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Old 05-04-2021, 10:00 AM   #1
Cp1
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Default J38 popping, no power

Have a J38 in a G2 that has me baffled!
It idles good, revs up with no load, but starting , it sometimes backfires thru the carb and then hard starting. Putting a load on it trying to drive it it has no power, seems to have intermittent spark , tried a different spark plug , has a good spark cranking, jumps at least 1/4 inch and steady. Checked compression, it’s 120. Adjusted the valves, checked and cleaned carb and jets.
Can something be intermittent with the Ignition system as it increases in RPM?
Any thoughts or have you seen one doing this?
Thanks
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Old 05-04-2021, 04:15 PM   #2
slonomo
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Default Re: J38 popping, no power

I have had ignition systems break up as they heat up, and have seen rpm related issues with cdi's. Not on the cart you are working on, just in general (cars, trucks, atvs, small engines). So it's possible to have an ignition issue. The problem is, you won't know until you try a new cdi/ignition module. As far as I know there is no way to test them reliably. I have seen ignition modules test good, then run like garbage, then see the vehicle run great with a different module.

If you know someone with the same cart, you 'could' swap one to another and give it a whirl.

What carb are you running? If it's a cheap $50 aftermarket carb I'd suspect that as a possible cause. They are not always good quality, some yes, but most of them are not nearly as good as a stock unit, usually Mikuni.
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Old 05-04-2021, 07:20 PM   #3
Cp1
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Default Re: J38 popping, no power

Thanks for your thoughts 😀
Seems so much like the spark is intermittent,when you are driving it. It’ll run pretty fair in neutral or the wheels off the ground. So I was watching the spark, making it jump a gap and really looks to be steady. Tried still another spark plug and still the same, choking slightly,still the same. So hooked up my timing light to it and made a temporary mark on the clutch. When it is running good the mark is steady. Then sometimes when it is acting a little strange there seems to be a timing light flash way advanced, also, with the normal flash at about 22 degrees advance at full RPM. The timing advances with RPM increase, so I’m thinking this is built into the module. But with all the backfiring, sometimes, I wonder if the module is throwing out some ghost signals for the coil to fire. Have to study more on this ignition system when I have time.
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Old 05-04-2021, 07:42 PM   #4
raydhd
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Default Re: J38 popping, no power

A compression reading of 120 psi is really low for that engine. The manual list the minimum @ 142 psi. Does it smoke ?
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Old 05-04-2021, 09:08 PM   #5
smallblock450sl
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Default Re: J38 popping, no power

Quote:
Originally Posted by raydhd View Post
A compression reading of 120 psi is really low for that engine. The manual list the minimum @ 142 psi. Does it smoke ?
I agree, compression is low. Did you do the test at WOT ? Might try this, unscrew the spark plug cap. Cut a little section off the coil wire and screw the cap back on. It's worth a shot for the ignition system.
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