Topic: CB350F Pilot Jet Sizing - Off Idle Response Issue

[Gibbit]

So my daughter just started riding and got me back into this project so I can ride with her.  Sorry for the long delay, bike sat for another year, but here's the latest.

I poured through the responses and started attacking every suggestion one by one.  I still have the same problem, but the bike overall is running better than ever after making a few improvements.

Done recently:
Replaced rubber Carb-to-Airbox boots and Carb-to-Engine boots
Ultrasonic cleaned carbs, full disassembly and reassembly (#35 pilot, #80 main, needle in third, screws 7/8 out)
Installed NOS genuine Honda condensers
Cleaned and lubed spark advancer
Installed new Sakura coils from parts-n-more
Timing was retarded slightly, I adjusted it and both idle timing and advanced timing are spot on
Compression test gave me about 150ish on all cylinders after a spirited ride (hardly any variance between cylinders)
Spark plugs after said ride were a nice tan color

After those few things the bike is running better than ever, but still same issue as before. 

Maybe a better way to explain the issue now I have some more time in the seat.   After the bike is fully warmed up it has no power under load at take off.  For example, if I gently rev to 3500-4000RPM from a standstill as soon as the clutch starts to grab the motor bogs down hard to near stalling and any more throttle stalls the motor.  If i pull the clutch during the bog, the bike goes right back to idle.  If i can lug the engine along 10-15 feet without stalling until i get a bit of momentum, then it's smooth sailing through all the RPMs and gears until the next stop.

Applying the choke during this bog does not help.  I tried a couple times at 1/4, 1/2 and 3/4 choke taking off from a stop up and this did not aid the issue at all. 

Next up:
Double check float levels (tube method)
NOS Honda air filter on the way (on the bike now is the garbage aftermarket one)
NOS Honda points on the way (the points might be ok, but they are the silly cheap daiichi ones)
Check valve clearance (i read in another post to tight clearance can cause some low RPM issues after warm)
Another compression test - best method for testing these bikes?

[M 750K6]

Your next steps look sensible. Probably not your issue, but have you definitely ruled out a weak battery?

[Gibbit]

I think I did rule out battery. Old lead acid was on the way out so I swapped it with a new ag about a month ago. I keep it tendered every night and it doesn’t even lose steam by the end of compression testing.

[HondaMan]

I re-read the posts here and just realized that the intake manifold's O-rings haven't been mentioned. These are in the little manifolds that bolt the carbs to the head. When they get old and hard (somewhere around 1990) they don't seal well: if the nuts holding those manifolds to the head were loosened at some point, then now they don't seal at all.

I think I have some of them: I gotta go dig thru the notes and then my O-rings to find them. They sometimes come in the gasket sets, too, but not always.

Did you, by chance, get to replace those? If not, they form a nasty vacuum leak below 1/4 throttle, which makes throttle modulation (like during takeoff) almost impossible to control because it goes from [idling OK] to [dead lean] when the throttle is cracked open. This then makes the engine rev some, but not with any strength, so applying the clutch then makes it fall back on its face again. We use to call it the "startup see-saw" action way back when, as it is common on bikes with leaking intake manifolds (not just these Fours).

-It also tends to wear out clutches when the riders just rev for the sky and clutch=slip their way to 5 MPH, like college kids without any $$ to actually fix the problem used to do when I had my own shop in the 1970s era...

[Gibbit]

HondaMan you nailed the description of the problem.  I was out today putting more miles on it, getting a better feel for what works and what doesn't and started the clutch=slip method to see if I could get to that 5mph.

I was convinced this was the issue so I replaced the whole "Intake Insulator Set" I think is the official part name.  2x 16212-333-030 - 2x 16213-333-030.  The rubber was so stiff on my old set that I needed a pry bar to push in and remove the carbs and the o-rings were embedded in their grooves. The "replica" replacement set looked good, the o-rings sized right, flexible rubber... I didn't see any noticeable red flags.  Installed them with new allen head screws just to make sure each one was nice and snug.

First trip after everything was put back together and same issue.

But I still think it's a vacuum leak.  When I had the intake boots off I noticed a little screw in each intake ports in the head.  Curious, I looked up the purpose of this and it's for an intake vacuum test.  I took the 4 screws out to inspect and clean them up when I noticed that one of the ports had the threads drilled out and re-tapped for a size up thread.  The hole threads looked pretty thin so I gently cleaned up the area and put the screw back in making sure not to strip it further.  That's a long way of saying; I think that might be where my vacuum leak is given it behaves exactly like your described.

