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Engine falling off just past 4k - carb limit?
Posted By: Secret Chimp
Engine falling off just past 4k - carb limit? - 02/13/12 06:02 AM
My new camshaft feels a good bit better than the old one around town, but the engine doesn't really want to go past 4200-4400 rpm or so now. It pulls strong up until that point, and it feels fairly consistent. It doesn't sputter or backfire or hit a wall, the power just kind of starts falling off and makes me have that "better shift now" feeling like when you start revving off the power band in a motorcycle or old car without a tachometer. It revs over 5000 sitting still in neutral no problem.
I started freaking on fuel pressure and replaced my slightly-kinked fuel hose and fuel filter before I started theorizin'.
The theory:
My loose old timing chain was letting the engine rev higher because it was pulling less air down due to delayed intake valve opening. My new chain and cam are "correct" yet worse because I'm actually hitting the airflow limit of my Carter BBD carburetor, which I can fix by installing the Performer and Carter AVS I have almost ready to go. My symptoms sound most the same as people reporting stuck secondaries on four barrels.
Reasoning:
The horsepower peak for my engine in 67 was listed at 4400 rpm. The biggest Carter BBD ever made was 325 cfm. The next step below that was 285 cfm.
Let's assume I have a 285 cfm since they also installed those carbs on 383s, and they'd probably use the biggest ones on the big blocks and save the smaller ones for 318s like mine.
If I use any one of the carb cfm calculators on the web (which all seem to estimate on the low side) and assume a terrible engine volumetric efficiency of 75%, they say I should need roughly 300-340 cfm with a max rpm of 4400. I.e., even though these are fudgy numbers, the stock engine and carb setup is peaking where it is because it's just about out of air to flow at that RPM.
Does this sound like a reasonable conclusion or am I talking out of my butt?
I just don't want to start chasing down nonexistent fuel or ignition advance gremlins if my "problem" can be fixed by adding on a carb with secondaries.
Posted By: GTX MATT
Re: Engine falling off just past 4k - carb limit? - 02/13/12 07:23 AM
What is the rest of the engine combo? It sounds like you've got a stock '67 383 2 barrel and just changed the cam and timing set. What cam? Speed Pro CS327?
If you've got a two barrel on your 383 (which is a big block) it probably wont spin much past 4000 rpm and keep making power, its a tiny carb. Don't get too caught up in the carb calculators, especially since they are geared for four barrel carbs which are actually rated differently. If I remember right when they flow test a two barrel they do it with 3 inches of vacuum, whereas a four barrel is flowed with 1.5 inches. So basically a 500 cfm two barrel is probably comparable to a 350-400 cfm four barrel.
I would recommend a four barrel on any big block, you don't have to go crazy. You wont suddenly have a race engine, but a factory four barrel intake with a Carter or factory Holley four barrel will give you some extra punch.
As far as the difference, if it revved out higher before it could be a couple things. For one the cam is almost guaranteed to not be the same specs, and the old one was worn. It is more likely that the new cam is bigger, so you should actually have more high rpm power. The old stretched out timing set would effectively retard the cam and may have shifted the powerband up. Its also possible that your new cam is too far advanced if you just installed it by lining up the dots. Does it have more low end torque?
You may also be encountering some valve float if the new cam is more aggressive and you're using stock springs.
Posted By: ScottSmith_Harms
Re: Engine falling off just past 4k - carb limit? - 02/13/12 07:25 AM
A fuel pump going bad will often give those symptoms.
Posted By: Secret Chimp
Re: Engine falling off just past 4k - carb limit? - 02/13/12 07:44 AM
Quote:
What is the rest of the engine combo? It sounds like you've got a stock '67 383 2 barrel and just changed the cam and timing set. What cam? Speed Pro CS327?
If you've got a two barrel on your 383 (which is a big block) it probably wont spin much past 4000 rpm and keep making power, its a tiny carb. Don't get too caught up in the carb calculators, especially since they are geared for four barrel carbs which are actually rated differently. If I remember right when they flow test a two barrel they do it with 3 inches of vacuum, whereas a four barrel is flowed with 1.5 inches. So basically a 500 cfm two barrel is probably comparable to a 350-400 cfm four barrel.
