HELP .. picked up a 1980 cb750c .. high idle when warm

[Adrians1980cb750c]

I just picked up a 1980 cb750c .. it has a vance and hines supersport exhaust on it and the k&n air pods


It wasnt running when I bought it .. but I cleaned the carbs .. the primary jets are #89 and the main jets are #102


are these the correct jets?  

my main problem right now is that the bike starts right up and idles fine when cold .. but as soon as it warms up the idle gets higher and higher .. anywhere from 3 to 5k and wont go down unless you put a hand over a carb to knock it back down or put a load on the engine (put it in gear and load it up)


ive read around the forums .. I have no throttle cable problems, carbs are clean, and no vacuum leaks

do I just need to sync the carbs? .. if you put your hand over the #1 and #4 they have more vacuum than #2 and #3 carbs just by feel

[TOOLS1]

Welcome to the forum. It does sound like a vacuum leak, but I have heard the if the carbs are really out of sync it will cause the idle to fluctuate. I would go on, and try syncing them. As, for the jets being correct, you are going to have to run it, and check the color of the plugs.
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[shinyribs]

I dont know for sure,so you should probably ignore me,but those primaries sound HUGE! I didnt download the particular manual for your model.Mine is different. Download the Factory Service Manual and see what it calls for. That would be a good start. If they are off nothing else will work right until they correct.Good luck

[Adrians1980cb750c]

from my research so far ive been seeing on jet sizing 75 / 110 are what i need for the air pods and the exhaust

I have 89 / 102  .. so I might be way off


I'm pretty sure the sync is way off since the #1 and #4 pull more air then #2 and #3 just by holding a hand up to them when the air filters are off


I'm going to sync them first .. then go from there

[LukeM]

I got a set of vacuum gauges for my bike last year, and after setting the little damper thingies, put it on my Shadow.  I had used a "bench sync" procedure, where it says to set the butterflies so only the top hole is visible.  Well, that was close, and it ran OK, but I decided to check it with gauges.  

End result: there's nothing like reading it with gauges.  The balancing screw needed quite a bit of adjustment.  When both needles were getting close to the same reading, the RPMs went up and up, necessitating the throttle stop screw turned WAY down...like maybe 3-4 complete turns.  Blipping the throttle, the engine revved much quicker, and a test ride confirmed it.  Nice burble on deceleration, ran nice and smooth, mileage was about the same, all wins in my book.

As far as your issue, pods and the DOHC carbs are a pain, as the carbs like to see a certain amount of vacuum.  The stock air box provides this. Rejetting the carbs to compensate for the more open exhaust and more open air will be necessary, but I think the previous owner went a little overboard.  
Check the plugs: If it's running rich, the plugs will be darker, almost a sooty dry black. If they are light to medium tan, then the mixture is probably about right.

I hope this helps.  Keep us informed, and we'll help if we can.
Luke M

[Adrians1980cb750c]

cool .. thanks

I work weekend shift .. so Probably wont see much action on the bike til sunday or monday - thursday

I ordered the m5 nipples to hook up the gauges to the motor to sync the carbs .. they shouldnt take long to get here .. so as soon as they arrive I'll be back tuning it


I guess I'll need to get the 75 / 110 jets .. where can I get these for cheap?  ... I'm looking around jetsrus.com but cant seem to figure out which are which .. I have a keihin cr29 right? .. I guess I need to look at the bike too

[Adrians1980cb750c]

another thing .. I'm trying to keep this build as cheap as possible  

but does anybody know what exactly is in the dynojet jet kit?

http://www.ebay.com/itm/80-82-Honda-CB750-Dynojet-carb-jet-kit-stg-1-3-/200592900864?pt=Motorcycles_Parts_Accessories&hash=item2eb444c300&vxp=mtr#ht_1408wt_698



I dont want to buy it and turn out its just $30 worth of jets .. has anybody used it before?

[Adrians1980cb750c]

http://www.z1enterprises.com/Dynojet-Kit--Honda-CB750-CB900-198082-4035.aspx


found this ..


so looks like a #75 primary jet should work just fine .. but which main jet should I get (the dynojet kit doesnt come with a 110) .. is that bc of the needle it comes with?

anybody know why it comes with drill bits?

[Adrians1980cb750c]

http://www.dynojet.com/pdf/1127.pdf


is that needle any different from stock? .. can I just get it on jetsrus.com?  



so basically I need to get 75/130 jets .. then an adjustable needle .. and drill out the lift holes with a #14 drill bit .. then put the mixture screws at 3 1/2 turns

[TOOLS1]

Honda sells the jets as cheap as the online stores. I have learned to go to the Honda dealer first.
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[sgtslag]

Most of the folks on the CB750Custom forum recommend not using pods, period, due to their instability on these bikes.  Stock airbox, with a K&N filter, is easier -- no mod's required.  Best of luck whatever you choose.  Cheers!

[Re-run]

If you DO stick stick with pods, either get K&N pods or Uni pods. They do work better than the cheap emgo junkers you see for $30 or whatever they cost.

[Thorby]

My bike idles faster when it's warmer out (say, 70).  Even though it does idle at substantially faster (topping at 1500 RPMs), I didn't think it was a big issue.

[Adrians1980cb750c]

I do have K&N pods


I'm going to keep them .. saw a youtube video of a cb750 with pods, exhaust similar to mine, and the dynojet jet kit and it seemed to idle just fine .. hung for a second but quickly returned to normal idle


1500 idle is fine with me .. just dont like the throttle hanging at 3 to 5k rpm


I'm going to try and source the parts that come in the dynojet kit .. #14 drill bit, #75 / #130 jets, adjustable needle, and some shims / washers that go with the needle

says install that and turn the mixture screws 3 1/2 turns out and your good to go

if I cant find the parts I'll just buy the dynojet kit

[Adrians1980cb750c]

anybody know which needle I need to get to match the dynojet kit?


I have found all the jets on jetsrus .. all the main and pilot jets comes to $24 .. drill bit is less than $3 .. just need to find what needle I need to get and some shims

[Adrians1980cb750c]

http://www.ebay.com/itm/CB750-CB-750-70-76-CARBURETOR-SLIDE-NEEDLES-KEIHIN-/160753885275?pt=Motorcycles_Parts_Accessories&hash=item256dada05b&vxp=mtr



will these work? or are they all different

[TOOLS1]

Those are, for the old style round top carbs. Doubt they will work in the CV carbs.
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[Adrians1980cb750c]

yea I figured they wouldnt work



anybody have any experience with the six sigma jet kit on ebay?


I emailed them asking exactly what is in their kit and if it compares to the dynojet kit .. they just email back saying they custom make the kit for my bike ... not what I asked at all .. lol  

[Adrians1980cb750c]

nevermind .. I just went ahead and bought the dynojet kit


got tired of looking for that damn adjustable needle after a few hours of looking



ordered the dynojet kit and a stainless bolt set with allen heads since a couple on my carbs are stripped out .. guess I'll polish them up some and paint the brackets black .. there already pretty darn clean

[Cataclysmic83]

I have a 1980 cb750c. I'm also going to try pod filters and 70/130 jets. Shimmed the needles about 3mm. I'm crossing my fingers lol