Bogging under full throttle - The 1947 - Present Chevrolet & GMC Truck Message Board Network

TheCougarine · 07-07-2016, 02:14 PM

Hey guys, so my 71' GMC wants to bog down under full throttle. It'll accelerate to about 45 then will just stay there but if you let off the gas it then has no problem going on to 70+ and runs great otherwise. Any ideas on what could be causing this? The 350 was just rebuilt and it has a new q jet and hei distributer along with new everything on the outside of the engine. Thanks in advance.

davepl · 07-07-2016, 02:19 PM

When you plant the throttle and it bogs, does it "catch up" with itself and take off in a few seconds or does it stay completely bogged out?

Normally this would be:

a) Accelerator pump not working or undersized
b) Vacuum doors opening too quickly

TheCougarine · 07-07-2016, 02:26 PM

It will catch up with its self up until 45 or so then completely bogs down. Its hardly smooth up until then though its a slight bog then go and does this repeatedly until it completes bogs out.

davepl · 07-07-2016, 02:42 PM

Could it be running out of fuel, like a weak pump or dirty filter? Sounds like it can't keep up with the demand somehow....

TheCougarine · 07-07-2016, 02:46 PM

Thats what I was thinking but the fuel pump is brand new, like put on today brand new haha. The fuel filter was also just replaced. The only thing I can think of to change out is the small inline filter right before the carb. Its also fairly new but I guess it could have something in it.

**Zeke's Garage** · 07-07-2016, 03:44 PM

Do you hear the secondaries open when it's bogging? Also you say that if you get off the gas, and ease into it, you can get it to go up to 70+.

Sounds like the jetting is too rich on the secondary metering rods when the secondaries kick in. I think that if it couldn't get enough fuel, you'd get lean backfire through the carb as it was running out of fuel.

TheCougarine · 07-07-2016, 06:50 PM

Well it does seem to be backfiring as well under full throttle.

garyd1961 · 07-07-2016, 07:24 PM

Where's the timing set at, maybe too much advance. I had to change out the bushing in my MSD because it was doing similar to yours. It would run fine until the timing advance came in then it would bog.
My crate engine only has 0 marks, no degree marks on the tab or balancer so I set it by ear. Runs great though.

TheCougarine · 07-07-2016, 08:15 PM

I'm not sure honestly, I'll have to check

garyd1961 · 07-07-2016, 10:30 PM

Quote:

Originally Posted by TheCougarine

I'm not sure honestly, I'll have to check

Just try backing off on the timing some and see if it helps. If so you will know you are on the right track.

TheCougarine · 07-08-2016, 07:52 PM

Update: Looks like this new fuel pump is bad. This is the second new pump thats gone bad within about 25 miles if that. Now I'm thinking it might have not been feeding the carb enough gas under heavy load. Anyone have any suggestions for a good mechanical pump? Seems like I'm having bad luck with the ones from the local parts stores.

07-07-2016, 02:14 PM	#1
TheCougarine Registered User Join Date: Sep 2014 Location: Johnson City TN Posts: 311	Bogging under full throttle Hey guys, so my 71' GMC wants to bog down under full throttle. It'll accelerate to about 45 then will just stay there but if you let off the gas it then has no problem going on to 70+ and runs great otherwise. Any ideas on what could be causing this? The 350 was just rebuilt and it has a new q jet and hei distributer along with new everything on the outside of the engine. Thanks in advance.

07-07-2016, 02:19 PM	#2
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	Re: Bogging under full throttle When you plant the throttle and it bogs, does it "catch up" with itself and take off in a few seconds or does it stay completely bogged out? Normally this would be: a) Accelerator pump not working or undersized b) Vacuum doors opening too quickly __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

07-07-2016, 02:26 PM	#3
TheCougarine Registered User Join Date: Sep 2014 Location: Johnson City TN Posts: 311	Re: Bogging under full throttle It will catch up with its self up until 45 or so then completely bogs down. Its hardly smooth up until then though its a slight bog then go and does this repeatedly until it completes bogs out.

07-07-2016, 02:42 PM	#4
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	Re: Bogging under full throttle Could it be running out of fuel, like a weak pump or dirty filter? Sounds like it can't keep up with the demand somehow.... __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

07-07-2016, 02:46 PM	#5
TheCougarine Registered User Join Date: Sep 2014 Location: Johnson City TN Posts: 311	Re: Bogging under full throttle Thats what I was thinking but the fuel pump is brand new, like put on today brand new haha. The fuel filter was also just replaced. The only thing I can think of to change out is the small inline filter right before the carb. Its also fairly new but I guess it could have something in it.

07-07-2016, 03:44 PM	#6
Zeke's Garage Senior Member Join Date: Feb 2010 Location: Kennesaw, GA Posts: 288	Re: Bogging under full throttle Do you hear the secondaries open when it's bogging? Also you say that if you get off the gas, and ease into it, you can get it to go up to 70+. Sounds like the jetting is too rich on the secondary metering rods when the secondaries kick in. I think that if it couldn't get enough fuel, you'd get lean backfire through the carb as it was running out of fuel. __________________ -- '67 Chevy Stepside 250 Inline 6 Megasquirt Fuel Injection T5 5 speed conversion Ferric Oxide impregnation throughout

07-07-2016, 06:50 PM	#7
TheCougarine Registered User Join Date: Sep 2014 Location: Johnson City TN Posts: 311	Re: Bogging under full throttle Well it does seem to be backfiring as well under full throttle.

07-07-2016, 07:24 PM	#8
garyd1961 Registered User Join Date: May 2016 Location: Henderson NC Posts: 975	Re: Bogging under full throttle Where's the timing set at, maybe too much advance. I had to change out the bushing in my MSD because it was doing similar to yours. It would run fine until the timing advance came in then it would bog. My crate engine only has 0 marks, no degree marks on the tab or balancer so I set it by ear. Runs great though.

07-07-2016, 08:15 PM	#9
TheCougarine Registered User Join Date: Sep 2014 Location: Johnson City TN Posts: 311	Re: Bogging under full throttle I'm not sure honestly, I'll have to check

07-08-2016, 07:52 PM	#11
TheCougarine Registered User Join Date: Sep 2014 Location: Johnson City TN Posts: 311	Re: Bogging under full throttle Update: Looks like this new fuel pump is bad. This is the second new pump thats gone bad within about 25 miles if that. Now I'm thinking it might have not been feeding the carb enough gas under heavy load. Anyone have any suggestions for a good mechanical pump? Seems like I'm having bad luck with the ones from the local parts stores.



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