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Author Topic: acceleration backfire  (Read 3319 times)
nlpirr
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« on: July 11, 2006, 04:53:51 PM »

I have minor problem with my 69 Camaro.

When I accelerate fast, I get a slight miss or backfire. It started to occur recently as far as I can tell. Itís very noticeable with heavy acceleration standing still even after a long warm up period.

I recently started to add Octane booster to my fuel and was wondering if this could be the reason?

If not any ideas and any thought on these octane boosters in general?

FYI...the engine not original and and the car now has a 406 ci.

Thanks for the help again!!

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rich69rs
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« Reply #1 on: July 11, 2006, 06:40:03 PM »

http://www.idavette.net/hib/fuel/index.htm

Some good information on octabe boosters.  General consensus in the article is that the 3 different types of commercially available octane boosters aren't all that they advertise themselves to be and that if you really need higher octane - AV gas is the preferred method.

Adding the stuff in recommended quantities won't significantly change the % blend of fuel and shouldn't cause a miss or backfire.

I think I'd start by checking all of the normal tuning parameters, i.e. timing, AF mixture, etc. to be sure nothing has changed.

« Last Edit: July 11, 2006, 07:09:06 PM by rich69rs » Logged

Richard Thomas
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hotrod68
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« Reply #2 on: July 11, 2006, 11:14:06 PM »

Rich is right--octane booster should have no bearing on it. After checking all tune-up specs, the first thing I'd check would be the accelerator pump adjustment on the carburetor or look for a vacuum leak--sounds like a lean spot when you suddenly hit the gas. If you have a vacuum-secondary Holley it's very easy to fix if you've perchance put a screw in the secondary linkage--just take the screw out. If not, it sounds like not enough initial pump shot if you have no vacuum leaks. The accelerator pump spring on a Holley should never be screwed in so tight that the coils can bind--this is a common mistake that actually shortens the pump shot and makes it less responsive.
   Next I'd check the valve lash--you may have a valve too tight if it backfires through the carburetor, but a cam lobe going flat can do the same thing. If you don't have a rocker arm ticking you may have a valve too tight.   
   It could be a dozen other things, but these came to mind first. Just my 2 cents. Good luck!
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HotRod'68  1968 SS350 coupe undergoing frame-off resto/rod. 386/350/4.11s
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nlpirr
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« Reply #3 on: July 12, 2006, 04:36:48 PM »

This is good stuff.

I think its time for me to relook at the valves (or at least have the experts do it) and Cam shaft too becasue I get alot of clap/slap which I cant for the life oif me figure out? 

Is it possible to have a too big of a cam Sahrt in the engine that causes these noises and misses?

also, I cant really tell if its exhaust driven or not sometimes? I do know I dont have exhause back fire and smoke at all.

How dows one marry the correct Cam sharf size to the engine  and or valves to optimize the running of the engine??

anyhow...thanks again for the help.

I see big bucks ahead of me on this one..

Nick P.

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3Zs
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« Reply #4 on: July 12, 2006, 09:13:33 PM »

Do you have stamped steel rocker arms?  If so, it might be a broken rocker arm.
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hotrod68
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« Reply #5 on: July 12, 2006, 11:40:25 PM »

Do know what cam you have? Is it hydraulic, solid, roller, what? What are the specs? Any cam over .490" lift with early stock heads is going to likely cause problems with coil bind, hence broken rocker arms, bent pushrods, etc.. But if you had a broken valvetrain component you'd have a dead skip on one or more cylinders. If you have a solid cam it'll be a little noisy from the lash chatter, but you haven't given us a lot to go on. Cams don't cause errrant noises and misses--mismatched parts do. I don't know what to tell you without knowing what is in your engine.
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HotRod'68  1968 SS350 coupe undergoing frame-off resto/rod. 386/350/4.11s
Butternut Yellow    black standard interior
nlpirr
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« Reply #6 on: July 13, 2006, 08:35:07 PM »

basically..I have no clue what makes this engine what it is. I never got the true spec's of everything and not sure if the previous owner even knew?

I simply need to get the pro's under the hood and check things out a little at a time to control the costs I suspect.

I brought up the cam only because I read that many people like to use cam's that are not correct for the other components and so forth.

I want to optomize this engine and guess starting with the noises it makes (if they are bad noises or actually normal) i figured would be a good start.

I live in the dallas area and my general mechanic is too busy to really offer me advice and help. So I have been doing a ton of stuff and learning myself, but there obviously is a ton of technical stuff I simply wont attempt.

thanks again and once I get the full specs of my engine, I'll follow up and start all over.

Nick p.
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