Is my 340 Overcarbed? Avg 5-6 MPG

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OK guys, sorry for the long delay in updates, but I was on det to San Diego. Here is the latest and I am making progress:

1) Jetting on the carb was returned to the original settings
2) The bushing in the MSD distributor was removed and a 10 degree was installed.
3) I set the initial timing to 24 degrees and the motor is very happy right now. I verified total at 3000 rpms and it reads 34 with no detonation or pinging. I tried advancing all the way to 34 degrees initial and the distributor never kicked back as stated. It was happy there as well with no detonation or pinging. Right now I backed it down to 24 degrees initial until I get further guidance.
4) Took the car on a test drive and throttle response was very snappy, power was there and there was no stumble at low speeds at all. I took readings from the AFR Wideband during my drive at highway speeds. Here are the results:
- Idle (800 rpms): 14.5 to 14.7
- 1500: 12.5 to 12.8
- 2000: 12.2 to 12.5
- 2500: 12.0 to 12.3
- 3000: 11.8 to 11.9
- 4000: 11.9 to 12.3
5) I am thinking that I need to jet down based on the rich AFR readings, but I am not going to pull that trigger till I hear from you all. My only concern now is that if you do tell me to jet down, I am going to get that low speed stumble back. Thanks for all the assistance.

Ken
 
It's a parts mismatch. The cam's too big, and the carb is WAY too big. Event he Comp catalog says a minimum of 9.5:1 and you're below that. On the carb - Compared apples to apples with an older carb you have more than 880cfm with that carb. The dyno shows it's a pig - no offesnse but 390/390 out of a 340 is terrible given the work done. Hence the inability to be efficient and poor mileage. Stick a cam in it that has 225 - like the Comp XR274HR10 - and put on the Avenger 570 and you'll pick up a shitload more torque and horsepower, and mileage.

Bingo ,it is way down on hp and torque , my little cam 340 made 398 hp 413 tq with x heads mild port and an xe262 cam , now @ 420hp 440tq with Ede heads same cam . What was said above makes sense , you should be making more than my little 340 ., plus I get 18 -20 mpg with Holley 670 Avenger and Air gap , 23 with x heads and AVS
 
You're still fat. Cruise numbers should be 14.5 - 16.0 assuming the vacuum reading is above the power valve number. Once you crack the throttle past 1/4 or so, and the power valve opens you should see those 12:1 numbers all the way to WOT.
 
A 770 ultra HP is a vacuum secondary carb, correct? List #86770? You mentioned a DP earlier. If it's not a Vac carb, which list number is the carb?

Cool that the idle and crispness are present, that's the first step.

Idle looks good as long as that's the point of highest vacuum. If it has a higher vacuum at a slightly lower idle number, that's OK. Set it there.

Try turning the float adjuster 1 flat clockwise on both ends and see if the low end numbers clean up below 2500. You will likely need to readjust your idle mixture screws as well. Do this How far out are the screw now? Once you do this and if it cleans up your low speed cruise, then go to the next step.

Take at least 2 jet sizes out of the primary, maybe three and see what it does. The secondary jetting if a vacuum carb won't come into play unless you are really into it.

There is a lot of power buried in the bad tune... No reason at all that you can't get it, BUT, it will take a little work.
 
A 770 ultra HP is a vacuum secondary carb, correct? List #86770? You mentioned a DP earlier. If it's not a Vac carb, which list number is the carb?

Cool that the idle and crispness are present, that's the first step.

Idle looks good as long as that's the point of highest vacuum. If it has a higher vacuum at a slightly lower idle number, that's OK. Set it there.

Try turning the float adjuster 1 flat clockwise on both ends and see if the low end numbers clean up below 2500. You will likely need to readjust your idle mixture screws as well. Do this How far out are the screw now? Once you do this and if it cleans up your low speed cruise, then go to the next step.

Take at least 2 jet sizes out of the primary, maybe three and see what it does. The secondary jetting if a vacuum carb won't come into play unless you are really into it.

