Do i need to rejet or whats the story.

ive posted about this before, somewhere but i cannot remember if it was this board,
when i put m,y cb on the road i needed restriction, the kit i got did not include jetting(new kit from ebay, kickstart engineering)

so ive lived with it like this for months with standard jetting but the bike does not like to tick over properly when its close, i usually have to drive it straight away or restart it several times, I paid someone to balance the carbs but at the time did not realise that the fuel tap was an air leak so they probabaly arent balanced right yet, at certain rpms though theres a lot of backfiring and for the first time the other day one of the plugs fouled (fuell tap side although theres a new tap on since january)

would fitting the lower jet as found in the honda and FI kits help cure this and maybe have it runnning a little better?i dont know if the valves have ever been done but theres only 20k on the clocks so not due for another 4k or so? would it be worth trying these either way? its reallly annoying me at the moment because my 2 stroke px200 vespa is a lot more reliable

assuming you're using an inlet manifold restriction yes you will need to rejet. You're reducing the flow of air being allowed into the engine so need less fuel too. Not sure if the jets need to be upsized of down on the CB it varies from bike to bike.

Although logic says you should need a smaller jet it doesn't always work out in reality. Because you're restricting the flow of air across the face of the jet it is a lot harder for the engine to "suck" the fuel through, meaning you actually need to up jet to allow it to flow properly

Personally I would sort out any leaks and balance the carbs first. Popping on the over run may indicate a leak on the exhaust side or a very weak mixture from an inlet side leak.

I doubt you will need to rejet - CV carbs are to a moderate degree self regulating - less suck means the slide in the carb will not rise - so less fuel for less air...


How old are the plugs?

Frog

I think popping and farting on the overrun is normal for CB500's.

what do you lads mean by "overrun" as in decelleration? im getting loud enough backfires whenever im running the bike in neutral, usually if i hold it around 2500rpm and sometimes at 4000 and othertimes just at idle, I removed the restriction washers there and the bike runs a hell of a lot better when its cold,checking the valves today and the carb balancers will be on order.

I agree some popping on the over run - yes when you close the throttle at speed, is normal (but any loud bangs is not).

Backfiring when in idling in neutral is deffo not right. Good idea to check the valve clearances and would also check timing.

If the plugs are seriously black and sooty it is well rich and it maybe unburnt fuel is igniting in the exhaust.

Keep us posted

Frog

have sooty plugs alright on both cylinders, valves were within spec anyway today so no worries there, somthing i forgot to mention earlier, when i had the carbs balanced last there was a standard dented exhaust on it, since swapped that for a dent free one and also repaired a vac leak that was there during balancing(bad fuel tap) so all that said i doubt the carbs are properly balanced any more, to back this up the left cylinder takes longer to warm up,

assumed the timing was static on these, wheres do i go to check it? is it a rotatable stator job?

The timing is static... it's electronic, so I suspect it either works or it doesn't.

As you were saying I suspect that the carb balancing has been adjusted by changing the exhaust etc.

You were metioning that the plugs were sooty, have you checked that the choke isn't stuck on (or partially on)?

If you are comfortable changing jets (sounds like you are) and you know what jet sizes the FI and similar kits use i would just try them out and see.


It will cost you less than a tenner, and CB carbs are pretty easy to get to.


I would also replace your air filter if you haven't already.

Sorry when I said timing in the sentence about valve clearances I meant valve timing while you are there - i.e. check the marks line up correctly in relation to each other. I had a GT550 kwak I bought that had been stood and ran crap, popped and banged and after a lot of head scratching and tinkering with the carbs etc (I thought it was just being stood that was the problem) found a cam was one tooth out - checked with the previous owner and yes indeed it had run crap and then been stood ever since he did the valve clearances
Just worth checking however improbable.

Frog

way ahead of you, took out the restriction and its still doing it, going to balance without them and see does it fix anything, took it for a rip up my lane without the washers in and the difference is amazingly different, I went into second gear and opened the throttle and i got a fairly scary power wheelie! wasnt expecting that the last cb never did it!

well i ended up breaking the little brass thing on the side of the intake so i ordered 2 generic ones from ebay. built my balancer out of tubes and jam jars and had a good play with it today, the balance seems to be slightly off still but its 100% better, the low speed shudder is gone and theres no more popping, one side seems to be a little strong still but the manometer wouldnt stop moving, it would either go one way or the other, dunno if that was normal but its a home made tool, im fairly happy anyways, will take some photos of my comedy balancer for the other thread. next mission is sorting the back brakes.

It doesn't matter if they aren't *exactly* spot on... as long as they are better than when you started!

oh they are, i would say the screw was almost a full turn out couldnt get them spot on though, the liquid was always moving, it never stopped dead like the ones you see on youtube, either moving one way or the other all the time. i reckon there might of been an air leak in my tool somewhere,not to worry,my stupid washers will be going back in in the next few days anyway so ill put them on again for good measure.

G3o wrote:assuming you're using an inlet manifold restriction yes you
will need to rejet. You're reducing the flow of air being allowed into
the engine so need less fuel too. Not sure if the jets need to be
upsized of down on the CB it varies from bike to bike.

This is a bit wierd. I fitted a set of ebay inlet restrictors and the bike continues to run smoothly, starts OK and ticks over at just under 1000rpm with no issues. I didn't rebalance the carbs or anything else. What have I done wrong?

was it 25kw or 35kw you fitted? just curious

from kickstart? same as mine if so. i havent put mine back in yet but i suspect the bike will drive the same, it made no difference to my specific issues when i had em out

CBsean wrote:from kickstart?

Yes, they are just a partially open throttle, and shouldn't make any difference to jetting at all. It just means that full throttle is not available...........................yet