Running Problems 750 K - Carbs ?

[Rake]

Evening all.  I picked up a 1989 750 K non starter that had been laid up for some years. I found it had no spark which turned out to be the CDI unit. Replaced it and got her to start but she will only run on 3, pot 3 being the naughty one.

occasionally pot 3 comes in but it’s intermittent.

I see fuel coming out of the carb breather tube as a dribble. 
 

she will start and run on choke but turn the throttle and she dies. Once warm on choke I can start her without choke but the throttle has to be wide open to get a start. I can keep her running if I good the throttle partially open but she does if I drop to idle speed.

I did a compression test but I’m not 100% happy with the fitting of the tester - the adapter kind of wobbles in the plug hole. Pots 1 2 and 4 give around 100psi pot 3 gives a very low PSI reading. I was kind of hoping that it was a tester issue or stuck valve that would free up with some running of the engine. I have not yet repeated the test since having run her up.

here is a video of her 

https://youtu.be/R4TaK2QD5cw

I’ve taken the carbs off to clean them but would value some help on whether this could be a carb issue or something else.

 

thanks in advance 

[dago]

Probably blocked pilot jets , also check float heights and valves. Give carbs a good clean and try again 

[Simbec1863]

After being stood for years I wouldn’t waste my time with anything else other than removing the carbs,fully strip,clean and rebuild with decent quality parts.

Then stick the carbs back on, balance and at this point start working your way through the possible low compression issues, these engines are pretty good even with some miles on them if they’ve been looked after over the years.

I took some carbs off my 1100K recently the bike still had an mot and was used a bit before I bought it and even this supposedly road worthy bike had the most shitted up carbs I’ve seen in years thanks to lack of use and modern ethanol content fuels.

Keyster normally do a good quality rebuild kit not cheap but decent quality you can get them from NRP in Manchester or contact Allen’s Performance in Notts for genuine mikuni spares. Worth spending the cash and taking your time you’ll find the bike will run 100% better afterwards, it won’t sort any potential engine issues out but is a step in the right direction.

Example of my 1100k’s float bowl with 30000 miles on would hardly run and leaked fuel out of the vac pipes plus most other of the rubber hoses

Edited October 3, 2021 by Simbec1863

[Rake]

Ok carbs are off, I’ll strip em asap and let’s see.

incidentally this bike only has 31k miles on her. 

Edited October 3, 2021 by Rake

[TonyGee]

i would say it could be a pilot jet blocked or the float valve is leaking.  but i would investigate the compression isue as well, 100 psi is not the best reading   did you have the throttle wide open when you did the test ? and the engine should be up to temp if possible. 

[Buzuki]

IMHO those carbs are in very bad condition and they ask for complete overhaul , obligatory all those internal O-rings need to be replaced , check for correct valve clearance also, and for good sealing between 4 carbs rubber boots and cyl.head .

[Rake]

21 hours ago, TonyGee said:

i would say it could be a pilot jet blocked or the float valve is leaking.  but i would investigate the compression isue as well, 100 psi is not the best reading   did you have the throttle wide open when you did the test ? and the engine should be up to temp if possible. 

The engine was cold on the compression test. Throttle was closed.

[Rake]

20 hours ago, Buzuki said:

IMHO those carbs are in very bad condition and they ask for complete overhaul , obligatory all those internal O-rings need to be replaced , check for correct valve clearance also, and for good sealing between 4 carbs rubber boots and cyl.head .

Checked sealing by spraying wd40 over rubbers which had no effect.

[Merijn]

On 10/4/2021 at 3:47 PM, Rake said:

The engine was cold on the compression test. Throttle was closed.

Test with wide open throttle. The actual readings are not very accurate on most testers anyway, as long as it gives a decent value and all 4 are on the same level it says enough to begin with. Since 1 is way lower, lift off the cam cover and check valve clearance and if there's anything different from the other 3 going on. It's not a lot of work on a 750K

[Gixer1460]

8 minutes ago, Merijn said:

Test with wide open throttle. The actual readings are not very accurate on most testers anyway, as long as it gives a decent value and all 4 are on the same level it says enough to begin with. Since 1 is way lower, lift off the cam cover and check valve clearance and if there's anything different from the other 3 going on. It's not a lot of work on a 750K

You could test compression again with the carbs off, they don't need to be fitted! Even cold, i'd expect numbers above 160-170psi and upto about 195psi if warm. Absolute minimum would be around 110-115psi and variation between cylinders - no more than 10-15%.