Topic: running issues

[Buffo]

I have installed A brand new Dyna S, 3. Ohm Dyna coils and 8.mm Dyna wires on my 1978 CB750K8. Set the timing and adjusted the cam chain tension. Freshly overhauled the carbs, set the float leveland turned the piolt screw out 1 1/2 turns.

Yet my bike only runs when you twist the throttle back and forth and will not idle. This was the first time that I have rebuilt these carbs as I have mentioned in previous posts and I am sure of all but 2 items.

1. The slow jet: It is pressed on and I can't see right through it...how do I KNOW it's clean?

2. The Main Jet that I got in my Keyster carb kit. There was no size stamped on the head of it.

The Pilot screw is turned out 1 1/2 turns.
Full tank of fresh gas.
New NGK 8's.
New fuel filter.
New battery kept charged on a tender.
Loads of spark.
Carbs cleaned with Berrymans and blown out.

Don

[TwoTired]

1. The slow jet: It is pressed on and I can't see right through it...how do I KNOW it's clean?

2. The Main Jet that I got in my Keyster carb kit. There was no size stamped on the head of it.

1. I use a spray can of carb cleaner that has the tube on the nozzle.  I imagine WD 40 could be used in the same way, though. I use my thumb and forefinger to make a seal around the pilot jet and the spray tube and apply pressure from the can.  Fluid should come out of the orifice in the carb throat near the slide (assuming your pilot screw is installed but opened a bit) and the air jet located at the air inlet of the carb.  You may have to block one or the other exits to see positive flow.

2.  Don't know why you thought it necessary to replace jets with an unkown orifice dimension.  But, you could check the sizes with jet guages, perhaps twist drill shanks, or drill rods from a machinists supply.  The jet numbers are metric referenced.  A #100 is 1 mm diameter.  A #115 is 1.15 mm.  a #98 is .98 mm, etc.

[Buffo]

I forgot to mention that it will only start/run(?) with the choke on.  I read the "boggy" post and put my Air filter on and it runs better, but still only at full choke...this means I'm runnig lean right?

 still no idle.



Thanks for the slow jet cleaning tip 2tired.


Don

[Bob Wessner]

If one or more of your slow jets it really blocked, you may have to gently poke some very fine, very soft wire through it. Two of mine were blocked even after two soakings in both ChemDip (nasty stuff!) and Yamaha (much better).

[Buffo]

tomarrow I plan on removing the carbs. AGAIN
I will put the old mains back in and verify that all my slows are free and clear.

Don

[eldar]

Get a carb tool if you do not have one. It could be possible that there is blockage that only a tool can remove. Take a look at your plugs to see what color they are. It could be possible that you are very lean but yet you never know, you could be rich.

[Buffo]

Took the carbs off and checked the mains...110, the same as it came with, at least to me.

I live at sea level and it has a K$N oem replacement filter in it ...would you think that these are the proper mains?

I am in the process of cleaning the slow/pilot jets. I stripped a length of brass wire and twisted it together (like a drill bit) then I put it down the jet and twist it so it "digs" in the crud. the twist digs in and hold some of the crud, I take it out, clean it off and go some more. When I stop getting crud I will try and force carb cleaner down the hole like 2tired said...slow/pilot jet are full of stuff...the berrymans didnt get.

Don

[TwoTired]

Be aware that the slow jet hole you see at the end of the tube is not the jet orifice.  The orifice is deeper into that tube and is only about .016 inch diameter.  Don't try to force something in there larger than that or you'll be changing the jetting.

I haven't pulled the pressed in jets, yet.  But, assuming they perform and are shaped the same as the screw in ones...  You're not going to like this.  On the other side of where it is pressed in, and formed into the jet, is another tube residing in a small cavity of the carb body.  The cavity is fed air from the air jet at the mouth of the carb.  That hidden jet tube is cross drilled with holes that aerate the fuel from the jet as it goes to the carb throat.  I don't know how to tell if that chamber, aeration tube, aeration tube holes, and passageway are fully cleaned, without removal of the pilot jet.
A suggestion is to feed carb cleaner into the air jet at the mouth of the carb.  There are two, one feeds the main jet emulsion tube and the other the pilot jet tube.  These air jets are #120 or #150 which makes them bigger holes to feed solvent into than the pilot jets.

