Author Topic: Fuel delivery symptoms? 1976 cb750k (Read 2168 times)

robdrobd · « **on:** August 10, 2011, 04:11:59 PM »

I have been trying to get my bike to run for an extended period of time so I can adjust the idle and air screws. The bike will start with full choke, and then die within seconds. The carbs are spotless. New Mac exhaust. Main jets changed from 105 to 120's. Vintage Honda velocity stacks. It seems like a fuel issue to me, but I am not sure of the symptoms. Also, what should be done with the air mixture screws to start with when I have 120's and the stacks on? Loaded question I know, but at least I am almost certain it's not clogged jets. Any answers. Thanks

Nortstudio · « **Reply #1 on:** August 10, 2011, 05:13:59 PM »

Stupid/simple things first.

Have you checked to make sure you are getting a healthy amount of gas in the bowls?

Battery is solid?

When you say the carbs are spotless, did you do this yourself?

Have you done the other things on the 3000 tuneup list?

Really? · « **Reply #2 on:** August 10, 2011, 05:24:19 PM »

I have not done the velocity stack part but don't you need to up the other jets and/or adjust the needle clip too?

Really? · « **Reply #3 on:** August 10, 2011, 05:25:33 PM »

Sounds like you are plain not delivering enough fuel to the amount of air you are feeding it now.

robdrobd · « **Reply #4 on:** August 10, 2011, 05:27:25 PM »

I just finished the 3,000 tune up. I cleaned the carbs myself and they are clean (I pulled and checked them since posting.) I am using a funnel connected to a fuel line for my gas tank. I didn't check the bowls. Do the bowls have to fill all the way up for initial start? Maybe I wasn't putting enough fuel in the funnel to fill them initially?

Nortstudio · « **Reply #5 on:** August 10, 2011, 05:32:49 PM »

The bowls would fill to the level determined by the floats. So there would definitely be plenty in there if set up correctly.

Not sure how big your funnel is, but you would certainly want to provide more gas that you would need, to rule out that as an issue.

Tipper is probably right, there are probably other moves needed to get in the ballpark with stacks.

Also, do some searching for the air screw starting point. Somewhere on here is a chart (basic, and NOT science) for where to start with jets for any changes you make from stock.

Really? · « **Reply #6 on:** August 10, 2011, 06:39:52 PM »

It's not necessarily how much fuel your pouring into the carbs, it more of getting more fuel into the throat of that carb because of the amount of air you have going through the throat because of the stacks.

When you have the choke on and it runs, you are limiting the amount of air.

From what I have read in many posts, it takes a while to tune it without the breadbox.

I have a Mac 4-1, 110 mains and the breadbox. Runs good to me that way.

To me, the excuse that it is too much work to pull the breadbox is a crock. These are way too easy to take off and put back on. I know you didn't say that.

.

If you insist on running the stacks, expect to spend a lot of time trying to get it right.

Danno · « **Reply #7 on:** August 10, 2011, 07:19:17 PM »

I agree with tipper there is no way that with 120 mains that you could have the air screws out more then half a turn with stock slows I would bump the slow jets up to 42 or 43 because i believe that stock is 40 on this bike and then try to get it to idle at one and a half turns on the air screw if it still won't idle and turn it back a half turn and see
if you have to turn it back a half turn or more the slows are still too small try up a point for each half turn
also if it is only a half turn try moving the need up one notch instead
to move the need up move the clip down

dtmmil · « **Reply #8 on:** August 10, 2011, 07:31:01 PM »

None of you guys have any experience with stacks huh? 120 mains is good, do not go up with the pilots, makes the trouble worse. Sounds to me like you don't have your carbs clean, sounds like something in pilot system is plugged on one or more carbs. Take your air screws out, are they solid or do they have a hole in them? If they have a hole in them plug it with jb weld. I can get you a tutorial on it. If the screws do have a hole in the end, turn it all the way in to start, the ones with a hole in them will run leaner then the solid ones, hence filling the hole with JB weld. Did you completely tear down the carbs when you cleaned them? Did you make sure all the pilot jets were clean and you could see through them, and you could blow compressed air through all the ports.

robdrobd · « **Reply #9 on:** August 10, 2011, 07:49:45 PM »

Jets are clean. Can see through them and the mains are new out of the package. My Air screws are turned out to about 1 and 5/8. Should I turn them IN since the stacks are providing so much air?

dtmmil · « **Reply #10 on:** August 10, 2011, 07:56:59 PM »

Yes, do you know if the air screws have the holes in them, or are they solid, makes a big difference with open pipes and intakes.

robdrobd · « **Reply #11 on:** August 10, 2011, 08:00:34 PM »

Not sure. Can I screw them all the way out and check or is it more complicated than that?

dtmmil · « **Reply #12 on:** August 10, 2011, 08:04:41 PM »

Nope, just unscrew, there will be a small spring in there also, that needs to be there, it holds pressure on the screw so it doesn't move on its own. but, those should have come out when you cleaned the carbs also.
http://www.hondachopper.com/garage/carb_info/idle_screw_mod/idle_screw_mod.html
if yours are the hollow ones, check this out.

