BP-35 engine/carb problem

[Francis]

20 yr. old Shindaiwa Brushcutter engine. Starts, runs, but only with the choke almost fully closed. With throttle fully depressed, max RPM can be achieved by adjusting the choke, but engine stalls after throttle is released. It is possible to achieve a balance between choke position and less than full range throttle response, but after 10-15 min the engine stops.
I have two carbs for this engine and swapped one for the other, but the problem remained. The engine runs smoothly at idle and at a set high speed and a set in-between, but increasing the throttle, causes the engine to stall.

Is this enough information to suggest the cause of the problem? Thank you for you help!

[StarTech]

First thing to do is to check the muffler for a clogged spark arrester screen.

[Francis]

Thank you. I’ll check and let you know.

[Francis]

Spark arrestor screen is clear. As usual, all screws were removed successfully — except one. BTW, compression, based on pull rope tension appears good. If you have time, what’s next? I won’t be able to respond until tomorrow.

[KE4AVB]

Two cycles can be a pain to troubleshoot if you don't have the necessary test equipment which is why I recommend getting some help from an experienced local two cycle tech.

[bgsengine]

as KE4 mentioned... or at the very least, needing uncommon tools - the first step in cases like this for any experienced shindaiwa/echo shop is going to be along the lines of the following:

1) Compression test - with an accurate non-automotive compression gauge specifically for small 2-stroke engines
2) pressure/vacuum test of fuel system, and any followup tests
3) spark quality test (might fire in open air but not fire under compression)
4) crankcase pressure/vacuum test (any air leaks MUST be fixed)

- most all (if not all) of the above are done before even thinking about wasting time on the carburetor, and most all - all put together, typically take less than 10 minutes to complete and give the experienced tech a LOT of information on what shape the engine is in before even tearing it apart.. which makes diagnosis of a problem much easier...

Unless you have the necessary tools ( or the means to get them or have the tests done) anything else we might do on here would be a "shotgun" approach of ideas of what may be wrong...

Your symptoms might be caused by a leak in the fuel system.. by a crankcase leak.. or, yes, an ignition problem.. beyond the aforementioned exhaust/spark arrestor checks.. so those 4 relatively quick tests would be essential to eliminate possibilities down to a few that we could then direct as to what specific things to look or test for.. Also bear in mind that Shindaiwa has used several different carburetors over the years on some of those models and not all of them are quite compatible with the engine build date.. (Most of the differences are EPA vs. Non-EPA adjustable compliant carburetors, but some are different jetting and different hookups and operation) So, you probably also want to have the serial number of the unit handy...

Which is a more detailed explanation of why KE4 suggested you would be better off getting an assist from a local experienced 2-cycle tech with the tools (and the know-how to use them properly) but if you have the tools, and the ability to understand what we're explaining.. and the mechanical aptitude... we (or I at least) would be happy to assist further...

[Francis]

I’m taking advantage of your offer to help. Thank you. This doesn’t have to work again till spring - so no rush. I think I have what I need to run the compression test, crankcase leak test, and spark strength test but I have a few questions.

What is special about a two cycle compression tester?

Is this compression measurement procedure correct?
Turn off the ignition switch
Open the throttle and choke
Screw the compression fitting into the spark plug hole and attach the gauge
Pull the starter cord until the pressure reading is constant.

Crankcase leak detection:
Seal air inlet and muffler outlet. I need to make a hand pump adapter to attach the pressure hose.
Pressurize 5-10psi - probably 5 psi to not blow seals. Wait 5 min. I’m thinking a loss of 1psi is acceptable.
What do you advise for max pressure and % loss?

Spark strength:
I have an in-line tester that has an adjustable air gap. If the spark will jump a 1/4” gap, it should be forceful enough. Do you agree and/or do you have a different method.

Thanks for your help. Enjoy your weekend!

[KE4AVB]

I'm quoting Stihl test numbers but both pressure and vacuum test are just as important.

Pressure Test

Pressurize to 7.25 PSI. Pressure must remain constant for at least 20 seconds.


Vacuum Test

Pull 15 inches of mercury (.5 Bar). Vacuum must not drop below 8 inches (.3 bar) during the same 20 seconds.

