Author Topic: High idle after basic maintenance and carb set back to stock (Read 3258 times)

bananaseat · « **on:** June 16, 2019, 01:00:05 PM »

Hi All,

Recently purchased a 76 cb750 that was modified by the p.o to be somewhat of a harley. I've spent the past few months getting the bike back as stock as possible but still running into some issues. Main one being now the bike idles high 3k ish rpm after carb cleaning, resetting needle to middle clip, points/timing check, valve adjustment, cam tensioner adjustment. The last thing on the agenda was a vac sync but unfortunately the vac sync tool I purchased came with two bad gauges. I used the bench sync method I've seen mentioned here with the idle screw adjustment for the main carb and adjusting the other carbs to that as well as using a light or 1/8 bit for the distance of the slide. The bike still idles high with that and super high 5k ish if it warms up.

My thoughts on the culprit of this issue from doing some research here on sohc is
1. Air leak (possible as my airbox upper and lower half don't fully mate but I've seen it mentioned by others there's does the same? I installed a new airbox gasket
2.Carb adjustment (if I press down on the carbs the idle lowers but they are set now using the 1/8 screw method and only have two threads from the top showing
3. Timing is still off (could be possible I set the timing using the instructions in the clymer)

This is my first cb750 I'm used to working on single cylinder dual sports so my hands on knowledge is non existent any insight would help. I've tried searching in older threads but only a few come up using the keywords associated with the issue.

Bike in question

jaytee-nz · « **Reply #1 on:** June 16, 2019, 02:30:51 PM »

A high idle after carb work is a common problem and it is often due to the slides being set too high during the bench sync. There are varying opinions on what size drill bit to use but I think 1/8 is too big which has set your slides high. The fact that your idle speed drops when you push down on them also indicates this.

ekpent · « **Reply #2 on:** June 16, 2019, 02:31:54 PM »

If there is no longer any ability to lower the idle using the large knurled head idle adjuster screw located on the top of the carb bank then that would mean that the slides are set too high from the double nuts on the top of each carb. You have tried the idle adjustment screw I assume for starters ?

cjespersen · « **Reply #3 on:** June 16, 2019, 03:06:38 PM »

I had the same problem with my Honda cb350f. It would idle's high and it would race out of control when I let the choke out. I bench synced the carbs without the idle screw touching the tang so the slides didn't close all the way with the idle screw turned all the way out. I went back and dropped all my slides down and then turned in my idle screw until I could slide in the 1/8th drill bit. Take your carbs off and look into the intake side, and see if they are dropped all the way down with the idle screw turned out.

pjlogue · « **Reply #4 on:** June 16, 2019, 04:35:22 PM »

You need to lower your slides using the slide adjustment on top of each carb. 1/8" is way too much of an opening for bench sync. 1/64th is more like it. The top of the threaded rods will have ~1-2 threads showing above the lock nut.

-P.

bananaseat · « **Reply #5 on:** June 17, 2019, 11:44:40 AM »

Thank you for the information guys. I will look into what you've advised and look at the slides again. Just to make sure the slides should be shut with the idle screw backed out?

Patrick · « **Reply #6 on:** June 17, 2019, 09:07:28 PM »

The point of synching the carbs is not too look for a special gap at the bottom, it is to get all the slides moving at the same time. When you set you synch, screw the throttle bolt in to open up the slides just a bit to give you some clearance. Then, when you are done, you should be able to let out the throttle bolt until the slides close all the way. Otherwise you are setting your minimum idle with your sync, and not your throttle screw.

Patrick

cjespersen · « **Reply #7 on:** June 17, 2019, 09:49:55 PM »

That is correct. If you turn out the idle knob the slides should drop all the way down. If not your getting to much air and that will cause your high idle and hanging idle. I had this problem with my cb350f because I wasn't taking into account the idle knob.

Adjust the idle knob until the number 2 carb slide is open enough to just let the 1/8" drill bit through on the intake side. Then adjust the _sync_ screws (screws at the top of the slide with a thin metal stamping connecting the two) on the other 3 carbs until the drill bit just fits them as well.

