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Help surge at 25-40 mph cruise


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I had been having some drivability issues / stalling hard start no start. I believe mostly due to vapor lock. Added 1/4 inch phenolic carb spacer and converted to Holley in tank fuel pump with return. Regulated fuel pressure 7 psi. Replaced starter with MSD. All the big problems resolved. However, now have great idle,almost too much throttle response off the line ( time to look at rear suspension:) and good response passing. Great acceleration, good cruise under 25 mph and over 40-45 mph. But surges at in town cruise speeds.

Q code 73 Mach1 - po states that oo replaced the 351 with 429 from a 69 Thunderbird 4V. And casting numbers confirm this is correct block and heads for a 429 "thunderjet". Engine believed to have had heads rebuilt, ported and polished. Intake is Edelbrock performer 429 which also has had some port and polish done. Bottom end and cam are stock 11:1 CR). Carb is prosystems modified Holley 4150 double pumper ( I do not know CFM - but most likely either 750 or 850 ). No choke. Bleeds about 1.5 turns out. 18 mmhg vacuum at idle (bounces a lot) idle screw all the way out. Idles well about 600-650 does not stall hot or cold. If I adjust idle to about 1k ( best guess from sound tach not fixed yet) smooths out and vacuum more steady- does not fix my problem so I put it back. Dizzy is magnetic pickup hooked to MSD digital 6Al and blaster coil. No vacuum advance. Timing is 6* initial plus 30* mechanical. ( this is per the correct timing per ford service manual. ) because there were some odd things going on with ford BB engines in the early 70 for emissions and it is possible I do not have straight up like I should have with an early 429. I have tried 4*-14* initial in two degree steps. And this made no difference in my 25-45 surge issue. I replaced all of the vacuum hoses and sprayed starting fluid around the base of carb intake and vacuum ports. No leaks found. All the smog stuff is gone except PCV valve. C6 4:11 rear gears. pulled plugs and do not look to lean or rich. Any ideas?

 

 

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I had been having some drivability issues / stalling hard start no start. I believe mostly due to vapor lock. Added 1/4 inch phenolic carb spacer and converted to Holley in tank fuel pump with return. Regulated fuel pressure 7 psi. Replaced starter with MSD. All the big problems resolved. However, now have great idle,almost too much throttle response off the line ( time to look at rear suspension:) and good response passing. Great acceleration, good cruise under 25 mph and over 40-45 mph. But surges at in town cruise speeds.

Q code 73 Mach1 - po states that oo replaced the 351 with 429 from a 69 Thunderbird 4V. And casting numbers confirm this is correct block and heads for a 429 "thunderjet". Engine believed to have had heads rebuilt, ported and polished. Intake is Edelbrock performer 429 which also has had some port and polish done. Bottom end and cam are stock 11:1 CR).  Carb is prosystems modified Holley 4150 double pumper ( I do not know CFM - but most likely either 750 or 850 ). No choke. Bleeds about 1.5 turns out. 18 mmhg vacuum at idle (bounces a lot) idle screw all the way out.  Idles well about 600-650 does not stall hot or cold. If I adjust idle to about 1k ( best guess from sound tach not fixed yet) smooths out and vacuum more steady- does not fix my problem so I put it back.  Dizzy is magnetic pickup hooked to MSD digital 6Al and blaster coil. No vacuum advance. Timing is 6* initial plus 30* mechanical. ( this is per the correct timing per ford service manual. ) because there were some odd things going on with ford BB engines in the early 70 for emissions and it is possible I do not have straight up like I should have with an early 429. I have tried 4*-14* initial in two degree steps. And this made no difference in my 25-45 surge issue. I replaced all of the vacuum hoses and sprayed starting fluid around the base of carb intake and vacuum ports. No leaks found. All the smog stuff is gone except PCV valve.  C6 4:11 rear gears.  pulled plugs and do not look to lean or rich. Any ideas?

 

 

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Look up "cam surge" I think you may be able to find some answers.

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Ok, thanks for the suggestion [emoji106]

 

 

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With a stock cam I doubt it is cam surge. I think more likely it is lean in that area of operation (which is usually the t-slot) and/or you are experiencing some detonation because the high compression ratio with the stock cam is going to require something higher octane that pump gas premium.

 

Try going .005 smaller on the idle air bleeds which will richen the transition and see if that has any effect on the surge. If it seems to help then you know you are on the right track and might want to consider increasing the size of the ifrs to get more fuel to the t-slot.

73 conv. 460, D0VE large valve heads, Performer RPM manifold, Voodoo 227/233 cam, Holley 950 HP carb, C6 trans, 3.25 trak-loc.

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i chased a highway surge in my 408 for a long time. Tried everything from plugs,wires,to jets and bleeders. And everything in between. Mine was always at 2800 rpm and 55-60 mph. Only thing that fixed it was a cam swap. Once i put in a different cam it drives perfect. And the new cam was a good bit more radical than the one that was in it. Not sure if its your issue but that was my experience. Like Roy said ....cam surge.

