Topic: CB750K 77-78 Carb "Sputter" when wrap throttle

[Kawahonda]

Yes, I am aware of how the float orientation. If I didn't know that, I shouldn't have opened this up.

Floats were off on all carbs, not even close. I wish I had checked float height before I started dissembling to confirm. I will use the clear hose method.

Not a bad idea to check if theyre's any spacers with the needle. Is there a way to check this without having to mess with re syncing again? If there's no spacers for the needle, then I'd be pretty shocked because this bike ran good with pods! ("good" as in, not as good with an air-box, but didn't stumble, hesitate, and rev'ed freely). Is that even possible to slap pods on a K8 and have it run good while only making adjustments from the outside?

I had it hooked up to the fuel tank last night, but couldn't get the accelerator pump to squirt. I did have some bowl leaks. Removed the bowls and found a gasket that ripped. Have new bowl gaskets on the way. Double checked floats and made some additional adjustments. That's where I left the carbs as I now await for bowl gaskets.

TwoTired: According to the plugs, it doesn't seem like I should play with jet sizing...

Today, I will install new D7EA plugs, and check tappet clearances.

[TwoTired]

If I recall, the accel pump should squirt at least 4 inches into the carb throats when the throttle is twisted to actuate the pump.

If you still have the carbs apart, do verify the jets haven't been drilled larger.  The stamping is in decimal millimeters where 100 = 1.00MM. 40 + 0.40mm, etc.

According to the plugs, you are getting soot fouling which is a too rich mixture.  However, throttle position and choke position history is unknown.
I agree it is too soon to claim jet changes are required, yet.    You need to establish all other engine functions and adjustments are correct before mucking with jet sizes, adjustments, and establish a baseline to change from - to?

Have you not sync'd the carbs yet?

Cheers,

[TwoTired]

* Carbs are assembled, sans bowls. checking float level @ 14.5mm right now. How do you guys recommend setting the floats? Do you go with the lay the carbs upside down and let the float rest on the spring, then test, or do you hold the carbs at an angle to where the float tab barely touches the float, then test?

I do the mechanical measurement with carbs on their sides.  The needle should be in its seat and the spring pin not compressed.
This works fine with the stock float needles.  The aftermarket needles have a different spring pin tension.  So, the clear tube test is best for them.

* Valves were checked about 600 miles ago. I suppose I could check them again...

Economizing?

What if they are off and you adjust all the carbs to run well with tappets that are out of spec?

Cheers,

[Kawahonda]

* I took a very close look at the jets before installing them. They didn't appear to be drilled out.

* I will recheck valves today or tomorrow.

* I noticed when I removed a float that the float needle's rubber came off the float valve and was stuck in the bore. Is this a sign of the floats needing to be replaced? I will ask re-builder if he replaced float valves, or if those are stock. I'm also asking the re-builder if he remembered any washers (spacers) on the needles.

* The carbs were indeed synced a couple years ago.

[TwoTired]

* I took a very close look at the jets before installing them. They didn't appear to be drilled out.

I'm impressed you can visually determine fractions of a millimeter differences.  How did you learn that?  Could you teach me?

* I noticed when I removed a float that the float needle's rubber came off the float valve and was stuck in the bore. Is this a sign of the floats needing to be replaced? I will ask re-builder if he replaced float valves, or if those are stock. I'm also asking the re-builder if he remembered any washers (spacers) on the needles.

I'm not aware of any Honda supplied rubber tipped float valves.  But, if they shut off fuel inlet at the correct bowl level, and don't leak, they are serviceable.

* The carbs were indeed synced a couple years ago.

Years?

Be aware that the owner's manual specifies a routine tune-up every 3000 miles or 6 months, whichever comes first.  Also recommends a Honda dealer to do that.  Dealers have the vacuum sync tool in their shop along with other specific purpose tools.  I know they include a carb vacuum sync with their tune up work.

I never had a dealer tune up my bike.  But, have done the tune up checklist on every bike I have with some frequency.  Certainly whenever there is a run quality issue, that's the first thing I verify before taking any other corrective action.

