• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Looking for some feed back.

Andy D

Member
Local time
5:44 PM
Joined
Dec 15, 2018
Messages
11
Reaction score
10
Location
Florida
I have a 68 plymouth GTX. Ever since I purchased it I have not been real happy with the performance. She has a set of 1972 motor home heads (3751213) it is also running the stock intake. And I believe this may be where the problem lies. When I purchased it there was a holly 750 on it and I was not happy with the start up and idle. After trying every adjustment it was time for a new 750 street demon. Start up is perfect and idle is perfect. It seems like it kind of gurgles I am not sure that would be the term but to me it somehow is not normal. I know the flow on stock intakes is quite low compared to most aftermarket intakes by a good margin. Plugs look fine. I just think upon reading the info I got with the car they said it had a mild cam and no notation as to what cam. I think what may be happening is with a larger than stock cam and the stock intake it may not be getting enough flow? Let me know what you think as I am in the process of finding a route to go. Also it has the stamped steel rockers and I am not sure of the cam or lifters.
 
I have a 68 plymouth GTX. Ever since I purchased it I have not been real happy with the performance. She has a set of 1972 motor home heads (3751213) it is also running the stock intake. And I believe this may be where the problem lies. When I purchased it there was a holly 750 on it and I was not happy with the start up and idle. After trying every adjustment it was time for a new 750 street demon. Start up is perfect and idle is perfect. It seems like it kind of gurgles I am not sure that would be the term but to me it somehow is not normal. I know the flow on stock intakes is quite low compared to most aftermarket intakes by a good margin. Plugs look fine. I just think upon reading the info I got with the car they said it had a mild cam and no notation as to what cam. I think what may be happening is with a larger than stock cam and the stock intake it may not be getting enough flow? Let me know what you think as I am in the process of finding a route to go. Also it has the stamped steel rockers and I am not sure of the cam or lifters.
You haven't given any specifics on the engine. Is it also a 72 motorhome engine, with a bigger cam? If so, sure some closed chamber heads will give you more compression, and better performance. All I'm saying is is if you want more information, we need to know more too.
 
You haven't given any specifics on the engine. Is it also a 72 motorhome engine, with a bigger cam? If so, sure some closed chamber heads will give you more compression, and better performance. All I'm saying is is if you want more information, we need to know more too.
Sorry I thought I put in it is a 440 with a stock manifold a mild cam (which is unknown) currently running a 750 street demon iron heads and stamped steel rockers. it has headers hedman. Stock distributor with vac advance and electronic point conversion. Orange box ignition. excellent compression all at 155 to 160 psi oil startup at about 80 psi warm 30 psi.Also the engine was built in 2005 while I have the work order they do not list the cam and I am sure they would laugh if I called and asked if they remember the build or had any further info on it.
 
IMHO, the 213 heads were the worst of the bunch. A stock intake manifold is not ideal, but should be OK until you get a pretty healthy motor, although maybe it's a really low perf intake version not the HP. Never liked the Orange box, seem to fall off at too low of an RPM. Cam timing may also be an issue. Your compression looks great.
 
If this is indeed a stock motor home engine that ports those stock heads, basically what you have is a gutless turd powering your bird. (GTX)

IF it was I, there would be a plan for a separate engine build up and then just swing out the old turd for the new and fresh engine.
 
Compression does look good, just guessing I would say somewhere around 9.3 to 9.5. I think you can do with better heads and intake and possibly a cam change to something more performance oriented. I have seen a lot of engines built with cams (RV) that have great low end torque but play out at a little over 4K rpms. I bought a done car one time very unreasonably because of 2 issues. #1was a 451stroker that fell on its face at 4000 rpms. #2 was solvent pops in a fresh paint job. Just don't be disappointed because everything you are saying can be cured quite easily and doesn't have to break the bank.
 
IMHO, the 213 heads were the worst of the bunch. A stock intake manifold is not ideal, but should be OK until you get a pretty healthy motor, although maybe it's a really low perf intake version not the HP. Never liked the Orange box, seem to fall off at too low of an RPM. Cam timing may also be an issue. Your compression looks great.
The 440 intakes were the same whether they were on an Imperial or a GTX. The only different intake was the 6 Barrel.
 
