Topic: 1976 CB750F - who wants to help me with my carb tuning?

[riffman12]

ok confirmed that all 4 floats are within 0.5mm of 26mm. Do I still need to clear tube test?

If not, my next step is going up from a 40 to a 42 pilot, correct?

[riffman12]

Ok so I know it's been a while. I've been slowly rebuilding the carbs over the winter with new gasket kits as well as new 42 pilot jets instead of the stock 40s. I also moved the needle clip down one position to richen up the midrange as I had a hunch that it was running a bit lean there as well.

Today I finally got the carbs back on and the bike running/riding.

However, now I'm getting a pretty substantial stumble off idle that I never had before. The bike seems to start and idle easier and smoother, but the bike hesitates/misfires quite a lot as I crack it just a tad off idle.

Note: I did use Keyster pilot jets instead of real Kehin ones.

I started with the air screws one turn out. After going an extra half turn out (leaner), it helped with the stumble but didn't fix it.

Couple of questions:

1) is the Keyster quality so bad that it could be causing this off idle stumble / hesitation?

2) is there a chance the moving of the needle clip was unnecessary? If so would that cause an off idle stumble?

3) I started with the air screw one turn out and it stumbled a lot. Turned it out another half turn and it seemed to help, but it didn't fix it. Should I turn it out more? What's the max I can turn it before it's apparent I have a different issue?

Thanks again

[seanbarney41]

One change at a time.  You made 2. Now you don't know which did what

[flybox1]

Ok so I know it's been a while. I've been slowly rebuilding the carbs over the winter with new gasket kits as well as new 42 pilot jets instead of the stock 40s. I also moved the needle clip down one position to richen up the midrange as I had a hunch that it was running a bit lean there as well.   

Today I finally got the carbs back on and the bike running/riding.

However, now I'm getting a pretty substantial stumble off idle that I never had before. The bike seems to start and idle easier and smoother, but the bike hesitates/misfires quite a lot as I crack it just a tad off idle.

Note: I did use Keyster pilot jets instead of real Kehin ones.

I started with the air screws one turn out. After going an extra half turn out (leaner), it helped with the stumble but didn't fix it.

Couple of questions:

1) is the Keyster quality so bad that it could be causing this off idle stumble / hesitation?  Maybe.  You havent done your clear tube test to prove the fuel level is correct.  Until then, your changes are just guesses.  Get your fuel level correct, or visually verify it is correct, and then proceed with an idle plug chop.  The plug chop results will give you a better idea of which direction you need to go with your IMS.  Right now, you're just guessing.

2) is there a chance the moving of the needle clip was unnecessary? YES If so would that cause an off idle stumble?  You need to verify bowl fuel level first, then correct your idle A/F mixture, before moving to needle clip adjustments.

3) I started with the air screw one turn out and it stumbled a lot. Turned it out another half turn and it seemed to help, but it didn't fix it. Should I turn it out more? What's the max I can turn it before it's apparent I have a different issue? Adjusting it more is futile until you clear tube test your bowls, get the fuel levels correct, and then proceed with idle plug chops, and adjust your IMS as needed by the chop results.

Thanks again

[riffman12]

ok I'll try and get clear tube test done this week, then move the needle clip back to middle position so I don't have so many moving targets.

Then I'll idle plug chop and see how things look. Should I start with the air screws one turn out when I do the chop?

[flybox1]

Yes.  1 turn out is fine and needle clips back to their stock position.
This will give you a good baseline to start your fine tuning from.

Also: please repost your jet brand and sizes, and your intake and exhaust setup.
Thanks

[riffman12]

Yes.  1 turn out is fine and needle clips back to their stock position.
This will give you a good baseline to start your fine tuning from.

Also: please repost your jet brand and sizes, and your intake and exhaust setup.
Thanks

sure. Here's the full specs:

Bike: 1976 CB750F
Intake: Stock airbox with K&N filter
Exhaust: Carpy 4 into 1 with metal baffle installed
Main Jets: Kehin 115 (up from 105 stock)
Pilot Jets: Keyster 42 (up from 40 stock) - this change was due to the lean idle plug chop at the beginning of the thread
Needle is currently one position richer, but will be changing that back to stock shortly.

[flybox1]

Thanks for reposting the details.
The #42s in there are fine, but I would certainly feel more confident with OEM jets in there.
I do understand its an additional expense and more days waiting.

Go ahead and leave your needle clip where it is.  Lets see if we can get the idle mixture dialed in first.
If your plugs go rich when we move to the 1/2 throttle plug chop, we'll know for sure it needs to go up one notch.

[SKTP]

This is a good thread...thanks all

[jonda500]

Subscribed! I follow all carb threads - I hope one day to see someone successfully tune their idle mixture by using an idle plug chop followed by airscrew adjustments and then verified that it worked by another idle plug chop... ??
This thread linked below (read hondamans post) has everything I thought I knew about the airscrews thrown away!

http://forums.sohc4.net/index.php/topic,164119.25.html

John

[evinrude7]

riffman, the bike sounds pretty damn sweet.  maybe check that all carb and airbox boots are tight. 

