68 KR tuning issue

[gt350bp]

Need help / advise on issue with a 482 custom stroker crate engine tune. I'm still getting a serious bog under load. I'm using a Quick Fuels Q-Series 750 with the following current settings. They are as follows:

Primary jet #74
Secondary jet#84 with jet extensions
Primary power valve #45
Secondary power valve: plugged
Primary nozzle: #31
Secondary nozzle: #31
Accelerator pump: 30 cc primary & secondary
Pump cam: Pink #2 position

Cam is a Comp Cams solid roller
Intake: .691
Exhaust: .702
Lobe separation 112 degrees

Idle set at 950 rpm with 9" vacuum. Vacuum increases to approximately 14" at 2,000 rpm (checked with no load).

When trying to get the car moving, I'm getting a terrible bog and a slight lean backfire, so I know I need to get more initial fuel with the accelerator pump and cam as well as increasing the primary nozzle. I'm think I need to go to the green cam #2 position and increase to a #35 primary nozzle. My question is do I need to change the power valve also or? I'm lost on how to best size the power valve. The idle vacuum seems like the #45 is OK, but I don't know if I should change to a #55 or #65.

The engine dyno #'s are very good, but it was dyno'd from 4,600 rpm to 6,600 rpm, so no info on low end.

Any help or advise is greatly appreciate. The neighbor are starting to get angry with the holeshots to get the car moving.

Thank you.

Don
gt350bp 

[shelbydoug]

I think it is bogging because it is getting way too much fuel off of idle. It will backfire with too much fuel.

First off go to a 6.5. Dial the main jets down to 68. Secondary jets somewhere in the 76 to 78 area.

Just because it has that many cubes doesn't mean it needs that much more fuel.

Put a hotter plug in it until you start to find normalcy. Plugs should read like a medium brown with a whitish tip. They should be dry, not wet.

You are probably washing down the cylinders with fuel as well at this point.

[gt350bp]

Now I'm totally confused with my tuning issue. Maybe the backfire description is incorrect, as I'm getting a bog with a sputter / pop and need to feather th throttle to keep the car running. I felt that the car at low vacuum under initial load was running out of fuel and needing to feather the throttle was indicating the need for additional fuel from the accelerator pump?

The car runs fine under no load and has a good idle as well as crisp throttle response. When letting out the clutch in first gear, if you don't dump it and be at about 1,500 to 1,800 rpm it sputters and can feel an actual shutter through the power steering.   

[Rukiddin]

What is ignition timing? Might try 24 degrees initial ,or more.

[gt350hr]

   Don ,
     Try a 78 primary jet. "Feathering the throttle" tells you that you need MORE fuel as you are dumping fuel from the accelerator pump when feathering. You need the jet change as the pump shot won't completely mask the fact that you are lean and it misfires because of being lean. If the bog were from a rich condition , the "bog" usually clears up as the throttle is closing  ( or at least less opening).

[Royce Peterson]

Sounds like timing is way off.

[gt350bp]

Checked the dyno sheets and it indicates 40 degrees timing. I have not changed the timing from when the engine was built and dyno'd. The change to a #35 nozzle and green accelerator pump cam helped, as the sputter is gone but still have a bog. I'm going to try the #78 primary jet but I'm still wondering if I should change to a 5.5 power valve?

Thank you.

Don
gt350bp

[gt350hr]

   Don ,
  Leave the 6.5 PV. "To me" you proved the engine needs more fuel with the squirter change because you are dumping more fuel! It may turn out that you only need 76 or 77 but a four jet change is better than going one at a time. If your carb had adjustable air bleeds we might have fixed it there but you WILL see a difference with the jet change.
   Randy

[shelbydoug]

It's really very difficult to diagnose this from afar BUT a sensible approach to this would be to substitute another carb that you know is good right now.

I can't believe that anything needs THAT much fuel off of idle. For all I know you have blown out power valves or some other type of a vacuum leak issue.

...and you are NOT going to be able to diagnose it long distance. You are going to need in person help with it.

[Side-Oilers]

My less than .000002 cents: 

I've been chasing a similar problem on my Kirkham's 482-inch 427. 

Best result so far has been by increasing the nozzle/squirter size (in steps) from .032 to .037 in both the primary and secondary sides. 

That eliminated 85% of the bog when dipping into secondaries, so I'll try .040 squirter next.   

My engine is already timed correctly.  Runs like a raped ape at WOT.   The increase in squirter size definitely increased the neck-snapping power.

[Royce Peterson]

You need to not rely on the dyno sheet. You don't drive a dyno. The car needs to have appropriate centrifugal advance, appropriate vacuum advance, and appropriate total advance. All of those things need to be tuned on the car, preferably on a chassis dyno. Then use an air fuel wide band meter to jet appropriately.

40 degrees advance is likely not the case any more, and is typically too much advance unless compression is really low or you are using race gas.

[gt350bp]

What jets, power valve and carburetor are you using in your 482 Side Oiler? Looks like we are chasing the same issue. Tomorrow, I intend to change the primary jets to #78's and I think the power valve to a 5.5.

Today with the increased nozzle size to a #35 and green accelerator pump cam in the #2 position, the car was at least somewhat drivable leaving the neighborhood. Once it is rolling, hold on!

I believe it is about 10.5 compression with the Edelbrock aluminum heads with a KCR stage II matching and porting. I'm using a fuel mix at about 96 calculated octane with the additive. I requested the additional information on the engine build and tuning today from the builder. The balancer is marked at 6 BTC, 34 degrees and 40 degrees. Have not touched or moved the timing as the engine was a custom stroker, pan to carburetor / water pump to aluminum flywheel.

Don
gt350bp

[Side-Oilers]

gt350bp Don:

Here's what I can recall is in/on my Kirkham's 482 FE.  Hope it's helpful.

Holley Ultra XP 950 
92/92 jets 
4.5 power valve
37/37 nozzles/squirters  (so far...stepping up from .032, trying .040 next)
Green accelerator pump cam in #1 position

Ignition MSD7AL-2
Timing 36 degrees @ 4000 rpm

Edelbrock Performer FE heads
10.5:1 compression

Comp Cam hydraulic roller
Duration @ .050 = 233/241
Lift @ .050 = .566/.573
LSA= 112+2

If your problem is like mine was, there's not enough accelerator pump squirt volume (caused by the size of the squirter orifices, not necessarily because of the accel pump itself.)  Plus, possibly not aggressive enough ramp of the accel pump cam. That made for a stutter-surge condition when getting into the secondaries. Too little fuel.

Stepping up in squirter sizes one size at a time (not jumping up 2-3 sizes at a time) helped me. Also changing from the pink to green pump cam, set on #1 setting. #1 gets the squirt going sooner than #2 position.

Let us know how it goes.  I'm no expert like Randy and some others here.  Just a trial-and-error guy.
Van

[corbins]

didn't see a mention of the initial timing, but 40 sounds a bit strong. I would think 36@  2500-2800 would work pretty well. Wonder about that initial...

[gt350bp]

Engine builder got back in contact and confirmed 10.5 compression ratio and suggested 38 degrees on the timing. Initial is at 6 BTC.

Made the carburetor changes yesterday but was unable to take for a test drive. The changes were that I installed a 5.5 power valve and changed primary jets to #78's. Increased primary nozzle to #37 and secondary nozzle to #35. Not sure if the weather is going to cooperate today as we are bracing for some bad thunderstorms moving into Central Florida.

Don
gt350bp