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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #1  
Old 29th September 2011
bhakakha bhakakha is offline
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Default starting with mikuni carb

I have been having a hard time kick starting my 72 XLH with 38 round slide mikuni carb, and am now more confused after reading some threads. The times when I have been able to start I have the enricher lever UP (so the plunger is down), and after the bike starts I push the level end down (so the plunger is pulled up). The bike then runs fine. Am I doing this backwards? The one time I did get it started with the lever down (and plunger up) it was fine until I pulled the lever up (plunger down) and the bike stopped immediately. I have looked at Piniongear's posts (thanks), and am just unclear. In the following picture to me the lever is down. Thanks for the help,
Ben

[/IMG]
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Old 29th September 2011
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flathead45 flathead45 is offline
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down is choked... i'll call it "choked" as its easier to comprehend.

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  #3  
Old 29th September 2011
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Quote:
Originally Posted by bhakakha View Post
I have been having a hard time kick starting my 72 XLH with 38 round slide mikuni carb, and am now more confused after reading some threads. The times when I have been able to start I have the enricher lever UP (so the plunger is down), and after the bike starts I push the level end down (so the plunger is pulled up). The bike then runs fine. Am I doing this backwards? The one time I did get it started with the lever down (and plunger up) it was fine until I pulled the lever up (plunger down) and the bike stopped immediately. I have looked at Piniongear's posts (thanks), and am just unclear. In the following picture to me the lever is down. Thanks for the help,
Ben

[/IMG]
The pic is postage stamp size and I cannot enlarge it so I cannot see the details of the lever.
But, applying the starter system to start the engine is done by pressing the lever down.
This raises the plunger, which opens up the starter jets and air passage.
After the engine starts you raise the lever.
This motion sends the plunger down where it blocks off the starter jet and air passage.

So, it sounds like yours is acting backward. Not right, although I have no idea why this is happening.

Let me ask you this...... When you try to start the engine with the lever down (plunger up) are you giving the engine any throttle when you spin the starter?
You are supposed to keep the throttle slide completely closed to maintain the strong vacuum necessary to activate the starting system.

Other than that all I can figure out is the carb adjustment must be way off....... but that said, I cannot imagine how that could be.
As I recall you have all the correct parts in the VM38 and this carb should start the bike with a simple 2 kicks at most or a short spin of the electric starter if yours is so equipped.
pg
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  #4  
Old 29th September 2011
bhakakha bhakakha is offline
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Point ignition. This bike sat for many years before I bought it. I spoke with someone at Mikuni and he thought maybe the pilot jet had become clogged and this would explain why this was happening. Does that make any sense? I have never worked on motorcycles before this one, but I am planning on cleaning out the carb and putting a new pilot jet in. As far as starting, I have been giving a couple twists of throttle to put gas in the bowl, giving a couple priming kicks, turning on ignition, then kicking without touching throttle. Maybe I should not give the twists before starting?
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Old 29th September 2011
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ditto on pinions answer....

unless your sold on the looks of the mikuni (i was) junk it for a cv carb. takeoffs can be found at swapmeets for 50 bucks. it'll be the single mostest bestest thing you do to your engine

I did, and i enjoy my 72 way more now that it starts, idles, and pulls like a freakin freight train
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Old 29th September 2011
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meanmechanic meanmechanic is offline
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The mikuni will be just fine. Clean it good, clean or replace the jets if ness., and youll be fine. I just hate to see this turn into another "this carb is better than that carb" deal. I dont run a mikuni or a cv. PG knows best when it comes to round slide miks.
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Old 29th September 2011
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Quote:
Originally Posted by bhakakha View Post
Point ignition. This bike sat for many years before I bought it. I spoke with someone at Mikuni and he thought maybe the pilot jet had become clogged and this would explain why this was happening. Does that make any sense? I have never worked on motorcycles before this one, but I am planning on cleaning out the carb and putting a new pilot jet in. As far as starting, I have been giving a couple twists of throttle to put gas in the bowl, giving a couple priming kicks, turning on ignition, then kicking without touching throttle. Maybe I should not give the twists before starting?
First of all, let me say that giving the throttle a couple of twists is not doing anything because a Mikuni does not have an accelerator pump.
Fuel is in the bowl because you have turned on the petcock and gravity carries the fuel into the bowl.
The floats rise to the proper fuel level and this closes the needle into it's seat and fuel flow stops....... until running the engine eats up some fuel, the floats drops and the needle comes unseated letting fuel flow back into the bowl. Up and down goes the float as the engine is running.

When you start the engine I advise you to not do any priming kicks at all. A Mikuni does not need priming kicks. All you are going to do is flood the engine.

Just get on the bike......
Turn on the petcock if it is closed.....
Turn on the key.....
If the engine is cold.....
Push down on the starter system lever...... give it a kick with NO THROTTLE (which you say you do....Good!)
It should start the engine within 2 kicks.


Since the carb seems to be 'backward' to the way it should be........... Yes, as others suggest do a good cleaning of the carb.
Let it soak overnight in carb cleaner.
DO NOT fool with, or try to change the float level. That will open a can of worms best left alone.

Install a new pilot jet if you wish.
The correct pilot jet is a #30.

Here is an explanation of how the pilot system works and it is an important part of starting the engine.......

The pilot system works in the throttle range of zero to 1/4 throttle opening.
At idle there is not enough of a strong vacuum to pull fuel from the needle
jet.
So the fuel/air delivery during low speed operation is done by the
'bypass' and the 'pilot outlet' .
These are two holes (passages) located respectively on the downstream side of the throttle slide (or the side closest to the engine) in the carb's throat. Look at the carb throat and you will see these holes.

Connected to these two holes are the pilot jet (pilot outlet) and the bypass, also connected to the pilot jet passage.

When the engine is idling at very low throttle the fuel metered with the pilot jet is mixed with air coming from the air adjustment screw and the mixture is mixed into fine particles. That is where the air screw fits into the picture.

As the throttle slide is opened further other stages of the carb operation begin to function.

So clean the carb and then try starting it as I described above and let's see if that does not get it going.
pg
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  #8  
Old 29th September 2011
bhakakha bhakakha is offline
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PG,
Thanks. I will try to start like you have outlined. If it doesn't work I will take apart the carb and clean it. I thought I had read about the priming kicks on this site. Maybe it does not apply to the Mikuni carbs. I can't thank the members of this site enough for all the info I have received. I hope to one day gain enough knowledge to help the newcomers as they come in.
Ben
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  #9  
Old 29th September 2011
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piniongear piniongear is offline
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OK.... good luck on the starting try. Let us know the results.
You say the bike sat for many years without being run.
If the previous owner just shut it off one day and did not run the fuel out of the carb that may be reason enough to do a good carb cleaning.
As I have said many times before and I will again..... A Mikuni is a very good trouble free carb.
pg
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  #10  
Old 29th September 2011
RRB RRB is offline
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This is just a wild guess, but could you have a big intake manifold air leak? Spraying propane around the manifold with the engine just above idle might give you an indication of this, if the rpms increase.

Have you checked your plugs for indication of burn mixture? Starving for gas and indications of lean mixture sometimes is indicative of a manifold leak.

Again, just wild guesses on my part. I'd go with whatever Piniongear recommends. He's the expert on Mikuni 38mm carbs. The Miks may not be the ultimate in high performance carbs, but they sure are dependable, easy to tune, and otherwise trouble free. And with all the voodoo it sometimes seems to take to keep an ironhead running consistently well, it is nice to know that you have a simple and dependable carberator on the bike. Good luck, and let us know what ends up working for you.
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