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RUNS ROUGH-BACKFIRES STARTS GOOD THEN DIES-IS IS IN NEED OF CARBURETOR ADJUSTMENT OR IS IT POSSIBLY VALVE ADJUSTMENT? THERE APPEARS TO BE NO WAY TO ADJUST CARBURETOR AIR FLOW AS OPPOSED TO OTHER ENGINES. DO YOU THINK THERE SHOULD BE A VALVE SPACING TEST DONE? THANKS FOR YOUR REPLY!

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In a 2-stroke engine, the valving is done by the mechanical relationship of the piston position in the cylinder. No adjustment is possible other than grinding the cylinder ports or changing the piston skirt shape. You need to check the fuel filter (in the tank), air filter, and muffler for plugging. Because the engine is designed to burn a small amount of oil with the fuel, the muffler tends to plug over time. Check the fuel lines for decay or other damage. The carburetor has a rubber diaphragm in the lower chamber that decays over time as well. Go to: http://www.drystacked.com for a 12 page article on Walbro brand carburetors that are typical of chain saw carburetors. Good theory and troubleshooting information. 2-stroke engines are picky about air leaks behind the carburetor. Hope some of this helps!

Posted on Aug 05, 2010

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How do you adjust idle speed 1991 johnson 100 hp


not important, what's important is that they are absolutely clean, including the four brass high speed jets located in the bottom center portion of the carburetor.

That model engine's carburetors has fixed jets, BUT if the slow speed jets are adjustable, earlier model carburetors have been installed. If so...................

The four adjustable needle valves control the flow of fuel in relation to the set air flow, adjusted as follows (if needed).

(Dual Carb V/4 With 4 (2 each carb) Slow Speed Adjustable N/Valves)
(J. Reeves)

The adjustment procedure of the carburetor slow speed needle valves follows. NOTE... if the needle valves turn too freely, replace the nylon bearing retainer (the nylon item at the front of the carb that the needle valve goes through first) with the newer type RED retainer #315232. If your engine has that weird linkage that connects all four of those needle valves, the newer RED retainers will enable you to discard that linkage. The RED retainers make it impossible for the needle valves to vibrate out of adjustment.

(Carburetor Adjustment - 2 Slow Speed Adjustable Needle Valves, each carburetor)

Initial setting is: All (4) Slow speed valves = seat gently, then open 1-1/2 turns.

Facing the carburetors, start with the top left needle valve, then the top right valve, then the bottom let valve, then the bottom right valve. It may be necessary to redo these steps to get the adjustments ideally set

Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.

Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.

Do not attempt to gradually adjust all four of the valves at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve.

When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.

Jun 28, 2017 | Cars & Trucks

1 Answer

What should I try next, I adjusted the valves, it turns over, then backfires, but still won't start


if you have the valves set correctly. try a shot of carburetor cleaner in the opening of the carburetor. if it starts you have a carburetor problem. need to be cleaned or rebuilt. the backfire is usually a sign of valves or timing. you might take the flywheel off and check the shear key to be sure it hasn't sheered . is the spark plug good and gaped right?

May 19, 2016 | Garden

1 Answer

Engine keeps backfiring under load motor is an enduro xl 13.5 hp


Need Make/Model/Year/Engine Model/Deck Size, and hours on meter if you have one. A short description of the events preceding the beginning of the problem is also helpful.

Several possibilities:
1. Needs a valve adjustment or a valve job. Start with adjustment.
2. Needs new plugs. Start with new plugs.
3. Shear pin under flywheel is slightly damaged throwing off the timing. Remove flywheel examine shear pin for any damage. Just go ahead and replace it; if it even looks nicked or sliced.
4. Coil, or spark plug wire, is going bad. Very hard to test a coil for weakness without test equipment that the average person doesn't have. Check the first 3 problems, first. If it still backfires, then replace coil.
5. The other thing that causes backfire is fuel mixture too lean. On a small engine this is not usually an adjustment problem. It may be the fuel system/carburetor is dirty or partially clogged. It may also be an air leak in the gaskets of the intake manifold. If nothing else solves the backfire, remove the carburetor and clean or rebuild it. This is the least likely of the options so save it for last.

If compression is good and cylinder leakage is minimal, but a cylinder is misfiring or shows up weak in a power balance test, it indicates a fuel delivery or ignition problem (fouled spark plug or bad plug wire). A power balance, or cylinder balance, test should be run on a twin cylinder engine.

