Author Topic: CB750 carb issues - rich at stock setting and poor idle (Read 8171 times)

zabzob · « **on:** July 23, 2015, 12:36:30 AM »

I am having some real trouble getting the carbs dialled in on my 75 750k. My bike is stock except for Mac 4-2 slip on exhaust, which I believe is not supposed to require rejetting. Ignition is Prestolite.

Issue 1:

Originally got the bike in 2011. Hadn't run in about 14 years, so the carbs were good and gummed up. Cleaned them really well (pretty much twice) before ever starting the bike. I used the Keyster rebuild kits for this, replacing all the jets and the needle. Bike ran okay for a while, then started fouling plugs quite regularly. Regularly as in I have about $120 worth of fouled plugs in a drawer in the garage. This year I cleaned the carbs again hoping to get to the root of the problem. Lucky for me I bagged all the original Keihin jets and needles, so I was able to go back to those. I also noticed the needles weren't cilpped at the middle position. I moved it to the middle, thinking this was the standard position for all CB750s. Float height was set with the tool from vintagecb750.com.

Ran the bike for a while and everything seemed okay. Bike felt on the hot side though. Last night I randomly came across a post in this forum stating that the clip position should be second from the bottom, a richer mixture. Not wanting to overheat my engine I switched to this clip position. Took it out on the road and notice the performance drop immediately. Pulled the plugs, and they were pretty dark. Decided to do a carb sync this evening. After warming up the bike I swapped out the plugs for fresh ones. Spent a good 40 minutes with it idling in the driveway, fiddling with it. When I was satisfied with the sync I pulled the plugs. White to light tan, so the idle circuit is a little on the lean side. Took it out on the road and again felt that performance drop... bogging down at 2000 rpm or so, feels like maybe 1/4 to 1/3 throttle. Pulled the plugs and they were black again.

This leads me to believe that for whatever reason, second from the bottom is not the right clip position for my bike. I don't think float height is off to the rich side since the idle circuit is a bit lean at 1 turn out on the air screws. Anybody have an opinion on that?

Issue 2:

I get a lot of cam chain noise at idle. Ignition timing dead on with a timing light. Cam chain was replaced when I rebuilt the head a couple of weeks ago. Valve tappets adjusted properly. Cam chain tensioned properly. A timing light check on all 4 ignition wires shows some misfiring in cylinder 4 only. I will have to double check that to be sure. Tested the coils and 4.6/24k (with caps on) for 1-4, and 5.2/25k for 2-3. I'm tempted to just replace the coils, because I don't know what else this could be.

harisuluv · « **Reply #1 on:** July 23, 2015, 02:44:33 AM »

1. Stop trying to go against Honda engineers and go with the stock spec. The needle clip is 4th from the top. Besides, needle is mostly 1/4 to 3/4 and you've already said your problem is 1/4 to 1/3. You have yourself pinpointed the range you have an issue but are addressing the wrong thing. Just the wrong way to go about it, ie you changed the needle, that didn't work, so what if i adjust by changing the float height that Honda found works best? That didn't work, their slow jet must need to be bigger/smaller. Eventually you chase the wrong cause of the issue and end up with franken settings. Stick with the stock settings. You are correct you don't need any changes really with a 4-2 only.

2. You said you went back to the keihin needles and jets, did you go back to ALL the brass? (excluding float valve/seat).

3. You are having a problem running too rich. There is absolutely no mention of your current setting of the mixture screw. Have you not adjusted this at all?

4. No mention of testing for vacuum leaks.

5. How is your air filter? Paper or foam, did you forget to oil it?

6. What sizes are your jets? Also just because they were the jets that were installed when you got them doesn't mean that they are Kei Hin parts. What is written on the jets, do they have the Kei Hin Logo?

7. 40 minutes fiddling with it in your driveway is an awful long time to not be moving, hope you had a fan on it..

If it were me I would have probably started off checking for vacuum leaks and then just adjust the mixture screw. On your bike: IN ---> Richer, OUT ---> Leaner. This is also assuming that your plug assessment is correct. If you want to be sure, you should post some pictures. Including the carbs on or off the bike.

zabzob · « **Reply #2 on:** July 23, 2015, 09:26:53 AM »

Thanks for the help harisuluv.

1. I'm still at the stock setting for the clip right now. I'm no carb expert, I admit! FWIW, this was my reasoning on thinking it was the clip: I thought air filter would be fouling across all throttle positions, as would weak spark. Idle is lean, so it's not fouling across all ranges, but fouls with some throttle. This page http://www.salocal.com/sohc/tech/carb/asmpg_mgs/jetgraph.htm led me to thinking that it's the clip, as the 1/4 to 1/3 throttle is right where clip position makes the most difference. I'd be happy to find out the problem is elsewhere.

