Strange acceleration problem - help needed...

[jarni]

Hello, I have a 1979 550GS with Mikuni BS32 carbs.
Carbs were cleaned twice, I installed k&n style separate filters, carbs were rejetted to suit filters... and now the bike runs strange...
It starts instantly with choke out... I can put the choke in after 5 seconds of running... it idles great... everything seems fine until I sat on it and wanted to go for a ride... It starts fine but when I opened the throttle halfway it hasitates and does not GO as it used to... but when I open the throttle totally, it goes with full acceleration... Then I put the bike on centre stand and removed air filters and looked in the carbs while revving on centre stand... when I revved the throttle in fast motion, the engine started stalling... more and more...
Does anybody have any idea what could be the problem?

Best regards, J

[seanarthurmachado]

Is a "separate filter" a little cone shaped air filter for each carburetor ?

If so that is referred to as a pod filter.

Check the "pods and pipes" thread

Sean

[Rob S.]

I suspect Jarni is not in his twenties anymore. When I first installed four K&N filters on my Z1 30 years ago, "pods" held peas in your Chinese food.

[JCT]

It sounds like you have a carburation problem.
You changed the filters to have more air flow I assume .... and then you rejetted the carbs to match the new settings.... (hopefully the jets are the right size,this would be very important).
OK so back on track this is what I can understand from what you have said...
your idle is working fine right? this tells us that the pilot jet and air jet are the right size
your 1/4 throttle is OK? pilot is ok needle starts to work
your 1/2 throttle is not OK; Needle is the reponsable for 1/2 throttle
your 3/4 throttle is Ok? Needle is still working but you main jet starts to take over
Full Throttle is Ok ? Main Jet is the one working on the final range of acceleration.
....So... I believe you are havien problems with your needle.... lucky for you, you don?t have to buy anything extra to change the fuel/air ration from the needle you just have to take out the plunger and needle and jump the needle to a higher "step" (the higher the needle sits the more fuel it will let through/ The lower it sits the less fuel it will let through)... there are videos on how to adjust those specific mikuni carbs look at them (they really help)
Good luck
(after the bike is running fine it is always a good idea to take it into a shop where they have a air/fuel ratio tester this helps to make sure you are doing fine in the whole throttle range. Also make sure your carbs are in sync with each other that way all your pistons are working equally)
Once again good luck and let us know how it goes

[Steve]

Well, the first "problem" is that the '79 550 did not have BS32 carbs.
I think you have an '80 bike that was built in '79. Look for the production date on the steering stem.
If it is 08/79 or later, it is an '80.

Also on the VIN tag will be the letter code for the year.
If that letter is "N", you have a '79. If it is "T", you have an '80.

Now to your carburation issues:
Welcome to the world of pods.
You now know why we REALLY encourage you to stay with a stock setup, but all is not lost.
You say the jetting was changed for the new filters. Any idea what jets were installed?
The carbs need to have a bit of restriction on the intake and a smooth airflow at the inlet to work properly. If you have removed the filter and are looking inside, they will operate exactly how you describe: idle well, but stumble with any throttle input. At least put the filters (pods) back on, as they will usually smoothe the airflow. Throttle response will still depend on proper jetting, but there are THREE circuits that need to be adjusted. Changing the main jets is only one of them.

.

[jarni]

I suspect Jarni is not in his twenties anymore. When I first installed four K&N filters on my Z1 30 years ago, "pods" held peas in your Chinese food.

Hmmm... yeah... kind of... ;-)

[jarni]

I will try your suggestions tomorrow... a friend of mine says that this could also be caused by old mikuni membranes... that they are not as flexible as new... what do you think?
Thanx for replys! J

[2BRacing]

If you have the "membrane" type carbs (flat tops) then they are the BS type (also known as CV type) as you said in your first post. This type of carb must have restriction over the inlets, as mentioned by Steve. You can put your pod filters back, or use a few layers of cloth held over the intakes to provide some restriction, if you want reasonable throttle response while revving the engine while the bike is on the centre stand.

Have you still got the original exhaust system on the bike, or has it been changed?

Your quickest path to proper carb performance with pods and maybe exhaust is to fit a Dynojet stage 3 kit, which uses larger jets and more importantly, needles with a different profile to OEM needles, to give correct midrange performance.

[jarni]

Yes my Mikunis have flat tops... and my exhaust is Sito aftermarket that is very loud.....but yi also have a silent origimal tcpe of exhaust... .
is it possible that it is the membrane fault?
thanx!

[2BRacing]

As long as the membranes do not have a tear or hole in, they will be OK. The flexibility of old vs new membranes is not the cause of your problem. The design of this CV carb makes it essential that there is a certain amount of "restriction" to airflow into the carbs. That is why, on a standard bike, the airbox must be sealed all around; the air filter must be lightly oiled and the rubber boots from carbs to airbox must also seal properly. All together, these factors will provide the required "restriction" to the airflow, and you will get the proper throttle response.

So as soon as you increase the airflow through the engine by pods and freeflow exhaust, all the original OEM design factors go out the window, and you must compensate properly for the changes.

Do you know what jets are in the carbs compared to standard sizes? You definitely need a Dynojet stage 3 kit with that open exhaust and pods. Only changing jets alone to larger sizes will not help. You also need a different profile (taper) on the needles to give correct midrange fuelling and response.

Use the "search" feature on the forum to look for "Pods" or "jetting for pods". You will find many hours of reading information, and the answer will be the same each time: DYNOJET STAGE 3.

Good luck!

[jarni]

What is the meaning of the tube that goes from top of the engine to the original airbox?
Since I changed the air filters and do not have the airbox anymore, my tube is opened hanging above the carbs...
thanx!

[2BRacing]

That tube is a breather tube for the engine. Sometimes a small amount of oil can come out of that tube. Most people connect a similar "pod" filter to the carb pod filter, but much smaller in size, to the end of that pipe.

[jarni]

I am sorry... I do not understand where should I connect this pipe to... is it bad that it is hanging opened above the carbs?

[jarni]

Oh... I read your post many times again... and I think I understand now... i should put a very small pod filter on the end of that pipe?

[Steve]

As mentioned, that hose is a "breather" hose. It ventilates the crankcase by removing fumes from the top of the valve cover. In stock form, it is connected to the airbox, where the air flow will draw the fumes through the carbs and burn them. Since you have removed the airbox and the fitting nfor the hose, there is no 'draw' through the system to vent the fumes.

As mentioned, some have merely added a small filter to the end of the hose to trap the oil in the fumes and let the air through. Others have extended the hose down past the back of the engine so it ends in the air stream, where the moving air will draw the fumes through. If you don't do anything, the oily fumes will collect on the bottom of the fuel tank and top of the engine, making it all quite messy.

.