Mikuni PHH 44 balance at idle

[jrusso07]

Spent the morning fiddling with the solex carbs on my 69 U20. Circuits 1,2 and3 are right on - about 3.5 on the snail air flow meter. #4 is at 2.5. Adjusting pilot screw won't get #4 to 3.5. Went through and set valve lash...no joy. Is there a way to adjust throttle plates on each circuit within a carb body? Else, I think I might have a bent throttle shaft?

Thoughts?

[bakerjf]

People have remarked about gently twisting the throttle shaft to bring things back in line. Not sure how brave I’d be to try that however.

[SLOroadster]

The only way to even the flow out is by flexing the throttle shafts back to where they should be. Its a delicate process, but if you are careful, its not too bad. Be gentle and make small adjustments and you should be fine. The pilot screws don't affect airflow, that is all the throttle butterflies.

Will

[jrusso07]

Thanks guys. What is the proper technique? Remove carb and push onthrottle plates with fingers?

[david premo]

Joe, do not twist the shaft or any other crazy half ass fix. The right way to deal with this is to get the carb off the engine and inspect it for the actual problem. It most likely will need a new throttle shaft and two throttle plates. You can do this without disassembling the carb, just change out the throttle shaft and plates. If you need to talk about what to look for and how to repair it I can advise you.
Dave

[ted heaton]

"or any other crazy half ass fix"


Ted

[spyder]

This is how I solved an unbalanced idle with a broken mounting ear on the carbs.

viewtopic.php?f=3&t=8965&p=61231#p61231

[jrusso07]

Thanks all. I appreciate the inputs. I will follow Dave's advice and pull the carb and diagnos the problem and then determine best, non-crazy fix.

[Gregs672000]

You could also remove the progression hole cover and visually inspect where the throttle plate is closing on each by looking at the progression holes while you move the throttle. I would imagine that you will be able to see a difference. Use a flash light for a good view.

Take your time inspecting this. Was there a problem before? New (to you) carbs? Recently off? How would this have gotten bent?

[jrusso07]

Greg,

I purchased carbs as rebuilt. I didnt go through them when they arrived. I just bolted them on to a SU U20, and added the B cam.

The number 4 circuit has always given me problems. Under low RPMs it "coughs". Not a back fire but just a soft cough, almost like it's missing a spark. Spark is good. I thought itmightbe too lean so I changed pilot from 57.5 to 60. Slight improvement. I checked plugs and they were a bit sooty, rich on all cylinders.

So I went back to zero, checked valve clearances, set carbs up again. Perfect balance 1 thru 3. #4 air flow low. I moved pilots up and down a bit, 52.5, 55, 57.5, 60, 62 but really couldnt get #4 dialed in.

Runs ok at speed, maybe a little flat on response from 60 mph to faster.

So, long story short, have no idea if shaft is bent or, if it is, how it got that way...

[Gregs672000]

Bummer man! That certainly will screw things up. My engine is so sensitive to balance that anything less than perfect will show up as a rich idle condition on the air/fuel meter. I'm glad you have the snail version of the available flow meters, you need to be able to hold the meter over the throat for some time to get a good reading. I kinda wonder if maybe someone like Dave who is very familiar and experienced with these would be money well spent, as I can imagine several "on and off the car, check and repeat" kinds of things will be necessary otherwise to see if you got it right... fun stuff. As I have been chasing some minor intake leaks and other issues, I've gotten pretty used to this (carbs on and off 4 times in one session), but it gets tedious. Good luck, and be patient as it will be worth it.

[spyder]

Greg,

I purchased carbs as rebuilt. I didnt go through them when they arrived. I just bolted them on to a SU U20, and added the B cam.

The number 4 circuit has always given me problems. Under low RPMs it "coughs". Not a back fire but just a soft cough, almost like it's missing a spark. Spark is good. I thought itmightbe too lean so I changed pilot from 57.5 to 60. Slight improvement. I checked plugs and they were a bit sooty, rich on all cylinders.

I had a similar problem when I mashed the gas, sometimes it would cough and sometimes it would emit a loud knock. I went with larger accelerator squirters and it fixed it. The squirters are very small. I would check that they all work.

[PandaBoy]

Hey Joe,

Did you ever resolve this problem? I have the exact same issue with the #4 circuit. Perfect balance at idle from 1 to 3 but #4 stays low & won't come up.

[jrusso07]

Hey Joe,

Did you ever resolve this problem? I have the exact same issue with the #4 circuit. Perfect balance at idle from 1 to 3 but #4 stays low & won't come up.

Hi Mike, we missed you at Watkins...damn border is still closed. We did a run and a picnic but that's all

No, I haven't taken the carb off yet. I got turned into a carpenter this summer...we did a complete demolition of the house interior and then rebuilt it...still working on some baseboard and window trim and waiting on the new kitchen countertop...we've been working on it since June... how much can one roadster mechanic do in a day as a carpenter - not much!

I have had very little time behind the wheel of any of my cars. Just a run with Joe and Holly (amazingjoe) and the Watkins Glen fun run. Hopefully I will get one out this weekend - supposed to be 75 on Saturday. My shop is full of furniture and cabinets and stuff that needs to go. I can still get the cars in there but only by a fingernail.

Once the house is done, I will pull the carbs, change some of the cooling components (new solex water T from Datsun Restoration Parts and NOS heater hoses). I will let you know what I find but will likely be in November