PeWe · « **Reply #8 on:** June 18, 2019, 02:21:59 AM »

Short throttle cable? Possible to adjust.
Idle screw to adjust idle....

bananaseat · « **Reply #9 on:** June 18, 2019, 08:33:13 AM »

Thanks for the insight guys. I think my bench sync fell short at the slider height adjustment. I think the issue is with the sliders and them being fully seated prior to adding the slider arms that hold the throttle cable and after. Without the bracket connected the slides close fully with the bracket on I cannot get the slides to fully seat closed without running out of threads on the slide. This issue I'm assuming is attached to me being able to drop the idle if I press down on the slides manually which shoots back up once released. I'm going to undue the 10mm nut with the bracket attached to the carbs and see how low I can get the slides to go down while still being able to secure the 10mm and 17mm securing nuts after. Once I have that figured out I will use the advice you guys have given me to get all the carbs synced to each other along with the idle screw being engaged. Does the carb bracket go bad? it's a relatively simple design and I've seen in the manual they mention greasing the ends I don't see anything that would cause it to not allow me to start from a completely closed carb and work my way up as needed.

TwoTired · « **Reply #10 on:** June 18, 2019, 10:20:50 AM »

Some models have a master carb position that you cannot adjust. All other slides are vacuum synced to the master carb.

For your model, you have to assign a master carb. This is done by adjusting your designate as master (usually #2) and assuring that the slide can travel all the way to the "floor" with the idle knob backed out, and then locking the slide adjuster down. Never adjust it again. All other slides are adjusted to what ever opening or vacuum level it indicates. The main idle knob can then regulate minimum air flow for all carbs.

Cheers,

cjespersen · « **Reply #11 on:** June 19, 2019, 09:19:34 PM »

I had the same problem, even with the idle knob turned all the way out my slides didn't drop all the way down unless I pushed on them. I dropped all 4 of my slides by adjuster at the top and screwed them down until all 4 slides hit bottom, locked them all down, then screwed in the idle knob until I could slide a 1/8 bit, or paper clip under the number 2 carb slide. After you do that then don't touch the number 2 carb or the idle knob. Go to the next carb, unlock the adjuster nut at the top for that carb and adjust until you can slide the drill bit under, then lock it down and move to the next carb. When your finished all your carbs should be synced and when you turn the idle knob out and its not touching the tang, your slides should be in the closed position. This Hondachopper guide helped me.

http://www.hondachopper.com/garage/carb_info/bench_sync/bench_sync.html

bananaseat · « **Reply #12 on:** June 22, 2019, 06:36:07 AM »

Thank you everyone I'm going to try your suggestions today.

bananaseat · « **Reply #13 on:** June 22, 2019, 11:00:00 AM »

Got the carbs off and reset everything using the second carb following the guide you guys gave. I've included two videos before and after the slides definitely were too high prior I'm about to put the carbs back on and see how it goes.
https://imgur.com/9k0Q77Z
https://imgur.com/Rtaesrk

bananaseat · « **Reply #14 on:** June 22, 2019, 01:45:17 PM »

Thanks again everyone after putting the carbs back on I'm able to get 1100 - 1200 idle after warm up. Anyone have tips for adjusting the idle mixture screw? I set it as per the clymer 1 1/2 turns out but after using the adjustment steps outlined in the book I couldn't get the bike to idle anymore. This happened after adjusting the screw in

TwoTired · « **Reply #15 on:** June 22, 2019, 01:54:18 PM »

Um, if it runs well with the stock setting, why change it?

Cheers,

ekpent · « **Reply #16 on:** June 22, 2019, 02:25:35 PM »

Hmm- I always thought the baseline for the air screw was one turn,that's what I set all of mine at with the roundtop carbs.

bananaseat · « **Reply #17 on:** June 23, 2019, 08:32:59 AM »

Thanks for the feedback TwoTired and ekpent. The reason for making adjustments would be the bike stumbles and dies during warm up on choke it also backfires on deceleration I thought it may be running on the rich side. The clumer mentiones 1 1/2 turn out for cb's 71-76 and 1 3/4 turns out for 77-78.

Patrick · « **Reply #18 on:** June 23, 2019, 08:41:31 AM »

You air screw only governs fuel richness at idle. Popping during deceleration is often a symptom of exhaust leaks around the head. The air screws have no effect on richness once you are moving.