IMHO, the 213 heads were the worst of the bunch. A stock intake manifold is not ideal, but should be OK until you get a pretty healthy motor, although maybe it's a really low perf intake version not the HP. Never liked the Orange box, seem to fall off at too low of an RPM. Cam timing may also be an issue. Your compression looks great.
3751213 400/440 Motor Home 1973 2.08 1.74...someone fucked him and kept the good heads...?
 
IF you want to keep it stock looking
IF you don't care about original looks,
there's a ton of stuff to boost the performance

you need to make that, stock or not, apparent & make the right choice

maybe;
get a set of 906 castings 68-70 383-440 HP heads (like it came with org.)
ditch the camshaft you know nothing about
get a good camshaft, for your specific needs & style of driving

get a better ignition box,
go to a Mopar performance chrome box at a min. for the CIE controllers
or an aftermarket controller/Ignition box, like a FOB or Rev-n-ator
get a hotter coil, (coils will cause a bunch of power loss sometimes too)
use low ohm resistance plug wires, like at a min. 500 ohm or less per ft.
lower the better

properly gap your plugs for the type of ignition you have
probably go a step cooler plug too
(depends on your usage & what ignition etc.)

a better manifold like a Edelbrock Performer dual plane intake
(it's closer to a stock low-rise, but alum.)
or better yet* if you don't have any hood clearance issues, you shouldn't
unless you have an N96 Airgrabber hood,
it may have some under-hood clearance issues
*great street manifold Edelbrock Performer RPM dual plane,
a have a heat shield under the carb,
maybe even block off the heat riser going to the manifold
if it has an electric choke,
disclaimer;
if it has a mech. stove choke
don't block off the heart riser, unless it's only driven in warmer weather
& the choke is disabled

check & see what the fuel pressure is
if the pressure is below 5 psi,
it'll probably never perform all that well

there's a lot of little tricks you can try,
that don't cost all that much

maybe even with what you have now
bump/put a tad of advance timing into it,
a couple degrees advanced
try like in the 15*btdc or try to bump it up farther, until it starts pinging
then back it off a tad/couple degrees

or old school try, with what you have now
set the idle screw at like 2800-3000 rpm,
loosen & turn the distributor slowly 'advanced' it
(you can leave the vac. adv. hooked up)
until you get the highest RPM
you need to have a tach to do it right, you can do it by ear too
(if you got a good feel for it)
& a timing light to know where it is too, for reference if nothing else
after performing that
tighten the dist. back down snug so you can move it with you hand still
but it won't move under acceleration
then turn the idle screw back to normal, shut it off
make sure it will still start normally, if not back off the timing a smidge
then go out & test it
once it's right tighten the dist. back up
& enjoy

all engines like a little different timing
your may like something some other combo doesn't & visa versa
it doesn't cost much if any to try

On the Street Demon,
make sure the needle & seat/float levels are where they should be too
accelerator pump setting too
it could be affecting the performance, if wrong
again like said above if the fuel pressure is below 5-psi
you can put a gauge on it inline somewhere fitting out there
that have a 1/8 pipe NPT, to screw a fuel pressure gauge into/inline
or right off the carb fitting
especially WOT (wide open throttle)
it probably won't perform well under hard loads

lots of stuff to try

the heads are the biggest choke point & an orange control box (shitty)
from what I read, what little information we/I have...

engine are just huge air pumps what goes in must be able to come back out

an exhaust blockage restriction or something like that,
may rob power too, feel like a turd especially going uphill
much like a clogged fuel filter does too

could have a restriction in the fuel lines, old fuel lines & old sending unit socks
are notorious for that ****

good luck

Wall of text.jpg
 
Last edited:
IF you want to keep it stock looking
IF you don't care about original looks,
there's a ton of stuff to boost the performance

you need to make that, stock or not, apparent & make the right choice

maybe;
get a set of 906 castings 68-70 383-440 HP heads (like it came with org.)
ditch the camshaft you know nothing about
get a good camshaft, for your specific needs & style of driving

get a better ignition box,
go to a Mopar performance chrome box at a min. for the CIE controllers
or an aftermarket controller/Ignition box, like a FOB or Rev-n-ator
get a hotter coil, (coils will cause a bunch of power loss sometimes too)
use low ohm resistance plug wires, like at a min. 500 ohm or less per ft.
lower the better

properly gap your plugs for the type of ignition you have
probably go a step cooler plug too
(depends on your usage & what ignition etc.)

