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620 ignition timing

J

jwerdy

Member
I got a 620 thats lacking some hp on the dyno, only about 42 hp. We rebuilt the carb and tune up kit and adjusted the valves. It has some blow by and idles great with no stumbling during acceleration and runs great on the dyno. It seems like the distributor timing doesnt sound right. Current timing is 20 degrees at idle, which idles and runs great. But if we set the timing to 20 degrees at fast idle (WOT) it runs different at idle- labors. Using timing light. So what am i missing here, and what is the correct adjustment? The carb adjustments are, idle needles set to run at the fastest idle when shorting each cylinder out (1 - 1 1/2 turns out) the load is 3 turns out per tech manual.
 
A good rule of thumb on a dist. ignition with centrifugal advance is to set dist. rotational adjustment to where engine starts the best , lock it down and then tailor the advance using springs/weights to get 25-30 degrees at fast idle on pump regular fuel. With access to a dyno just tailor advance inside dist for highest reading. The tractor will tell you what timing it wants for start timing....the dyno will tell you what max advance it wants for power. Make sure your springs/weights are working freely.
 
If the tractor sounds like it is laboring at idle you"re getting close to the correct setting. You have to have spark to retard fully and set it at the correct speed at recommended idle speed. I" was never much concerned with the full advance because the retard will give your cylinder the full burn time. If you idle is timed to high you will exhaust your fuel at hi rpm. Very common mistake to make. Too much advance you loose your torque and power.
 
Full advanced timing should be set at wide open throttle. I will sound like it labors a little bit at idle. Make sure you have the high speed no load rpms set correctly, a few rpms slow will cost you more hp than you would think.
 
When timing strictly by "ear" its my experience if you keep advancing to where she quiets down and smoothes out at idle THATS TOO FAST but if you retard it slightly till she just pops and labors a tad at idle that's about right.

John T
 
Deere and Company paid their engineers a lot of money to figure out optimum timing. The correct timing for a 620 gas tractor is 0 degrees at idle and 20 degrees at wide open throttle. Ears make poor timing lights!
 
Good Morning Tom 43 aka Barack and Happy Thanksgiving to you and yours. I respectfully disagree "Ears make poor timing lights" Perhaps that's so for your or an inexperienced ear HOWEVER having been a tractor dealer and user of forty years I found I could time by ear and usually get right on at least within a few degrees of correct. OF COURSE that's if you're out in the field or broke down or don't have access to a timing light WHICH I USED ONCE I HAD ONE BACK AT THE SHOP. YES I agree Mother Deere figured out optimum timing which I always adhered to.

John T
 
You have it on the dyno adjust the timing while running for max power. Then check with light to see where it ended up. Report back.

FYI: With some blow by your top piston rings may be shot as I have see it several times on those models ! One had ran so long with broken top rings they worked there way through the piston and ended up hammering against the cyl. head.
 

Cranking timing must not be above 0 degrees TDC..anything before TDC will end up causing the starter-drive to break from Kick-Back..
 
I disagree, the 630 timing was designed for gas that was available at that time.

Best 0 RPM timing is 0 BTDC so it does not kick back , 600 should be about 10 BTDC, then about 24 at 1500 RPM.
 
(quoted from post at 00:03:54 11/24/16) If the tractor sounds like it is laboring at idle you"re getting close to the correct setting. You have to have spark to retard fully and set it at the correct speed at recommended idle speed. I" was never much concerned with the full advance because the retard will give your cylinder the full burn time. If you idle is timed to high you will exhaust your fuel at hi rpm. Very common mistake to make. Too much advance you loose your torque and power.
Click to expand...

Retard will give full burn time , no you are incorrect . Timed too high will exhaust fuel at high rpm ? Total baloney.
 
Here"s what"s baloney....if you don"t set the timing at full retard 0 then run up you will get timing off as most of the aged tractors have aged springs and the timing can actual go slightly higher. Then when you crank at advanced timing above 0 you will break something. Best thing would be to replace the distributor springs and you probably will get accurate on each end. NO baloney here.....just BS but 55 years experience if that's worth anything.
 
B&D, agreed.

Retrarded timing causes too little burn time in the cylinders, it all happens in the exhaust manifold.

We do this on the big turbo engines to get the turbo's hot so they will work.
 

One of the reasons why Deere ran fixed ignition timing on the V4 starting engines that was not advanced as usual at high rpms.
 
The thing that needs checked before any timing is the advance the manual calls for oil under the rotor the large shaft is hollow and needs oil let the to shaft move inside one an another this is the most over looked lube point on the late model gas 2 cyls
 
I got back at the shop today, and adjusted the timing to 20 degrees fast idle (1300 rpm). Hooked up to the dyno and making about 48 hp, which is on par for its rating. Just sounds a little different at idle, thanks for all the advice.
 
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