Author Topic: '79 CB750 (Read 5067 times)

flybox1 · « **on:** July 06, 2015, 07:53:38 AM »

Helping a buddy out.
He brought it to me after spending too much at the mechanics attempting to tune out a rich/plug fouling condition.

In my experience with the bike for the last 3 days, it fouls all 4 plugs.
Its other symptom is that it bogs down while accelerating from 60mph.

Here's what I have verified/checked:
Bike idles well at 1500rpm. No popping. If i try to get it to idle lower, it chugs out.
The engine is quiet, otherwise, so i passed on the valve shim/gap verification.
New Honda paper air filter/original airbox, unobstructed.
Baffled 4 into 1 Kerker (I dont think this was stock). Its really loud...lol
Plug caps all are 4.5-6.0k ohms. D8EA plugs. Good spark on all.
Carbs cleaned. Keihin VBs. (VB42 maybe?)
Keihin stock jets: 68 primary, 102 secondary and emulsion tubes are clean and clear.
Fuel cutoff diaphragm clean and operational
Accel pump clean and operational
Fuel level confirmed at 4mm+/-. Fuel screws at 1.50 turns out.
Carbs vacuum sync'd.
I do have to say that the airbox/boots on this bike scuk b@lls.
I'm thankful every day for the ease of my SOHC CB750 airbox.

I will be pulling the carbs again over the next few days. The choke lever/linkage is out of whack. It does nothing. So I start the bike with a few twists to prime the carbs, and then feather the throttle to keep it lit until warm.
I'll ride around the neighborhood, 2/3/4th gear, 4-6000rpm, and the longer i ride it the worse it sounds, and sure enough, the plugs look like the circled one pictured below.

I appreciate the assistance so far from eigenvector (PM's), but I'm stumped. Would the Kerker be more restrictive than the original 4:4 pipes, causing the stock jetting to be overly rich?

Thanks for any input, guys.....
Rob

flybox1 · « **Reply #1 on:** July 06, 2015, 08:14:12 PM »

seanbarney41 · « **Reply #2 on:** July 06, 2015, 08:25:36 PM »

I am no expert on those carbs but have worked on a couple racks of vb's with a 50/50 success rate...I am thinking cursing and swearing is the general consensus....might double check/further research correct settings as I believe that looks like a K, not an F...are you sure the ignition is all good?, I am thinking you will need FSM and a multi-meter to check the various mysterious ignition boxes

KayOne · « **Reply #3 on:** July 06, 2015, 09:32:44 PM »

The VB carbs have problems with plugged pilot jets. Some are screw in, some are pressed in and need to be pulled with a small easy out. My 79 750 F carbs looked very clean, but 2 of the 4 pilots were plugged, resulting in the need for high idle settings and crappy performance These carbs are also very sensitive to float height setting.

The ignitors are also a weak spot, one of the 2 on the F worked intermittently, causing major trouble shooting headaches. This could explain the degradation on performance as it warms up. I finally diagnosed the ignitors by using an inductive timing light (you may see an intermittent strobe flash). You can also feel the ignitor cases and see if there is a difference in case temperatures. My bad one got very warm.

also verify the timing advance.

Verify the main jets and secondary jets are not reversed, this would make the bike very rich in the low/mid and lean up top....don't ask me how I know

Sorry for the Novel

flybox1 · « **Reply #4 on:** July 06, 2015, 11:06:06 PM »

Guys....I greatly appreciate the info. Thx.
I'll have more time in the next day or two to get into the carbs again.
the 102 jets were in the emulsion tubes, and the 68's were in their own carb post.
They were spotless. No corrosion. All holes clear.
More tomorrow.....