There is a lot of power buried in the bad tune... No reason at all that you can't get it, BUT, it will take a little work.

My 770 is a vacuum secondary carb listed in the link below:

http://www.summitracing.com/parts/hly-0-86770bl

I may have mislead you by calling it a DP. Sorry, still learning here. It is a double pumper, however, it has mechanical primaries and vacuum secondaries.

I picked up a vacuum gauge and the max vacuum I am seeing right now is 12 which is above the power valve reading of 6.5 which is what is installed C-130. I will rig up some tubing to run that gauge into the cabin and record vacuum at cruising speeds tomorrow.

I will adjust the float levels as discussed above in the morning and let you know how that works. Air mixture screws right now are 3/4 out on all 4 corners. I just pulled the primary float bowl and put in 71 jets from 73 main jets so I will go for a test drive tomorrow to see how things improve. I will leave secondary jetting alone for now.

OK, for those who think its a parts mismatch, I will agree to disagree! I personally think its tuning. I hope my engine builder would not lead me in the wrong direction. Factors in play:

1) Engine was tuned out of the car. A/F ratios were based on turbine on the carb vice my wideband meter sensor mounted in the collectors.
2) Engine was also tuned with a 2" open spacer under the carb. Engine in the car is not currently running a 2" spacer due to hood clearance. Would that affect A/F ratios and jetting?
3) There have been a lot of pms asking is my IFR's are adjustable and what orientation my metering blocks are so here are some pics. It looks to me like the IFR's are staked in and are not adjustable. The PV (only one on the primary metering block) looks fine once I removed it and I vacuum tested it and it works as advertised. Yes, I know my gaskets are bad! I have replacements, but until I get this thing dialed in I am not going to change them! Just trying to answer questions before they are asked!

As always, I am open to suggestions and not trying to piss anyone off. Every motor has different personalities so I am trying to get to best advice for my current setup. Thank you all for the help!
 

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Sure don't look like an air gap intake ? Must be the angle pic was taken .
 
Sure don't look like an air gap intake ? Must be the angle pic was taken .

I agree, but look at the 9th photo down.

SmartKen: To answer your original question, IMO, the engine is over-carbureted for street use. Besides better mpg, the smaller carb will give improved throttle response at lower rpm. For race use, the 770 is still a bit much for the 340 assuming 6600 max. rpm), but it's better than going down to a 650 and running out of breath at top rpms.

Your low end stumble may be a result of premature opening of the secondary. It's probably worse when the engine is less than completely warmed up. Let's see what others say about putting in a heavier diaphragm spring.
 
Its an AirGap, flash washes out depth. Weber/Percy adjust-a-jet...
Slide-18.jpg
 
Yes, it is an AirGap. Here are some better pics.
 

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Do you see in the picture of metering block with power valve removed the two holes behind power
valve? Those are power valve channel restrictions(pvcr). I can see how small they are. Think of those
as jets, because that's exactly what they are. When your power valve opens, those "jets" combined
with your main jets create your w.o.t. a/f ratio.
 
If you put in jets to give proper a/f ratio at steady cruise speed, you have to open up pvcr size to
give proper w.o.t. (or any throttle heavy enough that allows power valve to open) a/f ratio. I would think this would be easy with-(what do you's call it- a "wide band"?)
 
OK, now that I have my Duster fresh out of restoration, I am trying to figure out MPG and I am eating gas likes its going out of style! My last two tanks made it 81 miles from full to empty. Not good in my opinion. It is a factory 16 gallon tank. Here are the specs on the motor and the dyno sheets. I am looking at the Air1+2 SCFM column and the avg is 573. I am running a Holley 770 Street Avenger so am I over carbed? Thanks all.

