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Old 09-14-2021, 03:43 PM   #1
Speedbump47
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Default Flat spot that just won't go away

G'day All
I have been advised that the knowledge of the members on the site is beyond approach and can help solve anything!

Recently purchased a 1948 Mercury 100 HP V8 with 6 volt positive electrics.

The engine has a bad flat spot when accelerating, the PO gave up trying to solve after many tries. Looking for help to resolve as it is annoying and sometimes can be a problem.

Since I've had the car the following have been changed:
Plugs, wires, crab cap and rotor.
Petronix ignition and coil.
Rebuilt the model 59 carb following Mikes carbs on Youtube.
New battery.

Actions and observations:

Distributor brake connected and disconnected - no difference either configuration.
Intake and cylinder head bolts retorqued, no vacuum leaks detected.
Vacuum gauge reads steady 11psi, never changes despite fuel/ignition changes. Swings to zero back to about 18-20 when flipping accelerator then back to 11.
Mixture screw can be in all to way or out all the way but no difference to vacuum reading or engine idle.
Ignition timing set.
Petronix checked with manufacture who confirmed all working properly.
Engine compression 110 on all except nbr 7 at 105, dry, engine hot.
All grounds removed and cleaned the re-secured.

I think that is it. As you can see very frustrating.

NEED HELP PLEASE!

Thanks in advance.

Mike (Speedbump47)
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Old 09-14-2021, 04:30 PM   #2
pistonbroke
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Default Re: Flat spot that just won't go away

Hi Mike, The first thing I see is your carb is not set correctly. It's idle is set to high so the the idle mixture screws have no effect, also check your float level. With the engine NOT running actuate the foot feed while looking down the throat of the carb. You should see two nice solid streams squirting . One down each barrel. Second is who set the timing and how? With out the vacuum brake hooked up there should be a noticeable difference. Is the vacuum brake hooked to manifold vacuum ? It should be but I have folks mistakenly hook them to the carb like a vacuum advance. I hope this helps, Tim
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Old 09-14-2021, 06:44 PM   #3
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Default Re: Flat spot that just won't go away

Are you saying when you pull the throttle from idle it hesitates or are you saying at a higher rev it does it ,Ted
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Old 09-14-2021, 07:06 PM   #4
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Default Flat spot

One thing that really sticks out here is the engine manifold vacuum, ordinarily an engine in stock form in good tune will pull a steady 19 to 21 at sea level. If it won't there is a serious defect somewhere.
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Old 09-14-2021, 07:47 PM   #5
alanwoodieman
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Default Re: Flat spot that just won't go away

timing way off or valves out of adjustment for vacuum to be that low
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Old 09-14-2021, 07:53 PM   #6
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Default Re: Flat spot that just won't go away

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Maybe also leak at manifold gasket or leak into power valve ,wrong manifold ,
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Old 09-14-2021, 09:21 PM   #7
fordwife
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Default Re: Flat spot that just won't go away

Might possibly be a leak in the intake manifold or gasket between the intake and the exhaust passage.
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Old 09-14-2021, 09:50 PM   #8
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Default Re: Flat spot that just won't go away

Or perhaps a bad vacuum gauge; can you check yours out on another vehicle or perhaps borrow another?
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Old 09-14-2021, 10:00 PM   #9
Bill OH
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Default Re: Flat spot that just won't go away

Try blocking off the hose to the wipers or did you have your vacuum gauge connected in its place and if so try driving with the vacuum gauge connected. I have encountered flat spots with CG and Holley carbs. I would try a stromberg and see if that is the solution.
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Old 09-15-2021, 06:39 AM   #10
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Default Re: Flat spot that just won't go away

Speed,
Pert and 6v not always trouble free ,been there.
When you disassembled the base of your carb how much clearance did you find between the throttle shaft and the throttle shaft bore ? Once the wear approaches
.005 you are in a very gray area, many annoying issues with 94's can be traced to
vac leaks at the throttle shaft. The mixture screws must cause a change when turned in.......Set the idle speed at 550/600 rpm and starting with the screws 1 1/2 turns out
slowly advance until the engine slows or stumbles....back that screw out 1/2 turn.
Ditto on the other screw. If no change you have a vac leak in the carb.
The 11 in/hg is reflective of a worn out motor, not a motor with 100 +/cyl.
6v with Pert and a generator is risky.
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Old 09-15-2021, 09:10 AM   #11
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Default Re: Flat spot that just won't go away

And if all else fails then one of these fine products may help.


