engine and drivetrain Jetting Question - Main & Corrector

[Cru Jones]

Setup: I have a Weber 32/36 on a car with stock cam running unknown headers and a very custom exhaust. Exhaust is basically a straight 2" OD pipe from secondary to the end of the muffler. Primary corrector jet is 150 and main jet is 150 as well. Secondary corrector is 160 and main is 190.

 

Problem: Car won't rev in hard accel above 5500 rpm.

 

Thoughts: Car is running too lean main jets (at least) and possibly air corrector jets. At first I thought it was fuel flow, so replaced the filter, but no dice. Plugs when I changed them not too long after I got the car looked to indicate a lean condition (going by my motorcycle knowledge). Car runs well and is seemingly great right up until you hit 5500. I'm thinking I need to at least increase my mains +10 (160/200), but wasn't sure if it was possibly just my primary and/or both mains and if additionally I need to increase my correctors. I plan to get a wideband O2 setup to do this properly, but wanted to get on the right path as not being able to rev past 5500 is no fun. This car only starts getting fun above 4500.

[Cru Jones]

So, noticed this. Doubt it would cause what I'm experiencing, but interesting how much crap is in there.

[TobyB]

Put a fuel pressure gauge on it anyway.  I had a car that had a restriction in the fuel line that caused it to starve, but only flat out in 4th

or 5th on the straights.

 

Also, look under your cap- if the rotor's big and round, it's got a rev- limiter in it-

 

and if you're cutting out at 5500 in every gear, that's most likely.

 

t

[Cru Jones]

Thanks Toby. I'll check the dist cap and put a pressure gauge on if that's not restricted. Is it easy to remove the restrictor if it has one in place? I haven't seen that mentioned before (not that I was searching for that).

[Beaner7102]

@@eurotrash, Posted 20 March 2011 - 12:26 AM

Mine cuts out a 5500 and I tap it rather easily with the 3.91 LSD. I am thinking about bending it a bit to get it to hit at 6K...

 

 

It could be the rev limiting rotor, Eurotrash had his cutting in at 5500rpm, the same rev range as you.

BMW part with limiter #12 11 1 355 211 (Bosch part #1-234-332-280)

BMW part with NO limiter #12111354503 (Bosch part #1-234-332-074)

[Cru Jones]

I checked the rotor and it's not the rev-limited type. Also of note I was able to rev to 6K in 2nd, but not in 3rd (still). I'll get the pressure gauge next. Stupid question, am I only looking for the pressure at idle or will I need someone to rev it to 5500 while I monitor the gauge to get a true sense of what's going on?

[downhillwolf]

 

@@eurotrash, Posted 20 March 2011 - 12:26 AM

Mine cuts out a 5500 and I tap it rather easily with the 3.91 LSD. I am thinking about bending it a bit to get it to hit at 6K...

 

 

It could be the rev limiting rotor, Eurotrash had his cutting in at 5500rpm, the same rev range as you.

BMW part with limiter #12 11 1 355 211 (Bosch part #1-234-332-280)

BMW part with NO limiter #12111354503 (Bosch part #1-234-332-074)

 

The BMW distributor rotor made by bosch had rev limit at 6400rpm. If the car cannot rev undder that than there is a problem elsewere.

[Cru Jones]

Since I don't have my pressure gauge just yet I was wondering if the fuel pressure is low (should be ~3.5 psi, right?) would that cause the plugs to look like the jetting is lean? Or would they look normal? Just curious more than anything...

 

edit: I spoke with the guys at Pierce Manifolds and they said what I'm experiencing is most likely the result of the air corrector jets being too lean vs the main jets. So, I ordered some corrector jets and will update once I have a chance to drop them in.

Edited November 5, 2013 by Cru Jones

[Cru Jones]

Ok, got the new corrector jet and had some interesting findings.

 

Swapping the 150 corrector over to the secondary from the primary and putting a 140 into the primary I noticed my problem actually got worse. Instead of the prior 5500 in 3rd (and 4th assuming) I was only able to pull about 4300.

 

Since I had another 32/36 laying around with 160/165 corrector jets I swapped them into the car and now it has no problems revving above 6K in 2nd and 3rd. This is odd to me in that putting in more restrictive corrector jets made the car run better when they in theory would be leaning out the mixture in the last 25% of the rev range (per my understanding). At this point I need to get a pressure gauge and a WB02 to fully sort out what's going on.

[Cru Jones]

So today I got a fuel pressure gauge and all looks good. PSI at idle fluctuates from 3.75-4.5. This gauge is a 'Mr. Gasket' gauge, so I'm sure that it could running a bit high. Revving up to 4K the wifey said the PSI never dropped below 4, so I'm pretty confident that my pressure is not an issue.

[downhillwolf]

First thing i got on my mind after your report after changing the jets/corectors.

