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Old 06-08-2010, 06:57 PM
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Horrible gas mileage! LOL Also need some advice

I just went on a 400 mile trip all together going there and back. My engine is a 383 stroker built for around 500 horses give or take. Not completely sure havent dyno'ed yet. Anyway the transmission is a t56 which has 2 overdrives,
6th gear is .5. Rear end is a 3.42 posi. The carb is a holley 750 double pumper with a proform mainbody and 4 corner idle. I adjusted the carb for highest vacuum. Im now running vacuum advance and have it set up best i can. At idle I can remember for sure, but i think i have about 30 degrees of advance and at its highest point im getting 50 degrees. Centrifugal is at 38 degrees total all under 3000 rpm.

I averaged 12 mpg all highway going 55-75 mph. Just about all of the driver was done between 1600 to 2100 rpm's. I know double pumpers get bad gas mileage but i didnt think it was this bad... i think im going to just get another carb. What would you recommend for my engine? You can look at my previous posts for the exact specs of the engine.

Now a few things you can gather.. When i got WOT the car will go, but it just dosent feel like it has the power it should, also when i look behind me there is a cloud of black smoke. Evertime i stopped for gas i would fool with the timing and mixture screws a tiny bit to just see what works best. The best combination i found was with the centrifual advance at 40 degrees total with vacuum advance at around 54 i believe and with the idle screws at 3 quarters of a turn out. The car still bucks in all gears under 5th and 6th like crazy. I noticed before i turned the screws out a tiny bit, the car simply would not get out its on way. It was already running rich, but when i made the cruise and idle screws just a little more rich the car seemed alot more responsive and the acceleration was smooth.. mostly. Now when i go WOT the car still sputters, but still goes dectently. About as fast as a stock mustang gt. So its till not running as it should.

Im positive its a fuel deliver problem.. what else should i try?

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Old 06-08-2010, 10:23 PM
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Well I take it at least runs now? That`s a step up. Anyways, if you see black smoke coming out the exhaust when you kick it can be several things such as float set too high, jets too large on the secondary side, weak ignition or simply put the carb is too big for the application. The bucking and kicking is usually a ignition problem.
I`m not sure if I read your post right, but moving the mixture screws is effecting off idle performance? I don`t see how, the mixture screws control idle and that`s it. I agree you should get rid of the carb, I don`t trust nothing with proform written on it not to mention that downleg boosters are completely worthless. I would use a holley 670cfm street avenger. Your engine being a street machine would likely be happier with less cfm.
I don`t know what size cam is in the car, but if the powerband doesn`t kick in until 2000 or higher and the cruise RPM in 5th and 6th gear is below that it`s not good on the engine because it`s placing it in a lug which will cause detonation to set in. Have you checked the spark color? blue means healthy, yellow means weak. Since you`ve had alot of problems with it I would surely check it, as many times carb problems diagnose the same as ignition problems. What is the entire combo of the engine? Maybe with a broader look we can help further.
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Old 06-09-2010, 09:09 AM
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12 is pretty bad.

is the speedo correct?

try a 3310 (750 cfm, vacuum secondarys).
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Old 06-09-2010, 11:27 AM
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Quote:
Originally Posted by stroker444
I just went on a 400 mile trip all together going there and back. My engine is a 383 stroker built for around 500 horses give or take. Not completely sure havent dyno'ed yet. Anyway the transmission is a t56 which has 2 overdrives,
6th gear is .5. Rear end is a 3.42 posi. The carb is a holley 750 double pumper with a proform mainbody and 4 corner idle. I adjusted the carb for highest vacuum. Im now running vacuum advance and have it set up best i can. At idle I can remember for sure, but i think i have about 30 degrees of advance and at its highest point im getting 50 degrees. Centrifugal is at 38 degrees total all under 3000 rpm.

I averaged 12 mpg all highway going 55-75 mph. Just about all of the driver was done between 1600 to 2100 rpm's. I know double pumpers get bad gas mileage but i didnt think it was this bad... i think im going to just get another carb. What would you recommend for my engine? You can look at my previous posts for the exact specs of the engine.

Now a few things you can gather.. When i got WOT the car will go, but it just dosent feel like it has the power it should, also when i look behind me there is a cloud of black smoke. Evertime i stopped for gas i would fool with the timing and mixture screws a tiny bit to just see what works best. The best combination i found was with the centrifual advance at 40 degrees total with vacuum advance at around 54 i believe and with the idle screws at 3 quarters of a turn out. The car still bucks in all gears under 5th and 6th like crazy. I noticed before i turned the screws out a tiny bit, the car simply would not get out its on way. It was already running rich, but when i made the cruise and idle screws just a little more rich the car seemed alot more responsive and the acceleration was smooth.. mostly. Now when i go WOT the car still sputters, but still goes dectently. About as fast as a stock mustang gt. So its till not running as it should.

Im positive its a fuel deliver problem.. what else should i try?
My 454 78 chev pick up never saw the high side of 9.5 with an 800 CFM Q jet and a 4 speed. So 12 with a double pumper Holley sounds good to me.

There is a problem with big carbs and overdrive transmissions where the RPMs get real low. These modern OD trannys running with a high ratio rears are intended for electronic fuel injected engines where air flow thru the system has no effect on the quality of fuel metering, that being measured, timed and placed by a high pressure system under computer control. Carbs depend upon air flow thru the venturi to meter fuel. When that flow is reduced to a minimum against large size main metering and air correction jets, the whole system gets pretty sloppy with wide digressions from the ideal mixture ratios along with poor transitions with throttle movement involving lean lags on acceleration and rich overshoots when decelerating. Messing with the idle circuits has no effect on this. Why 4 corner idle? This is something for engines with radical cams that can't get enough air off two throttle blades without opening them so much the transition circuit become exposed.

