Topic: Replacement Petcock options

[papi_sosaa]

Hello,

I'm running a 75-76 CB750K gas tank and the petcock is M20x1.5 with a 1/4" outlet. Unfortunately, the carburetor inlet is 5/16 and I don't believe enough gas is getting to all 4 carbs. Cylinders 1 and 2 are running fine but 3 and 4 aren't getting enough fuel.

The carbs and bike is an '82 CB650SC and the OEM petcock is M20x1 with a 5/16 outlet. Due to the thread sizing, the 650 petcock will not thread onto the 750 gas tank.

Does anybody know of another petcock that has a matching M20x1.5 threads but with a 5/16 outlet?

Online, they only specify the outer threads but do the inner threads on these reproduction petcocks match? Could I buy a petcock meant for the CB650 tank and use the threaded nut off of the 750 petcock to mate it to the 750 tank?

[PeWe]

My CB750 K6 -76 had no fuel issues with its stock petcock.
836cc, ported head, cam.
No problem to cruise in 150-180kmh all tank long until the reserve kicked in and time to refill.
This on German Autobahn during my holiday tours. Made 4 trips in the 80's.

Used a braided fuel hose that needed clamps to sit safe.

Your fuel hose might have a kink to the starving carbs. Or blocked inside.
If filter on petcock is  blocked all carbs should starve.

No additional fuel filter?

[CycleRanger]

Check that the fuel lines are routed properly.

[papi_sosaa]

So I've done a little more testing and I believe the petcock is fine and providing enough fuel. All 4 cylinders are firing if I start taking it for a ride.

The issue that I am trying overcome now looks like a carb fine tuning problem that I will need to do a little more research into. I have to crank the idle screw so the idle is technically high just so the bike starts, it'll sound like it's only running on 2 cylinders and then the carbs starts to pickup and it'll start running on all 4. Idle will start to get high, as expected, and that's when I lower the idle to where it's supposed to be. It will idle okay for a bit and then it'll start to slowly die again. If I take it for a short ride after the idle gets stable, the bike will want to die if I come to a stop. Regardless of short ride or just idling, the bike will end up dying and I'll have to play with the idle screw again.

I bench synced the carbs when I rebuilt them by doing the nail method and then I modified the carbs using the Dynojet Stage 3 kit and followed those instructions.

Going to do some research and see if I can figure out what's going on. Won't be surprised if I'm going to have to get a barometer and sync the carbs while they're on the bike.

[M 750K6]

I bench synch'd my carbs, then did a synch with vacuum guages (a Morgan Carbtune) on the warmed up bike. There was a slight adjustment required, but not enough to alter the tickover and cold starting on 3 cylinders. My problem turned out to be a tired coil (2 & 3).

[papi_sosaa]

I bench synch'd my carbs, then did a synch with vacuum guages (a Morgan Carbtune) on the warmed up bike. There was a slight adjustment required, but not enough to alter the tickover and cold starting on 3 cylinders. My problem turned out to be a tired coil (2 & 3).

I'm pretty sure I'm having a fuel starvation issue on idle and it definitely seems like a fuel delivery issue. If I fill up a water bottle with fuel and just feed the carbs that way, no petcock/filter/tank, it runs just fine.

When I was testing it last night, I was only running on two cylinders. I cranked up the idle screw and it only started running right when I rocked the bike side to side.

I'm going to inspect the petcock and tank tonight, my guess is its either having an issue getting fuel through the petcock filter or maybe the gas cap is not venting properly and limiting the flow. You can faintly hear the gas cap whistling shortly after shutting the bike off and opening the cap releases air.

[papi_sosaa]

I tested it out today and cylinders 2 and 3 were firing but 1 and 4 were not. A friend came by with his wisdom of CB650s to help me out and we eventually got the bike running decently.

Turns out my pilot jets were clogged again, probably from me using a water bottle to feed the carbs with no filter. Cleaned the pilot jets for carbs 1, 3, and 4 and also cleaned the bowls again. Bike was now easier to start but was still misfiring.

Pulled spark plugs 1 and 4 and they were black and smelt faintly like oil while the spark plug for cylinder 2 looked much better and just a little white on the end of the electrode. My friend cleaned up the spark plugs for cylinder 1 and 4, we put it back together, and the bike is now running much better. Throttle engagement is much smoother, there is a huge power improvement, and first gear put a big smile on my face when I took off.

I installed these spark plugs 3 years ago when I first got the bike but they've barely been used and actually sat on my shelf longer than they were in the engine. I'm guessing 1 and 4 got bad because the cylinders were barely getting enough fuel for good combustion.

I would say I'm 80-90% there now, I only bench synced it so now I'm going to do a little more tweaking to make sure it's all good; I just bought a Digi Sync so I can vacuum sync it soon, double check my valve clearances, double check the timing, and also get new spark plugs again.

[papi_sosaa]

Since I "fixed" it last week, I noticed cylinder 4 was still running cooler than the other 4 cylinders. Bike would still bog/misfire a bit if I tried to blip the throttle at idle or it was bogging when I was, let's say, in 4th gear cruising between 2-3k RPM and I decided to accelerate.

As part of my tweaking from yesterday; Re-tightened the timing chain, checked to see that the spark mechanism was set right, set my valve clearances to the book (0.002 in and 0.003 ex), and I installed new spark plugs. Unfortunately cylinder 4 was still running cold so I was still having a slight misfire.

I checked the slow jet in carb 4 again today and sure enough, it was clogged again. I also checked slow jet for #2 and it was fine. I cleaned the slow jet from carb 4 by hand, ran it through the ultrasonic cleaner, and then used a tiny bristle brush through it just for it to get even more clogged. I tried soaking it longer in the ultrasonic cleaner, soaking it in carb cleaner for a while, blasting compressed air through it but nothing worked. I had the slow jets and main jets from the 4into1 rebuild kit that I didn't use so I took one of those slow jets and installed it into carb 4 and now it's running much better. Blipping the throttle at idle is smooth and accelerating while cruising didn't bog.

I'll keep an eye on it for now but I'll probably end up buying new genuine Keihin slow jets in the winter and just replacing all 4. It's running decently now but I'm still waiting on the carb sync tool and I'm curious to see how out of sync they actually are.

[newday777]

It sounds like you have rust in your tank and you need to de rust it. Even lite surface rust will flake off and clog your idle jets as you continue to have happening.
If you have sediment in your carb bowls like this and your tank looks like this(or worse) you have to de rust your tank if you want to run right.....

[papi_sosaa]

It sounds like you have rust in your tank and you need to de rust it. Even lite surface rust will flake off and clog your idle jets as you continue to have happening.
If you have sediment in your carb bowls like this and your tank looks like this(or worse) you have to de rust your tank if you want to run right.....

The tank is brand new and before installation, had a little bit of oil in it to keep it from rusting.

I rebuilt these carbs 3 years ago and for the times I ran the engine for testing since the rebuild, only unfiltered gas from a water bottle fed them.