If an engine is hard starting, runs rough, misses or lacks power, run a cylinder balance test to determine if both cylinders are operating to their full potential. You will need an engine tachometer, a spark tester, and an insulated screwdriver.

1. Using the spark tester, with the plugs in, start engine and verify spark is equally good on both cylinders. Remove spark tester.
2. Connect engine tachometer.
3. Run engine at top no load speed. Note rpm.
4. Ground one cylinder spark plug with screwdriver. Note rpm.
5. Ground other cylinder with screwdriver. Note rpm.

The difference in RPM between the cylinders should be less than 75 rpm. If the difference between the two cylinders is greater than 75 rpm, the cylinder with the least rpm loss is the weakest and is a potential problem.

Note: If the spark plugs are new this could be an indication of a weak coil. Of course it could also be compression (rings), or valves.

Mar 24, 2015 | Garden

1 Answer

18Hp v b&s backfiring or poping thru the carb almost start but wont i have fire so idk its a john deer gt 235 2000 the. Ignition mod went bad, i would just start turning over when i would the key to run...


Since the machine is a year 2000 machine, I assume it has several hundred hours on it? If this is the case, it may be your valves need adjustment.

Valves that are out of adjustment will allow the engine to spin more easily than normal because they are not locking the cylinder for proper compression. More importantly, they allow the fired gas vapor to flash back through the carburetor or down through the exhaust. This is heard as a backfire while cranking the engine and it results in a non-starting situation.

The good news is...it indicates you may not have a problem with fuel or spark.It's a problem of Air control.I am going to add my simple starting diagnostics below this post.

All engines that are working correctly internally need Air, Fuel, and Spark. Bad valves (out of adjustment or worn) give you problems with the Air component.

Prior Post:
Let's say you have less than 300 hours on the engine. Let's further assume it worked last summer, or last week, but now it's acting up. Let's also verify the battery is in good shape and fully charged.

A mechanically functional (meaning all the internal parts are working, i.e. pistons, valves, etc) engine requires 3 things, air, fuel, spark. So the quickest way to start your diagnostics is to remove the cover over the air filter and then remove the air filter. I do not recommend spraying Quick Start.
  1. remove air filter (set it aside for the rest of the diagnostics)
  2. put throttle on fast run (do not choke)
  3. pour an ounce or two of fresh gas into the carburetor
  4. crank engine (15 to 20 seconds, rest to cool the starter, repeat 3 times if it does not fire at first).
Removing the air filter and placing the throttle at full speed with no choke eliminates the "air" question. Dumping in the fuel eliminated all fuel related questions. What's left? Spark. If the engine fires and runs a bit, then dies, you have eliminated the spark question. The engine started so Spark and Air are good; you have to track down a fuel problem.

If the engine does not fire, you have eliminated the Air & Fuel questions; you must tract down a Spark problem.

Tracking down fuel problems.
  1. Make sure there is plenty of fresh fuel in tank.
  2. Remove fuel line from filter and remove the filter, gas should pour out of the hose coming from the tank. If it does not, remove gas tank cap. If fuel flows now, you need to unclog the cap vent or replace the cap. If the fuel still does not flow then blow air from the filter end of the hose back up through fuel line to blow out blockage inside the gas tank. Once you have fuel flowing out the line to the filter, PUT ON A FRESH FILTER. Try cranking the engine. If it still does not start move on to the carburetor.
  3. Note: many systems have a fuel shut off solenoid. This is a small black cylinder on the bottom of the carburetor. It has two wires running to it. It should make an audible click when you turn the key on.
  4. NOTE: if battery voltage drops below 9 volts when cranking or while the engine is running, the fuel solenoid will not function. That means the engine will not start or will die soon after starting. So check your battery voltage before, during, and after cranking. If the battery voltage is good before but drops below 9 volts while cranking this is generally a sign the battery has one or more bad cells. Replace the battery. If the battery is in fact good and the voltage is still dropping you may have a bad starter motor and it is drawing too much amperage.
  5. Ck to see that the carburetor bowl is full of fuel, if not...you may have to disassemble and clean the carb at this point.
Tracking down Spark problems.
  1. verify battery fully charged.
  2. verify the fuel cutoff solenoid clicks when you turn the key switch
  3. get fresh spark plugs and verify gap
  4. install plugs and crank. Perhaps verify spark with a spark testor
If these simple steps do not solve the spark problem, you have a Spark problem that moves into the arena of complex. You will need to check coil gap and coil performance. If it still doesn't start you will need to diagnose the electrical system for wiring and safety switch problems, key switch problems, fuses, etc.

