Mikuni outer venturi question

[PETEW]

Hey guys, 

I want to switch the outer venturi's in my Mikuni ph44 from the current 37 to the smaller 34 I have. I am not sure if you do this from the front taking the inner venturi out first or if you have to remove the carbs from the intake and take the venturi out from the back side. Any advice would be appreciated. I seem to be getting closer to getting these dialed in and I think the big problem I have on the transition from low thottle to full is air velocity. The motor just can't handle the bigger outer venturi, it seems. Thanks for the help.

Edited October 11, 2017 by PETEW

[ryant67]

You can only remove the outer venturis through the front of the carb, and yes, you will need to remove the inner venturis as well to do so.  Chances are good that you will trash the inner venturi gaskets when removing, so I would go ahead and order 6 new ones - they are only a buck each, so no biggie.  I would STRONGLY recommend removing the carbs from the intake manifold to do this work, as it will be close to impossible to install the inner venturi and gasket with them mounted.  

What does your engine build look like?  Mostly stock, or? 

Any other question, feel free to ask.  I've spent a lot of time with these carbs over the past few years and am very familiar with them at this point, happy to share any knowledge I can.  

 

[PETEW]

Thanks for the help ryant67.

The motor has a 2.8 liter with a p90 head, doomed pistons and mild cam. 6-1 header and 2 1/2 exhaust. I also went to a 280zx ignition. The Mikuni carbs are running with a 57.5 pilot jet a 160 main and a 200 air jet. the pump jet is a 140. I have the idle mixture screw 1 1/2 turns out.

I just hooked up a wideband to the car to dial the carbs in better. Everything seems to be inline but during the transition to full throttle the sensor goes lean and the car stumbles badly. I went to a 150 pump jet and had no change. From what I have read it sounds like it is a air velocity issue and I should switch from the 37 to a 34. I am hoping this will rid me of the horrible stumble I am having when going to full throttle. 

Where do you order your Mikuni parts? Thanks, again.

[ryant67]

Going by your engine specs, I reckon it's definitely a good call to switch to 34's.  You should certainly lose the stumble in doing so, and gain a little more driveability on the street.  

The jetting looks about right.

I've been ordering jetting from Upgrade Motoring out of California.  They have a good website that you can shop online through.  

[PETEW]

I had to go to a bigger main to keep O2 levels at a safer spot. I am now at a 165 main and 210 air jet. O2 levels are in the 12-13 range now. I am changing the outer venturis today and hoping it solves my bogging issue. I will let you know. Thanks.

[PETEW]

I have the new venturi's in and the Mikunis seem to run better with them in. I am still getting the bog when I start to lay into the throttle in lower or higher rpm. It almost seems like I need a bigger Pilot Jet. My idle is good and I am getting a mid 12 on the O2 sensor for idle. Light throttle applications (cruising) stay similar in O2 levels, as soon as I start to press the throttle to accelerate some my O2 sensor starts to lean out (14-15 range). If I get into the throttle more it goes off the readable range on the O2 sensor and bogs. When the car is floored it pulls nicely and cleanly with a high 11 to mid 12  oxygen sensor reading. Should I go to a larger Pilot Jet? I have a 57.5 in there now and was thinking a 60 or 62 might help. Is there another adjustment I can look at? Thanks for any help. 

[Rob L]

Im going to be switching to 40s over the winteron my stock L26,  but i have a feeling im going to wish i just left my holley on ....idk about triples but i see guys all the time having issues with them ....the holley i want to get rid of i dont like the look of it under my hood,  but it is super low low maintenance ....set it and forget it

 

[PETEW]

I put in a larger pilot to see what would happen. I wound up going to a 65 because the 62.5 I have are hand reemed and I have heard they are very inconsistant at best. I took the car out for a run. It is running pig rich down low. 10's and 11's. The mid range stumble is all but gone but is still leaning out badly. I am talking 16-17 range. I am going to try the 62.5  or maybe 60 pilots. I am not sure what to look at next. The overly rich pilot is obviously covering the mid throttle problem I am having. But I do not know what it is. Full thottle pulls cleanly above 3k rpm and O2 level is mid 12s. Any ideas? Thanks.

[rundwark]

Did you ever sort this out Petew?

 

It sounds to me like the bog only happens on rapid throttle changes. If you get on the throttle very slowly and gradually, does AFR stay around 13:1 (with then 57.5 pilot jets) across the range? If so, your jets are fine.

 

Most likely, you’re not getting enough volume in your accelerator circuit. I’m not sure which Mikuni’s have this adjustment but on mine the accelerator pump has three positions for volume. I got rid of the bog on my car by changing to the highest volume setting.