Mikuni PHH backfires at part-throttle

[EricB]

My set of 44 PHH recently started backfiring occasionally (let's say 3-50% of the time) in 2nd & 3rd gear in the following rev range 1500-3000rpm.

Basically when accelerating from a part throttle to full throttle in order to pass someone one, or accelerate from a light/stop sign.

I took them apart, cleaned them out as best I could and re-assembled them.

It did nothing for the backfire. It didn't get diminished nor did it increase.

I did recently step my main jet size up from 150 to 155 to take care of the slightest bit of detonation at wide open throttle. However on the flip side I am now running a bit rich at idle. Can't have your cake and eat it too as they say...

Is my backfiring at those low rpms simply due to the larger jet size?

Or does someone with a more accurate knowledge of the exact functioning of PHH know which part of the system I should be looking at cleaning again?

Because it only happens at such a specific rpm range and under a certain load I figured maybe that would point to a specific part of the carb I should look at?...

-e

PS: Can I send someone some $$ to have them photocopy their Mikuni factory service manula for me? I really can't find mine anymore - lost it in the recent move... Thanks.

[jmortensen]

Mikunis suffer from a hesitation that is very tough to completely eliminate right at this spot in the rev range. The problem is you snap the throttle open and the acc pump sprays a big pee stream of fuel at the valve and there is no velocity in the intake tract to atomize the mix right there. The solution is to run a larger pilot so that the pump nozzle can be relatively small, but I still haven't been in a car with 44's that didn't have a slight hesitation there. I'm running a pretty big pilot in mine, high 60's I think, maybe a 67.5. Pump nozzle is a 50 I believe. I think when I got my carbs they had a 50 pilot in them. It was WAY too small.

You say you have a BACKFIRE, not a hesitation. That I haven't dealt with before. Is it truly a backfire that you're dealing with?

[texasz]

PS: Can I send someone some $$ to have them photocopy their Mikuni factory service manula for me? I really can't find mine anymore - lost it in the recent move... Thanks.

I can probably dig mine up and copy it for you. Likely will not be until next week though (have to find it over the weekend then copy/scan it next week at work).

[Victor Laury]

Try a little more inital advance

[EricB]

Thanks guys. Glad to hear that what I am experiencing isn't an isolated case.

I'll try both suggestions.

Have a good weekend!

-e

[andrewg3300]

sounds like it is running too lean. Right?

/drew/

[texasz]

Ok, I got mine copied and in a pdf file. I posted this in the articles forum for all to have access to. Here's the link to it: http://www.classiczcars.com/forums/showthread.php?p=150299#post150299

I'll also place it on my website tonight.

[jmortensen]

sounds like it is running too lean. Right?

Actually it is a rich condition, but it will show up on an O2 sensor as a lean condition, which makes it kinda confusing. When the throttle is closed and the rpms are low, there is a strong vacuum behind the throttle plates and the air that leaks past the plates is moving very fast. When you slap open all 6 44 mm butterflies the vacuum goes down to almost zero and the air in the intake slows down as well. Right at the same time you dump a bunch of fuel in. You'll find that this problem doesn't happen if you roll onto the throttle. Only when you punch it from a low rpm. At rpms above 2500 or 3000 the velocity is already high enough that the engine won't stumble.

It is this combination of low vacuum and rich fuel condition that causes the stumble. The O2 sensor will read lean here because suddenly the air velocity drops. No O2 into the chamber, no O2 into the exhaust. You can see this effect on dyno sheets done with wideband O2s. Air/fuel will go from 14:1 to 10:1 until 2500 rpm or so, then jumps back into a reasonable range.

What's interesting to me is that the car will pull better through this spot with the bigger pilot. I spent a lot of time on this in my Z, because I had a lot of situations where I would come out of a turn at a low rpm at autox and the thing would stumble. I was reading the O2 sensor and it was showing lean when the throttle was opened, so I kept going to a bigger and bigger pump jet nozzle thinking that would help. It did at first, but at a certain point it wouldn't make any difference how much larger I went. Then I read online that the O2 would show lean in this spot despite the fact that the car was actually super rich, and it also recommended using a larger pilot and not the acc pump nozzle to fix. So I tried that. MUCH better. I was surprised at how big the pilot had to be. I kept making it bigger and bigger thinking that it couldn't possibly run better with a bigger pilot, and it just kept running better and better.

