What causes Backfire on acceleration?

Greetings...

89VR

Just synced the carbs (using my $3.50 homemade mamometer) this morning.

Starts very easily.

Idles perfectly like a bumblebee at just under 1000.

Accelerates wonderfully (after the chuffing stops anyway).

Sounds like it's in the airbox - and not in the mufflers.

In fact - all in all - other than this annoying low rpm backfiring - it runs perfectly...

It only happens in first gear, usually on a slight hill. It will not happen if I open the throttle very very slowly and ease it up to over 1800 rpm, at which point I can rocket it if I want. But If I try to beat the next light - It will POP POP POP until it reaches about 1800 - 2500 or so...

I'm thinking it might just be bad gas - as I did fill up last night, and did the sync this morning, and it's been happening all day.

But anyway - I'm really wondering just exactly what causes backfire on ACCELERATION - it does NOT backfire at all on decel...

Timing? But If it were timing, I would think I would have other problems too...

I plan to switch out the plugs tomorrow morning, and possibly all the filters too if I can find the cash. So maybe all the running around town will let me get a new tank of fuel from another source in and see if that's all it is...

And yes - she's already guzzled 2 cans of seafoam in the last few months - Fwew! The Price of that stuff has doubled in the last year!!

Are you sure that your homemade manometer is accurate? You really can't beat a Carbtune which lets you compare all four carbs at the same time.

My best guess -- you are way out of carb synch now.

Greetings...

Accurate enough. It wasn't starting instantly, It wasn't idling well, or accellerating well, had a lot less power and general quickness than it now does. There was a heavy gas smell yesterday, All that's gone now...

No - It's probably not my sync. It may not be "Perfect" But it's close enough for the girls I go out with. Besides the difference between the accuracy of my homemade tube and oil tool, and some overpriced fluffy bit of needlessly high tech will wear off in a week anyway assuming all other factors.

Anyway - What I really wanted to know is just exactly what causes backfire on acceleration in the first place. If I know the physical cause, I can come up with a diagnosis and a solution. It's not a problem I've experienced before.

I think it's the gas - I'll find out tomorrow after changing the plugs and filters...

If popping on acceleration...check for vacuum leak....the caps that seal the ports you hook your manometer hoses to, or the hose going to the pressure sensor for timing advance might be cracked or left off...sucking in air, leaning out intake.

backfiring into airbox is a sure sign of lean condition. Are your slides free, diaphragms solid?

Hey Kandaje,

I'm gonna' give you the list of other things to check.

Check the clamps on the Carb. boots for tightness, (there are two sets)

Check the boost sensor (connected to #1 cylinder)

Download the First Gen, Service Manual off of this site

Make SURE you put the 3 plugs back on the ports where you did the Carb. Sync. and make sure that are not cracked or leaking

Check to see if you pulled the line off of the boost sensor (behind radiator) while plugging the 4th line back up (#1 cylinder).

When was the last time you looked at the Carb. Diaphragms?

Have you checked the Idle mixture screws?

Now, all of these wonderful suggestions aren't going to amount to a hill of beans because what REALLY is going on is the FACT that the right bank and the left bank of your engine are not synced properly, and the low vacuum side of the engine is puking unburned fuel back into the airbox. So, unless you are wearing asbestos underwear while you ride, :crackup:check the VR.ORG members list, find someone with one of those overpriced "fluffy" needlessly high tech Carbtunes close to you, swallow your pride, ask them over for lunch, sync. your carbs. have a "victory" adult beverage, go for a ride, and be happy! You can do it. Just because you can drive in a nail with a 5 pound mall, why would you want to?

My thoughts, for what they're worth,

Earl

Also, loosen the clamp under each carb. The, remove the Air cleaner box.

Now lift the bank of 4 carbs UP , out of the Intake manifolds.

Now, check the circumferance of the top of each Intake manifold.

One of the Rubber Intake manifolds , where carb mounts might be ripped, broken, or baddly leaking.

Last week, are had Popping at idle on my 89 ( a new problem ) and I could just barley detect a sucking sound from the left side.

I found the #1 Intake manifold rubber ripped open, Had to replace the manifold.

See this thread: http://www.venturerider.org/forum/showthread.php?t=35810 see my photo

The correct setting for the Idle mix adjust jets, is 2 1/8 to 2 1/4 turn CCW from full bottom position ( DON'T OVER TORQUE WHEN BOTTOMING THE JETS !!!!! ) But missadjustment of these jet should not cause popping.

