weber jet change: what to conclude?

356 Porsche-related discussions and questions.
Message
Author
frank p hijmans
356 Fan
Posts: 37
Joined: Tue Oct 19, 2010 2:31 pm

weber jet change: what to conclude?

#1 Post by frank p hijmans »

Hi All,

Worked on my D engine today. The car was lacking torque below 2500 rpm, so I decided to experiment with some jet changes.

I started with 115/200/55 on my big bore, SC crack, sports cam, weber 40 IDF, 28 venturi engine. Everything is new and checked so everything should be in good condition. I am aware that the lack of torque could be caused by my sports cam. Was just wondering how the behavior would be on different jettings. This is what I did:

- changed my mains to 125's: no noticable change, then
- changed my air to 180: minimal improvement
- changed my idle's to 60's, this made an improvement. Engine performed better at steady speed, also in the lower RPM's. When using throttle, there was still a flat spot between 2000 and 2500, but with much less hesitation. Engine picked up much better as well. Above the 2500 rpm's the engine felt even stronger then before.

After this I played with the idle mixture. I noticed that the best mixture was with the screw 1/2 out from totally screwed in. To be honest the range from totally screwed in, till half a turn out was the same.

Question: because turning in (close) the idle mixture screw completely in did not cause the engine to stall, or popping sound or other problems. Does this mean that 60's are too large? There is quite some fuel smell, so I quess that settings are quite rich. Am I correct in my assumption that, given a good jet setting, there should be a noticable difference in idle mixture screws turned in totally and not turned in totally??

Whatever jets are being used, torque below and above 2500 differs massively. Above the engine is strong and pulls great. Below the engine hesitates a lot. All though is has become less by changing the idle jets to 60's I am in dought whether I am going/searching in the right direction.

Thanks a lot in advance.

Regards, frank
 

User avatar
Dave Wildrick
356 Fan
Posts: 1953
Joined: Wed Mar 26, 2008 2:10 pm
Location: Houston, TX

Re: weber jet change: what to conclude?

#2 Post by Dave Wildrick »

This is the setup I use, and it works great with Weber IDF40s and a big bore kit plus stock C/SC cam (on the C engine). Posted by John Willhoit in 2003:

[Subject: Re: To all you weber experts
From: John Willhoit <jbwillhoit@yahoo.com>
Date: Sun, 20 Jul 2003 16:09:55 -0700 (PDT)

First step in adjusting would be to screw the idle
bypass valves closed (the little screw with a nut on
it next to the idle mixture screw). Next make sure the
throttle linkage is balanced (back off the idle stops
until the butterflys just close and check that the
adjustment from side to side on the throttle rods is
even). Open the idle stops about 1/2 turn, the idle
mixture screws 1.5 turns and start the car. Adjust the
idle equally on both sides to get it at about 1K rpm.
Screw the idle mixture screws in individually until
the idle starts to slow down then back them out about
1/2 turn.

That should adjust the carbs correctly. Before doing
the adjustment, I would recommend changing to the
following setup with Weber 40s: 28mm venturis, 115-120
mains, 175 ac, F7 e-tubes, 50 idle. If you have an
050, set the max timing to 36 degrees (about 10
degrees at idle). We have spent a lot of dyno time
with Webers and this will produce the best running
engine, especially with a stock exhaust With the
smaller venturis you will give away about 2 hp above
5200 but will gain torque and drivability up to that
point. If you still have a hesitation it could be a
characteristic of the cam. Norris cams use older
profiles which have a slower ramp and a lot of seat to
seat duration. You can help this by opening your
valve adjustment to .008/.0010".

Try it. If you're not happy I'll eat a bug.

John Willhoit]


The only change I made was to go to 55 idle jets instead of 50s.
Dave Wildrick
Houston, TX
#10230
64C coupe
65C coupe

User avatar
Jacques Lefriant
356 Fan
Posts: 4740
Joined: Mon Dec 06, 2010 1:50 pm
Location: Washoe county NV

Re: weber jet change: what to conclude?

#3 Post by Jacques Lefriant »

Theorectically the 32mm venturies in a Solex should permit 30% more area for air flow but less velocity.
jacques
 

User avatar
C J Murray
356 Fan
Posts: 9286
Joined: Fri Oct 23, 2009 7:24 pm
Location: 30MI WEST OF PHILA
Contact:

Re: weber jet change: what to conclude?

#4 Post by C J Murray »

Frank-What are your cam specs? What compression ratio?
'57 Speedster
'59 Sunroof
'60 Devin D Porsche Race Car
'63 Cabriolet "Norm"
'67 911 S Original Owner
'03 Ferrari 575M
'09 Smart Passion

User avatar
Frank Meek
356 Fan
Posts: 249
Joined: Sun Dec 21, 2008 6:54 pm
Tag: live life on full...
Location: Rocky Mountains

Re: weber jet change: what to conclude?

