Backfiring thru all 4 carbs.

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fatbiker

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I have a 2005 Vmax with 15k miles on it. When I rev above 2500 rpm intermittent backfiring occurs thru all 4 carbs. If I let out the air/fuel mixture screw 6 times it will stop on all but one carb. I know that that is way too many turns and indicated an air leak somewhere. I have used two sets of known good carbs including one set that was recently rebuilt by DannyMax that run perfectly on my 86 Max so I don't feel the problem is in the carbs. I have installed new rubber between the airbox and carbs plus new rubber between the carbs and the intake. I have not changed the rubber located between the vboost throttle/intake sections. The bike charges great, gives a good spark, and is running a Initech TCI and COP kit supplied by Sean Morley. Plugs are gapped at .026 .

Any idea what the problem could be? Valves needing adjusting? New rubber between the vboost intake? Something I'm missing?

Thanks in advance!

Fatbiker
 
Ok.....well back firing thru the carbs is caused by a lean condition....with everything that you have done, I would suspect the carb boots. My 07 I recently changed the carb boots on, and putting the carbs on is SO much easier now.....and they really werent that expensive from Sean.

Also when the changed the carbs did you change the fuel filter? Is there a possibility that a bit of trash got sucked up into Jet that the carb runs off at idle and up to 4000 rpm....it is 37.5....and so very tiny. That is what you are cleaning out when you do a shotgun.
 
fatbiker said:
I have a 2005 Vmax with 15k miles on it. When I rev above 2500 rpm intermittent backfiring occurs thru all 4 carbs. If I let out the air/fuel mixture screw 6 times it will stop on all but one carb. I know that that is way too many turns and indicated an air leak somewhere. I have used two sets of known good carbs including one set that was recently rebuilt by DannyMax that run perfectly on my 86 Max so I don't feel the problem is in the carbs. I have installed new rubber between the airbox and carbs plus new rubber between the carbs and the intake. I have not changed the rubber located between the vboost throttle/intake sections. The bike charges great, gives a good spark, and is running a Initech TCI and COP kit supplied by Sean Morley. Plugs are gapped at .026 .

Any idea what the problem could be? Valves needing adjusting? New rubber between the vboost intake? Something I'm missing?

Thanks in advance!

Fatbiker
Click to expand...
If you have a carb sync tool hook it up (set your a/f between 2 1/2 and 3) and spray a small shot of wd-40 on the intake boots--all 8-- 1 at a time and see if you have a change on the sync.
I doubt your valves need adjusting at 15k unless you are hard on it.
Color of plugs? Any debris?
 
Carb boots are brand new. I have two sets of carbs that I have put on the bike and it backfires through all 4 carbs on both sets. One set was recently rebuilt and runs great on my 86 max. I know that set of carbs is in good shape as it does not backfire at all on the 86 Max. I know the problem is with the 2005 Max but I cant figure out what. Has anyone had problems with the rubber in the vboost manifold leaking? Can the valves being off cause backfiring?
 
fatbiker said:
Carb boots are brand new. I have two sets of carbs that I have put on the bike and it backfires through all 4 carbs on both sets. One set was recently rebuilt and runs great on my 86 max. I know that set of carbs is in good shape as it does not backfire at all on the 86 Max. I know the problem is with the 2005 Max but I cant figure out what. Has anyone had problems with the rubber in the vboost manifold leaking? Can the valves being off cause backfiring?
Click to expand...
I would say pilot fuel jets in the jet block are clogged.
 
Carbs are seated all the way in. When installing them, do you use any kind of sealant or silicone?

If the pilot fuel jets are blocked or clogged then the carbs would backfire on my 86 Max as well. Both sets of carbs run perfect on the 86 Max. They only backfire on the 2005 Max. I have taken the airbox rubber and intake boots off the 86 Max and moved them to the 2005 Max. They still backfire on the 2005 Max.
 
No sealer recommended. I use a little wd-40 to help installing though. Make sure the caps are on the vacuum ports. Map sensor line on? I know this is very basic,but simple stuff gets us all sometimes.
Steve-o

Also, yes the V-boost rubbers can and do crack, under the clamps too, sometimes. That would be something common to just the 2005. I'd inspect them.
 
Last edited:
I just put my carbs back on my bike after rebuilding the trans, actually had to take them off cause I forgot to hook up the v-boost linkage (duh), but when I pulled them off I noticed I actually pushed them too far down and they were past the little ridge on the inside of the boot, could this be your problem????
 
Traumahawk said:
Ok.....well back firing thru the carbs is caused by a lean condition....with everything that you have done, I would suspect the carb boots. My 07 I recently changed the carb boots on, and putting the carbs on is SO much easier now.....and they really werent that expensive from Sean.

Also when the changed the carbs did you change the fuel filter? Is there a possibility that a bit of trash got sucked up into Jet that the carb runs off at idle and up to 4000 rpm....it is 37.5....and so very tiny. That is what you are cleaning out when you do a shotgun.
Click to expand...

I think this is the correct diagnosis................Take some carb cleaner or WD40 and spray around the entire motor while idling and listen for a change in idle to find out for sure that your motor is vacuum tight. Spray a little and listen. Spray everywhere top, bottom, front, back...........good luck.
 
No yet. I'm going to send the carbs to DannyMax next month and let him clean them out. Probably a few gaskets need replacing also. I will update and let you know if that fixes the problem.
 
Well after removing the old gas in the tank from when i bought it, i just replaced the oil and filter and replaced the nasty fuel filter, took the bike out yesterday and noticed at lower rpm's a popping from the carbs every once in awhile, it was not there before i replaced the fuel filter, also I smell a bit of gas when I am sitting at the light, could this be to rich???
 
Check out P/U coil ohms, sometimes they go out like a light bulb, sometimes they start by your bike running bad. I had this in my Vmax folder notes, NOTE***This for a 90-07 model P/U coil check.

Checking is easy. Remove seat and l.h. side cover, find the two-wire (orange and black) pickup coil connector. Measure the resistance, on the coil side of the connector, between the terminals. Should be 80.8 - 121.2 ohms @ 20degrees C
(room temperature, bike cold). Last time that I checked mine it was 101.6 ohms.
Then I would start and warm up your bike to operating temps, or preferably till it starts to stall. Then quickly shut the bike off, and check the resistance again. If still within range, the p.u. coil is probably not your problem.
 
alorio1 said:
Check out P/U coil ohms, sometimes they go out like a light bulb, sometimes they start by your bike running bad. I had this in my Vmax folder notes, NOTE***This for a 90-07 model P/U coil check.

Checking is easy. Remove seat and l.h. side cover, find the two-wire (orange and black) pickup coil connector. Measure the resistance, on the coil side of the connector, between the terminals. Should be 80.8 - 121.2 ohms @ 20degrees C
(room temperature, bike cold). Last time that I checked mine it was 101.6 ohms.
Then I would start and warm up your bike to operating temps, or preferably till it starts to stall. Then quickly shut the bike off, and check the resistance again. If still within range, the p.u. coil is probably not your problem.
Click to expand...

Sounds easy to check, I will check this week Thank You!
 
I just wanted to add Sean Morley has a loaner program, where he will send you a CDI and a P/U coil. If one or both are the problem naturally he would expect you to buy the bad component and send the one you don't need back to him. Sean Morley is great about helping members on the forum by having such a program, I don't think you will find this any place but here on this great forum you are a member of..........Joe
 
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