Echo CS 8000 Backfires Through Carb

[miller755]

So I picked up this Echo cs 8000 with some hedge trimmers I needed. And it wouldn't start.
It back fired thru the carb or popped, but wouldn't start and would often flood

So far.
Gets fuel, carb kit seems newer and has the tan diaphragm in it.
Coil resistance good.
Has spark on spark tester.
Flywheel key not sheared
Crank seals look ok, holds pressure test, vac test can pull a vac but leaks.crank seals on order.

Had about 150 psi compression it was put together without a gasket by a previous person. Had some scoring I'm guessing straight gas, since it was everywhere in the cylinder.
Cleaned up piston cyl and rings, rings also on order, put together with motoseal let dry overnight 120+ psi on 3 pulls still about 150+ topped out.
Still only backfires thru carb
Ideas
I feel like the coil would either work or not, but it's like it has a timing issue.
Oh and coil doesn't appear to have been replaced, so it shouldn't be a coil/flywheel mismatch.
Could leaking crank seals cause this? Idk, might order a carb kit too.
Trying not to throw a bunch of money at it.
It's just a bonus saw. And if it needs hundreds in parts it would be better as parts
Thanks in advance if you have any ideas.

 

[miller755]

Oh and I've tried two different plugs both show spark when grounded.
Maybe another idea which may mean timing is that it seems like it pops/backfires easier when I have the spark tester inline, like maybe it's causing some delay? Idk it's a harbor freight spark tester.

 

[Old2stroke]

Oh and I've tried two different plugs both show spark when grounded.
Maybe another idea which may mean timing is that it seems like it pops/backfires easier when I have the spark tester inline, like maybe it's causing some delay? Idk it's a harbor freight spark tester.

So I picked up this Echo cs 8000 with some hedge trimmers I needed. And it wouldn't start.
It back fired thru the carb or popped, but wouldn't start and would often flood

So far.
Gets fuel, carb kit seems newer and has the tan diaphragm in it.
Coil resistance good.
Has spark on spark tester.
Flywheel key not sheared
Crank seals look ok, holds pressure test, vac test can pull a vac but leaks.crank seals on order.

Had about 150 psi compression it was put together without a gasket by a previous person. Had some scoring I'm guessing straight gas, since it was everywhere in the cylinder.
Cleaned up piston cyl and rings, rings also on order, put together with motoseal let dry overnight 120+ psi on 3 pulls still about 150+ topped out.
Still only backfires thru carb
Ideas
I feel like the coil would either work or not, but it's like it has a timing issue.
Oh and coil doesn't appear to have been replaced, so it shouldn't be a coil/flywheel mismatch.
Could leaking crank seals cause this? Idk, might order a carb kit too.
Trying not to throw a bunch of money at it.
It's just a bonus saw. And if it needs hundreds in parts it would be better as parts
Thanks in advance if you have any ideas.

Easy to check the timing if you have a timing light, a good tool to have if you are going to be repairing chainsaws.

 

[ZeroJunk]

I know nothing about Echo saws. But, a lot of newer model saws have delay built in to the coil to make them easier to start. Could be a timing issue in the coil itself, not a sheared key.

 

[miller755]

So using a timing light on a saw that doesnt start. How does that work? Mark the clutch and TDC and ±15° degreess and turn the saw over with adrill? Mark the flywheel?

 

[ZeroJunk]

Normally, if they have compression , fuel, and fire at the right time, but won't run they are flooded.

 

[Old2stroke]

So using a timing light on a saw that doesnt start. How does that work? Mark the clutch and TDC and ±15° degreess and turn the saw over with adrill? Mark the flywheel?

Yep, do a search on this forum, there is lots of info on how to do it.

 

[Captain Bruce]

So using a timing light on a saw that doesnt start. How does that work? Mark the clutch and TDC and ±15° degreess and turn the saw over with adrill? Mark the flywheel?

This one gets style points......how does anyone hold a timing light, in 1 hand, and a drillmotor in the other? Priceless humour.

 

[miller755]

Normally, if they have compression , fuel, and fire at the right time, but won't run they are flooded.

a possibility, ill order a carb kit, i have tried factory carb settings(L 2-3/8, H 3-3/8), choked/not choked, after airing out for a day etc. 1 turn out on lo jet, still no start. I have read that the tan diaphragm can pump too much fuel so idk.