What would be the best way to airtight seal up that screw hole, given that the threads are iffy, and I can't really put any torque on the screw?

[M 750K6]

Do you also have the thin washers on those screws? On the 750, those screws have a little washer, to help seal. I'm sure the washer alone works OK, but on mine, a couple of the threads seemed looser. I wanted to put a Dowty washer on instead, but my kit doesn't go that small, so I tried a small o-ring, between the washer and the carb body. Seems to work, so I left it alone.

[Rayzerman]

If you haven't got the original washers, use aluminum washers.

[Gibbit]

Come to think of it, that screw did non have a washer. Easy enough to try a washer and/or o-ring and see what happens.  Will be a few days, bike is apart for some other maintenance.

[HondaMan]

You can either Helicoil that hole for the screw (Ace probably has some near you?), or apply some hardening-style gasket goo like Permatex #1 (sparingly) under the washer when you put it back in so it seals around the threads. Permatex#1 is pretty fuel-resistant.

One other thing comes to mind, although it appears to be more prominent here than in most places (because of a lazy state governor here who forgot to renew certain EPA waivers, so our gasolines have become very screwy here lately) which is: adding a couple of ounces of oil to a full tank of gasoline will stabilize the burn rate of the gas at lower engine speeds. Around here, not having some oil in the gas makes for VERY sudden low-throttle engine surging problems, and poor idling on hot days. It is totally due to the gas we get now.

[Gibbit]

All right, near the finish line finally!

It was an air leak at the vacuum port screw holes, but ironically it was on a different intake port than I thought.  Originally I noticed that the #2 cylinder vacuum port hole was enlarged to m6, so I figured this had to be where the leak was.  So I removed all 4 intake screws, cleaned up the area and holes, applied some permatex no1 to all sealing surfaces and reinstalled screws with new aluminum washers. I let it sit for 24 hours to cure.  The next day I took her for a spin and the problem seemed gone at first... but it just took longer to come back.  After a solid 30 minute ride, the problem was back to itself and I was defeated.

When I got back to the garage I noticed some dark brown goop bubbling from the #4 screw hole.  When turning out the screw, I got about half the threads out and the screw just pulled right out of the hole.  The hole was completely stripped and the screw had just barely enough bite to go in, but there were no threads to seal any air.

I had some nice stainless m6 screws lying around so I decided to drill and tap all 4 vacuum port holes and install new screws, aluminum washers and some permatex no1 for extra good measure.  All 4 drilled and tapped with out a problem (#2 was already m6, so I just cleaned the existing threads with the tap). Again put everything back together and went for a test ride the next day.

This time the problem was fixed!!!  That was it, problem solved, but there was still some hesitation and relatively flat spot around the 1/8 throttle.  Very very similar behavior to the air leak, but not as unmanageable.  More RPMs fixed this new symptom, so going back through the thread I wanted to re-evaluate all previous suggestions since the problem was still similar.  Ultimately, I think it's still running rich.  I turned the air screws all out from 7/8 to 2 full turns out and the problem is nearly gone. 

I might try 2.5-3 out and see where it feels best, or move the needle from 3rd to 4th clip position.  At this point I'm just experimenting to find the right mix for that 1/8 throttle flat spot.

[Gibbit]

Oh ya, also done:

NOS Air Filter
NOS Honda Points
New NGK D8EA plugs
Timing dialed in with timing light
Clutch adjusted
Fresh Oil change

Some pics of the new hardware, shiniest thing on the bike at the moment.

[denward17]

^Good news Gibit!

[M 750K6]

Perseverance paying off. Think I've read on here that idle screw adjustment is meaningless beyond about 2 turns out. Sweet spot should be around 1 turn out. Can't remember what you've said about your pilot jets. If they aren't new ones it might be possible a previous owner has poked them out to clean them and enlarged the hole. It might also be the float set too high, giving a greater initial burst of fuel as the slide moves up?

What are your plugs like now? I've found even a moderately rich idle setting will give sooty plug electrodes.

It might just be the engine speed where you notice this. At low revs, 2,500 area, I don't think it's unreasonable for it to feel a bit snatchy on opening the throttle. Especially 3rd gear and upwards. By 3,000rpm and up it should be nice and smooth.