1st gen MK1 carb detailed info.

[timgray]

I am about to break into the carbs and change my diaphraghms. while I was in there I was going to adjust the needles and do the 1st gen Mk1 version of changing the spacer.

 

I need to know a couple of things. Which way do I adjust the needles? In or out?

Second everyone mentions to adjust he mixture screws to 2.5 turns Where are the mixture screws and 2.5 turns from bottomed out or completely loose?

 

Does anyone have detailed information about the carbs? a full step by step details on adjustment? The factory service manual in the tech section has no information at all on carb adjustment or even needle adjustment. only disassembly and rebuild.

 

Thanks!

[mbrood]

I think it's safe to say that it's far better to do one change at a time and note the changes... both mileage and spark plug condition.

 

If you NEED to change diaphragms, do that first with a good crab sync... then put a few miles on the beast.

 

The needle change that's recommended for the later units is to reduce the thickness of the white spacer below the needle... making the needle reach further into the carb... but this needs to be done after a spark plug check confirms that they are black/sooty... this "deeper" needle setting will cause it to run leaner and the plugs should reflect a more light brown color... but you really want to watch the plugs for the first few runs. I think the 83 actually had multi-notched needles so you can forgo the later mod and look at just moving the clip.

 

The idle mixture screw is located just below the carb diaphragm cover... untouched carbs have a brass sleeve installed to keep you out... but are easily drilled out...

http://bergall.org/temp/venture/1stcarbs3.jpg

 

And the setting is 2 1/4 - 2 1/2 turns OUT from a lightly seated needle.

[timgray]

Ok, so if my brass plugs are in there then I know that the previous owner did not go messing about in there.

 

I am certian the diaphrams need replacing, when I got the bike it ran good but had lots of backfiring. the cars were incredibly out of sync. I mean in a drastic way that the bike should not have been running. A sync made a difference, but it's running poorly synched right. I also am sooting the tailpipes pretty hard as well as getting 29mpg.

 

I'll check the sparkplugs before I do anything. If they are blackish with no traces of white anywhere I'll lean it out a tiny bit.

[skydoc_17]

Hey Tim,

Check out this link, it shows the tear down for the replacement of the Carb. Diaphragms on an First Gen. MKII but it's pretty darn close to yours. Mbrood is right on about ganging up on the mods, Do the Diaphragms first and then after the Carb. Sync and a new set of plugs, if it is still running rich, try adjusting the slider needles. I had to play with mine (a few different shim thicknesses) before I got it right. The biggest improvement I gained was from the replacement of the Diaphragms. With a good Carb. Sync. you will be surprised how much better your bike will run.

Here is the link for the Diaphragm replacement:

 

http://www.venturerider.org/forum/showthread.php?t=32052

 

Here is the link for the "Split the Difference" Carb. Sync.:

 

http://www.venturerider.org/forum/showthread.php?t=35274

 

This should get you well on your way, Any questions, just ask!

Earl

[MiCarl]

Ok, so if my brass plugs are in there then I know that the previous owner did not go messing about in there.

 

 

Not necessarily. Little brass plugs often come in the carb kits.

[Venturous Randy]

Tim, a biggie on carbs being out of sync is hoses or box leaking on the YICS. I took all that off mine and plugged the tubes and my 83 ran a lot better.

RandyA

[timgray]

I checked all the diaphragms all are in fine shape.. well now I have 4 spares waiting. I did not make the change but did sync, it was slightly off, and sync did help a bit. surprising how a tiny bit off can make a difference in throttle response.

 

I cleaned up the plugs (they were blackish) and will run a tankful. If they are blackish again I'll move the needles one notch to the lean side, and then run a tank and check again.

 

I think what may be causing a bit of the problem is that I have been running premium in the tank to make sure all is clean. And premium really does not work well in these bikes. The slower burn time premium has loses power for the timing the engine is set for. I have a 500 mile run planned for in two weeks so I want it running well.

 

Side note: my stock diaphragms look like they are rubber coated cloth. The replacements look like they are just rubber with no cloth inside.

 

also for future refrence, on the 83 is the big plastic screw for the needle in the slider left handed thread or a standard right handed thread? It looks really easy to break.

[Rocket]

also for future refrence, on the 83 is the big plastic screw for the needle in the slider left handed thread or a standard right handed thread? It looks really easy to break.