 

[miller755]

This one gets style points......how does anyone hold a timing light, in 1 hand, and a drillmotor in the other? Priceless humour.

Yeah thats what i was thinking, but idk it must be simple, since i was scolded for asking...

 

[Old2stroke]

Yeah thats what i was thinking, but idk it must be simple, since i was scolded for asking...

Here you go, took awhile to dig it out, just didn't feel like typing it all over again. There's a lot more good info relating to timing if you have the patience to do the searching.
https://www.arboristsite.com/threads/how-much-ignition-advance.356118/post-7621913

 

[leaf]

Yuh, and for timing, it is common to print out a "degree wheel" from one of the websites that provides them for scaling and printing on your laser printer, or on your friend's laser printer ( or on some other computer printer). Often done just on paper and then stuck onto a CD. ( Round plastic thing - used to be for storing data.) Mount by hole in middle with adapter onto crankshaft....

The timing light can "hold itself" by being put (even duct-taped) onto a sturdy enough stand...

Usually saw with a throw-out bar on it, clamped into vise.

Can even operate drill by sticking "trigger" on and putting switch in line cord.... (variable speed, if you got it) can be kinds nice .... "imitate" start (pull) speed.

 

[ZeroJunk]

He already said the flywheel key is not sheared.

 

[SteveSr]

He already said the flywheel key is not sheared.

Yea, but the ignition module may be defective and firing at the wrong time. These are NOT simple coils anymore. Most have processors in them to program the advance curve.

What did Reagan say? "Trust but verify".

 

[ZeroJunk]

Yea, but the ignition module may be defective and firing at the wrong time. These are NOT simple coils anymore. Most have processors in them to program the advance curve.

Waht did Reagan say? "Trust but verify".

They still idle OK, they just don't advance. It's not like they are going to fire when the magnets are 180 degrees away or something.

 

[leaf]

Agreed that seeing spark in a grounded plug and an intact flywheel key are good "indicators." But there are ignition systems that begin to sag when the plug is in the piston with the compressed mixture, there are coil failures when cranked under compression rotational speed conditions, etc. I thought the conversation had somewhat turned to how one does spark timing as such....

 

[Old2stroke]

They still idle OK, they just don't advance. It's not like they are going to fire when the magnets are 180 degrees away or something.

They actually have a number of different failure modes. The maximum advance is set by the position of the coil and FW magnets relative to TDC and is typically close to 30 deg BTDC, the electronics will retard the ignition from this setting to make for easier starting and it is often this retard function that fails, leaving the ignition set to the maximum advance and if that's the case, you can expect some kickback on the cord when pulling it over and even backpressure through the carb. Unfortunately, if all possible faults have been eliminated and you are convinced there is a timing problem, the only valid test is to check the timing with a timing light or swap the coil for a KNOWN good one. If you are thinking of a new coil, don't risk money on an AM module as too often their timing curve does not match the OEM one.

 

[ZeroJunk]

They actually have a number of different failure modes. The maximum advance is set by the position of the coil and FW magnets relative to TDC and is typically close to 30 deg BTDC, the electronics will retard the ignition from this setting to make for easier starting and it is often this retard function that fails, leaving the ignition set to the maximum advance and if that's the case, you can expect some kickback on the cord when pulling it over and even backpressure through the carb. Unfortunately, if all possible faults have been eliminated and you are convinced there is a timing problem, the only valid test is to check the timing with a timing light or swap the coil for a KNOWN good one. If you are thinking of a new coil, don't risk money on an AM module as too often their timing curve does not match the OEM one.

Back in the day, they were all 25 degrees or more BTDC.

Not sure what his problem is, but I'll bet you 5 bucks if the coil has fire at all it is not the coil.

 

[heimannm]

 

[leaf]

For whatever it may be worth -- and you can say it's not much (and that doesn't bother me) -- I have had the problem of spark timing at start (including 'kickback' etc.) when the plug has shown spark and the flywheel key is fine, and I have had the problem of very weak to no spark in compressed conditions at slow rpm when the plug has shown spark (admittedly a little funky-looking, not the sharp little blue snap). Agreed most of the coils that fail just completely fail - no spark at all, a winding opens, or a semiconductor eats it - but I don't think those are 100% of the ignition failures.