still have carb backfire

[first timer]

hey there guys, i recently replaced all my carb intake boots and o rings. i cleaned and synced the carbs replaced the plugs. the problem is the bike will stilll backfire slightly out the carbs once in a while, especailly when its cold. The bike runs strong all the way to 10k rpm but another thing i notice too, is if im riding it and keeping it at a steedy mid range rpm and let off the gas for a moment or so then hit the gas again the bike seems to hesitate for a moment, like it is clearing it's throaght before it takes off. i feel that these problems are related, could it be some how related to my mixture screws or needle heights or my air fuel mix. the bike has a jet kit, 4 into 1, k&n pods and accel coils. oh yeah its a 81 gs 550

thanks,
ryan

[Anonymous]

Carb Backfire

Ryan:

I'm sorry that I don't have an answer for you, but I wanted to clarify something: when you say "carb backfire", are you talking about a gust or mist coming out of the carb boot?

If so, I've noticed the exact same thing on my 1984 GS1150ES! I haven't the slightest idea what woud cause it. Very strange.

[AOD]

from Mikuni's guide on carb tuning:

http://www.mikuni.com/fs-tuning_guide.html

3: Backfires Through Carburetor

Common Causes:

Ignition: The factory Evolution engine's ignition can contribute backfiring through the carburetor.
Cam design: Long duration cams with early opening intake valves can contribute to backfiring.
Intake manifold air leak: A lean condition due to an intake manifold air leak can cause backfiring.
Carburetor jetting: An overly-lean low-speed circuit, non-functioning accelerator pump or clogged pilot jet can contribute to backfiring.


Ignition:

Harley ignition systems have been ?dual fire? for decades. Virtually all stock Evolution engines, Big Twin & Sportster, have dual fire ignitions. The exceptions are the EFI touring bikes and the 98 & later Sportster Sport models. All Twin Cam engines are fitted with single fire ignitions. Under normal conditions dual fire ignitions present no problems. However, when combined with high performance long duration cams the stock ignition can cause premature ignition of an air/fuel mixture entering the rear cylinder. This, in turn, results in backfiring through the open intake valve into the intake system.

Dual fire ignitions fire front and rear cylinder spark plugs together. One of the sparks starts combustion while the other is ?wasted? in other cylinder which is not on its firing stroke.

When the rear cylinder is getting a useful spark, the front cylinder?s spark is occurring near the middle of its exhaust stroke. There is nothing to burn in the front cylinder at this time.

However, when the front cylinder is getting its useful spark, the rear cylinder is on its intake stroke and a combustible mixture may be present. If that mixture is ignited by the ?wasted? spark, then a backfire occurs as the burning mixture forces its way past the intake valve and out through the intake manifold and carburetor.

Single fire ignitions can often eliminate carburetor backfiring since they do not produce a wasted spark in the rear cylinder. In fact, single fire ignitions can generally eliminate backfiring in any Harley. For instance, EFI and Twin Cam engines very seldom backfire through their intakes; both have single fire ignition systems.

Cam design:

The earlier the intake valve opens the more likely the dual fire ignition will ignite air/fuel mixture in the rear cylinder. High performance long duration cams open the intake valves earlier than the stock one. This is the main reason why modified Harley engines tend to backfire through the carburetor more frequently than stock engines.

Intake manifold air leak:

A common and continuing problem with Harley engines is air leaks around the junction of the manifold and the cylinder heads. Carburetor/manifold leaks are much less common. An air leak can cause carburetor backfiring.

Other symptoms of an air leak include a slow return to idle or an irregular idle.

Carburetor jetting:

Excessively lean carburetor settings can contribute to backfiring. If the mixture is too lean, it may burn very slowly and unevenly. This condition, in turn, may result in burning mixture remaining in the cylinder until the beginning of the next intake stroke when it can ignite the incomming air/fuel mixture.

A too-small or partially blocked pilot jet can bring about this condition.

An accelerator pump adjustment that starts the pump too late can cause this problem.

A partial vacuum in the fuel tank can reduce fuel flow and bring about a lean condition. The common factory Harley gas cap that incorporates a one-way valve (for emission purposes) sometimes restricts air flow into the tank. This restriction can result in a partial vacuum and fuel flow restriction.

