QFT 735 lean surging

[Nico Buchner]

Hi everyone,

I've been reading on this forum for a long time now and finally have reached a point, I need to ask for your help. I have already read every single article that included the word “surge".

I have a lean surge above 1500rpm that I am fighting for almost a year now.

Specs: Fully restored 68 Dodge Coronet with a mildly built 440. Comp cams XE268H, 9:1 compression, ported 452 heads, weiand 8008 intake with blocked heat, 1“spacer, QFT735HR, MSD6AL, distributor with mechanical and vacuum advance, total timing 36°, electric fuel pump with pressure regulator set to 6psi, 1 7/8 headers with 2.5“ exhaust, wideband O2

The problem: When I accelerate normaly, the engine surges. It feels like it has full power for one second and than no power for 2 seconds. During these 2 seconds the engine is shaking and goes way lean on the O2 (up to 16:1) until it goes back to normal for a second and everything repeats until I either lift the foot or go WOT. At cruise and WOT there is a very slight surge, too, but almost not noticable. It idles fine and starts up good. The problem seems to get worse, the longer I drive. Sometimes the surging is accomponied by a backfire through the exhaust.

What I did so far in that order:
- Had a Holley Street Avenger 770 on it before, that had the exact same issue. After messing with all adjustments for a while decided to change the carb
- Installed a QFT 735 HR
- Checked for vacuum leaks multiple times (including a smoke test)
- Checked transfer slot
- Checked if choke fully opens
- Adjust float level
- Adjust acc pump linkage
- Adjust idle mixture (best vacuum at 15 in N and 12 in D and A/F at 12.5 is at 1 turn out). The engine seems to like more fuel, the richer the idle mixture is, the better the engine accelerates. I was up to 2.5 turns out, but still had a very slight bogging and it seemed way too rich (tears in the eyes behind the car)
- Checked plugs with no signs of fuel problems
- Bumped up primary and secondary jets by 2 and than 4 sizes with very little improvement
- Played with timing of the secondaries. Later kick in made it worse, earlier made no difference
- Unhooked vacuum hose to distributor, which felt a bit worse

I am now out of ideas and would be very thankfull for every idea you have.

 

[wagonman]

Pretty much the same combo I have. Sounds like a carb issue for sure. More carb specs like PV and jet size would be helpfull. I run a 3310 with 72s and a 6.5 PV. Had an issue with a lean stumble and played with it for a while. Wound up removing the secondary throttle plate adjusting screw and using a machine screw that you can adjust without pulling the carb. Cracked the rear throttle blades a C-Hair and stumble is gone. Did not want to drill the primary throttle blades. Your O2 readings is a tellall Lean Lean Lean. Now to find out why??

 

[Nico Buchner]

Thank you very much for your reply!

The power valve is 6.5. I did not touch it, because according to my vacuum reading it already is slightly on the richer side, which is what the engine wants.

With jets on the primary I went from the stock 68 to 70 and then to 72. Secondaries from 80 to 82 and then to 84.

Due to my relatively mild cam and good vacuum and idle quality I did not think about drilling holes in the primary throttle blades (I would like to avoid that, too) or open up the secondary throttle blades. Wouldn't that make it being even more on the rich side?

 

[beanhead]

Your vacuum reading kinda surprises me. What's you idle rpm in gear? I have a similar 440 but use an 850 mech. secondary carb, one step larger cam (xe274) and get 15+ inches in gear @700 rpm. You definitely shouldn't need to be drilling butterflies with that small of a cam. (That leans the idle mix and lets you idle faster without opening the plates to the point too much transfer slot is exposed). You did not indicate that it happens while maintaining steady rpms while cruising so I would look at your squirter/cam/pump arm adjustments. (My carb needed different pump cams front and back). As for the backfiring...that may indicate not enough timing. 36° is fine for your all-in but you may need more initial and to re-curve the distributor to keep the 36 limit. Plug your vacuum advance and increase the idle timing until vacuum just maxes out, note the degree reading, then see where that puts your timing at all-in. Gotta sort the timing out first before moving to carb stuff or you'll be going in circles..

 

[wagonman]

Thank you very much for your reply!

The power valve is 6.5. I did not touch it, because according to my vacuum reading it already is slightly on the richer side, which is what the engine wants.

With jets on the primary I went from the stock 68 to 70 and then to 72. Secondaries from 80 to 82 and then to 84.

