Topic: Carb adjustment..........

[rlvitt]

I have a 1970 CB 750 Honda. I have just finished putting a new head gasket and seals in the tope end. I also installed a Web Cam 41A cam. I rebuilt the carbs along with everything else. I am running a Scientific Design 4 into 1 header and K&N pods as air filters. The engine has stock displacement. I'm using a set of 5 ohm Dyna coils and stock points that are set to teh stock marks on all four cylinders. Now, here's the problem.

Upon first start up I noted the exhaust was blowing black when I cranked the throttle. The plugs indicated an overly rich fuel mixture. They were black and sooty. The carbs were running 120 Main Jets and 40 idle jets. The air screw was 7/8 ths of a turn out. The engine ran well despite the rich mixture. I didn't have to use the choke to start the bike. The engine developed good power upon take off and seemed to run very well through 6K rpms.

During my search for the reason for the rich mix, I installed Main jets in decreasing sizes down to 102, put in #35 idle jets and dropped the needle to the top notch. There was no change in the engines performance. I discovered that I had installed the carb slides backwards. The bevel was facing the engine not the air box. I corrected the slides and hooked up the fuel lines. After that change the engine would loose power and sputter once it reached 4K rpms. I returned the main jets back to 120's and #40 idle jets. Still the engine would develop no power above 4K. 

I double checked my floats and set the fuel level in the bowl to 4mm below the top of the bowl. I checked that level by using a bowl drain plug/clear tube to physically see that level. The engine still refused to get above 4K rpm's. I have again torn the carbs down and completely cleaned the parts and carb body. I paid special attention to the air passage ways and I'm testing the floats for leaks in case there is one not working properly.   

When setting the floats for proper level at 4 mm from the top I set the carbs flat on their front venturi while adjusting the tang that pushes the float valve closed. However, I've noticed that the manual shows that the carbs are tilted to some unknown degree in the example shown in the book. I have see this on line also. Is there a certain position other that 0 degrees vertical that the carb body must be set at to do a proper adjustment? And if so how many degrees is it.

Has anyone had this experience? And how did you resolve it?  Any ideas or input would be appreciated.               

[martin99]

No specific angle as far as I am aware. The float height is adjusted that way so the weight of the float is not compressing the tiny plunger on the bottom of the float needle, which would cause an inaccurate measurement. The float tang just has to be touching the needle when adjusting.

I'm not familiar with your carbs, but reading through your changes you do not say whether or not you returned the needle clip to stock position after re-installing the slides. Could be with the smaller jets and lower needle position you are now too lean, which may account for lack of power.

[rlvitt]

Yes. One of the things I did to try to correct the carburation was return the settings to the stock specs. I moved the clip back to the third notch from the top and put the 120 mains in with the #40 idle jets. What is baffling me is that the cam change shouldn't cause such a drastic change in carburation. It might very well have to be re-jetted. But, it seems that the change is really drastic and I may have overlooked something in the carbs. I've dialed carbs in before and I can't get any changes to happen, good or bad, with jet changes. I'm scratching my head......

[flybox1]

as i read your original post, besides re-sealing the top end, you added a new cam.
all else stayed the same after the engine was reinstalled, no? (exhaust, carbs, intake, etc.)
you went down the path of slides back wards and jet changes, but you've ended right back where you started.
i mean...it ran with this setup before your top end work, and now it doesn't, and the big change you made was swapping cams.

IIWM, i'd revisit my cam timing....

[harisuluv]

How did you even get the slides in backwards?  If you have proper carbs for your 70 which I will assume as you haven't stated to the contrary.  The little guide on the inside of the cylinder bore only allows the slide to go in one way.  Also, k0 carbs have an idle stop screw for each individual carb, where the slide has a notch on one side.  I don't even know how you could get the slides in backwards, maybe if they were upside down.

I think you should post some pictures of your carbs for starters.  Also I think there is some confusion with some of the terminology you're using. 