2. They are all Keihin stamped parts, and the jet needle has the correct numbers. The float valve and seat are Keyster.

3. Mixture screw is one turn out on all carbs.

4. Haven't tested for vacuum leaks.

5. Air filter isn't super old, but it's not brand new either. Just the stock paper filter.

6. I want to say 105/40, but I will double check this. They are all stamped Keihin.

7. Had a fan on it the whole time, and it was a cool night. It was off for a few minutes a couple of times as well.

I'll check for vacuum leaks today. The mixture screws are on the lean side, I'm pretty sure, based on plug color and some mild popping on deceleration.

evanphi · « **Reply #3 on:** July 23, 2015, 10:14:35 AM »

105 mains are too lean.

110 should be stock.

Noise at idle is normal for these bikes, even with everything adjusted to spec. Does the noise go away when you pull in your clutch? I've ever only had ONE carb sync and 3K tuneup result in a perfectly silent, rattle-free idle. I can get it close now, but it isn't perfect like it was one time. I got it on video, though!

Something to check for your misfiring spark are your condensors. Get TEC branded ones. Diachi are crap and may start to fail.

When you cleaned your carbs, did you also clean the small holes in the emulsion tubes for the main jet/needle?

Also listen to Harisuluv... he's the carb genius around here.

zabzob · « **Reply #4 on:** July 23, 2015, 01:42:47 PM »

I'll add to this that the voltage on my quite freshly charged battery is now 12.34. I know the system is charging as I've tested it at various revs as described elsewhere. AGM battery is 4 years old and has been discharged a couple of times over winter. Think I should replace it to rule that out as a cause?

harisuluv · « **Reply #5 on:** July 23, 2015, 01:48:23 PM »

How about some pictures?

flybox1 · « **Reply #6 on:** July 23, 2015, 01:51:54 PM »

Quote from: zabzob on July 23, 2015, 01:42:47 PM

I'll add to this that the voltage on my quite freshly charged battery is now 12.34. I know the system is charging as I've tested it at various revs as described elsewhere. AGM battery is 4 years old and has been discharged a couple of times over winter. Think I should replace it to rule that out as a cause?

I would consider a replacement if you only get 12.34v after a charge.
You should be 12.7v +

zabzob · « **Reply #7 on:** July 25, 2015, 06:01:07 PM »

Okay, back with some pics and an update. I recharged my battery, and after a short ride read over 13 volts at the terminals, with the bike off. So, is that battery good?

Took it out for another ride and it still bogs down at the off idle range of the throttle.

Attached are pics of the plugs, in order 1 - 4, and the carb model and jets installed.

zabzob · « **Reply #8 on:** July 25, 2015, 06:06:11 PM »

Running a 110 main jet, and this says it should be 105:

http://www.hondachopper.com/garage/carb_specs/carb_specs.html

However this says 120, which from other posts I've seen here is totally wrong for a '75:

http://www.salocal.com/sohc/tech/carb/asmpg_mgs/jetgraph.htm

Tested for vacuum leaks with WD-40 today, all around both joints for each of the 8 rubbers. No change in RPM.

@evanphi: I'm using the Prestolite electronic ignition, so no condensors to worry about. The holes in the jets were cleaned with guitar strings and blown through.

dusterdude · « **Reply #9 on:** July 26, 2015, 04:46:46 AM »

The 120 was stock for the k1,not sure about the 75 model

zabzob · « **Reply #10 on:** July 26, 2015, 01:43:33 PM »

@dusterdude Based on more searching hear and the CSMNL parts fiche, it looks to me like #105 is indeed stock for this year of bike.

I'm working on tracking down #105 jets genuine Keihin jets. Keihin part number is 99101-393 according to Jetsrus. I could order from them, but I'm leaving on a road trip August 5th and I don't know if they'll make it to me in time (I'm on the west coast of Canada.) Z1 has them and is willing to do express shipping, but $1.75 seems on the cheap side for these, so I wonder if they are genuine. I will also call my local dealership to see how fast they can get them in for me.

If anybody has a set kicking around or knows a faster way to get them, feel free to chime in.

About issue 2, the poor idle. After some searching here it seems that the tach can be a good 200-300 rpm off. So, maybe I'm looking at 1050 on my tach and chasing an unobtainable 750 rpm idle. If I'm up at around 1200 on the tach everything seems smoother, so I'll set the idle stop screw there.

evanphi · « **Reply #11 on:** July 27, 2015, 06:41:43 AM »

Quote from: zabzob on July 26, 2015, 01:43:33 PM

About issue 2, the poor idle. After some searching here it seems that the tach can be a good 200-300 rpm off. So, maybe I'm looking at 1050 on my tach and chasing an unobtainable 750 rpm idle. If I'm up at around 1200 on the tach everything seems smoother, so I'll set the idle stop screw there.