Due to my relatively mild cam and good vacuum and idle quality I did not think about drilling holes in the primary throttle blades (I would like to avoid that, too) or open up the secondary throttle blades. Wouldn't that make it being even more on the rich side?

The way I understand it is when the secondary blades are cracked a bit it gives more idle air and a little more fuel. It is an easy almost no cost test and you can return to the original setting without pulling the carb. Just be carefull when removing the original screw as they are kind of staked in the plate and if you get to agressive it will pull the threads out. I could be wrong so please correct me if so.

 

[beanhead]

....drilling the primary throttle blades.....open up the secondary throttle blades. Wouldn't that make it being even more on the rich side?

No. It will lean the mixture. You're adding more air, without adding more fuel....not the fix for your lean condition! During that moment that it stumbles lean it wants more fuel, to cover that transition. As you accelerate, the plates open and gulp air quicker than the carb can add fuel in that moment through the transfer slots..you need enough fuel right then to keep the mixture balanced. Timing, squirters, pump cam/arm adjustment....

 

[Nico Buchner]

Your vacuum reading kinda surprises me. What's you idle rpm in gear? I have a similar 440 but use an 850 mech. secondary carb, one step larger cam (xe274) and get 15+ inches in gear @700 rpm. You definitely shouldn't need to be drilling butterflies with that small of a cam. (That leans the idle mix and lets you idle faster without opening the plates to the point too much transfer slot is exposed). You did not indicate that it happens while maintaining steady rpms while cruising so I would look at your squirter/cam/pump arm adjustments. (My carb needed different pump cams front and back). As for the backfiring...that may indicate not enough timing. 36° is fine for your all-in but you may need more initial and to re-curve the distributor to keep the 36 limit. Plug your vacuum advance and increase the idle timing until vacuum just maxes out, note the degree reading, then see where that puts your timing at all-in. Gotta sort the timing out first before moving to carb stuff or you'll be going in circles..

Idle rpm is about 700 rpm in neutral and drops to about 600-650 rpm in gear.
I checked the pump arm adjustment but did not touch the squirter/cam. My thougts were that if it was the accelerator pump shot, it would rather be a hesitation than a non ending surge. Do you think changing cams would help with a constant surge, which is also slightly noticable at a steady cruise at for example 2000rpm? The pump should not be operating at cruise.
My converter stalls at about 3000rpm. My distributor is set up to have the total timing at about 2800 rpm. This indicates that my timing is all-in during the surge. My initial is at 12° if I recall correctly. According to the spec sheet of the distributor, with 2 blue springs installed the mechanical advance is 24°. Do you think 12° initial is not enough? I totally agree with you, that the timing should be spot on, before messing with the carb and that the backfire indicates a ignition not advanced enough. I will definitely check highest vacuum reading depending on initial timing and report back to you. This might take a while though, because there are some busy weeks coming up.

The way I understand it is when the secondary blades are cracked a bit it gives more idle air and a little more fuel. It is an easy almost no cost test and you can return to the original setting without pulling the carb. Just be carefull when removing the original screw as they are kind of staked in the plate and if you get to agressive it will pull the threads out. I could be wrong so please correct me if so.

The way I understand this, is that I can add more mixture at an earlier time without richening it up too much. This makes sense to me, since it needs more fuel, although it is already running rich. I will definitely give it a try when I confirmed the timing.

Thank you very much for your help guys!

 

[beanhead]

Okay...you didn't mention originally that it happens during a steady throttle (cruise) condition...yes definitely get the timing nailed down first. I think you may find that you can add timing at idle then adjust the carb to keep you in that 650-700 rpm range and you'll have noticeably more vacuum, which of course will cause you to go through all your carb settings again but be patient and keep working your way around the carb, you get it dialed in. Those float levels and idle settings continue to affect the fuel curve beyond idle...but you have to have them right before moving to jetting etc.

 

[Nico Buchner]

I checked my timing today and I am at 15° initial at 700rpm idle speed and the total is all in with 36° at 2800rpm. At idle I had 14" vacuum today.

I then raised the initial timing in 2° steps up to 20° and adjusted the idle speed to 700rpm accordingly. The vacuum stayed at 14". I then took it for a very short spin with vacuum canister plugged on the carb and stayed below 2000rpm to not have too much total timing. The driving experience was the same as with 15° (probably worse, but the backfiring was gone).