Check if you can actually open the carburetors to max. The same thing happend to me with 2x40dcoe weber, first gear no problem till 6400, second was struggling to get to 6000. The problem was. I pushed the throttle to the floor and the carburetors was not fully opened.

[TobyB]

Ah, sorry, missed this for a while.  Unfortunately, you want the gauge to be visible when you have your foot on the floor

in 3rd and 4th under load at the problem spots.  It's not trivial to do- i usually use a tee and a length of

fuel line, and zip- tie the thing to a wiper arm or something like that.

 

 

 

 Also of note I was able to rev to 6K in 2nd, but not in 3rd (still). 

this is what makes me really think it's fuel; it sounds like it's burning faster than it can pump.

Or that there's a restriction at the needle valve- but you've found (and cleaned) the final- stage filter...

 

 

 

Since I had another 32/36 laying around with 160/165 corrector jets I swapped them into the car and now it has no problems revving above 6K in 2nd and 3rd. This is odd to me in that putting in more restrictive corrector jets made the car run better 

 nope, this is exactly what the air correctors do- they add AIR to the emulsified fuel mixture, so a LARGER air makes

the mixture leaner, a SMALLER air makes the mixture richer.

But they don't have as big an effect as the mains- the rule of thumb is that 3 jets (say, 15) difference is about 

the same as one (5) difference in mains, in the part of the range where they're affecting the mixture.

This is a bit of a guess, but it's the right idea.

 

Hey, are you sure the secondary bore is opening? 

 

t

[Cru Jones]

Ah, sorry, missed this for a while.  Unfortunately, you want the gauge to be visible when you have your foot on the floor

in 3rd and 4th under load at the problem spots.  It's not trivial to do- i usually use a tee and a length of

fuel line, and zip- tie the thing to a wiper arm or something like that.

 

this is what makes me really think it's fuel; it sounds like it's burning faster than it can pump.

Or that there's a restriction at the needle valve- but you've found (and cleaned) the final- stage filter...

 

 nope, this is exactly what the air correctors do- they add AIR to the emulsified fuel mixture, so a LARGER air makes

the mixture leaner, a SMALLER air makes the mixture richer.

But they don't have as big an effect as the mains- the rule of thumb is that 3 jets (say, 15) difference is about 

the same as one (5) difference in mains, in the part of the range where they're affecting the mixture.

This is a bit of a guess, but it's the right idea.

 

Hey, are you sure the secondary bore is opening? 

 

t

 

re: gauge - Thanks for clarifying. I was hoping that wasn't the case, but will sort that out sometime this week.

 

re: corrector jets - So my understanding is that the larger the jet the leaner the mixture. When I described the issue to the guy at Pierce he said that he doubted it was pressure related, but that the mixture was too lean up top, so suggested I go down a few sizes in the corrector jets. I did that, which made it worse (moved the problem down 1K in the rev range or so). So, at that point I wasn't sure what the problem was, but figured I'd try moving to the larger corrector jets and see if they made a difference. Surprisingly they made it run better. So, now I'm at the point where I doubt it's pressure related because the corrector jets being larger made it run better (one would assume that if it was pressure related they smaller corrector jets would have helped, not made it worse). With that said, I'll still run a longer piece of fuel line and verify the PSI when above 5500, but figure I need to get a WBO2 setup to get to the bottom of this.

 

re: seconday - never thought to check, but that's a good idea to check and will do that.

[Cru Jones]

First thing i got on my mind after your report after changing the jets/corectors.

Check if you can actually open the carburetors to max. The same thing happend to me with 2x40dcoe weber, first gear no problem till 6400, second was struggling to get to 6000. The problem was. I pushed the throttle to the floor and the carburetors was not fully opened.

 

I'll check that out. Wouldn't think that was the issue, but sometimes it's the most obvious and least checked that's the root.

[TobyB]

 

he said that he doubted it was pressure related, but that the mixture was too lean up top, so suggested I go down a few sizes in the corrector jets. I did that, which made it worse (moved the problem down 1K in the rev range or so). So, at that point I wasn't sure what the problem was, but figured I'd try moving to the larger corrector jets and see if they made a difference. Surprisingly they made it run better.

Well, if bigger correctors helped, maybe you're running RICH at WOT and lean at idle/light cruise.

 

A bog's as good as a lean, as far as power loss is concerned.  Black smoke out the tailpipe when this happens?

 

I agree, fuel pressure's maybe not the most likely, but it's also easy to rule out with one of those compound vacuum/pressure gauge

that I seem to accumulate...

 

A suggestion- get a main jet leaner and a jet richer than you have.  Try them.  One bore at a time.  If it's lean on the primary

and stupid rich on the secondary, that MIGHT be doing what you're getting.

 

Next random thought- power valve is good, right?

 

hth

 

t