1980 CB750k Weird Jetting & Idle issue

[Piczker08]

Hello fellow 750 lovers!

As a background story, I purchased a 1980 cb750k back in march and ever since it's had some problems with idling. It'll start and idle for a minute and slowly die off. If i give it throttle it'll stay alive as long as i keep throttle on. Recently though, it's gotten even worse and started to die on me at red lights/stop signs. Once i get going it has no problems, just that darn idle. I finally got angry though and took my carburetor off thinking its a gunked up pilot jet/screw issue.

Here's where i start getting to the point . When i took the carb off everything was nice and clean, however my jets were wayyy off factory norms.

My first three carbs had a main jet 201 & Pilot Jet 89. The last carb had main jet 102 & pilot jet 89. WEIRD. The pilot screws were also wayyyyy screwed out compared to stock settings.

The bike has a stock airbox, but a 4-2 honda exhaust. (which i don't think would call for such large jetting changes; if any).

I read online that stock main jet should be 68, pilot jet 35 and pilot screws 1.5 turns out. So why on earth would the previous owner change them so much, and would changing them back to stock jettings possible fit my idle issues?

Thanks guys, looking forward to getting some help.
Matt

 

[amc49]

You need to read better too. There are TWO main jets, one up front around 68 and the rear one is around 100 or so and pilot 35 is pressed in and not removeable on '90 models. All 4 carbs the same. The pilot is under a black rubber plug that MUST be in good shape and in place. 2-2 1/2 turns on the mixture screws but with jetting errors like that I expect the screws/seats to have been damaged and carbs may be junk. Not only do you need the stock airbox, you need the stock paper filter as well. Match all 4 carb bodies for the VB# on the side, somebody may have used a wrong carb there, more than one that look the same.

With that extremely rich in some spots it shouldn't even run at all, as in foul plugs to have them dead in not long at all. Trying to still drive it that rich as well could easily have a damaged engine as well and the first thing on these anyway is to always do a compression check to determine if the motor is junk or not. Nobody ever sets the valves and then they burn and people go after carb fixes thinking they are smart enough. Not likely. Sound familiar?

If the compression is off then the engine will try to die around idle a lot.

Why? Can that be so hard? Dumb-ss is the word you are looking for there. The PO had not a clue.

 

[Piczker08]

Hey Amc, thanks for the response.
Also thanks for the correction; we all have to learn somehow. I'll definitely not ride it until this gets figured out.
I guess it turns out I've a lot to check up on. I've got a compression guage coming in this week, so I'll check that as soon as I can. I'll take the carbs back out after to match the #'s and make sure the air filter is stock as well.
Once those things check out I'll do some research and post up with results and my next steps.

Appreciate the help.
Matt