A Problem of Air Control.
If you determine the Spark & Fuel systems are good, the next likely problem is Valve adjustment out of wack. This is a problem of Air Control (and its sister compression), the third element in the engine trinity. Usually this problem shows up as backfire; either back through the carburetor or out through the exhaust. It will backfire while you are trying to start it.HOWEVER, it does not always generate backfire. You may instead notice that the engine spins more freely than usual; or oddly enough just the opposite and the engine bind while you are cranking it. This is because the compression is lost by out of wack valves or the compression occurs at the wrong part of the stroke. You need a valve adjustment.

Once you get it running be sure to replace the Air Filter with a new one.

These simple steps solve most starting and running questions and problems.

Mar 04, 2015 | Garden

1 Answer

Running rough, backfire, hissing sound


Hi Anonymous, be sure to check your intake seals, and the usual suspects are:
1. Spark plugs in bad condition or partially fouled.
2. Spark plug cables in bad condition or leaking.
3. Spark plug gap to close or to wide.
4. Faulty ignition coil, module and or sensor.
5. Battery nearly discharged.
6. Damaged wire or loose connections at battery terminals especially the ground cable at BOTH ends, ignition coil, or plug between ignition sensor and module.
7. Intermittent short circuit due to damaged wire insulation.
8. Water or dirt in fuel system, carburetor or filter.
9. Fuel tank vent system plugged or carburetor vent line closed off.
10. Carburetor controls misadjusted.
11. Damaged carburetor or intake seals.
12. Loose or dirty ignition module connector at crankcase.
13. Faulty MAP, CKP, and or CMP sensors.
14. Incorrect valve timing.
15. Weak or broken valve springs.
16. Damage intake or exhaust valve.
For a prime suspect please visit website below and good luck:
Big Twin Carb EFI DTC Intructions codes Harley Davidson Forums Harley...

Oct 15, 2012 | Harley Davidson FLSTC - FLSTCI Heritage...

1 Answer

My 86 suzuki intruder 700 backfires out the exhaust at idle. and makes a tapping noise. i already did a valve adjustment an new plugs


It could be the speed of one carburetor is faster than the other. This causes the backfiring as well as forces one cylinder to run lean which causes the tapping noise. You will experience an identical tapping or "ping" when you get close to empty on main fuel and need to switch to reserve. Most likely you or the previous owner unknowingly adjusted the speed of the rear carburetor down because of the high stock idle not realizing the front carburetor needs to be slowed down to match. While running simply turn the idle adjust screw clockwise on the rear carburetor until the idle smooths out and the backfiring/tapping stops.

Sep 20, 2010 | 1995 Suzuki VS 800 Intruder

2 Answers

Vstar 1100 idling rough and backfiring letting off the throttle


It sounds to me that your carbs might be a little gunked up - you mentioned the gas may have been old, so it's possible that the smaller (idle) jets are slightly plugged up. Fresh fuel might do the trick and/or you could try using a fuel system cleaner and water remover. This problem might 'solve itself' with a little time.

As far as the backfire - this might be either from the carb issue or it could be related to that rattle you mentioned - you could have a loose connection at or near the head - this can **** air and cause a backfire.

I hope this helped!

Jun 09, 2010 | 2003 Yamaha V Star 1100 Classic

3 Answers

2002 buell blast is running rough - missing - backfiring


hard to say w/ seeing it; it does sound as if they replaced everything w/o really finding the problem first; try better fuel and clean petcock; when dies is it electrical( all lights out) or does it sputter and choke off? sorry; I know it can be frustrating check all wiring under seat looking for loose or broken connectors or ground wires

Jun 28, 2009 | 2002 Buell Blast

2 Answers

Snow blower starts right up but dies once i engage the blades?


if its a 2 stroke (gas oil mix) there is an air leak in the crank case. a new gaskit is in order

Dec 30, 2008 | Ariens 34 In. Zoom Series Riding Mower

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