Kevin
1971 Mach 1

408C Stroker - C4 w/3,000 stall - 8.8" Rear w/3.73's - Disc brakes all way around.

 

 

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With a stock cam I doubt it is cam surge. I think more likely it is lean in that area of operation (which is usually the t-slot) and/or you are experiencing some detonation because the high compression ratio with the stock cam is going to require something higher octane that pump gas premium.

 

Try going .005 smaller on the idle air bleeds which will richen the transition and see if that has any effect on the surge. If it seems to help then you know you are on the right track and might want to consider increasing the size of the ifrs to get more fuel to the t-slot.

 

 

I’ve always left air bleeds alone due to holley’s warning about catastrophic detonation- but I guess as long as I go smaller / richer can’t hurt. I’ll see if I can get some and try it. Shouldn’t my plugs read lean if this is the case? I’ve toyed with the idea of adding a o2 sensor to monitor A/F mix but seems a bit extreme

 

 

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Ok, thanks for the suggestion [emoji106]

 

 

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I did some reading on cam surge and although the description of the problems seems to match what I have going on to an extent, the problems seems to be focused around aftermarket cam ( which I guess PO could have mislead me about ) and at low RPM.  This occurs at 25-45mph and although I do not have a tach with the 4:11s and stock C6 I am well over 2500 and probably in the 3500RPM range. can it happen this high RPM ?   ::shrug::

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Even though it doesn't seem logical that someone would have that amount of head work performed and keep a stock cam, I tend to believe it, with a vacuum reading of 18 at idle. I'm assuming you meant 18 inches of mercury, not millimeters of mercury.

 

At 45 mph, your engine speed should be around 2500 RPM, assuming you have 26 inch diameter tires. You can play around with gear ratios, tire sizes, and so on with the spreadsheet in this thread:

https://www.7173mustangs.com/thread-tire-sizes--11566?pid=115406#pid115406

 

Did the P.O. also have adjustable valves installed? Whether or not they did, the problem may be a valve (or valves) too tight.

 

Having to back the idle screw all the way out to get the idle speed down may be another clue to a potential problem. As mentioned in another post, the problem may be because the throttle blades are in the transfer slots at that speed. It sounds like the secondary throttle adjustment may be off and the secondary throttle blades open too far, which would cause the primary throttle adjustment to not work right. You should be able to back the primary adjustment down enough to stall the engine. Having the secondary throttle open too far at idle would make it more likely the primary blades are in the transfer slots.

 

You should get an engine analyzer, timing light, or automotive multimeter that has a tachometer. Even if you have a dash tachometer connected it doesn't help much when your head is under the hood, and you really need to nail down the engine speeds better when trying to analyze problems like this.

 

EDIT: I'm assuming you meant 18 inches of mercury, not millimeters of mercury with your vacuum test.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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I'm probably (and more likely), talking out the back of my head here, but what the heck, I'll throw it out there anyway.

I know nothing about 429's and not much more about 351c"s, but recently and you may have read my post/saga, I had a misfire, cough, stumble, you name it on my 351C 4V. Couldn't figure it out. I knew my timing was set up for my engine specs and the Holley 670 carb was set up by a pro tuner. I tried swapping the power valve, increasing the squirter etc. etc.

 I then borrowed a friends Air / Fuel Ratio meter and ran several tests at idle, high revs stationary and on the road. Note; you need at least one O2 sensor bung installed in the exhaust at the prescribed location to use an AFR. I swapped out my plugs for one heat range hotter. With the AFR, I was able to improve the idle ratio, but on the highway, it was well in range for air/fuel ratio. Still I had this annoying cough, stutter, slump, whatever. Then I had a brain 'wave' check the distributor cap!! Low and behold, it was cracked, replace it with a high end brass contact cap and problem solved. Now it runs the best it has ever run, good power good fuel mileage, what more could a guy want living on a fixed income!!

Good luck hope it's something just as simple.

Geoff.

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Geoff.

 I learn something new every day!

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Can't hurt to check the distributor cap. Thanks for the tip

 

 

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Yea- inches of Hg— I deal In mmHg all day.

I actually have a multimeter with a tachometer- never thought to use it. This weekend I'll see if I can get true idle rpm and see if I can get the multimeter set up so I can read it while the car is being driven.

Thanks again- the speedometer gear calculator is great also- one of the next projects [emoji51].

 

 

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With a stock cam I doubt it is cam surge. I think more likely it is lean in that area of operation (which is usually the t-slot) and/or you are experiencing some detonation because the high compression ratio with the stock cam is going to require something higher octane that pump gas premium.

 

Try going .005 smaller on the idle air bleeds which will richen the transition and see if that has any effect on the surge. If it seems to help then you know you are on the right track and might want to consider increasing the size of the ifrs to get more fuel to the t-slot.

 

 

I’ve always left air bleeds alone due to holley’s warning about catastrophic detonation-  but I guess as long as I go smaller / richer can’t hurt. I’ll see if I can get some and try it.  Shouldn’t my plugs read lean if this is the case? I’ve toyed with the idea of adding a o2 sensor to monitor A/F mix but seems a bit extreme 

 

 

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You aren't going to hurt anything with low speed air bleed changes. You are looking to determine if the issue is the result of a lean condition. The easiest thing to change to help you do that are the air bleeds. If that change seems to help you know you are on the right track and that up sizing the ifr is worth the effort. If the air bleed change doesn't help or makes it worse they are easy to put back.

73 conv. 460, D0VE large valve heads, Performer RPM manifold, Voodoo 227/233 cam, Holley 950 HP carb, C6 trans, 3.25 trak-loc.

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+1 on TommyK's post. On the Idle Air Bleeds (IABs), make changes of about .005-.006 at a time to look for changes. NOT SO on the High Speed Air Bleeds (HSAB)! Only change .001-.002 at a time or leave them alone. Make these too lean at WOT can kill an engine. The reason is the relative sizes of the IABs and the HSABs. The IAB is around .070-.075 and the HSAB is about .033-.036 (calculate the surface area of the holes in the bleeds to see what difference a change makes). Chuck

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Thank you for the advice. I will try changing the air bleeds- much easier then changing main jets.

 

 

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  • 3 weeks later...

I had been having some drivability issues / stalling hard start no start. I believe mostly due to vapor lock. Added 1/4 inch phenolic carb spacer and converted to Holley in tank fuel pump with return. Regulated fuel pressure 7 psi. Replaced starter with MSD. All the big problems resolved. However, now have great idle,almost too much throttle response off the line ( time to look at rear suspension:) and good response passing. Great acceleration, good cruise under 25 mph and over 40-45 mph. But surges at in town cruise speeds.

Q code 73 Mach1 - po states that oo replaced the 351 with 429 from a 69 Thunderbird 4V. And casting numbers confirm this is correct block and heads for a 429 "thunderjet". Engine believed to have had heads rebuilt, ported and polished. Intake is Edelbrock performer 429 which also has had some port and polish done. Bottom end and cam are stock 11:1 CR).  Carb is prosystems modified Holley 4150 double pumper ( I do not know CFM - but most likely either 750 or 850 ). No choke. Bleeds about 1.5 turns out. 18 mmhg vacuum at idle (bounces a lot) idle screw all the way out.  Idles well about 600-650 does not stall hot or cold. If I adjust idle to about 1k ( best guess from sound tach not fixed yet) smooths out and vacuum more steady- does not fix my problem so I put it back.  Dizzy is magnetic pickup hooked to MSD digital 6Al and blaster coil. No vacuum advance. Timing is 6* initial plus 30* mechanical. ( this is per the correct timing per ford service manual. ) because there were some odd things going on with ford BB engines in the early 70 for emissions and it is possible I do not have straight up like I should have with an early 429. I have tried 4*-14* initial in two degree steps. And this made no difference in my 25-45 surge issue. I replaced all of the vacuum hoses and sprayed starting fluid around the base of carb intake and vacuum ports. No leaks found. All the smog stuff is gone except PCV valve.  C6 4:11 rear gears.  pulled plugs and do not look to lean or rich. Any ideas?

 

 

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You mentioned that the cam and bottom end are all stock. Was the bottom end of this 429 rebuilt or was it just the cylinder head work? If the bottom end has not been touched in recent history I would want to confirm that you do not have a worn timing chain. That would cause your surge issue. Ron

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Thanks I will check it out. I have not inspected it.

 

 

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tnfastbk,

Great ideas here on using o2 meter to see what the A/F is doing while you are surging.

You have not stated if you have this same surge in first or second gears.

You need to first find out if the surge is load specific, rpm specific or speed specific.

Yes, it is probably related to load (or the lack of it) but just another simple road test will tell you a lot.

Just because you have tried different base timing adjustments does not mean all is well in that distributor.

Nobody has asked about your advance curve here, what if your mechanical advance is sticking ?

One thing that I have done in the past, trying to figure out surge conditions with holleys and transition is to slip a piece of feeler gauge between the secondary throttle stop and then drive the car, without opening the secondaries (of course)

and see if it affects your surge, if it does not, you have not (lost) your secondary adjustment.

if it does, then perhaps an adjustment (bias ) is in order.

You are probably too lean with too little secondary and still being on the primary transition or too rich (too much secondary and just past the primary transition.

Have you checked your float levels since the carb spacer and in tank fuel pump ? 7 psi can be a little much for some

holley needle and seats and can drive float levels up and make it over rich.

                                                        Boilermaster

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Thank you for the suggestions. I never thought much about the secondaries. The float levels are good. I have clear sight glass so easy to keep eye on them

 

 

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