Classic over carburetion problem?

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Richie said:
Elevation: 342m / 1122feet
I still don't like vacuum secondaries though they never even seem to open up.
The only thing I can really answer right now is about the rpm 3000 and yes that's when it starts to go good.
Here's all the cam info.

View attachment 1715738847
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Well there you have most of it;
I'll guess;
Static compression ratio of................... guessing 9.5:1.
elevation 1122@ Ica of 63* (in at 108)
Effective stroke is 2.59 inches.
Your dynamic compression ratio is 7.64:1 .
Your dynamic cranking pressure is ................. 148.16 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and
boost of PSI is............................................... 7.42 :1.
V/P (Volume to Pressure Index) is ................... 119

That's a nice cam; especially so with the pressure up around 180 .......... in a 360..............lol............. like mine.
But predicted to be 148psi, the V/P of 119 shows it to be about as powerful in the lower rpms, as a 318LA

At 10/1 the Pressure climbs to 158, and VP to 128, so now the bottom end is feeling like a 5.2M might.
At 9.0 the pressure is predicted to be 138, and the VP is 111 decidedly weak.
So now you know why pressure is important.
Until you do a compression test, I sure cannot say what your solution is. I mean if the pressure is down at less than 140psi I know what I would be doing. But if the pressure was already 160, then I would give a different solution. But at 150 it's a tougher call.

Your choices at 150 are;
a higher compression ratio
a higher stall,
or a higher number rear gear.
Of these the higher compression ratio is an across the board power increasing fix; more real power all throughout the rpm band.
The higher stall effectively gives away efficiency below it's stall, and gives you nothing afterwards. It simply allows your engine to spool up to a higher rpm, where the power is.
The gears allow your engine to run up the power curve quicker, and so make the engine feel more powerful, altho it is not.

A lot of builders seem to recommend a compression ceiling of between 9 and 9.5 ; which, with a stock cam would be fine. But with a 230/236/110 cam is terrible, as you may have discovered.

As to the 750 vacuum secondary, hang on to that for a few months. It is the right size carb for your combo. It may just need some light tuning adjustments. If you can feel the Secondaries opening, they are probably opening too soon.
A DP carb may be more fun, but with a 2200 stall, that is gonna hurt performance just as much as the 2200 is hurting the VS carb. The thing is that below 2800 to guessing 3400, the carb don't much care about the secondaries as in most cases the Primaries are well able to meet the air supply demand. If you had a DP and opened all 4 barrels at 2200, the demand would simply be met at a slower airspeed thru the venturies. I mean if the engine, at a certain rpm, can only inhale 300 cfm, you can supply that 300 thru whatever holes will pass at least that much. If your holes are bigger, the same amount of air is still gonna get thru there, allbeit at a slower velocity, cuz 300 is all the engine can pull.. The 750, whether a DP or a VS is easily big enough to feed any SBM even most strokers. At the price of new carbs these days, I'd rather put that money towards getting the pressure up; it will be far better spent in doing that. Or even just a higher stall, cuz 2200 is way too low for that cam, even if the engine was up to pressure...... IMO
 
Richie said:
I was thinking of ordering that last night
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Get a handle on tuning before buying another carb thinking it will ootb run the way you expect it to.

Snappy throttle depends upon a few things, higher initial timing, the perfect pump shot, the correct secondary springs...in general...the tune.
 
Richie said:
Hi everybody:

Now that I have over 300 miles on this 340 I'm getting to feel that there might be a problem like I thought. I got my friend to drive it also and he thinks the same thing. It should go WAY better than it does especially in the burn out awards. It hardly breaks the wheels lose :(


1970 340
Ground and drilled rods
Speed pro forged pistons
ported 2.02 J heads
Comp Cam 488/491
750 Holley vacuum secondaries
Edelbrock Airgap intake
Doug’s Ceramic Headers
Super 44 Flow Masters
Exhaust 2 1/2"
727 with 2200 –2400 converter
8 3/4 Dif with 3.55 Eaton trutrac
HP haven't a clue
Click to expand...
 