Good luck,

[Buffo]

I belive my biggest problem is clogged slow/pilot jets. After spraying myself with carb cleaner for 5 minutes i took the plasic tube and with my Dremel put a point on the tube. I took off plastic on a gradiant for about 1/2 inch. It looks lik the end of a tooth pick now. This allowd me to push the tube into and seal in the orifices.

The "air feeder holes" on the mouth side of the carbs I got cleared (I think) because now when I spray the cleaner in that hole it comes out inside the carb by the slide freely. However on the 3 and 4 carbs if I spray the cleaner in the tube in the float bowl nothing comes out of ANY hole. NOTHING. The seal on the cleaner tube is so tight that nothing comes out of anywhere until i pull the tube out and it releases.

I didnt want to pull the carbs apart again. Should I just slide some berrymans chem dip in the slow jet from the float bowls?  Sense the holes are so small I would Put a piece or 2 of wire in the hole in float bowl and slide very small amounts of chem dip in the hole that way. a whole drip would never make its way down the hole due to specific gravity of the tube and the  surface tention of the liquid.

Any Ideas  would be great. I didnt want to pull the pressed jet out.

Don

[Buffo]

well two tired, the slow jets are as you described. I pulled all 4 slow jets out and they do indeed have cross drilled holes. I dont know what is in my #3 jet but it is packed solid. I threw them all in the berrymans chemdip and will let them sit for 45 minutes or so. sprayed cleaner throught the air holes and it ran freely up through the holes where the jets were.

the jets pulled out after a turn or 2 with a pair of plyers. any words of wisdom in reinstalling the slow jets? FYI the jets are stameped 135.

soon, soon

Don

[jtb]

Eldar, 
Please tell me about this "carb tool".  I need to get one, I have an extra set of carbs I'm working on, and two of the slow jets are blocked.  I tried some copper strands I pulled from a wire, but they only bend, they are too small.  If there is a tool that will help, I'll buy one.  Thanks.

John

[Buffo]

The only way I see to clean the slow jets properly is to remove them. I removed mine with a pair of pliers, cleaned them and they just tap right back in real easy.  Pull them out and do it right.

I just reassembled my bike and the 1978 CB750K8 starts and idles on the first kick!!!

twotired, your two cool!! Thanks!!!

It doesn't run great and it will only run with the choke on full, but it runs. It has not ben sync'd yet but I think it should run better than it does. I will adjust the valves tonight. what could be the problem?

Don

[TwoTired]

It doesn't run great and it will only run with the choke on full, but it runs. It has not ben sync'd yet but I think it should run better than it does. I will adjust the valves tonight. what could be the problem?

It still needs full choke after it is thoroughly warmed up?  Or only for cold start up?

The choke on full means the fast idle cam is fully engaged, which holds the slides a bit open.  The carbs begin to operate on the slide needle jets.  AND, the choke butterflies are closed causing high vacuum in the throats to pull fuel out of the slide needles and the pilot jets.

Do you have the air cleaner off?  I'll ask again if the head pipes are the even temped since you've made changes.

If it still needs full choke to run after warm up, and the air filter is installed, it sounds like the slow jet system still isn't working.
What is your idle mixture screw setting? and does turning them out help matters?

[Mikeshonda750F]

Sounds like a vacuum leak to me

[TwoTired]

Sounds like a vacuum leak to me

That's a good point and something to check.