[riffman12]

riffman, the bike sounds pretty damn sweet.  maybe check that all carb and airbox boots are tight.

thanks bud. It did run really nice, but my plugs weren't getting any color on them at idle. Also a ton of exhaust popping on lift off

I bought some little vaccuum attachments and will be doing a clear tube test this evening, then hopefully an idle plug chop

[riffman12]

ok just did the clear tube test to see where my floats are.

Carbs 1 and 2:




Carbs 3 and 4:




To me it looks like all are in the "4mm below mating surface" except for #4? Do you guys agree or do you think all of them are a bit low?

Edit: just opened up all 4 bowls and each float is dead on at 26mm. Maybe that #4 carb just wasn't perfectly level when i measured the tube

[seanbarney41]

yep, #4 is low...now you see that measuring floats doesn't always get you the fuel level you're after

[riffman12]

Best to have them floating and supported when measuring the fuel level (unless they were and that's a reference picture?).

Hmm I hadn't heard about floating them. Any pics of that?

[flybox1]

One thing that really jumps out at me.....Your slide heights are really different.
Just take a look at the picture of 1 & 2.

A proper bench sync is advised before these go back on the bike.

If you've done all the rest of the maintenance items, and you can get the bike to idle....vacuum sync them.

[riffman12]

One thing that really jumps out at me.....Your slide heights are really different.
Just take a look at the picture of 1 & 2.

A proper bench sync is advised before these go back on the bike.

If you've done all the rest of the maintenance items, and you can get the bike to idle....vacuum sync them.

hm I think it must just be the angle of the photo. I bench synced and vacuum synced last time I had them on the bike. Went back and checked them on the bench and they all look even.

What do you think about the float heights?

Sounds like I might need to suspend the carbs somehow. I don't have a vice but I'm wondering if I might be able to hang them somehow...

Oh and I did end up putting the needle clips back to stock position just so I'm not dealing with too many moving targets.

Thanks again for your help guys

[flybox1]

I cant say your wrong....Might be the angle....but that pic of #1&2 carb above, slides doesnt look close. 
Its easy to check bench sync choose a drill bit near 1/8"
turn idle set screw IN several turns to lift the slides
insert bit in #1 carb throat, and turn OUT the idle set screw till it barely pinches the bit.
Test fit the bit into 2, 3 & 4 for same 'pinch' as in #1.
If they are any different, you have work to do.

As stated above, #4 float height needs to be corrected to match the others.  1 thru 3 look good.

[Airborne 82nd]

One thing that really jumps out at me.....Your slide heights are really different.
Just take a look at the picture of 1 & 2.

A proper bench sync is advised before these go back on the bike.

If you've done all the rest of the maintenance items, and you can get the bike to idle....vacuum sync them.

hm I think it must just be the angle of the photo. I bench synced and vacuum synced last time I had them on the bike. Went back and checked them on the bench and they all look even.

What do you think about the float heights?

Sounds like I might need to suspend the carbs somehow. I don't have a vice but I'm wondering if I might be able to hang them somehow...

Oh and I did end up putting the needle clips back to stock position just so I'm not dealing with too many moving targets.

Thanks again for your help guys

A 2x4 and 20 mins. The board the carbs are on pivots to any angle. I have the jig C-clamped to the table.

[riffman12]

ok haven't had a ton of free time to work on the bike, but I was able to tackle the float heights today. I'm pretty happy with all 4 levels. Let me know what you guys think. This took a whole lot of trial and error 

note: I split the carb rack in half (testing 2 at a time) so they were a little easier to manage.

#1:




#2:




#3:




#4:




Does this look dandy? If so I'll go on to idle plug chop.

[flybox1]

#4 is good.   1, 2 & 3 id bump up a bit, but probably not too low to proceed with the idle plug chop.

[riffman12]

Quick update, got everything back on the bike yesterday. I did end up moving the needle clips back to the middle position to eliminate too many moving targets.

I warmed up the bike, put in fresh plugs and let the bike idle for ~8 minutes with the air screws one turn out. I didn't snap a pic but the plugs are still pretty clean. I'd say one shade slightly more tan than bright white.

Took the bike out for a ride and it was a whole lot better than last time. Hardly any hesitation off idle, just a little bit when I would get up to ~40mph and back the throttle down to idle.

Came back to the garage and backed the air screws an extra 1/4 turn out and it seems to be almost gone completely.

I'm just confused as to why my plugs are still so clean after an idle chop with these big pilot jets. Can anyone put a pic of what color they should be after a few mins of idling?

[flybox1]

I'm just confused as to why my plugs are still so clean after an idle chop with these big pilot jets. Can anyone put a pic of what color they should be after a few mins of idling?

A 1/4 (throttle position) plug chop to isolate the pilot jet mixture.
At idle, the IMS is helping meter the A/F mixture, and is not just the pilot jet.
Here's my 5 min idle plug chop.

[riffman12]

ok thanks. That's a bit darker than what I've got.

When you say 1/4 chop, I'm assuming I need to find a pretty long stretch of road and hold it at 1/4 throttle? Not in the garage?

[flybox1]

if still light, richen by 1/4 turn or so, and test again.
yes, the 1/4 throttle position chop should be under load, out on the road.  anything flat or slightly uphill, in any gear.