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Lets see if you guys can figure this one out. New NGK BPMR7A spark plug, new OEM fuel filter. Saw starts after about 4 pulls cold and runs a few seconds and bogs down and dies and takes another 4 pulls to restart. Have checked internal carb screen, checked carb adjustment, and checks fuel passages and carb diaphragms are in good shape. Also there is no loss of spark when checked with a spark tester while the engine is running and/or bogging down and dying.

If it not the carb then is probably the crankcase sealing causing a lean out condition.

@StarTech Good consideration but not that complicated. This thing acts like it is running out of fuel but it isn't a fuel problem.

Okay then there is the impulse hose that could loose or have a hole in it. I do not see any Dolmars here so I miss that on the IPL. Also could be a hole or split in the intake manifold. The image of the carb appear to have an oddball impulse setup.

I also been have a lot problem with Zama needle seats leaking here causing a flooding problem. Ever Stihl got involved with Zama their carbs are just failing a lot more than in the past. Needle seat and main nozzles problems.

StarTech said:

Okay then there is the impulse hose that could loose or have a hole in it. I do not see any Dolmars here so I miss that on the IPL. Also could be a hole or split in the intake manifold. The image of the carb appear to have an oddball impulse setup.

I also been have a lot problem with Zama needle seats leaking here causing a flooding problem. Ever Stihl got involved with Zama their carbs are just failing a lot more than in the past. Needle seat and main nozzles problems.

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Impulse setup is below the intake with a horn that sticks off the bottom of the carb Other end connects to the crankcase. Haven't had issue with this style of saw with split or damaged pulse lines or intake tubes. The impulse line looks similar in material to molded fuel lines used on other brands as well as Dolmar. The 6400-9010 I have seen a few inpulse lines split but not this saw. And normally when the carb goes goofy on Dolmar/Makitas they dump the entire contents of the fuel tank into the crankcase.

I have already diagnosed and repaired the saw. When I tell you what the problem is you will think I am wrong.

With some of the crazy problems I have seen in the last few year anything is possible but glad you have it fixed and it does sounds like one for the books.

@Rivets @slomo @Scrubcadet10 Any of you guys have any thoughts before I reveal the answer.

StarTech said:

With some of the crazy problems I have seen in the last few year anything is possible but glad you have it fixed and it does sounds like one for the books.

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I come across this issue enough that I have to keep several different parts numbers of this item in stock for the various models. I come across this issue on Husqvarna and Stihl and helped another shop diagnose this issue on a Echo a few months ago.

You state you have good spark at all times and it is not a fuel problem, so only thing left is compression, which I doubt is the problem. Can you narrow down which of these three system was causing the problem, or will it give away the answer?

Hmm... Is it a Valvetrain problem or a plugged exhaust screen? Never heard of this engine brand before.

If it is in the fuel system, I’m guessing a bad pickup tube on the fuel tank!!! Craftsman, this is a 2-cycle engine.

Craftsman Garage said:

Hmm... Is it a Valvetrain problem or a plugged exhaust screen? Never heard of this engine brand before.

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Chainsaw 2 stroke

Alright guys the cause of failure is a faulty ignition module. Welcome to the world of digital spark advance with integrated rev limiter ignition. So try to diagnose a faulty ignition without loosing spark. Symptom mimics a chainsaw or trimmer that is running out of fuel in the tank.

First time I came across this was with a Dolmar PB 251.4. Blower would run about 30-45 minutes and then would start bogging and would die unless you released the trigger. Would idle all day long. Since this blower is 4 stroke, I changed the fuel lines, replaced the spark plug, adjusted the valves and replaced the carb. Still would not fix the issue. Contacted Dolmar and talked to tech support. And after talking to the head guy he determined that it was a faulty ignition module. Replaced the module and ran like a new one. Since then I have probably replaced 20 modules over the last 10 years.

Ok, that’s a new one on me, but even at my age I’m not too old to learn. I’ve heard of this on a 4- stroke engine, but never on a 2-stroke. Thanks you for teaching this old dog a new trick

Ahh, almost said the coil but you tested spark. I hate all these DSAI systems. Such a gimmick.

ILENGINE said:

@Rivets @slomo @Scrubcadet10 Any of you guys have any thoughts before I reveal the answer.

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I haven't read the thread, I'll read through and see what I think

Oh wow, very interesting fix. I would not have considered that, unless I got into a "throw parts at it" mindset.

Scrubcadet10 said:

Oh wow, very interesting fix. I would not have considered that, unless I got into a "throw parts at it" mindset.

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They best I can determine is the spark advance chip goes south and throws the timing off to the point it won't run.

ILENGINE said:

Alright guys the cause of failure is a faulty ignition module. Welcome to the world of digital spark advance with integrated rev limiter ignition. So try to diagnose a faulty ignition without loosing spark. Symptom mimics a chainsaw or trimmer that is running out of fuel in the tank.

First time I came across this was with a Dolmar PB 251.4. Blower would run about 30-45 minutes and then would start bogging and would die unless you released the trigger. Would idle all day long. Since this blower is 4 stroke, I changed the fuel lines, replaced the spark plug, adjusted the valves and replaced the carb. Still would not fix the issue. Contacted Dolmar and talked to tech support. And after talking to the head guy he determined that it was a faulty ignition module. Replaced the module and ran like a new one. Since then I have probably replaced 20 modules over the last 10 years.

