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Author Topic:   Backfiring through Intake
roche314
Dirt Roller

Total posts: 18
posted September 27, 2005 06:16 PM  
I have basically a stock Chevy Small Block 350.
During the last race I started hearing a loud popping sound during heavy acceleration and loss of power. I got a flat tire so I only ran a few laps like that. Here is what I have done so far:

1. I thought it was an exhaust leak and found that where the exhaust manifold bolts to the exhaust pipes had come loose. I replaced the donut gasket and still backfiring.

2. I replaced the exhaust manifold gaskets. still backfiring.

3. Now I realized that it is backfiring out of the intake manifold. It only happens when you rev the motor, and the quicker you rev the motor the more it backfires. I can actually feel it in the throttle linkage and see gasses (not flames) shooting out the carb.

4. I moved the timing back to about 5 degrees advanced at idle, made sure all of the plug wires were connected correctly, checked the distributor cap for signs of wear, replaced carb to manifold gasket, set carb float level. still no luck.

Does anyone have any advice?

YAZ
Dirt Maniac

Total posts: 147
posted September 27, 2005 07:17 PM  
pull the valve covers off and check
for a broken valve spring.

way2fast72
Dirt Freak

Total posts: 303
posted September 27, 2005 07:31 PM  
check that your camshaft hasent gone flat on a exhaust valve.

andykmod
Dirt Freak

Total posts: 351
posted September 27, 2005 07:36 PM  
Did the timing chain jump a tooth or maybe distrubtor gear went to heck. You said the engine is stock, does it have a hydrulic cam maybe a lifter took a ****.

eenfield
Dirt Maniac

Total posts: 116
posted September 27, 2005 08:14 PM  
While your in there checking out springs, look over your rocker arms- exhaust in particular...Been there done that.

roche314
Dirt Roller

Total posts: 18
posted September 27, 2005 10:41 PM  
thanks few questions before i get back under the hood:

- what would cause the timing chain to jump a tooth, is the only way to check this by pulling apart the front of the motor to get the timing cover off?

- when i pull off the valve covers, what should i look for when i inspect the rocker arms?

- also i recently put new lifters & pushrods in, as one had sunk and the rocker arm fell off the pushrod. the motor ran fine for a complete race with the new lifters.

Xtreme12x
Dirt Forum Champ
Total posts: 874
posted September 27, 2005 11:04 PM  
Your balancer slipped the outter ring. When you time the motor it isn't timing in right. Check your TDC Mark with a TDC Stop, it'll come with directions.

sc1 racing
Dirt Freak

Total posts: 419
posted September 28, 2005 06:11 AM  
it will be either a sticking valve. maybe bent, or weak valve springs.

STREET16
Dirt Maniac

Total posts: 158
posted September 28, 2005 11:39 AM  
Broken push rod, broken rocker arm, or rocker arm backed off. What ever it is will probably be obvious when you pull the valve cover.

avila
Dirt Freak

Total posts: 217
posted September 29, 2005 09:50 AM  
try a compression test. I had the same problem this year. compression test found the dead cylinder, turned out to be a bent
intake valve.

roche314
Dirt Roller

Total posts: 18
posted September 29, 2005 12:49 PM  
Looked under the valve covers, everything looked fine.

Did a compression test, all cylinder's between 100-110.

Did a leak down test, all cylinder's in the green, between 10%-20%.

Next I am going to take a good look at the distributor, and timing, just to make sure I haven't overlooked something.

Could a leak in the intake manifold cause a lean condition, thus causing the incoming gas to combust before it makes it's way into the combustion chamber?

Could an exhaust leak be creating enough backpressure to prevent the cyliders from completely emptying and then when the intake valve opens cause the incoming gas to combust?

Since it only happens when I rev the motor, could it be from an intake valve staying open during the combustion stroke only during higher rpms? If so how would I detect that. Could I disconnect a plug, rev the motor, and if it doesn't backfire then that cylinder is at fault?

leapinlizard
Dirt Freak

Total posts: 402
posted September 29, 2005 02:45 PM  
When you do the leakdown, start lookin and listening around the exhaust and valve cover filler hole as well as the carb throat for scaping air. This will clue you in to what's leaking.

You might wanna check the pump and filter. You might have a lean sneeze.

roche314
Dirt Roller

Total posts: 18
posted September 29, 2005 03:19 PM  
It never ends.
I pulled distributor out looked at the gears, everything looks fine. Found compression stroke TDC on #1, double checked through the spark plug hole that the piston is at the top of the chamber, put the distributor back in ready to fire #1. Double checked plug connections. Motor will not start. I am getting fuel and spark. Is it time to pull the timing cover?

STREET16
Dirt Maniac

Total posts: 158
posted September 29, 2005 07:35 PM  
It might be easier to rebuild the motor. OK, does it even try to start? When you say you're getting spark. I will assume that you've checked with a spark plug on a spark plug wire. If so, I would check the timing chain. Have you ever rebuilt this engine? The reason I ask, is GM had some plastic looking timing chain teeth that were notorious for causing problems. I have pulled, rebuilt, and reinstalled a chevy 350 in one day. And you can rebuild one for less than $100.00 if you don't need any machine work. Something to keep in mind if you continue to have problems.

roche314
Dirt Roller

Total posts: 18
posted September 29, 2005 11:55 PM  
Timing chain was fine.
I am planning on building a new motor over the winter, but I would like to make it to the last 3 races with this motor this season.

sc1 racing
Dirt Freak

Total posts: 419
posted September 30, 2005 07:57 AM  
have someone check your timming.get it runing up to temp, then shut it off. pull plgs and pull a comp check. hold throttle wide open when you check compression(wet).if running a holley carb also check power valve.

roche314
Dirt Roller

Total posts: 18
posted October 03, 2005 03:54 PM  
well, i finally found the culprit. Bent pushrod and burnt valve.
Thanks for all your advice.

roche314
Dirt Roller

Total posts: 18
posted October 16, 2005 10:00 AM  
found the real culprit, lost a cam lobe on #6 exhaust.

goetzracing58/78
Dirt Newbie

Total posts: 3
posted October 28, 2005 11:17 PM  
have you checked your carbautor to see if a jet came loose or a power vaule went becuasethat happend to us. does the spark pulgs fual. well check those and let me know
Royce Goetz #78

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