Exhaust popping

[William_]

Hi all,

I'm just wondering if anyone would have any pointers regarding a popping which is occurring out of the right hand side exhaust pipe. It also sometimes backfires out of the right side, this backfiring starts after it has been running for about a minute but stops after the bike has been drove for a while.

The backfires and popping can be embarrassing!

When driving, it bogs down a little bit at lower rpm but still pulls hard at higher rpms. The bike also spits out of one carb occasionally when heating up (top right one near the ignition switch).

The bike is mostly stock except for delkevic back boxes and an aftermarket cdi box (carmo).

So far I have tried/checked:
-checking for air leaks around boots (a bit cracked but sprayed with fluid and appears to be no leaks)
-pea shooter
-carb sync (gauges are showing low vaccum as can be seen in video below)
-exhaust leaks - there is a tiny leak on the balance pipe, really small cant imagine this is the cause.
-gas tank - a lot of surface rust on the tank spout, cant see in very well but i imagine there is some inside too.
-fresh fuel
-new plugs

This is only occurring on the right side. Is there anything else I should check? Video link below



Thanks

 

[Fire-medic]

Pull the carbs, remove and clean all four of the pilot jets. The ones on the right side may-be obstructed.

Is your right front or rear exhaust pipe cold at idle? That would be, compared to other cylinders. That's a sign of a blocked pilot jet. At higher rpm's, the cyl fires because it's getting fuel from other circuits for fuel delivery. The engine revs normally, above idle or off-idle.

 

[William_]

Thanks Fire-Medic.

I never actually measured any of the temperatures, I'll check this first then pull the carbs and clean.

 

[Fire-medic]

My understanding of 'popping' is that is a lean condition, and if your pilot jet(s) is/are plugged, at idle and off-idle, that's when you should be experiencing what you have-been.

A temp probe, one of those Harbor Freight infra-red ones, is a good investment, you can use it for all-sorts of things, and one of those is checking your exhaust pipe temps.

However, you can do it the old-fashioned, 'shade-tree' way, and just spit on the pipe you think is not firing. A more-sanitary way is to use a squeeze bottle, and to spray the pipe in-question. The 'hot' pipes, the water vanishes, almost immediately. You hear and see it sizzle, it's evaporated quickly. The lazy, non-firing at-idle pipe will have the water run-down the pipe, evidence that it's nowhere-near normal operating temp.

This method is much-kinder to your finger pads, should you be brave-enough to touch the exhaust pipe in-question. You can use the back of your hand, in close proximity but not-touching the pipes, to sense the heat levels, but a squeeze bottle of water is graphic, audible proof of what's happening, when you spray the pipes.

Two other things which can affect your low-speed operation, once you have clean pilot jets, is to balance/synchronize your carburetors, and also adjust your air bleed screws, below your CV carb diaphragm caps. If you remove and split your carbs to clean the pilot jets (you only have to split them in pairs, front and back, you don't have to disassemble them into four separate carbs) then you need to synchronize the carbs when you re-install them. If you don't, your carbs can be so-far-off from the correct setting, the bike will run poorly. Also, setting your air bleed screws for best idle is quick and easy, and once you've done a synchronization and air bleed screw adjustment a couple times, it becomes easy, and a part of your maintenance.

 

[William_]

I have an infrared thermometer so I will check the temperatures tomorrow.

I wasn't sure about the rust in the tank so I am going to use por 15 tomorrow also so that should eliminate that as a possible cause.

Thanks again I'll report back with findings.

 

[Fire-medic]

If the tank is shiny, and no corrosion is visible, then treating the tank is probably not required. If you scoured the tank because you saw very-evident corrosion and you used Evaporust, cleaning vinegar, some type of acid, handfuls of small hardware nuts, ball-bearings, etc, to break-loose all the rust, and you've rinsed the tank thoroughly, then that's the time to use a tank sealant. Be aware that the water vapor left in the tank can cause a 'flash' of new surface rust, in minutes after a cleaning. Also, use a block-off plate in-place of the fuel level gauge. Some steel strap to make a plate, use the fuel sender switch to get the hole-spacing correct, and then smear some sealant on the tank, where the block-off plate will sit, and snug it down.

 

[William_]

Just to update this.

I pulled the tank and this was the result, excuse the photo hard to get a good one.



And a bonus picture of the "recently serviced" fuel filter.



Coated inside of tank with por-15, waiting on new fuel sender gaskets for re-install so fingers crossed this solves it.

 

[C2protect]

With my 85 Vmax it sounds like your carbs are not synced correctly. I would say sync your carbs make sure they're accurate as well as everyone else said clean and make sure the pilot jets are clean as well. The most inexpensive route is having it synced. If it CAN'T be synced... Then there is the reason, your pilot's are plugged.

 

[C2protect]

Also with the first generation Vmax has it is crucially important that they are synced correctly. And when they are synced correctly you will notice an amazing difference in performance and sound and how the vboost actually performs.

 

[William_]

Thanks c2protect. I am still waiting to put the tank in, I figured lockdown was as good a time as any time get all these other jobs done!

The gauges I have are a cheap ebay ones which it seems the smallest knock will put them out of calibration. I suppose it's a case of buy cheap buy twice. I'll retry them once I get the tank in and just buy a better set if they wont keep calibration long enough to sync it.

 

[William_]

Sorry for all these updates!

I dove into the carbs today, just attempting a shotgun before pulling them completely. I noticed that the pilot screws on the right side (cylinders 3 and 4) are a bit corroded. The pilot screw spring on cylinder 4 is corroded so bad that it is seized in the compressed state.

I guess I will replace the set on the right cylinders. Are the springs important?

 

[billy1ear]

My first thought was a air leak between the carbs and the cylinder head, or in the carb, but it looks like you've tried to test for that already. Have seen that cause popping more than once. I've seen the throttle shaft leak air before when it is worn.

 

[desert_max]

I am thinking that, based on the look of that tank and filter, and your observation of a corroded idle circuit spring, that a complete tear down and refurbish of those carburetors would certainly be in order. I would be seriously skeptical about the efficacy of a shotgun on carbs that are likely to be that dirty.

 

[William_]

Thanks for the responses billy1ear and desert_max. The work so far has stopped the popping but the idle is inconsistent and after a short ride a float needle had gotten stuck (likely from that crap that was in the tank still in the lines, drained bowels and still a lot of rust).

I have pulled the carbs and they arent particularly dirty but I guess it doesnt take much to block the idle circuit. In combination to this I have found the carb boots are badly cracked and the previous owner seems to have tried to solve this with instant gasket. So I've ordered new ones to go with the cleaned carbs. Thanks again everyone, hopefully it will soon run like it was supposed to!!

 

[William_]

Well this is now resolved the bike is idling perfectly at 1000ish rpm. No popping and no backfiring and it seems to rev cleaner (although that could be placebo).

Quite a lot was done in the end, tank resealed, new mixture screws, new fuel filter, carbs cleaned, sync and new intake boots. But at least its sorted now thanks everyone.