1978 cb 400 twin wont idle

[Restrick]

Hey all!

Looking for some advice, i have a 78 cb 400 twin with makuni carbs and pod filters also the exhaust has been moded to a 2 into 1.

i realize with those mods , it would need adjustments.
I've never played with jets or really cracked open a carb before. so before i go and make some mistake and make matters worse .Any tips on where to start ?
Your guys help is greatly appreciated.

-cory

 

[VonYinzer]

Need more info...

What do you mean it won't idle? Does it die or does it rev too high?

What Mikunis? Have they been modified?

What exhaust?

Are the pods the cheapo EBay bits? If so, throw them away. They're junk. Inconsistent flow, poor filtration, etc... They are also prone to falling apart which is a VERY bad thing to happen while out riding.

Is the rest of the motor in good running shape? Timed correctly? Valves adjusted?

Let us know all the details and we'll get ya shaped up in no time.

 

[Restrick]

excuse my lack of proper terminological or basic knowledge for the matter. with that said.

it has strong throttle response, sounds like its popping when revved up. the second you let go of the throttle it dies. i haven't ran the bike for more then a few minutes, fear of and damage it could do i guess.

as for filters http://www.mikesxs.net/product/15-0027.html this is whats installed. at least they bare the same shape and the xs stamp.

exhaust is stock headers and muffler, with a custom Y joining them 2-1

as far as i know the timing and valves were adjusted before the airbox and stock carbs where removed and replace with the makunis and pods.

do the carbs bare a mark or serial number to help identify them ?

the only photo i have at the moment is this

i will take more photos tomorrow.

 

[VonYinzer]

Ok. Right off the bat, you're running really lean. That's what the popping is from. You need to up the jetting.

Those pods are ok... But just ok. If you can swing it, get some higher end stuff. K&N or UNI.

You'll need to measure the carbs. If they're larger than the VM30s, it'll be a real pain to tune them.

The exhaust... Are both head pipes of equal length before the "Y"? They should be. Also, you say its a stock muffler, so the pipe after the "Y" is the same diameter as the head pipes. No bueno. Very restrictive. Basically, you're forcing both cylinders exhaust through a passage designed for one.

Long story short, you need to order a bunch of jets for the carbs (Sudco is a great source). Start high, and work your way down. I won't get into carb tuning theory and what jets you need here. There are plenty of threads explaining that in much greater detail and in a much more concise way than I'm capable of.

Before that, you need a proper exhaust though.

And don't assume that the work was done correctly by anyone. Until you check the motor to see if it's all kosher, assume its not.

Your manual will walk you through that process.

Good luck and keep us posted on the progress!!!

 

[Restrick]

Re: 1978 cb 400 twin won't idle

VonYinzer
i measured my carbs, turns out they are 34mm. everything i'm reading is suggesting they are over kill.
i would like to make due with these if i can. Any thoughts ?

also the exhaust, its not the stock muffler not sure of the brand and it's wide open, no baffle.

one more question, if Makuni has a left and a right how can i tell them apart?