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1979 CB750K Running Rough

knightcrawler

1979 CB750K DOHC
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Hey guys,

I'm new to the forum, hoping to get some help here and be able to help out a bit myself.

I bought a '79 CB750K last year for $300 with 12,000 original miles. Engine was off the bike and most of it was in pieces. I managed to get everything put together and cleaned the carbs. I got it running(not well) and it seemed like it needed air cut off valves. I bought a full kit and completely rebuilt the carbs, took them apart, soaked them, replaced needles, jets, adjusted floats to spec, replaced air cut off valves etc. I put the carbs on tonight and it still is running rough. It sometimes has a loud knock at low idle but after 2,000 rpm it smooths out. Sometimes I can hit the throttle and it comes up fine, other times it dies. Very inconsistent. I checked the ignition timing and it was right where it should be. Coils are good, need to double check the spark plugs.

Here is one thing that might be very helpful in figuring out the problem, when I hook up my carb sychronizer, all of the cylinders are in the red where it says (Late valve timing or leak at intake etc.). Last year before I rebuilt the carbs I checked and it wasn't leaking, but I still need to check this time just to be sure.

Any advice/ideas are appreciated.

Stock air box and a 4 into 1 exhaust.

Side note: anyone else in the Grand Rapids, MI area?
 
check your valves man
make sure the holes where you check your vacumn are plugged also check your float levels
 
Ok, I checked all my valve clearances and they are out of spec...(Spec is .08(+.05)(-.02), Most of my valves were .15-.18, a couple were .20-.23). Forgive my ignorance, but would that be enough to cause it to run fairly rough? I plan on getting new shims but I want to be sure I shouldn't be looking for something else as well.

I checked my float levels when I rebuilt the carbs a couple days ago and I adjusted them to 15.5mm according to they Clymer Manual.

The holes where I check vacuum are definitely plugged.
 
knightcrawler you have a great resource in big rapids.Check out cyclebuster.He is a member and also has a bike shop!He has helped me a lot on the forums.
 
Sweet. I'll have to see if I can get in touch with him.

I ran a compression check on my bike today just to make sure that wasn't and issue and it was 140+ on all four cylinders. I think I am going to get a fresh tank of gas and try to just run it for a while and tweak the carbs to see if I can get it running better. I'll probably spray some brake cleaner around the intake boots and see what happens just to be 100% sure everything is sealed off...
 
I would get the valves adjustments back into clearance. It can be done without too much fuss provided you have all the shims you need. Until this is corrected, adjusting the carbs will be tantamount to chasing your tail.
 
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