Topic: 73 cb 750 gas coming out the carb inlet? Fixed video

[izaleth]

Hey there guys, having some issues with getting the bike running, 2 main issues,
cylinder number 2 seems like it isnt firing regularly IE exhaust is much colder on that cylinder
and number 4 carb has fuel vapor coming out the air inlet, any advice would be greatly appreciated.
yes i still need to tune the carbs, but im worried about number 4 so i haven't done that yet

i just set timing so its pretty close probably not perfect
edit:
come to find out cylinder 4 seems to be down on compression
the motor has sat for at least 8 years so my guess is the rings may be stuck best case, should i try soaking the cylinder with some marvel mystery oil and hope for the best?
anyone had a similar situation and had an easy fix?

i have resigned myself to rebuilding the top end but im willing to give a few quick fixes a try first

https://youtube.com/shorts/WTRXDFgyHis?feature=share 
Better video

[ekpent]

Make sure all the carb vents are clear including the one at the bottom of the float bowls that is also the carb overflow.  Have you stripped and really cleaned the carbs and jets etc. well yet ?
  Is the #4 carb problem raw fuel coming out or burping out a vapor ?  All the valves set close to spec also ?

[izaleth]

Make sure all the carb vents are clear including the one at the bottom of the float bowls that is also the carb overflow.  Have you stripped and really cleaned the carbs and jets etc. well yet ?
  Is the #4 carb problem raw fuel coming out or burping out a vapor ?  All the valves set close to spec also ?

i had everything apart and cleaned the carbs out, ill double check the overflow vent.
more of a vapor, not raw fuel, you can see it just barely in the video clip
and you mean the valves in the head? i havent checked them yet to be honest

[newday777]

Make sure all the carb vents are clear including the one at the bottom of the float bowls that is also the carb overflow.  Have you stripped and really cleaned the carbs and jets etc. well yet ?
  Is the #4 carb problem raw fuel coming out or burping out a vapor ?  All the valves set close to spec also ?

i had everything apart and cleaned the carbs out, ill double check the overflow vent.
more of a vapor, not raw fuel, you can see it just barely in the video clip
and you mean the valves in the head? i havent checked them yet to be honest

Adjust your valves and cam chain.
Do you have a repair manual for the bike?
How did you clean your carbs? And what condition were they in before you cleaned them?

[PeWe]

When my K6 carbs overflowed, fuel ran into the airbox.
Clear tube test showed wich carbs to adjust the floats.

[izaleth]

Make sure all the carb vents are clear including the one at the bottom of the float bowls that is also the carb overflow.  Have you stripped and really cleaned the carbs and jets etc. well yet ?
  Is the #4 carb problem raw fuel coming out or burping out a vapor ?  All the valves set close to spec also ?

i had everything apart and cleaned the carbs out, ill double check the overflow vent.
more of a vapor, not raw fuel, you can see it just barely in the video clip
and you mean the valves in the head? i havent checked them yet to be honest

Adjust your valves and cam chain.
Do you have a repair manual for the bike?
How did you clean your carbs? And what condition were they in before you cleaned them?

i do not have the manual, i still need to order one.
the carbs were in ok shape just sat for a while fully disassembled and cleaned out, i didnt adjust float height though so that could be a problem?

[newday777]

Yes float heights are important.
I've seen so many replies by novices that they have cleaned their carbs and thought they had them fully cleaned, yet we're still having problems that showed they hadn't done a through job of it(I also am one!). A little bit of hardened gas in the tiny ports of these carbs, air screw holes/ports included, will give you fits to tear out your hair. It's very important to Fully strip each carb body bare of all jets, tubes and air screws(on pd carbs the little springs, washers and orings on each screw too). Failure to do so is quickly evident.
Proper adjustment of all screws and floats cannot be skipped.

