Engine Hesitation at high rpm's

[69%morewhoopass 10]

Hello all,

I am the next generation of foothilltom (he is my father), and i happy to be a part of this forum. I apologize for the obscene username. It was meant to be "69%more'stang", but unfortunately the other username is being posted. The more obscene one was not supposed to be kept, but an accidental stroke of a key saved it into the computer.

 

With that said, my dad and i have recently installed a new Edelbrock manifold with a holley 600 4 bbl carb on the orange car, which i have been told you are all familiar with to some degree. With all these new additions we have introduced a hesitation in the engine at a certain RPM just before the secondaries open up. The engine runs fine above and below this RPM range, but in this range the car falls on its face; almost cutting out completley before pushing past to a higher RPM. Also the engine runs very rough if not pushed past the hesitation or let off.

 

It is my intention to purchase pertronics, but budget constraints have hindered the effort. In the mean time i need a course of action. Any help is welcome

Edited January 13, 2010 by 69%morewhoopass

[Print Dad 58]

Hello 69%more,

Welcome to the forum. Your dad is a great guy and has learned so much.

You should be proud of him

 

No so that we don''t get confussed....Yours is the 302 and his is the 351?

Is this correct?

 

My first thought on hesitation is fuel starvation or even over fuel.

 

I think it would be wise to post your specifics on the motor and carb.

 

I get a little confused because I know your dad rebuilt a Holley but i forget if it was yours or his.

 

What is the timing set at...and how about the dwell?

 

I somewhat agree with most about the Petronix. They are nice but if you set the points correctly they work well also. remember for many years us old guys had no other choice. So my point is Yes it would be nice to get but the car will run without it till yo get some $$$.

 

Tell your dad Hi - -Update your profile and I know the gang here will help.

 

Once again........welcome...... Print Dad

[Swags63 13]

This will must likely stir the put with some people, but I'm not a big fan of holley for street use which generally sounds like your problem. For easy of tunning and a durable setup I prefer the Edelbrock Carb. No power valve just for starter. Here is a tip I picked up, if you go to Summit's site http://www.summitracing.com/parts/EDL-9905/ and type in Remanufactured Carburetors in the search engine you can pick one up with a warranty for under $200.

[Print Dad 58]

Hello 69%more.

Let's start simple. This is a 351W correct?

I think I would start off simple and advance the timing to 10BTD.

Try this and see how the car does.

 

Then if the car still has the hesitation, I would check the accelerator pump setting. How do you feel about pulling the car off?

The correct way to set the slight gap is to turn the carb upside down and

make the adjustment as per holley's web-site.

 

What I often do however is try to set the accelerator arm so it just lightly kisses the shaft on the pump. Many times people often run the adjustment too tight.

 

Look at the picture....see the piece that looks like an upside down bolt with a spring. That bolt is what activates the accelerator pump. If ti is set too tight or too loose it doesn;t work right.

 

I would check this out secondly. You want the head of the bolt to just touch the arm very very lightly.

 

This may be a case where you need to tweak things and it may take a little trial and error.

 

Print Dad

[LoneWolf2U 136]

we have introduced a hesitation in the engine at a certain RPM just before the secondaries open up. The engine runs fine above and below this RPM range, but in this range the car falls on its face; almost cutting out completley before pushing past to a higher RPM. Also the engine runs very rough if not pushed past the hesitation or let off.

 

It is my intention to purchase pertronics, but budget constraints have hindered the effort. In the mean time i need a course of action. Any help is welcome

 

What rpm range?

From your discription, I would check your vac advance first.

It runs fine above and below this rpm point somewhat rules out fuel flow and points toward a timing problem.

Areas to check are vac pot and breaker plate under points for wear or sticking that restricks movement for advancing.

[Print Dad 58]

Hello Mach1Rider,

Always good to hear from you.

What do you think about the timing? I think the car could run better at about 10. he is at 6BTD now.

 

This may require some different trials. Great point about the VAC.

 

Be well- - - Print Dad

[69%morewhoopass 10]

Thank you all!

I will look into the vacuum advance unit, i believe that may be a source of error. I will also check to see if my accelerator pump is maladjusted. The range, however, i cannot be sure of, for i do not have a tac on my car. a rough guess would be around the 3000 rpm range, which again is a very rough estimate. The engine only hesitates when I get on it a little bit. On that note, i know that my points are not correct, for my dad and I made a futile attempt at adjusting them correctly. needless to say it was 30 minutes, only to end up at the exact reading we started. anywho, I will be sure to check into all the possibilities you have all presented.

thanks again

[foothilltom 33]

As 69%more* (I'll call him the boy) said, the dwell angle right now is off. Repeated attempts to adjust those god-forsaken points resulted in readings of 24 to 36. Getting to the sweet 29 degrees seems beyond us. Right now, the dwell is 24 which we acknowledge is pretty "off". Do folks think this is fundamental enough to get right first before we start chasing other things?

