Topic: CB750 Idle issues

[brycegp]

This video was VERY helpful. Thanks!

LUCKY - Trying to address all the questions you raised in your latest posts.

Out of curiosity I pulled the plugs and checked the compression.  All seem good and no bleed down..schwew!
#1 = 135 psi
#2 = 135 psi
#3 = 130 psi
#4 = 130 psi

All mixture screws are 3/4 turn out.  And I'm going to shoot another video cold starting it again in a little while.  Its cooling down from the compression test now.

I am determining that #2 isn't firing by checking the heat on #2 header.  I can grab on to it...not hot...but warm.  Plus, while out on the street I can just tell, I'm not running on all cylinders.

The bike has new idle and main jets.  I put them in myself.  There shouldn't be any blockage...but I'll check.

To my knowledge the needle clips are in stock position.  The guy that rebuilt them, rebuilt them to stock specs.  SUPPOSEDLY.  But I never had a visual confirm on that.

[brycegp]

UPDATE: I haven't really done anything with the bike in a week.  Wife was outta town and daddy duty was exhausting!!

Anyway...watch the video.  All idle screws 3/4 turn out now.  I am thinking its time to pull the carbs and see what is going on inside. 

#2 cylinder still isn't firing and #2 carb is still leaking fuel (probably due  to sitting for a week)...even after I set the float height.  My diagnosis is that these "rebuilt carbs" weren't ever rebuilt or rebuilt and never cleaned properly.

Any other thoughts or suggestions are welcomed!

[lucky]

IF fuel starts dripping out, shut it off immediately!!! It can catch fire easily!!!

If it was my motorcycle I would take the carbs off and replace all of the float needles and seats.

Also I would check float heights and make sure all jets were clean and write down the size of the jets on a piece of paper. Also check all of the clip positions on the needles.
I would also check and make sure no brass overflow tubes in the float bowls are cracked.

That is what I would do no matter what anyone told me.

[brycegp]

IF fuel starts dripping out, shut it off immediately!!! It can catch fire easily!!!

If it was my motorcycle I would take the carbs off and replace all of the float needles and seats.

Also I would check float heights and make sure all jets were clean and write down the size of the jets on a piece of paper. Also check all of the clip positions on the needles.
I would also check and make sure no brass overflow tubes in the float bowls are cracked.

That is what I would do no matter what anyone told me.

This is the plan...and I was just marking this point on the thread so all can see where I was before pulling the carbs.

A resync will be necessary...but I don't have the gauges...

[brycegp]

I pulled the carbs and put in new stuff...5-slot jet needles, needle jets, needle valves, needle valve seats, idle jets, mixture screws, slide assembly seals and bowl gaskets.  All else cleaned and passages blown out of corrosion...and there were no cracked overflow tubes.

New Specs:
Set all floats to 26mm
Needles set at 2nd slot from bottom.
#40 idle jet
#120 main jet
Mix screws 1 turn out (for now)

But of all the parts I replaced...I did NOT see where that little O-rings goes.  Hints??

Tricks to getting carbs back on without a heat gun???

[lucky]

IF fuel starts dripping out, shut it off immediately!!! It can catch fire easily!!!

If it was my motorcycle I would take the carbs off and replace all of the float needles and seats.

Also I would check float heights and make sure all jets were clean and write down the size of the jets on a piece of paper. Also check all of the clip positions on the needles.
I would also check and make sure no brass overflow tubes in the float bowls are cracked.

That is what I would do no matter what anyone told me.

This is the plan...and I was just marking this point on the thread so all can see where I was before pulling the carbs.

A resync will be necessary...but I don't have the gauges...

Thanks BRYCEGP! Please let us know what you find if anything. Thanks

[lucky]

I pulled the carbs and put in new stuff...5-slot jet needles, needle jets, needle valves, needle valve seats, idle jets, mixture screws, slide assembly seals and bowl gaskets.  All else cleaned and passages blown out of corrosion...and there were no cracked overflow tubes.

New Specs:
Set all floats to 26mm
Needles set at 2nd slot from bottom.
#40 idle jet
#120 main jet
Mix screws 1 turn out (for now)

Tricks to getting carbs back on without a heat gun???

Getting the carbs back on. This is my method.
Put a tiny bit of Vasoline speared on the mating parts.
Get on the right side of the bike and tie the carbs up with zip tie to take the weight off.
Then push the #4 carb into position all the way and tighten the clamp.
Then go to the left side of the bike and push the #1 carb into position all the way.
Tighten the clamp.
The two middle carbs should take care of themselves.

