BB performance question: - The 1947 - Present Chevrolet & GMC Truck Message Board Network

franko72 · 03-26-2005, 09:59 PM

My 402 BB has me stumped. It seems to really run strong when it I punch it down at 30 mph. It doesnt turn the tires over from a stand stiill run. I thought that a BB should fry the tires from a starting position.I have the timing adjusted and the rpms are correct. I checked the vacuum at idle with motor warm and get about 12 to 15 inches of vac.I bought a rebuilt carb.hoping that it would help. no difference.The motor runs quiet , doesnt miss or stutter.Idles perfect,starts great. just doesnt seem to have the power that I think it should. any suggestions?motor has 125,000 but doesnt use any oil. Its a C20 with 3:54 gears 400 turbo.I have adjusted the valves also. need some answers.

HeavyD · 03-26-2005, 11:45 PM

With a 402 and those gears it should have no problem spinning the tires. Are you sure of that ratio in the rear?

Bob Moore · 03-27-2005, 12:13 AM

On the other hand, that is a heavy truck with large tires. Those 2 things are working against tire spin. Do you hold your foot on the brake, raise the RPMs to increase launch power, and then stomp it or do you just step on it from idle? On the positive side you won't be breaking parts.

Just drive it - Bob

Big Block · 03-27-2005, 02:10 AM

Make sure your vacuum advance is working properly. Hook up your timing light and see if the timing is advacing as you increase RPM of engine. If it doesn't check your distributor. You can bump up your timing and get an increase in power but your gas mileage will drop, and engine may run a little hotter. B.B.

franko72 · 03-27-2005, 10:11 AM

It has 16.5" tires and they wont spin from start either way . If i am going around a corner real slow and punch it the tires will white smoke and burn into second gear.It just has a lame hole shot.How much vacuum should I have at normal operating temp.?It seems to bog down when I punch it from a stand still position.

Rdrdave · 03-27-2005, 01:52 PM

i run my trucks advanced to about 14*. I think the stock timing is around 8-10. I was able to get a rolling scratch. That was awesome!

mr402 · 03-27-2005, 06:26 PM

First I'd start with a good tune up. Plugs, points (if your still running them etc). If an HEI check the centrifugal advance. as well as the vacuum advance. I personally run 12 deg advance (make certain to disconnect the vacuum advance when checking). When checking your timing it's a good time to also verify the advance mechanisms are working.

W/125k miles on it you should check the cam for wear. You might have a few lobes wearing down. Pull a valve cover and see just how much lift your getting. The rockers would likely loosen up as well.

With a 402 and 3.54 gear you should have no problem turning a tire. My GMC 3/4 with the same basic set up (except it's a 4spd and posi) will break them loose a little too easily. That thing gets lots of wheel hop with the leaf springs.

Good luck. This could be hard to track down.

03-26-2005, 09:59 PM	#1
franko72 Franko72 Join Date: May 2004 Location: Cincinnati,Ohio Posts: 471	BB performance question: My 402 BB has me stumped. It seems to really run strong when it I punch it down at 30 mph. It doesnt turn the tires over from a stand stiill run. I thought that a BB should fry the tires from a starting position.I have the timing adjusted and the rpms are correct. I checked the vacuum at idle with motor warm and get about 12 to 15 inches of vac.I bought a rebuilt carb.hoping that it would help. no difference.The motor runs quiet , doesnt miss or stutter.Idles perfect,starts great. just doesnt seem to have the power that I think it should. any suggestions?motor has 125,000 but doesnt use any oil. Its a C20 with 3:54 gears 400 turbo.I have adjusted the valves also. need some answers. __________________ 1972 C20 cheyenne Super 402 BB/400 turbo LWB A/C Tilt, Cruise Control, Saddle tanks, Tach My Home Page Better to burn out than it is to rust

03-26-2005, 11:45 PM	#2
HeavyD Senior Member Join Date: Sep 2004 Location: Wakaw, Sask, Canada Posts: 3,179	With a 402 and those gears it should have no problem spinning the tires. Are you sure of that ratio in the rear? __________________ "You can take the man out of PA, but you can't take PA out of the man"

03-27-2005, 12:13 AM	#3
Bob Moore Registered User Join Date: Feb 2004 Location: Tacoma WA Posts: 1,365	On the other hand, that is a heavy truck with large tires. Those 2 things are working against tire spin. Do you hold your foot on the brake, raise the RPMs to increase launch power, and then stomp it or do you just step on it from idle? On the positive side you won't be breaking parts. Just drive it - Bob __________________ 67 C-20 LWB, tilt, speed alert, vac, 454, T400 67 C-10 SWB, factory air with more to follow

03-27-2005, 10:11 AM	#5
franko72 Franko72 Join Date: May 2004 Location: Cincinnati,Ohio Posts: 471	It has 16.5" tires and they wont spin from start either way . If i am going around a corner real slow and punch it the tires will white smoke and burn into second gear.It just has a lame hole shot.How much vacuum should I have at normal operating temp.?It seems to bog down when I punch it from a stand still position. __________________ 1972 C20 cheyenne Super 402 BB/400 turbo LWB A/C Tilt, Cruise Control, Saddle tanks, Tach My Home Page Better to burn out than it is to rust

03-27-2005, 02:10 AM	#4
Big Block Big Block Join Date: Mar 2005 Location: Durham, N.C. Posts: 1,649	Make sure your vacuum advance is working properly. Hook up your timing light and see if the timing is advacing as you increase RPM of engine. If it doesn't check your distributor. You can bump up your timing and get an increase in power but your gas mileage will drop, and engine may run a little hotter. B.B.

03-27-2005, 01:52 PM	#6
Rdrdave Broke and still trying Join Date: Apr 2003 Location: Alameda, CA Posts: 562	i run my trucks advanced to about 14*. I think the stock timing is around 8-10. I was able to get a rolling scratch. That was awesome!

03-27-2005, 06:26 PM	#7
mr402 Registered User Join Date: Jan 2004 Location: Elmira, Oregon Posts: 449	First I'd start with a good tune up. Plugs, points (if your still running them etc). If an HEI check the centrifugal advance. as well as the vacuum advance. I personally run 12 deg advance (make certain to disconnect the vacuum advance when checking). When checking your timing it's a good time to also verify the advance mechanisms are working. W/125k miles on it you should check the cam for wear. You might have a few lobes wearing down. Pull a valve cover and see just how much lift your getting. The rockers would likely loosen up as well. With a 402 and 3.54 gear you should have no problem turning a tire. My GMC 3/4 with the same basic set up (except it's a 4spd and posi) will break them loose a little too easily. That thing gets lots of wheel hop with the leaf springs. Good luck. This could be hard to track down.



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