Here are the download manuals. Scroll down to what you need.

http://manuals.sohc4.net/cb750k/

[izaleth]

Yes float heights are important.
I've seen so many replies by novices that they have cleaned their carbs and thought they had them fully cleaned, yet we're still having problems that showed they hadn't done a through job of it(I also am one!). A little bit of hardened gas in the tiny ports of these carbs, air screw holes/ports included, will give you fits to tear out your hair. It's very important to Fully strip each carb body bare of all jets, tubes and air screws(on pd carbs the little springs, washers and orings on each screw too). Failure to do so is quickly evident.
Proper adjustment of all screws and floats cannot be skipped.

Here are the download manuals. Scroll down to what you need.

http://manuals.sohc4.net/cb750k/

thanks ill be sure to go over that again, also one thing i havent been able to figure out, the idle fuel mix setting, where is it supposed to be, is 3/4-1 1/2 turn related to flush with the carb or something else?

[Bodi]

Mixture screws are always turns out from gently bottoming the screw. Too heavy a hand will damage the seat.
"Fuel vapor" coming out of a carb is usually a leaky calve or a cam with a link or two out of sync with the crank. Something is causing a blowing back out of the cylinder.

[izaleth]

Mixture screws are always turns out from gently bottoming the screw. Too heavy a hand will damage the seat.
"Fuel vapor" coming out of a carb is usually a leaky calve or a cam with a link or two out of sync with the crank. Something is causing a blowing back out of the cylinder.

i can see the valve ill check them, but the cam chain being out would cause that on all cylinders would it not?
it seems to run decent enough aside from a backfire due to improper tune, with the exception of the vapor which i suppose could be a sticky valve not closing all the way?

[izaleth]

Mixture screws are always turns out from gently bottoming the screw. Too heavy a hand will damage the seat.
"Fuel vapor" coming out of a carb is usually a leaky calve or a cam with a link or two out of sync with the crank. Something is causing a blowing back out of the cylinder.

so i did a compression test and got almost nothing, opened the valve inspection port and turned the bike over and had tons of air coming out of the port, so im assuming i may have stuck piston rings, no visual damage to be spoken of everything seems to be in good condition otherwise im guessing that the rings got stuck from sitting for several years.
soak in marvels maybe and hope it frees itself?
and worse case which i would like to avoid, rebuild the upper end?

[newday777]

50/50 mix acetone and Marvel Mystery Oil as a soak for the rings

[izaleth]

50/50 mix acetone and Marvel Mystery Oil as a soak for the rings

will give it a shot thanks!

[TwoTired]

Has the cylinder cover been removed and then reinstalled without properly getting the the tappets ON TOP of the valve stems?  The shop manual warns about this to prevent bending valves.

Bent valves won’t close, leading to loss of compression.

[izaleth]

Has the cylinder cover been removed and then reinstalled without properly getting the the tappets ON TOP of the valve stems?  The shop manual warns about this to prevent bending valves.

Bent valves won’t close, leading to loss of compression.

dont know to be honest its a new to me project it sat for close to 8 years before i got it

it had an excessive amount of air coming out of the valve inspection port when i took it off and turned the bike over which leads me to believe its the rings rather than the valves, but cant confirm that untill the ring issue is fixed assuming thats my problem
but oll the valves seem to be properly adjusted from what i looked at

[TwoTired]

A leak down test will help diagnose the cause of low compression.

[Tracksnblades1]

Fuel vapor could be number 4 intake valve is leaking. It must have some compression to vapor backwards out of the carb.

 Check and make sure #4 intake has clearance. .002 is not much and can be subjective if you don’t have a lot of experience with lash type camshafts. I would back both adjusters (intake and exhaust) TEMPORARILY to .008-.010 just while checking the compression on number #4. Just to make sure the lash adjusters aren’t hold the valves off their seats. Too tight of lash doesn’t take very long running to ruin a seat or valve face. If Caught soon enough, a litter valve lapping may correct it. While your checking number 4’s clearance, I’d double check on #2 also. Sometimes when the lash on one cylinder is tight, they may have lash them all consistently “tight”.  Only check the compression with WIDE OPEN throttle position. I’d have all the spark  plugs out so the starter can spin it fast. This would get you the highest reading. 