[Spanner 1]

I'm surprised in 56 replies that no-one has suggested that your problem is actually electrical 
I would be very interested to know what the voltage @ the Black wire connection to the coils is ??
Low primary voltage will 'favor' 1 of the 2 plugs that fire off that coil as each side is never exactly the same impedance ( resistance ) allowing 1 cyl. to stay below firing threshold. Putting 3 ohm coils with resistors would give the same results as stock coils. The voltage would need to be checked and monitored with the bike idling until it stumbles and dies...... Wonder what the voltage is when you first start the bike and when the motor stops ?

[brycegp]

I'm surprised in 56 replies that no-one has suggested that your problem is actually electrical 
I would be very interested to know what the voltage @ the Black wire connection to the coils is ??
Low primary voltage will 'favor' 1 of the 2 plugs that fire off that coil as each side is never exactly the same impedance ( resistance ) allowing 1 cyl. to stay below firing threshold. Putting 3 ohm coils with resistors would give the same results as stock coils. The voltage would need to be checked and monitored with the bike idling until it stumbles and dies...... Wonder what the voltage is when you first start the bike and when the motor stops ?

11.5V 

The bike isn't running (haven't put the carbs back on yet) so no charge is being delivered.  Also, I took the voltage where the Black/White meets the Black to the resistor pack at the coil.  The resistor pack then feeds both coils...

[lucky]

UPDATE: I haven't really done anything with the bike in a week.  Wife was outta town and daddy duty was exhausting!!

Anyway...watch the video.  All idle screws 3/4 turn out now.  I am thinking its time to pull the carbs and see what is going on inside. 

#2 cylinder still isn't firing and #2 carb is still leaking fuel (probably due  to sitting for a week)...even after I set the float height.  My diagnosis is that these "rebuilt carbs" weren't ever rebuilt or rebuilt and never cleaned properly.

Any other thoughts or suggestions are welcomed!

Where did you get the carbs?

[brycegp]

Where did you get the carbs?

Off a local guy...why?

[brycegp]

Just ran the bike with the new carb bits.  It needed all the stuff.  AND #2 exhaust still isn't getting hot like the others.

AND...just call me worst case scenario.  I swabbed inside the baffle to see what I saw and there is oil inside the left side 2 into 1 exhaust pipe...the #1 & #2 pipe.    ..suggesting I threw a ring...??  ...valve seals?

But I tested the compression and #2 had 135psi and was within 5% of all the others!!!!  So how could I loose a ring AND have compression???

Note: I didn't rebuild the motor because it ran good when I tore it down.

#2 plug

[lucky]

Just ran the bike with the new carb bits.  It needed all the stuff.  AND #2 exhaust still isn't getting hot like the others.

AND...just call me worst case scenario.  I swabbed inside the baffle to see what I saw and there is oil inside the left side 2 into 1 exhaust pipe...the #1 & #2 pipe.    ..suggesting I threw a ring...??  ...valve seals?

But I tested the compression and #2 had 135psi and was within 5% of all the others!!!!  So how could I loose a ring AND have compression???

Note: I didn't rebuild the motor because it ran good when I tore it down.

#2 plug

I would not worry about that right mow.
Too much choke can produce oily looking deposits.

[brycegp]

UPDATE: 

I swapped #1 and #2 plugs.  I wanted to see if the symptoms remained the same.  AND #1 exhaust then wasn't warming up and #2 was hot.  So I eliminated the plug that seemed to be fouled.  I put a fresh plug in #1 and left the plug (originally in #1) now in #2.

All four cylinders are firing (exhaust good and hot) now.  And blips to the throttle are much more responsive, I noticed right away.

So about the oily-ness from the left hand exhaust pipe.  Could the unburnt fuel from the non-firing cylinder be turning the existing soot into an oily consistency??

[lucky]

Where did you get the carbs?

Off a local guy...why?

I just wondered if you got them on Ebay. Doesn't matter.
But I do have my list of bad Ebay sellers now.

[lucky]

No such thing as "threw a ring"

IF you had a broken ring you can usually her it making noise.

But the compression is even across the board. STOP worrying so much.

[brycegp]

No such thing as "threw a ring"

IF you had a broken ring you can usually her it making noise.

But the compression is even across the board. STOP worrying so much.

haha...I'm just sooo ready for this bike to be well sorted...AND I am a bid of a "worst case scenario" minded person. I'm working in that. 

new running video coming soon.  just waiting for it to cool down for a cold start.

