Author Topic: bike stalls after a hard brake (Read 1927 times)

aminemed · « **on:** June 11, 2018, 07:08:31 AM »

Hi

my CB 750 stalls when i brake hardly, coming to a stop for example. When i hit the brakes, and the clutch to keep it alive, it dies. Only way to prevent the engine from dying is to give it some gas even though clutch is pulled. If i don't hit the gas, bike stalls.

You would tell me that it is a gas related issue, bike might be lean but what is funny is that i upgraded recently my main jets from 115 to 125, with pilot jet set at 42. It is running pod filters with a single exhaust (4 to 1).

flybox1 · « **Reply #1 on:** June 11, 2018, 07:11:50 AM »

What process did you follow to determine the bike needed 42 pilot jets?

aminemed · « **Reply #2 on:** June 11, 2018, 07:24:36 AM »

Honestly, no process at all

My idle was perfect, but bike seemed to be a bit lean in the first half of the throttle. I wanted to try 42 as pilot jets. It run on these pilot jets for 2 months without any issue.

Idle is around 1200 rpm and carb levels set at 26mm. Timing is correct and clutch adjusted correctly as well. I'm running on Main & RES when needed, tank was cleaned perfectly. My only change is the contact points of cylinder 1-4 that i've replaced with DAIICHI contact points, but timing was set correctly after this change.

flybox1 · « **Reply #3 on:** June 11, 2018, 07:28:24 AM »

My thought is you are overly rich at idle, and the quick decel plus closing of the slides chokes out your engine.
How about performing an idle plug chop (hot engine, new plugs) to see what is really going on with your mixture.. . .

Deltarider · « **Reply #4 on:** June 11, 2018, 07:31:35 AM »

Flyboy is right, I think. Either open airscrews more or replace slow jets by smaller.

TwoTired · « **Reply #5 on:** June 11, 2018, 07:52:27 AM »

Hard braking forces the fuel in each vessel to move forward. Maybe into the carb throat.

Have you changed float needles? Are they still stock ?

Is the ignition tied into the brake lighting system?

Is battery voltage stable?

Just some ideas...

Cheers,

flybox1 · « **Reply #6 on:** June 11, 2018, 08:06:22 AM »

What are your air screws set at?
You have 657b carbs on a K2?
Daiichi points?!? Why?

These bikes can run like a top, if you are methodical in your processes.
Slow down. Ask more questions before guessing....and spending/wasting money.

evanphi · « **Reply #7 on:** June 11, 2018, 08:08:20 AM »

i had this happen when i was running lean because of a vacuum leak.

aminemed · « **Reply #8 on:** June 11, 2018, 08:18:55 AM »

Quote from: flybox1 on June 11, 2018, 08:06:22 AM

What are your air screws set at?
You have 657b carbs on a K2?
Daiichi points?!? Why?

These bikes can run like a top, if you are methodical in your processes.
Slow down. Ask more questions before guessing....and spending/wasting money.

i'm running 657b carbs, that is right.

My airscrews are set to 1 turn.

I do have pod filters and 4 into 1 exhaust.

I'm running daiichi (japenese, not chinese copy), because my bike fell very hard and the contact points of cylinder 1 - 4 were broke, so had to change them.

To answer the other question, my needle is set at the 4th notch (or 1 notch before the bottom) and needles are stock. Battery is 100% ok, run through all the voltage tests to see if my charging system is correct.

Before falling hardly and changing these points, i had no issue at all with the bike engine dying when braking. I have a small hole at the exhaust level (i can take a picture and show you), this because of the bike fall. Do you think that this can cause a vaccum leak ?

evanphi · « **Reply #9 on:** June 11, 2018, 08:30:15 AM »

Yes!

If you find it is dying during engine braking, you're likely running too lean to begin with (probably the case with pods anyway). Fix your vac leaks.

flybox1 · « **Reply #10 on:** June 11, 2018, 08:40:50 AM »

how about posting up a picture of your plugs....

aminemed · « **Reply #11 on:** June 11, 2018, 09:12:11 AM »

Quote from: flybox1 on June 11, 2018, 08:40:50 AM

how about posting up a picture of your plugs....

and of my broken exhaust

PeWe · « **Reply #12 on:** June 11, 2018, 09:44:13 AM »

Lean at first part of the throttle can be needles too lean. I should use stock pilot jet, needles 1 step richer (+1 notch down from top so needles raise).
Main jets 130 with pods and open exhaust, probably more, 135 might be a better guess.

