8.90 motor misfire/pop on top end

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8.90 motor misfire/pop on top end

Postby radracing » Sun Mar 31, 2019 10:01 pm

I am in need of advice. Have a 8.90 JR race car engine. Freshened over the winter and now has a pop misfire on top end. I have gone up and down in jetting, new coil, new fuel pump, new fuel lines, unhooked kill switches and it still has a pop. The severity has varied but there every run. Any thoughts?
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Re: 8.90 motor misfire/pop on top end

Postby WILD BILL » Mon Apr 01, 2019 7:00 pm

How far did you go with the jet change?

After years of running a 104 main in both my Zr 4's with no issues last year I ended up with a popping and missing down track. Got really bad at Bristol.

I ended up going through everything else before I changed jetting. I ended up with a 109 in it.
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Re: 8.90 motor misfire/pop on top end

Postby radracing » Tue Apr 02, 2019 6:27 pm

Its a 33mm and ive been from 185 to 205.
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Re: 8.90 motor misfire/pop on top end

Postby WILD BILL » Tue Apr 02, 2019 9:51 pm

Check for vacuum leaks around the intake and carb boot.
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Re: 8.90 motor misfire/pop on top end

Postby jtracr » Wed Apr 03, 2019 8:28 pm

How big of a motor? The smallest I run is a 205, I've gone as big as 210-220 on our 8.90 set up. Being a fresh motor I'd say it needs more fuel.
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Re: 8.90 motor misfire/pop on top end

Postby Goose6891 » Thu Apr 04, 2019 8:14 pm

Who built the engine? I ask because the last two seasons of 8.90 were a living ****... I'll be polite and just say our new builder is incredible and found out what what was going on inside/wrong... So glad to finally be in 7.90... running great with NO issues...
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Re: 8.90 motor misfire/pop on top end

Postby Pool Man » Mon Apr 15, 2019 4:46 am

I have had a leaking valve do just that. I know you said it was fresh. Do a leak down just to verify.
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