If either test fails you have a crankcase leak.

[bgsengine]

I’m taking advantage of your offer to help. Thank you. This doesn’t have to work again till spring - so no rush. I think I have what I need to run the compression test, crankcase leak test, and spark strength test but I have a few questions.

What is special about a two cycle compression tester?

they have a shorter hose length so they do not lose accuracy (remember these are a small volume of air, and air can compress in the hose before it reaches gauge)

Is this compression measurement procedure correct?
Turn off the ignition switch
Open the throttle and choke
Screw the compression fitting into the spark plug hole and attach the gauge
Pull the starter cord until the pressure reading is constant.

correct. Most 2-strokes will need a minimum of 100 PSI to operate well - 130 - 160 psi would be a generic operating range

Crankcase leak detection:
Seal air inlet and muffler outlet. I need to make a hand pump adapter to attach the pressure hose.
Pressurize 5-10psi - probably 5 psi to not blow seals. Wait 5 min. I’m thinking a loss of 1psi is acceptable.
What do you advise for max pressure and % loss?

Stihl's are similar to Echo's - crankcase pressure test should hold 7 PSI for 1 minute, and should hold 14 in. Hg (14 inches) for 1 minute (also, if that test passes, try shifting crankshaft in and out against bearings - end play may uncover bad spots) Reason being - if they cant hold pressure or vacuum, they can't properly operate the fuel pump (via impulse port) of the carburetor.

Spark strength:
I have an in-line tester that has an adjustable air gap. If the spark will jump a 1/4” gap, it should be forceful enough. Do you agree and/or do you have a different method.

Thanks for your help. Enjoy your weekend!

yes should be able to jump a 1/4 inch gap (with spark plug installed) on an inline tester - may even go as far as 10mm (each mm of gap represents approximately 1 Kv of firing voltage.. spark plug under compression will typically add another 10Kv or so)

Lastly - using the very same pressure/vacuum tester as for the crankcase test, with machine all together, fish out the fuel filter and remove filter (plan on replacing it anyway even if it looks clean.. they do plug up internally) remove filter and attach pressure pump to the fuel line - should be able to pump up and hold 6 - 7 PSI for 1 minute (pop off on a wet test may go as high as 13-14 PSI) if not, metering needle is leaking (maybe deformed diaphragm, or metering lever height wrong, etc), or you may have a pinhole or crack in a fuel line, leaking carburetor diaphragm or gasket, among other possibilities.. If there is a primmer/purge bulb in the system , then you can press the bulb once and release which should cause pressure to drop a bit and then hold steady (if it does, then no need to bother with a pop-off test, which in your case isnt needed anyway since you are getting fuel enough to run a bit) and then you can switch to vacuum side, and use the purge bulb to pull a vacuum and see if it holds vacuum.. (which should also test the purge system as well) and finally if all checks out, you'd put fuel line back in with new filter, and fuel cap, then unhook fuel line from carburetor and hook up tester, try to pull a vacuum (tank vent should prevent any vacuum from holding, though you might get the needle to move) , and then see if tank holds pressure (should be able to hold 2-3 PSI before pressure relief pops off - more than 3 PSI then you have a tank vent problem.. likewise if it holds any vacuum..)

Finally, check and verify the spark plug is the correct type and gap (If I recall it should be a CJ7Y or RCJ7Y champion or NGK BPMR6A or BPMR6Y.. or maybe a BMR6A or BM6A gapped at .025")

if all those check out fine, and you are certain you have clean, fresh gas with 50:1 mix (and not just any old cheapo "2 cycle oil", but modern blended oil meant for chainsaws & trimmers) then pretty much only thing remaining is either the carburetor, or possibly clogged exhaust port (remove muffler and check port for build-up and also check muffler itself for any clogging.. I've seen them plugged up by mud wasps / mud daubers and go through as many as 3 different shops or techs before coming to me with all kinds of expensive work done on them... and all I had to do was cleanout the mud... another possibility , though relatively unlikely would be a sheared flywheel key or bad ignition module (timing off or misfiring)

[bobodu]

See if a shot of carb cleaner helps.....my stand by method for trouble shooting fuel problems...but I'm old....