FV carb set up

[Don]

Good Morning........

Looking for info on FV carb set up. I have been racing for 3 years with the carb jetting that existed when I bought the car. Don't have ready access to a dyno so have only checked plugs and exhaust color. Appreciate that many will have their secrets but..............questions are:
The rules say that the accelerator pump boss must not pass a #56 drill. So is it a good idea to drill it out to one size under?
The venturi can be machine out. Should the ID be machined out as large as possible (really thin wall)?
The bowl is radiused, float weighted. Current jetting is pretty radical, 240 main and 220 air. Seems to run ok (2 seconds slower than the fast guys and 1 second faster than the slow guys). From what I've read its backwards, air should be larger than main. As I said don't have easy access to dyno and only get 50 minutes of track time per saturday and sunday so I am reluctant to play around with jets.
I do plan to send a carb to SR for dyno set up, but in the mean time any suggestions for a better starting point besides the jets I have?

Thanks, don

[SR Racing]

so have only checked plugs and exhaust color.
Either are helpfull but you can still be off a long way. The plugs have to be read immediately after a WOT run. So it's tough to stop on the track and read them. <g>

The rules say that the accelerator pump boss must not pass a #56 drill. So is it a good idea to drill it out to one size under?

Yes, drill it out as close as you can. This has nothing directly to do with fuel or A/F ratio. It simply allows for more CFM. Years ago some were drilling it out to the pint that was just a big hole. It did help air flow. So the rule was imposed.

The venturi can be machine out. Should the ID be machined out as large as possible (really thin wall)?

The easiest way is to replace the venturi with one specially turned. We have them. They allow more air, but still keep a decent profile to properly mix across the load and RPM range. Some people have completely removed them which can be done, but it may screw up mixture especialy at low RPM.

Current jetting is pretty radical, 240 main and 220 air.

On the surface the main seem pretty fat. However, some people have messed to much with the profile of the emulsion tube holder (filed off the eyebrows) This usually causes the need for a larger main. (and probably poor mixtures at some RPM points. )


From what I've read its backwards, air should be larger than main. As I said don't have easy access to dyno and only get 50 minutes of track time per saturday and sunday so I am reluctant to play around with jets.

See above.


I do plan to send a carb to SR for dyno set up, but in the mean time any suggestions for a better starting point besides the jets I have?


And we will improve flow as much as legally possible and then jet on the dyno.
If you want we can take a baro reading at the time of the run and then send you another couple mains (one above and one below the correct one for conditions.) Then if baro is very low or high you can adjust as required.

[Dietmar]

Don:

Since it was your post, you can delete yourself- but since you asked, it has been deleted as per your request.

Dietmar

[brian]

I personally don't drill out the accelerator boss. The Germans make the hole small on purpose to extend the duration of the squirt of fuel. Drilling out any of the passages will shorten that duration and may cause a stumble. We have all been seduced by flow benches only to find out we've left something off for the track.

Plug reading is quite an art and is still the best way to watch mixtures. Even Jack Roush still does all the plug reading and tuning at the track. Here''s a great article about plugs. http://www.dragstuff.com/techarticles/r ... plugs.html

[hardingfv32-1]

Are we talking about drilling out the jet in the pump circuit or the boss in the cab top?

While plug reading is a wonderful way to WATCH mixtures, an O2 sensor system calibrated on a dyno is much better for setting the mixture.

Brian

[SR Racing]

Brian,

Reading plugs on a Vee (unless on the dyno) is a waste of time. (Unless you stop on the track after a WOT run and remove the plugs. ) The modified 28pci has attrocious low end a/f control. By the time you get into the pits you cannot properly read plugs.

O2, or even EGT is far better.

The fuel flow to the pump cicuit jet is not controlled by the orifrice at the jet (in the throat) It is regulated by a small brass insert in the carb body under the screw plug. When the engine is running (especially at WOT) the air through the jet quickly emulsifies no matter what the size. (Actually I suspect it emulsifies better when drilled out.) And.. the CFM delivered through the carb increases the bigger that hole is. (Enough so that it can easily be seen on a flow bench. The original bug was designed for decent acceleration from a stop light and certainly never to 6000+ RPM. The amount of fuel delivered can be adjusted via the throw of the accelerator pump arm. (Usually by bending or adding a washer.)
While many of the things the German engineers did were pretty good, they never anticipated the CFM requirements and venturi gutting we do. They had bad requirement data presented to them. If Ferdinand or Adolph were FV drivers, the carb would have been a lot different.

[SR Racing]

Are we talking about drilling out the jet in the pump circuit or the boss in the cab top?

Since he refers to the #56 drill size, I assume he is talking about the boss in the carb top.

[brian]

I completely agree that an on board O2 sensor is the hot set up but not everyone has the several hundred dollars to buy one. Yea, it's a pain to push the car back to the pits but a WOT cutoff is not impossible and the article I gave a link discusses some of these issues and how to read more than just mixture. I think NASCAR forbids onboard data so that may explain why they still read plugs.

[problemchild]

Don:
Most people in the Northeast just buy a carb from Dave Carr at Autowerks. He sets it up on the dyno. You never need to check plugs, O2 sensors, or have any discussions about any of this stuff. It will be good on any engine. You can forget about it and worry about lots of other stuff. His contact info is in the banner above. Anybody that tells you they have to tune it to your engine, car, time of the month, track, manifold, time zone, etc, is blowing smoke up your a$$ and probably trying to sell you something. Rather than buying dyno-testing on your carb, or DIY, IMO, make that carb your spare, and buy one from one of the top established engine builders.
Cheers!

