Topic: Happy update on my '78 750K8, and of course a question :)

[Scout]

All  I gotta say is - I love Yamaha Carb Cleaner!!!

To make a long story short, my bike had to have the choke on 3/4th way to ride it, and idled for crap. I put Seafoam in the tank, however after 80 miles it didn't do anything. Today, I decided to try the good ol' Yamaha carb cleaner. Put it in for 4 hours, drained it, then started the bike. At first it was the same. Then I shut her off, for about two hours. My buddy came over to check out my new ride, so I started it again, and the RPMs jumped higher than normal, so I started pushing the joke in, and went all the way, and the bike started idling at 1,500RPMs!

Needless to say, I was a bit happy. Took her out for a ride, and wow, she's a screemer without the choke I also fixed the draging front brake by taking everything apart (bleeding is a #$%*!!)

Now for the question - she has developed a hesitation off idle. Above 3,000 rpm it's all perfect, but anything below and I have to be very gentle with the throttle, or she will die. Makes for fun U-turns

Scout

[Buffo]

sounds like a slow jet problem...they may be clogged a bit still...try to richen the mix with the piolt screws

[Buffo]

if you didnt take the slow jets apart and clean them properly...which is the only way to clean them small holes they are still dirty...and the problem will most likly remain untill you do so...do a search on slow jets...but the piolt screws might help

[Scout]

Well, the whole point is that I'm trying to avoid taking the carbs out I've never done that before, and I sure don't want to learn on this baby, unless I absolutely, positively have to...

So, which ones are the pilot screws? I got a couple of manuals on order, but they won't be here until after the weekend...

Scout

[TwoTired]

Are your exhaust head pipes all about the same temp now at idle?  If so, then your slow jets are probably working okay.

Just a guess, but you have an accelerator pump that shoots raw gas into the intake runners when you twist the hand grip.  This is because the carbs are set very lean at idle and won't pickup RPM very well without the extra shot of gas.

I *think* that accel pump can be removed, examined, replaced without taking of the entire carb assembly.   It's on number two (or is it three?) carb, hanging down from it.  Pretty obvious if you look down there.
If the diaphram is still intact and pliable, then maybe the circuit that feeds it gas is clogged.  When it works right, your throttle response should be pretty good.

 I'd leave the Idle Mixture Screws (pilot screws) alone until I verified the accel pump function.  They are difficult to set properly without a very sensitive tachometer.

Cheers,

[Buffo]

2tired is right (and usually is) about checking the accel pump..it is help on with 3 screws on the #2 carb...it would be tricky taking it off with the carbs on but could be done...if you can get your airbox off you can see if they are working by looking into the carbs while twisting the throttle...fuel should come out of brass tubes that point up from the mouth of each carb...make sure they are shooting a good amount of fuel in a good pattern...if they are not working then take the pump off and check the diapham...those tubes can get clogged to...

I have a K8 and the carbs are not hard to work on/over haul. If you want to keep these bikes you should get used to the idea of working on them.

If the pump is working fine......

I will warn you of one thing...if you have clogged or slightly clogged slow jets your bike will never run right. your slows are pressed in and can be pulled out with a good set of plyers...

...if it is the slow jets YOU WILL NOT EVER GET THE SLOW JETS CLEAN UNLESS YOU REMOVE THEM AND CLEAN THE JETS BY HAND AS WELL AS ALL OF THE SLOW JET PASSAGES.

you could soak your carbs for 3 weeks and the slows will still be clogged. the passages are to small for the cleaner to go all the way in and get at the gunk/crud thats inside...

I have posted many questions on the K8 carbs,(and 2tired answered most of them) do a search and start reading.

i will tell you that those carbs have to come off.

the pilot jets are on the bottom of each carb, in front of the float bowl...out for richer, in for leaner...stock setting is 1 1/2 turns out...DO NOT OVER TIGHTEN THE SCREWS WHEN CLOSING THEM...start with the stock setting and turn each one 1/4 turn each untill you notice a difference...if by 1 full turn past the stock setting you notice no difference your slows are very clogged


read all the posts, ask all the questions you want but if you dont remove the carbs you are wasting your time.

The clymer manual sucks for the cb750k8..I have a copy of JTB's shop manual and will burn you a copy for like 6 bucks including shipping...that is what I paid him...the clymer is wrong about most everything for our bikes...I have one of those too.

[Scout]

TwoTired - Yeah, looks like I'm going to be doing that today, I'll report back with the results. Hopefully I can take the accelerator pump without taking the carbs out.

Buffo the Magnificent - I know sooner or later, I will have to take the carbs out, even as part of keeping the bike for a while. I'm just not experienced, and have been delaying doing it, especially since Chicagoland has had nice weather last two weeks, so I've been able to ride it. Once the cold sets in, I'll have one more reason to take the bike apart... I've been reading all the posts, this site is quite an encyclopedia. As fas as the shop manual, I would love to have it. I live in Aurora, Illinois. Please let me know the  shipping and PM me your address, I'll send you money right away.

Scout

[Scout]

As far as the exhaust pipes, the two outside ones are still much colder than the inside ones...

Scout

[dusterdude]

arent  no.1 and no.4 plugs fired off the same coil?

[eldar]

! and 4 do fire on the same coil. Keep running the seafoam. It will clean things out. 80 miles is not that much. That is only about 2 gallons. You need to run at least a full tank through. Maybe a couple of times, but regular treatment will really stretch out the amount of time between cleanings. many people take their carbs off a minimum of once a year but unless you have to store your bike, you could do it eery 2 - 3 years with the seafoam and a filter.

One thing with the accel pump, I have never seen gas come out with the engine off as the carbs require a vacuum to pull the fuel in.

[cb350f_rider_73]

I know this is mostly unrelated to your post, but I'm happy to hear you could pour the Yamaha carb cleaner into the carbs with no damage.  I kinda am planning to do a full soak of my carbs in the stuff, and was not completely sure it wouldn't eat my floats.  Thanks. 

Wayne

[Buffo]

I put #38 pilots in my K8 today...carbs off...pilots out/in...back on the bike and riding in under 1 hour...

I checked the accel. pump and I had spray coming out of mine with the bike off...

I am gong to have to put larger mains in as soon as I get them...I think I will strt with 120's and then chop and increase...chop and increase as needed

I am running a K$N factory drop in the stock air box an just  4 into 1 header wth no muffer

Buffo the Magnificent

[eldar]

Wow I never had spray with the old pump or the new pump.  I have run carbcleaner through it and even used wire to make sure there was no blockage. Well since it is now getting to cold to ride, I can drain my fuel, repair dents in my tank and get decent float valves and check that out too. But I am certain that it has never done that even when i first got the bike.