VM33 smootbores

legrandemk18 wrote:

i must say i used to think that so long as the carbs where not sucking the bowl dry on top end all would be good.

I think i now have a better understanding, let me know if this statement is correct:

the higher the fuel level in the bowl the more air holes are cover up on the needle jet and pilot jet resulting in a richer mixture ?

I do run a fule pump and regulator, it was set at 2.5 psi,
turning it up to 3.5 richened up the mixture to 14.3 afr from 14.7, and had no effect after that, so i assume that at that point the bowls are full and operating off of the floats.

so currently i have 22.5 pilots, 140 mains and 3.5 psi with an afr of 14.3 am pretty happy with that and it runs great, egt's still running 1400, dont like that.

i got my other pilots in yesterday and swapped out the 22.5 pilot with a 30 expecting to go a little richer and the afr actually went leaner went to 14.6, so im a little confused about that, will play with fuel pressure some more and if that has no effect i guess ill try the 22.7 and 17.5 pilots again.

Thanks

Yea...I'm confused about that one myself. Very strange that going up 3 sizes on the pilot makes the AFR read leaner. I don't think I have much faith in the AFR at this point.

Take some duct tape and make wrap all the way around he pods. Just wrap the whole gang, no need to do each one individually. Leave only 1/4 of the available area open. Then see what the AFR says about that and if the motor likes it.

One more thing that might be worth having a look at your float bowl vents. Fuel can't get in if air can't get out.

You're not running some sort of ram air setup are you?

A way to tell if you have a large enough needle jet is to yank the main jets with the needle full lean clip position. If you can't make the motor go dead rich, a larger needle jet is needed.

legrandemk18 wrote:

i must say i used to think that so long as the carbs where not sucking the bowl dry on top end all would be good.

I think i now have a better understanding, let me know if this statement is correct:

the higher the fuel level in the bowl the more air holes are cover up on the needle jet and pilot jet resulting in a richer mixture ?

I do run a fule pump and regulator, it was set at 2.5 psi,
turning it up to 3.5 richened up the mixture to 14.3 afr from 14.7, and had no effect after that, so i assume that at that point the bowls are full and operating off of the floats.

so currently i have 22.5 pilots, 140 mains and 3.5 psi with an afr of 14.3 am pretty happy with that and it runs great, egt's still running 1400, dont like that.

i got my other pilots in yesterday and swapped out the 22.5 pilot with a 30 expecting to go a little richer and the afr actually went leaner went to 14.6, so im a little confused about that, will play with fuel pressure some more and if that has no effect i guess ill try the 22.7 and 17.5 pilots again.

Thanks

The higher the fuel level in the bowl the easier it is for atmospheric pressure to push the fuel into the low pressure of the venture area above, well, vice versa actually. Remember that carbs work on pressure differences.
I cant see how the fuel level would not be maintained by the floats, unless the pump over powered the float seats. Could this be what happened when u increased the fuel pres. from 2.5 to 3.5psi ? maybe the bowls are full now ,therefore there is no more rise in level , although if that were the case u should have evidence of this with overflow.
When u increased the pilots size, did u change the air screws settings? If u cant read an increase in a/f ratio when u richen it with the air/ fuel screws, then there isn’t much point in changing the jets. U must figure out why u are not getting a change first.
U are changing the pilots to satisfy the a/f gauge.
At some point u may have to dought the gauge and use another test method.
I would suggest that u try the test that I mentioned earlier for the pilot system, if u exhaust the a/f gauge tests. U will be able to FEEL the dif with this test.
Did the fuel pump come with any instructions? Is there a common psi for this ap ?

PSI for carbs is "generally" 4-6psi. Flow of pump in GPH is directly related to engine displacement, RPM with A/R in mind. If the motor can't get enough fuel at WOT from the fuel delivery system, it will be lean.
I would not run the engine with Main jets removed, as this is what holds the Needle Jet/ emulsion tube assy in, without them you could possibly stick the throttle open at least partly, if it comes loose it can rotate and then be up on its locating pin.
Drill out some old mains real big to simulate this test, it will be a better idea.
The 1400 EGT is not real far off for WOT with that A/R, some races tune for that area, but the engines don't live forever. Getting a bit richer would be good. If you block off the Pod's like suggested it should "Choke" to a richer ratio.
Its been asked a couple of times, Have you set the Fuel level in those carb's? Fuel level is very important.
Ignition timing can play into the EGT readings as well to a point.
Isn't tuning Fun! Love it!

Motor Head wrote:

PSI for carbs is "generally" 4-6psi. Flow of pump in GPH is directly related to engine displacement, RPM with A/R in mind. If the motor can't get enough fuel at WOT from the fuel delivery system, it will be lean.
I would not run the engine with Main jets removed, as this is what holds the Needle Jet/ emulsion tube assy in, without them you could possibly stick the throttle open at least partly, if it comes loose it can rotate and then be up on its locating pin.
Drill out some old mains real big to simulate this test, it will be a better idea.
The 1400 EGT is not real far off for WOT with that A/R, some races tune for that area, but the engines don't live forever. Getting a bit richer would be good. If you block off the Pod's like suggested it should "Choke" to a richer ratio.
Its been asked a couple of times, Have you set the Fuel level in those carb's? Fuel level is very important.
Ignition timing can play into the EGT readings as well to a point.
Isn't tuning Fun! Love it!

You've never had a rack of Mikuni Smoothbores in your hands have you? Do you honestly think I'm going to tell the guy to do something that will cause the throttle to stick......

Just so you know.....the needle jets thread into the jet block. The main jet doesn't hold them in.

If you've obviously never worked with a particular rack of carbs then how do you have any insight into tuning them?


