351W Tuning

[Gun Doc]

It looks like the weather here in Texas may finally be changing and my new PE built 351W is broken in, so I guess it is time for me to attempt to fine tune my Weber set up.

The engine starts and runs well at idle and at almost any rpm while holding the throttle steady. My problems are manifest when first accelerating from a stop, or when passing on the hi-way where the engine runs extremely rich. So much so that my buddies kid me about having a second job as a mosquito fogger! You can really tell during around-town driving by the strong fuel odor that the engine is very rich, as well.

My engine in not too radical; it is a basic 351W with AFR 185 heads, 9:1 compression, 32deg timing, cam has mid 500s lift with around 244 duration 110 deg separation. It produces right around 460HP @ 5100RPM and 500lbs/ft @ 4000RPM. According to the dyno sheet from PE, has fuel/air ratios between 12.74 and 11.87 at speeds between 3000 and 6000RPM. The carbs currently have F7 E tubes, 145 mains, .120 low speed air bleed, .70 idle jets, and .120 high speed air bleeds. I am pretty sure the choke tubes are 37mm.

I recently bought the Weber tuning manual, a synchrometer and a set of .60 and .65 idle jets. Just out of curiosity, I checked the air flow on all of the barrels yesterday (with the main linkage disconnected) and got readings from 5 on the front left two barrels to 10 on the left rear barrels. The right hand bank of cylinders was not quite as bad, but still ranged from 7 up front to 10 in the rear. Of course these readings were with old plugs, which I am sure are at least partially fouled.

I am wondering what the collective wisdom of the Weber experts is…Should I start by reducing the idle jet size to the .65, change the plugs and sync the carbs, or do you all think a change in the E tubes is also in order? Or maybe there are other changes I should consider?

Thanks,

Gun Doc

[traindriver]

Gun Doc,

First and foremost make sure your carbs are sync'd, the fuel pressure is 2.5-3.0 psi, and the timing is correct, and freshen the plugs. You should be able to get the air flows exactly the same, make sure when you reconnect the short arms or cross arm (depending on the linkage system type) that tie each bank to the bellcrank that you put no pressure on the linkage system.

When and only when you get the above done, I would start by dropping the idle jet, I have a similar setup to yours and I run a 55 idle. The idle jet size is affected by the amount of vacuum your engine makes, your cam sounds mild like mine so a smaller idle jet will be better. You list the air corrector at .120, I suspect you mean 1.20, this needs to be increase to 60 points above your main jet to start. The 145 main is about right, I use a 150/210 for the mains.

Do you know the size of your accel pump bypass jet?

Keep us posted on your progress

Chuck

[Gun Doc]

I have to correct my cam specs...I was trying to recall them from memory. Lift is 573 intake and 568 exhaust with 274 and 282 duration, respectively. Lobe separation is 114.

Gun Doc

[Gun Doc]

Chuck,

I have not gotten into the carbs, yet, so I don't know what all is in them. I have asked for the info from PE, but have not received a response. Looking at what is listed on the dyno sheet as far as the jetting is concerned, it appears as if the carbs are pretty much the way they come from Weber (according to the Weber tuning manual).

Gun Doc

[RRCOBRA]

Your high speed air bleed is small for a street setup. Most street setups are around 190-210. Weber tuning guides usually say that a good starting point is 40 larger than the mains. Also you need to check to see what size exhaust pump valve you have in the bottom of the fuel bowl. Sound like the carbs are stock settings from the factory.

Lou

[Gun Doc]

I attempted to sync my carbs the other day. I got them pretty close, but I think I may need to change out my linkage before I will get them spot on.

Just synchronizing the carbs and changing the plugs made it run like a different engine. It was scary! I got on it about ¾ throttle in second while going about 25, or so and the car launched to over 60 in a flash!

I ran the car through most all RPM ranges and I know it is still running rich on the idle circuit. I guess my next step is to drop the idle jet size. I am running .70 now. What is the collective opinion: .60 or .65?

Gun Doc

[JCoop]

Originally Posted by Gun Doc

I attempted to sync my carbs the other day. I got them pretty close, but I think I may need to change out my linkage before I will get them spot on.

Just synchronizing the carbs and changing the plugs made it run like a different engine. It was scary! I got on it about ¾ throttle in second while going about 25, or so and the car launched to over 60 in a flash!

I ran the car through most all RPM ranges and I know it is still running rich on the idle circuit. I guess my next step is to drop the idle jet size. I am running .70 now. What is the collective opinion: .60 or .65?

Gun Doc

They're easy enough to change so I'd go with 65 first. My 48IDAs on a 302 had 70s on them; changed to 65, then 60, and STILL is pretty fat. I'm switching to 55s and probably fiddle with the mains too. I'm also ordering 40mm venturis.
My guess is that you'll end up with the 60s. You may have to back out the mixture screws 1 full turn out. Then set idle speed. Next, do the increasing idle test. (turn idle screw up slowly up to about 2200 and see if the transition to the mains is smooth. If it gets rough and bogs down, you're still too rich.

[68gt500]

Before you even think about changing jets, you have to make sure that the synchronisation of the carbs is spot on. Not only during idle but very importantly also at the higher revs.

If you really want to tune your Webers do yourself a favor an get a WBO2 Gauge, it makes diagnostic so much easier and accurate. Innovate makes a nice unit that does not cost an arm & leg.

That will tell you exactly which circit to enrich or faten up to get the results you want.

Don´t let anybody tell yout that you have to put up with rotten driveability or crazy fuel consumption - the Webers can be tuned to deliver power, driveability and mileage.

There is an italian guy on ebay (alfa1750 or so) that has really great prices on original Weber jets.

The Idle holders are hard to get - I did solder mine shut and drilled new holes. I bought an nice set of carb reamers at a motorcycle shop - makes for much more precise holes than just drilling.

With 400+ HP I would think that you could use the 40 mm venturis - you will love (or be scared of) the new found top end.

[JCoop]

Originally Posted by 68gt500

If you really want to tune your Webers do yourself a favor an get a WBO2 Gauge, it makes diagnostic so much easier and accurate. Innovate makes a nice unit that does not cost an arm & leg.
...
That will tell you exactly which circit to enrich or faten up to get the results you want.
...
With 400+ HP I would think that you could use the 40 mm venturis - you will love (or be scared of) the new found top end.

Mike, what's a WB02 and where can you get it?
I like the instantaneous response with the Webers but miss the G force-induced grin I'd get above 3500rpm with the original Holley 4bbl.

[68gt500]

Originally Posted by JCoop

Mike, what's a WB02 and where can you get it?
I like the instantaneous response with the Webers but miss the G force-induced grin I'd get above 3500rpm with the original Holley 4bbl.

A WB02 is a Wide Band Oxygen Sensor. A neat gauge that will read out the instantaneous Air Fuel ratio. Great tuning tool!

I got one of these: http://www.innovatemotorsports.com/products/db.php


A sluggish top end could man that it is way rich, the WBO2 gauge will tell you what changes you need to do.

Once you are spot on, you will never miss the Holleys "Top End"

[JCoop]

Thanks, Mike.

I had kinda shelved tuning the Webers for a while as I was getting frustrated.
Maybe this will motivate me again.
Cheers!

[dlampe]

Traindriver (Chuck) is your guy. Send him a PM. He is a super guy and he knows more about webers than anyone I have ever talked to.

By the way chuck, 100 holders seems to be the magic number. My car is tuned!

[Gun Doc]

Update:

This past weekend I fiddled with the Webers and thought I had found the right combo…I First changed the idle jets to 65 and took the car for a short test. On deceleration, I had backfiring, so I turned the idle adjustment screws out and additional ¼ turn and the backfiring went away. The car ran well under heavy acceleration and also around town. When I took it on the hi-way, however, the problems started. Right at 2000-2100 rpm I got popping and sputtering through the carbs (stacks). Also, if I had to increase throttle to pull a hill, I would get the popping. I took the car back home and installed the 60 idle jets, re-synched the carbs (idle at 1200rpm) and turned the idle screws out an additional 1/4 turn (1 ½ total). Again, the car ran great around town and under heavy acceleration, but the popping and sputtering was still there at the same 2000-2100 rpm. Before I started all of this the car was running very rich (70 idle jets) and fouling plugs around town and left huge black clouds on acceleration. There was also a very slight stumble at around 1800-2000rpm, but at hi-way speeds the car ran well. I know I am close…Any suggestions?

Thanks,

Gun Doc

[traindriver]

Gun Doc,

With your latest testing, here's what I surmise....the top end of the idle circuit is current running lean. This was likely being masked when you had the 70 idles jets in. Basically it was very rich at idle and off idle, at the top of the idle circuit, the large idle air jet (120) would lean out the mixture to something reasonable as the 70s were buriying the idle circuit in fuel. The remedy for this is to reduce the idle jet holder size. My engine is very similar to yours, I'm now running 50/80 on the idle circuit with nice results. Small idle jet holders are hard to find, the easiest thing to do is to solder the 120 hole closed and drill a new hole at .80mm. I can solder and drill the holders on my mill for you if you need help. A quick test you can do while driving is when the car is in the bad spot, does it get better if you let up on the throttle a bit or does it get better if you give it a bit more throttle? My guess is the former (less throttle). This would confirm a lean condition.

I was looking at your posts from last fall, are your air correctors (main circuit) still 120? If so, these need to be opened up to about 60 points over your mains. This is easily done on a lathe.

Chuck

[Gun Doc]

Chuck,

Thanks for the info and offer to help. Unfortunately I am heading back to Iraq early next month, so I don’t think there is time for me to send you my holders. I did check Pierce manifolds and it looks like they carry 60, 100 and 120 idle jet holders. Of these, which do you think would work best (100?) with either the 60 or 65 jets?

Thanks,

Gun Doc

[traindriver]

Gun Doc,

I would buy the 60s and work my way up as needed. It's much easier to drill out an exisitng hole than solder and redrill. Use a #67 drill for the 080 size and a #60 drill for the 100 size.

Chuck

[Gun Doc]

Good point. I do have a small set of drills and a pin vise that should work okay.

Thanks,

Gun Doc

[Gun Doc]

It’s been a long time since I posted to this thread. Been on a little government sponsored vacation to Afghanistan…

I finally got around to trying what Train Driver suggested and changed out my idle jet holders (.60 drilled to .80) idle jets (.60) and air correctors (205) and just returned from a test drive.

The car is better, but still pops and sneezes a little around 2200 RPM under light to moderate throttle and much more around 3000 under heavy throttle. Overall it seems to drive better…Smoother.

I am beginning to wonder if I may need the third progression hole, as the problem seems to manifest during the transition from idle to mains?? I sure would like to get this thing running correctly for the summer. Any thoughts or advice would be appreciated.

Thanks,

Gun Doc

[edmonds007]

Gun,
I'm just made the move to put my webers back on and while it has been an experience with some ups and downs my initial engine issues were very similar to yours. What you're experiencing sounds to me like you need to check your total advance. Turn it up to 36-38 and this just may solve a lot of your problem. If your advance is wrong nothing else will matter with webers. As soon as I got this part straight the sneezing and popping went away.

Good luck and let us know how it goes!