- 340 Small Block Blueprinted and Balanced
- Forged stock crank
- SFI Billet Steel Flywheel
- Centerforce D/F clutch
- Keith Black 0.030 over cast pistons cut to 9.3CR
- Forged rods with ARP Waveloc bolts
- Kevko oil pan
- Melling HV oil pump
- Comp Cams XR286HR-10 Hyd Roller Cam at 105 degree C/L 13 degree @ 050
.544 lift / 286 Duration
- Comp Cams Double Roller Timing Chain
- Comp Cams 8920-16 retro hyd roller lifters
- Comp Cams Double valve springs (140/385 spring pressures)
- Hughes 1502 1.5 ratio roller rocker kit with holddowns
- Hughes custom built pushrods: 5/16" x .083" 4130 HT 7.900"
- Single groove valve stems
- Ported J Heads @ 68cc w/ 2.02" intake and 1.60 exhaust valves
- Edelbrock 7576 Dual Plane air gap intake
- Holley Ultra Street Avenger 770 double Pumper
- MSD Pro Billet Distributor
- MSD Blaster 2 Coil
- MSD 6AL-2 Ignition box

just put your specs in eng dyno and got 409tq @4750 448 hp@6259 , changed the compression to 10.2 and got 446 tq @4750 497 hp @ 6250 . @10.1 441 tq 481 hp

Maybe I made a mistake ? Or maybe the poster was right , to much cam
 
Holleys can be such a pain to tune--yeah they do make more HP then any single Carter carb. If your not looking for best HP, I would think about a 600 Carter Competition carb. The metering rods are easier to change to tune it and it will likely idle better to boot. Flame away, lol, I ran holleys for over 20 years and yes they can be the best carb out there but my dual Carters give me less fuss then any Holley-Demon carb I had...I'm sure one Carter would work just as well, just be down on HP.

Thats just my 2 cents, if you want another Holley, go for it..just make sure the metering block gasket doesn't have an internal leak that causes a rough idle--that was the last straw with my Holley days, freaken thing would be OK sometimes and then not OK after a jet change--even using a new gasket sometimes.

Then you might need to adjust the idle feed restricters or idle air bleeds to get the best idle...I had pin drills, mics, all that stuff and had had Holleys run great all season. It can be done, I just think a Carter carb would be easier to get running around town if max HP isn't needed. It idle better and get better MPG with less tuning
 
I think I'm starting to agree with moper here unfortunately it looks like you have some parts mismatches. That carb is very big unless you're turning 7000 RPM and I think the overall calibration of the carb to your smaller engine is too far off to be worth the trouble of tuning it IMO. I bet an OOTB 670 would run better than your current setup with all the changes.
 
OK I am working through it and think I have it figured out. I installed a spacer back under the carb and put a 2" height filter on for hood clearance. I also put back in the 31 squirter and the green cam which fixed the low end stumble issues. I was still getting some popping from the exhaust so I decided to change the spark plugs out because they looked pretty fouled the last time I had them out. Once I fired her back up it sounded like a different engine and all the A/F ratios went lean all of a sudden so I re adjusted mixtures to get them back. I took it for a spin and there is no more popping from the exhaust. AFR's throughout the entire powerband at steady cruise speed are all bouncing between 13.8-14.2. I think I am as close as I am going to get. Looks like I may have had a fouled plug or heading that way in addition to all the tuning.
 
Which spacer did you use? The 2" or a 1" open?

There's more sorting out to do, but, that's much better than the tune up that was in it before. A little elbow grease and minor changes can make a big difference in these things. There are some adjustments to do to clean things up a bit more.

Nice work and kudos for not giving up and putting in a great effort. :cheers:
 
Which spacer did you use? The 2" or a 1" open?

There's more sorting out to do, but, that's much better than the tune up that was in it before. A little elbow grease and minor changes can make a big difference in these things. There are some adjustments to do to clean things up a bit more.

Nice work and kudos for not giving up and putting in a great effort. :cheers:

Thanks Rob!

I put a 2" open spacer on it. I am going on det again so I will do some more fine tuning once I get back home.

