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DCOE Help Update: Problem Solved

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This post is an update on the power cutout problem I was having on my new car. I am happy to report the issue has been fixed! Thanks to everyone that replied, I learned a lot and now have much more insight into how Webers work than before. It turned out to be a weak fuel pump, which I had suspected all along. That explains why the PO had the pressure at 9 psi in an attempt to get enough flow.

Since my last post I checked synchronization with an STE synchrometer (thanks Lee) and it was fine. I put in a new Carter 4070 fuel pump with no regulator, which many have said works well. However, since I still had in the 225 needle valves it ran rich and puked fuel all over. I went back to 200 needle valves and a regulator and the problem is solved! I had two 30 minute sessions at Summit Point's Shenandoah Circuit with no power drops and good power between 4000-6800 rpm. The plugs look great. The AFR said rich with 140 mains and 170 airs so I went to 130 mains and 170 air. Now it reads a tad lean (13.2 - 14.1) at WOT and high RPM. At least now I can start to tune the carbs.

With the regulator (dial type Mr. Gasket) fuel pressure is about 4.0 at idle and between 1.0 and 2.5 psi on track, depending on fuel demand. This is reading pressure AFTER the Webers take fuel from the fuel rail. Reading the fuel pressure with no regulator showed the pressure cycling between 5.0 and 5.5+ psi at idle, which must be a function of the Carter rotary pump. The regulator damps out this pressure cycling.

I now understand that measuring the actual fuel level in the bowels is important, rather than relying on float depth settings only. Thanks to KFunk and 75duce for explaining this. While the float depth gives a general baseline, due to minor differences between carbs, float weights and likely other things, two "identical" carbs may have slightly different fuel levels for a given float setting. I need to take more measurements to find out what works best for my setup.

I met some great folks at O'Fest, including two other 02'ers that helped me out. Photo of 2002's below--Left to right: Fred, Alex and Scott. Photo shows the fuel pressure gauge I fitted outside my windscreen for diagnostic purposes.

Previous Posts in this thread:



Need DCOE help, loss of power on track (long)


Thanks, Fred '69 GT3 & '74tii



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those round hockey puck type regulators must have some production quality variance. when i was using one with my dcoe's, i set it at 2.5 on the dial, and the measured fuel pressure was 2.5psi all the time. idle or wot.

i don't see how people use the carter pumps without a regulator. way too much fuel pressure.

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uel pressure is about 4.0 at idle and between 1.0 and 2.5 psi on track,

That ain't right. It should stay consistent.

Spend the money, take the timeto plumb it, get a decent regulator-

the only one I've had work well over time is the Mallory 4309


which is $100 now.

It's worth the effort (and plumbing) to put it in front and plumb

the return back to the cell...

ANd yes, the Carter needed a regulator when I ran it with Webers, too.

And I still think your jets are too big...

Glad you've made progress, Fred.


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Hey Toby: Yes, progress finally made! Now that I have confidence in the carbs I am going to rebuild the engine to perform better. Compared to my tii it must be putting out only 120 HP or so now. Will do a Schrick 316 and perhaps 10.5 or 11:1 compression so I can run on pump gas or a blend of 93 & 100. And of course all the bits that will make this work.

With regard to fuel line pressure, I am not certain the lower pressure AFTER the carbs is an issue. Imagine a pipe with a given pressure and flow. If one withdraws a volume of fluid at some point (the carb needle valve), then the volume of fluid after this is smaller and I would think the pressure would be lower.

On page 72 of "Weber Carburetors - Tuning Tips and Techniques" (John Passini. 1992), it is stated that "Although the gauge pressure taken at idling drops when the engine goes on full power, the needle valve does not see it like that; some of the static pressure is now converted to flow energy (velocity head) and the equivalent total differs little from the static reading."

A way to think about how I am reading fuel pressure is to consider a bypass system with the regulator after the carbs. While pressure in the fuel rail should remain constant, I would think the pressure in the return line might vary based on fuel withdrawn. I will have to check this when I re-plumb. See diagram below for what I mean.

Here's something on dynamic pressure from the interwebs, a good reference page if for a different engine:

Fuel pressure/ filtering


"Webers need a pump that can provide a high volume of fuel at a low pressure. The fuel pressure should be regulated to between 1.5 and 2.5 pounds per square inch at high RPM and no higher than 3 PSI at low RPMs."

--Fred '69 GT3 & '74tii


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Yeah, it's not a big deal, but I found that varying the fuel pressure

at the carbs, what you show in #1,

did vary the way the car responded to jetting changes,

so I eliminated fuel pressure as a variable, since there's enough

to do on your average race weekend as it is.

This is just my advice, and it's worth everything you paid for it, but...

I'd tune the motor you have for everything it's worth before you take it apart.

And here's why:

I've NEVER made any power with a mechanical rebuild, with the

notable exception of the cam.

It's ALWAYS in the tuning.

Now before everyone jumps my shit, yes, with a higher compression

ratio, I can change timing and mixture and THEN get more power, yes.