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Sometimes the tank breather on a backpack blower will get dirty and start causing the engine to bog down. Ignition coils rarely go bad in two stroke equipment. This post makes me remember a Husqvarna I worked on last year with similar issue. I still have the chainsaw and will try a different coil. Thanks for the post.

As I said anything is possible. The first and last time I saw coil failure like this was a Homelite chainsaw about 13 yrs ago. So this is a very rare problem apparently.

Rare

StarTech said:

As I said anything is possible. The first and last time I saw coil failure like this was a Homelite chainsaw about 13 yrs ago. So this is a very rare problem apparently.

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Rare, but not as rare as I would like.

Incomplete history. Did it run well before changing the plug??? Or was this sitting for 5-10 years, then new plug and does not run. I do not know what the status was before you started this thread. When was the last time you used this?, and were you happy with it. Was it run dry, and stored in a dry place. Simple background is helpful.

jviews12 said:

Incomplete history. Did it run well before changing the plug??? Or was this sitting for 5-10 years, then new plug and does not run. I do not know what the status was before you started this thread. When was the last time you used this?, and were you happy with it. Was it run dry, and stored in a dry place. Simple background is helpful.

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The history on this saw is I sold it new to a neigbor customer that burns firewood for heat. This saw is run every year mostly in the fall like now, but does get used over the rest of the year for storm cleanup and various tree jobs. The saw presented to the shop with not running properly, and had the running out of fuel/ restricted carb/dirty carb symptom. But this thread was started to highlight the fact that you can have a faulty ignition system that will mimic a fuel system issue and not loose spark to the spark plug.

I own the high performace version of this saw the PS-5105 that has had 2 igniton modules fail since I owned it. The last one was it would idle perfectly but would die as soon as the trigger was pulled, but would go back to idle if released, or would restart on the first pull if it died. My nephew owns a Solo 694 which was made by Dolmar as the PS-9010 that saw will present with a 4 stroke rev limiter effect when it fails.

Years ago I had a similar problem on a 35hp 2 stroke motor on an ultralight airplane. It would start, run 5 min and quit. Problem there was it forced several emergency landings. New ignition module, problem solved.

I learned to avoid flying over large sections of forest, so that there was always somewhere to land. If I was 5000' altitude or above, it gave me more landing options because I could glide from that height. Engine reliability takes on a whole new meaning in the air.

ILENGINE said:

Lets see if you guys can figure this one out. New NGK BPMR7A spark plug, new OEM fuel filter. Saw starts after about 4 pulls cold and runs a few seconds and bogs down and dies and takes another 4 pulls to restart. Have checked internal carb screen, checked carb adjustment, and checks fuel passages and carb diaphragms are in good shape. Also there is no loss of spark when checked with a spark tester while the engine is running and/or bogging down and dying.

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intake crack or leak?

kjonxx said:

intake crack or leak?

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The answer is in post 13

ILENGINE said:

@StarTech Good consideration but not that complicated. This thing acts like it is running out of fuel but it isn't a fuel problem.

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This isn't a guessing game game show.
If you've already fixed it and know what the problem was then tell the whole story up front.
Many of us don't have time or enjoy the carrot game.
We come here to help people fix problems they can't figure out or two tell them what they're doing wrong or what they shouldn't have done to begin with.

I'm just glad you got it fixed. I have certainly seen Stihl ignition coils that had a timing issue. In fact there was a TSB or something on this a number of years ago. They would run but they were hard to start and they certainly wouldn't run well with stuttering and stammering and what felt like misfires but it did not act like it was starving her fuel like yours did.

I also ran into a weird situation one time on a Husqvarna where if someone puts the wrong type of plug in it will start and run but won't rev up and almost acts like it has a timing retard or governor on it.
Now I can't remember off the top of my head because I just don't mess with chainsaws anymore but I believe it was putting a resistor plug in a chainsaw that came with a non resistor plug.
Of course it could have been the other way around.
I don't think it affected all the Husqvarna saws either but there were a number of them and it was probably based on the type of coil they had or who manufactured it for them that month etc.
I have seen more than one of them do this and all you have to do is switch the plug for the other type and the saw runs beautifully.

It is a shame, and I greatly dislike, that they have gone to so many more needless electronics on these small engines.
I don't want to go back to points and condensers as I really despise those but these are what we always called magnetos and they shouldn't need a whole lot of electronics to make them work because it's all about a coil of wire and a magnet for the most part.

I don't like the new ones that as soon as you touch the kill switch it automatically dies even if you let go of the switch.
I know an argument can be made that this is better for the engine but we had equipment for decades that you could hold the kill switch down for a couple of seconds and let go of it and it would take off running again and that's what I like.
This obviously is a little bit simpler system than the new one that automatically turns off as soon as it detects the kill switch ground.

I don't feel that the coil should be able to affect the initial timing at all.
It should be so basic that all it can do is fire when the magnet rolls past and it's triggered and the only thing that can affect this is the speed of the engine which is proportional to when it needs to spark anyways or the position of the flywheel which should be in the right spot due to being keyed into the crankshaft.
It shouldn't be able to be messed up because there should be no capability and the timing should always be right as long as it fires.
Your story is a good example of what I complain about all the time with humans messing with things that don't need to be messed with.
In this case they weren't trying to reinvent the wheel but they were trying to improve something or fix something that wasn't a problem.