Very good deduction. I have a digital tach on mine and seems to idle best around 1100. You could always borrow a digital tachometer from someone to verify. You can get simple hand-held digital tachometers. Very simple to use.

http://www.amazon.com/CyberTech-Digital-Photo-Tachometer-Contact/dp/B001N4QY66

zabzob · « **Reply #12 on:** July 27, 2015, 10:27:44 AM »

@evanphi Yes, I'd definitely like to get a digital tach on it so I can figure out how far off I am for future tuning.

Got some #105 jets on order from the local Honda dealer. Apparently genuine Keihin, and should be here by the end of the week. Words cannot express how excited I am at the thought of going more than 200km without fouling a plug.

zabzob · « **Reply #13 on:** July 31, 2015, 03:55:39 PM »

New genuine Keihin #105s are here but not yet in the bike. After talking to some people it sounds like the needle is really at fault, which would explain why going up one clip position helped so much. Bike is at 34,000 miles, so I guess it is possible for them to be worn out. New needles and holders are on order from Yamiya and will be getting here Monday or Tuesday. I will try with the #105s and the new needles to see if that fixes it. If it doesn't, I don't know what else it could be other than my bike is an anomaly and requires the clip position one step leaner.

zabzob · « **Reply #14 on:** August 05, 2015, 09:25:57 PM »

New genuine Keihin 105s, new needle and holder set from Yamiya set at stock 2nd from bottom clip, new battery, new air filter, new plugs. Ride for 30 minutes. Result: blackish plugs.

Next thought is... advance the timing slightly and hope the extra heat helps?

HondaMan · « **Reply #15 on:** August 05, 2015, 10:17:54 PM »

Quote from: evanphi on July 23, 2015, 10:14:35 AM

105 mains are too lean.

110 should be stock.

Noise at idle is normal for these bikes, even with everything adjusted to spec. Does the noise go away when you pull in your clutch? I've ever only had ONE carb sync and 3K tuneup result in a perfectly silent, rattle-free idle. I can get it close now, but it isn't perfect like it was one time. I got it on video, though!

Something to check for your misfiring spark are your condensors. Get TEC branded ones. Diachi are crap and may start to fail.

When you cleaned your carbs, did you also clean the small holes in the emulsion tubes for the main jet/needle?

Also listen to Harisuluv... he's the carb genius around here.

The K4/K5/K6 mainjet in the 657B carbs is #105 size. The 7A carbs found on some K5 and a few K6 use #110.

One thing not mentioned: have you also pulled out the emulsifier tubes and made sure the tiny holes are clean like new? They are about .033" to .035" diameter in the upper holes, .025" diameter in the lower 2 holes. With ethanol-laced fuels, if these holes get smaller the plugs will foul pretty readily.

Also: if the rubber hoses between the carbs and head are hard (very likely) they can easily leak vacuum. This causes rich mixtures, due to the way pulse-feed carbs work. (The "why" is detailed in my book, Carb section.) So, make sure the clamps are in good shape, and tight.

HondaMan · « **Reply #16 on:** August 05, 2015, 10:24:52 PM »

Quote from: dusterdude on July 26, 2015, 04:46:46 AM

The 120 was stock for the k1,not sure about the 75 model

The K0 and early K1 had #120 jets, which proved too rich, especially if the bikes were run on premium gas and ridden in city traffic. This became #115 by February-March 1971 on the K1 in the 657A carbs (needles still in clip position#3). It then went to 110 on the later K1, and the needles then went to 4th notch from the top in the 657A carbs. The early 657B carbs with the holes in the tips of the idle air screws came with #110 mainjets, clip in position 4, with later "New Factory" K2 bikes usually having #105 mains, regardless of 657A or B series carbs. The 657B carbs with solid-tipped idle air screws had mainjets of #105 size, starting in the K3 sometime. By the K4, the mainjet stayed at #105 size until the 7A carbs mixed in the #110 sometimes: if the needle was the 27201 it had #105 jets, but if 271301, it had #110 mains.

The PD carbs have different numbers, starting in the F1 bikes.

zabzob · « **Reply #17 on:** August 05, 2015, 10:58:05 PM »

Thanks for all the great information Hondaman!

I did the WD-40 spray test around the carb to head rubbers and didn't notice any RPM change, but that test is not fool proof. Those are on the list to replace in the future even if they aren't leaking, to at least save my hands when I have to push those carbs back on.