Richie said:
Hi everybody:

Now that I have over 300 miles on this 340 I'm getting to feel that there might be a problem like I thought. I got my friend to drive it also and he thinks the same thing. It should go WAY better than it does especially in the burn out awards. It hardly breaks the wheels lose :(


1970 340
Ground and drilled rods
Speed pro forged pistons
ported 2.02 J heads
Comp Cam 488/491
750 Holley vacuum secondaries
Edelbrock Airgap intake
Doug’s Ceramic Headers
Super 44 Flow Masters
Exhaust 2 1/2"
727 with 2200 –2400 converter
8 3/4 Dif with 3.55 Eaton trutrac
HP haven't a clue
Click to expand...
One possibility is that the pedal to carb linkage is not opening the carb full throttle, happened to me once.
 
It sounds like you don't have alot of experience with this stuff - not having a compression tester or timing light suggests you've not spent time learning about tuning. And that's not a judgement, just an observation. Now's the time to get started. Buy the basic tools, learn how they work, and make small changes. I am by no means as knowledgeable as many of the folks here, but I have learned that SMALL changes in timing, float levels, idle mixtures, etc. can make a BIG difference, as well as help you learn what settings make what changes. Get that Holley book and learn how a carb works so you know what you're adjusting. You should have no trouble running a vacuum secondary.

After a while, you can be the one that can offer advice to the next guy.
 
Don't start throwing parts at it yet. Like suggested, check cranking PSI. Might as well check all cylinders while you're at it. They should be very close.

Timing is a big thing. I believe earlier in the thread you said you adjusted the timing to get rid of ping. If so, get a timing light on it (connected to cylinder one, the front driver's side plug wire) and see where you're at. Not enough spark advance could very well be your problem. YouTube how to check and adjust timing. Mine is happy at 36 degrees total advance. This is NOT initial advance!! You check total advance with the engine at a relatively high RPM.

If the engine pings with spark advance set correctly, you need better fuel.

Do you have the stock distributor? If so, there is a vacuum advance on the distributor. Is it connected to the proper port on the carb? Pictures would be useful.
 
"Things are looking up. I gave my brother a call. He doesn't drive like I do and back in the day 60s and 70s I can't count how many mopars he had. He jammed his foot to the floor, first the carb hesitated then it caught and sure enough it did a big long burnout. My foot was hitting the rubber mat I guess and I thought that was the steal floor. The carb didn't do that caugh when I tried either. I still don't like vacuum secondaries though they never even seem to open up."

So the problem was a "rubber mat" between the gas peddle and floor. Me thinks you should be driving a Prius:rofl:
 
Last edited:
69FBCuda said:
"Things are looking up. I gave my brother a call. He doesn't drive like I do and back in the day 60s and 70s I can't count how many mopars he had. He jammed his foot to the floor, first the carb hesitated then it caught and sure enough it did a big long burnout. My foot was hitting the rubber mat I guess and I thought that was the steal floor. The carb didn't do that caugh when I tried either. I still don't like vacuum secondaries though they never even seem to open up."

So the problem was a "rubber mat" between the gas peddle and floor. Me thinks you should be driving a Prius:rofl:
Click to expand...

Ouch! :lol:
 