I had no problem with Daiichi points back in the 80's except they were trickier to adjust. Ign changed when tighten the point screws. AND one point broke on a touring vacation thru France, the bakelit part broke so bike ran on 2 cylinders only until I found a Honda bike shop that could repair the point. No CB750 parts in stock back in 1988 when these bikes were cold.

aminemed · « **Reply #13 on:** June 12, 2018, 12:38:27 PM »

hi all
quick update: changed my 4 plugs that were black like coal with new plugs. Issue still here, bike does not hold idle. At every red light or stop i need to restart the engine. Im thinking that it might be carb bench related. Might need to pull off carbs and bench sync them again. What is weird is that before falling and changing daiichi points bike were running perfectly.

flybox1 · « **Reply #14 on:** June 12, 2018, 12:51:09 PM »

Quote from: aminemed on June 12, 2018, 12:38:27 PM

hi all
quick update: changed my 4 plugs that were black like coal with new plugs. Do you still think its lean? lol Issue still here, bike does not hold idle. At every red light or stop i need to restart the engine. Im thinking that it might be carb bench related. Yeah...id agree its carb related, but more like improper jetting.Might need to pull off carbs and bench sync them again. why dont you put your 40s back in while you're at it What is weird is that before falling and changing daiichi points bike were running perfectly. How do you know it wasnt rich then and you just hadnt yet fouled the plugs to the point it wouldnt run well?

IIWM, id follow this:
set the carbs back to stock spec...jets, ims, etc...all of it.
clear tube test the bowls to make sure the fuel level isnt jacked.
then start the bike.

If it will idle, set timing and advance, then put a new set of plugs in and then vacuum sync the carbs.....when done, pull out the plugs and check their color.
If they are good, great. ride on.
If they are still dark, turn the IMS leaner by 1/2 turn and go for a ride. see if it replicates the same issues.

aminemed · « **Reply #15 on:** June 12, 2018, 03:37:58 PM »

folks something terrible just happened to me and im still under the shock. i went again for a ride to test my air mixture that ice adjusted a bit to have the bike less rich hoping to have correct idle. It did not work. Therefore once i arrived to my parking spot and once again after restarting the bike twice or three times just because of this stupid idle i decided to adjust my carb main screw/ idle screw. This was my mistake: i turned the screw way too much. I started the bike with the choke on, with my left hand on the clutch and my right hand was turning off the choke. As soon as i choke lever was lowered, bike litteraly jumped. I went straight on my garage wall as i had not the time to do anything. Bike then went from the wall to the floor. Top triple tree is broke, tarozzi rearsets broken, gas tank dented and the transmission seems to be somehow broken from the inside as i cannot.move the bike anymore. Anyways ill post pictures tomorrow. Im really schocked by what happened and fortunately im safe.

aminemed · « **Reply #16 on:** June 12, 2018, 04:14:35 PM »

im not at all worried about money or the bike, of course really sad that this happened but thank god im not dead

i was not wearing any helmet..!! im really shocked as this is the first time something line this happens me

evanphi · « **Reply #17 on:** June 13, 2018, 06:53:42 AM »

Yoinks. That sucks.

I kickstarted in first gear with the clutch out once. Thought I was in neutral. NOPE. Bike rolled forward and cracked my alternator cover. Leaked a slow stream of oil all the way home. Patched up and still holding strong.

Bikes can be repaired. But you can't (mostly)! Take the time to fix these parts, and then take the time to revisit the fueling situation.

flybox1 · « **Reply #18 on:** June 13, 2018, 07:16:04 AM »

Sorry man...that sucks.. $:-\$

aminemed · « **Reply #19 on:** June 13, 2018, 07:18:40 AM »

Thank you guys for your support

aminemed · « **Reply #20 on:** June 13, 2018, 08:21:09 AM »

just a quick question, my front wheel does not want to roll at all now. Any suggestions on what to fix ?

flybox1 · « **Reply #21 on:** June 13, 2018, 08:28:37 AM »

Quote from: aminemed on June 13, 2018, 08:21:09 AM

just a quick question, my front wheel does not want to roll at all now. Any suggestions on what to fix ?

...probably because your forks are misaligned, and its twisting the rotor in the front pads.
Take off the front brake assembly

aminemed · « **Reply #22 on:** June 13, 2018, 08:29:22 AM »

Quote from: flybox1 on June 13, 2018, 08:28:37 AM

Quote from: aminemed on June 13, 2018, 08:21:09 AM
just a quick question, my front wheel does not want to roll at all now. Any suggestions on what to fix ?
...probably because your forks are misaligned, and its twisting the rotor in the front pads.
Take off the front brake assembly

Thanx a lot for your quick answer

aminemed · « **Reply #23 on:** June 14, 2018, 04:15:00 AM »

front wheel was locked not because of the calipers, but because my front wheel rim is bent. It's a real bad luck, you can see in another thread that i've posted 3 days ago pictures of my new alloy rims

I guess that a bent alloy rim is just something that cannot be fixed..it lasted less than a week

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Author Topic: bike stalls after a hard brake (Read 1927 times)