a better manifold like a Edelbrock Performer dual plane intake
(it's closer to a stock low-rise, but alum.)
or better yet* if you don't have any hood clearance issues, you shouldn't
unless you have an N96 Airgrabber hood,
it may have some under-hood clearance issues
*great street manifold Edelbrock Performer RPM dual plane,
a have a heat shield under the carb,
maybe even block off the heat riser going to the manifold
if it has an electric choke,
disclaimer;
if it has a mech. stove choke
don't block off the heart riser, unless it's only driven in warmer weather
& the choke is disabled

check & see what the fuel pressure is
if the pressure is below 5 psi,
it'll probably never perform all that well

there's a lot of little tricks you can try,
that don't cost all that much

maybe even with what you have now
bump/put a tad of advance timing into it,
a couple degrees advanced
try like in the 15*btdc or try to bump it up farther, until it starts pinging
then back it off a tad/couple degrees

or old school try, with what you have now
set the idle screw at like 2800-3000 rpm,
loosen & turn the distributor slowly 'advanced' it
(you can leave the vac. adv. hooked up)
until you get the highest RPM
you need to have a tach to do it right, you can do it by ear too
(if you got a good feel for it)
& a timing light to know where it is too, for reference if nothing else
after performing that
tighten the dist. back down snug so you can move it with you hand still
but it won't move under acceleration
then turn the idle screw back to normal, shut it off
make sure it will still start normally, if not back off the timing a smidge
then go out & test it
once it's right tighten the dist. back up
& enjoy

all engines like a little different timing
your may like something some other combo doesn't & visa versa
it doesn't cost much if any to try

On the Street Demon,
make sure the needle & seat/float levels are where they should be too
accelerator pump setting too
it could be affecting the performance, if wrong
again like said above if the fuel pressure is below 5-psi
you can put a gauge on it inline somewhere fitting out there
that have a 1/8 pipe NPT, to screw a fuel pressure gauge into/inline
or right off the carb fitting
especially WOT (wide open throttle)
it probably won't perform well under hard loads

lots of stuff to try

the heads are the biggest choke point & an orange control box (shitty)
from what I read, what little information we/I have...

engine are just huge air pumps what goes in must be able to come back out

an exhaust blockage restriction or something like that,
may rob power too, feel like a turd especially going uphill
much like a clogged fuel filter does too

could have a restriction in the fuel lines, old fuel lines & old sending unit socks
are notorious for that ****

good luck

View attachment 947573
Thanks for the wall lol. Lots of good pointers. I have played with the timing and set it at about 12 degrees and 38 total I believe it has been a while. I just checked the block casting and it is 36988304 which is 72 to 78 RB big block. I am thinking of starting with probably a manifold and a new ignition (distributor and box). This is not a daily driver and it is in Florida. I do not race and take it out on weekends and some car shows. She runs good but there is something not right and it bothers me. You know how it is when you just can't put your finger on what it is. In the back of my mind I am thinking that putting in the bigger cam and adding headers and not increasing the flow it is choking. While the 750 is more than I need I think the flow rate is low. I have seen bench tests with a stock manifold at about 200 cfm and even a basic aftermarket will flow about 240 or better. I think I will start there. and even if that is not the problem at least that has been done. Then move on from there. Thanks again.
 
Thanks for the wall lol. Lots of good pointers. I have played with the timing and set it at about 12 degrees and 38 total I believe it has been a while. I just checked the block casting and it is 36988304 which is 72 to 78 RB big block. I am thinking of starting with probably a manifold and a new ignition (distributor and box). This is not a daily driver and it is in Florida. I do not race and take it out on weekends and some car shows. She runs good but there is something not right and it bothers me. You know how it is when you just can't put your finger on what it is. In the back of my mind I am thinking that putting in the bigger cam and adding headers and not increasing the flow it is choking. While the 750 is more than I need I think the flow rate is low. I have seen bench tests with a stock manifold at about 200 cfm and even a basic aftermarket will flow about 240 or better. I think I will start there. and even if that is not the problem at least that has been done. Then move on from there. Thanks again.
Good deal, you are developing a plan, that is great.
 
Auto Transport Service
Back
Top