[KEITH KRAUSE]

I re-jetted my '79 GS1000 a few years ago with a Dynojet kit. I also went with pod's and a pipe. I got it running great at every rpm EXCEPT for what I call a little bit of 'spitting'. Only a few times at cold start ups and would happen a bit more if it was really cold in the morning. I tried going up several pilot jet sizes with no luck. I did have better luck adjusting the pilot and air screws, but could'nt totally eliminate it. I even called Dynojet and they said that it is characteristic of some stock carbs when adding pod's. I finally gave up trying to get the pilot circuit perfect. I can live with it. You may have to do the same. I don't think your pilot screws are adjustable like they are on '78/'79 models. If you try larger pilot jets you may have better luck than me, but don't count on it. Your mid-range hesitation would most likely be needle related. Are the needles in their stock position after the pod/pipe mod's? If they are, I can guarantee you the bike is running lean and hot. Pod's give a major increase in air intake. You have to change the needle position and increase the main jet size to have the bike running it's best.

[Anonymous]

backfire

I have the same problem on my '82 GS550LZ (12,600 miles) a slight pop or puff usually on deceleration and when hitting the throttle after decelerating. Also a lack of power above 6000 rpm. My hunch is one of two possibilities- 1) The valves are out of adjustment, or 2) The rubber carburetor diaphragms that lift the jet needles are old/tired/stiff and don't react quickly enough to provide the correct mixture. I pulled the valve cover and found that all but one intake valve are out of adjustment(less than .001" clearance), so I will do the adjustment/shims and repost with results.

[first timer]

what kinda filters are you runnin, did you modify your bike at all?


ryan

p.s i'm to tired right now to write replys in regaurds to my bike.

[Anonymous]

backfire

Ryan-My bike is all stock with the factory airbox. I think that if you modify the air intake with those CV carbs you are asking for jetting trouble, unless someone with a flow test rig does the testing and gives you the jetting recipe, like in a Kit. I'm waiting for the valve tappet depressor tool to come in, should be here early next week. No shops in the area would lend me the tool, or do the valve adjustment for me until the end of next week so i'm stuck till the tool comes in. I had to order the valve cover gasket from Suzuki anyway. I will post as soon as I do my valve adjustment. BTW the factory service manual says to check the valve adjustment roughly every 3500 miles, (it's been about 10,000 miles since my last adjustment oops!) so i'm not surprised they're out. Tom.

[first timer]

yeah tom let me know how the shims work, my bike has about 3500 miles on it and has never had a shim job. but its odd though cause the back fireing ( it sounds like a small *poof* noise) i'm just worried that it will blow out my carb boots agian. this poofing and slight hesitation are fairly new to the bike and the bike was running good when i first installed the jet kit. could the bike be running to hot? what are some signs of the bike over heating? could it just be my timing, even though the book sez you can't adjust timeing on the 81 + models. anyone have a anwser for that. what would be some syptoms of my bike running to rich, i'm talking not exceively rich syptoms but slightly rich. well i know left lots of questions but thanks for the help.

ryan

[KEITH KRAUSE]

You say you installed a jet kit. That does not mean it's been jetted right. If you simply followed the "base settings", this is just meant as a starting point. I've never seen any bike get jetted right on the base settings. If you have a valve(s) that are tight, that too can cause spitting out the carbs. That was the first thing I checked after putting about 50 miles on my new motor built by Vance&Hines. The clearances were good and I then discovered that "base settings" does'nt mean you're done jetting. Try checking the valves, it may be them, but it sounds to me like you are running too lean. Do a plug read. Spitting at idle/lower rpm's is not just the pilot circuit, the needle position can effect this too. It's called the overlap effect of the 2 circuits. You will also get some surging and hesitation most noticable at cruising/steady speeds and a spit or two. If your plug read shows a lean color, try raising the needle 1 position. If your carbs don't have an adjustable pilot screw, then do they have a mixture screw? Try adjusting these if you have them or try 1 size up on the pilot jet, this is in ADDITION to the needle change. As for the mains, they usually give you 2 sizes, so it's pretty easy to figure out which one works best. You asked about richness symptoms. Some black exhaust out the pipe when you rev' it. Dark plugs. Worse gas mileage. Sometimes taking off from a stop you will get a quick bog or cough. Loading up at a long idle and seeing lower idle rpm's or a stall. Not much need for the choke in the morning. There are a lot of things that can happen, but with the pod's and pipe and the symptoms you first described, you're running lean. Some jet kits will warn you: installing a jet kit may compromise "driveability". Which means sometimes you have to live with a minor annoyance or two, even if you have jetted it the best you can. After a LOT of trial and error, my bike runs strong at every rpm, good plug color, I lost only 2 mpg, etc, but I still get a few spits on colder mornings. I ended up going 1 1/2 needle positions richer than their base setting and I also got better top end performance using the larger main in the kit, NOT the smaller base setting one.