I haven't taken the time to read every comment or reply but let me says this ! You are in no way over carbed, over camed, or from what I see anything else with this build. At this time I didn't see any mention of an aftermarket Distrib. if you are using a stock type igniter, YOU NEED to find someone that can run a complete ignition function test on a computer diagnostics machine. You need to find out how much voltage is being dropped at each point in the secondary side from the coil to the distrib- from the center tower to the spark plug towers all the way to the plugs. I CANNOT IMPOSE ENOUGH IMPHYSIS ON HOW IMPORTANT IT IS TO KNOW THIS WITH THESE CARS ! I have run across several instances with Chrysler products where there was an issue with the phasing of the distrib shaft and points cam or electronic trigger wheel to the position of the ignition rotor to the towers of the distrib cap. What happens is this, the ignition timing so to speak is late. The rotor is already past the intended plug tower when the spark should be jumping, so the coil must deliver its maximum output when it should require much less. Under hard acceleration or throttle, the advance diaphragm cannot correct this without modifications to the link arm. This is the point of failure in the system. Once corrected you will find so much HP it will blow you away !!!! I know from experience with one 440 I had and many other peoples engines this to be a problem. Again what happens is the spark is coming according to the setting of timing but it is at a time when the rotor is pulling away from the plug tower on the cap and you have lost proper relation to the complete timing cycle for ignition and all the potential current to lite the fire when it needs to be lit. A timing light will not tell you what you need to know here ! By looking at the burn marks on the rotor and cap you will see that the arc marks are all the way to the trailing point of the total picture when they should be on the leading point so when rpm increase, there is a much shorter path to cross giving the coil the ability to build much lower required voltage thereby having much higher reserve under compression( which requires much higher output) under load. You actually get what is a massive amount of basically misfires which you cannot feel. That 440 I mentioned was not a stock engine and would barely lite the tires in a Barracuda before I learned what was happening by putting it on a scope myself in a shop I worked in back in the mid 80's, once I saw what was happening with the distributor and corrected it, it would smoke the tires for the whole 1/4 miles both sides and gaining speed all the way past 110 mph running 2.76 gears of course the speedo would be barried so the 110 was from the dragstrip clocks. Before doing the distrib to after, I had to have found at least 200 HP on that engine that was just not there. I couldn't believe it at first, the car was such a dog and I was so disappointed but after, it was a monster awakened. Trust me on this please. Do the test and confirm first , that the ignition system is completely in phase. You need to have a coil voltage output of less than 3,000 volts from the center tower to the plug towers of each wire than less than 1,500 volts dropped in the plug wires to the plugs so you can still have minimum loss of output building arc across plug gaps. With this, your coil will have high margin for output under max load to fire the plugs. Please do this testing before spending any money looking for a reason for low power production PLEASE ! I had a 318 before I got the 440 and I didn't have the scope to test the ignition system but that engine I built with a lot of factory parts from various SBM options and ported the heads, installed a thermoquad intake and carb from a late model 340 which gave me a learning curve and I raced it against all comers and nearly got in many fights about whether it was actually an 18er. People swore I had altered the numbers and it was actually a 340 or 360. It was amazing how it ran. I shut down LT's W30's Clevelands, you name it, I even shut down my share of BBC's with that little motor. I truly regret that day I let that little motor go down the road in someone else's car, a 72 roadrunner that had a 400 in it that didn't run. I gave up my trans along with the 18 for all the parts from the RRr with plans to build that BBM and never got it finished before going to a 440 that ran but was worn out. Every time I worked on any MOPAR or any other engine after that with a Distrib. I always checked the shaft and rotor phasing to be sure it was lighting on the lead cycle instead of the trailing so there was no suspected loss of ignition causing loss of power. I do not believe in over carbing any more especially with secondaries that require any vaccum to open which is pretty much all of them except Holleys. Over camming is another issue. Please check that distributor. Please check that Distributor! It will not cost you anything more than some time and diagnostic fees. I promise you will find some HP hiding under that cap, than if you still have doubts, you can go on to the next level which I doubt you will need to anyhow. Any 340 can make nearly 350 HP tuned right with near stock parts ! EVERY engine can make more power tuned RIGHT and that means getting the ignition right first then looking at fuel and other places.
 
By the way if and when you get the distrib phased right you will see vacuum numbers that would make a big v8 with a mild cam and 2 barrel cry even if you have a pretty big cam and a big 4 b in there.
 
@Bret Tschacher good post. I agree with it except the part that a “timing light will not tell you”. You can use a timing light to verify rotor/cap terminal alignment (phasing). Take a used old cap and drill a big hole in it adjacent to the number one tower. Shoot the timing light at it with the engine running and you will see where the rotor is firing. FYI MSD makes an adjustable rotor for their distributors and it’s worth it’s weight in gold when phasing is out.
 
Bret Tschacher said:
I haven't taken the time to read every comment or reply but let me says this ! You are in no way over carbed, over camed, or from what I see anything else with this build. At this time I didn't see any mention of an aftermarket Distrib. if you are using a stock type igniter, YOU NEED to find someone that can run a complete ignition function test on a computer diagnostics machine. You need to find out how much voltage is being dropped at each point in the secondary side from the coil to the distrib- from the center tower to the spark plug towers all the way to the plugs. I CANNOT IMPOSE ENOUGH IMPHYSIS ON HOW IMPORTANT IT IS TO KNOW THIS WITH THESE CARS ! I have run across several instances with Chrysler products where there was an issue with the phasing of the distrib shaft and points cam or electronic trigger wheel to the position of the ignition rotor to the towers of the distrib cap. What happens is this, the ignition timing so to speak is late. The rotor is already past the intended plug tower when the spark should be jumping, so the coil must deliver its maximum output when it should require much less. Under hard acceleration or throttle, the advance diaphragm cannot correct this without modifications to the link arm. This is the point of failure in the system. Once corrected you will find so much HP it will blow you away !!!! I know from experience with one 440 I had and many other peoples engines this to be a problem. Again what happens is the spark is coming according to the setting of timing but it is at a time when the rotor is pulling away from the plug tower on the cap and you have lost proper relation to the complete timing cycle for ignition and all the potential current to lite the fire when it needs to be lit. A timing light will not tell you what you need to know here ! By looking at the burn marks on the rotor and cap you will see that the arc marks are all the way to the trailing point of the total picture when they should be on the leading point so when rpm increase, there is a much shorter path to cross giving the coil the ability to build much lower required voltage thereby having much higher reserve under compression( which requires much higher output) under load. You actually get what is a massive amount of basically misfires which you cannot feel. That 440 I mentioned was not a stock engine and would barely lite the tires in a Barracuda before I learned what was happening by putting it on a scope myself in a shop I worked in back in the mid 80's, once I saw what was happening with the distributor and corrected it, it would smoke the tires for the whole 1/4 miles both sides and gaining speed all the way past 110 mph running 2.76 gears of course the speedo would be barried so the 110 was from the dragstrip clocks. Before doing the distrib to after, I had to have found at least 200 HP on that engine that was just not there. I couldn't believe it at first, the car was such a dog and I was so disappointed but after, it was a monster awakened. Trust me on this please. Do the test and confirm first , that the ignition system is completely in phase. You need to have a coil voltage output of less than 3,000 volts from the center tower to the plug towers of each wire than less than 1,500 volts dropped in the plug wires to the plugs so you can still have minimum loss of output building arc across plug gaps. With this, your coil will have high margin for output under max load to fire the plugs. Please do this testing before spending any money looking for a reason for low power production PLEASE ! I had a 318 before I got the 440 and I didn't have the scope to test the ignition system but that engine I built with a lot of factory parts from various SBM options and ported the heads, installed a thermoquad intake and carb from a late model 340 which gave me a learning curve and I raced it against all comers and nearly got in many fights about whether it was actually an 18er. People swore I had altered the numbers and it was actually a 340 or 360. It was amazing how it ran. I shut down LT's W30's Clevelands, you name it, I even shut down my share of BBC's with that little motor. I truly regret that day I let that little motor go down the road in someone else's car, a 72 roadrunner that had a 400 in it that didn't run. I gave up my trans along with the 18 for all the parts from the RRr with plans to build that BBM and never got it finished before going to a 440 that ran but was worn out. Every time I worked on any MOPAR or any other engine after that with a Distrib. I always checked the shaft and rotor phasing to be sure it was lighting on the lead cycle instead of the trailing so there was no suspected loss of ignition causing loss of power. I do not believe in over carbing any more especially with secondaries that require any vaccum to open which is pretty much all of them except Holleys. Over camming is another issue. Please check that distributor. Please check that Distributor! It will not cost you anything more than some time and diagnostic fees. I promise you will find some HP hiding under that cap, than if you still have doubts, you can go on to the next level which I doubt you will need to anyhow. Any 340 can make nearly 350 HP tuned right with near stock parts ! EVERY engine can make more power tuned RIGHT and that means getting the ignition right first then looking at fuel and other places.
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It's this kind of ignition kit
Proform Mopar Electronic Ignition Distributor Kit Dodge Chrysler 273 318 340 360 | eBay
 
TT5.9mag said:
@Bret Tschacher good post. I agree with it except the part that a “timing light will not tell you”. You can use a timing light to verify rotor/cap terminal alignment (phasing). Take a used old cap and drill a big hole in it adjacent to the number one tower. Shoot the timing light at it with the engine running and you will see where the rotor is firing. FYI MSD makes an adjustable rotor for their distributors and it’s worth it’s weight in gold when phasing is out.
Click to expand...
Yes that would work aside from the issue I never did a destructive test like that. However the timing light would never tell you the voltage drops across various parts of the system. When I did have access to the dyno and CRT scope I could pinpoint problems from ignition failure to carb issue without having to drill holes or shoot lights, I could do a printout or simply look at the display screen and know exactly what was happening. It was also much easier than crawling around over the top of a running engine especially with a rear mounted Distributor.
 