[rlvitt]

harisuluv, The slides will install backwards if you swap #1 to carb 3 or 4 or slide #2 to carb 3 or 4. And vice versa. I must have done it when I changed out the horsetail throttle cable. The carbs are either 7A's or B750A's. There is no stamped number on the carbs. The cam issue mentioned might be considered. I installed a Web Cam 41A grind for a little extra umph. However, there was no indication in any of the literature that it was not a drop in swap of cams. I was specified for use in a stock engine. No modification of the pistons was needed.

After doing some work to the carbs this morning I checked the plugs again and they appeared wet. There was no carbon or layer of burned fuel on them across the cylinders. I suspect that the carbs are running rich for some reason. I also found the #1 carb weeping fuel from the top of the bowl. So, the bowls may be overfilling and causing a rich fuel mixture to my mind. Another item I noticed was that the engine would give off a pop in the exhaust note regularly.

All this leads me to believe that the float level is wrong. Or that the float valve is not sealing properly and allowing the bowl to overfill. Would that not cause a rich fuel mixture? I'm just looking for answers and a way to be sure that my fuel level is at the proper height in the bowls. I'll attach a picture of the carbs in case it is of any help. Any advice is welcome.  Thanks.

[flybox1]

All this leads me to believe that the float level is wrong. Or that the float valve is not sealing properly and allowing the bowl to overfill. Would that not cause a rich fuel mixture? I'm just looking for answers and a way to be sure that my fuel level is at the proper height in the bowls. I'll attach a picture of the carbs in case it is of any help. Any advice is welcome.  Thanks.

if the float valves were not working, you'd have a continual flow of gas out the overflow tubes....
the only way to verify your fuel height in the bowls is correct, is to use the clear tube method

[bjbuchanan]

You could also have a vacuum leak. The balancing ports on my inner carbs weren't sealed and made the bike run horribly rich, my buddies all thought it was a two stroke. The vacuum leak didn't cause a rising idle or hanging idle and unlike a lot of cars and other stuff it doesn't make the bike run leaner.

[rlvitt]

OK. In this continuing saga of which way do I go now, I reset the floats to shut of fuel a little sooner. And I installed the float valves from my K2 to see if there would be any change in the engines behavior. It made no change. Wouldn't rev much above 4K still. I decided to throw on the choke just to see what it would do. I revved the engine to 6K without any misbehavior of the engine. When I threw the choke off it returned to the same 4K ceiling and the exhaust again began to pop again.

If the engine is actually lean now that I have the slides in properly what are my choices? The floats should be reset back to the 25mm level right off. The #40 pilot/idle jet seems to be working fine until 4K, so I'm thinking that I need to play with the mains and/or the needle. The mains are 120's right now and the needles are set on the 3rd notch from the top. Does the choke represent a certain size in main jets and clip position of the needle?

As far as checking the for a vacuum leak what would be good to use? Acetone or some other fluid? 

[rlvitt]

Thanks, I'll keep that one in mind!

[lucky]

What a mess!
Too bad because you have the best carbs made for that bike.

That is if they really are 1970 carbs.
It would be good to have a photo because everything about those carbs is much different that later models including the throttle cable.


Here is the complete story.

Make sure the choke lever and closing sides are working.
They are very delicate.

Float levels.
Set them at 26mm by measuring! Do not use any other method.
Make sure the float needles are new. IF you really want to have these carbs right.

Main jet 125  (120 was stock).

Leave the idle jet at #40.

Slide needle Clip position Middle or #3 slot.
If you find there is any stumble at all when giving it quick throttle when warmed up then drop the clips down one notch.

The mixture screws on these carbs are very responsive to small adjustments, Even 1/8 turn will make a noticeable difference.
Start at 3/4 turns out.

If there is any popping on decell after you rev it,  then open the mixture screws 1/8 turn until the popping goes away.

Now we get to a VERY IMPORTANT part of these carbs. SET UP.

There is a 1 into 4 throttle cable.
The first step is to make sure the 4 short cables going to the carbs have the two short cable going to the two middle carbs.

