Topic: bike wont idle below 2,000 rpm

[Austin4x4]

so here is the story. rebuilt carbs on a 1974 cb750.  i am running a 120 mainjet and a 40 slow jet, aswell as pod airfilters and a 4 in to 2 exaust.  and this is also at altitude here in denver.  other than high ideling the bike runs great.  if i try to set the idle speed screw any lower it just dies.  the bike was running ok before i changed the ait filters, exaust and rebuilt the carbs. so what ideas do you all have for me.

[SAEmiles]

Did you sync the carbs after rebuilding them? That has a huge effect on the quality of your idle.

[killersoundz]

Turn your mixture screws in a bit, it won't do much I guess they're only about like upping the size of the idle jet 1/2 step but might help. Did you raise your needle clip up a notch? You should. Also might want 125 mains and if you have to up the idle jet to like a 42. and a sync too

[SAEmiles]

Turn your mixture screws in a bit, it won't do much I guess they're only about like upping the size of the idle jet 1/2 step but might help. Did you raise your needle clip up a notch? You should. Also might want 125 mains and if you have to up the idle jet to like a 42. and a sync too

I could be wrong, but the bike will still run with the idle mixture very far from ideal. If it dying at idle, that would point to a clogged pilot jet, or out of sync carbs.  The other advise is probably sound for riding, but for idle it won't help.

[Spanner 1]

Sounds like the slow circuit in the carbs is kaput...... sounds like the bike will only run off the main jets = 2,000rpm. Might need to describe how the carbs were cleaned. Even with pods/exhaust changes, bike should be able to 'suck' fuel from the idle/slow jets 

[Austin4x4]

i haven't yet synced the carbs. didn't think i could do that until i got it to idle correct.  the carbs were cleaned well and rebuilt with new parts.  so if the slow jets are not working as they should what should i check, float level?

[Spanner 1]

Carb sync. is the next thing then, one slide a fraction higher will cause the 2k rpm 'idle' that you describe 

[killersoundz]

i haven't yet synced the carbs. didn't think i could do that until i got it to idle correct.  the carbs were cleaned well and rebuilt with new parts.  so if the slow jets are not working as they should what should i check, float level?

Yeah you should have bench synced them which would have gotten it pretty close. If you haven't even tried to match up the slide heights from the rebuild the could be so far out of sync that could be your problem

[iron_worker]

I would re-check pilot jets. People say they cleaned them well and so therefore they are clean ... well that's not always correct. I did the same thing. My bike ran well for about a week when it started to not idle well again. Pulled the carbs and found out I had a tiny piece of grit stuck in one of the pilot jets again.

If you don't have a fuel filter you could easily be pushing bits of junk down into the bowls. It doesn't take much at all! I have a fuel filter and it still happened.

My vote is to re-check your pilot jets. Pull them out and clean them with a guitar string or small gauge wire.

IW

[ekpent]

Next time you get it warmed up well touch a wet rag on the header pipes up by the engine to see if you get a good sizzle on all 4 just to make sure they are all working correctly. They can run on 3 but badly.

[flybox1]

i haven't yet synced the carbs. didn't think i could do that until i got it to idle correct.  the carbs were cleaned well and rebuilt with new parts.  so if the slow jets are not working as they should what should i check, float level?

No, check your slow jets, and assure us they are clear of any obstructions...

who rebuilt your carbs?

[Austin4x4]

the slow jets are new.  and i made shure all the passages were clean before i instaled them.  i did bench sync them and set the slides at 1/8".  i have checked and made shure that all four cylenders are fireing and they are.

[afkrejci90]

aswell as pod airfilters

Get rid of these and you use the airbox unless you have a more modern set of carbs.  It's better to getting working right with everything that was meant to be on there.  Never thought it would make a difference but it does.

[HondaMan]

the slow jets are new.  and i made shure all the passages were clean before i instaled them.  i did bench sync them and set the slides at 1/8".  i have checked and made shure that all four cylenders are fireing and they are.

If the new idle jets are the ones with just 6 holes in their emulsifiers (instead of , drill another set of holes in the emulsifier tubes.

If you also replaced the fuel needle valves (and/or seats), you may well have the too-stiff variety in the carbs, now. These cause the fuel level in the floats to sit about 3mm too low, which raises the idle jets right up out of the fuel. To check for this possibility, centerstand the bike, turn on the fuel, let the bowls fill, then turn off the fuel and unclip the bowls to see how deep the fuel is inside. If it is not more than half-full when you're looking at it, this latter situation has bitten you: it is EXTREMELY common now, as the float needles from Keyster (and some others) have too-stiff springs. In this case, either return to the OEM ones if you can, or change the float setting to 24mm to help compensate.

Then run the idle test again.

Where in Denver do you you live?

[Austin4x4]

the slow jets are new.  and i made shure all the passages were clean before i instaled them.  i did bench sync them and set the slides at 1/8".  i have checked and made shure that all four cylenders are fireing and they are.

If the new idle jets are the ones with just 6 holes in their emulsifiers (instead of , drill another set of holes in the emulsifier tubes.

If you also replaced the fuel needle valves (and/or seats), you may well have the too-stiff variety in the carbs, now. These cause the fuel level in the floats to sit about 3mm too low, which raises the idle jets right up out of the fuel. To check for this possibility, centerstand the bike, turn on the fuel, let the bowls fill, then turn off the fuel and unclip the bowls to see how deep the fuel is inside. If it is not more than half-full when you're looking at it, this latter situation has bitten you: it is EXTREMELY common now, as the float needles from Keyster (and some others) have too-stiff springs. In this case, either return to the OEM ones if you can, or change the float setting to 24mm to help compensate.

Then run the idle test again.

Where in Denver do you you live?

that is some good advise, thanks.  i still have all the old parts from the carbs.  i compared everything, but i could have overlooked a small diffrence.  i live on the south side of the metro area, in centennial, just north of C470 and Quebec.

[HondaMan]

the slow jets are new.  and i made shure all the passages were clean before i instaled them.  i did bench sync them and set the slides at 1/8".  i have checked and made shure that all four cylenders are fireing and they are.

If the new idle jets are the ones with just 6 holes in their emulsifiers (instead of , drill another set of holes in the emulsifier tubes.

If you also replaced the fuel needle valves (and/or seats), you may well have the too-stiff variety in the carbs, now. These cause the fuel level in the floats to sit about 3mm too low, which raises the idle jets right up out of the fuel. To check for this possibility, centerstand the bike, turn on the fuel, let the bowls fill, then turn off the fuel and unclip the bowls to see how deep the fuel is inside. If it is not more than half-full when you're looking at it, this latter situation has bitten you: it is EXTREMELY common now, as the float needles from Keyster (and some others) have too-stiff springs. In this case, either return to the OEM ones if you can, or change the float setting to 24mm to help compensate.

Then run the idle test again.

Where in Denver do you you live?

that is some good advise, thanks.  i still have all the old parts from the carbs.  i compared everything, but i could have overlooked a small diffrence.  i live on the south side of the metro area, in centennial, just north of C470 and Quebec.

You're down there in the shadow of the Faye Myer store. There's a bunch of other SOHC4 and Hondatwins members down there, too. I'm in Lakewood.