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 Post subject: High RPM miss
PostPosted: Sat Mar 14, 2009 2:37 pm 
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LUVTruck.com Lifer

Joined: Sun Jun 12, 2005 10:42 am
Posts: 684
Sbc 355 new motor has a slight miss (popping) under full throttle up around the 4500 RPM mark and up. I have the ignition timing set 12* initial 34* total all in @ 26-2700 RPMS. I dont think its ignition related. Could it be secondary jets causing a lean condition?


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 Post subject: Re: High RPM miss
PostPosted: Sat Mar 14, 2009 4:03 pm 
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Location: Pelahatchie, MS
What type of ignition are you running?

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 Post subject: Re: High RPM miss
PostPosted: Sun Mar 15, 2009 1:02 pm 
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LUVTruck.com Lifer

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Its an HEI but with the old motor I was spinning it up to 5500 with out a noticable problem.


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 Post subject: Re: High RPM miss
PostPosted: Sun Mar 15, 2009 6:24 pm 
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I suspect it is the problem,though. They tend to do that...

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 Post subject: Re: High RPM miss
PostPosted: Sun Mar 15, 2009 7:47 pm 
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LUVTruck.com Lifer

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what would make it work better in the old motor? I could spin it hard before. It seems to be be popping just under hard load in the lower 3000 ish RPMs now. Could it be something going bad in the dist. or is it because the coil isnt getting saturated?


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 Post subject: Re: High RPM miss
PostPosted: Mon Mar 16, 2009 7:25 pm 
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Location: East Central Iowa
Is it popping out the exhaust or the intake?

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 Post subject: Re: High RPM miss
PostPosted: Mon Mar 16, 2009 8:13 pm 
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LUVTruck.com Lifer

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From the best I can tell I am thinking it sounds like it is on the intake side.


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 Post subject: Re: High RPM miss
PostPosted: Mon Mar 16, 2009 8:16 pm 
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Location: East Central Iowa
Could be something in the valvetrain, flat cam lobe, bad lifter or rocker arm out of adjustment.

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 Post subject: Re: High RPM miss
PostPosted: Tue Mar 17, 2009 8:16 am 
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Location: Hermiston,Oregon
Check the ign. system if your new motor has more comp. then the old motor the ign. system wont handle it.Your total sounds a few deg. low to me you may want to put a MSD coil and module in the dist. it will help your spark a bunch.Also check your wires and plug gap.
I run a HEI that i modified on my Buick and have no problems up to 6000 but i also tossed the vac. adv. as well as the stock one will not drop out like they are supposed to when you floor it and they are for fuel economy anyway.Oh and if the vac. adv. pin is loose in the dist. it will let the timing flutter at higher RPM's.

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 Post subject: Re: High RPM miss
PostPosted: Tue Mar 17, 2009 5:33 pm 
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LUVTruck.com Lifer

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I had a little trouble adjusting my valves I am using some stamped steel roller rockers that seemed to make a lot of noise and at one time I think they may have been a hair to tight. When I had them a little looser and clattering a bit I dont recall the problem but with the noise catching my ear I dont remember flogging it hard either.

I dont know what else to check on the ign system. plugs are new and gapped to .045 wires are new msd factory crimped 8.5mm. cap and rotor are new bosch units with brass terminals and vac advance is disconnected from vac. As far as testing the rest of the parts in the distributor are there any simple tests I can do at home to check it out?

I am also using a moroso recurve kit with the softest springs giving me full advance (34*) by 2700 RPM.

I will try to get out and readjust the valves in a day or two. I will do them with the motor running and hot. Should the stamped steel roller rockers make more noise than the factory rockers?


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 Post subject: Re: High RPM miss
PostPosted: Wed Mar 18, 2009 10:39 pm 
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LUVTruck.com Lifer

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Re adjusted valves I didnt notice the popping but the stumbling remains. I dont know what else to do now. I also paid more attention to detail while test driving. cruising at around 2500RPM smash the throttle and thats where it starts I thought it was at a higher RPM but its instant.


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 Post subject: Re: High RPM miss
PostPosted: Thu Mar 19, 2009 1:48 am 
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Location: Minden, Nevada
Induction vacuum leak or carb problem. spray some water around gasket areas. Check power valve if holley.. Run a compression (always hot) test after running the valves.

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 Post subject: Re: High RPM miss
PostPosted: Thu Mar 19, 2009 1:09 pm 
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LUVTruck.com Lifer

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I have checked for vac leaks I thought of it as the first problem sprayed water evreywhere and nothing I even pulled the intake manifold and installed new gaskets for insurance. Carb is a Edlebrock performer.


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 Post subject: Re: High RPM miss
PostPosted: Thu Mar 19, 2009 3:37 pm 
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Sounds like 2 issues, and you solved one of them. The stumble is probably in the carb. It ran good on the smaller motor, but now it needs more fuel on the bigger one. An edelbrock is the old carter's they were known as thermobogs...

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 Post subject: Re: High RPM miss
PostPosted: Thu Mar 19, 2009 5:13 pm 
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LUVTruck.com Lifer

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Would stepping up on the secondary jet size help by giving it more fuel? I am going to go out in the garage and try to change out the distributor before my wife gets home. I have a known good HEI sitting on the shelf. Someone suggested to me that a magnetic pickup wire could be bad and causes the problem that I am having. So I guess that is my final fix until I come up with something else other than a "FOR SALE" sign.


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 Post subject: Re: High RPM miss
PostPosted: Thu Mar 19, 2009 8:27 pm 
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I would try a Holley, if you know someone with a good one to loan if the dist. swap doesn't work...

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 Post subject: Re: High RPM miss
PostPosted: Thu Mar 19, 2009 8:44 pm 
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LUVTruck.com Lifer

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I need the carb on it for emissions reasons. After I get it all set up I plan on switching to a holley. I dont know anything about the holley carbs or anyone I can borrow one from for testing. I got the dist switched out I need to set the timing and take it for a run tomorrow.


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 Post subject: Re: High RPM miss
PostPosted: Fri Mar 20, 2009 1:09 am 
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Location: Minden, Nevada
Some of the carters had three holes in the accelerator lever. You might try one of the others. If it does this above 3500 or so does it ever clear out? There is a weighted plate in the secondary that controls how quick the a/f get flowing.. Check to make sure its not stuck open, that usually causes a bog, however clears up as the rpm goes up.

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 Post subject: Re: High RPM miss
PostPosted: Sat Mar 21, 2009 12:09 pm 
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LUVTruck.com Lifer

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switched out the dist. with my old one on the shelf and the problem is solved. I need to swap out the recurve kit and new cap and rotor and she should be good to go.


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 Post subject: Re: High RPM miss
PostPosted: Sat Mar 21, 2009 12:51 pm 
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Location: Hermiston,Oregon
:roll: Hmmm

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