Set all of the IDLE screws so that the "T" mark is lined up.
Look for the "T" on the idle screw shaft.
This is so all the slides are bottomed out.
Did you find the letter "T" on that shaft of the idle screw? Line it up with the carb body when looking straight down on it.
Make sure the little cut out in the bottom of the slide is facing the rear of the motorcycle!  That is how it gets air to idle.

Make sure the throttle cable cylinder tube junction box is lubricated.

Using the adjuster on the top of the carbs make sure that there is NO slack in the 4 short cables. Put your finger into the carb and touch the slide. Start opening the throttle and make sure that as soon as the throttle starts to turn that the slide starts to move upward.

Set all 4 mixture screws to 3/4 turns out.

The part of the throttle cable going to the throttle has an adjuster to take the slack out of the cable. USE that adjuster to set the idle from now on. Then you will NEVER have to adjust the
cables on top of the carbs. You can just turn ONE adjuster to adjust the idle on all of the carbs. No lifter arms or shafts needed.

So to adjust the idle on these carbs when warmed up, simply turn the adjuster on the throttle cable near the throttle. Thats it. The carbs will always stay in synced.

When you attach the throttle cable DO NOT CLAMP any part of this throttle cable down.
This is very important You can put a zip tie very loosely around it to the cable does not stick out from the bike. Just one.

Throttle cable routing photos.
Click on photo for a larger image.
Notice the position of the throttle cable junction cylinder. Very important.

[lucky]

harisuluv, The slides will install backwards if you swap #1 to carb 3 or 4 or slide #2 to carb 3 or 4. And vice versa. I must have done it when I changed out the horsetail throttle cable. The carbs are either 7A's or B750A's. There is no stamped number on the carbs. The cam issue mentioned might be considered. I installed a Web Cam 41A grind for a little extra umph. However, there was no indication in any of the literature that it was not a drop in swap of cams. I was specified for use in a stock engine. No modification of the pistons was needed.

After doing some work to the carbs this morning I checked the plugs again and they appeared wet. There was no carbon or layer of burned fuel on them across the cylinders. I suspect that the carbs are running rich for some reason. I also found the #1 carb weeping fuel from the top of the bowl. So, the bowls may be overfilling and causing a rich fuel mixture to my mind. Another item I noticed was that the engine would give off a pop in the exhaust note regularly.

All this leads me to believe that the float level is wrong. Or that the float valve is not sealing properly and allowing the bowl to overfill. Would that not cause a rich fuel mixture? I'm just looking for answers and a way to be sure that my fuel level is at the proper height in the bowls. I'll attach a picture of the carbs in case it is of any help. Any advice is welcome.  Thanks.

The choke slides on 3 carbs are missing!
Unless the slides are in the open position, and I do not see the linkage so it is possible.

[rlvitt]

Lucky, Thanks for the input here. The picture I posted was of the carbs just after being polished. None of the guts or linkage was on them at the time. I'll post another picture of them, however. The carbs are the original carbs and came with the bike. There are no model stamps on the front venturies. The bike was bought in Japan and brought over on a carrier by the first owner. I have spoken with him and he told me all the history on the bike.

Through all the trial and error I have returned the bike to it's original settings (120 mains, #40 idle jets, clips on the 3rd notch and one turn out on the air screws.) and am using the stock points and coils. My intent was to see if there was any new part or "improvement" that was faulty. The stock configuration continues to misfire and develop no power above 4K except for short bursts and then cannot recover from the power loss. I took the bike out for a more extended run yesterday and even threw on the choke to see what would happen. It stalled the engine until I threw it off and then gained power to 4K again, gave me a quick burst to 6K before stumbling and misfiring.

Here's what I've done so far and haven't found the problem. I've checked the advance unit and it seems to be working well. Hits the marks on advance on time. What ever it is, effects all the cylinders though. All the plugs look identical. They're clean and without any trace of soot or white from being too rich or too lean now. Just a clean tan metal look to them. I'd say they look wet, but there is no smell a fuel on them and they are physically not wet.

