Surging 392

[Oil Scout]

this thread describes the same symptoms I have with my 800. Surging at most all speeds, all gears.

1) vehicle model. 1970 800 V
2) production date/or year model. 1970
3) two wheel drive or four wheel drive. FWD
4) type of transmission (and transfer case if applicable). wide ratio 4spd
5) if 4x4, which axles front and rear. Dana 44s, 4.09 ratio
6) which engine variation. IC392 Stamped - V 392 E, No. 1380624A
7) which ignition system variation. Holley Dist. with Petronix
8) which fuel system variation. saddle tanks, 3/8 stainless from tanks to valve, filter after valve, 3/8 stainless from valve to pump, 5/16 steel brake to filter at carb, three inches of rubber to carb. Holley 80457-10, with float adjusts on each bowl. Have size 31 pump nozzle, jests are what came with carb (I think there are 66), but have not confirmed what size yet. Running the pink accelerator cam, was running orange (came on Carb).

I had prior issues with lifter preload, solved with custom pushrods to achieve .040 preload. Solved some backfiring, timing and rocker clatter. Timing is at 18* BTDC. Has great power, acceleration, great idle, no miss, smells rich. I have checked only one plug since fixes and is good. Before fixes 6 out of 8 were black and wet.
9) which brake system variation. Rear drum, front disk
10) non-oem modifications present which May impact a response. running Scout II dana 44s, new plugs, wires, petronix, new wiring. new coil.

Have clear filter at carb. stays full while warming up, but after driving I notice it is only partially full, sometimes down to half. sometimes when warmed up or after driving, filter is not full and a few to a lot of bubbles. Once had a leak at pick up tube fitting sucking in air and had low fuel supply to the point of starving engine and dying. Fixed this and don't have supply problem anymore. can run 70 mph with room for acceleration. When trying to run steady speed from 5 to 75 it surges. Sometimes surges harder at 30 to 50, less hard at higher speeds. Slow corners and 35 mph are worst.

 

[Oil Scout]

Re: always surgeing

Sorry, need to add to post information.

Fresh rebuild by PO. Guessing an Isky supercam. Measured lift on both exhaust and intake at .281. This is a guess at the cam, based on lift specs listed for that cam and others listed on IHPA store. Estimated to have only 10 to 30 miles on new rebuild when this Scout was acquired. Went ahead with Cam break in procedure at start of trying to figure out what I had. Have driven about 140 miles since.

Will replace filters tomorrow. Have ordered .029 (29) pump nozzle and have 62 and 64 jets if this is a possible fix.

Thanks in advance.

 

[Scoutboy74]

Re: always surgeing

My first thought off the bat is 18* of static advance is too much, especially for a 392. 8* degrees is about max. I'll be back with more thoughts. It's a lot to digest.

 

[Oil Scout]

Re: always surgeing

After filter changes tomorrow I will check timing again. I had it at 15* and then advanced it while out driving trying to get a positive change. Anywhere close to 10* or 12* and it dies after running very rough. Appreciate the quick response.

 

[Scoutboy74]

Re: always surgeing

I had to do some quick research on that carb, as I wasn't familiar with the list number. 600 cfm is a lot of carb for an SV8, although with vacuum secondaries the engine will only use the delivery volume that it needs. That's if it isn't calibrated to be a high performance carb.
The surging at various speeds is usually indicative of a lean condition aka fuel starvation. So going down in jet size would only make that problem worse.
Have you hooked up a vacuum gauge to get some readings at idle and also while driving? That could really tell some tales.
What choke setup does this carb have and is it properly adjusted and functioning correctly?