Nortstudio · « **Reply #13 on:** August 10, 2011, 08:05:46 PM »

These guys obviously know their #$%* when it comes to stacks. But I would still suggest making sure you have enough gas in the carb to even get the thing running poorly, to then be able to diagnose your issue.

In a previous post you mentioned that you were not sure if your funnel provided enough gas to fill the bowls. If that's true, then there is no way of knowing whether you have enough fuel to run these tests.

robdrobd · « **Reply #14 on:** August 10, 2011, 08:07:02 PM »

Thanks, I'll give it a whirl tomorrow do I don't wake the kiddies:)

robdrobd · « **Reply #15 on:** August 10, 2011, 08:08:39 PM »

Quote from: Nortstudio on August 10, 2011, 08:05:46 PM

These guys obviously know their #$%* when it comes to stacks. But I would still suggest making sure you have enough gas in the carb to even get the thing running poorly, to then be able to diagnose your issue.

In a previous post you mentioned that you were not sure if your funnel provided enough gas to fill the bowls. If that's true, then there is no way of knowing whether you have enough fuel to run these tests.

The gas will be the first thing I check before I mess with anything , thanks

robdrobd · « **Reply #16 on:** August 11, 2011, 04:31:19 AM »

Quote from: dtmmil on August 10, 2011, 08:04:41 PM

Nope, just unscrew, there will be a small spring in there also, that needs to be there, it holds pressure on the screw so it doesn't move on its own. but, those should have come out when you cleaned the carbs also.
http://www.hondachopper.com/garage/carb_info/idle_screw_mod/idle_screw_mod.html
if yours are the hollow ones, check this out.

Air screws are solid, no holes. Should I turn them in all the way with stacks and work backwards from there to "fine tune"?

bollingball · « **Reply #17 on:** August 11, 2011, 05:02:37 AM »

Rob you rebuilt your carbs but didn't know how to check your air screws

maybe you need to go through them again.

Nortstudio · « **Reply #18 on:** August 11, 2011, 05:04:43 AM »

Quote from: bollingball on August 11, 2011, 05:02:37 AM

Rob you rebuilt your carbs but didn't know how to check your air screws
maybe you need to go through them again.

+1. If you didn't take the air screw out while cleaning - you might want to do another run through the carbs for good measure.

Also, what was the deal with the tank, tat you can't be using it as the fuel source???

robdrobd · « **Reply #19 on:** August 11, 2011, 05:26:46 AM »

Tanks is being painted by a forum member. I'll take the rack out...AGAIN... and clean it piece by piece. Can anyone with stacks answer the airscrew question? Should I turn them in all the way and work backwards?

dtmmil · « **Reply #20 on:** August 11, 2011, 05:30:04 AM »

Turn them in all the way, then out 1/2 turn to start with, but make damn sure your idle passage isn't plugged! None of the other guys here seem to deal with stacks, to them it seems if its not stock, its not right! I have stacks but I have 78 carbs, which are lean to begin with so I have larger pilots, you shouldn't need larger pilots, just make sure yours are clean.

Nortstudio · « **Reply #21 on:** August 11, 2011, 05:30:53 AM »

I think someone earlier in the thread said that you should start with them all the way in with stacks, if you have the holes. Since yours are not, you should maybe start with them at stock setting. Fine tune from there.

I feel your pain on the carbs - I recently took mine out 9 times in 2 weeks. Just be happy you have stacks instead of the stock air box, it cuts the time down to less than half. Then again, it probably ramps up the time to tune to over double

robdrobd · « **Reply #22 on:** August 11, 2011, 06:12:31 AM »

Yeah, I gave my airbox away to a friend because I knew I had the stacks. I can take the carbs out in less than 5 minutes, but tuning has proven to be more of a challenge.

dtmmil · « **Reply #23 on:** August 11, 2011, 10:09:05 AM »

The biggest part is to start with a solid clean set of carbs.

Nortstudio · « **Reply #24 on:** August 11, 2011, 10:21:51 AM »

Quote from: dtmmil on August 11, 2011, 10:09:05 AM

The biggest part is to start with a solid clean set of carbs.

+1. There is no doubt that you need clean carbs - and that they can be dirt, een when you "think" they are perfect.

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Author Topic: Fuel delivery symptoms? 1976 cb750k (Read 2168 times)

bollingball