Hi, hoping someone can help.

Riding a Zephyr 750 (1992) and its had a lot of work done on it, most of it ok cos she rides pretty well.  Trouble is at about 7k revs she runs out of puff and wont go much over 7.5k.  She will give a little burst, like she's really trying but then goes breathless again.  In top gear this means i can just scrape up to 95mph, but it happens in all gears.  acceleration is fine, with no popping or backfiring.

Standard airbox and filter, carbs have been stripped and cleaned i don't know how many times, spark plugs, coils and cdi all 'look' ok and i last balanced the carbs last september beforer its MOT.

Its almost like there is a restrictor on there!  I don't want to go superspeed (after all it's just a Zephyr) but its really bugging me.

Any help muchly appreciated.

Duke                                                                                                                                                                             

She same age as my XJ - they take a lot of looking after - well done. Have you thought about a tired or restricted  vacuum hose?On the XJ there is a small breather pipe which I clean at every oil change.Somtimes they go soft , and can collapse when higher load is demanded. Good luck                                                                                                                                                                             

Hi Gary   Interesting idea.  perhaps if i get someone to rev it to 7k and me watch the vacuum hose?  you do mean the one from carb to tank that operates the vacuum tap?   why would they collapse at 7k and not at some other point when i crank it on? say 5k?   I'll have a tinker with that one, but someone pm'd me about maybe the exhausts being restricted.  now this might be a problem cos i switched from a 4-1 with a kawa 636 end can; to a 4-2 system with cans from i think a gsx 1400 (least thats what i was told they were).  i dont fancy going back to 4-1 cos of all the welding and fettling it took to get this system working.  The Zeph750 is sposed to be 4-2 btw.  anyway.........   I got an XJ600 pre div btw.  customising it atm, looks bit like a bobber, but i stuck stonkin great pipes from a harley on it!!  got to get on and finish that one so i can start a new project with a z650.  thats gonna be a cafe racer.   cheers                                                                                                                                                                             

Hi DukeZ had a car did same thing only 3500 rpm ,rear back box had collasped inside, took it of to prove noisy but plenty of revs. Have a mate with 1100 zeph trike wont rev past about 8500 ,has a one of exhaust we all think this is his problem as engine was ok as a bike.Colin

cheers for that Colin

yeah its looking like the exhaust might be the problem cos i don't recall this happening when it was 4-1 thru the single silencer i had on it.  was wondering if i should try larger jets (or smaller?) currently they are 92's on the inside carbs and 88's on the two outer carbs.  this is correct according to the manual.

thanks                                                                                                                                                                             

Thanks to all who tried to help, still investigating but what i did find that on the throtttle, stood still it revs cleanly all tthe way to the red lne, no probs. so why does it do it under load? could i be misinterpreting the signals and maybe its clutch slip? it doesnt slip at all when accelerating, so i doubt its that.

cheers

Probably silly but have you checked that the gauze filter in the fuel tap is good and clean? 

Worth checking the throughput of the fuel tap whilst you're there too.

Also, is the filler cap clear of restrictions? They usually have a small breather hole or similar vent to promote the flow of fuel from the tank.
Even quite a small obstruction can initiate a lock.

I still think it's your exhaust Duke                                                                                                                                                                              

Agree with exhaust. Silencer collapsed on the xj and did the same thing.                                                                                                                                                                             

Thanks for all your suggestions so far!

I tried gary's suggestion about the vacuum hoses, but when its standing still and i rev the engine it can go all the way to the redline with no hesitation, so i don't think its that.

yeah i checked the filter in the tank and its clear, the breather in the fuel cap 'looks' ok.

Coming on to the exhaust: well....i was having a problem anyway with a flexible link pipe which kept splitting so i have gone back to 4-1 and now?  well, its pulling easier and stronger and i don't really need to go up to 7k revs cos its fine in all gears up to 6k with a good clean strong pull.  But.. its still hesitating at 7k as before but with more of a pulsing (can't describe the engine noise any other way).  did get up to a ton on the M61 yesterday so its a little better in small ways but still getting this running out of puff feeling.

any more offers?

cheers again

Duke                                                                                                                                                                             

have you had the plugs out? what color were they? if they are white then it's lack of fuel, if black it's lack of air/too much fuel, if they look about right and all the same do a compression test. being able to hit the redline in neutral isn't really much of a test as there is no load and they'll usually do that even if running on a very poor fuel mixture.                                                                                                                                                                             

Thanks for that, byt. it'll have to wait till the weekend now, got loads of other stuff to do. cheers

                                                                                                                                                                             

**UPDATE**

OK i have gone back to 4-1 and she seems to pull better.  started today intending to get the carbs off, clean, re-set and then balance them.  when i lifted the tank and went to pull of the vacuum pipe off the fuel tap i noticed a small kink in the pipe, and then further underneath...omg the pipe had been trapped against the rocker cover; hence the lack of fuel which caused the running problem.

New pipe, new route and better secured to prevent pinching and excess heat.............solved!!!

Thanks to all who helped with all the suggestions

:)

Duke                                                                                                                                                                             

    That's great news Duke   Just get out and enjoy it now   While this fine spell o weather lasts