Choke Circuit Diagram?

Hi all, further to a couple of threads on here, I think my cold start (ie NO start) problems might be related to the choke circuit.

Carbs have (once again) been stripped/cleaned. Valve clearances are spot-on (re-shimmed recently) and fuel levels appear there or there abouts.

The bike was running two days ago, when I synched the carbs with dial guages. All plugs and plug caps are brand new. I set the idle at a steady 1000rpm with a hot engine.

Now it won't start, and the plugs are bone dry!

Anyone got a diagram of the choke circuit, or can identify the relevant parts/drillings in the carbs?

Any other suggestions appreciated.

Thanls,
Nige

 

Even the "Drawing" in the Factory Manual does not fully illustrate the Circuit with enough detail to "Picture" it.

Beginning in the Fuel Bowl.
There is a separate Reservoir, ... a "Well" ... which fills to the Level of the Gas in the Bowl when the Bowl is full.

Two Metering Ports at the Bottom of the Fuel Bowl ... 1 within the Well ... control the Volume of Fuel and the Time required to Replenish the filled Well.

A Brass "Siphon Tube" ie guided into the Well when the Fuel Bowl is attached to the Carb Body.
The Siphon Tube has several very tiny Metering Holes drilled in it to regulate the Fuel.
One at its Bottom.
Another, ... across the Outside Diameter near the point where the Siphon Tube protrudes from the Carb Body.**
(** Not all tubes have cross drilling)

The Siphon Tube runs straight up the side of the Carb Body, it's open end at the point where the Enrichment Plunger's Seat and Plunger regulate how the Fuel from the Well will be drawn-up into an Opening which allows the Fuel to Enter the Combustion Chamber with the Throttles Closed.

Vacuum draws the Fuel to the Engine.
The Small amounts of Fuel is easily pulled into the Head.
Flooding is regulated by Fuel Bowl Bottom "Jets" replenishing the Enrichment Circuit Supply very slowly.

KEY to the Cold Start Enrichment Circuit working to start a Cold Bike is the
FLOW of Fuel to Fill and replenish the volume in the WELL.

If those Posts are Blocked ... the Cold Start System is disabled.

 

OK, thanks for that.

It looks like those carbs are coming off again!

Nige

 

Some time ago one of the members here did a cross section drawing of the Hitachi fuel bowl. The tiny orifice in the bottom of that well Rick spoke of is clogged often enough that I circled it and keep it handy for threads like this.

I forgot long ago who did the original drawing or I'd give credit.

 

If you can find a #79 drill bit, it is the correct size to clean out the orifice that is shown in the diagram above. It's a small drill bit. so you have to be very carfull with it. After cleaning, a strong flashlight aimed in the bowl, you should see a small pin of light down in the orifice. Chacol sells the correct bit.

Kenneth

 

Clean the Well and its Bowl-connecting Orifice so that when you shoot Carb Cleaner into the opening in the Bowl ... It will SHOOT >> OUT from the Well.
Be careful.
Those Zany Yamaha Engineers have the angle worked-out to give you an Eye-full of Carb Cleaner if you have the Bowl at the right angle.

Don't mess around.
If the Bowl Bottom Metering Ports are clogged ... Drill-out the Clog.

A Guitar String will Clean and Probe the Siphon Tube.

Shrink Wrap that fits over the Siphon Tube snugly ... can be Shrinked on to the END of a Carb Cleaner Tube ... then, fitted-over the Siphon Tube for a Power Flush.

Hold OPEN the Enrichment Valve if they are not removed.

MEASURE the DRILL Depth.
Don't drill through the Bottom of the Bowl.

 

Thanks for all the info guys (and the diagram). I will check these things out.

Nige

 

Right, carbs now off.

Have removed the float bowls - everything looks ok (in the little "well" etc) but I'll try the old guitar string method to make sure there's no gunk in there.

I've removed the cold-start plungers and everything is clear on the top, however, I'll deploy the guitar string up the brass tubes which extend down into the float chamber and then blow carb cleaner through.

After performing a bench-sync, I'll re-fit the carbs and try again.

Nige

 

Note at the top of the brass tubes there is a small hole that needs to be clear. Look right where the tube goes into the carb body.

MN

 

Thanks, will do.

Through a cursory spray of carb cleaner it would appear there is a blockage in one of the "well" passageways in the float bowl, and in two of the brass tubes or further up the passageways.

Will investigate further tomorrow.

 

OK I've made sure all the brass tubes are clear, and the little holes near the top.

I still have one float bowl blockage - I think its the actual pilot jet which lives way down in the tunnel...

Is there any way of removing these, as it doesn't appear to be slotted for a screwdriver - are the a push-fit, if so how do you get 'em out?

Nige

 

Press fit, drill or soak compressed air ultrasonic clean very tiny hole guitar string

 

Hello Nige...at this point all I have to say is get th carbs done right and you're well on th way to having th bike running sweet, take your time and follow th process ..

 

Phew!!!

After trying all sorts of bits of wire in the hidden-away cold-start jet, finally an Ernie Ball 9 gauge string came to the rescue... an 11 is too big.

I don't know what was in there, but I guess its been there all the time, as carb cleaner wouldn't shift it.

I will now double check everything before re-assembly.

I now wish I had kept hold of my two spare sets of carbs, as they would have been a good source of bits.

Thank you all for the shared knowledge and advice.

Nige

 

Everything now thoroughly cleaned and reassembled. Will fit to bike tomorrow.

Regarding butterfly position at cold start setting - should these be fully shut for cold-stating or (as I would think) should they be open a chink - if so, is there a datum opening measurement?

Nige

 

The butterflys are open just a 'chink' of light, that is the idle setting not affected by the 'choke' enrichment.

 

OK+,I'll set them up like that and take it from there.

 

At this point, you're supposed to "bench sync".
I use 2 strips of regular printer paper, which is 0.010 (inch)
This just happens to leave a good idle setting.

I keep tweaking the sync screws until the "drag feel" is the same on all 4.

 

Thanks, will try that.

Nige

 

Right, done that...

Looking through from intake side, just a small chink of light in each one.

Put carbs back on and.... NOTHING!

So:
Carbs thoroughly cleaned (again)
Cold-start passageways and jets all clear
All other jets clear
Floats are right way up
Float levels are good (checked)
Bench-synch done
Sparks are present - all new plugs and caps

LAST WEEK IT RAN PERFECTLY - STEADY IDLE, PERFECT PICK-UP ON THROTTLE etc.

I'm going to look again tomorrow - the only thing I can think of is that last time it was running I had the vacuum gauges and a slave tank on it and I now wonder if the caps on the vacuum take-off stubs are leaking (they've gone pretty hard). I suppose leaks here would be the path of least resistance for the intake airflow and therefore stop fuel being drawn up from the carb bowls.

As I really can't think of anything else that has changed since it last ran I am going to go with this as the potential cause and shall try and find something to give a good tight seal on the stubs and see if it solves this (now extremely frustrating) problem.

Nige