Flat spot help required .

[Mad4slalom]

Out in the 72 today I noticed a little flat spot had got worse, maybe since I lowered my idle speed. Not sure how a holley 650 double pumper works but am presuming there are idle jets and main jets. I am finding it hard to smoothly accelerate from a slow speed situation it is fine initially and if you keep on the gas , however, if you are tooltling along at 1500 revs and want to smoothly apply power, it hesitates and almost nothing happens then i am guessing the secondaries come in with a jolt and its all a bit manic. It is worse between 1800 to 2500 revs , any more than that and its all fine with plenty of power . Its just hard to get a smooth transition through the rev range.
i did read that a 650 dp holley is hard to tune out a flat spot on a motor with a manual box as they are better with an auto with more vaccuum.
are there any idle or other jets that i can remove and blow out or any other suggestions to get this running smoothly .

 

[Oneball]

I’d be looking at timing and distributor first.

If you’ve got a constant accelerator pedal position the secondary’s position won’t change as they’re mechanical on a double pumper.

 

[Mad4slalom]

I’d be looking at timing and distributor first.

If you’ve got a constant accelerator pedal position the secondary’s position won’t change as they’re mechanical on a double pumper.

Thanks oneball, I am lost already, just presumed carb as only flat between1800 and 2500 revs, comes in really powerful then. And lots of references to holley bogging and hesitation. Should i rig up a timing light and see where its at. Got some notes in the file on how they set it up i think.

 

[Roscobbc]

Firstly a DP carb is best suited for a manual transmission car. Great risk of overfueling on an auto unless you have a low axle ratio and/or show gas pedal restraint. Not saying they won't work but a vac sec is best suited for an auto. Have you checked out and optimised the primary 'squirter'?. Does it deliver enough of a pump shot to prevent a 'bog' when accelerating? is there too uch clearance between operating lever and cam?
When supplied new the carb would have come with a selection of cams which fit on the primary throttle shaft and allow you to tailor the pump shot. So one cam will pump more fuel in to the carb initially, another will deliver less but over a longer measured period. Presuming of course that the squirter pump is working. When you operating the throttle initially does the pump squirt neat gas into the primaries? - is it an equal amount both sides?. Probably not your problem but there is also a squirter for the secondaries - only likely to be used at higher rpm or when you give it the beans. It works exactly the same way as primaries.

 

[Mad4slalom]

Firstly a DP carb is best suited for a manual transmission car. Great risk of overfueling on an auto unless you have a low axle ratio and/or show gas pedal restraint. Not saying they won't work but a vac sec is best suited for an auto. Have you checked out and optimised the primary 'squirter'?. Does it deliver enough of a pump shot to prevent a 'bog' when accelerating? is there too uch clearance between operating lever and cam?
When supplied new the carb would have come with a selection of cams which fit on the primary throttle shaft and allow you to tailor the pump shot. So one cam will pump more fuel in to the carb initially, another will deliver less but over a longer measured period. Presuming of course that the squirter pump is working. When you operating the throttle initially does the pump squirt neat gas into the primaries? - is it an equal amount both sides?. Probably not your problem but there is also a squirter for the secondaries - only likely to be used at higher rpm or when you give it the beans. It works exactly the same way as primaries.

I have been thinking that it seemed to have got a lot worse since I slowed my idle to try to stop the run on issue I had. Could that have an effect on a hesitation higher up the rev range. Think i should raise the idle a tad more to see if it improves again. Seems strange that it got worse only after I interfered with it. Will do that this morning and report back. Thank you

 

[Oneball]

Idle speed shouldn’t have affected it assuming you’ve turned the correct screw (can’t think how you couldn’t on a DP)

what sort of choke have you got?

 

[Mad4slalom]

Idle speed shouldn’t have affected it assuming you’ve turned the correct screw (can’t think how you couldn’t on a DP)

what sort of choke have you got?

I wondered the same and didnt think it should affect it. I have No choke, can see the ( I presume) choke butterfly fully open , it has a lever assembly but no cable or electric control. Was actually thinking of closing it manually today and see if starting is easier than just a few pumps and then feathering throttle as I have been. If so then I would look at fitting a good old fashioned manual choke, ( did any of the c3.s have them ?) or best go electric ?

 

[Roscobbc]

Just to complicate matters - another question for you. Idle screws. Does you carb have two (on the primaries) or four (primaries and secondaries)?
Before going too far have you checked the float levels on front and rear bowls? (engine running - gas just seeping out of level plug opening)

 

[Mad4slalom]

Just to complicate matters - another question for you. Idle screws. Does you carb have two (on the primaries) or four (primaries and secondaries)?
Before going too far have you checked the float levels on front and rear bowls? (engine running - gas just seeping out of level plug opening)

No , I did read somthing about them tho, i will goodle it to check which ones there are, are they the flat brass thin bolt heads ? , thank you

 

[Mad4slalom]

Just to complicate matters - another question for you. Idle screws. Does you carb have two (on the primaries) or four (primaries and secondaries)?
Before going too far have you checked the float levels on front and rear bowls? (engine running - gas just seeping out of level plug opening)

I only saw one idle screw on the drivers side of the carb i will inspect closer

 

[Roscobbc]

I only saw one idle screw on the drivers side of the carb i will inspect closer

There will be a matching one on the other side which will also need adjusting when setting things up. Sounds as though you have a base DP carb with 'fixed' secondary idle jets in a metering 'plate' (rather than metering 'block' with adjustable secondary idle) - no issue, less to go wrong....

