More carb questions

[Bigfoot]

I found out recently that the engine on my 80 MNS is actually a 79. 3H3 vin number. This got me to thinking about the tuning of the carbs. The carbs are the later model (idle screw in the front bottom of the carbs.)

I have seen that there are differences in the two motors such as heads and piston crowns (I think). The carbs also had different jetting. So here are my questions.

1. Has anyone set up 80 carbs on a 79 motor?
2. Should I jet the carbs and set the float heights for a 79 or 80?
3. Should I just get a set of 79 carbs and pitch these?

[tarzan]

The carbs should be jetted according to the year of carbs not the year of the bike.
What carb problems are you having?
It doesnt matter what year carbs you have ,they should work fine on any year xs.

[Bigfoot]

Having a real hard time starting when cold. The problem is even worse if I try and use the "choke". Once started for the day it usually fires right up. It almost seems like it is flooded since enriching the mixture makes it worse. But once it is running it seems to be running a little on the lean side (slight popping on decel) and the looks of the plugs verifies it. I have cleaned the carbs several times and all passages seem ok. I have also synced the carbs.

[TopCatGr58]

Hey Bigfoot,

I just posted about the float bowl pressed in jet. Did you check to make sure that carb cleaner would spray thru that jet, and back up/out the tunnel in the float bowl, or vice versa, spray into the tunnel, and see the spray come out the jet on the inside of the bowl??

Secondly, some folks tend to want to turn the throttle when trying to start the bike, and that will actually make things worse. Our carbs do NOT have any kind of fuel pump, and opening up the throttle plates actually reduces the vacuum that allows the carbs to suck the fuel up thru the enrichener circuit to get into the engine to start.

But, what are your carbs jetted at, mains and pilots? Also, if they are the 80 carbs, are the brass caps OFF of the front top recesses of the pilot/idle SCREWS??? If so, how many turns out are your pilot screws set at?

If you have aftermarket pipes, it's common for it to pop some on decel.
T.C.

[Ivan]

I don't think his trouble is being rich, since I have seen him crack the airbox and give it a sniff of ether, which wakes it up nicely.

I have noticed when riding behind him he gets a bit of part throttle miss too. Pretty random, and not regular like a spark not firing.

From what I recall, the "choke" draws fuel directly from the bowl and mixes it with air drawn from under the slide diaphragm. I am guessing that, for some reason or another, he is not drawing fuel, and when the choke lever is pulled, he is only allowing more air through the carb, thus aggravating the situation.

That's all I have come up with. It did give me something to think about while riding home Sunday though.

[Bigfoot]

Yep, cleaner through all of the orifices including the tunnel in the float bowl. Got a facefull until I learned to not look while spraying. lol.

Throttle or no throttle doesn't seem to make a difference. You can hear it try and start for about 2 seconds and then just crank, crank, crank. Eventually it will start. Pretty hard on the battery and starter though. If I give it a little shot of starting fluid in the airbox she fires right up. Once started for the day, it is usually good until it sits overnight again.

Not sure what the carbs are jetted at. Assuming stock jets for an 80.

Brass caps are off and idle screws set at 1 1/4 turns out.

Stock pipes and airbox.

[Bigfoot]

Thanks Ivan, I didn't realize it had the part throttle miss. It seems to pull pretty strong when I twist the throttle. I have been told that I get a puff of black smoke when I get on the throttle as well but clears up quickly.

[Ivan]

Yeah, you get a slight puff of smoke. Nothing I would worry about though.

[John]

Harry

Just for s#its and grins, look at the gaskets for the float bowl. I had a cold starting problem with the Bag Lady for quite a while. I had pulled the carbs twice already checked everything. I knew things were clean, and all settings were proper, it ran great once warmed up. When it was cold it would just crank and crank, no hint of it firing. After I had the carbs out for the third time, I was checking to make sure the choke pick up tube and orifice in the bowl were clear when I just happened to notice that the air passage to the choke, in the carb body, from the bowl was blocked by the gasket. This passage is incorporated into the edge of the float bowl, on the side where the choke tube protrudes into the bowl. I had some old gaskets laying around. so I compared the two and that is what I found. This passage is the groove in the edge of the bowl where the choke tube sits. Once I punched the holes in the gaskets, it lit right off after two revs of the engine, and ran beautifully on the choke. This bike has a cable actuated choke, and it ran fine at all choke settings.
Just a thought, a long one perhaps, but just a thought none the less.

[Bigfoot]

Thanks John, I will check that. The symptoms sound identical to my problem. I need to check my float height anyway. I was looking through my manual last night and noticed that the height settings are different from the 79 and 80. I did not notice the additional section in the back of the clymer manual so I think it may be set wrong.

[John]

The 80 carbs have a float setting of 23mm vs the 25mm for the 78-79. I would set them up as 80 carbs. That's what they were intended to be. My carbs are 78's so everything was set to those specs. Then I had to rejet the mains to 140's and increase the pilot screws to 2 turns, because of the aftermarket air filter, Sketty pipes and big bore.

[Bigfoot]

Thank you John for the tip about the gaskets, that is what the problem was.

And thanks to Russ79 for his assistance, use of his sync gauges, lift, and most importantly BEER. She ran great on the way home. I will see how it starts in the morning cold but I am confident that it is all good now.