Flat spot help required .

[Mad4slalom]

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?
View attachment 9728View attachment 9729View attachment 9730

Thanks for the pics, i will check the blanking plugs and the vac pipe on dizzy. It is deffo on 8 as it was on seven prior to changing the plugs. I only remove one plug lead at a time , change plug , replace the lead then move to the next plug. It is smooth as silk until it reaches 1700 ish revs then it is just like a car with a speed limiter set, are there primary jets and secondary or main jets that come into play when the secondaries open ? Is the power valve an american term for a jet?

 

[Roscobbc]

Idle jets operate on primary side of the carb (and sometimes on secondaries of race spec carbs) These are fixed but adjusted by the idle mixture screws to provide a fuel source enabling the engine to idle and operate up to about 1000 - 1200 rpm from when there is a transition period before main jets start to operate. The power valve operates on the primary side of the carb (in most cases) and is an enrichment device that is controlled by venturi vacumn, sensing engine fuel requirements during this transition period.The power valve can 'blow-out' if you have an severe inlet manifold back-fire and the engine will accordingly run crap. I'm guessing the previous owner will have selected the correct power valve for the cam installed. On a modified engine with perhaps a 'cam' and typical lower vacumn readings an incorrectly rated power valve can make it run like a dog at lower rpm. The lower vacumn readings of a modified engine effectively mean that the engine struggles to draw-in enough fuel/air mixture to enable it to run smoothly and will generally require a richer idle mixture to compensate.
Secondaries. These to not operate at a fixed rpm. In general use they won't come in to play unless you depress the gas pedal more than half way or so. At that point the secondaries start to open, secondary squirters (accelerator pumps) kick-in a bit more gas covering the opening period and secondary jets opening-up.....and off you go accelerating in to oblivion
Contrary to some peoples belief the the transition period between the primaries and the secondaries opening should be smoooooth.......no sudden bog or hiccup!

 

[Forrest Gump]

Ross, Do you think it could be a damaged power valve allowing far too much fuel in at low speed? By getting on the throttle allows sufficient air in to restore the a/f mix.

 

[Roscobbc]

Only one way to check Andy - OP needs to 'pull' primary float bowl and remove power valve to inspect - perhaps suck against the valve to check whether functioning - problem being that he may have issues replacing items and remaining gas tight if the gaskets get damaged.

 

[Mad4slalom]

Idle jets operate on primary side of the carb (and sometimes on secondaries of race spec carbs) These are fixed but adjusted by the idle mixture screws to provide a fuel source enabling the engine to idle and operate up to about 1000 - 1200 rpm from when there is a transition period before main jets start to operate. The power valve operates on the primary side of the carb (in most cases) and is an enrichment device that is controlled by venturi vacumn, sensing engine fuel requirements during this transition period.The power valve can 'blow-out' if you have an severe inlet manifold back-fire and the engine will accordingly run crap. I'm guessing the previous owner will have selected the correct power valve for the cam installed. On a modified engine with perhaps a 'cam' and typical lower vacumn readings an incorrectly rated power valve can make it run like a dog at lower rpm. The lower vacumn readings of a modified engine effectively mean that the engine struggles to draw-in enough fuel/air mixture to enable it to run smoothly and will generally require a richer idle mixture to compensate.
Secondaries. These to not operate at a fixed rpm. In general use they won't come in to play unless you depress the gas pedal more than half way or so. At that point the secondaries start to open, secondary squirters (accelerator pumps) kick-in a bit more gas covering the opening period and secondary jets opening-up.....and off you go accelerating in to oblivion
Contrary to some peoples belief the the transition period between the primaries and the secondaries opening should be smoooooth.......no sudden bog or hiccup!

Wow Rosco, you have a load of knowledge and understanding of these carbs and their workings. The po in the us has said also that it sounds pos like a blown power valve , so have a new one and gaskets coming tomorrow. Hoping it is the cause . I saw a video that said to wind in the idle mixture screw /s whilst ticking over to see if the motor dies, if so then power valve good . Would you advise trying that first or just change it out .? it is a 125-85 (8.5 HG) arriving tomorrow with gaskets etc. I have had a few back fires whilst learning how best to start car with no choke. Thanks for your input and advice

 

[Roscobbc]

I saw a video that said to wind in the idle mixture screw /s whilst ticking over to see if the motor dies, if so then power valve good . Would you advise trying that first or just change it out .? it is a 125-85 (8.5 HG) arriving tomorrow with gaskets etc. I have had a few back fires whilst learning how best to start car with no choke.

We might be on the right track - not too sure about the the winding-in the idle screw to check power valve - may be applicable for older Holley carbs - newer Holley carbs (like bought in last 25 year) should have power valve protection - so they don't blow (theoretically).
Could be that the power valve you are buying isn't best suited for your engine IF it has a performance cam. Ideally you would use a vacumn gauge to measure the idle vacumn and select your power valve relative to that. 8.5 hg power valve will be fine for a stock engine with between 16 and 20 HG. If your engine has, say 10 hg to 12 hg at idle then a 4.5 or 6 hg valve should be used respectively. (yardstick rule is power valve selection should be half engine idle vacumn hg.)

 

[Chris Sale]

For a good overview of how Holley carbs work I can recommend 'Holley Carburetor Handbook' by Mike Urich. Have just checked on Amazon UK and they have 2 left in stock. Costs about £12.

Chris Sale
'64 Coupe

 

[Mad4slalom]

We might be on the right track - not too sure about the the winding-in the idle screw to check power valve - may be applicable for older Holley carbs - newer Holley carbs (like bought in last 25 year) should have power valve protection - so they don't blow (theoretically).
Could be that the power valve you are buying isn't best suited for your engine IF it has a performance cam. Ideally you would use a vacumn gauge to measure the idle vacumn and select your power valve relative to that. 8.5 hg power valve will be fine for a stock engine with between 16 and 20 HG. If your engine has, say 10 hg to 12 hg at idle then a 4.5 or 6 hg valve should be used respectively. (yardstick rule is power valve selection should be half engine idle vacumn hg.)