I would recommend a four barrel on any big block, you don't have to go crazy. You wont suddenly have a race engine, but a factory four barrel intake with a Carter or factory Holley four barrel will give you some extra punch.
As far as the difference, if it revved out higher before it could be a couple things. For one the cam is almost guaranteed to not be the same specs, and the old one was worn. It is more likely that the new cam is bigger, so you should actually have more high rpm power. The old stretched out timing set would effectively retard the cam and may have shifted the powerband up. Its also possible that your new cam is too far advanced if you just installed it by lining up the dots. Does it have more low end torque?
You may also be encountering some valve float if the new cam is more aggressive and you're using stock springs.
I have a 318 - I just edited my post, I saw how I made that kind of unclear originally
It has more torque - it's most noticeable outside of the stock stall/automatic shifting range. If I shift manually it really pulls stronger at RPMs beyond the stock converter stall, it just kind of starts giving up at 4200-4300. I'm using a Summit 6900 cam and new Comp 901-16 valve springs - nothing nuts. Again it revs willingly over 5000 without a load, if that makes any difference.
I just did a dot-dot lineup on the timing chain. I did note that the dot on my new gear was slightly further around from the key slot than the factory gear (by maybe a 1/2 tooth or so at most). I had the option to install it four degrees advanced via the crank gear but just left it in the stock location. I spun the engine around a couple times and the dots still lined up. The original timing chain was so loose I was within about an eighth of an inch from being able to pull it clear off of the teeth of the cam gear.
If this is a fuel pressure/supply problem, should the engine start acting up if I just run it up in 1st and stay on it for a few seconds? Or might it just hit a wall where I've just maxed out the available fuel flow without it leaning out enough to make any scary sounds? I don't have a fuel pressure gauge yet so I might as well try something out on my way to the store...
Posted By: GTX MATT
Re: Engine falling off just past 4k - carb limit? - 02/13/12 08:14 AM
For some reason I thought you had a 383 in one of your other posts too
Anyway your new cam is probably slightly larger than the original. What did it pull to before? I wouldn't really expect more than 4000 rpm still with that tiny two barrel.
Posted By: RapidRobert
Re: Engine falling off just past 4k - carb limit? - 02/13/12 02:11 PM
T into the carb inlet & run a neoprene line out to a vac gauge taped to your windshield & take it out & see what psi you have at 4K at the in hg vac # that it acts up at in high gear. Most if not all of the round hand held vac gauges have a psi side.
Posted By: Mr.Yuck
Re: Engine falling off just past 4k - carb limit? - 02/13/12 02:22 PM
What is your initial and full timing? What electronics are you running? What is your fuel pressure? I'd try to set your timing in full at 2400 at 36* (leave the vac advanced unhooked) and take it for a drive. See if that helps. Not sure what carb you are using but a 600 cater comp series carb would work fine.
Posted By: OrangeProwler
Re: Engine falling off just past 4k - carb limit? - 02/13/12 06:30 PM
Edelbrock 1406 would be a great one for that car as I would change the intake to a Weiand 8007. Also, make sure to get the calibration kit for it as well.
Posted By: Secret Chimp
Re: Engine falling off just past 4k - carb limit? - 02/15/12 06:51 AM
The hoses kept popping off of my gauge (really exciting with gas and exhaust manifolds) so I'm doing a solid run tomorrow after I get some little hose clamps from the hardware store. The nose-off does seem to be inconsistent - last night I got over 4500 before it started slowing down, earlier today I wasn't even past 4000. Sometimes it feels abrupt, sometimes the engine just climbs slowly without seeming to fall off a cliff. A new Airtex fuel pump plus an adjustable regulator (I understand those pumps run around 7 psi, which is too much for the AVS I have going on soon) will hopefully be a cheap solution to this if the gauge confirms my suspicions at this point.