 

The Holley 600 on the boy's coupe is used and freshlly rebuilt by a family friend who is a dang good gear head, but the accelerator pump is a fine thing to check.

 

Just after swapping in the 4b manifold and carb, we had trouble getting her to idle. Mixture screws all the way in had no impact. Vacuum was at 18in and steady, so we suspected jets. Swapped 62s out for 58s, didn't see much of an idle change, and eventually realized that we did have a vacuum leak at the PCV valve. The PCV valve and the PCV port on the new carb are now VERY close together and create a super sharp angle for the hose. This angle was sort of pushing the PCV valve "out of" the valve cover and creating the sucking. If I held the PCV valve physically in place, she idled real nice. We've since remedied this with a super long run of tubing.

 

So, all in all, we're not sure if the 58's were ever required, but that's what the boy is running now.

 

When the boy mentioned the problem we took a ride (I drove to his chagrin) and can definitely concur with what he describes. From my perspective, the engine "hesitates" substantially when climbing to higher RPMS. At a certain RPM/torque range, the engine just runs like crap. But if you push it through, she perks right back up.

 

The boy will spend some time this weekend wrenching around, methinks, but I wanted to throw in a little extra context.

 

Thanks for all your help.

Tom

[LoneWolf2U 136]

Reset the points first, as being off will dewell will just be worse as timing advances.

 

No changes in idle screws, rpms are too high and its off idle passages.

 

Start with resetting carb to basic starting point, idle mixture screws out from bottoming 1 1/2 turns.

Idle speed screw 2 turns down from screw contacting throttle plate linkage. choke fully open and engine up to operating temp.

Set rpm to 700-800 and adjust mixture screw for smoothest idle.

Check timing 8-10*bftdc

check advance by slowly moving rpm up to 3k looking for smooth advancing timing as rpm raises.

 

Accell pump should squirt fuel as throttle opens. timing this shot will help remove flat spot in off idle running.

[69%morewhoopass 10]

This is all great advice, and i will spend some time this weekend chasing down these soon to be nonissues. Thank you all

[bigperm2 10]

This is all opinion of course, but 10 degrees of initial timing would be perfect, with 32-34 degrees advance all in at around 2500-3000 rpm. This should all be checked with the vacuum advance unplugged. I think the dwell is close enough to say that it isn't the problem.

 

I have had a problem with the vacuum advance stumble in the past. Basically what happens is when the throttle comes off idle, the vacuum advance can gets a sudden surge of vacuum, which sets the timing at like 30 degrees instantly, with the corresponding stumble. This can be diagnosed by simply driving the car with the vacuum advance disconnected and capped. If you find this is the problem, post it here, and I am sure you can get good advice on how to fix it.

 

For the carb, it is unlikely that the jets are the problem. Jets really have no effect on the idle. The idle fuel circuit is seperate. It is more likely a combination of the idle adjustment and mixture screws. It is important that the idle speed adjustment (throttle blades) are not too far out of whack. The ported vacuum for the distributor should have no vacuum at idle, if it does, you have the blades open too far. Also, be sure the power valve is not blown. As print dad mentioned, a bad accelerator pump will cause major stumbleing problems. You should download a manual from holley if you don't have it. Barry Grant Demon carbs are very similar to holleys, and you can download their instructions also to help you to get the idle set right. Also be sure that you have no vacuum leaks, and all unused manifold and carb vacuum ports are plugged, you can check to see if the PCV valve is causing problems by removing the hose and capping the port and take it for a quick trip around the block.

 

For the pertronix, they work great. If you are on a super budget, get a duraspark II ignition setup at the u-pull boneyard. I have gutted the distributor and grabed the module and wires and paid less than $10 for the parts. Internally, all the ford V8 duraspark dizzys are the same, or if you can find the 302 or 351 dizzy, you can just swap the distributor. If interested, do a google search on how to do it, or let us know on the forum.

 

Good luck!

Edited January 17, 2010 by bigperm2
clarification

[foothilltom 33]

Hey guys, am giving in update on behalf of the boy as he gets busy with High Screwl during the week and doesn't type as fast as his old man! We spent some time on his car this Sunday, made some progress, and have just one lingering problem.

 

We wound up replacing the PCV valve which appeared to be stuck open and didn't fit too well in the valve cover grommet. Now we have a snug fit and a working valve. This was most definitely was a vacuum leak and the car idles much smoother now.

 

We also did a "once over":

 

* Set the timing to 10B per all the advice

* Rechecked the dwell (still sitting at 26 degrees, boy can't afford EI yet)

* Checked the carb floats

* Set idle-mixture screws to get maximum vacuum at the best idle (pulling appx. 18 in of vacuum at a smooth idle of 700)

* Verified correct throttle pump position per Print Dad's post

 

Off for the test drive.