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Old 09-15-2021, 10:17 AM   #12
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Default Re: Flat spot that just won't go away

Quote:
Originally Posted by Speedbump47 View Post

The engine has a bad flat spot when accelerating, the PO gave up trying to solve after many tries. Looking for help to resolve as it is annoying and sometimes can be a problem.

NEED HELP PLEASE!
Thanks in advance.
Mike (Speedbump47)
Have you checked the rich/lean settings on the accelerator pump?
When the symptom presents itself try partially operating the choke too see if there is an improvement.
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Old 09-15-2021, 06:16 PM   #13
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Default Re: Flat spot that just won't go away

11 inches of vac is a major indicator you say its been there for ever sounds like someone stuffed up on the build personally i would remove the timing cover and the heads then check cam timing and valve lift somethings out!!!! keys do split ang gears do slip also people use cams that they think are ok [looks good] but when you actually check them there literally little more that a wobbly stick Its still running shows how tolerant flatheads are to being out of sinc an ohv would have quit long ago
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Old 09-16-2021, 11:53 AM   #14
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Default Re: Flat spot that just won't go away

The accelerator pump and the power valve are the circuits in the carb that affect acceleration if all else is in good working order. The accelerator pump has to push a steady stream as the throttle opens. The modern replacement power valves can have trouble fitting against the seat in the carb body since they aren't made correctly for the most part. Charlie ny here on the forum has a fix for that. The power valves can blow out too and cause fuel to leak out of the float bowl after shut down making for hard starting plus they don't work when blown out.

Last edited by rotorwrench; 09-20-2021 at 08:21 AM.
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Old 09-17-2021, 04:57 PM   #15
Speedbump47
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Default Re: Flat spot that just won't go away

G'day all
Many thanks for all the suggestions.
Looks like the carb shaft has an excess amount of play.
Have ordered another carb. Won't be here until end of next week.
Will update once installed.
Did get another vacuum gauge but still same reading. Hopefully the new carb will improve the reading.
Redid the compression and still vey similar as before at 110.

Thanks and will keep you advised.

Mike
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Old 09-19-2021, 07:34 PM   #16
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Default Re: Flat spot that just won't go away

Mark the distributor location, with a magic marker. Try a little more advance. If it doesn't do anything to correct the situation, you can always put it back to the original setting.
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Old 09-19-2021, 07:54 PM   #17
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Default Re: Flat spot that just won't go away

Quote:
Originally Posted by Merc Cruzer View Post
Mark the distributor location, with a magic marker. Try a little more advance. If it doesn't do anything to correct the situation, you can always put it back to the original setting.
You know, I have used this technique several times over the years with great success on several different types of engines. It has been especially helpful when working on SBC's.
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Old 09-20-2021, 08:46 AM   #18
Merc Cruzer
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Default Re: Flat spot that just won't go away

Yesterday, I finally got around to moving my distributor over one tooth, since the vacuum advance was pushed up against the thermostat housing. This allowed me to add more advance to the timing and amazingly enough the acceleration stumble I had went away.
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Old 09-20-2021, 12:46 PM   #19
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Default Re: Flat spot that just won't go away

I think the preferred method is to move all the spark plug wires one socket over, in order to confuse any future owners.
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Old 10-27-2021, 09:06 PM   #20
Speedbump47
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Default Re: Flat spot that just won't go away

G'day All

Well, bad news. Put the new carb on and now the engine starts immediately and runs but is constantly popping back in the exhaust. Tried adjusting the carb all the way from full in to full out, all to no avail.

The vacuum is still at 11 jerking an inch or so either side.

Sprayed starter fluid along all potential leak areas, no change.

Tried adjusting the dist brake from full in to full out at 1/2 turn at a time. Adjusting the carb each time. No change.

Redid the compression test still 110 except one cylinder at 101.

The engine starts immediately so timing can't be to far off, it at all. What is the best way to check the timing, bearing in mind it has the Pertronix conversion?

Whats next?

Starting to get frustrated, willing to try anything.

Please advise.

Thanks Mike
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