2000 QuadMaster 500 has huge backfire/afterfire when turning off

[Brian Emerson]

Hi Guys, this Quadmaster has an extremely loud bang after turning key off (from idle), scares the stock, dog and missus, any ideas on what to check.

Thanks and regards,

Brian

[Mech]

Hi. I'm not familiar with a quadmaster.  Is that fuel injected or carburetor ?

Have you had it long and how long has this backfiring been happening ? Did it start abruptly or get gradually worse ? Does it do it every time ?

Is the backfire out the exhaust ?

Does it do it after a short run when the engine is relatively cold, or only when it's hot ?

If it has a choke, does putting the choke on stop it backfiring ?

Is it running too hot, or, is the sparkplug a too hotter heat range ?

[Brian Emerson]

Hello, quad belongs to one of my customers so not sure the history, I know he hasn't had it that long. A QuadMaster is very similar I believe to a Quad Runner, they call it an LTA500 (but not to be confused with the late model LTA500) it is year 2000 so 24 years old! It is a carburettor model, Mikuni CV type that I have had off a couple of times and also put a new kit in it, ie jets, needle etc. Have checked the valve timing, adjusted the valves, tried another CDI unit. It does a massive backfire out of the exhaust after you turn the key off so unburnt fuel is being ignited in the exhaust system and not by the spark plug, called 'afterfire' I believe.

[Mech]

Yeah it could be unburnt fuel from the last rotation of the engine, being ignited by flame in the exhaust. Doesn't have exhaust leaks near the head does it ?

Have you put a timing light on it and watched for a spark occurring on the crank rebound after it's been killed ?

Does it idle nice, and have you tried adjusting the mixture rich and lean to make sure it responds both ways ?

Are you sure the choke's right off ?

[Gwbarm]

Gas leaking past the needle and seat into hot combustion chamber, might be worth checking.

[1954JF]

You are right in stating the exhaust is holding fuel either a rich vapor collection or actual raw liquid. That muffler is extremely hot and will ignite either. When it's cold put your finger inside the tailpipe and wipe the surface to see if it's black soot. Indicating a rich fuel mix. If it doesn't backfire while running it is telling you it's collecting and holding a little bit inside that muffler and when the engine stops breathing/running the excess in the muffler ignites from heat inside of there. I've seen mufflers blow up an split out on cars.

I know you said you had the carburetor off a couple times but a couple things need checked again. Mikuni's CV series choke is an enrichment needle that sends extra fuel for starting from cold. Not like most American carbs that actually cut off the air intake to make the carb pull more fuel in. Set the correct slack in that choke cable and check that the spring is on that plunger needle at the end of the cable where it screws into the carb. It must close correctly when you move the choke lever to OFF. Next, I know the float setting is critical on these carbs so make sure it is set to the service manual float height. My 1998 500 Quadrunner is set at 17mm or 0.67" +/- .04" that's about 11/16ths of an inch. This is with the carb upside down measuring from the rim of the float bowl up to the highest point of the float. Use a machinist ruler or calipers. Bend the tang on the float that rides above the needle to adjust up or down. Be sure the needle has a good clean point with no damage and no obstruction in the seat, like a fine crumb of rust or something that might prevent a seal. Now go to the top of the carb and remove the cover and gently lift and inspect that metering rod diaphragm for fine splits or cracks. Also someone may have re-jeted at some point which could make it run rich so check those jets. The factory jets on my 98' are; Main #130 and Slow #55. The mixture adjustment screw is on the carb bottom on the engine side of the carb on mine. It's set about 2-1/2 Turns out and very hard to get at with the carb mounted. You need a #1 flat blade screwdriver about 1-1/2 to 2" length. If the engine fully heat up it's very hot work turning that needle screw. These are the things I would check if mine were running rich. When a float and needle aren't seating good you can get so much fuel it can hydraulic-lock from a full cylinder of gas when the engine is off. I am not a fan of the problematic vacuum petcock safety design.  I replaced mine with a common On/Off/Reserve petcock and plugged the vacuum port at the carb. Works fine. 

Good luck out there

 

[Gwbarm]

I did that with the  vacum petcock also had a lot of trouble with it.

[1954JF]

So your reply is saying you had trouble with the new petcock? Or something else is still a problem? Is the new petcock a vacuum design like the original or the manual type with ON-OFF-RESERVE? I just want to be sure of what we are discussing here. 

If you are saying you changed that vacuum petcock to a standard petcock or old style it will be just fine for what the purpose or intention is. You need to plug off the vacuum hose line or that affects the carburetion. I'm sure you already did that. The vacuum design petcock is solely for the purpose of shutting the fuel OFF when in a Rollover situation, by way of the engine dies and no more vacuum so that petcock closes fuel flow then. What if the engine doesn't die right away. This stinks in a way. You have 'PRI' position which is fuel 'ON' always like the normal ON and/or RESERVE would be, which they want you to use PRI to start your vehicle when there is no vacuum present, then they want you to move the lever switch to 'ON' and/or 'RESERVE' with no real 'OFF' position to mechanically close that petcock. This system relies on the rubber diaphragm inside the petcock to function as the shutoff when engine stops or is shut off. Now when that rubber goes bad and leaks (and it eventually will happen) the fun begins with issues. It will contaminate the fuel line and possible the fine screen under the needle seat or the needle seat itself and the carb too as rubber degrades or disintegrates. The issues begin.

