Author Topic: Cb750 runs in 1 km stretches, then dies, runs again after cool down. (Read 8652 times)

b1jackson · « **Reply #50 on:** July 02, 2017, 06:03:00 PM »

Floats all measured with micrometer. I consider that more accurate than a clear tube but I understand your theory.

scottly · « **Reply #51 on:** July 02, 2017, 06:11:04 PM »

Drain the tank until the level over the screen is just above where the problem starts, and then check the flow again. See what the level is when it stops flowing compared to the screen. Once it stops, check the flow with the petcock switched to reserve.

calj737 · « **Reply #52 on:** July 03, 2017, 03:07:36 AM »

Quote from: b1jackson on July 02, 2017, 06:03:00 PM

Floats all measured with micrometer. I consider that more accurate than a clear tube but I understand your theory.

That would be true if a micrometer showed the actual fuel level with the carb installed. The advantage of the Clear Tube method is it verifies what is actually occurring inside the float bowl as the floats respond to fuel. It is possible (perhaps not your situation) that a float is damaged, the float needle is hanging up, or the seat is worn. A micrometer won't confirm that.

Eydugstr · « **Reply #53 on:** July 03, 2017, 06:46:40 AM »

+1 for the clear tube test, for all the reasons calj737 just listed above.

Had a similar problem with my CB750K (a '72K with a '76K tank), which had the single outlet fuel tap on the LH side. Except in my case, the bike wouldn't die, it just wouldn't hold a speed above 60mph without cutting out. It would accelerate past it, but it wouldn't cruise for long periods of time at higher speeds.

Did three things that cured it. After scouring through Hondaman's book, there's a small section where it talks about the single pipe LH fuel taps. He recommends raising the float bowl levels to compensate some (1 or 2mm) for the loss of volume in the single pipe style taps.

Also, my carbs had aftermarket float valves. Replaced them with OEM ones.

The other main thing I did was find a dual outlet style fuel tap from a CB550 (16950-374-671) that used the same threading (19mm) as the later K tank that I have.

After all of this, the highway cutting out was completely gone and the plugs are a little on the sooty side!

HTH.

calj737 · « **Reply #54 on:** July 03, 2017, 07:08:16 AM »

To be clear (pardon the pun), I'm not saying the fuel level is the issue I'm only clarifying why it over micrometers is a more exact method.

b1jackson · « **Reply #55 on:** July 03, 2017, 11:03:28 AM »

Quote from: scottly on July 02, 2017, 06:11:04 PM

Drain the tank until the level over the screen is just above where the problem starts, and then check the flow again. See what the level is when it stops flowing compared to the screen. Once it stops, check the flow with the petcock switched to reserve.

I think my Saint has saved the day again. Just not sure the cause? This is a new petcock the PO put in. I imagine it's this one:

http://www.vintagecb750.com/parts/480/20-8038.jpg

Tank was filled and I started draining it off. Maybe got 7 liters (2 gallons) or so out and as the level approached the top of the petcock pipe screen, you could hear a squeaky sucking noise. Flow in the line then went all air bubbly and pretty much went to a dribble. Switched it to reserve. Full flow again.

So what the heck. Do I just run on reserve all the time? Not a big deal if I do as this isn't a daily driver and it's just a rip about bike but I don't understand the petcock? Why would it short change you so much on a tank?

TwoTired · « **Reply #56 on:** July 03, 2017, 11:14:09 AM »

Principle of a stand pipe. The reserve feed is from the bottom of the tank.

The normal feed is from a tube rising up some distance from the bottom.

That's how reserve works. Most of the fuel gets fed, and then the engine runs badly if at all. Switch to reserve and flow is restored so you can drive to the gas station and refill the tank.

b1jackson · « **Reply #57 on:** July 03, 2017, 12:21:37 PM »

Yes. I understand that and perhaps this aftermarket petcock pipe is just too dang long. I should see if I still have the original to see the pipe length on it if there is one.

TwoTired · « **Reply #58 on:** July 03, 2017, 12:59:24 PM »

How much reserve range is too much?

PeWe · « **Reply #59 on:** July 03, 2017, 01:46:40 PM »

CB750 reserve is about 5L.
I think my old type of petcock with dual pipes has a little bit more, maybe 6L.

I had never an issue with my K6 tank with single output petcock and std carbs feeding 836cc, ported head and cam (80rwhp). It delivered fuel on Autobahn with passenger plus luggage during long periods of high speed. Reserve came really quick when cruising 160-200kph all the time, about 150km until reserve.

b1jackson · « **Reply #60 on:** July 03, 2017, 02:07:25 PM »

Quote from: TwoTired on July 03, 2017, 12:59:24 PM

How much reserve range is too much?

Good question. As Pewe mentioned I figured it would be about 5L. Didn't expect it to be the opposite for me with a 5-7 liter run and a 15 liter reserve (all crude measures of course). Good to know as I continue to learn about this bike.

scottly · « **Reply #61 on:** July 03, 2017, 06:27:46 PM »

I only know what questions to ask because of the good info you provide.

b1jackson · « **Reply #62 on:** July 04, 2017, 01:09:29 AM »

Well thank you once again Scottly and to all that chimed in with suggestions and help. I think next week I'll finally have an inspection sticker on it and get it registered for the road.

