Duromax 440 Throttle Issue

[SquidBonez]

I've been having an odd problem with my Duromax 440. The engine has a stage 1 kit, billet rod/flywheel, and no governor. The engine idles just fine but will stall once any throttle is applied. However, if I open the throttle by manually moving the butterfly on the carb (little black piece on top of the carb) it will rev up but starts to backfire and sputter at high RPMs.

This makes no sense to me because regardless if I open the throttle with the throttle lever or on the carb itself both still cause the carb to open, but the engine reacts differently regardless. I have the coil gap set to 0.40 and I tried changing carbs or swapping emulsion tubes but nothing changes. Any ideas? I've never seen this before.

 

[ONE-EYE]

I'd think it'd be a jet issue but someone with more knowledge should answer soon. Its in the basics probably. Fire, fuel, or air.

 

[madprofessor]

Squid, I know you're aware of how 4-stroke timing works, so no need for me to go over the basics. Will just say you're describing 2 different problems, and I think explaining the second one will inadvertently explain the first one. Here's one of my hi-perf ignition coils..................
Predator 212cc / Hond Clones Performance Coil, Resister Free / No Retard Timing | eBay ...............see what the link title typo says about the resistors? The hi-perf coil doesn't deliver a hotter spark, that's the rare earth magnet in the hi-perf flywheel's job. The hi-perf coil has simply had its internal resistors disabled, because when you get up into ungoverned rpm the timing will start to retard. Check out this (212cc) package of different timing advance keys..............Chinese Clone, Predator Flywheel Offset Timing Keys, Go Kart, Mini Bike (3 Pack | eBay..............I think this and a resistor-disabled coil is what you need (for Duromax 440cc) for your...............

starts to backfire and sputter at high RPMs.

With the stock timing of the Duromax, nothing's supposed to be wrong while the governor is limiting the rpm. However, still brand new and sitting on the shipping pallet, unhook a governor spring and rev it up and I believe it'll do exactly what you describe above. Sputtering when the timing retards, unburnt fuel igniting in the hot exhaust (backfire). A resistor-disabled coil and advancing the timing should get that taken care of.
Not going to say a word about the idle/stall issue, because I'm hoping that the timing advance and hi-perf coil might fix it accidentally. Personally running a Predator 212cc hemi right now with a 28 degree PVL flywheel and a #6 (7.2 degrees) advance key for 35.2 degrees advance (meaning that is, folks can advance the timing further than a lot of folks think).
NOTE: Scroll down and check out the (red ink) chart Jerry has on the timing advance keys page linked above. Good stuff to learn from it, timing recommendations for half dozen hi-perf flywheels, etc.

 

[SquidBonez]

Squid, I know you're aware of how 4-stroke timing works, so no need for me to go over the basics. Will just say you're describing 2 different problems, and I think explaining the second one will inadvertently explain the first one. Here's one of my hi-perf ignition coils..................
Predator 212cc / Hond Clones Performance Coil, Resister Free / No Retard Timing | eBay ...............see what the link title typo says about the resistors? The hi-perf coil doesn't deliver a hotter spark, that's the rare earth magnet in the hi-perf flywheel's job. The hi-perf coil has simply had its internal resistors disabled, because when you get up into ungoverned rpm the timing will start to retard. Check out this (212cc) package of different timing advance keys..............Chinese Clone, Predator Flywheel Offset Timing Keys, Go Kart, Mini Bike (3 Pack | eBay..............I think this and a resistor-disabled coil is what you need (for Duromax 440cc) for your...............

