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Losing the idle when it heats up - new to me 1979 CB750L

NicksHonda

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Hello everyone,

I am new to the forum. I got a good deal on a 1979 CB750L.

It is in great shape overall. It just reached 15k miles on my 25 mile ride home. It runs ok, shifts well. So far, seems to need new front brake pads. These act like they were oil soaked (as in fork seals leaked on them). I plan to get new pads via Amazon. Unless there is a better place.


I was told this was sitting for a while. There was some surface rust in the tank. The rust wiped off what I could get to with the fuel in it. When it's cold, with the choke on, it takes two starts to get it going. Then I quickly turn the choke down. It seems to run and idle fine for a while. When it is up to running temp, it will slowly lose idle and die. The PO turned the idle screw up for me. Now it tends to idle around 1800 and "sticks" at 3000. If I snap the throttle it would go back down to about 1800.

I was reading some forums here. Seems I should check compression on all cylinders. It could be valve adjustment time. If the compression checks out, then clean the carbs. I am not 100% sure if I can just sync the carbs, or do you think they need a cleaning based on this information?

Is there a good place/brand to get carb rebuild kit?

Again - It seems to run well at all speeds and gears. Just an idle issue that I can tell.

TIA
Nick
 
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With that many miles you absolutely need to check the valves....and probably is close to having issues. And dont set them to factory specs they are too tight and will burn valves. If the engine checks out fine I suggest saving up some money and dumping the stock carbs, they are absolute junk and will never run right, Im pretty sure they never even ran right from the factory.

On a side note I see you have a 954...I have and o3 954 as well, still my favorite sport bike ever.
 
Hmmm, Can anyone confirm or elaborate while the stock carbs suck? The only thing I found is the stock carbs have a pilot jet that pushes in, instead of screws in. If ditching the stock carbs are the way to go, what are the replacement carbs to get?
Thanks
 
The stock cv carbs are very touchy, many passages to plug and hard to get open, engine condition plays a huge role in the way these carbs perform, very difficult to get them jetted correctly if you do mods to intake system, exhaust etc. Just do a search on the forum and look at other posts on these bikes, it seems someone posts with a problem with these bikes/carbs just about everyday. These things have been discussed many many times on this forum, just gonna take a little looking. Just look into Keihin CR carbs, they are basic and very tuneable.
 
OK, I guess I will do a search.

I am getting 120 or 125 on all cylinders. I found the air cleaner was saturated with oil. I changed the spark plugs. The old plugs looked good. It seems to run fine with the oily (was a paper element) filter out so far. I opened the tank trap line on the bottom and some water/oil came out. I drained the fuel. I put a fuel filter between the petcock and the carbs. I put new fuel in with techron. I ran it for a while and now I will let it sit with the techron in the carbs. It seems like it is still sluggish to get off the carbs. It seems like it needs a carb synch.

Thanks for the help.
 
120 to 125 psi is really not that good, its considered pretty low for these motors and will not give great performance with those numbers. Did you use a quality compression gauge and cranked with all plugs out and throttle wide open?
 
Yes. All spark plugs were out and the throttle was held open. The gauge is new from an auto parts store. The engine was stone cold.
 
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