Good morning/evening team, I have been trying to sort out a Victa Model 5 125cc Utility mower that I was asked to initially do a carburettor clean, but when I started it, I found it is occasionally backfiring through the carburettor and, when giving it full throttle, the machine starts to die until it is backed off to about half throttle. I have disassembled the carburettor several times in case I missed something, but the slide carburettors are really basic, and I have now replaced all internal gaskets, including the two o-rings in the fuel tap, but it still backfires and dies when pushed to full throttle. I've read the instructions on setting the points (Gap stated as 0.027"), and also tried various positions for the timing with the magneto plate (said to be 1/8" before TDC), but to no avail. Last change was to move the plate to the opposite end of the adjustment slot, but it wouldn't start. The points also didn't seem to fully close, and appear to open far too soon compared to the instructions I've attached regarding timing, etc. Would appreciate some assistance or suggestions as it's not my mower and our local mower shop has ceased trading and is now closed. I've attached photos of the instructions I read for this style of motor I obtained from this site and an overhead photo view of electronics. Hope the information above is clear enough to allow assistance. Will check back in the morning. Thank you for any feedback. Regards. Daryl.
Hi Bushy sounds like it is a timing issue or maybe condenser, what I would try is put an ignition module on it and see what that does. At least that will bypass the points and condenser. Nothing to loose
Could be coil , condenser , timing or spark plug (ignition) problem.
The points should fully close ,in the image with the rubbing block on the high part on the lobe the gap is set then rotate the coil plate so the points just start to open when the piston is at 1/8 of an inch before TDC.
The rubbing block on the points that rides on the cam lobe can wear down. This will affect when the points open and thus the timing. If it's significantly worn, the points may not open at the correct time or for the correct duration.
Even a clean gap on pitted or burned points can lead to a weak or inconsistent spark. Inspect the contact surfaces closely. If they are not smooth and clean, the spark will be unreliable.
I usually check the magnets on the flywheel with a screw driver to make sure they are not weak.
The small restrictor in the air cleaner housing is important as they won't run properly if another air filter is used.
Worn crankshaft seals can allow air to enter the crankcase, leading to a lean mixture and affecting scavenging, which could indirectly contribute to poor running. However, this usually manifests as erratic idle and poor overall performance .
One way I test the crank seals on these mowers to see if the mixture is lean is start the mower and while it's running you crimp the intake tube with 2 fingers and while the intake tube is about 50 to 80 percent restricted closed with your fingers see if the motor revs out fully and if it does you know the seals are leaking.
There are a few ways to check crank seals ,sometimes I use a feeler gauge that is a couple thou thick and slide that between the crank shaft and the seal and when the seals are old the feeler gauge slides in without resistance.
In essence, leaking crankcase seals create a chaotic environment in the crankcase and cylinder that disrupts the normal flow of the fuel-air mixture and combustion process, leading to the spitting back you're observing.
So, while ignition timing is a very common cause of backfiring through the carburetor, leaking crankcase seals are definitely another significant possibility, especially if you're experiencing lean running symptoms at higher RPM. The "finger test" or restricting the intake tube is a good way to further investigate this possibility.
Thank you NormK for your suggestion. As this is not my machine, any change from original setup will have to be discussed with the owner of the mower. In the meantime, I'll reset the electronics back to their original positions again in the morning so it's at least sort of running. Reading information from Max, it may be that the points are worn as they didn't seem to fully close and seemed to open well before expected at 1/8" (3-4mm) before TDC.
Thank you Max for your detailed information. I'll reset everything in the morning and have another go at getting the points in the right spot. The magnet on the flywheel seemed to be good as the key fell out of the keyway and was caught by the magnet in the flywheel. The mower does not have the original low mounted air intake assembly shown in Model 5 material, or the longer assembly shown in the Model 4. The mower takes the large Victa 225mm long paper filter in the housing, which I replaced with a Bynorm filter and pre-filter as the old one was floating loose inside the housing and wasn't effective. The air filter assembly starts at the carburettor with an original rubber hose (only goes about 30cm up the lower handle, but then connects into the standard rippled plastic air hose used on modern 2-strokes. I pulled the plastic tube off both ends and re-sealed it with silicone and wrapped each end with electrical tape to stop any excess air accessing the air pipes. As such, there is no restrictor in the air intake on this engine. I meant to take a photo of the mower to show the configuration, but forgot. I also cleaned the spark plug after I first started the mower when I found it was backfiring, but that didn't help as plug looks pretty new but was black. That said, I took a photo of it today and it is again black with the electrode looking dark brown. I may need to find the right part numbers so I can get a correct set of points and possible condenser and/or ignition module to match a motor with the 3F carburettor (Not a G3 or G4). Will see what happens in the morning. Appreciated. Regards. Daryl.
