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Discussion Starter · #1 ·
Running out of breath

As some of you might have read in another post, I did a driving school with my GT on Sunday (7/18). Engine ran great--no misses, etc. Could easily pull 1st and 2nd to my 6500 rotor rev limiter.

BUT...on the long straight, I would wind 3rd to 5000 (70 ish) and it would fall on it's face. It would still keep climbing, but very slowly. So I would up shift to 4th and it would jump to 90+ very quickly.

Could I be empting the bowl in 3rd gear? I do have a fuel pressure regulator on it set at 3lbs. But if that's the problem, wouldn't it do that same thing in 2nd gear too?

Ideas?

James
 

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I don't know the extent of your mods, but I've found that once you reach about 125-130 hp you'll empty a Weber's fuel bowl out somewhere in the middle/top of 3rd gear unless you use a higher volume (not higher pressure) pump and 3/8" fuel line.

Be cautious here, I used to run a lot of track events and if it weren't for my air/fuel meter I'd have melted a piston or burned a valve for sure. As it is I melted an electrode off a spark plug, luckily I had a spare set with me.

Bob
 

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If you had emptied the bowl in third than the upshift would have about killed it. This is a good one I've never heard of this particular problem before. I'll be watching and thinking. Sorry Bob posted at the same time.
 

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Discussion Starter · #4 ·
Sorry Bob, I meant to restate the mods. Duh, I know that would help!

2.0 with 11:1 Venolia
TGSI head W/ 44 & 38 Manley pro-flo valves
solid lifter 110 cam 256/250 .447 lift set straight up on marks
38 DGAS 155 mains 200 airs F66 e-tubes
header-- 2" all the way back w/ small turbo muffler
3/8 fuel line, 5lb 30 gal/hour electric pump w/ regulator set at about 3 lbs
Petronix w/ MSD box and coil, 12 deg static timing--just mechanical advance limited to 34 deg

I have heard of empting the Weber's bowl, but I don't understand why this would only happen in 3rd gear. Of course, I didn't have room to run 4th up that high.
 

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When increasing the size of the lines, don't forget to increase the size of the fuel outlet in the tank. The Carter P4070 pump works well BTW and doesn't require a pressure regulator, though it's a bit on the large side.

-Travis
 

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N61WP said:
I have heard of empting the Weber's bowl, but I don't understand why this would only happen in 3rd gear. Of course, I didn't have room to run 4th up that high.
It depends on engine load, not just rpms. Each gear takes progressively longer (time wise) to run through the rpms, and at the same time friction (tires) and aero drag increase the load on the engine (and therefore the fuel demand).

Also, a higher compression engine uses less fuel than a low compression engine (better efficiency).

You might want to reduce the air corrector size a tad, seems a bit big for high rpm use.

Bob
 

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Discussion Starter · #7 ·
Thanks for the replies, Guys.

This is very hard to duplicate on the street.

Travis, I will look into the tank fitting, which I did not change.

Bob, I probably will not fool with the jets on the 38 again as I am building a SSD 45 DCOE setup to install. You said the 200s were a tad large, but I swear it seems to run better with them then it did with the 180s that were in there. Do you feel that they could be the culprit?

Is there any possibility that this could be something more serious like cam timing? It does pull to red in 1 & 2 very quickly.

Thanks,
James
 

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Timing and jets . . .

N61WP said:
Thanks for the replies, Guys.

This is very hard to duplicate on the street.

Travis, I will look into the tank fitting, which I did not change.

Bob, I probably will not fool with the jets on the 38 again as I am building a SSD 45 DCOE setup to install. You said the 200s were a tad large, but I swear it seems to run better with them then it did with the 180s that were in there. Do you feel that they could be the culprit?

Is there any possibility that this could be something more serious like cam timing? It does pull to red in 1 & 2 very quickly.

Thanks,
James
Hmmm, and I was going to suggest simply switching airs to 175's but noticed you didn't mention your idles, actually better called transition jets. I DO think you're still running a bit rich overall. With your listed specs I'd suggest 145 mains, 165 airs, same E-tubes and 55 idles in your 38DGAS. Comments - R-Bob?

One other small detail, are you sure your total advance is limited to 34 deg? Before you do anything else, I'd try backing off the initial advance by about 4 degs and then running the car up through the gears again. A few degrees over-advanced total timing will exhibit very similar symptoms to what you describe. Again, not as much impact in the lower gears because of less apparent load to accelerate (gear torque multiplication), just as Bob said.
 

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on WOT acceleration the car feels like it hits a wall when it runs out of fuel

when it has to small of an air filter it will slowly creep up to speed
my MR2 would have a hard time going past 80mph until i realized the Air filter was way to small for a DOHC motor (took it off my FI GT)


a AFR meter is worth its weight in gold

JMHO
Davegt27
 

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AFR meter

davegt27 said:
on WOT acceleration the car feels like it hits a wall when it runs out of fuel

when it has too small of an air filter, it will slowly creep up to speed
my MR2 would have a hard time going past 80mph until i realized the Air filter was way too small for a DOHC motor (took it off my FI GT)


an AFR meter is worth its weight in gold

JMHO
Davegt27
Nothing you have to be HUMBLE about! ;)
 

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Discussion Starter · #11 ·
It does seem to "hit a wall" right about 5200 rpm. In the lower 2 gears and before 5000 in 3rd it was blitzing the tach (pardon the pun). If I get a chance over the weekend, I'll bypass the fuel pressure regulator and give it a try. I hate those things anyway. But for some reason my 38 is super sensitive to fuel pressure...way more so the the 32/36 was. I am running a 4 # pump and it still pumps over at idle w/o the regulator.