 

Right hand thread.

[Venturous Randy]

I think what may be causing a bit of the problem is that I have been running premium in the tank to make sure all is clean. And premium really does not work well in these bikes. The slower burn time premium has loses power for the timing the engine is set for. I have a 500 mile run planned for in two weeks so I want it running well.

 

 

I have never understood why people want to run higher octane than what an engine is designed for. Had a friend that had an old Tauraus that he ran premium in. He was convinced the gas was cleaner and his engine ran better. I run reg in my 83 and will ocassionally put midgrade in it if I am pulling my 450lb camper thru the mountains, but I probably do not need to do that.

Running premium will also make an engine run hotter as the slower burn tends to saturate the cylinder walls more with heat.

RandyA

[Marcarl]

Keep in mind that the mixture screws all not all that important,, they only come into play at idle. That's when the primary circuit is being used. Once above 1500 rpm's the primary circuit is no longer in play and so the mixture screw setting has no bearing at that point or above.

[CdnDave77]

Tim... I would set your pilot mixture properly first before doing the needle jet adjustment. I am not sure exactly where in the rpm range on these Mikuni's that the main (needle) jet kicks in, but I believe it is between 3000 & 4000 rpm. I am sure someone more knowledgeable will chime in here with the proper number.

 

But if you do not have your pilot mixture set properly to begin with, your entire rpm range will be affected. The pilot circuits feeds fuel to the air (and therefore your cylinders) throughout the entire rpm range, but your mains (controlled by the needles) only kick in at a certain point and/or when the carb experiences a big increase in vacuum (ie: you open the throttle, the diaphrams sense the pressure difference, the sliders/needles open and allow more fuel to enter the air mixture). So, if your pilots are set too rich, your plugs will not only be sooty, but you'll bog slightly when accelerating because your engine is getting a rich mixture.

 

Your spark plugs should tell you the entire story - a nice tan colour is ideal. If you search the internet on "spark plug chops", you'll get all kinds of information, but basically "chops" help you set your carbs up by looking at the results of running your engine at various rpms and then tweaking your carb settings for the best setting.

 

Just my 2 cents.. but it has worked for my machines.

 

EDIT *** I guess Marcarl and I differ here - I may stand corrected ****

Edited May 5, 2009 by CdnDave77
editorial comment

[timgray]

Ok so where do I set the pilot mixture? and does anyone have a procedure for it? I am assuming that pilot mixture is not the idle mixture and behind the brass plugs.

 

 

as for using the premium gas,

 

One of the gas stations here (Amoco) does have fuel cleaners in it. I ran 2 tanks of 97 octane simply to mix with the 87 that has sat all winter long and is now about 78 or lower octane. mix the two and you get back to what you are supposed to have. I simply wanted the stale gas out of the gas tank and forgot all about it. I agree though, most people know nothing about gasoline or even how engines work so they think more expensive is better. Nope. in fact most cars will run horribly on 108 racing gas because the timing and compression is not set up for it, the piston will be nearly 1/8 the way down the bore before the wave front would hit the piston. losing a lot of power and energy.

 

Anyways, I got 35mpg this last tankful.

 

I really need to unhook the fuel lines and empty the tank, I dont think it has ever been switchedto reserve so ther has to be a lot of gunk down there, and I'm wary of opening that valve and then having to dig out a fuel filter after it clogs up.

Edited May 5, 2009 by timgray

[CdnDave77]

What is referred to as the "idle mixture" in the earlier posts is what I am calling the pilot. My error for not referencing it or sticking with the convention.

 

I am having computer troubles trying to paste the diagram of the carbs from the service manual. It is the same screw in the center bottom of the carb, under those brass caps, if your bike still has them.

 

And if you have an 83 like I do, there is no reserve.

[5bikes]

Spark plug tip color

Black = rich, move needles further out the back of the slide, mpg

Tan, gray or clean = about right, mpg 40+

White = lean, move needle in, mpg high 40's

Do in this order: With clean carbs!!!

1. Check diaphrams

2. Set pilot screws (idle mixture) to 2-2.5 turns

3. Do carb synch

4. Check plugs

5. Adjust needles, may have to do several times...

Got 47 mpg going 65mph, 2 up last week.

Checked plugs, just a little white. Little lean = hotter combustion, more power, less pollution.