Posted By: Mr.Yuck
Re: Engine falling off just past 4k - carb limit? - 02/15/12 02:19 PM
do sb have heat risers in the mani's like a BB? if so maybe yours is stuck sending all the hot exhaust back up into engine... I also wonder if your ECM box is getting hot. You should in no way need an electric pump for that car. I'm going to guess it is not a fuel delivery issue.
Posted By: Secret Chimp
Re: Engine falling off just past 4k - carb limit? - 02/15/12 07:07 PM
Quote:
do sb have heat risers in the mani's like a BB? if so maybe yours is stuck sending all the hot exhaust back up into engine... I also wonder if your ECM box is getting hot. You should in no way need an electric pump for that car. I'm going to guess it is not a fuel delivery issue.
I have my MSD box mounted up on top of the inside of the fender next to the battery, not back on the firewall. I don't think it's an electronics problem.
The engine does have a heat riser, but it's not stuck (I actually have a problem with it rattling before the engine's warmed up all the way, time for new springs). When I start the car cold I do get a lot more vapor dumped out the left pipe than the right, but it looks and feels even to the back of my hand once it's warmed up a little.
If I end up finding a pressure or supply problem I'm replacing it with a new mechanical pump, not electric.
Posted By: Mr.Yuck
Re: Engine falling off just past 4k - carb limit? - 02/15/12 07:39 PM
Quote:
Quote:
do sb have heat risers in the mani's like a BB? if so maybe yours is stuck sending all the hot exhaust back up into engine... I also wonder if your ECM box is getting hot. You should in no way need an electric pump for that car. I'm going to guess it is not a fuel delivery issue.
I have my MSD box mounted up on top of the inside of the fender next to the battery, not back on the firewall. I don't think it's an electronics problem.
The engine does have a heat riser, but it's not stuck (I actually have a problem with it rattling before the engine's warmed up all the way, time for new springs). When I start the car cold I do get a lot more vapor dumped out the left pipe than the right, but it looks and feels even to the back of my hand once it's warmed up a little.
If I end up finding a pressure or supply problem I'm replacing it with a new mechanical pump, not electric.
is your gas tank vented? Maybe the vent is clogged???
Posted By: Dodgem
Re: Engine falling off just past 4k - carb limit? - 02/16/12 06:28 PM
To keep accelerating (with any life) a carb has to have say 25% more cfm available than needed at a specific RPM. so at 4200 at 100% is 386 cfm + 25% = 483 cfm so a 500 cfm carb would be kind of minimum to get to 5000 with some decent pull!
Just my
600 eddies and holleys work good on a 318!
Posted By: Secret Chimp
Re: Engine falling off just past 4k - carb limit? - 02/16/12 09:38 PM
Quote:
To keep accelerating (with any life) a carb has to have say 25% more cfm available than needed at a specific RPM. so at 4200 at 100% is 386 cfm + 25% = 483 cfm so a 500 cfm carb would be kind of minimum to get to 5000 with some decent pull!
Just my
600 eddies and holleys work good on a 318!
If I don't find any fuel problems this afternoon, I'm just going to finish putting my AVS together and get it on before I mess around any more. This carb ran perfectly up to high RPMs and this isn't that crazy of a cam, maybe cfm is all it is.
Posted By: Paul_Fancsali
Re: Engine falling off just past 4k - carb limit? - 02/16/12 09:53 PM
Did you change the valve springs? The carter 2bbl is barely 250 cfm Toss it and get the 4bbl of some kind without a stupid adapter. By the way the MSD can be a sourse of your problem I have had two that would slow the car down. Put a point dist in it and went 4-5 tenths faster. MSD are for race cars as far as I'm concerned too much heat from street driving and they act up
Posted By: Dodgem
Re: Engine falling off just past 4k - carb limit? - 02/17/12 02:35 PM
Then you would think it is running out of fuel. have you tried letting right off when you reach 4200 then getting back into it. this will allow the fuel to catch back up and if it then pulls up to 5000 from their fine it is a fuel delivery problem!
Posted By: Dodgem
Re: Engine falling off just past 4k - carb limit? - 02/17/12 02:48 PM
If the cam is to advanced the power will fall of quickly too.
Why is the chain loose? did you not change it?
DID you roll the motor over a couple more times to see if it is in fact lined dot to dot rolling and bring the dots back will show when you thought you were straight you were in fact not!
Posted By: Mr.Yuck
Re: Engine falling off just past 4k - carb limit? - 02/17/12 03:29 PM
Quote:
To keep accelerating (with any life) a carb has to have say 25% more cfm available than needed at a specific RPM. so at 4200 at 100% is 386 cfm + 25% = 483 cfm so a 500 cfm carb would be kind of minimum to get to 5000 with some decent pull!
Just my
600 eddies and holleys work good on a 318!
Ummm you should be balke to put a 2bbl on that thing and have it pull to 5k.
Posted By: Secret Chimp
Re: Engine falling off just past 4k - carb limit? - 02/17/12 09:44 PM
Quote:
If the cam is to advanced the power will fall of quickly too.
Why is the chain loose? did you not change it?
DID you roll the motor over a couple more times to see if it is in fact lined dot to dot rolling and bring the dots back will show when you thought you were straight you were in fact not!
The chain WAS lose, the new chain isn't. I was wondering if the old chain had done something to the cam timing that changed the rev range vs the new cam and new chain. And yes I did roll it over and the dots came back in line. The fact that it's never the same RPM whenever it hits the wall has me fairly certain I didn't goof up the installation.
Posted By: RapidRobert
Re: Engine falling off just past 4k - carb limit? - 02/18/12 03:21 AM
Quote:
The fact that it's never the same RPM whenever it hits the wall has me fairly certain I didn't goof up the installation.
post any new info
Posted By: Secret Chimp
Re: Engine falling off just past 4k - carb limit? - 02/21/12 05:06 AM
Couple of things:
I tried running it up until it seemed to nose off, backed off, then floored it again. Every time I did this it got up to around 4400-4500 rpm and started making an angry ping pong ball noise that sounded like detonation and power fell off with a "stop flooring it" type of feeling. I don't know if I ought to not be running 87 now or if it's getting so lean up top that it's pinging (maybe holding a higher RPM at lean-ish mixture heats the cylinders to the point of detonation will happen at higher RPMs but not when running it up in one go? I dunno, maybe something was rattling), but that's what happened.
I also tested my fuel pressure and I'm getting a good 6.5-7 psi at idle. I couldn't get my dang metal tees to seal well enough at driving RPMs in my garage to be comfortable actually driving with the gauge hanging out from my hood, but the pump seems strong.
My guess at this point is that the carb is just too low-capacity and too lean for the amount of air I'm trying to pull through it at this point. I'm all set to put my 650 Carter AVS on save for an air cleaner and a pressure regulator (didn't expect to have such a He-Man old pump but I sure need one now) so I'm just going to go ahead and do the swap and see what happens when it can get more fuel down.
Posted By: Secret Chimp
Re: Engine falling off just past 4k - carb limit? - 02/21/12 06:51 AM
Before I go throwing on the parts I have on hand and getting disappointed, should I run a fuel pressure regulator with the carb I'm using? I can't find any definitive specs for AVSes, only AFBs.
