‘39 Merc hesitating under load...driving me crazy

L

lurk

Junior Member
‘39 Merc hesitating under load...driving me crazy

Gents,

This one has me pulling my hair out...car is such a sweetheart otherwise.

Background: Stock ‘39 Merc. Original 33k mile car, never had an engine rebuild that I’m aware of. Fires up easy, idles smooth, shuts off cleanly, doesn’t run hot, even 90 psi compression on all cylinders, fully cleaned/replaced/rebuilt fuel system, new carb, rebuilt diver’s helmet distributor from All Ford, new coil, new plugs, generator puts out 6.7 volts, battery always reads 6.4+ volts.

Symptoms: has a hesitation under load in all gears. Not bad in first, worse in 2nd, worse still in 3rd. When motor revs out of gear there is no hesitation. Vacuum reads at 22 at idle (although it jumps around very slightly). Under load it drops considerably and is in the 5-10 range in 3rd. Can not identify any vacuum leaks and believe me when I say, I HAVE CHECKED.

I rebuilt the stock 94 carb and feared I may be having issues with the power valve. I temporarily installed a brand new 97...no change, of course. Also, it seems to get slightly worse as the car heats up. When it’s cold the hesitation is barely perceptible, as it reaches full operating temperature, it gets pretty bad. Could it be a valve issue?

Thoughts???

Any help appreciated. Thanks.

(I included a pic of the Merc in it’s current most enjoyable state: parked)
 

Attachments

  • 119AA71B-46E3-4801-8F76-B7CB67150156.jpg
    119AA71B-46E3-4801-8F76-B7CB67150156.jpg
    43.6 KB · Views: 51
Re: ‘39 Merc hesitating under load...driving me crazy

fluttering vacuum readings shows sticking valves. If the vacuum drops quickly and then comes back up, it may just be one valve. try some mmo in the gas and do a couple treatments of pouring some mmo down the carb till it smokes out. Worth a shot. Maybe a burnt valve but your compression readings are good. Sticky valves will run havoc under load but running in the garage sitting you wont tell.


Vacuum at idle should be around 19. 22 isn't an issue to your problem and you can adjust the carb idle screws to lower that easily.
 
Last edited:
Re: ‘39 Merc hesitating under load...driving me crazy

Will do. Easy enough! I did the MMO treatment when I first got it because it smoked really bad... un stuck the rings and smoking stopped. I suppose valves could be stuck still...
 
Re: ‘39 Merc hesitating under load...driving me crazy

I do it every spring on a rebuilt engine. Run the piss out of it helps too.


Another thought.... it Could be a bad condenser, but try the mmo first.
 
Last edited:
Re: ‘39 Merc hesitating under load...driving me crazy

Tinker said:
I do it every spring on a rebuilt engine. Run the piss out of it helps too.


Another thought.... it Could be a bad condenser, but try the mmo first.
Click to expand...

Could be condenser, but I doubt it. Had one on it that seemed ok but I’ve since swapped it for a brand new one... no change to any of the hesitation symptoms.

Will hit it with Marvel and report back. Thanks
 
Re: ‘39 Merc hesitating under load...driving me crazy

Do you have a known good (maybe from another car) distributor you could try? I have had similar symptoms from a helmet distributor. (And other types too) and it is the HT side tracking back to ground. On the helmet type it can ocurr between the rotor and the shaft. It was mentioned here only recently to help prevent it by putting a couple of layers of shrink tube or thick electrical tape over the shaft where the rotor sits.

Mart.
 
Re: ‘39 Merc hesitating under load...driving me crazy

I link misfire under load to secondary ignition system. Do you have a misfire or hesitation? Do you have new spark plug wires?
 
Re: ‘39 Merc hesitating under load...driving me crazy

I know you swapped carbs, but my initial gut feeling was low float level. Have you checked the float level in both carburetors?
 
Re: ‘39 Merc hesitating under load...driving me crazy

Tony said:
I link misfire under load to secondary ignition system. Do you have a misfire or hesitation? Do you have new spark plug wires?
Click to expand...

That’s been eating at me too...that’s the only part in the whole ignition system that isn’t brand new. Is there a reliable way to test them? They look perfect...

To answer your question though: at it’s worst it feels like an extreme hesitation, not a misfire.
 
Re: ‘39 Merc hesitating under load...driving me crazy

Automotive Stud said:
I know you swapped carbs, but my initial gut feeling was low float level. Have you checked the float level in both carburetors?
Click to expand...

On the 94 I set it myself. I haven’t on the 97 but there has been no change between the two carbs. I would expect at least a little difference. That said, I’ll pull the top off the ‘97 and check it.

Thanks.
 
Re: ‘39 Merc hesitating under load...driving me crazy

Re the secondary ignition. It's hard to test without replacing parts. You can help keep the loads down on the secondary side by using solid core wires, non resistive caps and keeping the plug gaps down at .025" i would pull the distributor and closely inspect the rotor and shaft. Unfortunately you have to dismantle it to do that. Do you have any other flatty powered cars that have a good distributor you could swap on - purely as a test to either rule it in or rule it out?
 
Re: ‘39 Merc hesitating under load...driving me crazy

FIXED!

Thanks for all the input. After much of the input here suggested something to do with plug wires or ignition, I decided to dig back into the distributor.

Initially I wanted to verify that I didn’t have any crossed wires... turns out, the left side distributor cap had a hairline crack in it! Luckily, I had an NOS set sitting in a box. Installed those, problem gone. ZERO hesitation upon acceleration.
 
Re: ‘39 Merc hesitating under load...driving me crazy

Hooray! For once a problem matched my experience and turned out to be related to the HT side of the ignition.

Mart.
 
You must log in or register to reply here.
Back
Top