Air cutoff help...

[Totaled108]

Hello,

I am having issues dialing in my carbs to perfect off idle response and idling on all 4 all the time.

I have taken the slow and main jets, along with idle screw adjuster and float needles. I sprayed every holes, both ways with carb cleaner, then compressed air (only on the last cleaning), then did it again the following day.

The engine idles great with choke at 3/4 when cold starting, and throttle response it crisp. But only 1,2,3 are firing and getting (exhaust) warm at the same rate, while 4 remains cold only getting warmer if I open the throttle a few times. This obviously revs the engine a bit, and the revs hang a bit till the # 4 drops and idle drops down again to ~2000rpm.

After about 30 seconds of that business, I'll push the choke in to about 3/5th and all four will purr away and have great throttle response, even when I roll off on my trip. As its running rich and is still cold, I stay pretty low in the revs, below 6,000 anyways. And throttle response is slugging beyond that, yet the bike is eager off the line and isn't bogging or anything during this, just over half choke and engine warming stage.

After a few minutes of riding, say 5-10 minutes, I'll push the choke in to about 1/4 from being all the way closed. This is we're it gets weird...

So idle is ok, seems to 'search' about 250 to 500 rpms from normal idle speeds, only sometimes, while others it will act normal. Then others it will idle seemingly well, but will be idling on 3 cylinders. At least I think it is... There is a low growling noise from the left side othe engine. I'll SLOWLY twist the throttle, the engine responds nicely enough, as the throttle is at about 1/8th throttle, the revs jump up a few thousand with no extra input. I'll let the throttle close and it will idle higher then normal, aroun 2500, sometimes even 2000 and the growling noise is gone totally.

If even after this bike is fully warm I put the choke any closer to opened all the way, the just off idle to about 1/4 throttle gets very hesitant till 1/4 throttle to WOT, which is responsive and pulls well to red line.
Ok, I'll get back to why I need help with air cutoff. I have done 3 complete cleanings of the carbs, no rack splitting yet. The last one I finally had access to the compressor.

When I sprayed air into the brass tube going from the air box side of the carb, heading to the air cut off chamber, carb cleaning sprayed out of the air cutoff cover. This is only #4 carb.

Please tell me this is my issue, and tell me what to do for sure to fix this problem. I'll split the rack if I must to get rid of the silly air cutoff diaphrams.

Timing chain is adjusted, valves adjusted, igniting gap adjusted, ignition timing is good, tank is good, fuel filter in place, new plugs and wires, charging systems charging perfectly well, newer battery, air box checked and sealed around the boot between the two air boxes with RTV sealant.

Thanks for reading. I'm getting frustrated, so am a bit scattered.

Any help would rock!

[Volker_P]

Did you verify you can blow through the idle jets/passages?

Note that the carb cutoff/accelerator pump diaphragms are not designed to withstand compressed air in any case.

[Totaled108]

Yes, idle jets and passageways are all getting carb cleaner moving through when spray via the cans pressure.

I completely forgot to mention what bike this is. 1980 CB650 with PD50B carbs that DO have air cutoff diaphragms.

If I get stuck in traffic for a few minutes, or down town and the engine gets a bit on the warm side, it seems to idle perfectly with no choke, along with no hesitation at any throttle position.

[Volker_P]

When I sprayed air into the brass tube going from the air box side of the carb, heading to the air cut off chamber, carb cleaning sprayed out of the air cutoff cover. This is only #4 carb.

Guess you have a leak/cut in the diaphragm there.

[MiGhost]

If I get stuck in traffic for a few minutes, or down town and the engine gets a bit on the warm side, it seems to idle perfectly with no choke, along with no hesitation at any throttle position.

Sounds like the idle mixture screws need to be tweaked just a bit. Try turning them in 1/8 - 1/4 turn. This will lean the mixture out slightly. Which should help the engine warm up faster, and get off the choke quicker.

Ghost

[jagans]

Time to remove your carbs and dip them for an extended bath. Good Luck, its a job, as you need to completely disassemble to remove all rubber parts. Take a LOT of digital pictures. The starting point on the pilot needles is 2-1/8 turns out for the 79-80 This came out in an addendum. The manual originally said 1-5/8.

I know, dont ask me on the 1/8ths bit, but thats what it says.

[Totaled108]

I was about to disassemble the rack, but ended up taking apart my spare rack and brainstorming on how the air cutoff system works. After thinking it over for the night and next day, I cam to the conclusion I'd have myself a bit of an experiment.

