Stihl 028AV Not Running

[Talltone]

Hello all, my first post on Arbtalk so hope you can help please.

Have used my 028AV over about 10 years for domestic tree felling and woodcutting. Suddenly, recently experienced the saw cutting out after a short time but would then re-start and cut out again moments later. In the end, suspected carb probs and cleaned / rebuilt it using new parts including new fuel line/filter and impellor. Also cleaned out exhaust box (needed it!). Saw was then very hard to start and when it did ran at v high revs before cutting out again.

Turned attention to electrics - change of plug, clean /adjust position of electronic ignition (credit card trick? - no, used feeler gauge). Spark seemed weak but was present. Now can't start at all - very frustrating. Compression seems very good and is expected as saw has had only light domestic use.

 

Any clues please on where to focus my attentions? I'm an engineer so am very careful about what I do and repair loads of stuff but this has stumped me.

Be ideal to swap either the carb or electronic ignition to help diagnostics. Carbs are on ebay for just £16 via Hong Kong - are they any good?

Anyone have a service manual?

Grateful for any help, saw is too good to abandon.

[Adamam]

I've used a couple of Chinese carbs and they've been absolutely fine.

 

Have you checked the impulse hose? They're quite prone to failure on 028.

[spudulike]

I would usually pop off the muffler and inspect the piston through the exhaust port - if it is scored, the lack of compression could be causing your issues despite it feeling OK!

 

Failing that and assuming you have fitted the carb parts correctly, it may be a split manifold, faulty seals or impulse line. The fast running sounds like a big air leak so don't just adjust the carb as it is likely to seize if not corrected!

[Talltone]

Thanks for advice given. Had changed the impulse hose as part of carb strip down ( was hoping it was the original fault source but no luck there).

Carb also checked and checked again against assembly instructions so should be OK.

Compression is really good though accept need to do a proper check.

Air leak possibility is interesting and will be investigated but as said all parts reassembled carefully.

Out of interest, carb kit was supplied with carb to manifold gasket where none was fitted previously. Comments online suggest that rubber manifold tube fulfills that role and no gasket was fitted originally - any comments?

Again, would like access to a service manual for the 028, any pointers to where I might find, have tried online without success.

[Richard Thompson]

I had similar problems with an 038 ( weak spark but present ), turned out the ignition module behind the flywheel was knackered so i replaced it with a new type of "small engine" ignition module from Ebay and the saws a fantastic runner now ?? the modules are so good i replaced the module on my 051 as well and its improved that saw as well

 

http://www.ebay.co.uk/itm/Lawn-Mower-Part-Mega-Fire-Ii-Ignition-Module-UNIVERSAL-/172422102653?hash=item2825287e7d:g:2zsAAOSw8gVX6j~x

Edited November 26, 2016 by Richard Thompson

[spudulike]

A very basic spark test is to bend the end electrode at a 45 degree angle from how it normally sits thus opening up the gap to 5-6mm and see if the coil will produce enough spark to jump the gap. If it won't then there is your answer.

 

It isn't conclusive as it cant tell you if the coil is breaking down under heat during use but is a good first step. I would check the coil to flywheel gap and try closing that down to one piece of thick A4 paper before writing it off. Sometimes the magnets get a little weak and this can get the thing working again!

 

The fast running thing is weird as a faulty coil will usually cause no running or faulting out after a few minutes.

 

The very basic diagnosis is compression fuel and spark, the engine must have all three to work! Fuel systems are the most common area of issue.

[Talltone]

Thanks for all advice given.

 

Not sure I can use the electronic gismo as this model has electronic ignition anyway - no contact breakers etc but will try to reduce the magnet to ignition module gap as recommended.

 

Will try the extended gap diagnostic too and examine the spark energy.

 

Have just purchased a s/h carb so hope that might give me another steer too.

 

Will let you know what gives.........

[Talltone]

Hello, calling all Arbs again!

Cold weather and vacations have interrupted recent work on my Stihl 028 but have the following progress to report BUT chainsaw still not working properly.

 

1. Adjusted the flywheel to coil gap with A4 paper as recco'd.

2. Checked spark with an old plug with electrode bent to 45 deg as recco'd. Not a fat, blue spark but still jumping the gap OK. NB:This was done with cordless drill and socket on flywheel nut giving saw a good turnover.

3. Fitted replacement secondhand carb. Saw started and ran for a minute or two, more or less normally, then died and would not re-start despite many attempts.

 

I'm inclined to think now that it is only a carburation or rather a fuel problem.

 

So, as the symptom persists, despite two carbs being used, it implies that the impulse line is not functioning properly despite new tube being fitted. Assume saw started initially because of fuel provided by many revs using cordless drill.

 

So again, need to know how impulse is provided to carb and whether there is a valve system to clean and what access is required? Are there any tests to assess if actually working properly?

 

Any help very gratefully received.

[peatff]

Study Walbro and Zama diaphragm carbs to see how they work. Here's a simplified youtube to get you started. You'll see that the valves are part of the carb. There's a test to see if the needle valve is sealing shown in the Walbro video (second one) Strip your carb and clean it out then put it back together and check everything is in the right place and the right way up.

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[spudulike]

On the bottom of the carb, there is a union that pushes in to a rubber hose (not the fuel line). This connects to a brass union sticking out of the crankcase just below the cylinder and under the inlet manifold.

 

In usual cases, the pipe is either connected or not and if new, shouldn't cause issues unless split on replacement.

 

If you strip and drain the carb, make sure the tank is depressurised by not plugging the fuel line - it may pump out a good stream of fuel. Connect the carb fuel line and impulse to the carb, pull over the saw 10-15 times. Strip the carb and see if it is full of fuel. If both sections of the carb are fuelled, the impulse and pump diaphragms and needle are OK.

 

You could just do a full pressure and vacuum check but that would spoil the fun:001_rolleyes: