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Engine Flat Spot on Acceleration

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New User

Apr 26, 2015, 8:02 PM

Post #1 of 9 (5079 views)
Engine Flat Spot on Acceleration Sign In

My friend has a 1958 Zephyr Mark II car which has a terrible flat spot on acceleration.
The carburettor dash pot works well when the accelerator is pumped, however it has been fitted with exhaust extractors and I suspect that the usual heat transfer by contact between the exhaust manifold and the intake manifold may be the problem.
Can anyone advise as to a method of overcoming this problem, short of replacing the original exhaust manifold.

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Apr 27, 2015, 3:17 AM

Post #2 of 9 (5054 views)
Re: Engine Flat Spot on Acceleration Sign In

Please tell me the purpose of these "exhaust extractors" and how they play into anything to do with choke and or engine warming up both on a long list of drivability problems.

Dashpot to me is generic so take it you mean and mentioned an accelerator pump that squirts extra fuel for RPM changes is the idea. That should be adjustable and will be unique to that set up and car.

Fuel stinks now for what was called for so long ago now. There will be things that will not like current fuel and some alterations called for.

Manual choke OE or retrofitted to one or automatic choke and does it work properly and on time? Those were then issues and still are now to deal with. Many used exhaust heat to shut off chokes and some also engine coolant. Some engines crossed exhaust over or under carbs thru manifold to warm them up.

Need to know how this is set up now and what was OE to the thing and what altered,


(This post was edited by Tom Greenleaf on Apr 27, 2015, 3:26 AM)

New User

Apr 28, 2015, 7:58 PM

Post #3 of 9 (5030 views)
Re: Engine Flat Spot on Acceleration Sign In

1 The purpose of the exhaust extractors was to increase exhaust manifold efficiency, mainly by reducing back pressure on the cylinders.
2 The normal Exhaust manifold is mounted below and in contact with the Intake manifold, for the purpose of transferring heat from the exhaust to the inlet to assist in the vaporization of the fuel injected by the dash pot (Accelerator pump).
3 This engine is fitted with a manual choke (standard from manufacture) and the use of this choke overcomes the problem of the flat spot, which is how the system should operate on a cold engine, but this flat spot continues even whilst the engine is hot.
4 The dash pot as I have called it is also called the 'accelerator pump' by some; and has the purpose of injecting extra fuel when the throttle is opened increasing the ratio of fuel to air (that suddenly was decreased by the throttle opening), to increase the power output.
5 As to whether the dashpot is adjustable I don't believe is the issue, as it looks like there is sufficient fuel injected, it just is not being utilized as needed to get the engine up and away.
6 Naturally the age of the vehicle has a certain bearing on this problem being as different fuel is used now compared to 1958 when it was manufactured, but I was of the thinking that if it was hot the fuel that is injected by the dash pot would vaporize more readily then unheated fuel.

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Apr 28, 2015, 11:13 PM

Post #4 of 9 (5027 views)
Re: Engine Flat Spot on Acceleration Sign In

OK - I'm with you but need some more info to help more. Please say what entire car this is and what engine. Zephyr was both a Mercury and a Lincoln depending on year I haven't looked up just yet - it's still that vintage of engine but may need to look up some details.

The word dashpot could and probably is fine to use or local lingo but doesn't mean anything to do with fuel by itself. Your eyes watching is are not good enough to know if it's enough or not. Fuel ratio cold thru warmed up is critical to be on target so with a manual choke meant to be manual (this part is good news) is fine, you get accustomed to their use and slowly shut them off which is a complete algorithm of actual temps and engine's temp. The exhaust might be a real problem and most alterations to exhaust are not helping anything but popular areas to fool with.

Fuel: Dang - impossible to know what you get now no matter what. In 50s thru 60s "regular" was 93 octane with lead which was just to increase anti-knock and make lesser fuel behave like more expensive grade of unleaded have the same anti-knock property. Hi-Test or Premium began at 98 octane and at least Sunoco sold 104 octane. It was almost all fossil fuel with the lead still needs discussing as it was really cheating on the gasoline using a lower grade forever claiming to lube valves and people told never to use "white gas" or unleaded which was sold on purpose to get the junk out no regard to if it was a good thing or not it was cheaper to make fuel with lead than without. Lead built up deposits, compression would quickly get way to high and plugs fouled were a few problems simply overcome by Spring and Fall full tune-ups and for lead and carbon build up you would just run a car hard down highway periodically up to assertively doing tricks to remove it - no joke running water down carbs but slow enough not to do damage was about like taking a glowing fry pan and dunking in a sink of water type thing. Don't do that, it was hack then and still is.

