Flat spot help required .
- Thread starter Mad4slalom
- Start date
Mad4slalom
Well-known user
Sounds very similar , right carb although not original but was running well before so something has changed i think.
Mad4slalom
Well-known user
i was going to check the free movement with a 5 thou feeler as i cant feel any movement there. Is there another check i can do, there is fuel being shot in when i blip the throttle cable lever. Its getting plenty of fuel when you put yr foot down but starved when applying a little throttle, awkward to drive slow and smooth then the secondries come in with a bang as i give it more throttle
Mad4slalom
Well-known user
not sure if my carb has
Vacuum secondaries or mechanical ones how do i tell? I can see a vac hose from the distributor to the base of carb or inlet manifold , is this just vacuum advance?, if it was a fuel blockage or pump issue then would be surprised it still goes like stink when the secondaries open ? Its hydraulic tappets and new plugs 2 weeks ago ?? I am baffled but know nothing about these carbs
Oneball
CCCUK Member
It’s a double pumper so you have mechanical secondaries.
By vacuum leak I mean air getting in where it shouldn’t so split brake vacuum hose, split blanking plug etc. My thinking behind this is if it’s not great at small throttle openings but ok at full throttle this can be a sign of air getting in when it shouldn’t
A blockage in one of the primary jets could be why it’s good when the secondaries are open.
if you’re cruising along and you depress the throttle half way what happens? Is there initial hesitation and then it picks up. Or is it just rough all the time and only accelerates well with you foot mashed to the floor.
The mechanical secondaries only open beyond a certain throttle pedal position, get a helper to find out at what point. You can then relate this to your driving experience and whether its ok with the secondaries open or if what you think is the secondaries opening is just it sorting itself out if that’s the case then Rosco is almost certainly on the money.
By vacuum leak I mean air getting in where it shouldn’t so split brake vacuum hose, split blanking plug etc. My thinking behind this is if it’s not great at small throttle openings but ok at full throttle this can be a sign of air getting in when it shouldn’t
A blockage in one of the primary jets could be why it’s good when the secondaries are open.
if you’re cruising along and you depress the throttle half way what happens? Is there initial hesitation and then it picks up. Or is it just rough all the time and only accelerates well with you foot mashed to the floor.
The mechanical secondaries only open beyond a certain throttle pedal position, get a helper to find out at what point. You can then relate this to your driving experience and whether its ok with the secondaries open or if what you think is the secondaries opening is just it sorting itself out if that’s the case then Rosco is almost certainly on the money.
Roscobbc
Moderator
Stand over the carb - look down the primary carb venturi's - operate the throttle linkage - do you see two continuous 'squirts' of gas from orifices up near the top opf the venturi? - if continuing the throttle operation does the same thing happen with the secondary venturi's?
Mad4slalom
Well-known user
Thanks all for your ideas , thoughts and suggestions I am reading all and taking it all in. At least I now know I have mech secondaries, vaaccum leak sounds possible, I dont have power brakes so not sure where to look other than dizzy vac advance pipe , where would i look for a blanking plug ?
if I gently accelerate, it gets to a point, maybe 1800 /2000 revs it feels flat like a car with a speed limiter set, if you try to gently press the pedal further then it feels a bit ragged, then you press a bit more which I presume is when the secondaries open, it then flies away no misses or anything and loads of power.
If i accelerate hard off the line and continue to through each gear so the secondaries open quickly then you almost drive through the flat spot ,
Mad4slalom
Well-known user
Thanks, do you mean per one pull of the throttle lever should give a continous stream of fuel? It does , i can see two more squirters further down in the carb they are working too.
Mad4slalom
Well-known user
Here is some info on the motor build and set up from the previous owners son who oversaw the build . 
Mad4slalom
Well-known user
Page 1 
Chuffer
CCCUK Member
Wow Gary ! You sure are lucky having so much previous detailed information about your car from the previous owners . Those guys really have gone the extra mile . I wish I had knew half as much history on my car . Hope you get the set up of carb right without too much more hassle . 
Oneball
CCCUK Member
Blanking plugs as below, you’re dissy will be connected to the little one underneath. Worth checking the plugs are both there and not cracked.
Suck on the vac hose to the dissy you should hear the diaphragm move
Did you get a stream of fuel in all four chokes?
With it running at idle look down the carb and check that you can see fuel going in in both of the primaries.
Are you sure it’s running on 8 cylinders? As you changed the plugs have you got the leads on right?
Suck on the vac hose to the dissy you should hear the diaphragm move
Did you get a stream of fuel in all four chokes?
With it running at idle look down the carb and check that you can see fuel going in in both of the primaries.
Are you sure it’s running on 8 cylinders? As you changed the plugs have you got the leads on right?
Mad4slalom
Well-known user
Yes, chuffer, it was the reason I risked buying sight unseen, father and son team having rebuilt 13 body off vetts of various years. Have got a file and club newsletters of the build progress over 2 years, sure this is one of many ongoing issues that are all part and pleasure of owning a near fifty year old hobby car. So glad i bit the bullet and went for it. Love the whole engagement of v8 and four speedWow Gary ! You sure are lucky having so much previous detailed information about your car from the previous owners . Those guys really have gone the extra mile . I wish I had knew half as much history on my car . Hope you get the set up of carb right without too much more hassle .
CaptainK
CCCUK Member
Ahhh... carb problems. I'll be watching this for all the ideas. Mine has had various issues over the years and I've never truly got to the bottom of it, despite changing the carb and dizzy and plugs and leads and other stuff.