Ken
 
If you got it driving decent, you're still going to have to open up pvcr in primary metering block
to get wot afr in the ballpark. Add 2-3 jet sizes to secondary before doing that. Also, you shouldn't
need that green pump cam (your main complaint in the beginning was poor fuel mileage, right?) To
me, if you need that agressive of a pump shot, your idle feed restrictions need to be opened up
.002" or so. It would be a shame to have such a nice motor, and not tune it to its' potential.
 
Thanks Rob!

I put a 2" open spacer on it. I am going on det again so I will do some more fine tuning once I get back home.

Ken

Awesome I'm glad it worked out too I had my doubts obviously this combo isn't exactly working in your favor... What are your WOT A/F numbers now? I agree with madmoparmick you need to keep the cruise ratio where it is or lean it a bit with jetting (primary) then get it fatter at WOT with more PVCR size and secondary jetting to match.

BTW the part-throttle ratio getting richer at upper RPMs is fine it won't really affect anything anyway as you are hopefully not cruising any higher than 3000.
 
Id say u need more timing
But mostly all aftermarket carbs come out the box all fucked up
U gotta bring it to a good carb guy so he can tweek it
My quick fuel runs in low 12s on the wideband when cruzin too
Watch, if u jus run primarys goin up a hill, it will run way lean 15-16+
Then wot is usually good around 12s

But carbs need to be tweeked jus how we do our motors
Im about to do my carb after im done with my 4speed swap

Find a good carb guy, cuz im pretty sure the meatering blocks gotta get all reworked
 
OK, now that I have my Duster fresh out of restoration, I am trying to figure out MPG and I am eating gas likes its going out of style! My last two tanks made it 81 miles from full to empty. Not good in my opinion. It is a factory 16 gallon tank. Here are the specs on the motor and the dyno sheets. I am looking at the Air1+2 SCFM column and the avg is 573. I am running a Holley 770 Street Avenger so am I over carbed? Thanks all.

- 340 Small Block Blueprinted and Balanced
- Forged stock crank
- SFI Billet Steel Flywheel
- Centerforce D/F clutch
- Keith Black 0.030 over cast pistons cut to 9.3CR
- Forged rods with ARP Waveloc bolts
- Kevko oil pan
- Melling HV oil pump
- Comp Cams XR286HR-10 Hyd Roller Cam at 105 degree C/L 13 degree @ 050
.544 lift / 286 Duration
- Comp Cams Double Roller Timing Chain
- Comp Cams 8920-16 retro hyd roller lifters
- Comp Cams Double valve springs (140/385 spring pressures)
- Hughes 1502 1.5 ratio roller rocker kit with holddowns
- Hughes custom built pushrods: 5/16" x .083" 4130 HT 7.900"
- Single groove valve stems
- Ported J Heads @ 68cc w/ 2.02" intake and 1.60 exhaust valves
- Edelbrock 7576 Dual Plane air gap intake
- Holley Ultra Street Avenger 770 double Pumper
- MSD Pro Billet Distributor
- MSD Blaster 2 Coil
- MSD 6AL-2 Ignition box


Over carbed no WRONG carb calibrations YES..............Common on most every carb sold today unfortunately but fixable for sure...............
 
Id say u need more timing
But mostly all aftermarket carbs come out the box all fucked up
U gotta bring it to a good carb guy so he can tweek it
My quick fuel runs in low 12s on the wideband when cruzin too
Watch, if u jus run primarys goin up a hill, it will run way lean 15-16+
Then wot is usually good around 12s

But carbs need to be tweeked jus how we do our motors
Im about to do my carb after im done with my 4speed swap

Find a good carb guy, cuz im pretty sure the meatering blocks gotta get all reworked

You just answered a thread that the last post was over 2 years ago

hagryy.jpg
 
You said your running a 6.5" power valve, but only pulling 7-8"hg at idle. Power valve should be half of vacuum so 3.5" to 4". It's definitely opening too soon based on those numbers.

Not true and even w/a p/v open at idle that circuit is not(shouldn`t) be active. You gauge p/v size on your average cruising NOT idle so if you cruise at say 10" of vacuum, then a p/v below that such as an 8.5-9.5 would be a good starting point but not the end all................
 
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