Next projects include another supersport and later this winter I'll tear into a barn find cb550.

PeWe · « **Reply #63 on:** July 04, 2017, 01:38:59 AM »

- Bike runs OK now?
- You checked the fuel flow from petcock?
Let fuel flow direct to a container and see if flow suddenly stops after a while.
I had a tank like that as I wrote earlier, stop after around 5km. Opened the fuel cap and fuel flowed OK.
My tank was almost full with fuel.

b1jackson · « **Reply #64 on:** July 04, 2017, 04:19:59 AM »

yes. See reply # 55 above. I think its solved now. Aftermarket petcock pipe provides a VERY generous reserve capacity.

Basically...I was running out of gas. There....I said it! $:-\$ $:-\$ $:-\$

b1jackson · « **Reply #65 on:** July 11, 2017, 02:58:02 AM »

Big thank you to Hondaman, Scottly, PeWe, calj737 and all others who have helped me on this latest issue plus my electrical short woes of last year. It has been a great learning experience to get this bike back to running condition and being able to have a vintage ride sit next to my Vstrom in the shop makes the decision on "what" to ride that much more difficult. My interest in CBs continues to grow as does my collection so stay tuned for future projects (and hopefully not issues!). This is an amazingly helpful forum.

So big day today. The bike was last road ready 5 years ago and had only been half running off and on ever since, finally got it road ready, licensed and insured. Nice long drive to get it inspected and it ran like a top! Here she sits after the return home from her true maiden voyage!

Thanks again all!

evanphi · « **Reply #66 on:** July 12, 2017, 08:24:18 AM »

Glad everything is sorted.

I bet the reason you were getting those 1KM stretches after it died were because you would stop and put it on sidestand and the bowl would fill a bit more. Rinse and repeat.

Ride until it sputters, then flip to reserve and find a gas station!

PeWe · « **Reply #67 on:** July 12, 2017, 09:48:06 AM »

Also possible to have an eye on the trip meter and change when reserve is expected to happen. I have got fuel starving way too often when decelerating in the city in low speed and do not hear in time that it's time to switch to reserve. Tricky situation to quickly apply full choke and turn to reserve, engine sputtering and hopefully not running harmful lean.

b1jackson · « **Reply #68 on:** July 13, 2017, 01:47:54 AM »

Yes. I topped up the tank before heading for my inspection and reset the trip. Since my tank is so sparkling clean and white inside from the sealer, it's easy to see the petcock pipe. I'll do some more riding and see what the magic # is to do the flip over to reserve ahead of time as suggested. No city driving out here. All country roads!

el pachuco · « **Reply #69 on:** July 15, 2017, 08:54:49 PM »

I've read this entire thread, including the first three posts and I'm with PeWE on the clogged vent in the fuel cap.
as your fuel level is lowered it must be able to draw air in to replace the missing fuel. if the vent is clogged, the fuel leaving the tank creates a vacuum in the tank and prevents more fuel from leaving the tank to fill the carbs. 'I know you said you checked it and it was clear, but when you hit what you think is the crazy high reserve level, try opening the fuel cap to relieve the vacuum, instead of switching to reserve. If you can, listen for the suction of the breaking vacuum when you open the cap.

I also experience this problem before.

PeWe · « **Reply #70 on:** July 15, 2017, 11:36:32 PM »

Funny, I talked about my clogged tank vent yesterday with the friend that came for rescue with his car and trailer when my bike stopped. I was so damn sure it was a clogged petcock due to dirty tank, rust. I changed tank and side covers to a new set and put my clogged tank on the shelf.

I found the real reason 6 months later when I drained tank from all fuel and it stopped flow after a while. I opened the fuel cap and fuel started to flow good again.
I could have driven the bike home if I had opened the fuel cap and kept it that way.

Posts like this are good since peoples experiences of similar problems pops up and will help others in the future.

el pachuco · « **Reply #71 on:** July 16, 2017, 12:21:16 AM »

my other bike (2001 Kawasaki ZRX1200) is known for this same problem and that is why it's no stranger to me. My Kawasaki has never experienced it but I was aware of it, so when it happened on the Honda I was ahead of the game.

To the O P, just because compressed air will pass, that doesn't mean that ambient air will be drawn in.

It is worth a closer look.

b1jackson · « **Reply #72 on:** July 16, 2017, 06:32:54 AM »

I'll keep it mind but for now it works great.

The tank "drain down" test I did the cap was off.

b1jackson · « **Reply #73 on:** July 16, 2017, 01:36:42 PM »

Update for any interested.

Long run on the bike today. Topped off the tank pretty full but not right to the cap and did 135 miles all highway. It wasn't until about 110 miles on the trip meter that I got the stumble. Flipped it over to reserve and kept on motoring.

So at least I know the magic # now.

el pachuco · « **Reply #74 on:** July 16, 2017, 04:08:53 PM »

sounds like all is well

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Author Topic: Cb750 runs in 1 km stretches, then dies, runs again after cool down. (Read 8652 times)