#5 Post by Frank Meek »

Hi there
While I inferred from your post that this is torque issue is a new one, then perhaps the carb jetting might be a cause. Here is a general setup which most folks can do initially and fairly easily:
http://porsche356registry.org/356talk/t ... n+class%3D
Of course, this presumes that other tuning options have been accurately completed.
There are a number of factors to be understood about these, (or any carbs), to make them function ideally. These suggestions are elemental. There are others.
Best regards,
FE Meek

User avatar
Walt Nolte
356 Fan
Posts: 205
Joined: Sat Apr 26, 2008 5:41 pm
Location: Mountain States

Re: weber jet change: what to conclude?

#6 Post by Walt Nolte »

Frank, you didn't say if you have F7 emulsion tubes. Although a stock 912 engine in a 59 coupe, I had some of the same issues you have described. Changing to F7 resolved almost all of them. I also changed the mains to 110 as I am at 5000 feet. This resolved the rest.

Walt Nolte
59 A Coupe
walt nolte

frank p hijmans
356 Fan
Posts: 37
Joined: Tue Oct 19, 2010 2:31 pm

Re: weber jet change: what to conclude?

#7 Post by frank p hijmans »

Hi All,

Thanks a lot for the replies.

Deducting from the replies, I would guess that I am running too big idle jets. Not sure what the compression is on my engine. Should be relatively high, heads are 912, bigger inlet and outlet, bigbore etc.

Nonetheless, my venturies are 28mm, so I would guess that running 60's make it too rich. Besides that ideal mixture is 0-1/2 a turn from closed. This indicates a too rich mixture (or am I wrong?).
Allthough the engine felt quite good with 60's (except for the fuel smell), I guess changing it back to 50/55's would be better.

Have a great weekend.

Frank
 

User avatar
C J Murray
356 Fan
Posts: 9286
Joined: Fri Oct 23, 2009 7:24 pm
Location: 30MI WEST OF PHILA
Contact:

Re: weber jet change: what to conclude?

#8 Post by C J Murray »

Hi Frank- A fairly wild cam with too low of compression will cause the engine to be very lazy at low rpm. I am not saying that this is your problem and without the hard numbers I wouldn't venture a guess.

Incorrect ignition timing can also make things sluggish.

Float level, accelerator pump output, and idle jets are the carb settings that are in play under 3000rpm. I assume you have checked the float levels. It is easy to do on Webers. Accelerator pumps only function when you are moving the throttle. The idle jets can only be evaluated by running the car at a steady throttle position. When the car runs best at say 2500rpm with no throttle movement then you have the right size. After the idle jet and main jet and air jet are chosen at a steady throttle then the last thing to do is to set the accelerator pump to give the best result when the throttle is "tipped in".
'57 Speedster
'59 Sunroof
'60 Devin D Porsche Race Car
'63 Cabriolet "Norm"
'67 911 S Original Owner
'03 Ferrari 575M
'09 Smart Passion

frank p hijmans
356 Fan
Posts: 37
Joined: Tue Oct 19, 2010 2:31 pm

Re: weber jet change: what to conclude?

#9 Post by frank p hijmans »

Hi CJ,

Thanks again for the help.

I guess I have to check my accelerator pumps. Engine is running fine at steady throttle. When I give throttle, the engine stumbles until 2400-2600 rpm and then accelerates fine.

Stumbling feels a bit like lack of fuel.

The one thing I do not get is why there is starvation below 2500 rpm and why not above this number. Does the fase change above 2500 RPM?

Regards, frank
 

User avatar
C J Murray
356 Fan
Posts: 9286
Joined: Fri Oct 23, 2009 7:24 pm
Location: 30MI WEST OF PHILA
Contact:

Re: weber jet change: what to conclude?

#10 Post by C J Murray »

Hi Frank
It is sometimes very hard to tell rich from lean. Do not jump to conclusions. Set the output of the pumps to a number near the Solex number. Test drive. Set the pumps lean one turn. Test drive. Set the pumps rich one turn. Test drive. This should tell you which way the to go with your final adjustments which will be made in smaller increments over a long series of testing and adjusting.

Before you do the above you need to have the jetting correct. Before adjusting the accel pumps try the smaller and larger idle jets at steady throttle one more time to be absolutely certain which works best.
'57 Speedster
'59 Sunroof
'60 Devin D Porsche Race Car
'63 Cabriolet "Norm"
'67 911 S Original Owner
'03 Ferrari 575M
'09 Smart Passion

frank p hijmans
356 Fan
Posts: 37
Joined: Tue Oct 19, 2010 2:31 pm

Re: weber jet change: what to conclude?

#11 Post by frank p hijmans »

Hi CJ, thanks.

Just put the 55's back in the car and it drives the same as with the 60's idle at steady throttle. Perhaps the 60's give slightly better oempf/smoothness, but with the 60's the engine runs best with the mixture screw totally turned in -1/2 turn out. Guess this indicates that the engine is too rich, right?? Should be near 1.5 turns out right?