Also: Check, the TWO Allen Head Bolts, Holding the Intake Manifolds to the Cylinder heads. The INNER bolts on the manifolds might be loose. They are hard to get too. Eaisest way is to Remove the Air Cleaner Box, and go in with 1/4 Extension. use Allen on a 1/4 socket, with a 1/4 Swivil drive

Each manifold has an O-Ring seal. But you might have to Remove Manifold, and apply gasket Sealant, IF, Leaking.

Greetings...

hehehe....

|| I'm gonna' give you the list of other things to check.

Cool!

|| Check the clamps on the Carb. boots for tightness, (there are two sets)

Clamps are Tight but the boots are cracked but not leaking. But now that you mention it - there IS a leak somewhere - I can hear a slight hiss that stops after a second whenever I throttle up right about that rpm range. I'll recheck those tomorrow when I change the plugs.

|| Check the boost sensor (connected to #1 cylinder)

You mean cylinder #2 of course - the left side forward one...

|| Download the First Gen, Service Manual off of this site

Got it. Did that a year ago. Same day I got the bike.

|| Make SURE you put the 3 plugs back on the ports where you did the Carb. Sync. and

|| make sure that are not cracked or leaking

They're all on.

Not leaking but definately cracked. Repaired with Black RTV.

|| Check to see if you pulled the line off of the boost sensor (behind radiator) while

|| plugging the 4th line back up (#1 cylinder).

It's still hooked up. I'll double check again tomorrow in the day light - Its a possibility that I may have yanked it off when I burned myself hooking up the mamo-tubes for #4 and #2 (right side/left side)...

|| When was the last time you looked at the Carb. Diaphragms?

Never. They were new a year ago, though I've heard rumors that backfires can punch holes in them. So that's a distinct possibility

|| Have you checked the Idle mixture screws?

Nope. Don't even know where they are...

|| Now, all of these wonderful suggestions aren't going to amount to a hill of beans

|| because what REALLY is going on is the FACT that the right bank and the left bank of

|| your engine are not synced properly, and the low vacuum side of the engine is puking

|| unburned fuel back into the airbox. So, unless you are wearing asbestos underwear

|| while you ride, :crackup:check the VR.ORG members list, find someone with one of those

|| overpriced "fluffy" needlessly high tech Carbtunes close to you, swallow your pride,

|| ask them over for lunch, sync. your carbs. have a "victory" adult beverage, go for a

|| ride, and be happy! You can do it. Just because you can drive in a nail with a 5

|| pound mall, why would you want to?

uhmmm.. 'cous then you only hafta hit'er once? hehehe....

No one within 350 miles, over the worlds largest flat top mountain, at least that I know of, who owns one of the gosh darn new fangled devices, and knows what the term Venture Royal actually means, or the term synchronization...

So - according to all the posts so far - the culprit for backfire on acceleration, the answer to my question - is Lean mixture. either due to calibration settings or Vacumn leak. Or unbalanced Carbs.

Well, I've performed this operation successfully maybe 20 times in the last year using the very same tools - So, I'm going to assume that by now I'm at least competent doing it and that the carbs are synched properly.

Though while I'm not ruling out having made a stupid error - I find myself doing that more and more these days - But I think I'm still going with the bad gas theory until I can definately rule it out.

If I could afford even one beer, or lunch for myself - let alone anyone else - I'd drink myself into forgetfulness and not worry about it. It's not pride that forces me to use home made tools - it's the lack of ObamaBucks...

|| My thoughts, for what they're worth,

|| Earl

Pearls of wisdom!

Infinitely more than what I paid for them! - Thank you Earl! I've definately got a lot more checking up to do tomorrow on the bike than what I was originally expecting.

Hey Kandaje,

Glad to see you have a sense of humor! I don't suggest this lightly, I would hate to envision you dancing around your VR in a fireball but you seem to have a pretty good head on your shoulders so here goes, start up your bike, roll on the throttle till you get to that "pesky" area around 2000 RPMS, (you could even adjust the idle screw) mist some Carb. cleaner around the boots, bottom of the air box, intake manifolds, etc. if you have a leak, the engine will rev somewhat when intaking the Carb. cleaner, my eyes aren't what they used to be but i can sure hear a motor rev up a few RPMS. This should localize the air leak pretty quickly for you. I have good used parts if you need them, shame you don't live closer, I'd buy you a beer!

Earl

Greetings...

Hey Kandaje,

Glad to see you have a sense of humor! I don't suggest this lightly, I would hate to envision you dancing around your VR in a fireball but you seem to have a pretty good head on your shoulders so here goes, start up your bike, roll on the throttle till you get to that "pesky" area around 2000 RPMS, (you could even adjust the idle screw) mist some Carb. cleaner around the boots, bottom of the air box, intake manifolds, etc. if you have a leak, the engine will rev somewhat when intaking the Carb. cleaner, my eyes aren't what they used to be but i can sure hear a motor rev up a few RPMS. This should localize the air leak pretty quickly for you. I have good used parts if you need them, shame you don't live closer, I'd buy you a beer!

