Weekend Freedom Machines banner

420 bogging down under load

13K views 29 replies 10 participants last post by  mdobson1 
#1 ·
Had a beautiful day today to finally cut my grass after way too long and ran into a snag. Seems like my engine doesn't have any power under load.

After start up, the engine seems to slowly rev up to a nice, smooth operating state but when load is applied, e.g. going up a hill or turning on the mower deck PTO, it bogs down, loses power and begins to backfire.

The start up bothers me, too. That slow rise to the normal operating speed. Seems to me that it would jump right up there before.

I removed the air cleaner assembly to watch the carb linkage movement as a load was applied. When I switch on the PTO, the governor linkage pulls back, I can see more fuel coming into the carb, the engine begins to bog down. When I turn off the PTO, the governor linkage begins to return to its previous position but not quite; engine begins to run smoother but still not that nice crisp sound. A second or two later, the linkage nudges just a bit further forward, almost to the positive stop, and then that crisp, smooth firing begins.

I've recently replaced the points and it was running great last time I used it.

Any ideas where to start on this problem?
 
See less See more
#2 ·
Recheck the points and carb settings. Almost sounds like thats the culprit.

Dom
 
#5 ·
possible condenser is bad, my 420 did similar with no power until I replaced it. if you can see the points when running, is there a slight blue spark or bright orange? orange is bad
 
#8 ·
I started work on this post before I saw some of the input from you guys, so bear with me. Some of you regulars are probably rolling your eyes. “Here’s this same Dobson guy with the same problem,…again”. But, in my defense, every time over the last year that I’ve posted, I’ve gotten the tractor back up and running by either repairing the wire between the coil and the points (twice!) or replacing the points themselves. I shied away from the Kirk Engine Point Saver mainly due to availability (I could get the stock points that day. Probably a case of short-term, short-sighted gratification) or diving into the fuel system. I spent some time on the site reviewing old threads with similar issues as well as my old posts and the feedback provided then. As so many have stated here, the wealth of knowledge and insight provided on this site is beyond compare. I am finally coming to grips with the fact I will have to dig deeper than the points and lead.

I had a good day on Friday. After removing the air cleaner assembly and verifying the ‘bogging down’ condition was still present, I did a compression test. Mainly because I saw a couple references in threads (and yours, K B) to this potentially impacting performance under load. Both cylinders came in at 100 psi +/- a couple of psi; definitely within 10 psi, probably within 5 psi. I did notice a difference in the spark plug conditions. One was a powder gray color and the other black. I don’t think this is related to my immediate issue (??) but I assume it relates to the oil consumption that I typically have with this machine. I also verified the points clearance at 0.016”, which actually needed a bit of closing. The new points have a pretty stiff spring to help maintain the setting, Glenn, and I’ve really got that spring compressed to get to the 0.016”. I think it’s almost fully compressed.

So, I was feeling pretty hopeful believing I wouldn’t have to address any valve issues. I removed the forward heat shields and points cover to access the carburetor. I checked to see if the bolts retaining the carb to the inlet manifold were tight (other threads suggested air leaks). They were, at least they seem to be, though access is pretty tight without more disassembly. I hooked the spark plugs back up to check the bogging behavior again, and it was gone. Very little ramp up to operating rpm, and good response to load application (turning on the mower deck pto). I immediately began to assume my problem was again the lead between coil and points. It’s an old wire so I checked the continuity from end to end, twisting it and such to see if the continuity changed – maybe a break under the insulation; but it checked out good. Insulation not cracked. I reinstalled the wire, with a slightly different routing. It has always come out from under the intake manifold, come up between the points cover and heat shield mounting plate, then squeeze between the plate and points cover again, to go down under and come up inside the points cover. This always seemed to me to be some tight turns with potential of cutting the insulation and causing a ground. So I kept the lead under the mounting plate and only coming up to the slot to enter the points cover.
Started the machine up again to insure everything was still working well. It was. So, I reassembled the heat shields and air cleaner assembly, ready to go cut grass. Started her up and problem was back.

I did change the condenser, Knotty. It was actually a day before Friday. I had changed the points with no improvement, so I changed out the condenser to. Same thing happened. Seemed like I had it fixed. Put everything together and the problem was back.

After the holiday, I’ll go back and check the points again, Glenn, but I’m ready to go buy a carb kit and dive in a little deeper.
Sorry for the long post. Just hoping to give you guys as much input as possible.
Thx again for all your suggestions
 
#10 ·
when it bogs down, does using the choke have any effect? if no effect then electrical IMO
 
#11 ·
Compression sounds decent at 100psi, at the very least 'its not dead yet'

I think the color difference in the plugs is telling you something, I'm just not sure what. did you clean them when you had them out?

when was the last time you replaced you plug wires?
 
#12 ·
Ditto on the plug wires, I just get copper core wires, pre made at the auto parts store. O'Reillys it was, give the guy the old wires and he goes back and matches them up. I never put carbon core wires back on.

BUT every time you mess with the wire from the coil to the points it works. I wonder if that connection terminal on the Coil is loose inside the coil???
 
