Surging while cruising at steady speed. Idle and acceleration great, Help!

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68Dart500

68Dart500

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Well I finally got my Dart up and running with a brand new 408 that I had dyno'd. I got it aligned and took it for a shakedown run and I've discovered that I have a surging issue while cruising. In fact any time I try to maintain a steady rpm (ex. if I push the clutch in and try to hold the rpm at say 2k or 3k rpm it surges and sounds like its missing). If I give it a tiny bit of gas to accelerate it smooths out immediately and accelerates nicely. Seems to idle great and it also has super quick throttle response at any rpm. WOT acceleration is right on the money too. Starts up when hot no problem and when its cold you almost don't even hear the starter go it starts so fast.

Here's what's on it:
408 Stroker, Edelbrock heads, Quick Fuel 750SS (the engine builders that dyno'd it jetted it at 68 and 74. 3/8 fuel line, Edelbrock 110 GPH fuel pump at 6.5 psi.

Timing was set at 19 initial and 34 overall. It's running an MSD billet distributor (no vacuum advance), MSD coil, MSD wires, plugs gapped at .045, and an MSD 6AL2.

My engine builder suggested it was probably running lean because I'm running open headers so they suggested I jet it up a bit. I installed 74 primaries and 78 secondaries. Problem almost seemed worse so I took out the 74's and installed 72's. I also double checked the floats and made sure they were set correctly. The problem is still just as bad. I have no other jets so I can't jet it down to 70 unfortunately.

I completely took for granted to check the timing that they did so I plan to do that tomorrow morning. Any other suggestions?
 
Wide band a/f meter ... Both banks ...awesome! To interpret an actual rich or lean condition throughout the power range is sweet .
 
Please consider this ..
Take a ten foot or however long you need section of washer or aquarium tubing ( opaque )
Hook one end to ported or manifold vaccum nipple ( vac source)
Other end seals pretty perfect along the length of a b12 chemtool red extension nozzle .
"Prime " hose to source with chemtool And make sure a quick spurt affects idle quality

Duplicate the surge and introduce your " added fuel" gently from the can during that instance
A very small amount will correct any true lean condition as you described BUT ..
Don't be surprised if you find its not fuel related !
 
Does the secondary side have an idle adjustment?
 
68Dart500 said:
In fact any time I try to maintain a steady rpm (ex. if I push the clutch in and try to hold the rpm at say 2k or 3k rpm it surges and sounds like its missing).
Click to expand...
Have you had a chance to check it driving at 30 to 40 mph and at 60 to 70 mph? Doing so will help identify which circuit to focus on.

68Dart500 said:
If I give it a tiny bit of gas to accelerate it smooths out immediately and accelerates nicely.
Click to expand...
Very interesting. esp. if this is true at 65 mph

68Dart500 said:
the engine builders that dyno'd it jetted it at 68 and 74.
Click to expand...
Engine dyno or chassis dyno? Do you have a file or printout of the pulls?

68Dart500 said:
was probably running lean because I'm running open headers
Click to expand...
Yea. maybe. Always possible, and hard to predict absolutely. Dyno pulls were almost certainly done at wide open throttle.

68Dart500 said:
I installed 74 primaries and 78 secondaries. Problem almost seemed worse so I took out the 74's and installed 72's. I also double checked the floats and made sure they were set correctly. The problem is still just as bad.
Click to expand...
* Leave the secondaries alone. They are completely irrelevant until the the throttle is much further open and rpms are up.
* Was the primary jetted 68 - > then tried 74s, seemed worse --> tried 72s?

A dyno pull with the AFR graphed will reveal whether the main airbleeds and e-holes are about right.
A wideband for what you are trying to figure out is hardly necessary. You know you're at the limit and they only way to figure out which circuit(s) to focus on is methodically change one thing and observe the effect. A logger with AFR, RPM, MAP and/or TPS is handy to track and see the relationships and how they change. So don't take it that I don't beleive they are handy tools, they can be.

I'm going to suggest two possibilities.
1. The main air bleeds and primary jet combo is such that the main circuit doesn't start early enough. A WOT pull in 3rd or 4th will often show that.
2. The idle circuit tapers off just a little too early.