I believe the emulsifiers were clean... though, I always doubt myself when questioned. Not sure if I'll have time to check them before I leave tomorrow. If not in the garage, I think I can get the emulsifiers out without removing the carbs from the bike so I can check them when I've stopped for the night.

AshimotoK0 · « **Reply #18 on:** August 06, 2015, 11:22:40 PM »

I have four NOS needle kits genuine Honda listed for the German ect market CB750 (K0 and sandcast). The needles in the bags are 271301 type. Anyone know if these needles are correct for a later CB750 model e.g K2?

PeWe · « **Reply #19 on:** August 06, 2015, 11:45:34 PM »

My K6-76 had 40 pilots, 105 mains and needle clip in the middle (3:rd).
Mains to 115-120 with Russ Collins 4-1.

When engine got 836, ported head w bigger IN valves, hotter cam and Pods: Needles one step richer (4:th), main jets 147.5 that was not too rich! Mostly touring with 2 persons and luggage like photo below.

zabzob · « **Reply #20 on:** August 07, 2015, 10:48:13 PM »

PeWe, you were running stock components with clip in the middle, with no ill effects?

Update:

Decided to do a baptism by fire of this bike, with a road trip from Victoria, British Columba, to Saskatoon, Saskatchewan. I'm in Valemount BC right now, and here are how the plugs are looking after a 760 km ride. Pulled 1, 3, and 4, and they all looked about the same. Pics of 1 and 3 attached. How am I looking? Is that grey a good colour, or am I looking a little lean?

PeWe · « **Reply #21 on:** August 08, 2015, 12:07:12 AM »

Yes, stock and needle clip in the middle.
Your problem reminds me about the problems I had in the beginning of my restoration 2013-2014.

Ignition
Problem: Full advance way too early, at around 1500 rpm. Detected with a timing lamp.
Reason: Sloppy adv unit springs .
Action: Tighten both springs by cutting them 1/2 coil turn. Full advance now at 2600-2700 rpm after I set ignition spot on 'F' at idle

Fuel
Problem: Bad idle with black smoke from exhaust and fouled plugs
Reason: Floading carbs. Measure float heights is not enough to ensure corect setting. Level was 0-2mm OVER float bowl gasket.
Action: Verify the reality with clear tube and adjust so the fuel level is 2-4 mm UNDER the float bowl gasket.

Photo show corrected float. Carbs Mikuni VM29.

Ignition wires and spark plug caps must be good to ensure good ignition. I had problem with a new Dyna supressed wire (graphite) with intermittent function.

flybox1 · « **Reply #22 on:** August 08, 2015, 08:12:23 AM »

Yes, youre lean. Be careful you dont hole a piston.

These were my plugs from my ride yesterday...

HondaMan · « **Reply #23 on:** August 09, 2015, 12:24:14 PM »

Quote from: AshimotoK0 on August 06, 2015, 11:22:40 PM

I have four NOS needle kits genuine Honda listed for the German ect market CB750 (K0 and sandcast). The needles in the bags are 271301 type. Anyone know if these needles are correct for a later CB750 model e.g K2?

Those are for the F0/F1 bikes with the #7A carbs. Some of the K6 also got there carbs. They are too lean for the early carbs, and will cause a flat spot between 1500-3500 RPM that's very annoying (much like when Keyster needles get installed).

HondaMan · « **Reply #24 on:** August 09, 2015, 12:29:07 PM »

Quote from: zabzob on August 07, 2015, 10:48:13 PM

PeWe, you were running stock components with clip in the middle, with no ill effects?

Update:

Decided to do a baptism by fire of this bike, with a road trip from Victoria, British Columba, to Saskatoon, Saskatchewan. I'm in Valemount BC right now, and here are how the plugs are looking after a 760 km ride. Pulled 1, 3, and 4, and they all looked about the same. Pics of 1 and 3 attached. How am I looking? Is that grey a good colour, or am I looking a little lean?

They look a little lean if you are running gas WITHOUT ethanol. If it has ethanol, they look close to correct.
You can experiment by using premium for a tank, then midgrade for a tank. Compare the plug colors after each tank. If the premium makes them a little darker, you're very close to correct in that RPM range where you mostly run. Note: at hiway speeds, these bikes run lean, by design. If you are worried about overheating during this test, add about 1 ounce of oil to the gas, especially if it has ethanol, to cool things off a bit while testing. (I usually do this anyway, as ethanol is a solvent that washers the oil off the valves and piston rings.)

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Author Topic: CB750 carb issues - rich at stock setting and poor idle (Read 8171 times)