#13 ·
I'm finally going to get some time today to work on this machine. Thanks for all your inputs.
I'll be checking on the choke effect and playing with the plug wires to see if that causes or improves the behavior. O'Reilly's, eh? I have one right down the street.
I'll tweek the coil terminal too and see what happens.
KB, I'm with you...those spark plugs are telling me something. I wish I knew what!
Thx again.
 
#14 ·
Wow! First thing I looked at was the spark plug wires. They're old and crackly, so I know I have to change them. But also, as they leave the coil, one was laying right on top of the other. I did a minor rerouting of the wires and put some of that ribbed conduit around the wires to give them more separation, as well as to keep them from perhaps arcing (??) to other metal parts.
Fired right up and sounds great.
Dumb question though, is there any alignment between coil terminal and cylinder? Another way to ask it, does it matter which coil terminal is wired to which cylinder or are they interchangeable?? I could eliminate the one wire laying across the other by swapping coil terminals.
This really makes my day!!
Off to the store for some plug wires.
Thanks again to all of you for the help.
 
#17 ·
Well, you learn something new every day. I went to remove the spark plug wires to find that they just don't pull off the coil!! They're threaded caps and the end of the plug wire is just cut off; a flat cut. And on of them looks like they don't have such good contact - a little dark.

So, when I go to install a set of new wires, how much conductive material should be sticking out of the end of the wire to insure that I have good contact?
 
#18 ·
cut them flat or with wire sticking out a mm

sparks jump across so good contact is not needed

if they are cracked, the spark can jump

carbon wires work fine as all the gaps will still let the spark jump unless it has cracks and the carbon falls out

there has been lots of myths/stories about wires vs carbon over the years I just get whats cheapest unless its HEI that requires a thicker insulation wire.

my dads old gravely which never had points, condenser or wires changed runs fine even after 40+ years
 
#19 ·
mark
it sounds that you have the original screw top
coil towers.
they have a small tapered post inside.
your original wires were stranded solid core,
with the resistor built in the end.
the originals had a red boot on the plug end,
that was stamped whitacre, or onan.
the ORIGINAL wires are VERY good, compared to
the current P style wires.
thank you boomer
 
#20 ·
I've Never replaced carbon wires with carbon on a small engine. Resistor plugs or the resistors like Robert said came OEM, not both. Carbon wires are a Bad thing, if you put them on and clamp so they don't move or never flex them when removing the plugs (fat chance) they are fine, otherwise NOT.
Knotty your dad had copper core and a lower output HV ignition vs modern.
 
#21 ·
I now have new wires - sort of. I must have had original wires as well as original screw top coil towers. The boots are red and say Whitacre. I had copper core plug wire left over from a plug wire kit for my Ford 600. I pulled the boots off the old wires and got the plug end connector from them as well. You say that bulky thing on the plug end is a resistor, Boomer? Anyway, I assembled them back onto the new wire, cut the coil end flat. I had to do a little cleanup on one of the coil's tapered posts. It had some corrosion on it. I then put a dap of di-electric grease on the end of the wire and pushed it in. The 'grommets' the screw-on tower caps are suppose to squeeze down on the wire to provide a tight seal are very hard but I've wrapped the ends with some electrical tape for the time being.

Runs strong now. Even got some grass cut before dark.
Thanks again for all your help, guys!
 
#23 ·
I've mentioned this in the past. You really want the suppression. It slows/lengthens the spark duration for a much fuller burn. Some correct lean fuel mixture engines will hardly run when the suppression wires, plugs or rotors are replaced with solid items. Some people will increase fuel mixtures to overcome the solid ignition they just installed and say the eng can now make more power. Wrong, and at a greater consumption of fuel per HP. I've dealt w/ combustion analysis & emissions 60+ yrs. No room for guessing or replacing parts without prior diagnostics. DCD
 
#25 ·
I hardly think an engine designed in the 1960's is a lean burn model. My guess it came with copper wires and resistors were added to keep the RFI out of the TV's and AM radios of the day. Now today with the high output ignition systems it can overcome the resistance in the HV system. My other guess is the lean burn is a result of EPA regulations not for performance.
 
#26 ·
Mr. George,
I can't fathom your post. "I hardly think", "my guess", "my other guess".
Lean burn is nothing new, way before '60s.
Secondary voltage was "suppressed by law on passenger cars in early '50s to the best of my knowledge, at least on GM cars. Other larger vehicles(trucks) a little later. I remember testing wires for resistance then as part of tune-ups. Mishandled wires by DIYers and un-knowing mechanics were the biggest problem. Secondary resistance is necessary for good combustion. Modern high output ignition has no advantage as far as resistance wires, etc are concerned. All my engs have them including racing. We're only talking 1,000-2,500 Ohms. Old ignition coils from the early '50s have plenty of poke. The suppression wires actually let MORE spark get through. It may be hard to grasp but it's true.
A tool box which includes a chassis dyno capable of running 100MPH+ under load, a full diagnostic analyzer where you can see the spark burn thru the fuel in miliseconds also helps design chamber shapes plus a multi-gas exhaust analyzer have been essential to me for a long time. DCD
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top