But without adjusting the jets and the ignition, the new motor

ALWAYS made what the old one made.

Until it was adjusted to take advantage of its construction.

Even the cam wanted some tweaks to optimize it, but a cam swap alone

will make more power somewhere, usually at the expense of somewhere


So if it was me, I'd spend some days with the WBo2, the right foot, maybe

a dyno or 2 and get this one really wrung out before I cracked the engine.

Because let's face it- if you burn a valve or a piston crown by accident on

this engine, it's almost free. On a fresh rebuild, it's not.

Once you've got it dialled, a new longblock will take only a few tweaks

to be at full snuff- and then it's spanky- fresh, to boot.

So maybe put a new cam into this head, break it in and then tune it-

but put all your tuning hours onto this thing before you drop the

$3-10 large that a race motor costs.

That's just my take, having done it wrong myself in the past...


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Toby: good advice all the around. Makes sense that running a return regulator with constant pressure to the carbs is best. You are probably right that tuning the carbs with the current configuration first is the way to go. The reality is that it runs like crap below 4,000 RPM, so it is not set up correctly to begin with. When I start it the engine simultaneously backfires out the carbs and exhaust, driving around the paddock even when it is warm it sounds like poo. And good point that tuning on this engine is free, since I have a different long block with an E12 head that I want to build up to use with these carbs.

CONS: Reasons I may not try to tune these carbs with the current engine: (1) without mechanical changes this engine will never make the power I want. The cam is very mild (Norris 301), and while compression said to be 10:1 I am not confident this is correct. Head likely has no porting. Without taking this engine apart to see what is inside (which I am planning to do over the winter), I don't want to spend any more time on it; (2) I am planning significant mechanical changes to the engine, so tuning the carbs for this power level may not be all that instructive; (3) I want to take the carbs apart, clean and rebuild before going further. I will also eliminate the cold running mechanism with a block-off plate.

The new engine for the '69 will have a different head (E12), major porting, possibly +1 valves, much bigger cam (Schrick 316), higher compression (10.5:1 or 11:1), and a less restrictive header (perhaps the IE 4-1 step header). I have been driving track cars with about 140 HP for just about 20 years and want to move up to the next level. So the target for the '69 is ~180 HP on pump gas, and the target for the '74 is in the 240 HP range in 112-114 leaded race gas (year 4 of the build, perhaps it will actually happen this winter); (4) since I have two engine builds to complete this winter, I don't have a lot of spare time or patience to dial in the carbs on this unknown engine. In addition, the '69 needs a lot of additional fabrication and maintenance to be ready for next season. At a minimum I need to do short struts, the front is now 2" higher than the rear!

PROS: On the flip side, per your suggestion, perhaps I'll rebuild the carbs, pull the '69 engine apart and inspect it, put it back together with new bearings and go tune it on the dyno. I need to do a compression test before I pull it, the oil pressure is very good on this one. There is a place I have in mind for the big engine, which has both engine and chassis dynos (a SuperFlow SF-902 Engine Dynamometer and I believe the chassis dyno is superflow also). If I do the big engine on the engine dyno first I will have to fab a bellhousing for them anyway. Doing the small engine on the dyno would be a good way to gain experience with this shop and tuning on an engine dyno. To tell you the truth, if I could get 140 out of the old '69 engine I would be happy for a while, as that would be the same power to weight as my '74tii with the current engine (both cars weigh about 2000 lbs).

Thanks again for your input. It has given me a new perspective on what I might do this winter. Experience gained with the old engine will help me both with the 180 HP and 240 HP engine. And I'd much rather learn with the old engine than either of the new engines. As you can imagine, with all the systems I am putting together for the big engine, the total expense for that build is significant. Then add a second race engine....The photo below shows what keeps me motivated, I want to put this bad boy on track soon, and at the same time want to be confident the engine will hold together.

I welcome any additional comments/questions on my engine program(s) and DCOE tuning.

Thanks, Fred '74tii & '69GT3


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So the target for the '69 is ~180 HP on pump gas, and the target for the '74 is in the 240 HP range in 112-114 leaded race gas

Jeezuz, that's into the 'real money' range! I don't think I've ever had

more than 160 with a tailwind, and it took years for my wallet to un- pucker from THAT!

I think your thinking's sound- about the only thing I'd add is that when I

got carbs tuned for a 120hp engine, they bolted onto the 160 version

and needed VERY little adjustment for it. AND I was very comfortable

working on them by then, had the linkage all sorted, blah de blah...

Who's your engine builder?

Me, I'm thinking that F-Prod and going to tii spec's the way to go for my 2002.

EFI at 12:1 and the tii throttle body seem to be a viable combo.


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Toby--good to hear that the 120 HP DCOE tune translated over to the 160 HP motor with ease. More reason for me to get these carbs working properly on the current engine.

I'd like to respond to your other question offline. Please drop me a note using my e-mail button above. --Fred

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