I macgyvered in the craziest easiest way. I used 2 tooth picks, snapped the ends off so I had a pointy end with about 1/2 inch of the shaft still on it. Doing this with 2 picks gave me 4 short pointy pieces of wood. I proceeded to put each one in he brass tubes, one in each carb, on the air box side, the tube (as looking from the air box side) on the left, neither of the tubes on the bottom, leave them be. Double checked pilot screws were at 2 and a hair out, snugged it all up and started her up. Idles nice, revs clean all with choke to warm it up. I put my gear on excited for a test ride, but not expecting any improvement.

In about 3 minutes I was able to have the choke fully off. What's this, throttle is responsive off idle and to WOT! I'm smiling ear to ear, hop on the freeway, still no hesitation when cruising like before, it was really bad cruising at all before the tooth picks.

You might be waiting for the bad part, well there is none as of yet. Rode the 17.5 miles to work, then shopping for food and home. If feels so nice having a motorcycle that works as it should!

Conclusion, disassemble as suggested, but after cleaning and before assembling, have someone with experience plug weld the ports that go from on top of the air cutoff diaphragm to the intake side of the carb body, and the ports that I put the tooth picks in.

[Volker_P]

Thanks, so now we know which are the relevant channels.

[Totaled108]

Indeed. I'll report back when it's done to confirm.

I left out that I had also plugged the left of the bottom two brass tubes, on the air box side holes, and it wouldn't idle at all without the throttle being up a little. So we know that this hole is integral to the idle circuit.

Thankfully I was able to pull them out without having to take the carbs off again, just pulled back the air box boots enough to get the long angled needle nose pliers to them.

[my79650]

Hello; i did the block off of the cutoff channels and it riched the hell out of my original pd50a set i had; plus it made them a pig on gas. There is a channel under the cutoffs at an angle which goes directly to the slow jets. I had to go get another complete rack and transfer all my good jet, needles, etc. after a cleaning. My idle holds at 1400rpm and gives over 40mpg running at 50mph travelling to work and back. Plus this replacement rack had great shape cutoffs. With mine it turned out that replacements was easier than trying to get my originals back to decent running. were you able to get any photos of the block job you did? Here's one of mine.

[Totaled108]

Which channels are blocked off? It's hard to see. Looks like the one I did, but there looks to be sn extra piece of wire looped in the throat of the carb.

I'll try to take a second today and grab a picture of what I did on my spare rack.

I had a look at the plugs when I got to work yesterday and they are all nice and tan! . During the road trip a few weeks ago (225 mile each way) plugs 2 and 4 were pretty white compared to now.

I have only had about 3/4 on a tank with the tooth pick 'upgrade'. I have anl app on my ipad mini and use it to log maintanence and fuel, and its costs. Well, when I filled up after using the 3/4 of a tank, the mileage went from ~35mpg to 41mpg. Not a massive jump, but even going super easy on the throttle before, I could never get better then 36mpg, usually 30-35mpg.

[Totaled108]

Here is a picture of a spare same model carb that I have sitting around. This is exactly what I did. The tooth pick is about 1/2 inch deep with a quarter inch e posed, to make it easy to remove when I fix it with JBWeld, or similar.

[my79650]

Yours was the same one as i did on mine. I also plugged the side hole from the cutoff cover to the carb throat. I took the picture and can't remember if that was wire or a reflection.

[cb650]

on a side note where are the "engineers didnt design it that way" crowd???
Heaven forbid you would do this and use PODS.
Sorry back to my box.

[Totaled108]

I was thinking of blocking off that second hole you blocked off too. Now I'll maybe not do that. My set up has been wonderful. Though I just feel a few small adjustments in the carb syncing area will smooth out some VERY minor things. I'll report back soon if that does anything. It was synced before this bandaid, so I'm sure it is thrown off a little since changing air flow through the carbs with the tooth picks.

Engineers have their place, trust not sitting on my bike. Why is it that some of these carbs have no diaphragms and other did? Which one did engineers design and which ones weren't!

I'm keeping everything else stock, air box and all.

I am surprised too I wasn't told that I was doing it all wrong. Though its hard to argue with a working bike.

Finished a 120 miles of riding only 1 hr ago. Feels nice, and I had a great day feeling that smooth throttle roll on, sexy!