Back to fuel/gas ratio to air. Cold to already atomized fuel on heavy metal parts would quickly recondense to liquid such that you choked off ordinary air mix (why it's called a choke) to overcome loss and maintain ratio till warmed up which this very well might not do well or fast enough. Keep in mind to an engine even 100F air is cold an engine still wants to uniformly be usually about 180-195F or so still by thermostat and way to new to do without one or run the common 160 people thought was helping and never was - book on that and there was a reason but not for automotives and was for marine use then and ongoing.

So, this "flat spot" almost certainly is lack of proper fuel to air mix if only for that moment. Any decent carb set up was designed to cover all that from idle, mid range, quick changes in RPM and WOT (wide open throttle) where the accelerator pump came in handy - any fuel mixing method had to deliver the right ratio for the assorted conditions and I say this isn't. Common all over the place same car beside another you learned each one's character either got used to it nicely or at some point fix or adjust it.

So let's learn more about what this is and I'll try to look up intended design and how exhaust played in not just for warm up but the whole time it runs. All sorts of carbs in use on even similar engines near all needed very careful adjustments from new and now think of how many (likely) have been totally replaced just because of the years or some really did make them run better. Do say it altered for more power so popular too and most tricks really failed but looked "cool" so remained popular.

Again, bear with me, I'm gettin' old so have to trust hands on memory for some of this which still works! I have a good running engine 10 years older now!

This is generally not allowed but a link to Elvis no less in 1960 (it was cold then too I was there) and this spot for good fun and information. Enjoy and let's get this car right.................;list=RDQkMVscR5YOo


New User

Apr 30, 2015, 7:41 PM

Post #5 of 9 (4981 views)
Re: Engine Flat Spot on Acceleration Sign In

1 The attached web site will give the details of this vehicle;
2 The lingo used is Australian.
3 Choke is fine.
4 Exhaust is a problem; hence this enquiry.
5 Fuel used has no detrimental effect on other vehicles or indeed on this vehicle prior to the extractors being installed.
6 Cold inlet manifold does have the effect of not atomizing the fuel.
7 The inlet manifold remains cold (relatively) even after the engine is hot; Hence this enquiry.
8 What we have learned here is; to revert back to the original exhaust manifold.

Ultimate Carjunky / Moderator
Discretesignals profile image

Apr 30, 2015, 11:50 PM

Post #6 of 9 (4979 views)
Re: Engine Flat Spot on Acceleration Sign In

Exhaust extractor? Is that a heat riser? Does this have a heat riser valve?

How about ignition advance? If the ignition doesn't advance on acceleration, that will cause a hesitation also. Does your distributor have a vacuum advance servo?

If you have a timing light, check to see if the ignition advances by applying vacuum to the servo. Make sure vacuum is reaching the servo when you blip the throttle. Also check that the mechanical advance inside the distributor is functioning and base timing is correct.

Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.

(This post was edited by Discretesignals on May 1, 2015, 12:02 AM)

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

May 1, 2015, 1:14 AM

Post #7 of 9 (4973 views)
Re: Engine Flat Spot on Acceleration Sign In

Wow! Was not familiar with this car and please leave that link as informative but will try to post some observations and specs from it...............
1957 Ford Zephyr Mk II overdrive
    • Ford Zephyr Mk II overdrive, model year 1957, version for Europe North America
    • 4-door sedan body type
    • RWD (rear-wheel drive), manual 4-speed gearbox
    • petrol (gasoline) engine with displacement: 2553 cm3 / 155.8 cui, advertised power: 67 kW / 90 hp / 91 PS ( SAE ), Torque net: 186 Nm / 137 lb-ft
    • characteristic dimensions: outside length: 4547 mm / 179 in, wheelbase: 2718 mm / 107 in
    • reference weights: base curb weight: 1243 kg / 2740 lbs, gross weight GVWR: 1633 kg / 3600 lbs
    • how fast is this car ? top speed: 140 km/h (87 mph) (declared by factory);
    • accelerations: 0- 60 mph 15.6 s; 0- 100 km/h 16.9 s (a-c simulation); 1/4 mile drag time (402 m) 20.2 s (a-c simulation)
    • fuel consumption and mileage: average estimated by a-c: 10.5 l/100km / 27 mpg (imp.) / 22.5 mpg (U.S.) / 9.6 km/l

    In Australia now but clearly made for non US use. Low compression, terrible HP, 4 speeds unheard of for a six, on a fairly heavy chassis for the power it has listed.

