leak down testing - I'll read up on that.

Growler, you've only had this bike for a month? Do you know if it sat for awhile before you bought it? You mentioned in your first post that it would only run for about ten seconds. That normally would lead me to think that the valves may be tight enough that your loosing compression once the exhaust valves warm up and everything is expanding. I've seen this a lot with single cylinder engines like on scooters. It will run for 5 to 10 seconds then nothing until it cools down. once it cools down it will start again, but only run for the 5-10 seconds. When was the last time the valves were adjusted? I'd try doing a compression check and a leak down test and see if your leaking out the exhaust valves. Also it's possible the cylinders and valves may have a lot of carbon build up. Some of it may be breaking loose allowing the valves to close a little more which would reduce the amount of combustion escaping causing less popping. if that is happening. some Seafoam in the fuel tank with the next few fill up might help clean that out. One other thing I'd look at would be to see if the carbs are synced together.

[Big T]

Growler, it's a 23 year old bike. You need to give it all the maintenance. I'd also blow $50 and replace the carb holders and exhaust gaskets