Topic: (Solved for now)..Cb750 stalling when slowing to a stop

[Xander^]

Hey guys..
 i got a 1977 cb750k, stock carbs, with a carpy exhaust 4into1 and a cognito moto airbox.. i have rebuilt the carbs and put in 120 jets and 40 jets.

Bike was tuned and synced by a shop and idle and runs amazing, this work was done over the winter so i havent been able to ride until yesterday.
While riding the bike was running great until about 3 minutes into the ride when it began to stall almost every time coming to a stop. it stalled out suddenly usually while i downshifted from 3rd -2nd and 2nd-1st.

I assume it may be the slow jet? Might have to go to 42?

[BenelliSEI]

Have you put fresh gas in it yet? Don’t be afraid to reach in and adjust the idle speed, with the knurled knob. I find I often have adjust it up or down a bit, depending on the day.....

[Xander^]

I topped up the tank in november and poured in the required amount of fuel stabilizer so i thought the gas would be fine. Im going to play with the adjusters next time i ride. It’s annoying though, it only happens after riding a km or so then it keeps happening 90% of the stop/downshifts i make

[BenelliSEI]

The gas we are all buying today is very suspect.

When you pull out the choke on most cb750’s, the mechanism also advances the throttle, very slightly. It could be your idle is just set to low. Most of the guys here will tell you, we’ve all become good at just reaching under the tank (while seated) and adjust small amounts almost daily! Join the club.....

[Xander^]

Would i need to go up to 42 slow jets or back down ? The stall is a sudden one

[newday777]

I topped up the tank in november and poured in the required amount of fuel stabilizer so i thought the gas would be fine. Im going to play with the adjusters next time i ride. It’s annoying though, it only happens after riding a km or so then it keeps happening 90% of the stop/downshifts i make

Most likely fuel delivery to your carbs..

Pull the tank off and empty the gas into you fuel injection truck. With the tank empty use a good LED flashlight(350 lumens or brighter) to inspect your tank for rust and debris.
What do you see?

[Xander^]

I topped up the tank in november and poured in the required amount of fuel stabilizer so i thought the gas would be fine. Im going to play with the adjusters next time i ride. It’s annoying though, it only happens after riding a km or so then it keeps happening 90% of the stop/downshifts i make

Most likely fuel delivery to your carbs..

Pull the tank off and empty the gas into you fuel injection truck. With the tank empty use a good LED flashlight(350 lumens or brighter) to inspect your tank for rust and debris.
What do you see?

Tank was cleaned throughly with a week long derust and cleaning method, and i have a nice big transparent fuel filter where the gas seems very clean when it goes passed filter into the carbs.. i doubt its that.. going to consider it though thank you

[Xander^]

Can this be an electrical problem?

[BenelliSEI]

Probably not, but pull the main fuse, clean it and the terminal clips and make sure it is nice and tight in there...... also, if you have an aftermarket ignition key switch (Emgo) they are notorious for failing, but usually not intermittent. They just fail...... The clue to the switch is if you have two keys (the one for ignition and the original for seat lock).

[Robbo]

I topped up the tank in november and poured in the required amount of fuel stabilizer so i thought the gas would be fine. Im going to play with the adjusters next time i ride. It’s annoying though, it only happens after riding a km or so then it keeps happening 90% of the stop/downshifts i make

Most likely fuel delivery to your carbs..

Pull the tank off and empty the gas into you fuel injection truck. With the tank empty use a good LED flashlight(350 lumens or brighter) to inspect your tank for rust and debris.
What do you see?

Tank was cleaned throughly with a week long derust and cleaning method, and i have a nice big transparent fuel filter where the gas seems very clean when it goes passed filter into the carbs.. i doubt its that.. going to consider it though thank you

Run it without the fuel filter and see if the improved fuel flow solves the issue.


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[HondaMan]

No, do NOT go to #42 pilot jets or the problem will get worse.
You can adjust the air screws on the carbs instead, which have lots more range than a #2 jet size. And, if the tiny O-rings up inside those screw tips are old and dried out, you will need to replace them: most are. This makes it really hard to adjust the idle, as they don't regulate the air real well. The 'sweet spot' that will make it idle with old, hard O-rings will be about 1/8 turn full range. Try it at 1.5, 1/75, and 2.0 turns out from [gently] full closed, on all 4 carbs. It sounds like the fuel level might also be a bit low in the float bowls, which can happen if someone rebuilt the carbs with the incorrect settings. The PD41 carbs are 12.5mm deep, the PD42 are 14mm, but run better at 12.5-13.0mm instead.