I have also bought a AFR meter and bung. I am going to have it welded on when I get my new exhaust system built. Soon, I hope....It's on the list...right behind the 45 SSD, the up-sized M/C and booster, relocating the battery, adding the roll cage, racing seats and belts, etc, etc.......
Overwhelmed Jc
 

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Discussion Starter · #12 ·
Otto, I meant to add that I plan to take your advice and check the timing also. The bypass hose is first, 'cause it's easiest!
Thanks for the help.
James
 

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AFR meter . . . which one did you get?

N61WP said:
It does seem to "hit a wall" right about 5200 rpm. In the lower 2 gears and before 5000 in 3rd it was blitzing the tach (pardon the pun). If I get a chance over the weekend, I'll bypass the fuel pressure regulator and give it a try. I hate those things anyway. But for some reason my 38 is super sensitive to fuel pressure...way more so the the 32/36 was. I am running a 4 # pump and it still pumps over at idle w/o the regulator.

I have also bought a AFR meter and bung. I am going to have it welded on when I get my new exhaust system built. Soon, I hope....It's on the list...right behind the 45 SSD, the up-sized M/C and booster, relocating the battery, adding the roll cage, racing seats and belts, etc, etc.......
Overwhelmed Jc
OOC, which AFR meter did you get and where from? :confused:
 

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Total timing degrees

N61WP said:
Sorry Bob, I meant to restate the mods. Duh, I know that would help!

2.0 with 11:1 Venolia
TGSI head W/ 44 & 38 Manley pro-flo valves
solid lifter 110 cam 256/250 .447 lift set straight up on marks
38 DGAS 155 mains 200 airs F66 e-tubes
header-- 2" all the way back w/ small turbo muffler
3/8 fuel line, 5lb 30 gal/hour electric pump w/ regulator set at about 3 lbs
Petronix w/ MSD box and coil, 12 deg static timing--just mechanical advance limited to 34 deg

. . . .
Just now looked at your engine specs again, are you saying your total timing (static + mechanical) is 46 degrees?!! . . . or 34 degrees (12 + 22)? :confused:
 

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Discussion Starter · #15 ·
Okay Guys, here's what I found out today. I put the light on it and have 12 deg static at idle (1000) and the full 36 at about 3000. That's about what I remembered it having. Man, I'm glad I made a mark on the front pulley when I built this thing!

I then bypassed the fuel pressure regulator and went for a ride. No problem at all getting 6500 in 3rd gear (90+). Went back home and pulled the top off the air filter and it was dripping fuel. It wound up killing after a few minutes of idle because of the raw fuel. I the rehooked the regulator and adjusted it up until it started to drip and then backed it off some. I haven't found my gauge yet to check, but I'm guessing it starts to drip at a little over 3 pounds. This carb has always been over sensitive to pressure. I think I'll just leave it alone for now and get back to work on the SSD setup.
Thanks for all the help,
James
 

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i used an Autometer AFR gauge

i was playing around with my MR2 trying to get it to pass emissions with out a Cat

I finally bought the AFR meter and it saved the day wish I had got one along time ago :)


Davegt27
 

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Has anyone tried to figure out WHY the 38 DGAS is more sensitive to pressure than the 32/36? I have the 32/36 on there now, but just got a 38 that I need to rebuild before putting on.

Looking at the Weber exploded view diagrams, they seem to be the identical design, and even share the same part number for the needle valve. What else would limit the flow into the bowl?
 

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Pressure vs. volume

asdasc said:
Has anyone tried to figure out WHY the 38 DGAS is more sensitive to pressure than the 32/36? I have the 32/36 on there now, but just got a 38 that I need to rebuild before putting on.

Looking at the Weber exploded view diagrams, they seem to be the identical design, and even share the same part number for the needle valve. What else would limit the flow into the bowl?
Since Webers are "volume priority" carbs, this is perhaps not as much a pressure issue as it is a volume issue. Weber carbs require low pressure so that you don't overpower the needle valve when the float bowl is full. Weber needle valves are available in a variety of sizes, so you might try a larger one (allows more volume to pass in same amount of time). Go one size up (volume) on fuel line between tank, pump and carb if you're experiencing "running the float bowl dry" problems at high RPMs under load.
 

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Thanks Otto, but my question is more along the lines of why the difference between the two different Webers. It sounds like the 32/36 is much more robust to higher pressure.
 

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How about Grose jets? I remember seeing ads for them years ago. I've been doing some searching for info and come up empty handed for the most part. Solution for a non existent problem?

Thanks,
Harold
 
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