Backfire Issue

[Josh Stanley]

Hi from Texas,

I am almost done with my Vmax. It is a 97 and when I got it, it was running on two cylinders. I cleaned the carbs, cleaned the tank, put a fresh inline fuel filter in, checked timing, checked all electrical connections and put a new batter in. It just wont start! I'm not sure what else to try. The issue I am having is that the front two cylinders are backfiring through the carbs. Has anyone had this issue before? Is it the timing? What should I look for? Very frustrated!!

Josh

 

[liptoss]

How'd you clean the carburetors? Using a disassembly or pea shooter method posted on the forum? Sounds like possible plugged carbs to me.

 

[caseyjones955]

Does it have COPs? If so the two front cyl cross. Disregard if you have OEM coils.

 

[MaxMidnight]

Given that the ignition is triggered by a single pick-up and the timing is fixed (i.e. no adjustment provided for) then I would rule out ignition.

Plugged idle jets I would expect to cause poor or erratic idling rather than a back-fire.
My immediate thought was cam timing on the front inlet...have you checked that?

 

[Traumahawk]

Someone else will probably have to chime in.......but doesnt the early Vmax's have 2 pick-up coils, so one can go out, and 2 cylinders will still fire on the other pick up coil?

 

[[email protected]]

Someone else will probably have to chime in.......but doesnt the early Vmax's have 2 pick-up coils, so one can go out, and 2 cylinders will still fire on the other pick up coil?

85-89 dual pickups
90-07 single pickup

 

[Josh Stanley]

Thanks for the replies. I am not sure what a COP is. My bike has two coils un under the instrument panel. My thought initially was that maybe the previous owner has them hooked up wrong. Do the plug wires cross? Does the left coil provide spark for the right front cylinder? Or does each side provide spark for the calendar on the same side?

When I checked valve clearances, we checked the timing. Using the manual, It looks as though all the valves are opening correctly and are in time with each cylinder.

I took the carbs to a mechanic who methodically went through each one cleaning them as he went. I am considering bringing it back to him and make him figure out why it wont run. However, I would like to be able to get this thing running on my own.

Any advice would be helpful
Josh

 

[davesax36]

Front coils cross, yes. Will it sort of run and just backfire through the carbs or will it not even turn over? I bet that mechanic got some stuff wrong, especially if he didn't take the jet blocks apart.

 

[Josh Stanley]

It wont even start. It will crank and stumble and try and then backfire through the carbs. The mechanic did take them completely apart and cleaned everything. He was very methodical about it. The only questionable thing is that the air fuel adjustment was 3 turns out. I remember reading somewhere that they should be between two and two and a half. Is that correct?

As I look at the bike, the coil wires do not cross but run down to the spark plugs on their respective sides. Maybe that is it.

JS

 

[Traumahawk]

Looking from the back of the bikes towards the front, the coil that is on the right front......the plug wire from it, will go the the spark plug on the left front, and vice versa.

Do you know if they were changed? Or swapped?

 

[davesax36]

It wont even start. It will crank and stumble and try and then backfire through the carbs. The mechanic did take them completely apart and cleaned everything. He was very methodical about it. The only questionable thing is that the air fuel adjustment was 3 turns out. I remember reading somewhere that they should be between two and two and a half. Is that correct?

As I look at the bike, the coil wires do not cross but run down to the spark plugs on their respective sides. Maybe that is it.

JS

The rear ones go straight to their plugs, but the front ones cross.

There are two A/F screws on each side (one per carb). It could be a really bad sync issue. If they're not reassembled perfectly level, you can get all kinds of weird stuff happening. Those sync screws are sensitive. You might try to bench sync them just to get all the butterflies opening the same amount when the throttle is closed. I use a single piece or wire cut off a strand of 16ga or something. Take the lid off your airbox and see if they flies are open the same amount when the bike is sitting. If the front two are off, you could have issues.

I still mostly vote for crossed coil wires, though.

 

[Josh Stanley]

That was it, the front wires were not crossed. Looking from the seat forward. the coil on the left went to the left spark plug and the coil on the right went to the right plug. I crossed them and it fired. Thanks for all the assistance.

 

[Bill Seward]

That's one of those times you scratch your head and wonder what the hell they were thinking.

 

[liptoss]

Good to hear you were able to resolve it. Happy riding!

 

[Josh Stanley]

I realized that the wires on my front two cylinders were going to the wrong cylinders. I crossed them and the bike fired up and ran for the first time in a year. Now I can move on to fork seals, brake master cylinder and paint. I hope to have it on the road in a week or two. Then, its up for sale!!

Thank you all for your help!!!

Josh