1981 cb 750 with pod filters

[Eric]

My son just bought a 1981 cb750 the previous owner" bobbed". It has no airbox and is running pod filters . I am having trouble getting it to idle. Has a aftermarket exhaust also. Could it not be causing enough vacuum to draw fuel at idle? Bike runs great above idle.

 

[MickMick]

I tried pods on mine and found just the same happening. The pods have less air resistance to were reducing the vacuum level in the velocity chamber so on idle, drew less fuel. I tried to install restriction plates in the pods but then the top end was affected. Fine balance these bloody things. Went back to standard and all good again. Don't give up though, you do need different jets in the carbs to use pods or low resistance exhausts. Again ,fine balance and every bike will be different dependant on the combination. Also assuming the carbs are balanced and no blockages in thee idle jet under the butterfly.

 

[BB74]

I put pods on my bike two weeks ago and I'm about to put the airbox back in for the same reason. I can get it to idle smooth at under 1200 rpm.

 

[fg1972]

Although POD's look cool on a custom build, this is the reason I stuck with the original air box as these CV carbs are quite fiddly to get right at the best of times. Often there are a number of other problems contributing like vacuum leaks via the intake boots or throttle shaft seals. Then there is the dreaded blockages of the idle circuits which sounds simple to rectify but is a lot of effort. Might be a bit of a compromise if you're going to go the POD route with the stock carbs. I hear others having success with the right rejetting but most have some sort of issue.

 

[dirtdigger]

Start looking for a stock air box now, only way you will get it to run decent.

 

[Eric]

Ok, thanks for all of the advice. I purchased a factory airbox, now I need to find out what lines hook up to the air box. I know the drain line . But have 2 other hookups on the airbox. In the pics the previous owner has 2 small pod filters on what I believe is the crankcase vents that need removed for the airbox installation, I need to know where they originally went. Any help would be greatly appreciated.

 

[fg1972]

If I'm not mistaken, the crankcase vents (after removing the PODs) should go to a tee piece, the middle of the tee to the top of the airbox.

 

[cashmachine777]

All,
Can any one confirm airbox fitment between DOHC CB750's. I have a 1983 CB750C that is having the same type of issue, and am currently trying to source a stock airbox. Can anyone speak to the campatibility of airboxes between the 1979-1983 CB750 K F FII C ETC..? Thanks!

 

[kirky]

cashmachine interesting that you are in japan, this is regarding emissions. what i do know is i have a JDM 1979 dohc 750FZ. the manuals i have for the 750's state main jet at 102 and mine had 75's in. i got them rebuilt with 102's and the bike would not rev past 5000rpm, long story but i also found a cracked coil and melted CDI's so thought this was the problem. the problem was the size of the air box intake tube that was smaller than the european intakes. i cured the lack of air flow by leaving off the air filter screwed on cover allowing it to pull more air in around the push on cover. so what i say is there are differences in air boxes regarding a JDM bike and a european one. i would check your jet sizes they seem to be matched with the air flow as i found out... the hard way