[SR Racing]

Don:
You never need to check plugs, O2 sensors, or have any discussions about any of this stuff. It will be good on any engine.

True. Once jetting is done, it will be good on any Vee engine.

Anybody that tells you they have to tune it to your engine, car, time of the month, track, manifold, time zone, etc, is blowing smoke up your a$$ and probably trying to sell you something.

No one told him he had to tune to HIS engine. We can tune it on any FV engine. I am not sure where you come up with the time zone, time of the month etc. stuff.

Rather than buying dyno-testing on your carb, or DIY, IMO, make that carb your spare, and buy one from one of the top established engine builders.

Thanks, but why should he buy a carb? His may flow fine and/or be upgraded slightly. If it doesn't appear to flow well and can't legally be improved, we can talk about options.
BTW, If Dave doesn't tune them on a dyno, exactly HOW does he do it?

On our dyno runs of carbs, we can check A/F via 02, actual fuel flow and air flow, AND EGT. There is no better way of doing it. I might also add, it is important to know where the engine is used most often. 500 ft of elevation change will typically mean a 1/2 jet size change. And while not quite as important, relative baro pressures can also make a jet change optimum.

BTW, IF someone calls and just buys a carb off the shelf with no dyno tuning and not knowing what aprox baro it was set at, then SOMEONE really is blowing smoke.

But thanks for your input.

[SR Racing]

Off thread a little, But blindly buying a carb or manifold (or heads) from anyone is a crap shoot. I will normally not sell a manifold or carb unless that person sends me their existing part. I will flow each and make SURE that what he is buying is better than what he has. (or advise him of what the differences are.) There are probably several people on here that have bought these items from me and can vouch for the above.
I have had several people who have bought manifolds and carbs from others to replace what they had and we find that the original one was BETTER than what they bought. We will not do that.
I also invite people here if logistics permit to see their engine run on the dyno and/or flow anything on the bench. From A/C1200's to BB drag motors all owners are invited to watch the runs.
While it costs a bit more, we have had a few people want to see before and after runs (either on the engine dyno or chassis dyno). BTW, we also set up the dyno to make programmed runs at different rates of acceleration. So there is no operator variables. (As I have seen on lots of other dyno sheets.) (We also log EVERY variable room baro, air temp, oil temp, etc. so that runs are comparable. Not to mention all the on-board inputs.)

While most of our $ come from the large block customers, where they demand some of the above, A/C FV/FST/Vintage engines are a labor of love and thus get all the same benefits.

Also over the years, we have had the chance to dyno other builder's engines (chassis and engine dyno). (Plus flow parts). They are ALL comparable with little differences in a good race prepped engine. (We sometime question some things as to reliablity in engines that we have torn down. But, there ARE no HP secrets left in the FV. It is just a matter of good measuring, machining, assembly and tuning.

When the run-offs were at Mid-O (and even after) we had some of the top cars here in the shop for dyno testing. (on their way to the track) I KNOW what they made in HP, RPM, etc.

At this point in the FV game, have whatever pro shop (or experienced builder) you are comfortable with or closest to, or offers the best service/price do your engine. You will be fine. That is the truth. I don't knock the other builders, because I have seen most if not all of their work. I have seen a few questoinable, but don't always have the full story behind the build. But, IF we didn't build engines, I would be comfortable with most any of them. We just try to do better, just like everyone in a competive business.

[SR Racing]

I completely agree that an on board O2 sensor is the hot set up but not everyone has the several hundred dollars to buy one. Yea, it's a pain to push the car back to the pits but a WOT cutoff is not impossible and the article I gave a link discusses some of these issues and how to read more than just mixture.

Yes. I agree. and I read the article. (I questioned a couple things there though. <g> )

I think NASCAR forbids onboard data so that may explain why they still read plugs.

Yep. Not mentioned in the article, but NASCAR engine builders do LOTS of in depth testing. I think they ONLY read the plug to see if something has broken somewhere. These guys KNOW what the a/f ratio is going to be at under any circumstance before they ever take the track. But reading the plug could detect a poor cylinder, etc.

One thing they do is regrind the distributor cam lobes for different timing of each cylinder based upon its requirments. I suspect that reading the plug at the track may help them optimize that.
(BTW, We thought about doing that in a Vee. Due to it's poor induction and cooling system, it could be very helpfull. We woulld NEVER do that though since it would be outside the rules. <g>)
(and don't tell me about the old retarded #3 cylinder thing. I have never seen an 009 with it and I haven't seen any 010 distributor in use that have it.)

[VORT94]

FV racers might not have to change jets once the carb is dialed in if they run around sea level but come out to Colorado and see if your jetting is good. I agree with most everything that Jim at SR said except the half jet size per 500 feet of elevation change. That would make a 220 main at 500 feet change to a 180 at 4500 - WAY TO MUCH. Also as a side note- see what your oil temp does at 5000' on a 90 degree day with 20 percent humidity.

[SR Racing]

Yes. I should have pointed out that the 1/2 jet size for 500 ft is probably ok from SL to 1500 feet, (given equal relative baro), but it's probably not going to be linear. So you wouldn't use that at very high altitudes.