The biggest things that helped my car were:
1) Having it setup by someone on a rolling road.
2) I installed a heat insulator gasket between the carb and manifold as after a while (especially on hot days), the carb would get heat soak and the fuel would percolate and cause all manor of fun issues.
The only remaining issue I have now is trying to pull away fast from a standstill - revs hard, pulls away a little but before the clutch has completely engaged the engine just loses all power, you dip the clutch, it pops out the exhaust and recovers, re-engage the clutch and power and away she goes. However, if the clutch is fully engaged and I'm at at low speed (e.g. moving at idle speed in 1st) and I give it the beans then its all fine. So it doesn't really bother me, as I don't do many hard starts from a standstill. Could just be down to my poor ability to launch the car
The biggest things that helped my car were:
1) Having it setup by someone on a rolling road.
2) I installed a heat insulator gasket between the carb and manifold as after a while (especially on hot days), the carb would get heat soak and the fuel would percolate and cause all manor of fun issues.
The only remaining issue I have now is trying to pull away fast from a standstill - revs hard, pulls away a little but before the clutch has completely engaged the engine just loses all power, you dip the clutch, it pops out the exhaust and recovers, re-engage the clutch and power and away she goes. However, if the clutch is fully engaged and I'm at at low speed (e.g. moving at idle speed in 1st) and I give it the beans then its all fine. So it doesn't really bother me, as I don't do many hard starts from a standstill. Could just be down to my poor ability to launch the car
Mad4slalom
Well-known user
All part of owning a classic I guess, I have noticed when i switch off hot and want to restart after say half an hour then I have to pump some gas in as what was in the carb has percolated away, . Maybe needs an insulated spacerAhhh... carb problems. I'll be watching this for all the ideas. Mine has had various issues over the years and I've never truly got to the bottom of it, despite changing the carb and dizzy and plugs and leads and other stuff.
The biggest things that helped my car were:
1) Having it setup by someone on a rolling road.
2) I installed a heat insulator gasket between the carb and manifold as after a while (especially on hot days), the carb would get heat soak and the fuel would percolate and cause all manor of fun issues.
The only remaining issue I have now is trying to pull away fast from a standstill - revs hard, pulls away a little but before the clutch has completely engaged the engine just loses all power, you dip the clutch, it pops out the exhaust and recovers, re-engage the clutch and power and away she goes. However, if the clutch is fully engaged and I'm at at low speed (e.g. moving at idle speed in 1st) and I give it the beans then its all fine. So it doesn't really bother me, as I don't do many hard starts from a standstill. Could just be down to my poor ability to launch the car
Forrest Gump
CCCUK regional rep
....and check the the whole carb is tight on the manifold. The four corner bolts should only be snugged up, not over tightened otherwise you risk distorting the carb base. It’s not unknown for these nuts to become loose creating an air leak.
Roscobbc
Moderator
Accelerator pump jets. Think they are all the same - wrong, many different sizes. If jet sizes too small - it can bog - especially with a tuned engine - and even more so if you have a 'cam' installed. If accelerator pump jet sizes are too big it will not really matter too much (in fact in might make things a little easier starting the engine if you don't have a choke) Following will give an idea of typical sizes for accelerator pump jets.
www.speedwaymotors.com
Whilst the previous owners of your car have given plenty of detail relating to the build-up and timing etc there doesn't appear to be any detail about carb tuning (or cam specifications). Most performance Holley carbs will operate straight out of the box and are very tolerant to things like jet sizes, power valves and the like - however if that carb has been used previously and then remained unused for a couple of years or so there is a good chance that fuel could have eroded some of the internal components like gaskets, seals and rubber items.
Holley Accelerator Pump Discharge Nozzles
A good selection of pump discharge nozzle sizes is indispensable to the proper tuning of the accelerator pump system. The nozzle hole size determines the rate of fuel discharge. A larger hole will "shoot" the fuel at a faster rate with less pressure than a smaller hole. Sizes are stamped on each...
Mad4slalom
Well-known user
Ah ! Thats an easy one to check, cars out of shed so i will do that now . Thank you
Mad4slalom
Well-known user
Ithanks rosco, I am now in touch with him in the states and he has asked for videos of sounds to listen to it at varous revs, he did say the power valves can go bad , and he told me which one is in there. I have a lot of cam info in the file, I will have a look through it. Will update this eve. Thank you for your continuing advice and suggestionsAccelerator pump jets. Think they are all the same - wrong, many different sizes. If jet sizes too small - it can bog - especially with a tuned engine - and even more so if you have a 'cam' installed. If accelerator pump jet sizes are too big it will not really matter too much (in fact in might make things a little easier starting the engine if you don't have a choke) Following will give an idea of typical sizes for accelerator pump jets.
Whilst the previous owners of your car have given plenty of detail relating to the build-up and timing etc there doesn't appear to be any detail about carb tuning (or cam specifications). Most performance Holley carbs will operate straight out of the box and are very tolerant to things like jet sizes, power valves and the like - however if that carb has been used previously and then remained unused for a couple of years or so there is a good chance that fuel could have eroded some of the internal components like gaskets, seals and rubber items.Holley Accelerator Pump Discharge Nozzles
A good selection of pump discharge nozzle sizes is indispensable to the proper tuning of the accelerator pump system. The nozzle hole size determines the rate of fuel discharge. A larger hole will "shoot" the fuel at a faster rate with less pressure than a smaller hole. Sizes are stamped on each...