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BF5A revs up then wants to die after throttling up or closing choke

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Samav

Contributing Member
Title says it. At first, the engine would start but couldn't maintain idle, so I cleaned the carb, replaced failing parts, and inspected all jets/passages. Replaced/cleaned fuel system components and fuel lines. Now the engine will idle, but wants to die out after throttling up or closing the choke. It also isn't idling very smoothly, it flucuates by 200 or 300 rpm, maybe more. Seeing as closing the choke makes the engine rev up super hard and run really well, I would say the engine is running wayyyy too lean. I played with the pilot screw and several turns in or out really didn't change the performance of the engine much if at all. Not sure where to go from here, would love some suggestions. Could a vacuum leak at the intake manifold cause this issue? I have attached a video of the issue, make sure sound is up to hear the rev changes.

 
Hi Samav,

Yes, an intake leak might cause something like this. You should be able to find a vacuum leak by "spritzing" carb spray at the manifold and gasket flanges and listening for the rpm to pick up briefly if you hit a bad spot. The hard carburetor mounting spacer on these can crack and cause a leak so make sure you focus there for sure. Usual precautions about flammable liquids and running engine applys.

Do you know the year model?

I see that the older ones have what looks like an anti-stall dashpot. Those operate on vacuum and could also be a leak source.

Have you checked valve adjustment? Compression?
Just curious.

Good luck.
 
jgmo said:
Hi Samav,

Yes, an intake leak might cause something like this. You should be able to find a vacuum leak by "spritzing" carb spray at the manifold and gasket flanges and listening for the rpm to pick up briefly if you hit a bad spot. The hard carburetor mounting spacer on these can crack and cause a leak so make sure you focus there for sure. Usual precautions about flammable liquids and running engine applys.

Do you know the year model?

I see that the older ones have what looks like an anti-stall dashpot. Those operate on vacuum and could also be a leak source.

Have you checked valve adjustment? Compression?
Just curious.

Good luck.
Click to expand...
Thanks jgmo, gotta say you are one of the best contributors on this forum, so just want to let you know I appreciate your help every time. I tried the first thing you said, spraying carb cleaner around the intake manifold and other areas of the carb with no changes in rpm. I will try it again just to double check. I will check the mounting spacer as well. The engine is a 2015, I believe all models 2007-present share the same parts. I don't know if it has an anti-stall dashpot, to be honest I don't know what that is, so please fill me in. Checking valves and compression is the next step for me. Will be looking at that in the next couple of days.
 
Thank you Samav,

Well, to be honest, I really wasn't all that hopeful for this being an intake leak but it never hurts to check and rule it out instead of wondering.

Yes, yours doesn't have the dashpot but I will post the link to the older model that has a depiction of one so you know what I was referencing. Item 13 in the link for a 2001 model below:


I completely agree with checking compression and valve clearance. Are you working with a shop manual? I don't have one so I can't provide you with any specifications..

I also want to point out that you might need to look at the crankcase venting system as it is listed in the troubleshooting section of the carburetor manual for this type of symptom. Although, it doesn't look as if it is very easy to access.
See items 3, 4, 5, 8, 15 and 16 in the link below. You can ignore item 2 as it is for a D model.


In the event that you do find yourself revisiting the carburetor, a couple of questions:

Did you remove the idle mixture screw and spray into the passage while observing that the idle delivery orifice in the top of the carb throat delivers fluid?
Your description of the idle mixture screw making little difference has me wondering why not.

I always like to "backflush" all the delivery orifices when cleaning to ensure that they will pass fluid backwards through the passages that feed them. Flushing them all forwards and backwards, usually more than once, is the only way to get them truly clear.

Another area often overlooked are the high speed and low speed AIR JET tunnels. Those are accessed at the carb inlet mount flange. The low speed is the one recessed further into the carb throat while the high speed entrance is flush with the mount flange on this BC model carb.

Just pointing out some things I know I have missed before.

Good luck figuring this out. I know it's not easy sometimes.

As always, I will be following and cheering for you.
 
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