I have a 75 sportster that I do not believe is getting gas. I believe that the carb may need to be replaced. This leads to my question...are bendix and mikuni interchangeale? which is the "better" carb? any and all opinions are welcomed and appreciated.
thanks,
Joshua

 

It is unlikely that the carb needs to be replaced. More likely it just needs a good, proper cleaning.

Note that the following is written for the Keihin butterfly style carbs. The procedure is basically the same for any carb. Details for the Tillotson, Bendix and Keihin carbs are in the Factory Service Manual 99484-78, and in the Factory Parts Catalog 99451-78B.

Cleaning a Carb

1. Removing it from the bike should be straight forward. First thing when it is out is to check the pilot screw setting. Turn it all the way in until gently seated counting the number of 1/4 turns; then write this number down; then reset it.

2. I put mine in a vice to remove the screws, and for much of the following work. Wrap in a shop towel; close the vice gently taking extra care with the choke and throttle linkages. The vice is a needed extra pair of hands.

EDIT: A better choice than a plain vice ...


Better one ...



3. You must be very careful handling the float so as to not change the level. You should have the official specs for setting the level as in the FM, and check it, every time you dismantle the carb, as the last thing before putting it back together.

4. The jets are made of brass, a soft metal that is easily damaged. Use an exact correct size screwdriver. I ground a medium flat blade screwdriver down to exact size on my bench grinder to access the slow jet.

5. The general appearance of the inside of the carb is not necessarily a good indication of its condition. It can look spotless and have clogged jets, or look cruddy and have clear jets.

6. Clean each individual part. Do not allow any solvents to contact any rubber parts [tip of needle, o-ring seal for bowl]

7. Make a list of all of the jets and passages for your carb using the carb manual or the FM for the bike. Then ensure that you can blow either compressed air or carb cleaner thru each one.

8. Remove the pilot screw and clean the parts and the passage. The passage contains in this sequence: pilot screw, spring, washer, o-ring. These are very small parts, especially the washer and o-ring. Usually the spring will easily fall out. One time i thought the spring was not in there because it would not fall out; i obtained a new spring and could not get it in! The technique for removing the washer and o-ring is to use a pipe cleaner: stick it in the hole, twist it around, remove it - you should see the washer and o-ring on the end of the pipe cleaner.

EDIT: the washer and o-ring are part of the pilot screw assembly for 1979 on. They are not in the 1978 and earlier Keihin carbs.

Remember that the purpose of the washer is to protect the o-ring from the spring and you will always get them back in in the correct sequence.

9. Dismantle the accelerator pump assembly noting carefully the sequence and orientation of the parts. Clean and inspect the parts. Replace the diaphragm if it is cracked.

10. Check the float level then carefully put it back together. I personally find it very confusing trying to decide which way to bend the tang if it is not correct. If the fuel level is low is the float high or low? Do i need to bend the tang up or down? On the bench the carb is usually upside down, adding to the confusion. Sort all this out before making an adjustment.

Best to replace the original Phillips screws for the bowl with stainless steel socket head screws.

 

I tried that, but one of the screws is completely stripped & I can not get the float off...any suggestions?

 

I had on-going issues with my stock keihin ('78). I don't believe it is un-fixable but I wanted something "new" that would be less of a variable in my refurb. I went with a mikuni, a kit can be purchased that makes it "interchangeable" with its own adaptors.

I still have the keihin, one of my "someday" projects (aka when I have an indoor workspace that won't be meddled with by cats or children) is to fix it up. I love the mikuni, and even though its new, its the same design from the 70's and represents a period-correct upgrade.

 

It also depends on whether or not you want to keep it stock. The Bendix (also known as the Zenith) carb was stock between 1972 and 1975. Both carbs are good once they get cleaned up and tuned up.

 

1. Take it to any machine shop. They will have the screw out in 5 minutes and, if there are motorcycle guys in the shop, will probably do it for free.

2. Use a needle nose vice grips.

3. There are 2 or 3 types of screw extractors. One type has a reverse thread drill bit. First you drill a very small hole in the very center [difficult to do] of the stripped bolt then apply the reverse thread bit. I am not very experienced with these. Get them at Princess Auto or Harbor Freight.

For experienced guys a stripped screw is a mere annoyance. It is not a problem to get them out.

[Maybe look thru Google earth for a machine shop with a motorcycle parked out front]

 

if your current carb is a bendix, the float is just on a little dowel that slides through the holes. If it is stuck, it is likely just varnish (old dry gas) that is preventing if from coming off).
If you are having difficulty getting it off then it needs a good clean-up because the rest of the carb will be in similar condition and you will likely find a significant improvement in your ride once it is done.
Once it is apart boil it in lemon juice. It is remarkable how clean it will come out. Works better than any clarb cleaner I've tried and easier too.

 

The Mikuni is the best carb there is in my opinion.
It is simple to work on (if you need to ever work on it, which is highly unlikely).

The VM38 Mikuni is going to start your bike with a single kick every time.
Someone say that about a Bendix, S&S, or any other brand.

The only problem with converting over to a Mikuni from another carb are the following:
1) You need a throttle set up.
2) You need an air cleaner set up.

Other than those two considerations it is a no brainer in my view.
pg

 

What do you mean throttle set up and air cleaner set up?

 

Here is a link to my YouTube account (Listed as 'ThePiniongear')
and you can watch some of the videos I have made recently showing what is involved switching to a Mikuni from the original Bendix and it's controls and air cleaner set up.

http://www.youtube.com/user/thepiniongear/featured

The Mikuni has no provision for attaching an air cleaner (easily at any rate) and the throttle is an issue because the Bendix is a butterfly carb and the Mikuni is a round slide type carb.
These require different controls for the throttle.
pg