My 78 CB750 Idle/Throttle Changed Suddenly

[jsmits]

Hello again everyone. So I took my bike for a ride today. Rode it yesterday and it rode like a champ. The slight missing here and there I always get, but other than that running nice.

I start it up today and let it warm up, as I'm about to get on the bike, I hear it's idle drop really, really low, then pick back up. I figured the fuel was off overnight and the bowls were a bit empty. Seemed fine so I get on it. A couple miles down the road it starts to really bog down and the throttle gets jumpy. Around 4000 rpms, at about 1/3rd to 1/2 throttle it just stops accelerating. I give it more throttle, and it starts to bog a bit then jumps, like it's lurching into acceleration. Then I let off the throttle and bam, on it's face it goes. I manage to get it home by carefully massaging the throttle and idle screw and keeping it in 2nd gear, but the bike simply does not want to idle anymore.

I pulled the plugs and they look golden brown as they normally do. One was a bit more lighter brown then the others but they are all look acceptable. Needless to say I'll be pulling the carbs tomorrow. My best guess is that I've collected some particles in the carbs? I've been running it without an inline fuel filter so it's a definite possibility. I only put premium in it.

One other thing, my oil pressure seems to be lower at idle than it used to. It could just be because it's idling rough (ie barely idling), but its around 15-20 when it used to be around 40-50 at idle and then max out by 2500rpms.

Do you guys have any thoughts what could of changed besides the jets getting dirty? I'm going to carefully check fuel levels when draining them and look for deposits, and examine all the jets carefully as well, but something suddenly changed over-night and its a bit discerning.

Thanks for any help and insight.

[TOOLS1]

Sounds just like dirt in the fuel system. And yes if it is not idling fast enough the oil pressure would be lower.
TOOLS

[Re-run]

I would second the crud in carbs, notably the idle circuit since it won't idle. You can blow some carb cleaner through the mains but I wouldn't bother any further with them. Then you won't need to worry about syncing. Just pull the idle jets out and clean them and spray cleaner through the passages. Probably all you need.

Get a decent inline, not the cheapest you can find. Or, if you can find it, the oem in-tank filter.

Oh, and octane of fuel doesn't make it cleaner, it all goes through the same pump and so can pick up dirt.
Also, in these old bikes, premium won't get you anything. It can actually do worse than regular, especially if you still have the old points system. These bikes would only benefit from premium if you did a high compression 836 kit.

I would get things clean and try a tank of regular.

[jsmits]

Thanks guys, i'll try cleaning the idles without pulling them and checking the bowl levels. I'll head to a local powersports store and pick up an in-line ASAP as well.

[LukeM]

+1 on using regular gas.  It's a bit more volatile than midgrade or premium, which in my case means quicker/easier starting. I notice a difference in starting when I put premium in it.  One of the only reasons to move up in grade is if the engine knocks or pings noticeably under load (higher gear and heavy throttle). Knocking sounds like a rattle, or like small pebbles shaken in a tin can.  Ping is like knocking, only higher in pitch.  Light and occasional knocking or pinging is not harmful, but if it does it a lot, it's hard on pistons and bearings.

Also +1 on the inline filter.  Better safe than sorry.

Hope this helps.  Good luck.
Luke M