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ac 620 loss of power

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So I was having trouble with having loss of power and hard starting. i cleaned the carb and soaked it overnight in the paint can of carb cleaner. After reinstalling the carb and trying to restart with unfavorable results, i discovered a weak/intermittent spark. Got some new points and condenser, installed, timed the engine and it ran like a new machine. Went through taller grass without a complaint.. Came back a couple days later and upon starting out I noticed I had a subtle loss of power, it complained about having to cut the taller grass, so I thought it was a loose belt-nope. After about an hr of cutting, I noticed loud popping out the exhaust like gunfire. my wife was 1/2 mile away and thought I was shooting my rifle.

And still not much power, popping seemed to get worse with giving the engine more gas. Does it sound like I have a sticky exhaust valve? Not sure how to replace/repair valve....

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Brettw

Seems odd that you would have all of these issues show up at one time. Carb, points, condenser, valves. I would check to makes sure the new points did not loosen up? Sounds like the timing started to go bad, and with the new points installed, I would look there first.

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CleanBee

The Onans have a habit of the points boxes becoming loose where they mount on top of the motor. Doesnt take much to an erratic timing.

Dan

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compostibles

Went out to check it again after work this afternoon. It started up ok. It idled fine without the misfiring and popping. I checked the timing, and of course since it was running fine, the marks lined up. I gave it gas, lo and behold, it was running perfectly. So I thought, well, maybe it was a fluke. Having a little over 1 acre left to mow, I hopped on and started out with lots of power. After about 50 feet, it started misfiring and making the loud popping out the exhaust. Used the timing light, and for the most part the marks lined up except for during the misfire cycles. Points box was tight. Couldn''t see any oil in the box or any areas where a short might be, gapped to 0.02".

I called my local onan shop and he thought I should do a compression test-quoted me about 400-500$ for a valve job at the shop. Yikes.

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powerking_one

Jason,

You mentioned:

"After about 50 feet, it started misfiring and making the loud popping out the exhaust. Used the timing light, and for the most part the marks lined up except for during the misfire cycles."

This strongly suggests you simply still have an ignition problem causing the misfire and is probably not valve or compression related. It could be the ignition coil going south/internally shorting out when it gets hot. Perhaps the +12v supply/wire to the coil is shorting out or the points wire to the coil; check their routing & insulation for chafing.

If the coil is suspect, then maybe pickup a used Harley Davidson coil like this to test it (pretty cheap and lots of them out there on ePay)not to mention it's nice to have in one's toolbox if troubleshooting other points ignition systems on single/twin cyl engines:

http://www.ebay.com/itm/Harley-Dual-Fire-7-Ohm-Coil-/130956418924?pt=Motorcycles_Parts_Accessories&hash=item1e7d9c5f6c&vxp=mtr#ht_23wt_919

If the coil is the problem, then wait till you see Onan's price of a new/used one. Case in point, my brother put one of these HD coils on his Kohler KT-17 (and made a simple retro-fit bracket) and it worked perfectly. Kohler's price for a new one was out of this world, like $175 !!.

My 2cents worth,

Tom (PK)

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JP

I would follow Tom's suggestion. Onan's are 'WHACKO' on timing. it needs to be spot on. When coils fail, they go when warm. I would try a coil see if it makes it 'Happy'. One last, what do the plugs look like? Are they Champions?

JP

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compostibles

I think I paid about 90-100$ for a new onan coil a couple of years back.

I'll try your suggestion for the HD coil. Cheaper!

Plugs are champion h8c new last month.

Is there any diagnostics besides different coil that can be done in the meantime?

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Scott Salmons

Watching the discussion closely. My 720 is doing the same thing, rebuilt carb, new plug wires and cleaned points. It idles fine but pops when the throttle is raised, also seems to get worse the longer it runs.

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JP

Coil testers have gone the way of 'Drive in Movies', You could try the 'midnight' test. Running the motor in the dark to see if there's any grounding/arcing going on. Lastly, what color is the spark? White/blue/red?

Scott, Timing timing timing..... (?)

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compostibles

Nuts. I missed out on the end of the above auction by a couple hrs. I see a few other auctions for coils, but I'm a little fuzzy on what specs I would be looking for so as to get the right one.

The last post thought there might be a gasket leak somewhere. I did reuse the gasket between the carb and intake. Maybe?

What are the symptoms of a defective valve? Similar to what I'm experiencing now?

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Scott Salmons

I am waiting for new head and intake/exhaust gaskets from Sandy Lake so I can't say for sure but I think I found the problem with my 720. I removed the heads and pulled the valves, the exhaust valves were really worn so bad they could not be reground, it seems that the exhaust was not closing properly. I could have backed the lifter off and it may have run OK but I went ahead and ground both intake and exhaust, then lapped them in the seat. Hopefully this will fix the problems that I have been having. I should know in the next few days.

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daveb

I just bought a hi output coil and wires for my 720 on ebay for $60 plus shipping but haven't put it on yet. Twisted my ankle real bad and not able to get around.

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Scott Salmons

I put the 720 back together this morning and it runs like a Swiss watch. I ran it all afternoon spraying and knock on wood the problem seems to be gone. After looking at the exhaust valves I know what has happened, they were hammered so bad on the seat that they were not closing all the way thus the popping out of the exhaust. I agree that most of the time it is ignition or carb but sometimes you have to go a little deeper.

Compostibles, just my opinion but I would check my valve lash, like mine your exhaust could be worn down so there is little clearance cold and when it heats up you have negative clearance. Its not hard to check, intake should be .006 - .008 and exhaust should be .015 - .017. Also I would check the valve seat like Acfarmer said, they can work loose.

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compostibles

Alrighty gearheads! I've been a little busy with life and started poking around at things when I can.

1.Regarding compression testing, I cranked the engine when cold and found the cylinders gave me 105psi on one side, and 115 psi on the other side. squirted in a little oil and that only increased the psi by about 5# both sides.

2.Did the after dark test and found that there was no errant arcing anywhere that I could see. Both plugs have spark, blue in color.

3. The engine starts only after cranking on it for a while~15-20 sec and futzing with the choke. When it sputters to a start, it will only run without the loud popping out of the exhaust at low idle and it sounds like it is running on one cylinder. I confirmed this by pulling the wire on the right hand plug and it continued running the same. Any attempts to increase the throttle will cause it to start erratically popping out of the exhaust.

4. The exhaust pipe coming out said right hand side of engine is relatively cool compared to the left side. Did I mention I have spark on both sides? I also have decent pressures as well.

5. Weird thing-I pressed on points box and then it started running smoothly both cylinders. Ok. That would indicate the points box needs some looking at, maybe tightening up to make it have a better connection with the block? However, why would the right hand side of the engine be consistently cooler if the points box seems to be the source of the problem?

Puzzled.

Jason

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Brettw
quote:However, why would the right hand side of the engine be consistently cooler if the points box seems to be the source of the problem?

Puzzled.


id="quote">
id="quote">

For whatever reason your points box is causing problems, it is very possible it is affecting the right side only. Unless you are really good with the ears, it is hard to tell which cylinder may be "bad". One way is to check spark plugs, another would be if one cylinder is "cool" compared to the other. You can't check long, but from dead cold you could get it running and feel exhaust manifolds. They should both heat up about the same, obviously.

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compostibles

Well don't I feel a little silly. Turns out the inside of my points box had a thin film of oil, enough to short it out sporadically. Cleaned it out and it runs like new! Well, new in the sense that it's nearly 40 and is running yet. Simple fix, but it had me baffled. Thanks to all of you.

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