Tri-Power JEtting

[kevinl1058]

WEll after reading post on weber 34 Ich/ICT jetting decided to go with 175 main jet / 70 idle jet for the main carb, and 155 main / 70 idle for the primaries. It seemes that most people were still getting some lean reads with a 170 main jet so I just upped it to a 175. should get the car fired up in a couple of weeks so I'l let everyone know.
Kev

 

[JOHN G]

If I'm reading right, you have the outer carbs set up for idle as well? You should have the outer idle circuits blocked off(idle screws turned in all the way/no power to idle cutoffs if equipped) If all 3 have idle capability, it will be WAY TOO RICH and you will never get the rest of it right.

 

[kevinl1058]

Exactly.. The outers will be closed for Idle. then open progressily.

 

[CobraSix]

You still need the idle jets...they contribute to the overall fuel contribution under acceleration. And you don't want the idle screws closed on the outer carbs either. If you do that, you'll lean out the car since each of those carbs will act as a huge vacuum leak. It'll be even worse under partial acceleration when you don't have the outer carbs opening up yet. The engine will pull air right around the throttle plates. Gotta remember the throttle plates do not completely close off the flow of air in any carb. It has to allow for air to flow through else your car wouldn't run at idle.

Just on a Tri-Power set up, they'll be closed down a bit, but not completely closed.

 

[drag-200stang]

I agree with Cobra Six!

 

[kevinl1058]

What was the initial Idle screw setting for a first start up? for both the Primary and outer carbs..
Kev

 

[powerband]

What was the initial Idle screw setting for a first start up? for both the Primary and outer carbs..
Kev

I haven't worked with Webers on the 3X1. For 3X1 on new engine used good running YF carb with outers blocked for cam run in on initial startup . Once running well (trouble with distributor problem fixed first) , found out two of the three Holley carbs planned didn't run well individually in center position . Took a while to get three good running 1904's together.

With three tight good running Holley 1904's, Basically each one is tuned to run the car at idle, but outer throttle plates are shut tight at idle. Opening either outer even slightly raises idle RPM way too much. I don't think much atomization is happening but maybe the amount the air screw bleeds in - is maybe correct for the gas dribbling in mix?... . Off idle low vac performance is decent.

Off idle response and return to low idle is a challenge but the Offy linkage allows lots of variable in linkage ratios. Surprisingly, the Holleys will idle fine at @ 7-900 RPM with lots of return springs and tweaking . Setting outers to flip open at specific throttle positions is like a kick in the pants switch. .

The Webers you are using have a much better rep for consistency, keep it simple for initial tuning for drivability. The Offy 3X1 is always ready for WOT, hard to find enough road to tune that capability. 8)




Have Fun
Powerband

 

[CobraSix]

What I did to start it up was to block off the outer carb plates with some metal plate and gaskets and get each carb tuned on the center port only. I also advise doing this if you daily drive at all. I once had a major problem with one carb and the car was undriveable. I took the block plates and installed them and was able to drive the car home.

 

[Gene 64 2dr]

I haven’t gotten to the point were I have chosen what size jets to use but now have it running the way I like.

Here are some of the things that I’ve done along the way that may apply.

Starting out I ran on the center carb with the outer two outer two blocked with a plate.
After fixing the vac leaks under the manifold which involve taking it on and off many times and trying various sizes of O rings. One problem was the manifold was perfectly flat and the manifold would not lie flat in the back. Solution was to cut it in haft just behind the center carb and fill the gap with JB Weld. The mounting surface was also increased around the mounting hole with some JB because the O ring would eventually push out due to the small surface area.

When I installed the outer two carbs there was no way I could get it to run below 1500 rpm. With the idle screws on the two outboard carbs all the way in it would run very rough and then only with the center idle screw way out. I could get it to smooth out some by backing out the idle screw on the outboard carbs but there was no way to get the idle anywhere low enough to use.

After quite some time I finally found out that the problem was that the throttle plate edges were rough enough so that they would not seat well when completely closed and let too much air get by. With the idle screw all the way in the carb acted like a big vacuum leak and the only to get the engine to run was the open up the center idle screw too much and it ran rough.
The solution was to remove the throttle plates and touch up the edges with some 1000 grit paper. If you shined a light thru the carb you could see quite a difference.

With everything back together I started with the idle screws about 2 turns out and it ran nice smooth. It idles much better with all three idle screws out.

I bought a carb synchronizer from Summit because I new I was never going the setup just right without it. It works great, once you spend a little time figuring out how to set it up and use it (no instructions came with it). You can get all three carbs set up the same in about 5 to 10 minutes.

When this snow goes away I’m going to get a few different size jets a head off to the strip and figure which one works the best.
I’ll let you know the results then.

The biggest problem with setup is vac leaks. They will drive you nuts, but when it’s running right it’s the best.

One question. You said you were using 155 main jets on the outer two carbs and a 170 on the center and are going to 175. Are some of the lean reads on the outer cylinders? If so wouldn’t be better to increase the size of the jets on the outer carbs so that they match the center one. I would think that because each carb is feeding two cylinders it sits between it would be best if at wide open throttle the fuel supply should be even throughout the head.

Gene

 

[kevinl1058]

I made the jetting based of the jetting chart that was supplied by other members that were running the same carbs in there 3 x 1 set up. I used to run SU's on my datsun 510 motor and still have my syncro guage.. Is there a way to have the initial run without blocking off the outer carbs. I agree on the valve sticking. I had to readjust them to get them to seat and release smoother as well.

 

[Gene 64 2dr]

Like Cobra Six said.

Cut some plates from sheet metal to match the carb base. All you have to do is lift up the carb, slide in the plate with a gasket under the carb and bolt it back down. Block off the fuel line and don't forget to disconnect the linkage to and your good to go.
There was a guy I saw once that had his offy setup just like this. He kept the carbs on just for show. He said he got tired of fooling around with them and unless you look closely at the linkage you would never know. The fuel lines were blocked internaly.

Gene