1976 F1 carb problem

[Mike CB]

Hi all,

 Could anyone out there shed some light on a problem I am having I have just installed a new Motad 4 into 1 exhaust system on my bike,and am now having running problem the Motad part number is correct I checked with them,the bike is now in the UK but was originally from california it was running ok with the standard exhaust but that had past it's best,The bike runs ok at slow speeds up tp about sixty mph but then seems to struggle for fuel,it splutters and misses,and loses power if I slow down,or stop it recovers again.I have checked the manual and it states that the main jets are (should be) 105's I note that these are different from other models that have 120's could this be the problem? or am I barking up the wrong tree any help much appreciated thanks in advance

[CB750Winger]

  115 main jets might do the trick. Do you have the stock air box or pods on the carbs for intake?

[Mike CB]

Thanks for the advice this is a great site for all running CB 750's the bike has a standard airbox I will try to get hold of the jets in the UK

[Re-run]

Before you throw money at it(always the worst way to go), run a few tests first. When the bike starts to die, pull the choke and see if it helps. If it does, then you are probably lean.

What did the old exhaust look like? Was it rusted out badly with a lot of holes? If so, the bike may have ended up tuned for that and by putting on a new exhaust, you may be restricting things.
On top of that, in most cases, if the carbs are set right, you rarely need to make a jet change when changing exhaust. A lot of people think you have to and that might be true for other bikes but not these.

But if you really want to test things, do a plug chop.
warm the bike up and then pull the plugs and clean them. Put them back in and go to an open road.
Rev the bike and keep it up and go through the gears. Once in 5th at high rpm, hit the kill switch and pull the clutch and coast to a stop without releasing the clutch.
Pull the plugs out and see what they look like. If they are pretty dark on the insulator, then you are running rich. If very light, then you are lean.

[Mike CB]

Thanks for the advice rerun took the bike out and tried the choke trick this did improve things, the bike was originally from L.A. I checked the main jet size in the Haynes and Clymer manuals, they state that it had 105's(have I already posted this!!) any how after checking with Motad(the guys who made the replacement exhaust system they have recomended increasing the jet size as the new system is manufactured for the UK market,The old exhaust was the standard one fitted to the bike,longer than the usual fitted on earlier models I have had that Honda had to restrict the engines to meet with the USA emissions regulations,once again guys thanks fot all the great advice thsi site is a great help