Technical Pulling my hair out, engine stumbles under load.

Kinda pulling my hair out on a problem and would appreciate some input to get an issue on my flathead powered A running smooth.

'37 flathead running electronic Mallory distributor and dual Holley 94s..

I have smooth idle and it revs fairly OK, I think i can detect a slight stumble but it may not even be there it is that slight. I replaced the spark plug leads which were old and the engine runs a little smoother but my main issue remains...

The problem comes when I get the car on the road. Within 1/2 a mile it feel like it is going to die, I lift off the gas and the engine 'recovers'. At that point you need to coax it up the rev band, if instead you just hit the gas pedal and it will likely almost die again. Drive it into the workshop and park, get out and it revs fine once more from the carb linkage. I got it up to about 3k and held it there for 10-20 seconds, no problem. Didn't falter at all.

As crazy as it sounds, it is only when the car is moving that there is an issue. Maybe gas starvation under load?

Any pointers appreciated as this has been bugging me for weeks.

 

Do you have a pressure regulator? HRP

 

The Mallorys are notorious for having a very lazy advance curve. Not at all suited for a flathead engine. Need about 22-24 deg all in by 2200 rpm or there about.

 

Weak coil,
Check with a spark tester or just put a philips screw driver in one of your spark plug boots and see how far the spark will jump to a good ground.
On a good coil 1/2 to one inch easy

 

Could be the Power Valves in the 94s.

 

You got those cheap ass air filters on the Holleys?

If so, pull the filters and see what happens.

Rich

 

maybe check you gas tank venting

 

I'm with Grahamsc!
Change the coil, I bet it fixes it.

 

Sounds like a coil.

 

New build? Or a problem that has just started from no where? Only happening on the road - possibly float levels may not be correct.

 

OK, so I got the car a few months back and it was barely on the road before I pulled the grille and switched to a 32. This did involve a few minor changes but now that it is back on the road the issue is apparent. Whether it existed prior to my stripping the car down is a question mark.

I did replace the coil (Mallory 29219) as I thought that might be the culprit. Fresh leads, rebuilt the carbs with NOS kits. I will run some tests over the weekend to check the spark out. I think I have been fighting it for long enough that I am losing perspective on what it might be.

The only other thing that sprngs to mind is that I managed to step on the original ballast resistor and broke it. I posted on this back in May (God, has it been that long!). The original measured 2.3ohms but I replaced it with a 0.7 ohm resistor as the Mallory paperwork indicated a need for a TOTAL of 3 ohms. I dont see how this would cause my issue as surely this would result in a STRONGER speak surely? That original thread is here:

http://www.jalopyjournal.com/forum/...eplacement-for-flathead.922916/#post-10428445

Thanks to everyone for their input. I will run some tests in the morning.

 

I will put the car up in the air tomorrow and check underneath for one. I have no confidence in anything right now. Haven't had much time to spend on the car but do need to check the fuel pressure to ensure it is constant AND correct.

Thanks,

 

you should run the recommended resistance to the coil as it can overheat and break the insulation down internally and it can short out or lower the spark output under load by not developing a strong magnetic feild and these things do heat up a lot . short periods like starting is ok , but long term no and you can blow up your condensor in the process ( which might have failed causing the problem .) check the spark output when the engine is warm and been running awhile if its not blue then ignition problems

 

Did you do this ^ yet? Start with the easy shit first. If the engine can't breathe, it will die. Post a picture of your intake system so we can help.

 

Fought a bad condenser for days,ran better cold (not great either) got worse when hot.
Had one fail in a newer Mallory Fh distributor and more recently a vertex mag.
Anyone says that their either good or bad is entirely wrong.
Try to find a quality nos brand name condenser,all that I find is cheep crap in the stores now.
Good luck,
Gary

 

If you did not change the power valves from the ones in the kit to 4.5 or 3.5 then they are opening too soon and causing your stumble. ^I would also check the air filters.

 

OK, fired the engine up in a darkened workshop. No fireworks display, doesn't appear to be any wires sparking to ground etc.

Removed air filters, no change. I doubt they help much but they don't appear to be the cause.

I didn't switch the power valves out. I can have a look to see what are in there but it is unknown at this point.

The distributor is a Mallory breakerless unit so no condenser.

http://www.speedwaymotors.com/Mallo...thead-3-Bolt-Electronic-Distributor,7964.html

Plugs look fine. I will take one of the carbs apart tihs evening to check on the power valve/jets.

Pulled a plug lead and have just about a 1/2" spark, whitish blue. Based purely on this I don't think speak is the issue.

The coil is brand new. I will try the one in the wife's 55 Thunderbird while she isn't looking!

Attached are two shots of the offending engine.

 

Kinda lost when it comes to selecting the right power valve but the ones in there now are 85s. No idea what this means in terms of fuel delivery.

 

If both carbs have the 8.5 then they are opening almost as you crack the throttle, giving you a stumble from too much fuel. I would start at 4.5 PV's in both.

 

Fuel pump

 

I just looked at your images....plug the open holes for the choke shafts...you will suck in dirt.
If you fish you can use the split weights to fill the holes.....smash them.

 

Have 4.5 power valves en route and will switch over to them to see what change that provides. Will also look to get a pressure gauge installed temporarily to help pinpoint whether fuel delivery is the issue. That should cover the fuel regulator and pump comments above. What is the ideal fuel pressure for 94s?

 

You really should have the choke plates in the carbs. They promote proper air flow through the carbs.

 

no more that 3 psi.....

 

Drop by a tune up shop with a scope. Easy hookup, they may charge you an hour for diagnostic...
The scope can see invisible goings-on that rivals the government's watchful eye...

 

Finally found time to pull the jets. they are 47s.

 

You also might consider installing a vacuum gauge in the car so you can see it while driving. This will help you to see when and why things like power valves and vacuum advances have an effect on your tune-up and driveability. Make sure you connect it to a source of full manifold vacuum.

 

Thanks, I have a vacuum gauge ready. Fuel pressure gauge, 051 jets and 4.5 PVs on the way. Looking to spend some time this weekend seeing if I cant get this sorted out.

 

Since there's no history and you don't know if this was there or something you did- start with a compression test. It takes only mins and tells you volumes. I've seen cars with low compression run like tops and sound like Swiss watches in neutral but falter dramatically under load.

10 min compression test can save you lots of tail chasing and a few hair follicles

 

Makes perfect sense. Ran a dry compression test last night. All cylinders were in the 105-115 range.

Plugs were VERY sooty.

Carb rebuild kits with 4.5 PVs en route. Also new plugs and rotor. Will rebuild the carbs and see if the new PVs helps any.

Can then also redo the compression test with engine hot. Will do a wet test too.