Can't colortune a plug?

[CatatonicBug]

A local guy I recently met has a '78 with what appear to be newer-model carbs in it (see pics below). He asked me to help him tune the carbs, and after sync'ing them (very little vacuum across the board), I tried a colortune. No matter what position the mixture screws are in, the color stays bright orange! I can't get blue no matter what I do. I only tried one plug, so I don't know what the rest look like, but what could cause the mixture screw to not change the mixture?

I took the screw out, and the tip looks fine, and seems to seat fine when I screw it all the way down. Could it be a broken tip from a previous screw (blame it on the PO) stuck on the hole? Am I correct in the belief that the tips on the later model carbs generally don't have that problem?

[DGXSER]

Its all in the jets

IF the mixture screw can not lean the mixture out, then it must be that the jets are to big for what the engine wants. I'd open those carbs up and see what jets are in it.

When I got Banshee, the PO told me someone else had rejetted it for him to make it "more powerful". It was about the most fubarred jetting I have seen.

[motoman]

First all......those aren't the late model(81) carbs. They have obviously been off as the clamps are all array. If those look on the inside like they look on the outside........better off starting with a removal and tripple clean.......least you could straighten out the clamp mess.

[CatatonicBug]

First all......those aren't the late model(81) carbs. They have obviously been off as the clamps are all array. If those look on the inside like they look on the outside........better off starting with a removal and tripple clean.......least you could straighten out the clamp mess.

I agree with the clamps. I was half tempted to take care of that just fo the sake of the clamps, but at least they are in there, doing their jobs. The current owner said he has had the carbs out and "cleaned them". I also agree that there is a good chance that he didn't do a very good job. The thing that points me to the fact that they are the later-model carbs ('80-'81) is that the mixture screws are the same as my MNS's, there is no vent line from the float bowls, and the idle adj. screw is set in the lower front/center like mine (instead of the upper rear/center like the older style). The float bowls, however, look like the earlier style to me...

I guess my main question is, could there be broke-off tips stuck in the mixture screw holes? Could it also be that they were not cleaned sufficiently to allow the mixture screws to do their jobs?

[XS1100 Newbie]

That's the way my 80 MNS carbs are as well.

[DGXSER]

Hey Bug, those are the 80 model carbs from what I see, if the bowls are stock. Mikuni did not change the drain style until the 81 model year to the side screw bottom drain config.

It is very very uncommon to have the needle tip break off in the newer style carbs like those and yours. They changed the tip design of the screw so it is a much looser fit in the hole than the older style screws. So, is it possible, well anything is possible, but not very probable.

While the clamps are not lined up per say, from what I see they are tight and not a vacuum leak.

As to the cleanliness being your issue, well, could be yes. Although typically, dirty carbs seem to run lean more than rich from what I have seen. And what you describe is a rich running condition. A good cleaning is never a bad idea. But again, if adjusting the mixture screw makes no change, then it does tend to indicate the jets are mis-sized.

[WMarshy]

Anything is possible...

Sounds like your goin to have to take those babies off. First thing I would do is verify the carb bodies are the newer or later style and then make sure the bowls are for the right style, they are not interchangable! Then move onto the jetting. The main jet needle will be an obvious sign which era/style the carbs are (unless someone put a dyno jet kit on the new style). Im betting the the jet sizes are messed up. GL

[CatatonicBug]

Thanks guys. Now I wish I had the bike at my house so I could take them apart tonight and look at them. Unfortunately, it's 15 miles away. Maybe I can get the owner to remove the carbs and I can go and get them from him to do a "tripple clean" on them and check the jets.

[Bullet]

Mikuni did not change the drain style until the 81 model year to the side screw bottom drain config.

80's have them too ..

3U9 - Canada 80 SG - I got 2 sets with side drain screws and liquid level tube spout ..

If your idle screws are recessed, they are later style with old style bowls .. (early 80 models) ..

[DGXSER]

3U9 - Canada 80 SG - I got 2 sets with side drain screws and liquid level tube spout ..

Learn something new every day. Might be a canadian thing or something or a late model year change. That is the first I have seen or heard of the 80 year with those drains.