If correct, then I will see what can be achieved with changing the acc pump output.

Would changing the valve inside the fuel chamber help as well? I have several sizes and a 'zero' one as well. Not sure how it works, but these deliver the fuel to the pump, right? So fast or slow etc.

My books are not very clear on this topic.

Have a great weekend!

Frank
 

User avatar
C J Murray
356 Fan
Posts: 9286
Joined: Fri Oct 23, 2009 7:24 pm
Location: 30MI WEST OF PHILA
Contact:

Re: weber jet change: what to conclude?

#12 Post by C J Murray »

The inlet valve at the bottom of the float chamber can be used for tuning but rarely is. The hole size or lack of hole allow fuel to bleed back into the chamber or not which mostly changes the phasing of the squirt. I can't remember what Weber recommends but I would stick with what they say unless you have exhausted the changes to the usual tuning tricks.
'57 Speedster
'59 Sunroof
'60 Devin D Porsche Race Car
'63 Cabriolet "Norm"
'67 911 S Original Owner
'03 Ferrari 575M
'09 Smart Passion

User avatar
C J Murray
356 Fan
Posts: 9286
Joined: Fri Oct 23, 2009 7:24 pm
Location: 30MI WEST OF PHILA
Contact:

Re: weber jet change: what to conclude?

#13 Post by C J Murray »

I would expect the 55s to be correct. IDFs should have the idle screws set in the 1/2 to 3/4 turn out position according to Redline and others. I have found this to be the case when the correct idle jet is installed. If the jet is too large the engine usually will idle ok when adjusted to do so but extended periods of idling may see the engine tend to lose rpm or stall.
'57 Speedster
'59 Sunroof
'60 Devin D Porsche Race Car
'63 Cabriolet "Norm"
'67 911 S Original Owner
'03 Ferrari 575M
'09 Smart Passion

User avatar
Juha Vane
356 Fan
Posts: 329
Joined: Tue Jul 01, 2008 3:06 pm
Location: Sumiainen, Finland

Re: weber jet change: what to conclude?

#14 Post by Juha Vane »

I'm trying to get rid of the mid RPM bog in my 356. The Weber 40 IDF jetting is now not correct,
the car run very rich and there is a stumble around 2500rpm. I plan to try with 28 venturis, now
32, but what about the auxiliary venturis? I have some Weber jets and have collected the various
jet combinations from this forum, it seem that there are several combinations that work well.
Or are the basic engines so different that they need different jetting?

My Weber's are Italian 40 IDF 70 L and have now aux venturis 4.50. Should I change these to 4.00
if I go from 32 ventiris to 28?

My egine is nothing exotic, 1720, perhaps the biggest difference is the Sebring relatively free flowing
exhaust. I also have a 123 distributor.
KTF,

Juha Vane
Finland

'59 308
'63 356
'85 911

User avatar
C J Murray
356 Fan
Posts: 9286
Joined: Fri Oct 23, 2009 7:24 pm
Location: 30MI WEST OF PHILA
Contact:

Re: weber jet change: what to conclude?

#15 Post by C J Murray »

Juha Vane wrote: Fri Apr 19, 2024 1:54 am I'm trying to get rid of the mid RPM bog in my 356. The Weber 40 IDF jetting is now not correct,
the car run very rich and there is a stumble around 2500rpm. I plan to try with 28 venturis, now
32, but what about the auxiliary venturis? I have some Weber jets and have collected the various
jet combinations from this forum, it seem that there are several combinations that work well.
Or are the basic engines so different that they need different jetting?

My Weber's are Italian 40 IDF 70 L and have now aux venturis 4.50. Should I change these to 4.00
if I go from 32 ventiris to 28?

My egine is nothing exotic, 1720, perhaps the biggest difference is the Sebring relatively free flowing
exhaust. I also have a 123 distributor.
First, make sure the float level is correct.

Look at the output of the accelerator pumps. Too much or too little causes problems with "tip-in". Italian Webers may not have an external adjustment and may require parts to adjust.

I have never changed from 4.50s because Weber and "all" the experts say that is a mistake. I have no opinion.

What emulsion tube do you have? F7 is very rich. F11 is much more appropriate for engines our size.

Main jets almost always work in a range of 4 to 4.3 times the size of the venturi. Race engines will be larger.

Air jets can be the key to low speed/high speed transition as well as high speed mixture. Bigger air jets allow more atmospheric pressure which can help the main circuit activate sooner, and vice versa.

Idle jets are keyed to throttle plate size and for 40mm throttles they are likely to be 50 or 55. 60 could work for larger throttles.
'57 Speedster
'59 Sunroof
'60 Devin D Porsche Race Car
'63 Cabriolet "Norm"
'67 911 S Original Owner
'03 Ferrari 575M
'09 Smart Passion

Post Reply