Engine Troubles Runs Rough Flat Spot

C

cbmind

Member
Hi there, looking for some help with my 1980 IH Scout II traveler.
Specs-
1980 traveler
1982 ih345 v8
1991 GM 4l60 (not an e)
1991 np241c
edelbrock 1406 4brl
efi converted Holley points with GM 4 pin module and vacuum advance enabled.
Msd blaster 2 coil.
New wiring harness.
New spark plug wires, cap, plugs probably have 4 hours of run time. Autolite 303s.

Issue- idles rough at 750rpm, flat spot under load (in gear) at 1800rpm to 2100rpm.

Vacuum test with engine at 195f checks out at 21hg idle and doesn't bounce.

Compression test results- performed after 15 minute run time with engine to 195f then shut off throttle blocked open and all plugs removed.
Cylinder 2-
(1) 140psi
(2) 150psi
(3) 148psi

cylinder 4-
(1) 160psi
(2) 160psi
(3) 158psi

cylinder 6-
(1) 150psi
(2) 150psi
(3) 155psi

cylinder 8-
(1) 147psi
(2) 150psi
(3) 148psi

cylinder 1-
(1) 160psi
(2) 160psi
(3) 159psi

cylinder 3-
(1) 150psi
(2) 152psi
(3) 158psi

cylinder 5-
(1) 152psi
(2) 149psi
(3) 153psi

cylinder 7-
(1) 148psi
(2) 149psi
(3) 149psi

engine was pulled out of a bus. Installed in Scout 2 years ago. Only got it running in the last few months due to other work on driveline and wiring.

Any ideas are appreciated. Thanks!

Edit- wasn't quite sure where to post this.. Psi was obtained with 5 revolutions of the crank.
 

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Sounds like lean on the primary jets. Plugs look black from to much fuel, but that could be from the rest of the fuel mapping. Is this a new problem or has it been that way since you got it running? Was this carb tuned to the engine?
 
It's been that way since I got it running.

The carb has been tuned only setting choke, the ims screws, and idle. No other changes. Carb isn't new, but purchased new six months ago, took it out of the box and bolted it on. Engine is power timed. For what it's worth, I'm just shy of sea level
 
The edelbrock carbs come with a manual.

If you did not receive one with your new carb, they can be downloaded from the internet for free.
 
1975ih200 said:
the edelbrock carbs come with a manual.

If you did not receive one with your new carb, they can be downloaded from the internet for free.
Click to expand...

Thanks for the input everyone. I followed the manual for the initial installation and tune. I was chasing down leaks and other issues, solving them one by one. I haven't yet messed with any other changes or tuning to the carburetor based on previous experience that these are pretty decent out if the box. My fuel pressure is 4.5psi as measured at the carb fuel inlet from a new mechanical pump installed after mine cropped out. I'll go through the installation manual again. Maybe I missed something.
 
cbmind said:
issue- idles rough at 750rpm, flat spot under load (in gear) at 1800rpm to 2100rpm.
Vacuum test with engine at 195f checks out at 21hg idle and doesn't bounce.
Click to expand...

I'm thinking the power piston is coming up to soon and the engine is Rich while you experience the flat spot. Not uncommon on the performer/carter afb carbs. Acquire the afb power piston spring kit. Take the little piston covers off and see what color spring is installed, the 1406 should be equipped with the orange spring 7in/hg. Meaning the power calibration comes in at 7in manifold vacuum. You want it to come in later or 6-5 in. The set has those springs and a vacuum rating chart for the other spring colors. In stall one step lighter spring.
Please make one change at a time. No jet or other change until you have a chance to drive the Scout with the new springs. If it gets worse, go to the 8in spring set.

See below for some other items we May need if the springs don't have the desired effect.

Rough idle but make 21in/hg at idle? Damn good and still idle rough. Doubt you have a vacuum leak.

Flat spot in primarys only or when you try to dip in to the secondarys?

Have you run it with a stock points equipped distributor?

I don't know what the GM 4 pin module is and how you enable vacuum advance. Hard to tell what the advance curve is. Maybe you could explain.

I assume you still have the working stock Holley mechanical advance?
 
1975ih200 said:
the edelbrock carbs come with a manual.

If you did not receive one with your new carb, they can be downloaded from the internet for free.
Click to expand...

I did. Followed steps for tuning, those that didn't require additional parts if needed. I May be at the point that they are.
 
Robert kenney said:
I'm thinking the power piston is coming up to soon and the engine is Rich while you experience the flat spot. Not uncommon on the performer/carter afb carbs. Acquire the afb power piston spring kit. Take the little piston covers off and see what color spring is installed, the 1406 should be equipped with the orange spring 7in/hg. Meaning the power calibration comes in at 7in manifold vacuum. You want it to come in later or 6-5 in. The set has those springs and a vacuum rating chart for the other spring colors. In stall one step lighter spring. I have a kit, I'll work on it this Saturday and report back.
Please make one change at a time. No jet or other change until you have a chance to drive the Scout with the new springs. If it gets worse, go to the 8in spring set. will do.

