Restricted Fuel Flow

I'm having some trouble with fuel flow to the carbs. A valve job and a good vacuum sync are in order to get everything fine tuned, but she was running fine and then all of the sudden started to not run right. When I throttle it wide open I just kind of here a clicking sound from the carbs and you can tell its not getting fuel. The engine is just kind of stumbling along like this. Hard to start and a rough idle. (But remember I've just rebuilt the carbs and she was running okay for a little bit).


Petcock is checked and good. Fuel lines were replaced. An inline filter was added. And there are no clamps on the fuel lines (its a pretty snug fit, but not perfect).

So, I'm thinking (correct me if I'm wrong) either:

1. Fuel filter is restricting flow

or

2. No clamps on fuel line is causing a vacuum leak and its not "sucking" fuel

Any thoughts?

 

A fuel filter can restrict fuel flow if installed to high. The XJ bikes are gravity fed and a filter too high will block fuel flow. Sometimes a filter that is too large will cause the same problems. I use filters that are 1" or smaller in diameter.
I hope we can assume the carbs are in good order with no sticking floats and clean jets.

 

I think your "clicking sound from the carbs" is actually lean detonation from the motor or intake backfire...carbs don't make noise.

Either way, try to avoid doing that. Can hurt it bad.

I've seen people put new lines on, and make bends with them that work until heat/fuel soften them up a bit, then they fold and kink. Check for this first, just as an easy visual check...

 

Done that one myself... twice. :lol:

 

I hope that we can assume the carbs are in good working order too cause I worked really, really hard on those.

Could you elaborate on installing a fuel filter too high?
I'm using a smaller one. Its a little clear w/ crushed bronze filter.

And I was very careful about not the routing of the fuel line. Nothing should be getting pinched. But I'll recheck when its hot.

@jeffcoslacker. I understand that carbs don't make noise, but I was just trying to describe what it "felt" like and sounded like. Lean detonation? That would make sense, I suppose. I only tried it open throttle like that for a second to see what would happen, because at lower throttle it just kind of bogged down and died out. I kept having to play with the throttle waiting for it to "catch" and when I played with the choke a little it did. For a little bit anyways, can you tell me more about predetonation (in case that is the case)?

 

An overly lean mix will detonate early, and burns faster and hotter than the correct mix. When I had the big air leak in one boot, my first clue was it actually made that plug's high temp paint I put on my spark plug boots fluff off...it made the plug that much hotter than the other 3.

You get lean backfire through the intake when it's so lean that the incoming lean charge is lit off by the heat of the cylinder before the intake valve even gets a chance to close, or valve/ignition timing is wrong. It usually sounds like random clicking/snapping in the intake, as it's muffled by the carbs and airbox. I suspect that's what you are hearing.

Pinging detonation is the sound of the pistons being hit with a flame front before coming all the way to top dead center, which smacks them hard on the way up and rocks them in the bore. Usually more of a timing issue than mixture.

 

You don't think the cam timing jumped, do you? Not wanting to scare you, but it's a possible explanation for what you describe too.

 

I had one of my bikes acting up like that and there was an air bubble in the fuel filter. once I got that out the problem went away.

MN

 

What I mean by too high is when the filter installed above the carb hats. The fuel has to flow down through the petcock , up to the filter then down to the carbs. Its a good place to get a air bubble and that will stop fuel flow.
The small filter you have is perfect, just be sure the fuel flows down. Not down, up then down.

 

I don't think the cam timing jumped. Not entirely sure how that would happen. Everything seems to be moving in proper time and order.

The filter and fuel line do go down and then up over the carb hats. Its possible this could be the problem. Its all squeezed really tight in there.

I'm debating taking the filter off just to see. I lined my tank and everything's clean so I'm not too worried about it, but I do like having it in there. If nothing else than for piece of mind on the work I just did to the carbs.

I think I'll try that, and I'll shim my valves, and then try and vacuum sync this baby and I think that should eliminate most of the variables we're discussing. What do you think?

 

jeffcoslacker is a smart cookie on this one. coils fire the plugs around the top of the compression stroke and another "dead" fire at the top of the exhaust stroke, with cam timing out you can get some gnarly backfires due to valve overlap. That magic time when both valves are open and some of the outgoing exhaust flow helps draw in a head start on the fresh intake charge. Backfires from this can be mighty wicked, slamming CV slides up in their bores and such. From the OP's tone and description I don't get the impression this is it but plusses for the though Jeffco. Unitl/if we get some new evidence for or against, I'll cast my lot in with folks suggesting mixture issues.
Any amount of pinging or ping-like behavior will feel like a power loss, especially in contrast to the good amounts of power being made just before predetonation starts occurring.

 

put the fuel valve on run, pull the line off the out end of the filter, get a can to catch the gas and turn the valve to prime, let some gas squirt out till there's no bubble in the filter, back to run
now take the vacuum line off the motor and give a little suck on it should be the same flow, now plug the port, start the bike and let it run out of gas, shut the key off, connect the lines all back up and turn the valve to prime watch the filter
for any little bubbles to tell when the carbs are full, still on prime, start it up, should be lots of gas for it to run, then valve to run
both of mine have a bubble in the filter, doesn't seem to matter

 

The Fuel Filter can look like its Full and perfectly clean and NOT be sending the fuel down to the Carbs.

I pulled-off a Inline Filter of my 900 that was made with a NON-porous Paper Filter Element.
I would not let Gas through it.
Made in China.

A FRAM Inline Filter worked like it should.
Adding a Filter can cause Fuel Lines to Kink.
If you think a Fuel Line is at risk for Kinking, ... Splice-in an angle fitting or make a FORM out of a length of Coat Hanger wire and Tape the Fuel Line to it to prevent the Fuel Line from being able to kink and shut-off or slow-down the flow.