The later 2 stroke Victa filter housings still have a restrictor in them and work with a Victa 18.
A black spark plug is generally more indicative of a rich fuel mixture or incomplete combustion rather than a weak spark. However, a weak or inconsistent spark can contribute to poor combustion and cause the spark plug to foul more quickly, leading to a black, carbonized appearance.
Some of the tachometers have a spark analyzer that makes it easy to diagnose a faulty coil but cost a little more so it's usually cheaper just to try another coil from a later full crank Victa if you suspect a weak spark after servicing the points and condenser or if you try a module that still looks like a weak spark.
Thank you Max for the information on the air hose housing and spark plug. The spark plug is new, and I spoke with the owner yesterday who said the motor was rebuilt about 10 years ago. I reset the points gap at 0.024", where initially I found it to be 0.021" compared to stated gap of 0.027". An in between option as a starting point (No pun intended). With the timing plate, I initially set it about 1/8" from TDC (as indicated), which is about the width of the shear pin keyway, and started the mower. It ran on mid throttle, but when pushing up to full throttle it started to backfire and die, which is where I started with it after the carburettor was cleaned. I rotated the timing plate about 1mm anticlockwise and started it again, but the backfire issue was still there and again dies going to full throttle. I changed the timing two more times by moving it about 1mm anticlockwise each time with the backfire getting worse and it still starting to die when pushing to full throttle. I made 4 short videos that are between 15-50 seconds with each change, but they are all too big to link directly to the conversation, being 33-103mb each. If I go too far anticlockwise, the mower won't start, and I can't move the timing any further clockwise from where I started, so am rather stuck. Do you know what the correct points and condenser part numbers are for an engine with a 3F carburettor? All the ones I found only mention G3 & G4. Thank you again for your assistance.
Thank you Norm for your additional information on the Ignition Module. That may now be an option if the reason for the backfire can't be resolved and the owner agrees to modify his mower. How does the later module know the firing timing vs points and condenser, or it it all just based off the magneto and coil?
Hi Bushy, I have no idea how the module picks up the signal it senses it as the magnets pass the coil (very clever little things) It may not solve your problem if the coil is the problem but at least it will rule out your points, condenser being the problem. This is step one in fault finding and points and condensers are a bit hard to find these days
Hi Bushy, I have no idea how the module picks up the signal it senses it as the magnets pass the coil (very clever little things) It may not solve your problem if the coil is the problem but at least it will rule out your points, condenser being the problem. This is step one in fault finding and points and condensers are a bit hard to find these days
Thank you again Norm for your advice. Are there any instructions on how to swap out the points and condenser without cutting existing wiring, and where the best place would be to locate the replacement ignition module? My thought would be to cut out and shape a flat piece of metal to replace and match the existing condenser and points plate, allowing them to be kept in case of other issues, and mount the new ignition module on this plate using the screws and mounting point of the old plate. This would keep the mower looking original, but will it then need a cut-out switch or just let the fuel shut off like it does now???
Hi Bushy, you can leave the points and condenser in place , You can put the module inside the fan wheel if you want as long as you earth it. Not sure on yours if yours has the kill switch wire coming out of the housing, which is the kill switch wire, you can connect the module to that wire outside if you want to. Module is so small you can hide it anywhere
Hi Bushy, you can leave the points and condenser in place , You can put the module inside the fan wheel if you want as long as you earth it. Not sure on yours if yours has the kill switch wire coming out of the housing, which is the kill switch wire, you can connect the module to that wire outside if you want to. Module is so small you can hide it anywhere
Thank you Norm. There is no kill switch on the machine. It only stops when the throttle cable to the carburettor allows the slide to shut the fuel off. I have been able to upload the video to YouTube showing how the mower is running at first reset of points and timing. You can hear the occasional backfire and small puff of blue smoke come out from under the mower, and how the engine starts to die when you try to throttle up. Hopefully this link will work, as it has just been uploaded with a start time for public viewing of 2:45pm. In the meantime, it's been a challenge to get this far, and I can hear a cold refreshment calling out from around at my brothers house. Have a great weekend. Cheers.