If its really close, then slightly raising the primary fuel level may help. Just a 1/6 or 1/3 of a turn will make all low throttle slightly richer. My guess is that is too subtle of a change for the condition you are describing.
 
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As above, the secondaries are not any where near to being active in this phase of operation. Your carb's issues are probably in the transition phase, between idle and full primary operation. This is verified by not seeing any improvement with the primary jet changes.

I would first be working on the secondary throttle stop position and then readjusting the idle around that. The secondary stop screw is perhaps a bit to far open and sucking in more air than the primary transition slots can compensate for.

The next are is the idle air bleeds which are changeable. The point would be to enrich the idle well mixture, which is what feeds the transition slots in the early part of the transition to the primary mains operation. You can temporarily enrich the idle circuit by inserting some fine wire down through the primary idle air bleeds to block off a part of the flow area; try to block maybe 20% of the area as a test. (I wrap the wire around the air cleaner stud to keep it secure.) Then drive and see if this has changed the situation; if it does, then you know to buy and try some new idle air bleeds. If you don't have that fine wire, then buy a few steps smaller air bleed and try that. BTW, every time you change the idle air bleeds to change the transition mixture, you need to re-adjust the regular idle mixture.

I see that some of these carbs also have a 3rd set of air bleeds, for the transition circuit. If this is the case, then I would work on those rather than the idle air bleeds for this problem.

One other area is to mod the air bleeds in the emulsion tubes in the metering block. But this does not sound like where your issue lies; that area more effects going from mid load to heavy load conditions.

Having a manual trans is exacerbating the engine surge due to drivetrain slop; but the missing at mid RPMs with the clutch in shows that the engine is indeed going lean. With all the adjustments on this carb, then it is not unusual to have to change a lot of small parts once on the road and under real world loads. (And as said, keep in the back of your mind that this can be in the ignition..... the needed spark energy to fire of the mixture varies all over the map with changing loads and mixtures.)
 
Mattax said:
If its really close, then slightly raising the primary fuel level may help. Just a 1/6 or 1/3 of a turn will make all low throttle slightly richer. My guess is that is too subtle of a change for the condition you are describing.
Click to expand...
Yes, this will make a small increase in the richness of the transition circuit (by raising the fuel level in the idle well) and would be a good test.
 
nm9stheham said:
As above, the secondaries are not any where near to being active in this phase of operation. Your carb's issues are probably in the transition phase, between idle and full primary operation. This is verified by not seeing any improvement with the primary jet changes.

I would first be working on the secondary throttle stop position and then readjusting the idle around that. The secondary stop screw is perhaps a bit to far open and sucking in more air than the primary transition slots can compensate for.

The next are is the idle air bleeds which are changeable. The point would be to enrich the idle well mixture, which is what feeds the transition slots in the early part of the transition to the primary mains operation. You can temporarily enrich the idle circuit by inserting some fine wire down through the primary idle air bleeds to block off a part of the flow area; try to block maybe 20% of the area as a test. (I wrap the wire around the air cleaner stud to keep it secure.) Then drive and see if this has changed the situation; if it does, then you know to buy and try some new idle air bleeds. If you don't have that fine wire, then buy a few steps smaller air bleed and try that. BTW, every time you change the idle air bleeds to change the transition mixture, you need to re-adjust the regular idle mixture.

I see that some of these carbs also have a 3rd set of air bleeds, for the transition circuit. If this is the case, then I would work on those rather than the idle air bleeds for this problem.

One other area is to mod the air bleeds in the emulsion tubes in the metering block. But this does not sound like where your issue lies; that area more effects going from mid load to heavy load conditions.

Having a manual trans is exacerbating the engine surge due to drivetrain slop; but the missing at mid RPMs with the clutch in shows that the engine is indeed going lean. With all the adjustments on this carb, then it is not unusual to have to change a lot of small parts once on the road and under real world loads. (And as said, keep in the back of your mind that this can be in the ignition..... the needed spark energy to fire of the mixture varies all over the map with changing loads and mixtures.)
Click to expand...
Absolutely!
 
