Need more top end (UPDATE)

You could try 30 chokes

Your jetting looks pretty close. I'll bet you could go to 30 chokes and maybe not even change the jets. It will be close, you can check before buying new ones but don't kill the engine with a lean mixture. If it lags or has flat spots then it would need larger main jets. I assume it idles around 1300 RPM's or so.

Charlie
 
Wow this is getting fun. She idles best at 1100 rpm. I like your analysis, looks like ill try the 175 mains and 32 chokes. I found some 24 chokes in my parts collection Im going to get them cut to 32mm and see what happens. That way I have the 28,s to fall back on or cut to 30's. I'll also try 190 air Correctors. Lots to experiment with this weekend.
 
1100 RPM

at idle is quite good for that cam. That's a good sign and means that the cam is timed probably pretty close if not right on. If it were off 2 teeth the idle would be horrible. When you solve this problem you will say why didn't think about that in the first place, it was so easy.

Good luck
Charlie
 
Agreed, idles great and no hesitation or studder when I give her gas either. Still need to verify Tach is correct. Haven't had any garge time this week. But I suspect it's correct. Also need to get those advance curve numbers. The idea of the chokes being to small has grown on me. I have carb parts on order so we will see.
 
Update

Made some good progress the last few days. I verified the Tach is dead on. So taking advise from many on this board I decided to work on the Carb. Old Fiat Driver suggested I go larger on my chokes and many others had some questions about my air cleaner size. So I ordered some new Main Jets and Air Correctors. I had a pair of 24mm jets in the garge that I got machined to 32 mm. I also decided to go back to my old setup with Velocity stacks and air cleaner on top for more air.the results were very positive. She now pulls strong to 6,500 RPM's. Here is the current jetting.
Single 42 DCNF
190 Air Correctors
F24 Emulsion Tubes
175 Mains
.50 Pump Jet
.45 Idle Jets
80 F7 Starter Jets
32mm Chokes
I tried going up on the Idle Jets to .50 but after fine tuning it idled much better with the .45's
I'm a little further out on the idle mixture screws than I woul like (3 1/2 Turns) but it seemed to run best with them there. I will have more time early next week to fine tune some more but I'm pretty satisfied at this point. I still need to borrow a timing light and get the advance curve numbers so that's next on the checklist. I also have a right rear carrier coming, bad bearing, that need to replace. Thanks for all the help and if anyone thinks my jetting doesn't sound right let me know.
 
Cool Art

It looks like it's starting to come together. The total timing number will be important. Idle jets are a function of the displacement of the engine, 1500 CC. .45's do seem a little small. The idle mixture screws should be out about 3/4 to one turn when right. It's not set in stone but should be pretty close. If the idle screws a out less it indicates the idle jets are too big. If they are out more it indicates they are too small. This is all assuming that the carb throttle plates are staged correctly in regards to the progression circuit, ( holes in the throttle bore just in front of the throttle plate except for the last hole which should be covered by the throttle plate. ) Your main jetting at 175 looks pretty good for a 32 Mm choke.

Keep up the good work
Charlie
 
Agreed on the Idle Jet, I'll try the .50 again and see if I can get the mixture screws closer to where they should be. Tell me what the Air Corectors do. I have 220's I can try with the 175 mains but I'm unsure how that affects things.
 
This is the first I've ever read about this crank case breather vacuum. Very interested, could anyone post a picture or maybe try to describe how to do this mod. I've always ran my breather hose to a small k&n filter sitting close to the drivers side, side air inlet.
 
Not trying to hijack the thread but very interested in a lot of things that have been discussed as my X also suffers from the exact same 5400rpm wall. Narrowing down all the possible causes discussed, I've concluded that it's either the fuel pressure or the Bosch box? I'm running twin Weber 45DCOE's and keep my fuel pressure regulated to 2.5psi so I will try increasing it to see if it helps. "artz1731" was told that fuel pressure could be a cause among other things and he was able to find and fix the issue but not sure if adjusting the fuel pressure was part of the fix?

Also the Bosch box of certain years having a rev limit is very interesting to me. Do the Bosch boxes have different markings or identifiers from different years. How would I know if I have one of those Bosch units?

Congrats for being able to fix the issue and happy revving.
 
Hey Pete, Fuel pressure wasn't the problem it was my Chokes being too small, not allowing enough air into the carb. So when I went from 28mm to 32mm chokes I also needed to increase the Main Jet size from 160 to 175's. I also changed the air filter setup. You shouldn't need more than 3 lbs of fuel pressure.
I still need to do a few things yet but I'm pretty satisfied.
 
Carb set up

As I have asked in this post before what are the venturi or Choke sizes you are using.

You can have great bottom end and off the line starts with a small venturi. This will also give you fairly good fuel economy. Be careful as it is possible to go too lean and run the engine too hot. I have seen the exhaust ports start to bleed aluminum droplets.

So the basics are as follows small venturi great bottom end and off the line starts with good fuel economy, the engine will starve at higher RPM's. Larger venturi's require larger jets, have poor bottom end but great top end, poorer fuel economy.

You need to always tell us what venturi size you are using, what main jet and what air correction you are using. The idle circuit is totally different. If your carb is set up correctly you can remove the emulsion tubes while the engine is running off of the idle circuit. If you remove the emulsion tube and the engine dies then your throttle plate is opened too much on the stop and you are running partly on the main fuel circuit. Correct this. You can also change the accelerator pump jet as well to prevent stumble when punching the gas pedal.

So let us know what set up you are using, idle jets, main jets and air correction jets. Then we can arrive at a solution.

One other thing. All stock Fiat engines were under carburetted for emissions reasons, the stock cam will restrict flow just above 5,500 rpm. So cam timing is also an important variable.

TonyK.

Grimsby Ontario Canada.
 
Hi Art

The air correctors work in conjection with the fuel jet and the emultion tube. Here is what happens. The total amoun of fuel is mainly controlled by the fuel jet and the air correctior. The fuel jet supplies the fuel but the air corrector weakens the mixture by adding air thru the air corrector jet. The fuel jet has about a 3 to 1 effect on the fuel rario and the aire corrector has about a 1 to 3 effect on the fuel ratio at the lower RPM's. As the RPM's rise the ratios reverse and the fuel jet has about a 1 to 3 effect on the fuel ratio and the air correctir jet has a 3 to 1 effect on the fuel ratio at the high RPM's. The emultion tube also has air corrector holes that come into play at different throttle settings ( load ) and RPM's. By changing the location, amount and size of the holes you can alter the fuel ration at all those different load points. Emultion tube holes are the most difficult part of the fuel citcuit to get right. Making the fuel jet larger adds more fuel all through the RPM range but more at the lower RPM's than the higher RPM's. Making the air corrector smaller richens the fuel ratio all throught the RPM range but more at the higher RPM's than the Lower RPMS. The 220 air corrector in your setup seems a little lean at the higher RPM to me, just a guess. You might want to try a smaller air corrector from say 210 to 180 or so. You'll need to experiment if you don't have a wide band fuel sensor.
Good Luck
Charlie
 
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Good question on the Bosch part number

I don't know. The way to tell is to get a timing light with an advance read out as well as an rpm read out. Then run the enging thru the rpm's and take a reading.

Charlie
 
One other thing Art

When you set the idle with either of the idle jets you choose check the progression orifaces at idle. They should be exposed except for the last one which should be covered by the throttle plate.

Charlie
 
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