Patrick

bananaseat · « **Reply #19 on:** June 23, 2019, 08:56:22 AM »

Quote from: Patrick on June 23, 2019, 08:41:31 AM

You air screw only governs fuel richness at idle. Popping during deceleration is often a symptom of exhaust leaks around the head. The air screws have no effect on richness once you are moving.

Patrick

Thanks for the info I'll have to double check everything up there. I put in new crush washers and replaced some janky hangers but the replacements weren't oem I hope that's not the source $:-\$

ekpent · « **Reply #20 on:** June 23, 2019, 09:23:27 AM »

Here is a nice chart showing some cb750 carb specs. According to this it says 69-76 is 1 turn out and 77-78 1 1/2 turns. http://www.hondachopper.com/garage/carb_specs/carb_specs.html

bananaseat · « **Reply #21 on:** June 23, 2019, 09:54:01 AM »

Thank you ekpent.

I went back and adjusted the screw to one turn out to see if there would be a change in the stumble on cold start with no luck. I do have stock jets 105 and 40 and moved the needle clip back to the middle as the p.o had it running slightly richer. She still stumbles and struggles to idle at 800 before dying. I'm thinking it could be a larger issue with the adjustment screw as the clymer says adjust either way starting with carb 2 until the idle raises but the idle doesn't move at all no matter if the screw is all the way in or all the way out.

PeWe · « **Reply #22 on:** June 23, 2019, 10:45:48 AM »

1200 rpm idle is easier to handle at take offs and it will ensure oil pressure, especially when engine is very warm.

I have noticed that the air screw setting is dependent of the exhaust. Honda recommendation is most likely with stock 4-4 with everything in it.
Another exhaust with better flow need richer setting.
My CB750 K2 with K6 carbs has the earlier model of 4-4 that has better flow (HM300 type). It was possible with 1 turn out with the wool in. NOT without it. I had to decrease to 1/2 + 1/8 turn.
I have ordered 42 pilots so I can use 1 turn out. I'm convinced that the pilot circuit need air for good mixture and good flow.

Stock CB750 with HM341 4-4 with end plugs will probably work with 1 turn out, +/- 1/8 turn with stock air box.
Pods need probably larger pilot jets like 42.

TwoTired · « **Reply #23 on:** June 23, 2019, 12:29:19 PM »

Have you read the spark plug deposits? If it's too rich it will have black soot on them. Of course, throttle position matters with mechanical slide carbs. That position determines what jet is dominating fuel supply.

"Stumble" is a bit ambiguous. Is it burbling? or does it just kinda shut off? What do you do to make it recover? How does it recover? Cleanly? Or burble back to smooth? This will tell you if it was too rich during "stumble" or too lean.

The bikes that don't have PD carbs or accelerator pumps. Need the idle mix to be over rich, as when the slides lift, vacuum is initially lost, stopping flow from fuel jets while there is plenty of air available. If adjusted too lean, application of 1/2 of throttle travel under load will make the bike fall flat, like it was shut off. It will recover cleanly when the throttle is reduced. From idle under load, don't expect the snap to WOT to do anything but make the bike fall flat. You don't need to snap the throttle full open to get full power from low speeds/RPM. This is an SOHC4 trait. It wasn't until the carbs with accel pumps (or CV carbs) were installed that snap throttle was allowed by rider.

If you manage to enrich the mixture for the early carbs that allow snap WOT, then expect the plugs to carbon foul during idle periods.

Open exhaust, compared to stock, will also require idle fuel enrichment, as will an air filter that offers little flow resistance. Honda tuned the carbs for these items on the bike. Bike modifiers have random success changing these things.

Cheers,

bananaseat · « **Reply #24 on:** June 23, 2019, 02:52:02 PM »

It's like you read my mind. I actually pulled the plugs earlier today to see what condition they were in. I also started contemplating adjusting everything back to stock although it's running 4 to 2 exhaust and a uni filter in the stock airbox. In any event I added photos of the plugs below. First photo is 1-4 starting on the left, second photo is cylinders 1-2 last photo being 3-4.

News:

Author Topic: High idle after basic maintenance and carb set back to stock (Read 3258 times)