Lets try and figure what else might cause your problem or problems. 

[Brian Emerson]

No this petcock/fuel tap is fine, someone has replaced it previously, I did try the quad with the tap set on 'Prime' and the vacuum line to the intake blocked off but made no difference, still the huge backfire after turning key off..and yes, a leaking diaphragm in the tap allows unmetered fuel to enter the intake manifold via the vacuum line..have checked all of this and it is fine..

[1954JF]

OK you have some other 'loading up' problem causing this. It's not uncommon in carbureted vehicles, there is more fuel going through the engine than gets cleanly burned. RICH. By loading up I'm speaking to the muffler and the bang. Burning rich causes excessive heat that leads to the bang pow or puff you hear from the exhaust pipe when you shut it off. It depends how much is in there (the muffler). We have to find which little item is not right. I've seen this in so many cars and trucks.  I'm thinking something in the carb itself now. It took me awhile to solve this the first Mikuni CV I worked on I had it out and checked it over three times finding and fixing obvious things like the diaphragm, then tiny little parts that had been left out by the previous owner. It took me a long look on a zoomed in diagram of that carb to spot this. Then I found a video on Youtube I believe titled Mikuni Carbs 101. Like a class online about Mikuni's. I ordered a kit with a new metering rod diaphragm that had many little brass parts and a few tiny washer and O-ring that goes on the adjustment needle. In the end I got it regulating fuel properly. If your problem gets worse it could load the muffler enough to do this backfire while riding it and could blow that muffler out, maybe hurt you.  

FYI...  https://www.pure-gas.org/  I can tell you from experienced small engine shops mechanics I've talked with and they will tell you and my own knowledge of this, most gasoline today contains ethanol. It's another joke on the citizens. Our taxes are for the rulers to play with. Ethanol is grain alcohol and alcohol destroys rubber components sooner or later things fail. The ethanol enhances octane cheaply so they put it in every grade these days from low to the highest octane. I have tried to tell guys having vacuum petcock or Mikuni Carb issues about this debacle. I often hear the words from their mouth, "I only buy the best high octane premium gas" and they don't believe me when I say "so what", it still has alcohol blended in. A new petcock that may be the Chinese duplicate fake copy has the Chinese rubber in it. They make rubber from the tree and clay is added as a stiffener. The Chinese use more clay to make the parts containing expensive rubber cheaper to produce. This higher clay content makes the rubber stiffer and it cracks sooner than expensive more pure rubber. Ask any devout DUCK Hunter which chest waders last the longest. Not the Chinese ones. When ethanol was introduced to the gas pumps most all of the auto industry had prepared for it.  The small engines industry did not, and years later they had to played catch up. Older carbureted engines still work fine but you need "Pure Gas" which can be found everywhere in the US and Canada. But not without looking for it. Only two states in the US have it commonly by opposing government rules to change to some alcohol in all grades while farmers and other smart people know it is necessary for some older equipment or you buy the newer stuff. Here is a link to good gas. https://www.pure-gas.org/ This is super helpful for everyone. Put in any area code or city and they map or list out stations that provide non-alcohol gas at one of their pumps. This is to satisfy us the customers who need this. We filed consumer complaints to get this accomplished. You will pay more at the pump. I've got over a dozen older carbureted small engines.  They keep running for a reason. I don't mean to bore people with this sort of comment but I am sure many younger folks don't know this history. 

[Mech]

It's also possible that there is something wrong with the electrics such that the coil sparks one last spark after the engine bounces backwards off the compression. Something like a bad connection in a key switch, a bad earth on the cdi or the coil, or a bad kill switch.

The easy way to eliminate it being a spark at the plug doing it, is to watch a timing light as the engine comes to a rest after the kill switch is flicked.

Check the easy things first. If you run through those questions in #2 and #4 it will lead us in the right direction I think..

[Brian Emerson]

Thank you, I have an inline spark tester, will connect that and then see if I am getting any sign of a spark after turning key off, will let you know..

[Brian Emerson]

Hello Mech, connected up my in-line spark plug tester, ran the quad to hot, when I got the bang out of the exhaust when turning the key off there was no evidence of any residual spark or spark plug firing happening so I can rule that out. Will concentrate on the carb, maybe too rich, did notice that when running I only need to open the choke control a little and it causes the engine to bog...

[Mech]

Test it from cold, see if it backfires cold..

Warm it up and while it's warming get a rag and jamb over the exhaust outlet so you will hear leaks up at the head.

Check the choke operation, make sure it's going right off down at the carb, pull the plunger out by the cable and let it go making sure the plunger snaps back in against it's stop.  Adjust the idle mixture in and out making sure it makes a difference both ways and finding the best spot, and do it at a low idle speed. Then check for backfires.

[Brian Emerson]

Looks like I may have fixed this quad, even though I had the carb off numerous times and set the float height to specifications I decided to do the fuel level check with the clear hose up the side of the carb, fuel level was way too high, above the seam where the float bowl screws on, so ended up adjusting the float height until I got the fuel level about 2mm below this seam, tests so far have been good, the only backfire I have had was a really small pop/fluff sound, no huge bang..I will take it for a good run, check deceleration for noise, then check for any back fire after key off. Thanks to all who gave me some suggestions here, very much appreciated...take care...

[Mech]

Good one.

 

[Gwbarm]

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