The book says your slow jets are # 35, or .35 mm (0.35 millimeter = 0.0137795 inch).  Assuming the bike has all original exhaust and intake parts, this should run ok.  But, if something has been changed here, you may need bigger slow jets such as the earlier carbs had, like # 38 or so.  If you can't steal pressed in jets of larger diameter from earlier carbs you may have to drill them to a larger size.  But, don't do this untill there is absolutely no doubt that the slow circuits are indeed throughly clean and you run out of adjustment range of the idle mixture screws.
Also, jet sizes are determined for engines thoroughly warmed up.  Did I mention you adjust carbs last after all other tuneup adjustment items are totally dialed in?
Cheers,

[MRieck]

Sounds like a vacuum leak to me

I'll bet 100.00 that's it. Spray some brake cleaner around the boots when it's running and listen for a change in idle speed.

[Buffo]

the pilot screw are turned out 1 1/2 turns.
I have no Idea what exhaust came on this bike but it has a 4 into 1 now.
It also has a k$N factory replacment filter. And it is on the bike.
I dont have a suitable temperatur gauge to check the head pipes but they all feel the same.... Hot.

I ran it for about 10 min. and it need to be put on half choke cause it started to gain rpm. If you put it on no choke you have to rev it a little to keep it running. I will have to go to sears to get a screw driver that will allow me to adjust the pilot screws, I dont have one that will fit.

Is there a specifc tool for the job? Or is it just a small screw driver?

I tried to adjust the #1 exhaust valve and I think I did it right but I want to buy the tools. It was awkward to turn this while holding that, then reach for the other wrench while holding  that perfectly still...all the while saying "did that just move?"

Don

[TwoTired]

the pilot screw are turned out 1 1/2 turns.

That is the proper setting for a bike that is factory equiped.  The 4 into 1 and the filter change means you will have to find the proper setting for this set up.

I have no Idea what exhaust came on this bike but it has a 4 into 1 now.

The bike had 4 into 4 mufflers on it from the factory.

It also has a k$N factory replacment filter.

The K& N is likely less restrictive than the stock paper element.  This leans the carbs by reducing jet flow.  The choke adds restriction back into the inlet ar flow.

I dont have a suitable temperatur gauge to check the head pipes but they all feel the same.... Hot.

That's a good indication that all the carbs are behaving the same way.  Progress.

I ran it for about 10 min. and it need to be put on half choke cause it started to gain rpm. If you put it on no choke you have to rev it a little to keep it running. I will have to go to sears to get a screw driver that will allow me to adjust the pilot screws, I dont have one that will fit.

Is there a specifc tool for the job? Or is it just a small screw driver?

As the engine warms, it gets more efficient with the fuel provided and normally runs faster with the same throttle setting.  Yes, there is a specific tool for the mixture screw adjustment.  Since this is adjusted with the engine warm. A short screwdriver will likely burn your hands.  A right angle screwdriver that converts horizontal rotation to vertical rotation is what is needed.  It's kind of a specialty tool. I made one from pieces of an old slot car rear axle and brass tubing.  Not pretty but it worked.  I wouldn't think that Sears would have one.  Here's what expensive ones look like:
http://www.yamahafz1oa.com/motionpro90degreescrewdriverreview.shtml
http://www.carbtune.com/bevel.html

I tried to adjust the #1 exhaust valve and I think I did it right but I want to buy the tools. It was awkward to turn this while holding that, then reach for the other wrench while holding  that perfectly still...all the while saying "did that just move?"

This is an acquired skill.  Practice helps.

Do check that the carb intake rubber couplers are not leaking and sucking air into the runners.
Do turn the idle mixture screws out farther.  But, be aware that the adjustment range on these carbs is limited.
Honda intended this engine to be a "lean burn" engine with the stock components.  The exhaust and air filter change may require jetting changes to richen up the carbs.  Note that the K7 had #115 mains and earlier Cb750 carbs had #40 slow jets while yours are #35.  When you exhaust all other adjustments and have synchronized the carbs, you may find that larger pilot jets will be needed, either by replacement or drilling.