Ok I ran across a car a while back that had that exact part installed in a 66 273 with a solid cam that was supposedly rebuilt in a shop near here. The shop apparently did not know how to set the tappet lash and the engine ran fair but would lurch at low speed. I was told the engine was a 318 by the owner. So I assumed it had hyd tappets, I did a comp test and all seemed to be even but a bit low. So we thought perhaps a valve grind might be in order. Later on the covers were removed to pull the heads and that's when it was discovered it had adj rockers and the valve las was too short. A warmed engine ran worse. We adjusted the valves and comp was still low. Valves got redone comp came up and once we got that distrib properly calibrated, WOW I wouldn't have believed a 73 could make that much power but then I thought about those old D stockers at 275 HP, same as the factory rated 340s later on. Anyway the 73 was too quite before, couldn't hear valve tapping. That's why I originally thought it was an 18 with hyd. not a 73 flat tappet. So point is look closely at the igniter no matter what model or make, they are mass manuf. and there's always a chance for issues, which is a big reason nobody makes distribs anymore.
 
pishta said:
I ran a Holley DP spreadbore "TQ replacement" on my last 340 for a few months. That carb had thee best idle and best low end of any Holley I ever had including a 625 BG/Demon. And it was a beater, nothing done to it but bolting it on from a $30 swap meet table.
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I got one of those spreadbore Holleys direct from Holley that was a refurbished carb. It works great on my daughters Dart with 360. Funny thing is, on the Holley website, the listing clearly stated "not for Chrysler applications" for some reason. Maybe Holley corporation doesn't know everything. Haha
 
The spreadbore was marketed as a TQ replacement!!! Todays Holley is clueless what they built 50 years ago
 
I have seen so many people modify an engine and set the timing to factory specs and wonder why it won't idle and runs like a turd. Timing first.
 
gregcon said:
**I think the Titanic sank because it had too much carburetor.
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giphy (6).gif
 
Just to add some actual race facts to this. ....Back in 1982 I was running a no's match '71 340 street/strip Cuda, stock everything, 3.55's, plus the motor had an ally intake of which I can't remember now but I swapped it out for a Holley Strip Dominator, it had a 750DP and small 1.5/8" hdrs and a .484 hyd. purple cam and 3500 stall verter. It ran real good but I wanted more as we all do so I got hold of an 850DP of which all my race buddies said was too big for such a small motor, well it picked up almost 2 10ths in the 1/4.:thumbsup:
 
Hot Metal said:
Just to add some actual race facts to this. ....Back in 1982 I was running a no's match '71 340 street/strip Cuda, stock everything, 3.55's, plus the motor had an ally intake of which I can't remember now but I swapped it out for a Holley Strip Dominator, it had a 750DP and small 1.5/8" hdrs and a .484 hyd. purple cam and 3500 stall verter. It ran real good but I wanted more as we all do so I got hold of an 850DP of which all my race buddies said was too big for such a small motor, well it picked up almost 2 10ths in the 1/4.:thumbsup:
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I like that..Wait for July after I'm broken in
 
Update:

Had a mechanical secondary 750 Holley Brawler DP installed and it's like I'm driving a different car. When I stomp on it and it will now do a burn out no problem and gets a big chirp in in second.

Holley-Quickfuel-Brawler-750-CFM-Double-Pumper-4-_1.jpg
 
Just to add something. When I backed out of the garage tonight I didn't mean to but I even layed a patch then. Just so not use to the immediate throttle response. It's a good thing I was lined up well with the doors. Would have been slapstick if the garage doors went flying :lol:
 
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