    Of note: You noted cold air intake manifold to the touch, and that use of choke covered the "flat" spot. Smacks of just what I thought - too lean of many issues that probably need tweaking or go back to the OE exhaust is probably the cure.

    TMK and might find info on this. US alone used higher octane and better gasoline. Taxes virtually world wide outside of the US purposely raised cost of fuel for automobiles and taxed the CID (cubic inch displacement) so there was a motivation to make smaller engines and use cheaper fuel with far less octane rating hence the low compression.

    (Trivia on fuels) A barrel of crude oil is 42 gallons US measure and you can't get just gasoline out of crude. It is refined to make it useful at all so you get assorted products from it and takes more gallons of crude to make the sweet "Octane" chains {think 8} than lower #s like "Kerosene" chains {think 7} and so on. I don't know the %s but you get lots more low grade fuels for that 42 gallon barrel if you want fuel at all. It makes much higher and lower also well needed down to tar - no wasting it.
    Back on track. Simply the objective was to go as far as possible on the least fuel expensive or not and for this car not have to squish your azz into a tiny umbrella car (sorry) to do it.

    ? Now what to do? It wants what any want is the right mixture for the situation and Discrete mentioned timing as well. It all matters. You should be able to do wild adjustments to fuel delivery and timing for this. You can't always run on what it thinks is a cold engine though so I'd get rid of the exhaust manifold as it would still work fine as originally intended but set it for that.

    Fuel mixture is also greatly changed by the float level and how it meters what I'll call jets for off idle running. Think carbs as three steps of design but 5 steps of function. All this after it's warmed up!

    1. Idle circuit alone. If that's all that was needed the whole game would be over for an engine.
    2. Mix now at first part throttle to use both idle mixture (adjustable) and the next phase is mid-range now using some of the float level for mixture and some of the idle settings.
    3. Just mid range only the fuel ratio is dictated by the float level mostly and idle mixture no longer in use.
    4. Changes from this to more power now and some before will want that squirt to cover the flat spot of sudden change of request for power and change of circuits.
    5. This is WOT (wide open throttle) totally the fuel mixture is based on float level staying the same and jets unobstructed by any metering rods probably not used in this car which act to change the size of the hole fuel passes thru the jets which are just drilled holes to a specific size.

    This lead right back to that accelerator pump's volume of squirt and how fast the fuel can atomize always counts but not too much or it would knock or pre-detonate.

    By 1958 this all should be set once and forget it stuff. Older you would actually have adjustments at the steering wheel and dash for timing and use choke for fuel mixture a little for fuel available, temperatures, load on engine and even what altitude! Yikes!

    In short as I can you should carefully set all these adjustments for the most likely conditions you will use this car if only by smelling (go easy mate) the exhaust you can tell you have some things about right and looks of spark plugs will tell also with some use.

    Troubles will be one person will get used to how to do what and when with this car and driving it to make it run its best and may change settings for seasons of expected temperature changes. Was routine for ages in the general era. Now unknown fuel character is just going to be a pest to put up with.

    If this is new to you check for some features you may not have noticed it might have. If choke is a knob you pull try turning it and see if it locks the throttle or gas pedal. Some/many did as your fast idle instead of a separate throttle lock by the year.

    So IMO the changes to the exhaust were either a bad idea or worked well for someone along the way who constantly had higher to hot ambient temps. IMO again, it was better as designed in the first place and would aim to get it back that way.

    Great machine BTW. You'll either totally love this thing if any good or hate it all day long!

    Sorry for the total novel. It could take volumes more too. This car and engine design was well made to adapt to a wild assortment of fuels and conditions not suitable for a passive owner/driver,


    PS: One more. If this does have a knob for overdrive it may not be "overdrive" as we know it. It would allow transmission/gearbox to free-wheel when going downhill or slowing down as to not turn the engine faster NOT to change the gear ratio! Owned one and reason was to save fuel that way, not a higher gear of any sort............

    (This post was edited by Tom Greenleaf on May 1, 2015, 1:18 AM)

    New User

    May 3, 2015, 8:29 PM

    Post #8 of 9 (4935 views)
    Re: Engine Flat Spot on Acceleration Sign In

    Thanks for your assistance!
    However the problem seems too difficult to rectify so the answer seems to be that the original exhaust manifold should be replaced.

    Tom Greenleaf
    Ultimate Carjunky / Moderator
    Tom Greenleaf profile image

    May 4, 2015, 4:03 AM

    Post #9 of 9 (4930 views)
    Re: Engine Flat Spot on Acceleration Sign In

    Right. I totally believe this will run its best in assorted conditions and fuels back to original and only tweak out anything as needed. Good luck with it,



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