Sounds like you are making good progress and are closing in on your goal.  Just a little more and...
Cheers,

[Buffo]

thanks 2tired...My wife is going to love the new $125.00 screw driver.

I do not believe that carb boots are leaking.

Besides a rough Idle, would having slow/pilot jets that are a little lean hurt the bike or greatly hinder performance?

Don

[TwoTired]

thanks 2tired...My wife is going to love the new $125.00 screw driver.

Besides a rough Idle, would having slow/pilot jets that are a little lean hurt the bike or greatly hinder performance?

Show her the prices of a new motorcycle, the insurance for same, registration fees, and local shop maintenence rates.  The screwdriver is a bargain compared to any of that.  But, again I made my own for about $5 in parts.

As far as the lean question, it is a matter of severity.  First, you need to run without choke after it is warmed up.  Overly lean engines run hotter, the heat can destroy the oil, and destroyed oil won't lubricate the engine.  You may experience pinging, and/or after shut down run on.  Or, more severe, detonation, which is destructive.  Exhaust valves tend to get crispy.  I don't associate lean with a rough idle, though.  Uneven carburation among the four carbs is more likely a cause of that. Idle pickup power will likely be lacking with lean idle. You'll Rev it higher than normally necessary to get going.  And, probably slip the clutch to keep it in a power band causing higher clutch wear.  I'm not saying all these symptoms will occur, but they may.

Given your pipe and filter changes, you may need to rejet the main and/or change the needle clip position, too.  Now that you have the slow jets clear, move the clip a position to enrich the needle operation range.  There may be enough bleed at idle to richen that up enough, too. 
Learn to read your plugs. And, how to do a plug chop. At various throttle positions.  If you can spring for a Dyno test, it will speed the whole jetting/tuning process.

[Buffo]

I dont believe that my carbs have "clip positions" to alter...

I 'm going to set my vavle clearance, get my drive chain and ride the damn thing. After which I will attempt to set my pilot screws.

Don

[TwoTired]

I dont believe that my carbs have "clip positions" to alter...

Oh yes, I heard about the 78 not having other clip positions.  But, keep an open mind about the needles, as the the new exhaust and airfilter may require a change there. Adding grooves to your needles or getting needles from a set of 77 carbs, or rebuild kits for the 77 CB750 K may be options you'll wish to consider.

I 'm going to set my vavle clearance, get my drive chain and ride the damn thing. After which I will attempt to set my pilot screws.

Sounds like a plan.  Hope things go well for you.  I sense you'd rather ride than work on the bike and that you are close to your limit on the wrenching aspect.

It does make me wonder just how many of these bikes were cast aside due to these types of issues.  I can imagine the sequence of events:
1. Original mufflers rust through.
2. Cheap replacements are installed.
3. Engine doesn't run right.
4. Some fiddling with the carbs frustrates the owner.
5. Shop wants more than book value of the bike to fix it.
6. Disgusted owner sells it or parks it in favor of another bike.
7. New owner gets a "killer deal" on the bike.  But, won't spend more than he paid for bike to get carburetion right.
8. Bike sits until another "killer deal" is made.
9. Repeat 7 & 8 until bike is parted out to recover from the "killer deal".
10. End of bike.

Sad stories make me cry...

[73cb550]

Very sad indeed, I bought a $7.00 welding torch cleaning tool which has numerous small wires or rods of varying sizes that has been very useful for carb jets. Just thouyght I'd pass it on

[Lumbee]

..hey Buffo, heres a cheap was to adjust my the air screw.  I had a screwdriver that I can change out the fitting (phillips/flat on one side, allen on the other).  The flathead was to thick to fit the air screw, so I took my Dremel and lightly filed it down til it would fit.  Then just use the fitting without the driver, using u'r thumb and forefinger.  Its a tight fit, but it works, and it a little easier on the wallet.   

[eldar]

The burns from this method suck though! Get some gloves that have a thick back.