I've had 2 stroke mowers run like that when the spark plug is no good ,even if it's new, I always test a mower with a spark plug that runs well in another mower
They run like that if there's a spark problem ,fuel problem ,air leak (port covers, crank seals,gasket problem)
The points and a condenser can be used from a later 160 cc Victa 2 stroke ,they all interchange.
I converted a seventies Victa 2 stroke from points to electronic ignition and used a coil off a side pull 2 stroke but the coil had to be put on the core backwards ,I put images on this site but can't find that post.
Sometimes you need to remove the muffler and make sure the port isn't restricted with carbon and the muffler isn't blocked.
Thank you Max for your review. I'll see if I can get the muffler off to check on any possible carbon or blockage issues. I was trying to avoid doing this as it runs okay on half-throttle, and I didn't want to chance breaking the mounting bolt off. Will be slowly, slowly, with extra care working for this one. After I check the muffler, if there's no change, I'll try with a new spark plug. Not sure if I have a points assembly in a mower engine. One step at a time I guess. Appreciate the assistance provided from yourself and Norm. Have a great day. Cheers.
Good to see you still posting !! You know your stuff.
Regarding the clever little ignition module you said above and how it works. This is more relevant to your comment and the Powertworque flywheel magnet, ignition coil, armature etc. I'm unsure how the older Victa magneto etc is configured.
I've been going deep down a rabbit hole trying to understand how the ignition system works on the Powertworque Victa and in particular the ignition coil, armature and most importantly the silver little square ignition module you are saying is a "clever little thing"
I ended up finding a very good free book online from the 1960's that is solely about ignition coils and magnetos. I'll post the link below and you can read for free. If you go to page 221 I reckon this is what the inside of the replacement Victa module ignition module is made up of. Some diodes, capacitor and thyristor that get triggered once a certain voltage is reached gets triggered. I have taken a screenshot below.
There is one last great Australian electronics magazine Silicon Chip magazine and I emailed one of the staff writers and I said the way the magneto looks on the Victa Powertworque with 2 magnets on the flywheel and the U shaped armature (not E shaped) he agreed the internal circuitry would be similar to what I've posted.
I've also posted a photo from a Paul Dempsey book of the system and and explanation of how that circuit works Normk and it explains how the circuit works and I have taken a photo of the explanation.
I've vaguely thought about spending $30 and taking one apart but I reckon this circuit is pretty much and you can read the photo and someone with some electronic knowledge will see how once the SCR is triggered at the gate it dumps the voltage in to the primary.
And to be honest to really understand you need to know a decent amount about electronics, magnetic flux lines, Faraday's laws of induction etc
On the newer Victa 2 strokes you can't even see this part of the coil visually as it potted inside the coil
Just as a general rule whenever I hear of a backfiring issue I reckon it's normally ignition related like norm and max said.
Norm and max are spot on with their advice on these Victa mowers.
It's probably either the points or the ignition coils are bad. With ignition coils they talk about resistance tests on coils but I reckon a lot of the time they are useless tests. Ignition systems need testing under load to stimulate the cylinder pressure on the ignition system which is at its greatest when engine is under load.
If it was me, I've got one of these cheap spark testers were you disconnect the HT lead from spark plug and clip to engine ground and set a decent gap and see what the spark looks like. It should be a nice blue spark that jumps a decent distance on the tester. If you have a working mower I would look at how that spark looks visually versus the mower you posted about. Would give some clues
Then I would buy the ignition module normk is talking about and replace points. Points are a stuff around. Old timers like them because it sounds romantic but they always go bad. Here is a link or even better someone on this page might give you one or sell you one cheap.
Then if that ignition module doesn't fix your issue of misfiring under heavy load I would buy a new ignition coil.
Either the module or coil will fix your issues is bet.