"Don't try to describe the ocean if you've never seen it" - Jimmy Buffett

When normal tuning methods don't have the expected results it's time to revisit the most basic functions and figure out what's wrong.

The A/F meter is questionable at this point but the EGT would worry me.

Since this is a car and the engine appears to be in an enclosed area, there might be some pressurization happening that's causing odd behavior and funky readings.

I notice the fan right behind the NACA duct. What is it's purpose.

If there is any under hood pressurization happening, the float bowl vents will have to be addressed.

update from this morning:

revisited the fuel tank vent, is clear and fitting in tank is clear, so fuel tank vent system good.

drained all four bowls into clear plastic cups, all four bowls had same quantity of fuel in them.

made a rig to check bowl level with pu,p running with clear plastic hose, all four bowls had the same fuel level noted, which was level with top of bowl where it mates to carb body.

blew through the bowl vent hoses with bowl plugs removed, and all bowl vents are clear.

put bowl caps on and ran fuel pump to fill bowls, removed pods and blew threw bowl vent hoses and fuel came out air jet, so air jet passage is clear.

bypassed inline fuel filter as i thought this could be restricting flow, and that made no difference.

finally removed main jets, left jet extenders in, and ran with no main jets in place, made no difference in afr or performance of car running up to wide open 9500 rpm.

taped pod individually but only taped 1/2, this also made no difference in performace or afr up to wide open.

taking a break now, will tape 3/4 and see if that does anything, i also rechecked for intake leaks at carb boots, none noted.

I did call the previous owner of the car who ran it in colorado, 6000 ft and it ran with a 10.0 afr their and 1200 egt's, this was with 17.5 pilot and 130 mains
im currently up to 30 pilot and 140 main.

A, i overlooking something yall can think of, thanks again for all the suggestions.

heres a pic of another car im building, but its a long way from done yet.

lets try that again

larry

forgot to answer your questions.

the fan is for the oils coolers at low speeds, they come on when oil temp hits 140, theirs one on either side. their is a ram air set up to the oil coolers, but they vent to the wheel wells.

the engine bay is pretty open, the body work is just a shell, and the engine bay is open to the top, bottom, and fender wells, their is no pressurization of the engine bay
and i do not run a ram air set up to the carbs.

:huh:

legrandemk18 wrote:

update from this morning:

revisited the fuel tank vent, is clear and fitting in tank is clear, so fuel tank vent system good.

drained all four bowls into clear plastic cups, all four bowls had same quantity of fuel in them.

made a rig to check bowl level with pu,p running with clear plastic hose, all four bowls had the same fuel level noted, which was level with top of bowl where it mates to carb body.

blew through the bowl vent hoses with bowl plugs removed, and all bowl vents are clear.

put bowl caps on and ran fuel pump to fill bowls, removed pods and blew threw bowl vent hoses and fuel came out air jet, so air jet passage is clear.

bypassed inline fuel filter as i thought this could be restricting flow, and that made no difference.

finally removed main jets, left jet extenders in, and ran with no main jets in place, made no difference in afr or performance of car running up to wide open 9500 rpm.

taped pod individually but only taped 1/2, this also made no difference in performace or afr up to wide open.

taking a break now, will tape 3/4 and see if that does anything, i also rechecked for intake leaks at carb boots, none noted.

I did call the previous owner of the car who ran it in colorado, 6000 ft and it ran with a 10.0 afr their and 1200 egt's, this was with 17.5 pilot and 130 mains
im currently up to 30 pilot and 140 main.

A, i overlooking something yall can think of, thanks again for all the suggestions.

Good Test Process

If you are able to run up to 9500 without any main jet installed, the needle jet / needle setup is too lean hmmmm imagine that :laugh:

Regardless..... here's the plan

I think you mentioned having P-0 Needle jets for those carbs. If so, they are your next tuning component that must be tried.

First though, and this is a pain but you better do it anyway, verify the needles. Number and clip Position.

Here's Sudco's jetting specs on those carbs now [they've changed over the years]

Main Jet 132.5
Air Jet .6
Needle Jet O-6 [normally a good needle jet]
Pilot 17.5 [dam that threw me a curve]
Needle 6DP30 on 4th clip position
Slide C/A 2.0

If the needles check out to be proper number...great. But put them in full lean clip position. First groove down from the top of the needle.

I would use .6 air jet if you have it , if not use the smallest / closest you have.

Assemble again with NO main jet and head back out on the track.

You are looking to achieve having the carburetion go TOO RICH from about 3/4 throttle on up. If that happens, you now have a workable midrange jetting setup.

So if that works out as planned, Install the mains. It's up to you which to use but I would start with the 130's first. Test drive again. Work it from there.

Note: I can't tell you what clip position the needles are going to end up at. You're just going to have to sort them as you go. Normally, those carbs on a KZ 1200 I end up at 2nd clip from top in my area at 700' elevation with lots and lots of good air...

With a good working AFR meter, to fine tune once you get well within the ballpark, the Air Correction Jet is your quickest / easiest tuning component to work with and should affect noticable meter readings.

Holler back and good luck

Here's Sudco's jetting www.sudco.com/Diagrams323728/expvm33.gif

Larry C

made some progress finally, raising the needles to the 5th clip took me to 14.0 from 14.8 afr, and the egt's are running 50 degrees cooler.

dont understand why going up to p-2 needle jet wouldnt have done the same thing tho, but oh well.

question, since im on the last clip, can i raise the needle more with a shim ?
these carbs have a cap above the needle and a plate that hooks to the throttle lever with a tab that holds tension on the needle cap, can i shim the needle with a washer and bent the tab a little, or should i go with a different needle, the 6dp30 needle doesnt have any richer or leaner needles in that series of needle, and im not sure what other needle would be comparible to it.

thanks again guys.