gundj
New Member
Posts: 8 
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Post by gundj on Aug 27, 2023 9:19:39 GMT -5
Dear all, I´m sure this question has an answer somewhere, but I did not look enough maybe. I would like to replace the carburator on my 1991 SR500, German model from the beginning. The reason is that it has been fettled with at some stage and I do not get a nice running engine as it is now. I could just rebuild the one I have and hope for the best but I would like a Mikuni or similar just for the looks and maybe be able to get rid of the airbox in the future. The bike now has a shorter more open silencer and 167,5 ( think) main jet. It runs a bit fat and hesitates and so on acceleration. Smaller jet makes it worse and larger run just fat. If a new unit also would provide an hp or so more would be nice.
Suggestions?
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Post by gcrank1 on Aug 27, 2023 12:01:06 GMT -5
Fwiw, I am most certainly not a guru, but Ive been doing my own wrenching for over 50yrs and had over 30 bikes, also worked in a couple shops for a while. All that to say that Ive sorted a few out.
The factories have lots of resources for development and the oem air box is well designed for all round drivability, dropping it to use a pod often results in a loss of hp in the ranges (low and mid) where we spend most of our time. Putting a high flow air filter in the box is a good thing though.
The shorter, more open silencer/muffler has probably shifted the power higher in the rpm range, a long overall pipe length tends to keep it lower in the powerband. I like a longer pipe and can.
The jetting needs more than just a change of the main; the most useful for street use is the needle taper and/or needle jet.
Does your existing carb have an adjustable (maybe capped off) low speed mix screw? If it is capped off the cap needs to be removed the next time the carb is off.
The float height needs to be properly set too.
You Need to Know what jets/etc are inside, low, needle, needle jet and main to be able to sort it, as well as know just what model and size the carb is.
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Post by stevep on Aug 27, 2023 13:56:57 GMT -5
The factory carb on a 1991 is likely Mikuni BST34. Kedo make a great rebuild kit for this carb BUT they supply a #60 pilot air jet with the kit and you really need an #80. With luck your carb already has an #80 so keep hold of it for re-use. In kedo catalogue it's part number 29337. You may need other parts too!
Edit:That would give you the standard carb spec baseline. From there you could experiment. Can you post a pic of your carb? Or look up BST34 Mikuni to check if that is the carb you have and confirm here. The main jet is a #165.
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gundj
New Member
Posts: 8
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Post by gundj on Sept 5, 2023 10:32:29 GMT -5
Fwiw, I am most certainly not a guru, but Ive been doing my own wrenching for over 50yrs and had over 30 bikes, also worked in a couple shops for a while. All that to say that Ive sorted a few out. The factories have lots of resources for development and the oem air box is well designed for all round drivability, dropping it to use a pod often results in a loss of hp in the ranges (low and mid) where we spend most of our time. Putting a high flow air filter in the box is a good thing though. The shorter, more open silencer/muffler has probably shifted the power higher in the rpm range, a long overall pipe length tends to keep it lower in the powerband. I like a longer pipe and can. The jetting needs more than just a change of the main; the most useful for street use is the needle taper and/or needle jet. Does your existing carb have an adjustable (maybe capped off) low speed mix screw? If it is capped off the cap needs to be removed the next time the carb is off. The float height needs to be properly set too. You Need to Know what jets/etc are inside, low, needle, needle jet and main to be able to sort it, as well as know just what model and size the carb is. I´ll put some pictures of the little bugger here, maybe this can help to identify it. Attachments:
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Post by stevep on Sept 5, 2023 14:40:02 GMT -5
Yes, that's a BST34 Mikuni. There isn't the range of jets for the BST34 to go playing around with. It's very much a factory carb designed for standard airbox and exhaust with a narrow range of jet options. Then it gives very good fuel economy and not so much in the way of sporty performance. Put another way, it's not the sort of carb most folk would continue to use if they were going for performance upgrades elsewhere on the bike. So if you want to get rid of the airbox and you already have a shorty exhaust silencer, you're probably also looking to move to a different type of carb. Probably a Mikuni roundslide or flatslide. I'm not necessarily recommending Vintage Spoke, but they have a page of SR carb options here: www.thevintagespoke.com/shop/SR500/Carburetor.htm If you do keep the BST34: Worth checking condition of the main diaphragm (it's a CV carb,) and that little cover on the side held by three screws is a coast enricher, also hiding a little diaphragm. Stops fuel starvation when throttled off, say down a long hill. Probably the most overlooked part of the carb from the point of view of maintenance/replacement. There are kits by Tour Max and others for these parts or Kedo do them. Not expensive. Where are you based? That will help us to help you with shopping outlet suggestions. |
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gundj
New Member
Posts: 8
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Post by gundj on Sept 6, 2023 10:18:25 GMT -5
Thanks a lot!
I´m in Sweden, but do shop from Kedo now and then.
eBay.de is also one of my evening distractions/attractions.
Buying from the UK is not so fun anymore, post Brexit.
I´m not really after max performance just a bit nicer bike, therefore the exhaust had to go.
Removing the air filter box is not in my plans unless I find a carb that works well with filter pods.
I will take the carb apart again and go through it once more and get me a new diaphragm anyway.
I´m also a bit concerned if the choke can get stuck and enrich the fueling.
Thanks for the note on the coast enricher, I´ll give it a look, or just simply replace anyway.
I´ll start looking for another carb, preferably a used one to save some money, I have spent far too much on my other bikes and car repairs that was not planned for.
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Post by gcrank1 on Sept 6, 2023 11:33:05 GMT -5
Something often overlooked is the vac line to the petcock.
If you pull it off the petcock and ANY gas dribbles out you have a holed diaphragm in the petcock letting gas through and enrichening the mix; maddening to find on my '78.
The bike ran great accelerating but balky at steady throttle (high vac operation).
Btw, any diaphragm inspection should be with a light behind and pref with a magnifier.
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gundj
New Member
Posts: 8
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Post by gundj on Dec 7, 2023 15:24:31 GMT -5
Some feedback.
I got me a Mikuni TM36, K&N filter, bespoke bracket and a filter for the ventilation from crank housing.
It will be installed soon and I hope for a nice start up to warm me up in the darknes and cold here.
Just need to find the time to do the work. There are other projects around and christmas cleaning of the house seems to be important if you ask my wife.
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Post by stevep on Dec 8, 2023 15:47:49 GMT -5
Ha ha, I hear you. Christmas takes up a lot of time and effort that could be used for worthwhile projects. Let us know how it goes when you find the time to fit your new parts.
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gundj
New Member
Posts: 8
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Post by gundj on Jun 15, 2024 7:16:39 GMT -5
Dear all,
I forgot about this thread, sorry.
My bike now has a TM36 flatslide installed including a large K&N airfilter.
It runs really well with a nice sound out of the slightly shorter and more open silencer I put on.
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