I then went the opposite direction all the way down to 10° and still had 14" with the same driving experience (running pig rich down to 10.5:1 on the A/F with black smoke and having lean conditions up to 16:1 in between for a few seconds with no power).

Please let me know what you think and thank you very much for your help!

 

[Nico Buchner]

I also tried opening the secondary blades a little bit, which did not make a change except the idle speed.
It still has this lean surging while cruising and during slight to normal acceleration (gets worse under load going up a hill). Under WOT it pulls good.

Any help is highly appreciated!

 

[wagonman]

Try plugging of the PCV if you have one.

 

[beanhead]

Do you recall your a/f readings when you had it jetted way up? The thing with the O2 sensors is they will show a misfire as a lean condition...you could be having an ignition issue. You've done some good troubleshooting, maybe ignition system needs to be gone through. Your statement that it gets worse the longer you drive, can be a clue...Coils & ignition controllers can get erratic with higher temps if they're failing. You checked for vacuum leaks; that's good. Was the engine hot or cold? And wagonman just brought up a good point too...the PCV can cause weird issues if it's dirty/clogged/failing.

 

[wagonman]

Do you recall your a/f readings when you had it jetted way up? The thing with the O2 sensors is they will show a misfire as a lean condition...you could be having an ignition issue. You've done some good troubleshooting, maybe ignition system needs to be gone through. Your statement that it gets worse the longer you drive, can be a clue...Coils & ignition controllers can get erratic with higher temps if they're failing. You checked for vacuum leaks; that's good. Was the engine hot or cold? And wagonman just brought up a good point too...the PCV can cause weird issues if it's dirty/clogged/failing.

I have had weird driveability issues with aftermarket PCV valves. If it came from PEP Boys ect get an OEM valve for your app. Even an old one from a wrecking yard cleaned up is better than a Vachina valve.

 

[Nico Buchner]

Thank you very much for your replies.

I have had weird driveability issues with aftermarket PCV valves. If it came from PEP Boys ect get an OEM valve for your app. Even an old one from a wrecking yard cleaned up is better than a Vachina valve.

I have a new, adjustable PCV-valve from Wagner, that is adjusted as per instructions. To exclude the PCV valve, I also performed a test with the PCV, the brake booster and the distributor (all vacuum sources hooked to the carb) removed (source by source) and the port plugged. The problem did not go away.

Do you recall your a/f readings when you had it jetted way up? The thing with the O2 sensors is they will show a misfire as a lean condition...you could be having an ignition issue. You've done some good troubleshooting, maybe ignition system needs to be gone through. Your statement that it gets worse the longer you drive, can be a clue...Coils & ignition controllers can get erratic with higher temps if they're failing. You checked for vacuum leaks; that's good. Was the engine hot or cold? And wagonman just brought up a good point too...the PCV can cause weird issues if it's dirty/clogged/failing.

The problem is, my a/f readings below 3000rpm (including idle) are all over the place, I get everything from 10-16 at rpms between 1500 and 3000 and about 12.2-13 at idle depending on the idle mixture. The a/f smoothes out above 3000rpm and is pretty constant there (on the 68 primary jets about 12.6-12.8, on the 72 jets about 12.0-12.4. The vacuum tests were performed on the engine running (hot) and on an engine shut off but still warm.

There are 3 more things I discovered/tested:

• The surge is even present, when the engine is just idling in neutral. It is very little, but the engine is shaking slightly for 2 seconds and then runs good for 1 second. This also seems to get worse when hot. The a/f is moving accordingly, 1 second too rich and 2 seconds too lean.
• I unhooked my electric fuel pump while idling when hot, to exclude the fuel pressure (let it just empty the fuel bowl) from the equation. I did this, because I can watch the fuel pressure climb to about 7psi, then the float valve opens and it drops to about 5psi and starts climbing again to 7psi when the valve shuts (not in correlation/in sync with the surge).
  
• On friday, when I performed the test to adjust the secondary plates, the test was negative. When I adjusted them back to shut today, to get back to my baseline, suddenly the car was pulling much better and the surge almost went away. I figured I found the problem and the blades were the answer. I then made some further test runs (8 total) and on 4 the engine ran like crap, on 2 it was ok and on 2 it was nearly good. The only things I changed during these runs, was the idle mixture by about 1/8 turn, the blades in minimum steps (in the end it felt like it was not doing very much) and filled up the tank. There was no logical reaction of the engine to the changes I made. F.ex. on the way to the gas station it ran pretty good, on the way back it ran like crap with no changes done other then to fill up.