Thanks for all the suggestions. I'll be looking into a lot of these over the next few days
 
BTW, OP, read post #6 and beyond in this thread for other suggestions on this issue which showed up similarly in a 454:
Quickfuel 750 part throttle hesitation - Hot Rod Forum : Hotrodders Bulletin Board

This carb has a lot of adjustable bits and I'll be darned if I can find good documentation online to point you to. (I just know about them because of my time with Weber DCOE's back-in-the-day which had all these adjustments available back in the 60's...glad to see this come to the V8 carb scene!)
 
Mattax said:
Have you had a chance to check it driving at 30 to 40 mph and at 60 to 70 mph? Doing so will help identify which circuit to focus on.


Very interesting. esp. if this is true at 65 mph

Engine dyno or chassis dyno? Do you have a file or printout of the pulls?

Yea. maybe. Always possible, and hard to predict absolutely. Dyno pulls were almost certainly done at wide open throttle.


* Leave the secondaries alone. They are completely irrelevant until the the throttle is much further open and rpms are up.
* Was the primary jetted 68 - > then tried 74s, seemed worse --> tried 72s?

A dyno pull with the AFR graphed will reveal whether the main airbleeds and e-holes are about right.
A wideband for what you are trying to figure out is hardly necessary. You know you're at the limit and they only way to figure out which circuit(s) to focus on is methodically change one thing and observe the effect. A logger with AFR, RPM, MAP and/or TPS is handy to track and see the relationships and how they change. So don't take it that I don't beleive they are handy tools, they can be.

I'm going to suggest two possibilities.
1. The main air bleeds and primary jet combo is such that the main circuit doesn't start early enough. A WOT pull in 3rd or 4th will often show that.
2. The idle circuit tapers off just a little too early.

If its really close, then slightly raising the primary fuel level may help. Just a 1/6 or 1/3 of a turn will make all low throttle slightly richer. My guess is that is too subtle of a change for the condition you are describing.
Click to expand...

Yes I've driven it at every speed and it surges regardless of the speed as long as you attempt to hold a steady rpm under light load.

It was an engine dyno that they tuned it on.

Yes I jetted the primaries from 68 to 74 then down to 72. I don't have any other jets so that's all I could do at this point.

My dyno graph just has hp and tq on it but I do have the sheet so I've uploaded it.

20180604_090724.jpg
 
68Dart500 said:
Yes I've driven it at every speed and it surges regardless of the speed as long as you attempt to hold a steady rpm under light load.
Click to expand...
If by surge you mean it revs and wants to die, that's a behaviour of too lean (at light loads).
If it only occured at interstate speeds, my experience with cars has been that's when a 4150 carb is supplying fuel mostly from the main circuit. That is, a jet change will make a noticible difference.
Since you are finding it at all speeds, that strongly suggests fuel is mostly coming from the "idle" circuit.
68Dart500 said:
Yes I jetted the primaries from 68 to 74 then down to 72. I don't have any other jets so that's all I could do at this point.
Click to expand...
This ^ further supports the conclusion about the surge or miss relates to the idle circuit tapering off too early. (You can put it back to 68s if you want and call that your baseline.)

Change the primary idle air bleeds to something smaller - about .003" diameter change is enough. For example, if they measure .073" diameter, then install .070" diameter.
If they are not screw in types, stick an .016 to .020" diameter wire in each. That will reduce the area of a typical .70 to .76 IAB to to roughly one a little smaller.
I run my wires under the aircleaner gasket.
You may have to slightly readjust the idle mix screws. But make no other changes. One change at a time. Changes don't always do what we think - especially with mixed fluids (in out case fuel mixed with air from the bleeds). Take it for test run.

68Dart500 said:
My dyno graph just has hp and tq on it but I do have the sheet so I've uploaded it.
Click to expand...
Actually this is good. Give the dyno shop a call and ask if the Lambda readings printouts are converted to AFR for gasoline (looks like it, but the one says Volts so ??). My guess is one is the right and the other the left bank, but just ask the. It's their equipment - they'll know.

For some reason the one marked AF1 starts rich, then stabilizes around 4500 rpm assuming that represents AFR.
If so, we can address that later.
Don't worry about the absolute number (11.4).
 
