MGB mgb stromberg re-jet for hi-flow air filter

[eschneider]

On a 1980 MGB with a stromberg 175, emissions deleted:

I'm fitting a high-flow air filter. At low RPM, the car really wakes up (amazingly so); throttle response is great, power is good.

Of course, at 3000 rpm, it goes lean and power is flat.
Tried adjusting the needle to full rich.
Tried a TR6 needle
Timing is at 10 degrees
no vaccum leaks, carb & water choke are 100% functional
fuel pressure and flow are good.

Before I go drilling holes in the exhaust manifold for an o2 sensor to start playing with needle profiles, I wondered if anyone else had already found the needle that works when fitting a high-flow filter.

Question: what needle do I need?

 

[DrEntropy]

You check the timing at full advance yet? And you've made sure the diaphragm in the carb has no pinholes in it.

Welcome to the forum, too, BTW! /ubbthreads/images/graemlins/savewave.gif

 

[AlanT]

It leans WAY out at higher RPMs with a full flow air filter. Somewhere in my notes is the fix. Just from recall it seems the needle should be about .010 thinner at the taper. Careful or your combustion temps will get in the danger zone.

Alan T

 

[Stewart]

Your going to have to modify a needle to have a chance of getting it to work. I tried a K&N pancake filter and the results were horrible. Full rich on the 2 available low alt needles and it she wouldn't go above 3k in first 2.5k in second and wouldn't go in 3rd or 4th at all. Unless your limited by smog or are trying to keep the car stock your really best off doing a carb swap (Weber or Su's or a single SU on the stock manifold) than trying to get the zs carb to work with the filter unless you really enjoy a challenge.

 

[swift6]

When I had trouble with the needle profiles on my TR6 with Triple ZS 175's, I found help through Joe Curto . I called him after I spent hours pouring through needle profiles in the back of my Haynes ZS Manual. He came up with a solution AND had the right needle in stock! Give him a try, he might have the answer for you.

 

[eschneider]

thanks so far!

>You check the timing at full advance yet? And you've made >sure the diaphragm in the carb has no pinholes in it.

Yep and Yep.

>Somewhere in my notes is the fix

I'll be waiting!!!!!!!

>than trying to get the zs carb to work with the filter >unless you really enjoy a challenge

Bingo. Call me challenge-boy. I've heard so many people call this carb "junk", and that I couldn't install a low-flow filter. Might as well have hung a steak in front of a starving man. I'm actually very impressed with the carb without the flow restriction of the stock filter housing.

>Give [curto] a try, he might have the answer for you

Unfortunately, no. He suggested the TR6 needle, but otherwise wasn't much help. Ditto for the B-hive and a handful of other sources.

I did find a stromberg needle chart, and mapped out profiles for about 10 alternative needles. If I have to figure this out myself, I'll install an a/f meter and start swapping needles..... But I know someone has done it before me. I can't be the only one.

 

[kyreb1862]

I have a K&N pancake filter on my ZS carb. I have found the rebuilt carb and airfilter combo to be very responsive. I honestly don't remember making any changes to the stock needle. I did however make sure there were no vacuum leaks. i have revved as high as 5 grand with good response snd no problems.

 

[DrEntropy]

A/F meter, rig an on-board 3-gas analyzer, a cordless drill with a chuck to accomodate the needle shank, some 0"~1" micrometers, 600-grit wet-or-dry... you already have the charts. /ubbthreads/images/graemlins/grin.gif

...I'll be watching 'The Inquirer' for reports of a madman in a sportscar doin' high speed midnight runs thru Conshohocken..../ubbthreads/images/graemlins/devilgrin.gif

 

[eschneider]

>>...I'll be watching 'The Inquirer' for reports of a madman >>in a sportscar doin' high speed midnight runs thru >>Conshohocken....

hey - ever tried a straightjacket? Those things are warm!

 

[kennypinkerton]

Yep... if the charted ones aren't what you want, then it's custom needle time... like doc said.

 

[eschneider]

Thanks everyone-

When I first read the post about attacking the needle with 600 grit sandpaper, I thought "cha, and monkeys might fly out my butt!!!"

After further consideration, I am here to report that 320 grit works best. I chucked the needle in the ol' drill press and took about .010 off the narrow end, and about .003 off the idle end (progressive in-between)

Turns out the .003 at the idle end was too much. I had another needle, and repeated except leaving the idle stations at .098.

The car is nearly unrecogisable in the way it drives. Excellent throttle response, and a very noticeable increase in power. i was giggling like an idiot the whole time I drove it. (top down, 22 degrees farenheit here)

Thanks again to everyone! I'm going for another test drive!!!!!

 

[kennypinkerton]

oh... and you might (maybe - maybe not) need to retune for warm weather. Just keep that in mind. /ubbthreads/images/graemlins/thumbsup.gif Glad everything worked out.

 

[jlaird]

And you are trying to tell us you got it perfect the second time. Wow, now get some more needles and see what you can really do. hehe.

 

[kennypinkerton]

beginners luck? /ubbthreads/images/graemlins/jester.gif

 

[eschneider]

8) I'm sure it's not perfect. In fact, there is a slight burble in the exhaust at 4000-6000 that probably means I'm a tad rich.

I took notes and measurements so I can make more - how many would you like? 8)

 

[DrEntropy]

[ QUOTE ]
Thanks everyone-

When I first read the post about attacking the needle with 600 grit sandpaper, I thought "cha, and monkeys might fly out my butt!!!"

After further consideration, I am here to report that 320 grit works best. I chucked the needle in the ol' drill press and took about .010 off the narrow end, and about .003 off the idle end (progressive in-between)

Turns out the .003 at the idle end was too much. I had another needle, and repeated except leaving the idle stations at .098.

The car is nearly unrecogisable in the way it drives. Excellent throttle response, and a very noticeable increase in power. i was giggling like an idiot the whole time I drove it. (top down, 22 degrees farenheit here)

Thanks again to everyone! I'm going for another test drive!!!!!

[/ QUOTE ]

"Oh ye, of little faith..."

320 is a bit 'aggressive' if you're going for accuracy, but at least you got the 'raw data' down right. It's an ~old~ racer's trick. I've a range of needles to choke a pony here... but all SU. Some are 'custom' profiled, some are new in the container.

All my gasoline fueled cars are side-draught Weber aspirated. It takes a bit more than a drill press and some sandpaper to get those right.

I have NO advice regardin' them monkeys tho. Let us know how you get on with that, 'k?

/ubbthreads/images/graemlins/devilgrin.gif

 

[Guest]

[ QUOTE ]
In fact, there is a slight burble in the exhaust at 4000-6000 that probably means I'm a tad rich.



[/ QUOTE ]

Burble? I think that that is a cry for help nearing the 6K rpm playground.

Have you tried 3" K&N's?

 

[kennypinkerton]

I'd kinda figure it'd be more of an off-key scream at 6K than a burble. /ubbthreads/images/graemlins/grin.gif

 

[DrEntropy]

Soon to be funny "V" shaped splits in exhaust valves if it's too lean! 6K is "Never-Never Land" if the fuel mix is thin "up there"!!!

...and "stumble" may be a better term than "burble".

 

[jlaird]

I got real news, if this 998cc A 40 does not get 7K it's gona get pulled and some serious work.