Board logo

Mikuni Carb Issues
phelpsa - 9/6/10 at 09:22 AM

My dad stripped down the carbs (mikuni BST40SS) on the gixxer engine, cleaned them up, unblocked all the jets etc, and had it running beautifully on all 4 cylinders. Started it up again last night and once again the float on carb 2 isnt cutting off the fuel supply, so the fuel is pouring back out of the air jet on the front of the carb and out of the vent at the top.

We've had a fiddle with the float heights and as far as we can see they're all set the same, and all the seals seem fine. Any other ideas?

coozer - 9/6/10 at 09:26 AM

What did you clean the carbs with? Not thinners I hope!

matt_gsxr - 9/6/10 at 09:28 AM

Adam,

You have my sympathy.

I had similar problems on and off with the equivalent carbs on my gsxr1100wp. My solution was to clean them again, and again. I assume you have a filter before them, as any crap in the lines might cause problems.

It may be that the GSXR1100 carbs being gravity fed don't like the slightly higher pressure that you get from a motorcycle fuel pump. Or the different angle of the engine (and hence carbs).


You can get carb refurb kits that include all new seals and such.

Matt

phelpsa - 9/6/10 at 09:32 AM

quote:
Originally posted by coozer
What did you clean the carbs with? Not thinners I hope!


I found this product specifically for cleaning carbs. I think it was called something obvious, maybe Carb Cleaner?

coozer - 9/6/10 at 09:36 AM

quote:
Originally posted by phelpsa
quote:
Originally posted by coozer
What did you clean the carbs with? Not thinners I hope!


I found this product specifically for cleaning carbs. I think it was called something obvious, maybe Carb Cleaner?


Ok, so your needle jet valve seats maybe worn.. only asking cause thinners caused me a very similar problem that ended up with the carbs going in the bin

phelpsa - 9/6/10 at 09:39 AM

quote:
Originally posted by matt_gsxr
Adam,

You have my sympathy.

I had similar problems on and off with the equivalent carbs on my gsxr1100wp. My solution was to clean them again, and again. I assume you have a filter before them, as any crap in the lines might cause problems.

It may be that the GSXR1100 carbs being gravity fed don't like the slightly higher pressure that you get from a motorcycle fuel pump. Or the different angle of the engine (and hence carbs).


You can get carb refurb kits that include all new seals and such.

Matt


Matt, I cleaned out your old filter and am using that at the moment. I have been wondering if it's just that the fuel pump is too powerful, it's the more powerful of the ones you gave me that im using. Carb refurb kits for these appear to be very difficult to get hold of as they were fitted to one model of bike for one year! The float bowls are as flat as possible, I don't think that's the issue although its worth investigation.

It's supposed to be going to Big CC soon to be setup properly so we might just leave it to them to investigate, but it would be nice to have it running properly before it goes.

phelpsa - 9/6/10 at 09:41 AM

quote:
Originally posted by coozer

Ok, so your needle jet valve seats maybe worn.. only asking cause thinners caused me a very similar problem that ended up with the carbs going in the bin


I would have thought they would leak a little bit all the time if that were the case. As it is they were fine for a while, then suddenly one started pouring fuel out of it, as if the valve is getting nowhere near the seat.

matt_gsxr - 9/6/10 at 09:47 AM

Adam,

Agreed one pump does seem to be a little fruitier than the other.

Get your dad to rebuild it again!

Just because the carbs are a bit different, it may be that some of these seals are common.

Good luck,

Matt

phelpsa - 9/6/10 at 10:04 AM

quote:
Originally posted by matt_gsxr
Adam,

Agreed one pump does seem to be a little fruitier than the other.

Get your dad to rebuild it again!

Just because the carbs are a bit different, it may be that some of these seals are common.

Good luck,

Matt


Got him on the case as we speak. Taking intricate things apart and cleaning them is his favourite hobby...!

bbwales - 9/6/10 at 10:50 AM

I have had the same problem off and on with my Bandit 1200 carbs, I have found that by taking the diaphragm cap off and putting it back on has cured the problem, I don't know why or how it does it but it has.

Rergards

Bob

phelpsa - 9/6/10 at 03:03 PM

Took them apart, put them back together, all fine. Had some lunch, came out to start it, fuel pump on, fuel leaking out of the air jets again! ARGHHH! What is going on?

deltron63 - 9/6/10 at 05:58 PM

sounds like the fuel pressure is to high.
are you using the proper bike pump for the carbs. from the same model

phelpsa - 9/6/10 at 06:02 PM

quote:
Originally posted by deltron63
sounds like the fuel pressure is to high.
are you using the proper bike pump for the carbs. from the same model


The bike was gravity fed. The pump is from a ZX9R (? matt would know better than me!).

Have now booked it in at Big CC for next week and they're gonna have a look at it and fit a fuel pressure regulator if need be.

tony-devon - 10/6/10 at 08:43 AM

have already mentioned my work around on another post, but my method is to run the pump to a nice smart tank under the bonnet with the feed to the carbs at the right height, with a return to the tank from near the top of the underbonnet tank

this way its pumped from rear tank, but the carbs are getting fed under normal gravity conditions, not force fed

also a swirl pot

phelpsa - 10/6/10 at 08:48 AM

quote:
Originally posted by tony-devon
have already mentioned my work around on another post, but my method is to run the pump to a nice smart tank under the bonnet with the feed to the carbs at the right height, with a return to the tank from near the top of the underbonnet tank

this way its pumped from rear tank, but the carbs are getting fed under normal gravity conditions, not force fed

also a swirl pot


That's a very good idea and had I thought about it before I would have run a return down the tunnel as well.

tony-devon - 10/6/10 at 09:11 AM

yes that is the downside

but I kept the pot quite small, with heatsinks around the outside and an airduct pointed at it, kept it nice and cool