So far I've set the valve clearance and cam chain, swapped out the Dyna ignition and coils for the old stock units, pulled the carbs apart, reset the floats to 25mm again, moved the needle from the top notch down to the middle (3rd) notch, put in #40 idle jets, tried 102, 110, 120, 125 and 135 main jets. I did a test for a vacuum leak of the carb boots using both carb cleaner and acetone and got no definite change in the idle of the motor. No loss or gain of rpm's. Maybe a little change in the exhaust popping. Now I'm wondering if there is some problem with how fast the fuel is being supplied to the carbs as I throttle up to the mains coming on line.

As part of this rebuild, I resealed the fuel tank with Caswell fuel tank sealant. Is it possible that the seal is interfering with fuel entering the petcock? I drained the tank when I checked the petcock fuel was draining from the rear line feed but not the front feed. The front was sucking air and mixing it with the draining fuel. The Open setting seemed to drain fuel faster than the Reserve setting also. I don't know if that is normal or not. There was no blockage or crud in the petcock bowl or screen and the petcock is new.

It seems the last thing I can do is replace the carb boots and hope that they are actually leaking enough to cause this problem.

As for your questions and suggestions I shall take them in order if not already addressed. I set the float levels at 25mm using a plastic gauge made for setting them. I will set them for 26mm as you recommend. The float needles are new but not made by Honda or KeiHin. I have read that some replacement part manufacturers have less than accurate measurements. I installed KieHin parts from my K2 and they are serviceable parts. I wonder if a float needle with a plastic tip would seal better. The choke lever is working smoothly and retracts the choke slides completely.

There is some "popping" on decelleration, but no backfiring. I have the short cables to the middle carbs, also. I routed it through the triangle in the backbone of the bike and then to the carbs, not under it. It lays next to the coils in the frame. I will change that to below the triangle. It is a sealed unit with a plastic junction cylinder. To open it I would have to break it. It's operation seems smooth.

The "T" marks on the throttle stop screws have faded and/or disappeared. I have set those to just barely touching the slides when the slides are let snap closed. I have been using the adjuster at the hand grip to set the idle. Also, there seems no slack in the cable. The adjusting nuts in the slide rods are still set from a sync last week and pull 8 lbs of vacuum at idle across the board.

A last thought. Since the bike runs pretty well on the pilot (idle) jets and only breaks up after the mains should be coming to bear, do you believe that it is caused by a lack of fuel feed to the carbs or the 5ohm coil breaking down with higher demands on it. I ran the coils with the points as well as the electronic ignition. Either way the engine refused to gain power when drawing on the fuel.

Thanks for jumping in on this. I appreciate it.

[lucky]

OK...A lot of ground to cover.

If you had the stock air filter and stock air box all you would need is a mixture adjustment even with the aftermarket exhaust. It would run perfect. Think about that.

I still do not see the choke slides in the closed position. I will assume they work.

The gas tank... Yes the Caswell's tank liner can clog up the outlet.
If you got the petcock screen in the tank it is not too clogged up.

It is a tight fit into the hole. Check your filter .
Make sure the gas tank filler lid is venting.
Run it with the cap off and see if it behaves differently. Do not do it out on the road
unless you are very careful.

It seems odd that you cannot see the "T" marks on any of the idle screws.
Look again carefully.


OK...K&N pods and aftermarket exhuast.

You will have to drop that slide needle one notch.
god help you if you loose that black clip inside the slide holding down the needle!

Also change to a #42 idle jet.

You can remove the slides while the carbs are on the bike.
BE very careful when putting the top ring on the carb so that you do not cross thread it!!! Make sure the rubber gasket is in place.
You can remove the slide. Then compress the slide spring and remove the needle to change the clip setting.

I wish I could show you how to do that. Too bad.

[harisuluv]

I don't think that is a more recent picture of the carbs, so I would disregard it.

The slides in 2 and 4 are still backwards which he says has already been corrected.  I would remove the picture, it's just going to confuse people.

[rlvitt]

harisuluv,
     Yes, that is in fact a more recent picture of the carbs. In attempting to remove the jewelers rouge in a bath of hot soapy water and ammonia it turned the carbs to an ugly gray that couldn't be polished out easily. I decided to shoot the bodies black to match the color scheme of the engine. As far as the slides, I swapped the other two backwards to make sure I screwed them all up equally (Ha).

