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Strange carb issues on STIHL
#1
D
dingdiddydong
D
dingdiddydong
Im reviving this dead bg 75 and it's proving to be stubborn. Replaced fuel lines cleaned out gas tank ultra sonic cleaned the carb. Gaskets seemed reusable and the diaphrams were pliable. Primer bulb is not cracked. Cleaned off the air filter too and replaced starter cord. The carb is a walbro wt 413. Not made anymore and seem to be rare. Walbro says start at 1.25 turns out for the adjustment screws in their manual. Stihl says 1 turn out. The engine was flooding nonstop with both of those settings. So i tried tightening the screw clockwise about a quarter turn. Unflooded engine cleaned spark plug. Engine starts after first pull. I fully tune the carb and set idle speed to correct rpm. Put it in my car on its side. I come back and their is a puddle of fuel on my floor mat. The carb, air filter box, plastic housing, fuel lines, and gas tank were covered in gas so I cant pinpoint the leak. Let it sit upright overnight and didnt notice any leakage at all it seemed to mostly dry. Anyway i tried starting it again and it kept flooding probably from the leaked fuel. I had to tighten the adjustment screws more to get it to start and quickly open them more to avoid running lean. Anyway the thing wont stay running unless the idle low screw is about 3/4 turn out and same with the high speed. Since this contradicts both companies manuals Im apprehensive to run this thing adjusted at 3/4 because i dont want to burn up the engine. The only part of this blower I luckily havent had to crack open yet. Anybody have any tips here? Should I just not worry about it and run it like this because it seems to run right. Or is this carb or gaskets the culprit and not to risk it because itll burn up the engine. I guess I'm wondering if I could tell if its running lean. It just goes higher in rpm the leaner it goes so i dont want to try and push it to see if it does because 7k rpm with no oil will hurt this engine.
#2
B
bertsmobile1
B
bertsmobile1
If it is leaking fuel when sitting fuel cap down, that is normal at best or a bad cap at worst.
The settings given in the spec sheets ARE THE STARTING POINTS and are usually a little rich.
They are there just so you can get it to start and run for long enough to do a proper tune
The tune is to wind in the screws till the engine sounds like it will stall out then screw the needle out till it is about to stall out rich.
The correct setting is 1/2 way between the two and erring on the rich side rather than the lean.
Having set the H you then do the same with the L
Because the L affect the H you then repeat the process for both jets
After this set the throttle stop to give the desired idle speed.
It is not uncommon to go through this sequence quite a few times before you get it right.
If you have access to a small pressure pump with a reasonable gauge, pull the return line off the carb and pump up the tank
IT should hold 10 PSI without leaking
And some where between 10 & 15 the tank should vent
Running pressure is around 5 to 7 psi
The entire fuel system is pressurised
The needle is set to around 15 psi, any more than that and it will not be able to shut the fuel off.
This is called the "pop off " pressure but should not be tested as doing this can damage the carb.
The settings given in the spec sheets ARE THE STARTING POINTS and are usually a little rich.
They are there just so you can get it to start and run for long enough to do a proper tune
The tune is to wind in the screws till the engine sounds like it will stall out then screw the needle out till it is about to stall out rich.
The correct setting is 1/2 way between the two and erring on the rich side rather than the lean.
Having set the H you then do the same with the L
Because the L affect the H you then repeat the process for both jets
After this set the throttle stop to give the desired idle speed.
It is not uncommon to go through this sequence quite a few times before you get it right.
If you have access to a small pressure pump with a reasonable gauge, pull the return line off the carb and pump up the tank
IT should hold 10 PSI without leaking
And some where between 10 & 15 the tank should vent
Running pressure is around 5 to 7 psi
The entire fuel system is pressurised
The needle is set to around 15 psi, any more than that and it will not be able to shut the fuel off.
This is called the "pop off " pressure but should not be tested as doing this can damage the carb.
#3
D
dingdiddydong
Ok cool so I should just tune the thing by feel then. I dont have much doubt itll run good then as long as its not lean. I assumed that if it was too lean it would just stall and die but wasnt sure if I should push that if it would damage the engine. I'll try again in the morning by running it leaner than i had but still feeling on the rich side without affecting the performance negatively. Thanks a lot for your input
D
dingdiddydong
If it is leaking fuel when sitting fuel cap down, that is normal at best or a bad cap at worst.
The settings given in the spec sheets ARE THE STARTING POINTS and are usually a little rich.
They are there just so you can get it to start and run for long enough to do a proper tune
The tune is to wind in the screws till the engine sounds like it will stall out then screw the needle out till it is about to stall out rich.
The correct setting is 1/2 way between the two and erring on the rich side rather than the lean.
Having set the H you then do the same with the L
Because the L affect the H you then repeat the process for both jets
After this set the throttle stop to give the desired idle speed.
It is not uncommon to go through this sequence quite a few times before you get it right.
If you have access to a small pressure pump with a reasonable gauge, pull the return line off the carb and pump up the tank
IT should hold 10 PSI without leaking
And some where between 10 & 15 the tank should vent
Running pressure is around 5 to 7 psi
The entire fuel system is pressurised
The needle is set to around 15 psi, any more than that and it will not be able to shut the fuel off.
This is called the "pop off " pressure but should not be tested as doing this can damage the carb.
Ok cool so I should just tune the thing by feel then. I dont have much doubt itll run good then as long as its not lean. I assumed that if it was too lean it would just stall and die but wasnt sure if I should push that if it would damage the engine. I'll try again in the morning by running it leaner than i had but still feeling on the rich side without affecting the performance negatively. Thanks a lot for your input
#4
B
bertsmobile1
B
bertsmobile1
Running lean will destroy the engine in a very short amount of time.
Running rich will clog the spark screen on the muffler & build up carbon on the exhaust port which can fall off & score the bore.
The latter will usually take a lot longer than the former.
The trap most "by ear" tuners fall for is not using stalling rich to stalling lean.
They tune to the highest speed which will always be too lean.
I usually put a sock over the end of the blower outlet to apply a load to the engine when tuning them.
On string trimmers I run a full length of heavier line fr the same reason
Chainsaws get tuned cutting wood, that is a tad more difficult.
Running rich will clog the spark screen on the muffler & build up carbon on the exhaust port which can fall off & score the bore.
The latter will usually take a lot longer than the former.
The trap most "by ear" tuners fall for is not using stalling rich to stalling lean.
They tune to the highest speed which will always be too lean.
I usually put a sock over the end of the blower outlet to apply a load to the engine when tuning them.
On string trimmers I run a full length of heavier line fr the same reason
Chainsaws get tuned cutting wood, that is a tad more difficult.
#5
D
dingdiddydong
D
dingdiddydong
I will try and meet in the middle of rich and lean and then edge very slightly richer to keep from any potential damageRunning lean will destroy the engine in a very short amount of time.
Running rich will clog the spark screen on the muffler & build up carbon on the exhaust port which can fall off & score the bore.
The latter will usually take a lot longer than the former.
The trap most "by ear" tuners fall for is not using stalling rich to stalling lean.
They tune to the highest speed which will always be too lean.
I usually put a sock over the end of the blower outlet to apply a load to the engine when tuning them.
On string trimmers I run a full length of heavier line fr the same reason
Chainsaws get tuned cutting wood, that is a tad more difficult.
#6
D
dingdiddydong
D
dingdiddydong
Got it running good today. I adjusted it to slightly rich on high and low ends and lowered the idle speed to the minimum i could without it dying. I did all this with the engine under the load of the blower tube which is a duck foot looking one. Will be using a tachometer at the shop tomorrow to set idle speed and fine tune the carb.