[Xander^]

No, do NOT go to #42 pilot jets or the problem will get worse.
You can adjust the air screws on the carbs instead, which have lots more range than a #2 jet size. And, if the tiny O-rings up inside those screw tips are old and dried out, you will need to replace them: most are. This makes it really hard to adjust the idle, as they don't regulate the air real well. The 'sweet spot' that will make it idle with old, hard O-rings will be about 1/8 turn full range. Try it at 1.5, 1/75, and 2.0 turns out from [gently] full closed, on all 4 carbs. It sounds like the fuel level might also be a bit low in the float bowls, which can happen if someone rebuilt the carbs with the incorrect settings. The PD41 carbs are 12.5mm deep, the PD42 are 14mm, but run better at 12.5-13.0mm instead.

Thanks HondaMan, i watched a few videos on this but somehow you still explained it better lol, going to try this out later today. The carbs and al it’s pieces were rebuilt with new brass all around so the rubber orings should be fine!! Going to try those increments and see !

Also will try to without the fuel filter also why not does hurt to try.

Thanks guys, stay tuned

[PeWe]

Carb air vent tubes not clogged?
I guess there are 2 of them. 1-2, 3-4.
If your carbs works as my old K6 carbs it should bubble gas in float bowls if you blow into the hoses.
Not with high pressure. compressed air, even if you try to blow just a little. Risk to blow gaskets or fill airbox with fuel. The latter feels familiar

Idle rpm OK?
I prefer 1200rpm. Ensure oil pressure when hot and easier to ride in the city.

[spotty]

I always kept the carbs cleaned and balanced but eventually got used to reaching under the tank to tweak the idle most rides ( usually city riding, rarely country )

The only other words of wisdom I have to offer are
- most of us have probably checked the tension on the chain whilst sitting at the lights if you think it's maybe sounding a bit rattly......don't get stoned and check the tension whilst you're actually moving........

[Xander^]

Hey guys thanks for all the tips..
sooo i think I've solved the issue! I turned up my idle by 100rpms, it may have bit just slightly low but with the exhaust and airbox mods, im getting a bit of hang with the throttle and when the rev comes back down it bounces a bit lower than normal and that was what seemed to cause the stall.. so i bumped up the rpms a bit and now it isnt stalling at all.

[Jarrett_Honda]

Hear me out. My bike was stalling when I came to a stop too. It was because my cam chain wasn't adjusted in the correct spot. Which more than likely threw everything else off.

It would idle fine, but if I ride it and stop at an intersection that bike would struggle and want to die. The bike even completely died a few times.

Once I redid everything correctly the problem went away.

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[HondaMan]

Hey guys thanks for all the tips..
sooo i think I've solved the issue! I turned up my idle by 100rpms, it may have bit just slightly low but with the exhaust and airbox mods, im getting a bit of hang with the throttle and when the rev comes back down it bounces a bit lower than normal and that was what seemed to cause the stall.. so i bumped up the rpms a bit and now it isnt stalling at all.

Here's another hint: if you are running premium (or even midgrade) gas, drop down to the next lower octane. Reason: our modern gas burns much slower than 1970s gas (mostly because it helps light off catalytic convertors now), which leaves unburned fuel in the intake tract for a while after throttling down. After my last rebuild (when I upped the compression a bit and hemi'd the chambers) I noticed it dropped to idle slower and tended to 'roll on' more when I cranked back on the throttle (mind you, I've had this bike since new in 1972, 130k miles now...), so I started using midgrade gas. Later I've learned that in-town riding works much better with regular octane, idling better and less over-run, but I use midgrade for freeway work and premium when burning up the long roads.

[Xander^]

Here's another hint: if you are running premium (or even midgrade) gas, drop down to the next lower octane. Reason: our modern gas burns much slower than 1970s gas (mostly because it helps light off catalytic convertors now), which leaves unburned fuel in the intake tract for a while after throttling down. After my last rebuild (when I upped the compression a bit and hemi'd the chambers) I noticed it dropped to idle slower and tended to 'roll on' more when I cranked back on the throttle (mind you, I've had this bike since new in 1972, 130k miles now...), so I started using midgrade gas. Later I've learned that in-town riding works much better with regular octane, idling better and less over-run, but I use midgrade for freeway work and premium when burning up the long roads.
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Oh wow super interesting, this is my first bike, ever, and since ive started 2-3 years ago rebuilding I’ve heard countless times to use midrange-premium only.. I’ll have a try with SHELL regular 87 next fill up to see if I noticed anything