For these reasons, and because I changed and adjusted almost everything on the carb I can with my knowledge (except power valve), it almost feels like I should concentrate on the ignition system, as per your suggestion, beanhead. Especially, since the variations of the problem cannot be related to any changes done to the fuel system.
In which order and how should I test the components? Do you agree, that the MSD 6AL is one of the main suspects?

Thank you very much for your help guys, it is highly appreciated!

 

[wagonman]

7 psi is a bit high. what kind of Fuel pump? I like to see 5-6 psi. If it runs well at WOT I doubt the MSD ect is the issue. Do you have a regulator to use? Power valves are cheap put a new one in. Use a Holley Hi-flow PV and correct gasket.

 

[Jcharger]

I missed whether the surging always been there, or whether this is a new development. If a new development, I would still consider checking the ignition (cold and hot) and after that rechecking vacuum. Something has to go bad/get dirty or a leak develop for a carburetor to change settings like jets, restrictors, and bleeds.

When checking the ignition, check it hot and cold. The coil can do funny things to the way the engine runs and can change cold to hot and so can the MSD unit. Buying one and having a spare is not necessarily a bad thing.

Do I understand correctly that the surge occurs for the idle and at cruise and not at WOT or above 3000/3500 RPM. If the WOT power (air fuel) is OK and the higher RPM 3000/3500 RPM conditions are OK, rule out jets and PV. The accelerator pump would be on tip in and also ruled out if that is OK. If it is still the carburetor, it sounds like it is on cruise (primary) and idle metering circuits. The Air fuel of 16 is lean and enough to cause a surge, but not extremely lean. Is the value 16 at cruise?

Even though you have checked vacuum, I also find that a reading of 15 and 12 for an XE268H sounds low. Did your tests rule out a leak on the underside of the manifold - where it mates with the head. I assume if you had a leak on the valley pan side you would see oil in one or more cylinders. If it is not ignition and is a fugitive a vacuum leak, it is conceivable could mess with the idle and primary metering circuit, but that the WOT and secondary circuit is able to flow enough gas and air to mask it.

 

[beanhead]

..."I get everything from 10-16 at rpms between 1500 and 3000 and about 12.2-13 at idle depending on the idle mixture. The a/f smoothes out above 3000rpm and is pretty constant"..
OK this has me thinking along the lines of a distributor/advance mechanism problem, since you've had the same issue with two different carbs. That's the rpm range your distributor advance is operating, the random A/F numbers there still look like a misfire too so maybe replace cap & rotor and check the reluctor gap at every position. See how easily the advance plate operates and if it snaps back when you let go of it. Try to wobble the distributor, to make the bushing isn't wearing(although this usually causes erratic timing and bouncing of the reading on the damper). Again I know you checked for leaks, but 440s can get them under the valley pan as Jcharger stated, so watch your vacuum gauge when the problem happens--it will bounce around more than it should if there's a vacuum leak. Oil in the chambers is a dead giveaway(as long as your valve stem seals are good)

 

[Nico Buchner]

7 psi is a bit high. what kind of Fuel pump? I like to see 5-6 psi. If it runs well at WOT I doubt the MSD ect is the issue. Do you have a regulator to use? Power valves are cheap put a new one in. Use a Holley Hi-flow PV and correct gasket.

I have an electric fuel pump with 7psi max pressure and a volume of 80g/h. I use a fuel pressure regulator to lower the pressure and checked the surge with the engine just running from the carb bowls to eliminate any fuel pressure related problems.
The power valve is about 4 weeks old, just like the whole carb.

I missed whether the surging always been there, or whether this is a new development. If a new development, I would still consider checking the ignition (cold and hot) and after that rechecking vacuum. Something has to go bad/get dirty or a leak develop for a carburetor to change settings like jets, restrictors, and bleeds.

When checking the ignition, check it hot and cold. The coil can do funny things to the way the engine runs and can change cold to hot and so can the MSD unit. Buying one and having a spare is not necessarily a bad thing.