Posted By: RapidRobert
Re: Engine falling off just past 4k - carb limit? - 02/21/12 04:14 PM
OE avs/afb's have the same type float leverage system so the psi limits would be the same. Can I persuade you to check psi at the carb inlet at the rpm/pedal depression (load)AT SPEED that it acts up at. Need ~several psi. I still think you have a fuel delivery prob
. I'd set a can of gas in the eng compartment plumbed to the mech pump inlet & retry it & that takes the hard lines to the rear of the pump and the 2 neoprene connector hoses and the sender hard line pinholes and partly plugged sock all of it out of the equation. The mech pump may be weak & cannot handle the full throttle fuel demands of 4K+ but will handle 5K no load. less likely would be if there's a partly plugged screen filter in the carb inlet. Check float drop spec also. To ans your Q My "airtex" parts house pump puts out over 7 psi & I'm getting a reg (mileage is poor). eddy 1406
Posted By: Mr.Yuck
Re: Engine falling off just past 4k - carb limit? - 02/21/12 04:26 PM
Quote:
Couple of things:
I tried running it up until it seemed to nose off, backed off, then floored it again. Every time I did this it got up to around 4400-4500 rpm and started making an angry ping pong ball noise that sounded like detonation and power fell off with a "stop flooring it" type of feeling. I don't know if I ought to not be running 87 now or if it's getting so lean up top that it's pinging (maybe holding a higher RPM at lean-ish mixture heats the cylinders to the point of detonation will happen at higher RPMs but not when running it up in one go? I dunno, maybe something was rattling), but that's what happened.
I also tested my fuel pressure and I'm getting a good 6.5-7 psi at idle. I couldn't get my dang metal tees to seal well enough at driving RPMs in my garage to be comfortable actually driving with the gauge hanging out from my hood, but the pump seems strong.
My guess at this point is that the carb is just too low-capacity and too lean for the amount of air I'm trying to pull through it at this point. I'm all set to put my 650 Carter AVS on save for an air cleaner and a pressure regulator (didn't expect to have such a He-Man old pump but I sure need one now) so I'm just going to go ahead and do the swap and see what happens when it can get more fuel down.
did you swap the cam out when you changed the t-chain? are you sure you didn't bend a few push rods when the old chain was stretched out? Have you tried higher octane gas? I ran a bone stock 318 2bbl 69 coronet to 15.69 1/4's shifting at 5200 w/ the stock 2bbl on 93. Have you checked the plugs? Maybe go run it hard and get it back to the garage and take a look at a few of them.
Posted By: Secret Chimp
Re: Engine falling off just past 4k - carb limit? - 03/01/12 09:16 PM
It was the carb. I just took my car out for its first run with the AVS on - HOLY F^*# this thing GOES now! Runs like the stink with the secondaries open and has more poke in normal driving too. But holy crap this car is fast now. I'm still grinning.
Posted By: JohnRR
Re: Engine falling off just past 4k - carb limit? - 03/01/12 09:24 PM
Quote:
It was the carb. I just took my car out for its first run with the AVS on - HOLY F^*# this thing GOES now! Runs like the stink with the secondaries open and has more poke in normal driving too. But holy crap this car is fast now. I'm still grinning.
What carb did you originally have on it that was causing this problem ?
Posted By: Secret Chimp
Re: Engine falling off just past 4k - carb limit? - 03/01/12 09:46 PM
The original Carter BBD 2 barrel.
Posted By: RapidRobert
Re: Engine falling off just past 4k - carb limit? - 03/01/12 09:50 PM
Quote:
It was the carb. I just took my car out for its first run with the AVS on - HOLY F^*# this thing GOES now! Runs like the stink with the secondaries open and has more poke in normal driving too. But holy crap this car is fast now. I'm still grinning.
You sound pumped 
. Didn't you get like that once before!
Posted By: Secret Chimp
Re: Engine falling off just past 4k - carb limit? - 03/01/12 09:58 PM
LOL yes. Now I just need to get rid of these 2.93 gears for some 3.55s and I'll really be grinning...
Posted By: Mr.Yuck
Re: Engine falling off just past 4k - carb limit? - 03/01/12 10:03 PM
Quote:
The original Carter BBD 2 barrel.
ohh I thought you had a small 4bbl on it.. go get some gears son
Posted By: Secret Chimp
Re: Engine falling off just past 4k - carb limit? - 03/01/12 10:11 PM
This is my first run in it (empty road by the local ce-ment plant, no animals harmed during filming)
http://www.youtube.com/watch?v=xJMXl09IAEs&feature=youtu.beI'm sure that looks glacial to you guys but my fastest car previously was my 1.8 liter 140 horsepower Miata :P