With the stock timing of the Duromax, nothing's supposed to be wrong while the governor is limiting the rpm. However, still brand new and sitting on the shipping pallet, unhook a governor spring and rev it up and I believe it'll do exactly what you describe above. Sputtering when the timing retards, unburnt fuel igniting in the hot exhaust (backfire). A resistor-disabled coil and advancing the timing should get that taken care of.
Not going to say a word about the idle/stall issue, because I'm hoping that the timing advance and hi-perf coil might fix it accidentally. Personally running a Predator 212cc hemi right now with a 28 degree PVL flywheel and a #6 (7.2 degrees) advance key for 35.2 degrees advance (meaning that is, folks can advance the timing further than a lot of folks think).
NOTE: Scroll down and check out the (red ink) chart Jerry has on the timing advance keys page linked above. Good stuff to learn from it, timing recommendations for half dozen hi-perf flywheels, etc.

Wow, thanks for the in-depth reply. Only problem is I cannot find any resistor disabled coils for the big blocks online. I do, however, have an advanced timing key somewhere in my garage. My reasoning was that since I'm keeping the cam stock that the timing should be stock too. Should I try swapping in that key? The billet flywheel for the big blocks has the keyway in the stock position (24 degrees I believe). My key is a 4 degree key I think. Is that what I should put on?

 

[karl]

I cannot find any resistor disabled coils for the big blocks online.

Non digital coil for gx340/390 is honda part #30500-Z1C-023

 

[madprofessor]

The billet flywheel for the big blocks has the keyway in the stock position (24 degrees I believe).

The flywheel itself is the only unknown factor for me in that timing equation. Did you check out the flywheels/advance keys chart on the advance keys link? The timing built into the flywheel itself has to be known, and that chart is Jerry's recommendations for how much advance to add to each of those half dozen variously timed flywheels.
You can get at least some idea how much timing there is in your Duromax's billet flywheel from the chart if your brand is listed there. Really need to check with your flywheel supplier of course, to get the exact timing info for it. Advance keys aren't expensive, so you could get an assortment like that pack of 3 above (but for your Duromax of course) for very little cost, and experiment around.
Oops, should have said, you also need to find out for certain what the stock timing was on your OEM flywheel, so you'll know for certain if the billet is the same like you say, for how much you're advancing past whatever that was stock.

 

[madprofessor]

My reasoning was that since I'm keeping the cam stock that the timing should be stock too.

That's sensible of course, but not tattooed to the block. The OEM timing would be optimized for the intended use of the engine, with cam timing duration/lift, the bore and stroke, and the valve sizes all factored together for..............???..........but definitely not for gokart racing.

 

[SquidBonez]

Non digital coil for gx340/390 is honda part #30500-Z1C-023

I'm using a coil off of a Predator 420 which is non-digital.

The flywheel itself is the only unknown factor for me in that timing equation. Did you check out the flywheels/advance keys chart on the advance keys link? The timing built into the flywheel itself has to be known, and that chart is Jerry's recommendations for how much advance to add to each of those half dozen variously timed flywheels.
You can get at least some idea how much timing there is in your Duromax's billet flywheel from the chart if your brand is listed there. Really need to check with your flywheel supplier of course, to get the exact timing info for it. Advance keys aren't expensive, so you could get an assortment like that pack of 3 above (but for your Duromax of course) for very little cost, and experiment around.
Oops, should have said, you also need to find out for certain what the stock timing was on your OEM flywheel, so you'll know for certain if the billet is the same like you say, for how much you're advancing past whatever that was stock.

Just double-checked my flywheel. According to ARC the keyway is in the stock position which is 24° Just ordered the key set you linked.

 

[SquidBonez]

Update - I came across a review on OMB warehouse talking about the 0.42 jet that comes with the 301/420 kit and how it's supposedly "way too lean" and only running on half choke/at idle. One of the reviews mentioned that they drilled out their jet with a 1/16 drill bit and it solved the problem. Decided to take one of the stock jets I had and drill it out with a 1/16. Fired it up and now the engine revs fine. No breaking up on top end either. I'm still going to probably do a timing advance anyway just for the added power, but I think I solved my issue.

 

[madprofessor]

0.42 jet that comes with the 301/420 kit and how it's supposedly "way too lean"

Definitely filing that one in the dark annals of my organic memory card, good to know for a 420 expected sometime in my future.