1974 cb750 won't idle among other things

[Krullmitch]

Hey guys.
First of all, I must say I do love this forum. I have spent a lot of time reading the threads here during my one year with a cb750.
But now I feel like I've run into a wall and it's time for me to actually post something and hopefully get some help.

The bike is a 1974 cb750 k2. By some weird Swedish logic it is registered as a cb750 k2 f or maybe cb750 f2 k. Don't really remember. I've rebuilt the whole bike and most of the parts where K parts. I found some F parts here and there I suspect tho. The forks and maybe the top.

Anyways. The history of the bike is as follows. I bought the bike about a year ago. Drove it about 15 km to get home. The PO had it bored out to 822cc, pod filters, and 4-1 exhaust. I had a few short runs and then I went on a 4 week vacation. After that the bike wouldn't start. I cleaned and rebuilt the carbs and bench synced them. Nothing! Ordered a rebuild kit and rebuilt them. Nothing! Turned out it was a faulty ignition switch. I bypassed the switch and got it running. Can't remember now if it was idling properly.
I then started to tear down the bike:
Rebuilt the top end, adjusted the cam chain, new valves and guides, adjusted valves, Dyna ignition,new switches and harness. Oil pressure is good.
No compression test yet, wanted to give the valves some run time to seat properly first.

Anyways managed to get the exhaust back on today and started it up to sync the carbs. Couldn't get it to idle. It would just die if I let go of the throttle. Tried to turn some on the mixture screws with no effect.
I managed to ride around the garage some. Only on first gear. When I pull the throttle it bogs down and I need to work with the clutch half way in a bit to get it up in any speed. The bike revs fine in neutral.
I tore down the carbs again and had a look at the float level. One was a bit of.
Checked the pilot jets and they where al fine.

I don't really know what more I can do. With this set up. I would like to buy a stock air box and rejet the carbs but I really want to get it on the road asap since I need to get out of the garage I have borrowed.
I'm running 40 pilot and if I remember right 120 main.

I hope you all can help me. I really feel like I'm about to give up and just pay some mechanic to do this for me. If I can find someone.

Here is a pic I took before I fit the exhaust:

All the best / Kenny

[TOOLS1]

You just described the symptoms of a vacuum leak. Does it run any better with the choke on? Check the condition of the intake boots and clamps. That is most likely the source of the vacuum leak.
TOOLS  

[Krullmitch]

Hey Tools.
Really cool to get a reply this quickly. posted and went to sleep, woke up and there it was.

It does run better with the choke on. One of the things I forgot to mention. The boots look ok, no cracks but maybe a bit stiff and old. The clamp bolts bottom out so i guess their not sealing right. I'm gonna buy some new clamps today and try to get a better seal.
Also order a new set of boots. Would like to find a good dealer in Europe for faster delivery but I can really only find dealers in US. Any tips on that part would be great.

/ Kenny

[TOOLS1]

Here is a link to an ebay seller I have bought from several times and highly recommend. http://www.ebay.com/itm/Honda-CB750K-K1-K2-K3-K4-K5-Carb-Intake-Set-4-Intake-8-Clamps-21-CB750KINBT-1-/131797962237?hash=item1eafc54dfd:g:XQAAAOxygj5SjvtD
TOOLS

[Re-run]

Kenny, for your year of bike, I would really check with davidsilverspares.co.uk
cmsnl is in the Netherlands, but they seem really expensive sometimes, but if you have that rare part you just can't find, they likely have it.

[Krullmitch]

Thanks for the tips. I'm gonna see if I can fix it temporarily tonight and order the parts from ebay. If I cant fix it I'll go for the, maybe quicker, more expensive alternative.

Any tips on tracking down the vacuum leak. Thought i might spray the boots with something. Heard starter fluid was a good thing to use so I might go buy some.
Although I have some 5-56 and some Butan/propane. Anything there that will work?

[TOOLS1]

I like to use a propane, or acetylene torch to test for vacuum leaks, but they do not always show on small engines like these motorcycle engines.
TOOLS

[Re-run]

I wouldn't use starting fluid. That stuff is very flammable, like steroids flammable. I've used carb cleaner in the past. Little squirts right around the boots.

[Krullmitch]

Ok wont use that stuff 😦

So this evening I replaced the clamps with some new ones. The ones that are threaded on the metal band so you can tighten them as hard as nesessary.
Started it and it was much better. Almost idled for a while at 1K but wouldn't hold it for long.
I tried riding it a bit in the garage and it runs much better under load.

I noticed tho that the nr 3 header was much colder than the other once. Not cold but you could touch it, kind of.
I pulled the plug and it looked a bit wet. Quite black but I think that might be from my attempts yesterday.
Here is an image:


Pulled the nr 4 for comparison, it was dry but black. Still maybe from before tightening the boots. The tip of the ignition pin still looked good.
Here is an image of that:


Tested both of them for spark and they looked the same and really strong.
Tried to use some propane on nr 3 boot but I got no change. Maybe I couldn't direct it at the leak (where ever it is) or I had not enough gas flow. I don't know. It was hard reaching in there with the torch.

What should I look for when squirting carb cleaner on the boots? How will the engine react?

/ Kenny

[Krullmitch]

Ohh and also. At some time I was suspecting a leak at the header  tried to look and feel but I couldn't see anything. Thought I heard some puffing sounds tho. Any tips on how to better test this?