50 degrees of timing is way too much, a 454 even with open chamber heads shouldn't need more than 36-38 degrees total. Centrifugal and vacuum advance are not additive, at least they shouldn't be. These are systems for different operating regimes of the engine. Vacuum for low RPMs with a mostly closed throttle where the centrifugal hasn't come in, the manifold vacuum is high and the RPMs low. At this time the mixture density is low, that is molecules to volume. This is a slow burning mixture that requires more advance go get power out of it. Centrifugal comes up with higher RPM and opening throttle which lowers manifold vacuum and increases mixture density. This is a faster burning mixture that needs more advance because the time allocated to the burn gets very short so if it isn't started early enough unburnt mixture goes out the tail pipe instead of making power. Consider in all cases making power within the situation, also, develops the best fuel economy possible with that situation.

You have a slow tuning engine at cruise which is the worst of all worlds for your set up. Not knowing the vehicle I'll have to take a guess at two opposite but equally bad scenarios. First; low RPMs with a mostly closed throttle, typical of an engine like this in a lightweight chassis. In this case the engine is not using much air and the throttle is mostly closed. This leads to poor fuel delivery because there just isn't enough signal strength for the carb to discriminate fuel ratio requirements very well. Add to this, the mixture density in the cylinders is low. This among other things leads to miss and late fires. Aside from through mixture adjustments done on a dyno, the next best thing is a multiple spark ignition to insure that the thin mixture is lit off to avoid miss and late fires that waste fuel by not using it. On an exhaust gas analyzer the engine will appear rich even though the main metering may be correct or even lean.

Second; is a heavy vehicle with high final gears and low operating RPM. Here the throttle will be greatly opened with no increase in RPM. The centrifugal advance is not yet operating and the vacuum is low while the air flow is also low. This upsets main metering something awful as there still isn't enough air flow to properly allow for fine metering to the condition while uncovering the idle and transition circuits so these more discrete low air flow systems are shut down making the whole system dependent on main metering which is designed for higher air flows to operate closely to optimum. The timing is now going down because there isn't enough manifold vacuum nor RPMs to operate it to the engine's needs. Ignition computers can figgure this out, mechanical systems have a really hard time with this situation. Fuel is being wasted along with it power lost due to late and missed fires in addition to the sloppy carb metering. Again this will test rich on an analyzer even if in an ideal world the jetting is at correct to lean. Of course if the jetting is rich it will just drown the motor. Again a carb pro and a multi-spark ignition will be most helpful to you. In this situation the motor will tend to clean up when you get on it.

If you're going to tackle this yourself, you need to study up on the theory and operation of Holley carbs to learn what circuits are operating when. Generally, as main jets get bigger or smaller the air correction jets need to change in the same direction. A pro can game the air correction jets and metering tubes to force abnormal solutions, I don't recommend you take this on. A multi-spark ignition is something you can put in and should help. The vacuum advance needs to be adjustable, along with a vacuum gage you can dial this in. Another solution can lead to a smaller carb to get the venturi signal up at low speed, God love a Q-Jet in this situation. Or you can stiffen up the overall gearing to get the revs up to where the current state of "tune" is more compatible with good metering and proper timing. Of course there's electronic fuel injection solution which is ideal for what you've got here. Compression ratio is most important. The static should be at or a bit above 9 for cast iron heads and 10 or a bit more for aluminum, CR also plays to getting the most out of low density mixtures so it's a huge player at low RPMs. The Dynamic Compression Ratio (DCR) should be about 8.5, this is the Static Compression Ratio (SCR is that which you measured by total cylinder volume divided by the volume above the piston) adjusted for intake valve closing pint and connecting rod length. A decent calculator can be found at Keith Black http://www.kb-silvolite.com/calc.php?action=comp2


Bogie
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Old 06-09-2010, 11:38 AM
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Do you have a blown power valve?
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Old 06-09-2010, 12:31 PM
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yes, mechanical and vacuum advances are additive.

read this.

http://www.corvette-restoration.com/.../Timing101.pdf


yes, a blown power valve can cause really bad mpg. if you are using a big cam then use a lower numbered power valve (like a 4.5).

my 454 c10 truck gets 14 mpg at 70mph, 13 mpg at 75, and 12 at 80 mph with a 3.08 gear and th400. I use a 3310 holley and 50 degrees of timing (40 mechanical plus 10 from the vacuum advance). but only 8 mpg in the city.
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Old 06-09-2010, 01:48 PM
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Quote:
Originally Posted by oldbogie
. A decent calculator can be found at Keith Black http://www.kb-silvolite.com/calc.php?action=comp2


Bogie
Great reply 'OB'. Turning an engine too slow under a load will kill mileage as much as anything, especially one modified for high performance.

One thing, when using the KB / United Machine calculator, do not use the cam timing at .050" plus 15 degrees. Instead use the intake closing point from the cams advertised duration directly. This gives identical results to Pat Kelley's calculator which most people regard as the most accurate.
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Old 06-09-2010, 04:17 PM
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Quote:
Originally Posted by 454C10
try a 3310 (750 cfm, vacuum secondarys).
x2. Easier to tune, and compensates for mismatched components. 1600-2100rpm is kind of low. I know you have your engine set-up in another thread, but isn't 1600-2100 at the low-end of your engine/cam's operating range?
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Old 06-10-2010, 08:49 AM
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Horrible mileage

What is your fuel pressure? That may be the best place to START chasing this problem. Too much pressure will show up as blackish smoke and bad mileage.....as well as poor performance and MANY other issues.
Good luck and keep us posted.
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Old 06-10-2010, 09:30 AM
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the problem with 2000 rpm drive is there isn't enough mechanical ignition timing at 2000 rpms.

you may need a programable ignition system to get enough timing at that low rpm.
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