Lucky,
     The stock air box has since been reinstalled. I used the pods first as an experiment to see if I could make some sort of change, good or bad, in the carburation, and, second, to give me better access to the bottom of the carbs. I know exactly what you are talking about concerning the "T" marks on the throttle stop screws. They are no longer there on two of them and there was only one with a trace if a "T" and the other I could make the "T" out. I figure that they were rubbed off during the polishing they were given. All the screws were left in to keep the jewelers rouge out of the inside of the carbs along with metal tape over the venturi's.

     I have tried to count the number of turns to the existing "T" on all of the screws and also watched for any movement of the slides moving up in the venturi. I have also adjusted them to just feel the slides striking the screw, while touching them, while letting the slide snap back down into the venturi. They should be in the ball park. Which begs the question, can they be set more accurately during a synchronization of the carbs?
 
     I used last evening to run the bike on the street and really assess what symptoms it is displaying after reinstalling the Dyna coils again. While there is some popping during an idle there is essentially none when I rev the engine and let it wind down it idle again. And no outright back fires. The air screws at at one turn out now. would you suggest adding an 1/8th turn until the popping at idle disappears? The engine has a major stumble when snapping the throttle WFO suddenly. If I roll the throttle on at a more normal rate it gathers RPM's up as high as I wish......and when held, at 7K for instance, begins to misfire and loose RPM's. I will lower the clip (raise the needle) one notch and install the 125 mains as you suggested see what happens. I'm wondering if that misfire and loss of RPM's is a fuel supply issue. And, though the brass tube is clear of the bottom of the tank, if fuel flow into the petcock may have something to do with it somehow. I haven't run the bike on reserve to see what happens yet. 

     One point of concern is the float bowl gaskets, Lucky. During the many times I have taken teh bowls off to change jets I noticed that sometimes the gaskets misaligned and the bowls would weep fuel. Would it be advisable to use some Permatex to stick the gaskets to the top of the bowls to help keep the gaskets in their proper position? 

     Thanks for all the advice here also. It gives me some direction to figure out this problem.

[harisuluv]

harisuluv,
     Yes, that is in fact a more recent picture of the carbs. In attempting to remove the jewelers rouge in a bath of hot soapy water and ammonia it turned the carbs to an ugly gray that couldn't be polished out easily. I decided to shoot the bodies black to match the color scheme of the engine. As far as the slides, I swapped the other two backwards to make sure I screwed them all up equally (Ha).

You don't want them all to be screwed up equally (backwards) you want them all facing the right direction.  I think this is what you meant but what you said means that you turned the two that were correct backwards to also match the other two that were backwards.  From that picture we are referring to they all need to have the concave slide viewable as seen, which would be facing the air intake as pictured.

Ok you painted the carbs too, there are a lot of holes there to properly tape off, looks like you got the vent/ports painted too, you sure you covered everything that you needed to?  If you just plugged the hole for the vacuum sync access now there is paint on it and it might not seal properly, it's meant to be air tight hence the crush washer.

Lucky made suggestions based on pods, so if you are going to keep the airbox on then what he suggested does not apply anymore. 

From what would seem to be your most recent test drive and status the bike is fine aside from some popping and hesitation from a SUDDEN snap of the throttle.  These bikes don't have accelerator pumps, and you shouldn't expect immediate response if you squeeze the throttle suddenly.  All your symptoms point to mixture screw/idle jet.

I bet you probably have semi clogged slow/idle jets.  Clean them, or buy new ones.  If you have already done that, do it again, reassess them, physically LOOK through the jet and see what it looks like.  Don't see a perfect circle?  That's a clogged jet.

If your bowls are weaping fuel at the gasket, your floats are not set correct and before it gets to that point it should be draining from the overflow, so that might be clogged.

Have you synced them at all yet?  Bench sync, vacuum sync? 