Do I understand correctly that the surge occurs for the idle and at cruise and not at WOT or above 3000/3500 RPM. If the WOT power (air fuel) is OK and the higher RPM 3000/3500 RPM conditions are OK, rule out jets and PV. The accelerator pump would be on tip in and also ruled out if that is OK. If it is still the carburetor, it sounds like it is on cruise (primary) and idle metering circuits. The Air fuel of 16 is lean and enough to cause a surge, but not extremely lean. Is the value 16 at cruise?

Even though you have checked vacuum, I also find that a reading of 15 and 12 for an XE268H sounds low. Did your tests rule out a leak on the underside of the manifold - where it mates with the head. I assume if you had a leak on the valley pan side you would see oil in one or more cylinders. If it is not ignition and is a fugitive a vacuum leak, it is conceivable could mess with the idle and primary metering circuit, but that the WOT and secondary circuit is able to flow enough gas and air to mask it.

That is a rather long story, I will try and sum it up. I had the engine rebuilt and did put it in a while ago (along with the MSD6, a new carb and a new ignition coil). It did not run very good at that time, but no surge like I experience now (could be just masked by all the other issues, though). I then had some issues with it not related to this topic and installed a new camshaft, a high stall converter, new water pump, headers, new ignition wires and plugs and a new distributor at that time (about a year ago). The surge is present since then. Nevertheless chances are high, the problem excisted before that and due to the converter and the cam just showed in a different way. I than added some parts to exclude components (like a new fuel pump, pressure regulator) and some parts to help me analyse the problem (like the A/F-meter).
You are correct. The higher the rpms climb, the better the engine runs. While Idling and at cruise speed (especially under load on an incline) it is surging in all variations. The A/F at idle is jumping a little, at cruise it is jumping pretty good (10-16), at WOT it is pretty constant in the 12.5 area. The worst thing I can do is to go up a hill with the engine hot at about 1500rpm. Sometime I have to manually shift to 2nd to make it up the hill. Therefore I would also rule out jets, power valve and acc pump. With the idle mixture I was playing a lot and had the impression that the richer it gets, the better it runs. I had it up to 10:1 and it was smoking like my Cummins and still had very little surge, though.

For the vacuum test, I did the following:

• Spray break cleaner around the intake
• Spray starter fluid around the intake
• Used propane around the intake
• Had a smoke machine attached to a full vacuum port on the carb and checked for smoke coming out of the intake

I agree with you, that 12/15 is low vacuum for my setup. The question is, is it the cause or a symptom of the surging?

 

[Nico Buchner]

..."I get everything from 10-16 at rpms between 1500 and 3000 and about 12.2-13 at idle depending on the idle mixture. The a/f smoothes out above 3000rpm and is pretty constant"..
OK this has me thinking along the lines of a distributor/advance mechanism problem, since you've had the same issue with two different carbs. That's the rpm range your distributor advance is operating, the random A/F numbers there still look like a misfire too so maybe replace cap & rotor and check the reluctor gap at every position. See how easily the advance plate operates and if it snaps back when you let go of it. Try to wobble the distributor, to make the bushing isn't wearing(although this usually causes erratic timing and bouncing of the reading on the damper). Again I know you checked for leaks, but 440s can get them under the valley pan as Jcharger stated, so watch your vacuum gauge when the problem happens--it will bounce around more than it should if there's a vacuum leak. Oil in the chambers is a dead giveaway(as long as your valve stem seals are good)

Now that you are saying it, the vacuum gauge is indeed a bit erratic (rapid bounce of about 1 inHg at idle). As soon as I raise the rpms just a little bit, I get a steady 20.
I think I will check spark plugs first. Stem seals should be good, since the engine has about 300 miles on it, since it was rebuilt. In case I do not find anything on the plugs, is there a different way to check for a vacuum leak under the valley pan (since the catch can of the oil seperator between the pcv and the carb is full, I suspect no oil in the chamber due to the obviously working pcv-valve)?

My distributor is brand new and has less than 200 miles on it. That does not say much of course and I will check the gap after looking for a vacuum leak once more and checked the plugs.

Thank you very much for your help guys!

 

[beanhead]

If there's a leak under the manifold the pcv won't suck it all out, you'll still get it in the chambers. You'll have to at least pull the carb, but with some imagination you can rig up a way to see into the chambers, or what I've done is (securely) tape a cotton ball or piece of a rag to a small solid wire or coat hanger, and feed it down the intake runners...it will soak up some oil if there's any in there you just have to be certain you got the swab all the way in to the valve. Or just yank the intake at that point, then you have to deal with new gasket/sealing..