Stev-o · « **Reply #5 on:** July 07, 2015, 06:31:36 AM »

The Kerker would not be more restrictive than stock. The Kerkers I've had were not super loud, is the baffle removed?

flybox1 · « **Reply #6 on:** July 07, 2015, 08:08:15 AM »

Quote from: Stev-o on July 07, 2015, 06:31:36 AM

The Kerker would not be more restrictive than stock. The Kerkers I've had were not super loud, is the baffle removed?

i'd have to take a closer look. It does have a holed diffuser of some sort at the end. I'll post a picture later today.
If the baffle had been removed, it would certainly make it more lean. Yes....its loud

cb650 · « **Reply #7 on:** July 07, 2015, 01:09:18 PM »

What are stock jets? Looks like a anniversary edition too.

flybox1 · « **Reply #8 on:** July 07, 2015, 01:58:25 PM »

Quote from: cb650 on July 07, 2015, 01:09:18 PM

What are stock jets? Looks like a anniversary edition too.

Quote from: flybox1 on July 06, 2015, 07:53:38 AM

Keihin stock jets: 68 primary, 102 secondary and emulsion tubes are clean and clear.

flybox1 · « **Reply #9 on:** July 08, 2015, 09:13:33 AM »

OK guys, I made some headway last night.
carbs off. under the primary main jet(#68) there is a hidden emulsion tube. I didnt see it before.
Similar to if our pilot jets are cut in half. The tube with holes is a separate piece screw'd in first, then the jet goes over it.
Anyway, its deep in there, but machined on the end to accept a screwdriver.
I twisted them all out and cleaned them up. They were actually pretty clean.
Another step mentioned for these carbs is to blow out the slow jet passageway.
It requires building up one end of spray can straw with tape, jamming it in the primary jet hole, covering the secondary jet hole, and carb cleaner spraying it clean. This forces the cleaner in one direction. I got a bunch of carb bowl gasket material out. (I can thank the professional mechanic for this

)
Double-checked the float heights. (15.5mm) some needed further adjustment.
Cleaned off the old plugs. Fired it up. Took the kids for rides around the block.
Now..i'm lean on all 4. None of the plugs had black sooty deposits....which is OK. At least they are all the same, and I have some adjustment with the fuel screw.
I need to pick up a hand full of fresh plugs and do proper chops, and work on tuning out the lean-ness.
More evidence its lean....once idling, choke kills it instantly, and the bike wont start with any choke.
I'll report more in a few days.
Rob

Head · « **Reply #10 on:** July 08, 2015, 12:44:42 PM »

Have you joined cb1100f.net, 750f-900f-1100f are all covered there. Very informative site!
I cleaned my DOHC carbs and had some issues till I took a E string wire and rubbed it on a diamond stone to reduce the diameter a bit. I then held it with a pair of Hemostats. Placed it in the slow jet holes on 1 an 4 and saw the depth that it went in and marked it with a piece of tape. On 2 an 3 carbs it only went in a bout 3/4 of my measurement, fiddled it in an out with a little pressure and broke thru some corrosion and resprayed more cleaner thru them. made all the difference in the world. Before the E string spraying into the slow
jets and the flow seemed very close to being equal from the 4 carbs.

faux fiddy · « **Reply #11 on:** July 09, 2015, 07:00:30 PM »

Quote from: cb650 on July 07, 2015, 01:09:18 PM

What are stock jets? Looks like a anniversary edition too.

That's what I was thinking. Would that make it a '78?

They are the ones people call more 'collectable.'

flybox1 · « **Reply #12 on:** July 09, 2015, 10:17:50 PM »

No its a 79 750K
10year anniversary of the 1969 750K.
The 78's were K8's...or F3's

Plugs 1,3&4 are perfect brown. I have a wonky float on #2 making the fuel level really low.
It's catching on something. Thinking the tang has a wear spot, the float pin has a burr or the valve spring is bad. With a clear tube hooked up, I can get the float to release, but upon bowl drain and refill, it hangs up again. My plug is still bone white.
Gotta go in one more time

NobleHops · « **Reply #13 on:** July 09, 2015, 11:13:26 PM »

You've got this bike on the run now!

flybox1 · « **Reply #14 on:** July 10, 2015, 07:16:31 AM »

Quote from: NobleHops on July 09, 2015, 11:13:26 PM

You've got this bike on the run now!

haha...yeah!