Sealer to fix a gasket is a makeshift fix, nothing more.  The problem isn't even necessarily the gasket though, as your fuel level seems too high.  Address the problem, not the symptom.  If you need to address the bowl gaskets, buy new bowl gaskets.

Please take a moment and step back and stop trying random things.  You are getting guidance and it really doesn't sound like you have major problems.  You are changing things so rapidly that the last response given to you no longer even applies.  You already have a large number of variables which doesn't help your diagnoses. 

[lucky]

I agree with Harrisluv. Slow down. Focus your thoughts more.
Syncing the carbs is to be done at the very end after everything else is complete.

So lets gather all the mess up now.

You will have stock air box and stock air filter. Good!.... 80% of the problems gone.
Mixture screws all at 1 turn open.

Idle screws all backed off.

 If you open the throttle slide even .007 thousandths it will make a difference. You almost cannot even see .007 thousandths.
Try to control the idle with the adjuster on the throttle cable near the handlebar.

Is there any baffles in the exhaust at all?
Never put permatex on a motorcycle part.

[rlvitt]

harisuluv and Lucky

     Sorry if you missed my lame joke about the slides. First, let me thank both of you for your help. And I'm sorry if it seems like I'm moving too fast and at random goals. I'm retired and have lots of time to work on this carb problem. I can do a complete change of jets 102 through 135, if necessary, and test the results in a day. But more than anything else I want to get "My Mistress" out on the road. So please forgive me if I seem to be moving too fast.

And, OK. No Permatex on the gaskets. Promise.

I made the changes suggested by Lucky and moved the clip down to the 4th notch and installed the 125 mains. The idle stop screws on the sides of the carbs are set barely touching the slides. The air screws remain at 1 turn out from the seat. The adjustment screws on top of the carbs are set so there is no slack in the cables. I used my finger to feel for movement when I adjusted them. The cables have also been lubed with oil. I learned the convenience of using the idle adjustment at the throttle a while back. The 4 to 1 header looks like it is a free flowing glass pack

The carbs were synched in at the beginning of all this, harisuluv, right after I spun the slides around to their proper position. I figured that they would need it to run properly. I got 7 to 9 lbs of vacuum at idle and adjusted them all to within 1 lb of each other. The #2 carb (I think) was a little higher that the rest at 3500-4000 rpms. And it was during the road check that I noticed the poor performance above 4K.

Also the air screws have been replaced with ones of the original design. With the tips drilled across them. They are clear and clean. The idle jet is brand new and is clean and clear by visual inspection as is the main and the emulsifier tube. The float valve is also clean and clear. The air circuit from the air screw is cleared by wire and clean down to the bowl. The screw landing are clean metal and the vent is open and patient. All screw seats, vents and jet seats have been blown clean with compressed air after any other cleaning. The paint on the carb bodies should be of no issue. 

I road tested the bike today with the carb settings Lucky recommended and there was no change with the stumble above 4K. I also tried it with 135 mains with the same results. I did notice it puff black exhaust during a rev in the garage with the 135's.  No crackling on deceleration, but the popping remains during idle. Fuel flow looks sufficient to the carbs.

Now here's a side question. What other things in the motorcycle could cause the engine to misfire at higher rpm's? Something electrical?

The reason for my question is, that even with all the adjustments, engine performance has not changed good or bad. And the plugs have not changed in appearance darker or lighter. Could a coil breaking down be the problem? Or a rectifier or stator that isn't supplying enough power? I'm just asking because nothing has changed. Could the coils not making enough of a spark, or not sparking every time, cause a power loss at a higher RPM for some reason? Or cause the popping noise at idle?

[rlvitt]

I totally rebuilt the carbs again today. I re-installed the old parts. They were still serviceable. I replaced them just to put new parts in the carbs. I reset the floats to 26mm as suggested and made sure that the mains were 125's and the pilots were #40's. It fired up right away and, except for a float that stuck, ran fine. Fixed the float and took a spin. No break up above 4K. Ran strong through 7K. Still pops during idle and now crackles during deceleration. All in all a good step in the right direction.