If #2 isn’t firing with the engine running it’s probably like already mentioned, a carb mixture problem. Sometimes the idle circuit causes an idle miss fire. The intermediate and wfo circuits may have the correct mixtures for the cylinder to fire. If you can ride the bike, sometimes a greater open throttle position (more compression) can cause the dead cylinder to crack and pop before possibly picking it up completely (firing again). It would drop back out at an idle though if it’s an idle circuit problem.

Like already mentioned and recommended, a leak down test, and a few squirts of oil will remove all doubt whether your rings are stuck or your valve(s)  aren’t seated.

If it’s running I can’t see the cam timing off much. Especially if it’s only running on 1&3 and a weak 4…

[Tracksnblades1]

Your video doesn’t work on my end 🤔

[izaleth]

Fuel vapor could be number 4 intake valve is leaking. It must have some compression to vapor backwards out of the carb.

 Check and make sure #4 intake has clearance. .002 is not much and can be subjective if you don’t have a lot of experience with lash type camshafts. I would back both adjusters (intake and exhaust) TEMPORARILY to .008-.010 just while checking the compression on number #4. Just to make sure the lash adjusters aren’t hold the valves off their seats. Too tight of lash doesn’t take very long running to ruin a seat or valve face. If Caught soon enough, a litter valve lapping may correct it. While your checking number 4’s clearance, I’d double check on #2 also. Sometimes when the lash on one cylinder is tight, they may have lash them all consistently “tight”.  Only check the compression with WIDE OPEN throttle position. I’d have all the spark  plugs out so the starter can spin it fast. This would get you the highest reading. 

If #2 isn’t firing with the engine running it’s probably like already mentioned, a carb mixture problem. Sometimes the idle circuit causes an idle miss fire. The intermediate and wfo circuits may have the correct mixtures for the cylinder to fire. If you can ride the bike, sometimes a greater open throttle position (more compression) can cause the dead cylinder to crack and pop before possibly picking it up completely (firing again). It would drop back out at an idle though if it’s an idle circuit problem.

Like already mentioned and recommended, a leak down test, and a few squirts of oil will remove all doubt whether your rings are stuck or your valve(s)  aren’t seated.

If it’s running I can’t see the cam timing off much. Especially if it’s only running on 1&3 and a weak 4…

https://youtube.com/shorts/WTRXDFgyHis?feature=share

fixed video

and upon closer inspection seems to be an idle vapor, under throttle its fine. leading me to assume possibly carb related but ill try to check the valves to rule that out

[izaleth]

Your video doesn’t work on my end 🤔

sorry, uploded a youtube vid try that

[Tracksnblades1]

fixed video

and upon closer inspection seems to be an idle vapor, under throttle its fine. leading me to assume possibly carb related but ill try to check the valves to rule that out
[/quote]

That’s sounds like a classic leaking valve..if it’s a leaking intake, it’s still leaking under throttle too. Even if the reversion doesn’t have time to make back through the carb for you to see it..

If the intake valve is the source, a leak down test will allow air to leak out of the #4 intake valve and back through its  carburetor as long as you supply air to #4 cylinder. It makes leaking valves easy to find..

[izaleth]

fixed video

and upon closer inspection seems to be an idle vapor, under throttle its fine. leading me to assume possibly carb related but ill try to check the valves to rule that out

That’s sounds like a classic leaking valve..if it’s a leaking intake, it’s still leaking under throttle too. Even if the reversion doesn’t have time to make back through the carb for you to see it..

If the intake valve is the source, a leak down test will allow air to leak out of the #4 intake valve and back through its  carburetor as long as you supply air to #4 cylinder. It makes leaking valves easy to find..
[/quote]


guess ill have to figure out what i need to buy to do the test, is there any place i can get a leak down tester without breaking the bank?

[Tracksnblades1]

I didn’t see the shipping on the eBay scam..

Google cheap leak down tester…

Make sure your spark plug thread size adapter is included..

[izaleth]

I didn’t see the shipping on the eBay scam..

Google cheap leak down tester…

Make sure your spark plug thread size adapter is included..

thanks, ill probably pick one up soon get it tested!

[newday777]

You can also look at autozone to see if they have one in the loaner tools??