Topic: fuel starvation once warmed up

[chink57]

Gentleman,  Longtime looker or lurker here, so bear with me,not very computer literate.  I have a 1977 CB750 f supersport with about 11,000 miles on her.  I've had it for about four years.  I had the top end done and put pods on it, along with the usual upgrades for me: new tires, steel braided brake lines, changed the handle bars.  Kind of pseudo-cafe.  I fell in love with the bike. I've ridden for 33 years; standards, then sport bikes; Suzuki gs 450,Suzuki gs550es, yamaha fz700, yamaha fzr1000, Kawasaki zrx1100, now 2003 Suzuki  sv1000s, 1983 CB1100f supersport, 1981 CB900f supersport, and what was the love of my life the 1977 CB750f supersport, but i digress.
    I am not mechanically inclined, but I live within a block of an old Honda dealership, with an old Honda mechanic, he's done all the work on it.  We have tried 4 different petcocks.  The bike runs fine for 10 to 12 miles and then she starts getting fuel starved.  I have to rev the throttle and limp her home.  Any insight into what her woes might be would be greatly appreciated.  She's breaking my heart.

Thanks chink57

[harisuluv]

It seems that you and your mechanic have diagnosed the problem as fuel delivery from the tank, can you explain why?

I don't know what diagnostics that were done but it sounds like textbook plugged/clogged gas cap. 

[chink57]

Not the gas cap.

[strynboen]

open the cap. and tap..and let it run for 5 minuts..or more...to thek fuel flow..vaccum in the tank can do it..
mine vas doint the same..after have run hard fullsspeed it Loose one cylender firing..it vas fuel starvasion in one kaburettor..kleaned all 4 up..and took the tank off.. and flushed it ..even it vas klean looking..and problems vas gone

in line fuel filters can give strange problems..so take them off the bike...(even they looks fine)

[Airborne 82nd]

If anyone can help with a fuel issue  harisuluv would be one of the top 5 but you didn't seem to add much more information. of just what you or your mechanic has done. The more information you provide the better. How does your in tank filter look? How did you test your fuel cap? With a full tank and the cap on tight pull the fuel line off and do a flow test for about a gallon to see how that looks. Then do it like strynboen said with cap open see if there is any change.

[Don R]

 I had an inline filter that I could blow air through into the tank but fuel wouldn't flow back out. Removed the filter and all was fine. Mine was an F1, I'm convinced the alcohol fuel gelled or something like that.

[Cqyqte]

Gentleman,  Longtime looker or lurker here, so bear with me,not very computer literate.  I have a 1977 CB750 f supersport with about 11,000 miles on her.  I've had it for about four years.  I had the top end done and put pods on it, along with the usual upgrades for me: new tires, steel braided brake lines, changed the handle bars.  Kind of pseudo-cafe.  I fell in love with the bike. I've ridden for 33 years; standards, then sport bikes; Suzuki gs 450,Suzuki gs550es, yamaha fz700, yamaha fzr1000, Kawasaki zrx1100, now 2003 Suzuki  sv1000s, 1983 CB1100f supersport, 1981 CB900f supersport, and what was the love of my life the 1977 CB750f supersport, but i digress.
    I am not mechanically inclined, but I live within a block of an old Honda dealership, with an old Honda mechanic, he's done all the work on it.  We have tried 4 different petcocks.  The bike runs fine for 10 to 12 miles and then she starts getting fuel starved.  I have to rev the throttle and limp her home.  Any insight into what her woes might be would be greatly appreciated.  She's breaking my heart.

Thanks chink57

Not sure you have a fuel starvation problem.  When a motor reaches operating temperatures the mixture gets slightly richer.  I am interested in the symptoms when you say you have to rev the throttle and limp home.  I have a 1975 750 with '77 keyhole carbs and pods, crosswinds and turbulence would cause the bike to fall flat on it's face.  In order for me to ride it I too would sometimes have to feather the throttle or shift gears to change rpms to keep going.  Tell me more about what happens.

[2wheels]

In the gas tank lid cover, do you have a foam filter?  It covers the gas lid vent hole.
If you do then remove the foam,  or drive with the lid open.
See what happens.  My 78 F had this problem, took me all summer to figure it out.  Most because I didn't believe that little chunk of foam could be my problem.

[harisuluv]

If you had narrowed the problem down to fuel starvation (your claim/assertion) then what led you to try 4 different petcocks?  If you are swapping the petcock, surely you took the fuel hose off and just opened the valve and observed something wrong, therefore swapped 3 more petcocks.  Now however, you're asking for help, despite narrowing the problem down to your petcock.  A petcock is pretty easy to diagnose, frankly I suspect flawed troubleshooting.  As pointed out, any suggestions at this point would just be speculation until you provide more information.

[chink57]

All right,

First, and foremost, thanks for the input.  Second, originally the petcock was bad, so he replaced with a chinese one that i bought online, that didn't work, then another aftermarket one, then a different Honda one, but don't ask me which,  now I believe it is an original.  The diagnostic was based on the fact that once the petcock was replaced the problem began.  I will do due diligence with all of your advice and suggestions and let you know how it works out.  Again, thank you.

chink57