[TOOLS1]

Like I said, sometimes you will not be able to identify a vacuum leak on these small engines. You just have to go by the running problems to diagnose. As for a header leak, look for black soot around the connection. Also the "puff" you are hearing could be a bad exhaust valve.
TOOLS

[Krullmitch]

Got some work done on the bike today.
Now noticed that nr 3 wasn't very hot at all. In fact just getting warm front he other cylinders.
Checked the obvious and there was no fuel in the carb.

Managed to fix that and now the bike idles at 1k and runs great.
Although when taking the bike for a short ride and as soon as the bike got up to working temp it idles at 3k.

Didn't have time to do anything about it tho. Hoping it's just a small turn on the idle screw.

Thanks for the help guys

/ Kenny

[Krullmitch]

Ok. I need to take back my previous joy.

Ran the bike today. I started idling at 3k almost at once. I took of and after about 7 minutes of riding it stalled and died. Wouldn't start. I had to go to work so I didn't get the chance to take a diagnose.

What do you guys think. Vacuum leak still? Wrong pilot jets?
I've read some similar threads where the guy had A hanging advance unit?
What would you say is the right order to diagnose? New intake boots? Gonna check the gas flow also since that was the problem with nr 3.

[Re-run]

Sometimes you just run into weird walls.

I would start with the basic tune-up. This covers a few things and can help rule out obvious problems.

I would go over every connection in the ignition system. I would test the plug caps and inspect the ends of the plug wires, maybe clip a few mm off the ends. The plug caps should test out at about 5000 ohIf you have not replaced the plugs and don't know how old they are, new plugs might not be a bad idea, but wait till you have exhausted other things.

When you cleaned the carbs, did you clean every jet, including all their holes? Did you remove the air screw and clean in there? Before removing the air screw, turn it in till lightly seated while counting how many turns it took to seat it. Then you can set it back to how it was when you put the screw back in.
I tell people to get the insides clean enough to drink through.

Make sure float heights are right. Check this with clear fuel tubing connected to all 4 and turned upright. Then crack the drain screw. They should all be about the same height.

Make sure your clamps are tight on the carbs, but also the engine.

I would also double check valve gap. I have solved issues just by doing these steps.

[Krullmitch]

Ok back from my summer holliday and back on the bike.

Before I left of I ordered new original carb boots from UK. Took a closer look at the old ones that I fitted some hose clamps from a local hardware dealer.
I had tighten them down pretty hard as a quick fix to my leakage problem.

Dont ever do this!

Turns out that when the engine got hot and all the parts expanded. The clamps cut right through the rubber and cut the clamps of clean.

Anyways. New boots in place and the bike runs much better. It holds the idle and rides fine. I managed to do a sync with my gauges Fixed an oil seal leak at the tach connection.

However. When i blipp the throttle the rpm still makes a slow return to idle. It inst that bad any more but its still not very pleasant riding around the city.
I think this problem was here when I bought the bike but I could force the rpm down with the throttle return cable. Now I have removed that cable. (The controls where not functioning so I ended ut buying some new ones, not CB parts, that didn't have place for two throttle cables)

My throttle return spring seems quite weak and my cable isnt the smoothest one but the slides still return to their resting place with no fuss. (I can still force them down another millimeter tho)

Is this normal behavior for these carbs?

Can this be a symptom of something else, jetting? vacuum leak still? Valves?

[Krullmitch]

Ohh yea forgot to mention.
I did all of your suggestions except clear tube check. (I opened and checked with a gauge)
Didnt check valve gap but i did that wen I assembled the engine, might do that later. Still want to exhaust every other possibility.

[Krullmitch]

I replaced the avancerad springs with new ones from yamiya also. (I keep forgetting things... my brain isnt up to speed after 5 weeks of vacay jet)

Thanks for the replies  :)

[TOOLS1]

Two things that can cause a slow return to idle. First is too lean. You might be able to adjust the Fuel/Air screw, which ever your carbs have to cure this, or go to a richer pilot jet. The second thing is the carb sync. If you have round top carbs, you should have two threads protruding out the top of the adjuster. If you do not, readjust them to two threads, and then re-sync them.
TOOLS

[Krullmitch]

Found the gremlin haha!

First of... I had 2 threads showing on the top of my carb adjuster.
Tried fiddling with the air mix screw but that didn't really help anything.

Then I found it... The slides were snapping back as they should while the bike was not running.
But while running... They didn't...
Maybe because of the slight pressure in from the gas in the carbs while running. I really have no idea. They didn't behave as they did when the bike was off.

I ended up cutting two turns off my throttle return spring and that seems to have fixed the problem :)
Gonna take it for a longer ride this afternoon before I'll call this a win.

[Krullmitch]

One side note. Should maybe be a separate thread, but i'll post it here for now.

Some while ago while trying to fix this problem I found that my throttle arms was binding up on my petcock. I spotted this after changing the intake boots. Donno if it was there before.
The topp of my throttle adjusters also hit the tank then I pull the slides fully open.

Donno if some PO rebuilt this bike with spare parts from other bikes or what the problem is. The tank is resting nicely on the front rubbers and locked in place correctly by the rear rubber.
There seems to be a bit of a miss match tho.

I have chimed up the right side of the tank about 6mm but that doesnt help with the adjuster nut hitting the bottom of the tank.

Is there different models of tank bottoms or petcock placement within K2-K6? Intake boots?
Any ideas? Its nagging at me that I need a piece of cardboard to ride smoothly.