Hoping to finish this evening.
here's a idle chop plug picture. Probably my third mixture screw adjustment of about 5 adjustments last night (fan on)
#1 is just starting to show color. 3&4 are much closer. #2 stayed just like that
(I eventually ran a q-tip down the plug holes to get the PO's copper paste lumpies out)

flybox1 · « **Reply #15 on:** July 10, 2015, 04:07:53 PM »

Carb 2 offending parts.
Float tang indention. Float valve seat indention ring on the rubber tip.

Head · « **Reply #16 on:** July 10, 2015, 07:51:51 PM »

I cleaned and adjusted the carbs on my DOHC, I was having a similar condition, My carbs had just been completely disassembled, cleaned, reassembled and adjusted. Pulling the carbs for the 3rd time, checking why there is this lean problem is when I discovered by looking closely at the slow jet holes that the E string was not going into the slow jet holes as much on #2#3 as the depth of #1#4,,maybe 3/8" difference,, I fiddled with the string and felt some corrosion break up,the string dropped down like on #1#4 and blew out some dust, resprayed carb cleaner thru the passages, didn't change much in carb spray flow.
But starting and running was similar of my bike that you stated about yours. After getting the slow air passages clean, the motor really woke up! A lot work but its what it needed.
On my float valves I used 2 Qtips with some Semi Chrome and polished the bore and the needles until they gleamed #4 was having a float issue. Ever thing else checked out and this made it behave.

flybox1 · « **Reply #17 on:** July 13, 2015, 09:05:07 AM »

Got this bike out on the road over the weekend.
Carb internals and settings are perfect. Everything is stock except the 4:1 kerker.
Under 5500rpm, this bike hauls. crisp response, no popping. Plugs a perfect medium brown.
Above 5500rpm, it stutters and coughs. Plugs dry black carbon fouled.
Stock jetting, and intake. if anything, i should be lean at WOT because of the kerker.
The 750C board has me going thru electrical and ignition.

cb650 · « **Reply #18 on:** July 13, 2015, 09:53:44 AM »

Have u tried with out the air filter cover? It works on some of the 650's with cv's

flybox1 · « **Reply #19 on:** July 13, 2015, 10:24:28 AM »

After i get thru the ignition tests, and necessary fixes done, i will certainly try.

cb650 · « **Reply #20 on:** July 13, 2015, 02:16:31 PM »

If the pulse gen gap is too big it will miss.

tennesseebreeze · « **Reply #21 on:** July 30, 2015, 06:59:59 AM »

I'm working on the same carbs- '79 cb750F. I'm here to observe, but also ask- where the hell can I get the tiny washers that go with the pilot screws?

flybox1 · « **Reply #22 on:** July 30, 2015, 09:38:47 AM »

Its part of 16016-410-601 SCREW SET A
http://www.partzilla.com/parts/detail/honda/HP-16016-410-601.html
http://www.cmsnl.com/products/16016463601-screw-set_16016410601/#.VbpS8KRViko

or...you can get a caliper on one from another carb, and scour the internet for a match.
places like tacomascrew, fastenal or grainger would be where i'd look if i had the specs.

tennesseebreeze · « **Reply #23 on:** July 30, 2015, 09:55:41 AM »

Thanks Flybox. I just found that two out of four of my main emulsion tubes were stripped of their screwdriver slots by a p.o.

I guess they are stuck in there pretty good. I found replacements here:
http://www.justcarbs.com/parts/partsshop_v3.html
Other source for individual carb parts here:
http://4into1.com/carburetor-parts/

flybox1 · « **Reply #24 on:** July 30, 2015, 10:09:55 AM »

Quote from: tennesseebreeze on July 30, 2015, 09:55:41 AM

Thanks Flybox. I just found that two out of four of my main emulsion tubes were stripped of their screwdriver slots by a p.o. I guess they are stuck in there pretty good. I found replacements here:
http://www.justcarbs.com/parts/partsshop_v3.html
Other source for individual carb parts here:
http://4into1.com/carburetor-parts/

thanks,
your 'main' emulsion tubes are actually main jets, screwed into emulsion tubes. separate and clean.

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Author Topic: '79 CB750 (Read 5067 times)