 

In my opinion, the engine ran noticably better than before. The "hesitation" was much less pronounced, but there. The "hesitation" always occurs when you put your foot into it. It just sort of "stumbles", then responds.

 

We hooked up a vacuum gauge and taped the the face to the windshield so we could see it on our next drive. Every time the engine "hesitated", the vacuum reading was about 10 in (perhaps 9). We repeated this experiment about 20 times and the reading was totally consistent.

 

So, in my layman's explanation, as the vacuum decreases to 10 in as we open the throttle, we get that hesitation.

 

It started to rain heavy, so we abandoned ship for the day. A few more parting thoughts:

 

* We don't know what our Power Valve is rated at, but perhaps it is not opening "soon enough"?

* We haven't figured out where the secondary vacuum advance spring is located as we'd like to know if it's a "light" or "heavy" spring

* We realize our dwell isn't perfect yet

 

At this point, we'd really appreciate any wisdom on ironing out that last little nagging hesitation. My concern is the boy will need to put his foot into it to merge or get on the highway and the 302 won't respond the way she should.

 

Thanks to all in advance.

Tom (for the boy)

[foothilltom 33]

Oh, and to bigperm: thanks for your excellent post. We totally forgot to try a test-drive with the vacuum advance disconnected. Will do that later today when the boy returns from school.

 

tom

[LoneWolf2U 136]

Sec spring is located under the cap on the pass side, it has 4 screws holding it down.

Too light of a spring will creat a bog, as secd open too soon.

 

If your bog is off idle or with slight accel, try tweaking the acell pump screw till it jus touches the pump arm.

This will give it a squirt just as you hit the throttle.

[Print Dad 58]

Hello Tom,

I agree 100% with Mach1 Rider and was going to suggest the same thing.

Try different setting with the screw for the accelerator pump. Try to tighten it then drive - - then if no better go the opposite way and test it.

 

Tom - where are you guys pulling the vacuum from for the dizzy? Is it from the side of the metering block or below the bowl?

 

You could try swapping between the 2 ports and see if that helps.

 

As far as the dwell - In my opinion you are fine - not perfect but fine.

 

Print DAd

[foothilltom 33]

Yo PD, we have the dizzy vacuum advanced connected to the side of the metering block...just near the mixture screw on the passenger side. I used the vac gauge to make sure it pulls basically 0 inches at idle and a whole bunch with the throttle open, so I'm pretty sure it's in the right place.

 

I'll test out bigperm's idea of driving the car with the vac advance disconnected to see if that neutralizes the hesitation (implicating a vac advance problem to be researched).

 

We shall also goof with the accel pump today if this damned rain would ever stop. Will report back on that.

 

Finally, I'll open up that port on the secondaries and see what kind of spring we have in there. That's another fertile pasture to explore.

 

Thanks!

Tom

[LoneWolf2U 136]

Try gapping the points at .016-.017 for better dewell.

24 is not good enough and will create problems as rpm increases.

The port near the mixture scres is the correct one, using the lower port will cause the dist to advance at idle.

 

as for the sec opening setting slide a loosely bent paperclip on the rod going down to the sec throttle plate connection(right side of carb)up tight to base of dashpot. Do a test run at wide open and check to see if the clip has slid down the shaft, if it has they are opening and you can do spring replacing accorddingly.

[foothilltom 33]

On behalf of the boy, I'm happy to report that his 302/4V is now purring along just dandy! We took all the advice here quite literally and reset everything:

 

* Dwell (still have points, he's saving for EI)

* Timing (set to 10B)

* Verify best vacuum with idle-mixture screws and idle screw

* Checked float levels

* Checked/Adjusted Accel Pump spring

* Replaced the PCV valve and grommet (we suspected it was bad)

 

Since we had the vacuum gauge out, we rigged it with a longer hose and extended it out of the hood compartment so we could see it while driving. On our road test, we saw that the "hesitation" would consistently occur at the same vacuum reading (about 10 in). In drivability terms, whenever we'd start to put our foot into it, she'd really hesitate and then launch. Below this vacuum reading and above it, she'd run just dandy. We were even able to get it to run rough at a steady cruise with that same vacuum reading, so that was new.

 

After some Googling, we suspected that at the 10 in of vacuum, the Power Valve wasn't opening -- causing a brief lean condition. We had a newly rebuilt Holley 600 from a family friend and he didn't know what kind of PV came with the rebuild kit (shouldn't the PV have this stamped on it somewhere?), so I bought a PV rated at 10.5 in of vacuum thinking that might open "sooner".