[brycegp]

So here is my next start test after new spark plugs and new carb components.

New Specs:
Set all floats to 26mm
Needles set at 2nd slot from bottom.
#40 idle jet
#120 main jet
Mix screws 3/4 turn out
4 New NGK Iridium spark plugs

Charging at 13.5-14.5V thru RMP range. 
Notice that no choke was needed to start it from cold.
No more fuel leak for any of carbs.

NOTE: There isn't a fine sync on these carbs yet.

[Spanner 1]

So, basically your problem was a bad sparkplug in #2 cyl. I bet it would have fired and ran on all 4 if you had changed the plug(s) back on July 19th. when you started this thread...... just sayin' ..



Always the basics first.... cyl. not firing ??..... change the plug

[brycegp]

So, basically your problem was a bad sparkplug in #2 cyl. I bet it would have fired and ran on all 4 if you had changed the plug(s) back on July 19th. when you started this thread...... just sayin' ..



Always the basics first.... cyl. not firing ??..... change the plug

#2 not firing wasn't the original issue.  #2 not firing happened later...just sayin.    besides that...those were brand new NGKs...who knew one would be a dud!!??

The original issue was that the bike wouldn't idle.  And the carbs would leak fuel on occasion.  AND it was a great excuse to upgrade...cuz now...I've got new electronic ignition, new high output coils, wires, caps and plugs...and fully rebuilt carbs.  All verifiable and done by ME.

[Spanner 1]

Bike won't idle ( cold ) on 3 cyls.... back on July 23 you said 'spitting in #2 cyl. ".... fess'up.... should have put new plugs in waaay back when.....

I'm not arguing  with you, just pointing-out that the very basic task of trying new plugs should be done before even asking a question here..... you didn't even try a different plug until Aug. 14th. 
Sorry, but sometimes you have to call someone out when they haven't done the (very) basics.....live and learn.

[lucky]

A couple of comments.

In the video I see you using the choke and you flip it on and off.
It is also meant to be partially on or off also. Very important.
Back when these machines were sold cars and motorcycles had these mechanical chokes and people really learned to use them. They knew if the outside temp was a certain about they could start with half choke in the summer and full choke in the winter, People got to know their choke very well. It was not a On or OFF kind of deal.

There was no air filters on the bike.
The exhausts look like straight pipes. do you have baffles in the exhausts?

[brycegp]

Bike won't idle ( cold ) on 3 cyls.... back on July 23 you said 'spitting in #2 cyl. ".... fess'up.... should have put new plugs in waaay back when.....

I'm not arguing  with you, just pointing-out that the very basic task of trying new plugs should be done before even asking a question here..... you didn't even try a different plug until Aug. 14th. 
Sorry, but sometimes you have to call someone out when they haven't done the (very) basics.....live and learn.

Spanner My first reaction to your post is to say FU...but then I think I should be more civil.  BUT HEY...sometimes you gotta call people out when they're being an A-hole.

I don't need you to insult me.  And unless you're going to do something other than play "hindsight is 20/20"...go away until I ask you a question...which I haven't...live and learn.

[brycegp]

In the video I see you using the choke and you flip it on and off.
It is also meant to be partially on or off also. Very important.
Back when these machines were sold cars and motorcycles had these mechanical chokes and people really learned to use them. They knew if the outside temp was a certain about they could start with half choke in the summer and full choke in the winter, People got to know their choke very well. It was not a On or OFF kind of deal.

Yes I was flipping it ON and OFF...but I was also moving it incrementally at times.  This is a good suggestion...but it seems my bike only responds if the choke is either ON or OFF...and in between does very little.

There was no air filters on the bike.
The exhausts look like straight pipes. do you have baffles in the exhausts?

Well...the 'air filters' are velocity stacks which are open flow.  since they are a pain to get on and off...I tested it without them.  the bike performed the same when I had put them on later.

The exhaust is not straight pipes.  On the end of the header, there is actually a slip on tip with an 8" baffle.  cuz it was just stupid loud without the baffle.  the shorty baffle just takes the sting off a bit... 

[bollingball]

Well...the 'air filters' are velocity stacks which are open flow.  since they are a pain to get on and off...I tested it without them.  the bike performed the same when I had put them on later.

The exhaust is not straight pipes.  On the end of the header, there is actually a slip on tip with an 8" baffle.  cuz it was just stupid loud without the baffle.  the shorty baffle just takes the sting off a bit...

Say what I didn't think v-stacks had air filters? Can you explain I don't know
Ken