Earl

Got the can o' carb cleaner right here! In fact - had to run over to my neighbor's house to borrow it and a can of beer! Bonus he even threw in a half pound of roll yer own tobacco that he no longer smokes...

As my Grampa used to say - Gettin older just means that I'm getting better at hiding my mistakes from others - But the Tersiphone catches all and allows none. Without a sense of humor - I'd be ugly and ornery! LOL!!

The beautiful thing about motorcycle mechanics. Cut's though all your self delusions and gives you nothing but truth...

You know - I did try something this afternoon that sorta solved the issue - I adjusted the idle UP to 2k and narry a hiccup on the test drive. But then I thought that's not a real fix - that's just treating the symptom and set it back down to normal...

(BTW - the only carb that was out of sync was #1 this morning, I only had to fiddle with one screw... Well, 2 - I also played with the idle speed adjustment too...)

I had to do that last winter just to get the bike to run in the cold and wet. My TCI was staying damp all winter and giving me constant fits and that was the only solution short of a total tear down to get it out that I could find. I'll reread the manual tomorrow morning - I remember reading that there's another air system that operates under 2500 rpm? Now I'm wondering if maybe something blew in there in last nights 60 mile an hour winds we had. The humidity has been 0% for the last few weeks, and the dust has been really bad out here...

... and knows what the term Venture Royal actually means, or the term synchronization...

 

I've been to Yamaha Dealers that don't know any of that. One told me that carbs never need synchronization, the only solution is complete carb rebuilds. He must be buying a new boat and needs to make extra money for the payments.

Greetings...

It rained yesterday, so I had to wait till today-

But - The new spark plugs solved the chugging effects...

#'s 1, 3 and 4 were all black - definately lean. #2 still looked brand new though it's a year old. But then, I've run a lot of seafoam through her since last winter - and that alone can really foul the plugs. So No real definate cause can yet be scientificly determined.

She runs like a top now - a very very quiet top too... Blessed silence..

But I guess we can add fouled plugs to the list of things that can cause backfire on acceleration...

Greetings...

 

It rained yesterday, so I had to wait till today-

 

But - The new spark plugs solved the chugging effects...

 

#'s 1, 3 and 4 were all black - definately lean. #2 still looked brand new though it's a year old. But then, I've run a lot of seafoam through her since last winter - and that alone can really foul the plugs. So No real definate cause can yet be scientificly determined.

 

She runs like a top now - a very very quiet top too... Blessed silence..

 

But I guess we can add fouled plugs to the list of things that can cause backfire on acceleration...

Kandaje....

black plugs are from running rich, causing carbon to deposit on the tip....

You say #2 cyl looked brand new...if you mean still white....that generally means running very lean causing high firing temps and overheating tip....(or is that cylinder actually firing??)

A tan brown color is what a normal plug tip should look like after being in service for awhile.

http://www.ngksparkplugs.com/tech_support/spark_plugs/faqs/faqread.asp

Greetings...

Oops - I was thinking faster than my fingers could type...

I based my lean judgment on Plug #2 - Too lean. The porcelain was white - not the light chocolate that it should have been...

I suspect the black deposits on the rest were a result of a combination of 2 other things - me screwing with the idle and sync all winter dealing with a wet TCI and excessive overfeeding of seafoam...

In any case - As soon as the sun comes out again - I'll take her for a spin over the Grand Mesa and pull the new plugs and see what's up...

Well, I have a sync tool in SLC - I just saved you 70 miles!!

Let me know if I can help but it sounds like you're almost there.

Dave

Greetings...

Well, I have a sync tool in SLC - I just saved you 70 miles!!

 

Let me know if I can help but it sounds like you're almost there.

 

Dave

hehehhee...

SLC being about as far away as Denver, just in the other direction!

But hey!

I'm up for the ride just to put my bike on your tool - just to prove to everyone that my home made tool is "accurate" enough!!

Unfortunately - the 4 tanks of gas that it would take, are more than I can afford at the moment...

But I think that when my economic situation gets a little better - It sounds like a fantastic idea - That should be completely documented! You got a video camera?

I come to SLC, we go down to Lowes, I buy the materiels and construct the device - sync my carbs with it, then hook up to your High Tech machine and check out the sync.

A true battle between Good vs Evil.

The trailor trash practical vs the High Tech.

Simplicity vs the overly complicated corporate beast system...

hehehehhe.....