not getting Fuel to the carbs ?

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The short question is why my cbx750f 1986 runs nicely on idle but seems to run out of fuel after a few minutes and the red oil light is left on.
If i wait for a few minutes it runs again and then stops.

Here is what I have done recently since it last ran proplerly, I hope this isn't too long winded but I need to put everything in:

I took the tank off to oil the brake cable a while back. That when fine. We noticed a crack in the vaccum pipe so I replaced that too. (this seemed to explain the flat spot I was getting when moving off)
The bike ran for a few minutes, I turned it off to get ready I Came back and it wouldn't start.

I was suspicious of the spark caps as I had problem one of them before so I replaced all off them and the sparkplugs.
I also replaced the air filter and oil and oil filter too while I was at it.

There was also a ready oiler fitted with a t-bar on the vaccum pipe from the peatcock to the carb vaccum to get a vacumm for the oiler.
I took this away as it seemed to be something else to troubleshoot.

Sometimes it would start a bit. But not properly it felt like a fuel problem so I took off the petcock, took int apart and it seemed clean so I put it back together.
It them seemd to start properly.
I hade to play around with the idle to get it to run. I finally got it running properly and left it idling then suddenly the engine revs went to 4k on idle ( where it prob should be given how much I twisted it)
So it felt like something cleared and it was running properly.

So on a test run the bike starting loosing power and then shut off with just the red oil light on. I couldn't get it to start until I opened and closed the tank cap to let air in (this was a problem on a cb400 I had before ).

I was then able to drive home and the bike cust out just when I got back. ( it seemed run the same distance). I tried starting it again and no joy so I opened and closed the fuel tank again and it started.
I don't know if this is fixing the problem but it seems to each time.

I was able to drive 7.5miles taking it easy and it then stopped ( at the top of a hill)

I then tried driving much quicker and it stopped after 1 mile.

I took the plastic from the fuel cap as reccomended here:
http://www.fj1200.info/CBXmods.html

But I still have the same symtpoms.

The next thing I was going to try is taking away the vaccum pipe and taking out the large diaphram from the petcock so it just runs on gravity as the guy mentions in the same article.

But I wanted to ask here first if there was anything else I should try.

thanks

[steve murdoch icoa #5322]

It sure does sound like a vapor lock preventing the fuel flowing which i thought would have been cured by your removal of the tank cap liner.
Also surprising that it happens not long after starting. Usually it takes a while for heat to build up and contribute to the vapor lock.
Wish i could be of more help.

[NobleHops]

By the way I ought to point out that this site is dedicated to the 6-cylinder CBX, and while you may find some helpful info here, you might be better served by a group with more direct experience with your bike.

Hope you get it sorted without too much thrashing!

N.

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Thanks for the reply. This is the closest forum I can find, so fingers crossed someone else can help.

I'd be intererested to see if the advice from this guy looks solid: http://www.fj1200.info/CBXmods.html
Paticularly the bit about removing the large diaphram from the petcock. under " FUEL TAP VACUUM BYPASS "

[daves79x]

I would by-pass the vacuum system altogether and direct-feed the carbs, if you can. Then you'll know where the problem is quickly. You can set out to repair the vacuum valve later if that proves to be the problem. Are you positive you got all the vacuum lines installed correctly when you put the tank back on? Remember, you have to plug any open vacuum lines if you do remove/bypass the valve.

Dave

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Thanks for all the help. I have got it running nicely now and did 110 mile run yesterday.

I bypassed the vacuum in the petcock and plugged the vacuum hole in the carbs with a short piece of pipe with a screw in it. I just need to be sure to turn off the petcock when not in use.

I followed these instructions which he recomends as an upgrade anyway. (http://www.fj1200.info/CBXmods.html) a very useful page for cbx750f , which I am quoting here incase the other site disappears for someone in the future. I also attached the pic from the site.

FUEL TAP VACUUM BYPASS

My bike was suffering from fuel starvation at 6.000 rpm and higher constant throttle, so I did this mod in order top detect if the vacuum tap was working correctly. In few words, the mod is to convert the vacuum tap into a gravity only tap. I discovered than the bike accelerates harder and fast. In fact, HRC recommended the supression of the vacuum system when a carburetted Honda was raced. You only have to be careful not to leave the fuel tap open when not riding the bike, but if your bike works correctly, there would not be any problem.

The basic procedure is to disassemble the fuel tap, disconnect the small rubber diaphragm (3) from the large rubber diaphragm and aluminum piece(4), disconnect the plastic retainer from the large diaphragm (4), connect the plastic retainer to the small diaphragm (3) and re-assemble it, discarding the spring and large rubber diaphragm (including the little aluminum piece).

You no longer need a vacuum line from the engine to the fuel tap. You will need to remove the vacuum hose and plug the hole on the intake port. Follow the hose to the intake. Remove the hose. Remove the metal part the hose attaches to and plug the hole with an appropriate screw and washer (look at the other intakes for an example of the screw/washer). The vacuum hose cannot be attached to the fuel tap anymore.

After completing this procedure, the fuel tap should not leak, and if the carburettor floats and float needles are operating correctly, it should not overflow the carburettors if left sitting for a while. Also, initially, monitor the fuel tap to make sure it doesn't develop a leak soon after making this change. Leaks may not appear until after riding for a while. STRONGLY RECOMMENDED.