BF75S (Pre 1997) Wont Start

[presleymarkw]

This engine was running fine last week. Had just replaced the carb due to the wrong carb being on the engine. Someone had done some fancy work. It ran but crappy, new carb would start in a couple few pulls no problem and running great at full speed and lower speeds as well.

10 days between trips and it wont start at all. Oil light flickers green when pulled. Engine is in neutral. Carb bowl has fuel. Has only ever had clean new gas. Just was out two times on new carb this month. Tank is properly vented. Both plugs show spark when grounded to case.

It doesn't even sound like its almost gonna start. Would I get spark if the neutral switch or the safety switch were issues? Not sure how these operate on a pull start engine.

Haven't checked compression yet. Was just running strong so I wouldn't expect a compression issue to cause the engine not to start that quick especially with no concerning event such as low oil.

 

[ahicks]

Is the choke closing COMPLETELY?

 

[MattFL]

Usually on pull starts the no-start-in-gear protector locks the pull rope. My guess is either not enough fuel or flooded. Also double check the emergency kill switch if it has one.

 

[presleymarkw]

Is the choke closing COMPLETELY?

Yes. New carb. It does have a small arrester that bolts to the carb and to the engine with same bolts so I cant remove it and see directly. The arm is pushing it well to the closed position.

 

[presleymarkw]

Usually on pull starts the no-start-in-gear protector locks the pull rope. My guess is either not enough fuel or flooded. Also double check the emergency kill switch if it has one.

It looks like there was one (empty hole on casing) but its not obvious it was bypassed. This was my question, would I get spark if that was a problem?

 

[presleymarkw]

Yes. New carb. It does have a small arrester that bolts to the carb and to the engine with same bolts so I cant remove it and see directly. The arm is pushing it well to the closed position.

 

[MattFL]

Testing spark is easy enough, just take one of the spark plugs out, put it in the plug cap, make sure the metal part of the plug is touching metal on the motor and pull the rope. You'll see and hear the spark.

 

[presleymarkw]

Testing spark is easy enough, just take one of the spark plugs out, put it in the plug cap, make sure the metal part of the plug is touching metal on the motor and pull the rope. You'll see and hear the spark.

Yup already did as I stated in the original post. Both plugs show spark when grounded to the engine.

 

[MattFL]

Then the problem is likely not a spark issue. Are the plugs wet? If yes then it's flooded, pull the rope with the throttle full open. If no then try a shot of gas or starting fluid in the carburetor.

 

[presleymarkw]

Then the problem is likely not a spark issue. Are the plugs wet? If yes then it's flooded, pull the rope with the throttle full open. If no then try a shot of gas or starting fluid in the carburetor.

Been trying at various points for a couple days so I don't think its flooded. I have not tried with throttle full open I will see if I can manage that. My next thought was also starting fluid.

 

[presleymarkw]

Usually on pull starts the no-start-in-gear protector locks the pull rope. My guess is either not enough fuel or flooded. Also double check the emergency kill switch if it has one.

I have confirmed the emergency shut off switch would not allow spark to the plugs if that were the issue. I have not been able to confirm how the neutral safety switch works on a pull start engine. Mine will pull in gear.

Update: Wiring from the NSS goes to the CDI module. I expect if the neutral switch were an issue I would not be getting spark either.

 

[presleymarkw]

Been trying at various points for a couple days so I don't think its flooded. I have not tried with throttle full open I will see if I can manage that. My next thought was also starting fluid.

So using some starting fluid helps it sound like it wants to start. Starts a little then just dies.

 

[presleymarkw]

So using some starting fluid helps it sound like it wants to start. Starts a little then just dies.

I took off the carb and opened it up and spray carb cleaner through all orifices (I didn't remove the float and the pin and spray under that). Then blew the same out with the air compressor. Put the carb back on and same result. Spray gas into throat of carb lets it start a little then die.

 

[ahicks]

Primer bulb hard? If you disconnect the fuel line at the carb, can you use the primer bulb to push a bunch of gas through the line easily to that point?

 

[presleymarkw]

Primer bulb hard? If you disconnect the fuel line at the carb, can you use the primer bulb to push a bunch of gas through the line easily to that point?

Yes I have drained the bowl and use the bulb to push new fuel into the carb. I can hear the bowl fill and the bulb gets hard.

 

[presleymarkw]

Compression test at 125 exactly even on both sides. A bit low for a 4 stroke but I haven't been able to find the expectation for this specific engine. Also this is just getting the tester finger tight and using the pull rope until it no longer increases. But that should be enough to start and spraying gas in the carb seems to agree with that.

Update BF75 compression ratio is 9.7:1. Using a compression calculator online taking into account elevation (3400ft) works out to 126 PSI.

Guess its time to take apart the carb in full even though it was brand new and working..

 

[presleymarkw]

Final Fix: There is a low speed jet or pilot jet near the idle mixture screw accessible from the outside of the carburetor. I unscrewed it and ran a wire guitar string (G) from the bottom till it grounded out. Then through the larger hole on the side. Screwed it back in. Started second pull and ran perfect.

I had removed and sprayed the channel behind it. I never blew air or anything through its channels didn't look close enough at it.

So if your BF 75 needs to be primed to run then check this jet before a full carb cleaning.

 

[MattFL]

Great job sticking with it and thanks for posting the final answer.

 

[presleymarkw]

Great job sticking with it and thanks for posting the final answer.

No probs. Its a bummer when people don't post the answer if they get one. Read so many dead threads trying to figure this out.

I would have sworn it wasn't the brand new carb! lol But now this engine has really been gone through. Knew it was likely to teach me some lessons when I bought it.

Checked again this morning from dead cold. Took about 5 or 6 pulls. But then once warm its a 1 pull start. Not too bad for an old engine.

 

[ahicks]

No probs. Its a bummer when people don't post the answer if they get one. Read so many dead threads trying to figure this out.

I would have sworn it wasn't the brand new carb! lol But now this engine has really been gone through. Knew it was likely to teach me some lessons when I bought it.

Checked again this morning from dead cold. Took about 5 or 6 pulls. But then once warm its a 1 pull start. Not too bad for an old engine.

There's a lesson to be had there too....