 

[Bill Bennett]

Re: always surgeing

I would question if the distributor mechanical advance was optomized for that cam? If you are pushing the base timing out too far plus adding the vacuum advance, could be a cause of the engine 'surge'. Try disconnecting the vacuum advance and plug the vacuum can on the distributor and at the source. This will give just the mechanical advance. There is a number on the arm of the vacuum advance. Will be a #3, #5 or #7. Meaning the vacuum advance will provide 6, 10 or 14 additional degrees spark advance over the mechanical advance. Don't change that #31 jet yet. That Pink accelerator cam in the #1 position is rated at 30cc. In the #2 position, it is rated at 37.5cc. ( rated at 10 strokes ) The Orange cam in the #1 position is rated at 19cc. In the #2 position, it is rated at 24.5cc. ( at 10 strokes ). Consider going back to the Orange cam mounted in the #1 position, Block the vacuum advance to the distributor and go for a ride for the purpose of feeling for the 'engine surge' at constant speed. Not for a 'performance test' yet!. Can you add a vacuum gauge to show engine vacuum while driving?

 

[Oil Scout]

Re: always surgeing

Okay,will add longer hose for vacuum gauge to view while driving. I was considering this as a permanent gauge on a new dash panel anyway. So I will change filters, go back to orange cam and try driving with Vac Adv plugged.

Another item I found in my notes was two months back had a power valve go bad (we thought) so was replaced. All we had was a 3.5. IF the surging is a lean issue, wouldn't the 5.5 or 6.5 be the better valve?

Current vacuum is running 12 to 13 at idle. Bouncing in-between tick marks. I was as low as 10.5 before the new pushrods and bouncier. Depending on timing set position, I have achieved 14.

 

[FDChappie]

Re: always surgeing

The accelerator pump and power valve should have no effect on the fuel mixture if it is surging while cruising on flat ground. I did some extensive tuning work on a new Holley 390cfm carb and now have great results on a 345 with a comp cam and headers. You might want to consider putting a standard O2 sensor in the exhaust so you can read what the actual fuel mixture is. The standard one can be read with a multi-meter while driving the vacuum gauge is a necessary component too. See this thread for details http://forums.ihpartsamerica.com/showthread.php?t=11807

 

[Bill Bennett]

Re: always surgeing

Try the orange cam in the #1 position first as that will be the smallest fuel shot of the 4 choices you have. The 'power valve' will only open when the manifold vacuum drops below the rated value of that valve. Knowing which power valve is in the carb and having the vacuum gauge reading available while driving normally (?), will show when the power valve opens and starts dumping in the additional fuel - which could be part of the 'over rich' engine condition or the smell of excess fuel and the 'sooty' sparkplugs.
Do you know what size # main jets originally came in that carb?
What # main jets are in the carb now and what other # main jets do you have?
Have you determined which cam was installed in the engine? Is the cam just too hot for the way you want to drive?

 

[Oil Scout]

Re: always surgeing

About to head home and start on Scout (2:40 CST). Y'all are awesome quick on the help. Very much appreciated!!!

Carb:
Previously 6.5 Power Valve
Currently 3.5 Power Valve
Pump Nozzle:31
Main Jet 66 according to literature attached
I have 62s and 64s
Accelerator Cam Changing back to Orange on #1 position. I have all cams available if there is an opinion to change.

Vacuum Gauge:
Will rig something to view while driving. Hopefully next to Tach and video driving it.

Will change filters, reset timing if possible to 15 or less, change to orange cam, warm engine and oil and go for test drive. Will report back this evening. Thanks

 

[Oil Scout]

Re: always surgeing

I posted a reply earlier this afternoon, but it apparently got lost in the black hole of the internet.

I replaced the orange cam, set timing to 15* BTDC, set up vacuum gauge next to Tach and went for a drive. I will try to post pics and video soon.

In summary, nothing changed except accelerator is more sensitive. Surging is most prevalent when trying to maintain constant speed or rpm, but is strongest when slight acceleration to maintain rpm. When letting off just the slightest bit or coasting in gear there is no surging.

Vacuum rarely drops below 5, so I'm not sure if PV (3.5) opens very often. I noticed when rolling quickly on accelerator only twice that vac reached less than 5. I was able to find a sweet spot of the surging at 2200 rpm in third gear and observe. It really seems like it is starving. Not understanding how this carb could be starving the engine.?