 

And OUILA!!! We installed the PV yesterday, checked our vitals, and headed out for the test drive. Damn if the old man didn't finally fix something! The 302 accelerated smoothly out of the chute with nary a sign of hesitation. The boy was stoked and I was feeling pretty good too.

 

As always, we learned a TON through this process thanks to the advice of the fine human beings on this site. Right now, as I type this, the boy's Stang has never run better.

 

Tom

Edited February 2, 2010 by foothilltom

[foothilltom 33]

On behalf of the boy, I'm happy to report that his 302/4V is now purring along just dandy! We took all the advice here quite literally and reset everything:

 

* Dwell (still have points, he's saving for EI)

* Timing (set to 10B)

* Verify best vacuum with idle-mixture screws and idle screw

* Checked float levels

* Checked/Adjusted Accel Pump spring

* Replaced the PCV valve and grommet (we suspected it was bad)

 

Since we had the vacuum gauge out, we rigged it with a longer hose and extended it out of the hood compartment so we could see it while driving. On our road test, we saw that the "hesitation" would consistently occur at the same vacuum reading (about 10 in). In drivability terms, whenever we'd start to put our foot into it, she'd really hesitate and then launch. Below this vacuum reading and above it, she'd run just dandy. We were even able to get it to run rough at a steady cruise with that same vacuum reading, so that was new.

 

After some Googling, we suspected that at the 10 in of vacuum, the Power Valve wasn't opening -- causing a brief lean condition. We had a newly rebuilt Holley 600 from a family friend and he didn't know what kind of PV came with the rebuild kit (shouldn't the PV have this stamped on it somewhere?), so I bought a PV rated at 10.5 in of vacuum thinking that might open "sooner".

 

And OUILA!!! We installed the PV yesterday, checked our vitals, and headed out for the test drive. Damn if the old man didn't finally fix something! The 302 accelerated smoothly out of the chute with nary a sign of hesitation. The boy was stoked and I was feeling pretty good too.

 

As always, we learned a TON through this process thanks to the advice of the fine human beings on this site. Right now, as I type this, the boy's Stang has never run better.

 

Tom

Edited February 2, 2010 by foothilltom

[Print Dad 58]

Tom,

Thanks for sharing. When you get old like me you'll look back at these troubles and smile. Well done my friend and even "French" in the response. Very hip LOL

 

Great news - - tell "the boy" us dads can still rock - - Print Dad

[Print Dad 58]

Tom,

Thanks for sharing. When you get old like me you'll look back at these troubles and smile. Well done my friend and even "French" in the response. Very hip LOL

 

Great news - - tell "the boy" us dads can still rock - - Print Dad

[69BossMach1 10]

The rebuild kit likely came wit a 5.5 or a 6.5 power valve.

 

The correct one IF you are really pulling 18inches of vacuum at idle (REALLY??), would be an 8 or 8.5. (18/2 = 9 - 1 = 8)

 

a 10.5 WILL be an improvement over the 6.5, but the 8 would be ideal.

 

Also, you need to set your timing at 3000 RPM, not at idle. A mild 302 needs about 34 degrees at 3000 RPM, and no more beyond that. (34 degrees at 3000 RPM and maintain 34 up through 6000 RPM.) whatever that gives you at idle, as long as its within 6-12 degrees.

[69BossMach1 10]

The rebuild kit likely came wit a 5.5 or a 6.5 power valve.

 

The correct one IF you are really pulling 18inches of vacuum at idle (REALLY??), would be an 8 or 8.5. (18/2 = 9 - 1 = 8)

 

a 10.5 WILL be an improvement over the 6.5, but the 8 would be ideal.

 

Also, you need to set your timing at 3000 RPM, not at idle. A mild 302 needs about 34 degrees at 3000 RPM, and no more beyond that. (34 degrees at 3000 RPM and maintain 34 up through 6000 RPM.) whatever that gives you at idle, as long as its within 6-12 degrees.

[foothilltom 33]

Hey BossMach, I never knew the timing should be checked at 3000 rpms! I've done it at idle since high screwl auto shop.

 

I'll check the timing and dwell on the boy's car at 3000 and see what gives.

 

With regard to vacuum, my gauge is pretty cheapo...but it did square with a friend's equally crappy gauge, so I trusted it. I appreciate the formula, however. I'm going to get into this same situation with my car this weekend, so I'll definitely be able to use this good info.

 

Thanks again.

Tom

[foothilltom 33]

Hey BossMach, I never knew the timing should be checked at 3000 rpms! I've done it at idle since high screwl auto shop.

 

I'll check the timing and dwell on the boy's car at 3000 and see what gives.

 

With regard to vacuum, my gauge is pretty cheapo...but it did square with a friend's equally crappy gauge, so I trusted it. I appreciate the formula, however. I'm going to get into this same situation with my car this weekend, so I'll definitely be able to use this good info.

 

Thanks again.

Tom