I think the pilot works better than the acc pump because the pilot only puts fuel in when there is air to burn it, where the accelerator pump will pump a big stream of fuel with no air to burn it.

Mine still has a tiny bit of a hesistation there, but I just don't think it can completely be alleviated on the 44's.

That's my understanding of the problem and its causes anyway.

texasz--thank you for the .pdf. The info there combined with the info in the How to Hotrod Your Nissan/Datsun OHC Engine book should be enough to give anyone a good understanding of whats going on, but those Mikuni manuals are few and far between.

[texasz]

Actually it is a rich condition, but it will show up on an O2 sensor as a lean condition, which makes it kinda confusing. When the throttle is closed and the rpms are low, there is a strong vacuum behind the throttle plates and the air that leaks past the plates is moving very fast. When you slap open all 6 44 mm butterflies the vacuum goes down to almost zero and the air in the intake slows down as well. Right at the same time you dump a bunch of fuel in. You'll find that this problem doesn't happen if you roll onto the throttle. Only when you punch it from a low rpm. At rpms above 2500 or 3000 the velocity is already high enough that the engine won't stumble.

It is this combination of low vacuum and rich fuel condition that causes the stumble. The O2 sensor will read lean here because suddenly the air velocity drops. No O2 into the chamber, no O2 into the exhaust. You can see this effect on dyno sheets done with wideband O2s. Air/fuel will go from 14:1 to 10:1 until 2500 rpm or so, then jumps back into a reasonable range.

What's interesting to me is that the car will pull better through this spot with the bigger pilot. I spent a lot of time on this in my Z, because I had a lot of situations where I would come out of a turn at a low rpm at autox and the thing would stumble. I was reading the O2 sensor and it was showing lean when the throttle was opened, so I kept going to a bigger and bigger pump jet nozzle thinking that would help. It did at first, but at a certain point it wouldn't make any difference how much larger I went. Then I read online that the O2 would show lean in this spot despite the fact that the car was actually super rich, and it also recommended using a larger pilot and not the acc pump nozzle to fix. So I tried that. MUCH better. I was surprised at how big the pilot had to be. I kept making it bigger and bigger thinking that it couldn't possibly run better with a bigger pilot, and it just kept running better and better.

I think the pilot works better than the acc pump because the pilot only puts fuel in when there is air to burn it, where the accelerator pump will pump a big stream of fuel with no air to burn it.

Mine still has a tiny bit of a hesistation there, but I just don't think it can completely be alleviated on the 44's.

That's my understanding of the problem and its causes anyway.

texasz--thank you for the .pdf. The info there combined with the info in the How to Hotrod Your Nissan/Datsun OHC Engine book should be enough to give anyone a good understanding of whats going on, but those Mikuni manuals are few and far between.

I greatly respect your opinion as I know what you have been able to do with your Z car. Would you mind taking a look at the first page of that pdf and tell me what you think about what Todd has done to set up my carbs? I know that he is very knowledgable in this area also and would just like to see how what he has done compares with what you have done.

[jmortensen]

Thanks for the vote of confidence, although all I've done is autoxed for 10 years. That jetting looks pretty similar to mine, except the pilots and the pump nozzles are smaller. There is no doubt that Todd knows way more than I do about these carbs. One thing I can say is that road racing guys generally don't care at all about transitional response since they are always WOT at high rpms, so they might not take the time to tune it in as closely at low rpms.

I've got 44s set up like this IIRC:

37mm chokes

50 pump nozzle

67.5 pilot

155 main

210 air

OA blocks

Also the fuel requirements are going to be pretty drastically different based on what cam you have. Your Z should definitely run well enough to figure the fine tuning out with those jets. I'd suggest you stick with the small pump nozzles for now, get an O2 sensor installed before you mess with anything so you can see the changes. If you were going to put money in jets first thing to try would be bigger pilots.

I really like Todd's idea about "zeroing" out the linkage by loosening up the arms first, then synchronizing.

Another tip on pilots I've picked up is that the pilot should be sized so that the car runs best with the pilot screw adjusted out 1.5 turns. If you need to back the screw out farther than that then you need a larger pilot.

[EricB]

thanks so much for sharing this info with us - exactly the kind of tips i was after... very much appreciated.

-e

[texasz]

thanks so much for sharing this info with us - exactly the kind of tips i was after... very much appreciated.

-e

Yes, I second that!! Thank you!!