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05-02-2016, 03:51 PM
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#1 |
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Registered User
Join Date: Mar 2016
Location: Riverside, California
Posts: 11
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High RPM
Hey guys, so I finally have my '72 C20 back on the road but I have one issue with the truck running at high Rpm when I'm running at highway speeds. Everything seems run fine when I'm cruising below 40 mph but when I try to pick it to highway speed it jumps to almost 4000 RPM and stays there. Just wondering what the possible culprits could be.
Thanks in advance for any responses! |
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05-02-2016, 04:03 PM
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#2 |
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Registered User
Join Date: Feb 2003
Location: Santa Ana, CA
Posts: 2,191
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Re: High RPM
Manual or auto tranny? Carb or EFI? Throttle linkage sticking? Auto tranny kick-down cable not adjusted properly and kicking down too soon? We need more info on your setup to make a diagnosis.
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05-02-2016, 06:50 PM
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#3 |
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Registered User
Join Date: Apr 2013
Location: South Westerlo, New York
Posts: 1,325
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Re: High RPM
A C-20 with 350 CI comes with a 4:10 gear ratio I think, 4:56 optional. That would do it...As cleszkie says, we need more info.
__________________
Thanks, Joe.. 1969 C/10, 348 C.I., 3X2 bbl. V8, 2004r , LWB. |
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05-02-2016, 06:55 PM
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#4 |
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Shortbed club member
Join Date: Jan 2008
Location: Halethorpe, MD
Posts: 439
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Re: High RPM
Sounds about right, unless you replaced the transmission with one with an overdrive.
__________________
71 custom/10 6.0/4L80E SWB ece 4/6 72 custom camper 350/3 spd 2020 Chevy Express 3500 2023 Hyundai Tuscon 1986 GMC 3500 Bucket Truck Sent from my Rotary Phone via 14.4kb/s modem. |
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05-02-2016, 07:14 PM
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#5 |
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Registered User
Join Date: Mar 2016
Location: Riverside, California
Posts: 11
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Re: High RPM
So it's a 4 barrel quadrejet, stock automatic transmission and don't think the throttle linkage is sticking.
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05-02-2016, 08:13 PM
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#6 |
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Registered User
Join Date: Mar 2013
Location: Campbellsville, KY
Posts: 888
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Re: High RPM
By "jump" do you mean engine speed increases all of a sudden without an equal increase in ground speed, or that it just has to spin 4K to do highway speed?
__________________
Alex V. ------ 1967 C10 Suburban, 350/NP435, Green/Green, PS, PB, HD cooling, charging, shocks, and springs. 1985 GMC C3500 SRW, Sierra Classic, 454/TH400, white/blue. |
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05-03-2016, 12:28 AM
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#7 |
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Registered User
 Join Date: Jul 2015
Location: 2nd left past the stump on a dirt road.
Posts: 2,629
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Re: High RPM
My stock 350 with turbo hydramatic 350 transmission runs at approximately 3200 at 60 -64 mph.
At what speed are you going when you are hitting 4000 rpm? If my truck were doing that kind of rpm I would be hitting 80-85 easily. Sounds like you are stuck in 2nd gear, or the kick-down on the transmission is engaging and may need adjustment at the carburetor. Most Motor heads may agree. |
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05-03-2016, 06:51 AM
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#8 |
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Registered User
Join Date: Sep 2015
Location: danbury, ct.
Posts: 568
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Re: High RPM
My 1970 C30 with a 350 and 4 speed with 4.10's turns 3000 rpm at 62 mph.. I think your tranny might be toast and is slipping or you aren't getting into 3rd gear...
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