Thanks Rosco, apparently all his builds have anti blow protection but he said they do just go sometimes regardless. They seem very thorough and definate which power valve to use . I will look through my file to see if any reference to how much engine vacuum There is . .

 

[Mad4slalom]

For a good overview of how Holley carbs work I can recommend 'Holley Carburetor Handbook' by Mike Urich. Have just checked on Amazon UK and they have 2 left in stock. Costs about £12.

Chris Sale
'64 Coupe

Thanks for that Chris

 

[Mad4slalom]

....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.

I tweeked the carb nuts , they all went a gentle 8th turn , havent tested yet but thanks for the tip

 

[Mad4slalom]

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 suggestions

Here is a bit of cam info Rosco, i guess its a mild street cam ?
Thanks sgain

 

[Forrest Gump]

I tweeked the carb nuts , they all went a gentle 8th turn , havent tested yet but thanks for the tip

No, that won’t have made any difference, the nuts were tight enough.
I would bet an 8.5 power valve is too big on your engine, it’s likely only making 12hg of vacuum at idle with your cam. That size power valve might not be shutting off fully and you’re over-fuelling.
I’d say a XE 274 is a performance cam in a 350, vacuum won’t be very much at idle.

 

[Forrest Gump]

I’ve got a similar cam to you and currently got a 6.5 P/V. Ive been thinking it’s borderline too big for my setup.

 

[Roscobbc]

Agree with Andy's post above /|\ on this - the cam appears to be a Comp Cams Extreme Energy flat tappet 230/236 @ .050" hydraulic cam. Its probably as far as you'd want to go on a relatively small capacity small block with GM heads with manual transmission and still be relatively streetable.

 

[Mad4slalom]

Agree with Andy's post above /|\ on this - the cam appears to be a Comp Cams Extreme Energy flat tappet 230/236 @ .050" hydraulic cam. Its probably as far as you'd want to go on a relatively small capacity small block with GM heads with manual transmission and still be relatively streetable.

Thanks all, it is all new to me so just trying to get any info I can . Re the size of power valve , from a detectives point of view , surely , if it was too large then it would have been this way all the time, they ran it for 4000miles in the us with out issue and it was fine when I got it, something has changed surely, ? I think I should replace like for like to see if it cures the problem, then run the car for a bit and can always go smaller later as not a dear part. Saying that, I dont know what mpg I should expect with this motor ? would a smaller pv improve things without lowering performance.?
the po and his son seem very experienced so I wonder why they went so big, maybe I should ask the thinking behind it?
Thanks all

 

[Mad4slalom]

No, that won’t have made any difference, the nuts were tight enough.
I would bet an 8.5 power valve is too big on your engine, it’s likely only making 12hg of vacuum at idle with your cam. That size power valve might not be shutting off fully and you’re over-fuelling.
I’d say a XE 274 is a performance cam in a 350, vacuum won’t be very much at idle.

I tried to look up in my file any reference to the amount of vacuum my motor is running , only thing I found was that it has a vacuum canister connected to manifold vacuum and not timed vacuum as it had from the factory. I dont understand the difference vacuum novice here

 

[Roscobbc]

Power valve selection shouldn't make too much difference to fuel economy. As for fuel economy - depends on rear axle ratio to a degree - but think perhaps 18 - 20 mpg on a run and perhaps 14 -16 mpg around town.

 

[Forrest Gump]

Thanks all, it is all new to me so just trying to get any info I can . Re the size of power valve , from a detectives point of view , surely , if it was too large then it would have been this way all the time, they ran it for 4000miles in the us with out issue and it was fine when I got it, something has changed surely, ? I think I should replace like for like to see if it cures the problem, then run the car for a bit and can always go smaller later as not a dear part. Saying that, I dont know what mpg I should expect with this motor ? would a smaller pv improve things without lowering performance.?
the po and his son seem very experienced so I wonder why they went so big, maybe I should ask the thinking behind it?
Thanks all

That is true re running okay previously and something has changed. We’re maybe mixing up different thoughts here but maybe interrelated. When your carb was new it would almost certainly come with a 6.5 which would suit many applications. Your fellas have deliberately gone bigger which will add fuel sooner in a stock motor so even sooner again in your modified motor. Not saying this wouldn’t work but curious to know the thinking behind it, but agree with your logic to replace same and eliminate that.
As was mentioned earlier, it is worth checking the timing before you get too much further.

 

[Mad4slalom]

Power valve selection shouldn't make too much difference to fuel economy. As for fuel economy - depends on rear axle ratio to a degree - but think perhaps 18 - 20 mpg on a run and perhaps 14 -16 mpg around town.

Hmm, I havent checked acurately but it “feels” a lot worse than that ,

 

[Mad4slalom]

That is true re running okay previously and something has changed. We’re maybe mixing up different thoughts here but maybe interrelated. When your carb was new it would almost certainly come with a 6.5 which would suit many applications. Your fellas have deliberately gone bigger which will add fuel sooner in a stock motor so even sooner again in your modified motor. Not saying this wouldn’t work but curious to know the thinking behind it, but agree with your logic to replace same and eliminate that.
As was mentioned earlier, it is worth checking the timing before you get too much further.

Yes I have a timing light here and there are some quite specific timing advice in nicks letter to me., it may well have slipped a bit as I was having very slight run on symptoms prior to slowing the idle. Will a blown pv be apparent on inspection when removed?
thanks