See below for some other items we May need if the springs don't have the desired effect.

Rough idle but make 21in/hg at idle? Damn good and still idle rough. Doubt you have a vacuum leak. I agree.

Flat spot in primarys only or when you try to dip in to the secondarys? I think into the secondaries as well, will verify this Saturday after spring change noted above.

Have you run it with a stock points equipped distributor? no.

I don't know what the GM 4 pin module is and how you enable vacuum advance. Hard to tell what the advance curve is. Maybe you could explain. IH efi junkyard mod with IH Holley IH points distributor, Ford duraspark guts, GM 4 pin module.

I assume you still have the working stock Holley mechanical advance?
Click to expand...
I think so? I'm ignorant, not sure... This distributor was what I was using for full timing/ fuel control efi junkyard swap prior to engine swap 2 years ago.

I'll make the changes and verifications and report back soonest. Thanks!
 
If you did the Holley dizzy mod with the Ford duraspark parts and GM 4-pin ignition module, and are still using the carb, then it should still have mechanical advance & vacuum advance installed and functioning.

I would check the proper operation of these advance mechanisms while you are working on the Scout.
Both are quick and easy to check/verify proper function and operating smoothly.

If the mechanical advance & vacuum advance were disabled for the FI you had on earlier, then you switched back to the carb, you would have running issues without the advance mechanisms available. There advance mechanisms are needed for the carb engines,.....the FI engines have timing controlled/adjusted by the FI computer, therefore the advance mechanisms are disabled for them.

Please post up photos of the areas you are working on, it helps us understand what you have to deal with.
 
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Thanks for the heads up.

I'll look into it.

At some point during putting in fresh plugs and reconnecting spark plug wires, I've developed a no start condition. Noticed a wire down to the starter battery terminal that was arcing out on the firewall. Working through the mess to get it running again. Frustrating. Anyway while going through wiring diagnosis, discovered the 4 pin module b wire was connected to coil negative at coil. Swapped to coil positive. Not sure at what point I did this. While it was still able to run or after? Not sure.

So at this moment I'm trying to get it to start again, it turns over, but no spark at the coil output. Wondering if I fried the 4 pin module.
 
Auto parts stores can quickly and easily test the GM 4-pin module.

Worthwhile to carry a spare one of those anyway.
 
1975ih200 said:
auto parts stores can quickly and easily test the GM 4-pin module.

Worthwhile to carry a spare one of those anyway.
Click to expand...

Yes, just got back. Tested good. Took my junkyard spares with me while I was at it, also tested good. Going through basics. Already have checked fuses, need to check power to coil and such.
 
So I'm back in the saddle so to speak. Ignition issue resolved. Certain I was a bad module. As I was unable to replace it, I converted another distributor per bill's instructions for the GM 4 pin/ duraspark pickup for carburetors, leaving the mechanical advance intact.

That being said, still have same issue regarding fueling. Performing changes to carburetor as suggested then will report back. Happy to have it running again..
 
Edelbrock tuning kit and manual in hand..

Discovered the carburetor step up springs from factory are yellow (4"/hg) via the manual, and are installed. Not orange. I went ahead and followed advice and have installed the spring that coordinates with recommended 5- 6", this would be the orange spring at 5". Will report back findings. Thanks.
 
So I made the changes. Seems that there's some improvement, but not what was expected. The flat spot is gone, but I'm still experience a slight hesitation, and feels rough through the entire rpm range, especially at lower rpm. I haven't pulled the fresh plugs to look at what's going on, but I'm confident in the ignition now. Also, I changed the fuel filter, and noted 4.5psi fuel pressure at the fuel inlet to carburetor.
 
Just so I understand the situation when you experience rough running. Try these four running regimes and let us know the out comes.

In gear, running and rolling (5 mph), up to normal running temperature, if you just add a bit of constant throttle, like 1/2 of an inch of gas pedal, it is not smooth?

In gear, running and rolling (5 mph), up to normal running temperature, if you just add a lot of constant throttle, like 1/2 of gas pedal travel (primary bores only), it is not smooth?


Out of gear, running at normal operating temperatures, applying a little throttle just off of idle, it is not smooth?

Out of gear running at normal operating temperatures, momentary 1/2 throttle from idle, it is not smooth? Watching tail pipe area while doing the above, do you see any black/brown smoke?
 
Sorry for the late post. Finally got around to trying it out.

So not in gear either condition yields a smooth climb in engine speed.

In gear, both conditions give a rough climb in engine speed/ acceleration.

No smoke observed from my beautiful assistant!

Appears to be only under load/ in gear.
 
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