 

[Mad4slalom]

There will be a matching one on the other side which will also need adjusting when setting things up. Sounds as though you have a base DP carb with 'fixed' secondary idle jets in a metering 'plate' (rather than metering 'block' with adjustable secondary idle) - no issue, less to go wrong....

Thanks for the info, i will take a few pics and show you what i have .

 

[Mad4slalom]

Just to complicate matters - another question for you. Idle screws. Does you carb have two (on the primaries) or four (primaries and secondaries)?
Before going too far have you checked the float levels on front and rear bowls? (engine running - gas just seeping out of level plug opening)

Hi roscob, i took thes pics, i only have one screw on the drivers side that i tweeked. With screwdriver in pic.
i am pointing to i presume the float level plug ( brass one) i will do that test in a bit . The little lever i have pictured also , this is the one i nedd to check with a feeler guage, ? They both feel up against the stop with no movement now.
the other pic of the drivers side of the carb shows a hole in the front body with a small brass jet type adjuster but none in the rear body of the carb and the same the passenger side so presuming thats normal and not required?
I am sure you have better things to do on a sunny sunday but I appreciate your time. And advice , i am definately a holley novice

 

[Roscobbc]

Hi roscob, i took thes pics, i only have one screw on the drivers side that i tweeked. With screwdriver in pic.
i am pointing to i presume the float level plug ( brass one) i will do that test in a bit . The little lever i have pictured also , this is the one i nedd to check with a feeler guage, ? They both feel up against the stop with no movement now.
the other pic of the drivers side of the carb shows a hole in the front body with a small brass jet type adjuster but none in the rear body of the carb and the same the passenger side so presuming thats normal and not required?
I am sure you have better things to do on a sunny sunday but I appreciate your time. And advice , i am definately a holley novice

Sorry - no images shown - I'll check-in later. Levers only need a very small amount of clearance - literally just touching - when you lightly tweak the throttle pedal the primary squirter operates. This is designed to cover 'bogging' when initially cranking throttle at any point of operation.

 

[Mad4slalom]

Oops sorry.

 

[Mad4slalom]

Oops sorry.

I checked the fuel levels which seem spot on, going to look at the feeler gauge test on the pump lever next .

 

[Roscobbc]

Adjusting the idle mixture is simply a case of rotating the idle mixture screws clockwise (in turn) each side while the engine is running. These are the screws that are located in the metering block that's sandwiched between carb body and float chamber)
Normally this will mean the engine faultering and possibly stopping when you do this.
Ideally the idle mixture screws will normally be set at between 1 1/4 and 1 3/4 of a turn anticlockwise from closed.
When the engine is idling simply rotate each side 'clockwise' untill the engine just starts to falter (lean idle mixture) - then rotate anticlockwise a 'knats cock' to find the sweet spot (not too far or it'll be too rich an idle mixture). Repeat for the other side.
Repeat once more if you need to for the original side if there is a big difference in running. Finally adjust idle speed (the one with your big screwdriver) to your preference. Job done.
Primary squirters should give a prolonged 'squirt' of gas when you crank open the throttle. Its not unknown for carbs to get crudded-up if not used regularly and left with stale fuel. A carb rebuild kit with new rubbers, gaskets and seals can be an ideal if you intend stripping and cleaning it. Holley carbs are not rocket science.

 

[Mad4slalom]

I havent touched the two idle mixture screws yet i just dropped the idle with the throttle stop screw. Learning a little bit more ftom a couple of utube videos. .

Adjusting the idle mixture is simply a case of rotating the idle mixture screws clockwise (in turn) each side while the engine is running. These are the screws that are located in the metering block that's sandwiched between carb body and float chamber)
Normally this will mean the engine faultering and possibly stopping when you do this.
Ideally the idle mixture screws will normally be set at between 1 1/4 and 1 3/4 of a turn anticlockwise from closed.
When the engine is idling simply rotate each side 'clockwise' untill the engine just starts to falter (lean idle mixture) - then rotate anticlockwise a 'knats cock' to find the sweet spot (not too far or it'll be too rich an idle mixture). Repeat for the other side.
Repeat once more if you need to for the original side if there is a big difference in running. Finally adjust idle speed (the one with your big screwdriver) to your preference. Job done.
Primary squirters should give a prolonged 'squirt' of gas when you crank open the throttle. Its not unknown for carbs to get crudded-up if not used regularly and left with stale fuel. A carb rebuild kit with new rubbers, gaskets and seals can be an ideal if you intend stripping and cleaning it. Holley carbs are not rocket science.

thanks for your help, just been out for a test and still the same, it hesitates on the transition from primary to secondary then when they come in it takes off like a scalded cat and is smooth as anything. It is also revving smoothly up from idle when stationary. Trying to drive gently with little throttle is hardest as it feels like its getting starved of fuel and is hesitating and flat as f@ck then Suddenly gets fuel and she’s away. Dont think its gummed up or stale gas as it has been fine til the last couple of times out. Consequently It is burning fuel like no tomorrow as you have to be on the throttle to get past the flat spot. Any suggestions from anyone who may have had similar appreciated.

 

[antijam]

I had the 'bogging' problem with my '71. Put your foot down to accelerate away and the engine practically stalls - only to come back in with a bang a moment later. My carb was a Rochester quadrajet although not the correct one for the car. A change to the proper carb cured the problem. The cause is usually a delay in fuel supply as the secondaries are opening.

 

[Oneball]

Did you check the timing?

If it’s changed recently though I’d put money on it not being mixture adjustment or timing.

My first port of call would be to check you’ve not got a vacuum leak.

I’d then check for fuel blockages, fuel pump, spark issues and tappets assuming its mechanical.