The off color spark plug tip you showed, could easily have an out of sync cause.  Not the only reason.  But, one possibility.

One repair philosophy is to keep eliminating possibilities and accumulating items in the known good column, until the offensive symptom goes away.

Cheers,

[Kawahonda]

How would I go about measuring a hole that small? Get a drill bit set? Even this one doesn't appear to have bits small enough...

https://www.harborfreight.com/115-pc-high-speed-steel-drill-bit-set-with-index-61620.html

I could certainly check the sync of the carbs (again), but that will have to come later. I will need to borrow a sync gauge.

I would need a set with the range of #55-80.

[TwoTired]

I've been using an xacto micro drill set.

https://www.micromark.com/The-Rogers-Drill-Bit-Set-61-80-Set-of-20

There are less in this set:

https://www.ebay.com/itm/17pc-FINE-TINY-NUMBERED-61-80-MINI-JEWELERS-DRILL-BIT-SET-PIN-VISE-CHUCK-VICE-/172076133863?_trksid=p2385738.m4383.l4275.c10

Both are cheaper than a pin gauge set.  Which you can google.

Cheers,

[Kawahonda]

Alright, while my jeweler's bits and carb bowl gaskets are on order, I took care of the tappet adjustments and the spark plug installation.

The plugs don't lie. Here's what the clearances were set to:

#1 Intake: .0027, Exhaust: .0025
#2 Intake: .002,   Exhaust: .003
#3 Intake: .0023, Exhaust: .003
#4 Intake: .018,   Exhaust: .023

#1 and #4 required the most adjustments. Notice that #1 and #4 plugs were also the only carbon sooted plugs.

Well, that should make a pretty big difference. That may have been what's going all along...

So either 4,000 miles ago in late 2012, I did a rushed job, or 4,000 miles was enough to to need an adjustment, or a combination of both.

D7EAs installed and gapped to .028. They came gapped around .026.

I also ordered a Morgan CarbTune.

[Kawahonda]

Two, I received one of the drill sets in. Still waiting on the other one so I can test the mains.

I mic'd This bit it at about .38. It's able to go through the pilot jet barely (not using force). I think the pilots have been drilled out?

Will test main soon. Opinions? Sounds like for sure, I will need to order a 35. Highly likely the mains are drilled out too.

[TwoTired]

Was that 0.38 mm?

What was the jets stamped size?

Never force a metal object into the jet orifice. FYI.  Just use the shank, not the cutting end,

Did you verify/measure the shanks you used?

Cheers,

[Kawahonda]

Yes. This is the largest that is able to slip through the pilot hole.

It is a 35 jet. Doesn't sound like it's a 35 anymore....

My Matco calipers also measured the bit at .38.

[Kawahonda]

My other drill set came in.

The .98 bit slips in just fine.

The 1.132 bit does not.

I feel that the main jet has not been drilled, at least with two damned sets. I do not have a bit that is >1.0 and <1.13. The pilot jets apparently are larger than they should be, but the only way to confirm is to have a stock unmolested jet to measure. I guess it's forseeable that someone would leave the mains alone and drill out the pilots?

At any case, it sounds like I may need to place an order for 4 new .35 push-in jets, as well as snab some 1/4" clear tubing and get this thing going. Just looked at the prices at JetsRUs....WOW, $11 a pop. Starting to wonder if this is necessary? These are much cheaper: https://www.ebay.com/itm/Replica-Keihin-Slow-Jet-Honda-XL350-CB550K-CB750-1977-1978-35/172403650989?hash=item28240ef1ad:g:-FMAAOSwImRYI2gX&vxp=mtr

I heard terrible things about the keyster slow jets, though.

My inclination is to button it all back up as-is.

[Kawahonda]

About to button the bowls back on.

The floats are set at 14.5mm. It does NOT line up exactly with the pilot jets as I’ve read in other threads.

[TwoTired]

float height measurements are made without depressing the float valve spring pins.