I will try to post video and pics when phone finishes updating.

 

[Scoutboy74]

Re: always surgeing

What's peak vacuum at idle?

 

[Oil Scout]

Re: always surgeing

Question: I noticed when I pulled the vacuum plug off the base of the carb to plug in the vacuum gauge that it looked like something in the tube, I touched the tube and a few drops of gas dribbled out. Should there be gas here? this was on a cold engine that had not run for 24 hours.

 

[Scoutboy74]

Re: always surgeing

No.

 

[Oil Scout]

Re: always surgeing

Peak vacuum at idle is 14.

 

[Scoutboy74]

Re: always surgeing

That's pretty low unless you're at a high elevation. I wouldn't expect that cam to be lumpy enough to effect it that much. Fuel at the vac port, low vacuum... something's up with that carb

 

[Oil Scout]

Re: always surgeing

I think I will pull carb and rebuild. I already have kits. Should I at this time go back to 6.5 PV that came with carb? should I change main jets from 66 to 64 or 62 or at all?

Question: Does someone move these posts or copy them to more relevant threads? I'm only asking since this is in the ignition thread. Or should user (me) be aware and copy or move as appropriate?

 

[FDChappie]

Re: always surgeing

The records I have show that IH used 2.5 PV's in the carb on the 392. In a heavy vehicle/truck a lot of time could be spent with the PV open. Instead they jetted a bit on the rich side and opened the PV under really heavy loads.

Really the answer to your question depends on what do you plan to use this vehicle for. Then the carb can be tuned for that purpose. In my carb tuning adventure I found lots of info for hot rod tuning, but a dearth of info on tuning for trucks. In the carb thread referenced, my goal was to tune for MPG by having the primary's toward the lean side and a 2.5 PV and when the power was called for have the secondaries run rich. Unfortunately with a 600 cfm carb this type of tuning is not as viable as the secondaries will rarely open.

Running the vacuum gauge for a while will give you an indication of the vacuum level in your actual driving conditions. Decide where you want the extra power to come on and install the PV to match the vacuum level in those conditions.

 

[Robert Kenney]

Re: always surgeing

Surging is most prevalent when trying to maintain constant speed or rpm, but is strongest when slight acceleration to maintain rpm. When letting off just the slightest bit or coasting in gear there is no surging.

Vacuum rarely drops below 5, so I'm not sure if PV (3.5) opens very often. I noticed when rolling quickly on accelerator only twice that vac reached less than 5. I was able to find a sweet spot of the surging at 2200 rpm in third gear and observe. It really seems like it is starving. Not understanding how this carb could be starving the engine.?

I will try to post video and pics when phone finishes updating.

In the low end of the cruise range, you're on the pilot jets, then mid cruise range you're primary main and power valve. The 3.5 will delay the enrichment staging. This can leave you lean in cruise. Go back to the numerically larger PV. Test,

Leave the primary mains asis for the time being.

Leave the following alone for now. I'm just elaborating on tuning mistakes.
I have to assume that pilot jets are as supplied by the factory (Holley) and probably OK. A lot of would be tuners like to drill out the main air jets. This helps with generating a smoother transition where you would normally have a hole or hesitation, while going from the pilots to the mains under light acceleration on largely cammed engines. In that case the main jets are increased. On a small cammed engine with good manifold vacuum, it can end up lean in pilots light cruise.

 

[Oil Scout]

Okay, Carb was pretty clean. Not unexpected since I rebuilt it six months ago. However, there was one plugged hole on metering block see picture attached with picks pointing out plugged path.

Also, the jets were 50% plugged, picture attached to see comparison of dirty and clean.

Correction on previous post: Jets are 65(s) not 66. and PV is 4.5 not 3.5.

Question: see pic of carb/manifold gaskets, which one should be used, thick pressed paper or thin paper?

Square one on right was what was on this when I purchased vehicle. Ive seen a couple maybe three different styles of manifolds, is there any detail about this one that someone could add color for me.