Good evening.
Recently i am trying to fix an issue with my fuel mixture, the motorcycle keep making big bangs ramdomly in decel. Not every decel but often.
The carbs are clean, filters are aftermarket but in the original case and stock exaust. I have tried to back up the mixture screw to 2 turns out and seems getting better but not completely. Getting 3 turns out make the issue worse. Some ideas? Thanks.

Look into marbling or coasters. I'll post a link or two in a few. Or I can be lazy and tag in our handy dandy resident AI, @ForaFrank . Tag Frankie, you are it

 

https://www.vn750.com/threads/fix-popping-marble-method.109794/?post_id=1332328#post-1332328

 

Does this mean I am faster than an AI?

 

@OleDirtyDoc It seems my circuits were still warming up while you were already on the case. Nicely done.

For everyone following along, the "marbling" or "coaster" mod is one of the most common and effective fixes for deceleration popping on the VN750. The bike has an emissions system that injects fresh air into the exhaust ports to burn off any unspent fuel. While noble in its intent, this system can cause loud bangs when the bike's air/fuel ratio is even slightly off, which is common with aftermarket filters.

The mod simply disables this system by plugging the vacuum lines that control the air valves. A marble, a ball bearing, or the little rubber plugs from a carb kit (which some call "coasters") are used to block the lines, preventing the valves from opening. No extra air in the hot exhaust means no sudden ignition of fuel vapor, and thus, a much quieter deceleration. It's essentially telling the bike to stop trying to have an after-party in its exhaust pipes.

 

Could have a vacuum leak, mine will do that if I leave a vacuum port open. Backfires like a cannon on long decels.

 

Hi, more than a month is past but i still can't figure out the big backfire in long decel. I have done the marbling mod and closed the emission tube, checked for air leak but couldn't find anything. More suggestions?

 

When you say “air leak” were you referring to an exhaust leak?

 

No i checked if there was an intake air leak. I checked if the carbs are sitted properly, the airbox are properly connected, the hose also and to be sure I washed the air filter sponges.

 

Big bada boom. What are your air screws set at?

 

Did you spray starting fluid around the boots at the heads?

Doc is right, lean idle mixture can cause popping, though normally it doesn't go nuclear. It could also have some dirt in the pilot jets making it lean, it seems like those clog up first.

It can only help to use 1oz of Seafoam per gallon of gas to help clean things up.

I've also read on here that exhaust leaks nearest the cylinder can cause backfire. When the engine is ice cold, start it up and quickly feel for exhaust air at the head and downpipe. Open the throttle slightly and close it repeatedly while checking the flanges for leaks.

It will get hot quickly, so you may have to do one pipe, then let it cool down.

 

The mixture screw is at 2.5 turn out as adviced in the forum, as mentioned i cleaned the jets about 2 month ago and since than in addition i used a bottle of product to clean carburetor fuel system. I didn't yet try with start fluid to check the air leak but only by inspection of the hoses and I will use the soap water (as used for the fas tube in the house)to check the exaust leak. Thanks for the suggestions.

 

Soap won't help with a vacuum leak but water is ok. The difference will be the engine will stumble and RPM will drop instead of rising. You may eventually see some steam at the pipes using water.

It's practically impossible to tell by looking. Tiny cracks sometimes only leak after full warmup.

You running premium or regular gas?

 

For water and soap i mean for the exaust leak so if fumes coming out will make bubble, for the intake i will buy some start fluid or brake cleaner is also fine? A mechanic told me is almost same thing long ago.
I normally use 98 octane fuel often without ethanol but is not always avaiable. Here in netherland there is the 85 ron fuel with 10%ethanol or the premium 98 with 5%ethanol. Just a tank station near my house have the no ethanol gas but i am not always there.

 

Don't use brake cleaner anywhere that it can cause fumes or steam. The vapor from brake fluid is highly toxic especially steam from it

 

Yeah...inhaling ether is preferred.

 

Here's a good read. Safety Alert! Brake Cleaner = Phosgene Gas

 

High octane fuel will cause carbon buildup and will contribute to pre-ignition and backfire.

Never burn brake cleaner that contains chlorine, it creates phosgene gas, aka mustard gas, which was used as a weapon in WWII. Chlorine has been removed from some brake cleaners but there's still some chlorinated out there.

Almost gassed myself with it once, cleaning a throttle body with the engine running. I barely got out of there in time.

 

Do not get me wrong but are you suggesting to run on normal gas 85 octane containing 10% of ethanol always? It will save me some cash.

 

No you do not need high octane fuel. The VN750 calls for 87 octane , if you can get it without ethanol that would be best, but if you have to get fuel with ethanol than it’s best to add a fuel system cleaner like Seafoam.

 

87 with at least 10% corn, is what is common for regular gas here stateside. Thought the 85 was more corn additive for the new wave undesirable vehicles. Would love for you to save cash but the numbers rating for overall octane/ ehtanol have me confused. Probably somebody better versed in this will chime in.

 

Fixed. Main issue was a slight bit of gas trapped in the coast enricher on the right carb. Probably when I moved the carb some gas flow in the wrong direction. Just few minutes job on clean and quite some hours detecting the problem. Anyway I made a mistake about the octane number on the E10 gas, it is not 85 octane but 95 with a max 10% of ethanol. If i use it, can i let the gas in the tank for few weeks? Off course if have to stay for more than a month i will take care to empty the carb and take it out from the tank.

 

I would try to get some Seafoam into the fuel and carbs before leaving it parked. I've had my carbs get plugged up just sitting a couple of weeks. Temperature swings can make fuel degrade a lot quicker. Also, the fuel evaporates a lot quicker due to the carb design with the separate jet chamber.

I'll have to go back and read, but can you not get a lower octane without increasing the ethanol content?

87 octane with 10% ethanol would be better than the 95 octane.

 

No more big bada boom