[luketrash]

My friend told me that he thinks my high RPM cutting out issue on my CB350 above 7k rpm is due to the little inline fuel filters I'm using on each fuel line to the carbs.

I'm using these little ones

He said the flow from the petcock on these things is so poor that you'll starve out the float bowls if you don't have a direct line to the carbs.

My petcock won't shut off even with a new whizzer valve gasket installed, so I have two of these also inline:

This way I can shut off fuel for good, but again he's saying that I have too much stuff between the fuel tank and the carbs to allow the fuel bowls to remain full at wide open throttle.

Does this make sense? Is there a way to check for that? I was looking down at my fuel filters while the bike was cutting out and they had gas in them.

I'm wondering if there's some sort of other mechanical issue I haven't addressed yet.. cam chain stretch or the advance weights being out of spec and letting it advance too far or not enough?

 

[tbpmusic]

I suppose it's possible, but there may be other causes.
When the bike acts up, try slipping it a little bit of choke - if it helps, then you may have a fuel starvation issue. If not then you may be in a fuel-rich condition, for any of numerous reasons.
But it also may be jetting - you don't say what air cleaners and mufflers you're using, so can't tell - 350's are much more sensitive to those changes than a 450 would be.
Check the timing with a strobe - that will let you know if the advancer is working right. The advancer is usually fully "engaged" by about 4,000 rpm.......

 

[luketrash]

I'm running a set of clean OEM air filters with the stock airbox configuration.

The mufflers are straight through glass pack shorties:

So yes, rejetting is probably an issue. I'm at 1000 ft elevation so things already run on the lean side with factory settings down here.

However, pulling the choke in a cut-out scenerio doesn't do anything. Doesn't help it recover or flood it out.

And I've done plug chops (shutting off the engine when this happens) and they look brown, not crusty white (lean) like I'd expect.

So it's probably spark going away, not fuel delivery. The bike has performed identically regardless of which set of coils I'm running. I've got a set of blue Tec brand and a tan set of ND brand. My breaker points are new. Advance according to my timing light is working. Condensers are new as well, but shouldn't matter much. Runs the same with new or the old condensers installed.

I've got a brand new set of aftermarket XS650 coils that I was saving for my XS650. I suppose I could check those but have been too lazy to fabricate the bracketry to fit them.

 

[tbpmusic]

Maybe it just doesn't like those shorties.........
Ever check the compression?

 

[luketrash]

Compression was about 190psi in both cylinders when the engine is warmed up.

The mufflers helped it actually. I was running gutted OEM mufflers, but due to the fact one was gutted more than the other, things were out of balance. Now that I have the balance of two matching mufflers I should probably upjet the mains by about two numbers.

Still need to verify the spark at high RPM somehow though. I'm not sure of a good way to do that other than rev the heck out of it with a timing light.. I might do that.. I'm not sure the high RPM issue exists when revving it in neutral though. That'll be my next check. I just threw the 'too short' battery into my CL and bought the correct 14 amp battery for this bike, but again, that didn't fix the issue.

 

[dougqt]

Looks like the same setup I have on my 71 CB350 and I don't seem to have trouble with it. 1/4 inch inlet and outlet on the filters, screen mesh. No problem.

 

[luketrash]

The difference is that it looks like your petcock is changed from OEM. Apparently the flow through the petcock is the issue. If I had a way to fit pipe to the opening, I'd probably go that route. I hate the petcocks on these things because all the ones I have leak even though I've rebuilt the parts in them.

 

[dougqt]

Very true. My original petcock was shot. The high quality aftermarket replacement part you see in the picture is actually a steel natural gas line threaded tee that I smoothed the outside of on the bench grinder and threaded in 1/4 inch brass nipples. Works great and only cost 5 bucks. Much higher flow than the original petcock and the fuel shutoffs serve the same purpose. Just no reserve so I have to pay close attention to my fuel level. I have like a $0.00 motorcycle budget, so sometimes I have to improvise a little . The Honda dealer wanted 60 some dollars for one and this fix got me on the road till I can track one down cheaper (maybe, because I actually kinda like it the way it is.)

 

[luketrash]

Does the fitting fit on there without modifying the threads of the tank? If so I think I'll go the same route.

 

[dougqt]

No modifying the threads at all. Threaded right on. I sanded off all the lips and casting marks and numbers on the outside with the benchgrinder. It's really thick so you can grind on it a lot. I took the petcock to Home Depot and tried threading it onto pipes until I found a size that fit. Then I found a tee that fit the pipe. Trial and error and unfortunately I cannot remember the size, but they had one that threaded on perfectly. When I get home I will take a closer/clearer picture.

 

[luketrash]

Awesome. I'll be trying this over the weekend. I took all the paint off my tank this week, so I'm not too worried about leaking gas onto anything!

 

[boogiesg]

Well, I think the way you have the filters mounted is definitely going to pose a starvation problem if you have a particularly dirty tank. Run the fuel lines down from the petcock in a low 'U' shape like you have now, and place the filters below the carbs so that the fuel is traveling up through them. This way, the fine particulates will wash away from the filter surface and settle in the low spot on the fuel line. Try it! I had a 360 that starved because the filter would clog. The "previous owner" did the worst creme job in history to mask the rust. It didn't work.

As for jetting and fuel delivery, I'll give you a race proven cb350 setup and you can adapt to yours. My racer tank is 100% rust free (on the inside) and sealed with POR-15. There is no filter sock on top of the oem petcock, all of the screens and rubber nonsense are gutted except for the valve, which is necessary for obvious safety reasons. I have a buddy that uses a gas-stove valve for his petcock and it works great. Make sure the filler cap can breathe, if fuel delivery improves with the cap off, locate the breather hole and clean it out. I use zero filtration because as your friend pointed out, they do restrict fuel flow. Filtering dirty gas is just complicating the real issue and making things a headache, I totally sympathize with your situation, POR-15 is the fix.

I have the same exhaust setup as you except my headers are an old set of Hooker Headers, with no baffles.
I don't use any air filters whatsoever and neither does anybody else racing 350/500gp on honda 350. I do use the oem rubber carb boots though, and I recommend that you do as well, even with pod filters. The problem with running pods straight into the carb is that the air is turbulent. K&Ns breathe better than paper, but the oem airbox has a lot of paper with a lot more surface area. Those holes on the carb inlets need the air to flow smoothly or your slide will never function right, that's what those boots are for. Some may call it a poor-mans velocity stack. The second I remove them, the bike runs poorly. This is quite possibly the most free breathing setup that can be obtained on cb350 without rule breaking mods like valve work or intake porting. (stocker class). With all that in mind, my main jet only went up in size to #120! All the little rubber o-rings are kept nice and fresh too, those things can cause some hiccups if they leak. This post went a little long, but I hope it helps get you on your way.