Cheers,

[Kawahonda]

That’s correct. I’m just noting that they are not the same height as pilot jets at 14.5.mm.

[Kawahonda]

Bench testing carbs now. No leaks, which is good. Had a leak on #3, but after re-seating the gasket/bowl and testing the float, all seems good.

The only problem now is that #3, the squirt does squirt out as cleanly as the rest. I took a video to see what was going on. Looks like the squirt is hitting the carb exit and "splashing" out, not quite making it's way out in a clean stream, so maybe a pressure issue?

I checked the float balls, and the mickey mouse ears...not a problem it seems with the that area.

The rest seem to squirt just fine. I want to say that #2 or #1 (cannot remember) squirt a little further than the rest, but 1, 2, and 4 all squirt at least a foot and produce a thin stream.

I pulled the connection between 2 and 3 and saw that there were restrictors inside and that they were clear.

After watching the video, that's probably why I thought #3 was leaking...you can tell that the fuel would easily run down the bowl at that point after a few squirts.

Now what? One more issue to solve!

[TwoTired]

If I understand, you checked for presence of restrictors, but not if they were placed in the correct position.  I don't think the orifice sizes are the same for each of the three.  To maintain pressure, the diameters should decrease with distance.

Other than that, the most obvious it that you have a partial blockage at the "dribbling" station squirter.  And, since you aren't the original owner, you need to check if the squirter post exit orifice has been altered.  Cleaning those is not easy and mechanical aggression can cause damage.  If the exit hole is made larger, it won't squirt as far.

I sure miss the carb cleaners that had methylene Chloride in them.  They were chemically aggressive on organics but harmless to metals.

Cheers,

[Kawahonda]

Ah! Thanks TwoTired, I will definitely check to see if the restrictors are placed correctly. I'll remove that stuff and do some measuring tonight.

I will also get a clear image of the squirter outlet to see if anything obvious is amiss.

FYI, I talked to the rebuilder and he wasn't aware that the restrictor tubes differed in ID. This is likely the culprit and an easy fix. Will keep you guys updated.

[Kawahonda]

My findings. Please advise.

2-1 appear to have same exact size restrictors as 3-4.

Only 2-3 are longer, probably because the hose of 2-3 is the longest...

[TwoTired]

What are the orifice sizes?

[Kawahonda]

No difference in I.D. in any of the restrictors.

[TwoTired]

That so?
Well, then that leaves only the discharge nozzle being different than the others.

I really thought the inline restrictors would be different.  What are the chances they were changed from original?

Cheers,

[Kawahonda]

Idk. I searched my ass off last night and found no reference that inline restrictors have different I.D. The only thing I found was that the 2-3 connection restrictors are a bit longer. Best I can guess, this is bone stock.

I sprayed some carb cleaner out the #3 squirt nozzle. I'll do that to the other squirt nozzles for good measure. I'll re-hook everything up and see if that made any impact.

Is your your opinion that all squirters should spray the exact same distance and amount, no excuses? How "perfect" should I expect?

[robvangulik]

  I don't think the orifice sizes are the same for each of the three.  To maintain pressure, the diameters should decrease with distance.

That isn't logical, fuel is a noncompressible fluid, when you apply pressure to the system that pressure will be equal throughout that system, and when there are 2, 4, or 10 EQUAL openings, fuel will squirt in the same amount and over the same distance through each opening. Enlarge 1 or 2 openings and pressure will drop in the whole system and the larger openings will squirt more fuel over a smaller distance....

[Kawahonda]

Got it all hooked back up and same results.

2 squirts the furthest. Very healthy squirt.
1 and 4 are a few inches from 2's squirt, but squirt the same healthy distance.
3's squirt still doesn't make it very far, just splashes around at the end of the opening.

Should I attempt to stick something in #3 squirter nozzle?

One thing, is I'm using my thumb. I will try to use a screw diver to get more torque and retest.

Edit: Nope. Thinking at this point it's time to run something through the accelerator jet. What size guitar string for that?