DBXL Fuel pressure problem with video

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, and others.
Bryan K.

Bryan K.

Well-Known Member
Messages
268
Location
Richmond Hill, Ontario
I did a few runs over the weekend and had a fuel pressure issue with the priming bulb or diaphragm. Towards the end of the video, you can hear the loss of power as the engine barely revs and stays in that flat power curve. Eventually, the motor just stalls from lack of fuel pressure. Lots of bubbles in the fuel line and the fuel will fill about 1-2mm of the priming bulb. When priming I had the same issue, just barely got any fuel to stay in the bulb. So brought it home and swapped out the priming bulb and diaphragm base out of a spare 668 carb I had laying around. Primes just fine now and holds fuel at idle. I haven't tested it on a proper run yet, but seems to have fixed the issue. The odd thing is I put the old bulb and diaphragm back into my spare carb and tried to prime it and fuel filled up the bulb and pushed fuel through the lines with no air bubbles at all. What's the deal? I'm still learning about 2 stoke motors as I'm pretty new to them. I've fully rebuilt the internals of a few already (pistons, crank/rods, rings, bearings, etc) but I have yet to touch the carbs or really know how they work. All my lines and even the tank are brand new and I've tested them all for leaks. The cap is also brand new and vents properly.

So what am I looking at here? I understand that the bulbs crack or become brittle over time, but this doesn't seem to be the case here. What else should I be checking for? Why did this happen on one carb yet it works fine when swapped to another?

In the past I noticed the priming bulb on my Hpi Baja is like 90-95%% full when running yet my DBXL's priming bulb was always 50-60% full while running. Is this an issue and already a sign that something was wrong?


thanks,
 
Last edited:
21bangbang said:
Little air leak most likely. Going through it sealed it up again.?
Click to expand...

Where is it most likely to leak from? Is it the base where there's a diaphragm? Does a gasket or something need to be cleaned or replaced periodically? TBH, with the relatively low cost of a new carb, I don't want to spend too much time fiddling and maintaining them when they can be replaced with a trouble free unit pretty cheaply. Little things I don't mind, but tearing the whole thing apart I will pass!
 
Your carb will need rebuilt from time to time. The diaphragms get brittle and wont flex as much over time. The primer bulb is just there to prime the fuel system. It really has nothing to do with how the carb performs. You changed the metering diaphragm. That's only half the the carb circuit. The fuel pump is on the other side of the carb. It has a diaphragm as well, and is the most overlooked and important one. I just did an extensive write up in carbs. If your not used to working on them I suggest you give it a read.
 
Jshaff16 said:
You ended your video with "but, ..." And that was it. What's up your sleeve
Click to expand...
That video was part of a video for a mod I did for the heat problems with the 5iveB. Its still in the making. Turned out the manifold was loose and while running I was having problems tuning. No matter how much fuel I gave it on the LSN it kept idling up. And the problem kept getting worst, the longer I drove it.
 
Last edited:
all the above could be the issue mate, but also try the fuel tank vent,
if the vents blocked, then after a while of running flat out it can create a vacuum in the tank and the engine/carb cant pump/pull enough fuel through, hence the bulb empties and car starts running like crap, then will stall out as in yer vid,
also you can get heatsoak from the engine as which can also lead to an empty bulb and then stopping,
i've found in the past with heatsoak that the engine will run wide open , but then wont run, or is hard to keep running,
then when engines cooled right down the bulb will fill easy again and it will run well untill heatsoak happens again,
heatsoak is more common if your using an alloy manifold as well, happens a lot less with plastic manifolds,
 
crucialRC said:
Intake manifold is loose. I just ran into the same problem with my losi 5b

Heres a video I made, Bryan
Click to expand...

I checked the bolts on my mani's on all my rigs, they are all very secure. I'm usually pretty anal about my builds. I either thread lock everything or use brand new spring washers after removal. Good tip though.

Seandonato73 said:
Your carb will need rebuilt from time to time. The diaphragms get brittle and wont flex as much over time. The primer bulb is just there to prime the fuel system. It really has nothing to do with how the carb performs. You changed the metering diaphragm. That's only half the the carb circuit. The fuel pump is on the other side of the carb. It has a diaphragm as well, and is the most overlooked and important one. I just did an extensive write up in carbs. If your not used to working on them I suggest you give it a read.
Click to expand...

Will definitely look into this. If it's not too much work I'll give it a shot.

spents said:
all the above could be the issue mate, but also try the fuel tank vent,
if the vents blocked, then after a while of running flat out it can create a vacuum in the tank and the engine/carb cant pump/pull enough fuel through, hence the bulb empties and car starts running like crap, then will stall out as in yer vid,
also you can get heatsoak from the engine as which can also lead to an empty bulb and then stopping,
i've found in the past with heatsoak that the engine will run wide open , but then wont run, or is hard to keep running,
then when engines cooled right down the bulb will fill easy again and it will run well untill heatsoak happens again,
heatsoak is more common if your using an alloy manifold as well, happens a lot less with plastic manifolds,
Click to expand...

I actually thought about heat soaking issues while I was running. It was a really hot day and the added load from the resistance of the surface I was running on wouldn't help either. I don't think that was the issue though as when I got home, hours later I opened the fuel cap and started to prime the bulb with no success. It still wouldn't fill with fuel until I swapped the bulbs. Good info for future reference though.
 
probably the bulb was knackered then mate,
other things for future ref are clunk fell off inside the tank and intake fuel line floating to top,
split fuel line inside the tank so as fuel goes down it starts sucking air as well as fuel,
the gauze filter inside the carb (small circular filter fixed inside the carb ) they do collect grass and crap and restrict fuel flow after a while,
 
crucialRC said:
Intake manifold is loose. I just ran into the same problem with my losi 5b

Heres a video I made, Bryan
Click to expand...
I’m having to much fuel pressure and I’m getting fuel spitting out of the tank when I give it throttle any advice for me?
Tlr 5b tank running a 46gt with a wj71
 
BrapppnBuds said:
I’m having to much fuel pressure and I’m getting fuel spitting out of the tank when I give it throttle any advice for me?
Tlr 5b tank running a 46gt with a wj71
Click to expand...
you sure its pressure mate ?
sounds more like it could be vibration from the 46gt when ya give it the berries :D,
does the wj71 have a return to tank fitting? i may be wrong but i thought the 71 only had a fuel in fitting,
 
Back
Top
✖
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock    No Thanks