Running rich below 4000 RPM - 1974 CB750K4

[CR125Honda]

1974 CB750K4 - OEM carbs, exhaust, air filter & airbox

I've had a rich running engine (from idle to about 4000 rpm, runs clean above that) for a while now and haven't been able to pinpoint the issue.

Thought it might have been a ignition issue (original OEM coils that didn't pass the resistance checks) so I installed new Dyna S ignition system with new Dyna coils and Dyna wires. No change...

The springs on the OEM spark advance are a bit weak though. When checking the timing with a light, the timing is spot on a higher rpm but doesn't return all the way to "F" at low rpm. The advance moves freely, no binding. Thought about cutting off one coil on the spring to stretch it but haven't done that yet. New springs are "unobtainable".

Air bleed adjusting doesn't help.

I've cleaned the carbs a couple times (checking all passages, soaked the jets, correct float height, etc.).

Thinking the previous and original owner may have reamed out the pilot jets to clean them (the bike sat for a while before I purchased it) I just bought a set of Keyster carb kits.
Planning on rebuilding the carb with all the pieces in the kit (pilot, main, air bleed screw, jet needle and float valve needle and seat).

When I took the Jet needle out, I found it on it's highest setting - of the 5 clip settings on the needle, it was at the fourth one down (closest to the taper side of the needle).

Looked through all the manuals and couldn't find the OEM clip setting. It it the 4th clip position or is the middle setting correct?

Also read in another post that some are having issues with the Keyster Jet needle.  
Is that correct?
My old needles "look" good. i.e., no wear marks on the taper. Should I just reuse them?

Thanks in advance, Ken

[seestheday]

Someone else will need to chime in on your clip position setting (I don't believe it's adjustable on my stock DOHC carbs), but it sounds to me like you're getting too much gas at idle.  If it were me I'd stick in smaller idle jets and see if that fixed it.  Changing the clip may also help, but again, someone else will need to tell you.

[CR125Honda]

Thanks. Been doing a bunch of research on this in the last couple days...

What I've found out so far was...

Don't use the Keyster jets (main, pilot, needle and needle jet, etc.) Tolerances vary too much from OEM.
The float valve that comes in the kit is usable but the spring tension is stiffer than the OEM float needle.
Gaskets, o-rings, washers, etc - good

Main helpful piece of info that I found was that my OEM needles and needle jets are probably worn.
This makes good sense to me. I've tried many things (through carb cleanings, air screw adjustments, checking spark/timing, etc.) and I've never been able to take the "Blurble or Hesistation" out of this bike from idle to 3500 - 4000 rpm. Never really needed choke to start and the issue became more pronounced as the bike warmed up.

Never made sense to me that the main, slow or pilot jets could "wear out" just by gas passing through them. Now if someone enlarged them by reaming while cleaning, that's a different story.

The needle and needle jet do rub against each other, that they could wear makes sense.

What I understand now is that an excessive about of gas may be coming through the needle jet when the needle is lowered in the hole (i.e. idle to 1/4 "ish" throttle). Gas flow through the needle jet at low throttle settings should be minimal if any. The side of the needle is flat in that range and the taper doesn't come into play until the throttle is opened more.

Just an FYI - This is the long tapered needle on the bottom of the slide for those that might be confusing this with the float valve (sometimes referred to as a needle jet).

My needles are shiny in this area and there is a small ridge I can feel with my fingernail.
Recommendations are to only replace with Honda OEM parts (Needle and Needle Jet).

They are a bit spendy ($80 to $100 for the full set) but I've got to give it a shot.  Fingers crossed...

Ken

[robs750]

The only way to determine if the jets are worn/modified are to measure the ID's with guage pins and refer to a stock size chart.You could even possibly use a set of number drills and a mic.Now part of the needlejet and needle can show signs of wear just like you discribed.
I'm willing to bet that there could be a difference on the taper of the new needle compared to the stocker.

My son bought aftermarket carb kits for his DOHC .We found that the jets were not machined the same as the stock jets so we just reused them.

Another way to get some original jets may be to buy a set of used carbs.

I would try lowering the needles.This should lean out the lower/midrange of throttle.

[CR125Honda]

Update time..

A close inspection of the old needles showed some grooving near the top side of the needle (about a 1/4 below the E clip area, where the needle would be at idle or just above idle).

I removed all the needles from the slides and the needle jets from the carbs. Then performed a "wiggle" test of those two pieces in the area that looked shiny / grooved.

Seemed to me to be a fairly loose fit (some worse that others) so I bought 4 new Honda OEM needles and needle jet sets. I'm  pretty sure this gamble is going to pay off. According to my "wiggle test", the new needle & jets are a much tighter fit.

Next issue I need to clarify are the springs on the spark advance. I'll post that question in another thread.

Will update again once I'm able to get back at the project.