Picture showing wires in primary & secondary main air bleeds, and secondary idle air bleeds. Using one wire for both sides takes a little more time to form, but then is easier to handle and less likely to get lost. If its easier, just make one for each IAB.

F3310-2018-05-18_7800.JPG
 
Mattax said:
If by surge you mean it revs and wants to die, that's a behaviour of too lean (at light loads).
If it only occured at interstate speeds, my experience with cars has been that's when a 4150 carb is supplying fuel mostly from the main circuit. That is, a jet change will make a noticible difference.
Since you are finding it at all speeds, that strongly suggests fuel is mostly coming from the "idle" circuit.
This ^ further supports the conclusion about the surge or miss relates to the idle circuit tapering off too early. (You can put it back to 68s if you want and call that your baseline.)

Change the primary idle air bleeds to something smaller - about .003" diameter change is enough. For example, if they measure .073" diameter, then install .070" diameter.
If they are not screw in types, stick an .016 to .020" diameter wire in each. That will reduce the area of a typical .70 to .76 IAB to to roughly one a little smaller.
I run my wires under the aircleaner gasket.
You may have to slightly readjust the idle mix screws. But make no other changes. One change at a time. Changes don't always do what we think - especially with mixed fluids (in out case fuel mixed with air from the bleeds). Take it for test run.


Actually this is good. Give the dyno shop a call and ask if the Lambda readings printouts are converted to AFR for gasoline (looks like it, but the one says Volts so ??). My guess is one is the right and the other the left bank, but just ask the. It's their equipment - they'll know.

For some reason the one marked AF1 starts rich, then stabilizes around 4500 rpm assuming that represents AFR.
If so, we can address that later.
Don't worry about the absolute number (11.4).
Click to expand...

The surging doesn't seem to effect engine rpm at all, just feels like its missing basically but I really don't see how it could be ignition related because everything has been checked.

That makes sense about the idle circuit. My idle air bleeds are 70/70 according to my Quick fuel carb card. Not really sure what that means and my hi speed bleeds are 31/31. Needle and seat at 110. Power valve 6.5. Primary and secondary nozzles are 31. When you say adjust the primary idle air bleeds would it be these? Quick Fuel Air Bleeds

Going off what my carb sheet said my air bleeds are 70/70 so that probably means .070/.070 right which means I should try the .067's? Do I just need 2 or will I need to order more then that as I see they come in pairs or a pack of 10.

I will check with the dyno/engine builder and ask them about the two AFR lambda are and if its 1 for each bank etc.
 
68Dart500 said:
The surging doesn't seem to effect engine rpm at all, just feels like its missing basically but I really don't see how it could be ignition related because everything has been checked.
Click to expand...
Timing and fuel mixture are a hand in glove relationship. For now, lets see about getting the fueling to go with the timing. If changes have no effect, then going to have to look at timing.

68Dart500 said:
That makes sense about the idle circuit. My idle air bleeds are 70/70 according to my Quick fuel carb card. Not really sure what that means. .. so that probably means .070/.070 right which means I should try the .067's?
Click to expand...
You got it. Unlike Holley Jet sizes which are numbered based on flow, air bleeds are numbered strictly on diameter. Yes I think .067 or .068" would be the next step to try. 069 is too similar to what you have. Or, if its quicker and easier, stick some wires in for now. Then depending on the results, buy a pair the sizes that are close and fine tune. If you're just tuning this one or two carbs, an assortant of sizes on hand rather than a lot of one seems to make more sense to me.

The air bleeds reduce the pressure difference driving the fuel. Fuel pressure in the bowl is bascially atmospheric, and fuel in the idle circuit flows toward the low pressure we describe as manifold vacuum. An air bleed reduces that pressure difference.
For idle circuit, its generally true that larger air bleed will make the fuel to both the idle port and transfer slot leaner, and taper off earlier.
Main circuit can not be so simply generalized, but if the fuel mix goes leaner or richer as the engine approaches redline, it generally relates to the air bleed sizes.
 
If it has an idle adjustment on the secondairies close them some and see if it helps. You may have to remove the carb and access the screw from the bottom. When I had my Q-750 on the 340 I was adjusting it and got the secondaries open to far and it would surge as described.
 
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