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Appeal to the carb gurus for insight.

  • Thread starter Thread starter spyug
  • Start date Start date
S

spyug

Guest
Strange situation going on at the moment with the CB650 project.

I've been trying to fire it up and after 3 dunkings and pokings, I think I have the Keihin PD 50 mechanical slide carbs as clean as I can get them. However, she still won't run off choke or come down to idle. She won't idle because cylinders 3 and 4 are not firing or perhaps firing only intermittently .

I have confirmed there is gas in those carbs as blowing into the vent tube caused them to **** into the airbox. On blowing in to the vent tubes the bike will also fire right up without choke and will run until the gas is burned up.

The carbs are fitted with stock jets, cleaned with proper carb dip and have new O-rings. The airbox is stock and tight and air mixture screws are turned out 2 full revolutions.

I have confirmed there is proper spark at each "dead" cylinder and the plugs are bone dry.

I think I have experienced something like this before with my Kat project but I can not recall what the issue was.

So I'm wondering if any of the "experts" can point me toward a solution. I have plumbed both the CB650 and CB750 forums but don't find anything similar.

As always, any and all thoughts are welcomed and appreciated.

Cheers folks!
 
Best guess is the carb synch is so far out of balance that two of the carbs are basically shut off.
 
Don't know anything about the Kehin carbs, but are those "air screws" like on the Mikuni VM carbs or "mixture screws" like on the Mikuni BS carbs?

If they are truly "air" screws, the number of turns will depend on how far the FUEL screws are turned. Not sure if the same 2:1 ratio holds for the Kehin as it does for the Mikuni.

However, if they are "MIXTURE" screws, 2 turns might not be enough. Try 3 turns. Earl also has a valid point about carb sync. It will be a bit difficult to get any kind of decent vacuum reading until the engine is running, but you might get a decent comparison at cranking speed.

And, I hate to say it, but somebody has to ... :-\\\ in spite of all your careful dipping and poking, you might have missed something.

.
 
What year CB is this?? I had a 76 750 Hondamatic that acted similarly. Guys on the SOHC forum suggested the passage between the front air jet in the throat and the needle jet was still clogged...even after a full 24 hrs in Berrymans.

So my remedy was to dump straight MEK into the hole and repeat that a few times to try and eat the lodged scale out. Also a fine wire digging around to further dislodge crud. This seemed to clean the passages well and it ran better and wasnt as rich. Dont know if this will be of any help but thats my Honda Kehein story.
 
Thanks guys. I appreciate it. I can't believe the amount of crap I already got out of it but I guess there still could be more. These carbs are a right bastard to get off and on and a pain to take apart but I guess I need to have another go.

As far as the synch, as Steve mentions, it would be difficult to get an accurate reading without all things firing but I suppose I can try. I'll let you know what transpires.

cheers.
 
Be well certain the air passages are CLEAN and the vents are open also. The air passage doesnt take much on these carbs to be causing problems
 
Provided you've done a visible synch on the carbs - all reaching top of bore at the same time - that won't be a factor.
If it runs on four when you've flooded it by blowing in the vents, i'd raise the float level until it runs. Sounds very like starvation due to too low a level.
You can do your test runs without the airbox or connecting hoses, once you're happy it will run, put them back on and do the final adjustments.
 
Try this. With the float bowls off, place the rack upside down on a level surface, then lightly spray ( by very lightly depressing the button) aerosol carb medic onto the top of each pilot jet. The fluid on the top of the jet will be slightly dome shaped for a second then should slowly seep down into the jet if it's clear to the carb throat. Try it on one of the good carbs first and you'll see what I mean. Note that the carb spray won't unclog the jet, it only verifies whether the passage is clear or not. If the pilots turn out to be clear, possibly a carb to head vacuum leak? It might be worthwhile to check the O rings between the manifold and the head.
Good luck.:)
 
Thanks guys.

I think I am finally getting it. I did pull off the carbs again this morning and opened up the offending 3 and 4. I pulled the slow jets again and sure enough I find the passage on #4 fully plugged and that on 3, partially. I shot some acetone into each and after a minute or so was able to easily blow through each. I think that has been part of my problem. I have the idle jets sitting in acetone right now to see if they come cleaner too.

It could also be that the airmixture screws are too open at 2 turns so I will tighten them up to 1.5 turns to see how that might work.

Hopefully this will be it.

I will let you know.

Cheers and thanks for the interest and encouragement.
 
On my 750 Hondamatic the 1 out was too lean. finally was happy at 2 out. of course your bike is its own animal and youre gonna be wherever it ends up. Pluck a wire off an old wood handled wire brush and poke the jets out good..and go in the tip as well to be sure that pinhole is also well opened.
 
Thanks Chuck,
I am finally happy that the passages are clear including the air intake tubes. The idle jets are clean but I'm a little troubled that I can't get a small enough wire to push all the way through. I think (hope) I am hitting a restriction about 3/4 of the way down. It is so small an opening I can't blow through it but I can seem to suck air through a tiny bit.

As far as air mixtures I am trying 1.5 turns this time and I'll see where that gets me. I also re-synched the 1 and 3 slides which were off about 1/16".

I am just about to hang them on again so in another hour or so I'll know (they are a bastard to get on I tell ya.)

Cheers.
 
Bread tie wires work well too. Youll notice a small hole where the emulsion tube goes into the body..about half way down the hole. Bend a wire and poke the hole out from the emulsion tube side as well. point is to poke it as best you can.
 
Well it sort of worked but a new problem has arisen.

I am firing on 4 and it will idle, but presently around 1500. The problem is that I now have gas peeing out of every orifice in carbs 1 and 2. Its almost like Niagara Falls:mad:

I did not fiddle with the floats at all so I don't know now why this is happening. I am running my gas in from an I/V bottle with no filter in place but I hardly think that has anything to do with it as worked last week with absolutely no leakage.

man, these things are kicking my butt.

I have given up for today in sheer frustration. Maybe will try again tomorrow
 
Hey thanks Chuck. Moral support is always welcome.

So I did try again. Rapped on the bowls and that sorted #2 but #1 still peeing and now #3 is as well.

I did come across something interesting on Youtube that I will try tomorrow. They advocate polishing the float valve seats to get a better seal. Basically, use a q-tip chucked up to a drill, add a dollop of metal polish and run it in the seat for a minute or so. apparently it will take care of any varnish or crud on the seat face. It can't hurt.

I also will check float heights. I am sure they are ok but I might as well be really thorough at this point. I'm wondering too if I should bring the airmixture screws back to 2 full turns out.

Well hang in there my friend. Hopefully I will soon be able to report success.

Cheers man.
 
recheck the gaskets arent rubbing against the floats while youre back in there. In fact, trim them back to the inner edge of the carb body regardless of if you think they are or not..eliminate that outright by trimming them.

1 and 2 were pizzzing fuel and 3 wasnt. Then you tap on the bowls..1 stops..2 continues..and 3 joins in. Damned if that doesnt sound like gaskets or floats hanging up to me. I mean think about it a second. WHY would 1 stop with a tap ( because the float got freed up ) and 3 start ( because the running bike used up fuel from the bowl and the float snagged going back up when trying to refill the bowl ). Does that make any sense to you??
 
Yes Chuck it does and great minds think alike as I did just that this morning. These carbs actually have the rubber o-ring gasket but mine were shot and I haven't had time to get the real ones (local parts guy for these is 1 hour away out of town) so I fabbed up paper ones as a temp fix.

I am just about to measure float height now and button them all up again for yet another test.

I'll let you know how this goes.

cheers.
 
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