Subject:
Backfiring

From: Diode <me(at)aintgonnatellya.com>

Subject: Backfiring

Lines: 32

Date: Sat, 05 Jun 2004 03:00:52 GMT

NNTP-Posting-Host: 24.47.238.100

________________________________________________

OK, I have had this car a year and 3 days now. Little by little I`m

getting things done. However, there`s 2 problems that haven`t gone away

(yet). One of them is backfiring when I get off the gas. It`s the

worst when I`m really winding it out. between each gear, POP! POP!

POP!. Usually it means that air is getting sucked into the exhaust

system pretty far upstream. The connection of the exhaust pipes to the

manifolds is solid and the pipes are brand new. Any ideas? The only

thing that I`ve come up with is a partially malfunctioning diverter

valve on the AIR system. It`s backfiring on both sides and the diverter

valve is common to both I know the valve is at least partially working

because I can feel air blowing out of the muffler on the valve if I open

the throttle and let it snap closed. The part I don`t know (yet) is if

it`s COMPLETELY blocking the air flow to the manifolds on decel. If I

have time this weekend, I`m going to disconnect and plug the AIR hoses

at the check valves. If the problem stops, then I know that the

diverter valve needs help. If it doesn`t stop, then I`m totally lost..

Next problem, breaking up. When a full tune up didn`t fix the

problem, I thought for sure the new carb would. Didn`t. What the hell

is left? I haven`t changed the coil (yet). It just doesn`t sound like

a coil problem. It doesn`t break up until I hit 5,000 RPM, which sux

because it makes max power at 5,500 RPM. That being said, I`m replacing

the coil because it`s the only thing left. Again, any ideas? I was

going to get one of those higher voltage coils from Corvette Central (40

KV). The blurb says that it "fits the stock bracket". Does anybody

know if it fits under the shielding cover? Thanks gentlemen.

--

-|>|- Diode -|<|-

`68 L-79 Coupe

`79 Triumph Bonneville

Shut up, dave.

Professional driver on a closed course. Do not attempt.

Actual mileage may vary.

From: 194377`n`1Z37W2 <please_no_emails(at)myaddy.com>

Subject: Re: Backfiring

Date: Fri, 04 Jun 2004 23:58:32 -0700

Mail-Copies-To: nobody

Lines: 21

________________________________________________

In article <EHawc.37$c76.238282(at)news4.srv.hcvlny.cv.net>,

Diode <me(at)aintgonnatellya.com> wrote:

> Next problem, breaking up. When a full tune up didn`t fix the

> problem, I thought for sure the new carb would. Didn`t. What the hell

> is left? I haven`t changed the coil (yet). It just doesn`t sound like

> a coil problem. It doesn`t break up until I hit 5,000 RPM, which sux

> because it makes max power at 5,500 RPM. That being said, I`m replacing

> the coil because it`s the only thing left. Again, any ideas? I was

> going to get one of those higher voltage coils from Corvette Central (40

> KV). The blurb says that it "fits the stock bracket". Does anybody

> know if it fits under the shielding cover? Thanks gentlemen.

By breaking up, do you mean that it misses at high rpm`s? Seems I recall

reading something on the lines of it could be weak centrifugal advance

springs or the weights are too heavy and swinging out prematurely

throwing off the timing. It may be a distributor problem...

Here`s waving to ya - ||||

Owen

From: "Steve G" <NospamforSteve(at)Steve-Garner.com>

Subject: Re: Backfiring

Lines: 61

Date: Sat, 05 Jun 2004 15:57:27 GMT

NNTP-Posting-Host: 24.71.223.147

________________________________________________

Ignition timing is extremely unlikely to be the problem (read that as

impossible). Early ignition advance will result in engine ping, not uneven

fireing/misfire. In addition, the properly operating ignition system will

have it`s timing advance all in long before you get to 5000.

When you say after tuning it does this do you mean shortly after or

immediately after. If you mean you tuned it a month ago and it was working

fine the day you did it and now it`s breaking up in the high rpm I would

look at the plugs again. I`ve seen numerous occasions, especially on older

engines with lower voltage secondary ign and a few years since any

freshening of the engine itself, where the plugs will start to carbon up

soon. Wrong heat range of plugs, platinum where the engine was not designed

for them, etc. If you`ve covered all these I would next look to the plug

wires themselves. How old are they, any possibility they`re shorting

through the insulation as the firing voltage creeps up. Are you using good

quality ign points if the car still has the points type ign in it. Point

bounce at high rpm was a problem with older systems. Who did the tune up

and did they know how to properly instal points? Seems like a no brainer,

but I`m amazed at how many people are unaware that you must lube the dist

cam and points rubbing block and you must do it in such a way that the lube

won`t spatter onto the contacts. If there was no lube used in no time at

all the dwell will change dramatically and will certainly cause your

problem. Yes, coil can be the problem, but again you said it had all been

tuned. I presume the tuner had it on a scope and tested coil output ( pull

a plug wire and read the voltage spike). If your tuneup was sticking a set

of points and plugs in it under the shade tree and didn`t go any further

than that, go back to the proper tune-up procedures. Run through all of the

checks including vac and mech advance, dwell angle, firing voltages etc.

There is also the possibility of mechanical breakdown. High rpm on weak

valve springs and lifters can have this affect as well. I cut my teeth on

this era of engine and my observations have been that 9 out of 10 times the

problem is related to the tuning mentioned above. The fact that you

replaced the carb looking to correct this problem suggests that there wasn`t

a thorough understanding/knowledge of the problem and system and there may

be an oversight in the tuning aspects. Carb problems are seldom manifested

as high rpm break-up.

Steve G.

"194377`n`1Z37W2" <please_no_emails(at)myaddy.com> wrote in message

news:please_no_emails-122AED.23583204062004(at)corp.supernews.com...

> In article <EHawc.37$c76.238282(at)news4.srv.hcvlny.cv.net>,

> Diode <me(at)aintgonnatellya.com> wrote:

>

> > Next problem, breaking up. When a full tune up didn`t fix the

> > problem, I thought for sure the new carb would. Didn`t. What the hell

> > is left? I haven`t changed the coil (yet). It just doesn`t sound like

> > a coil problem. It doesn`t break up until I hit 5,000 RPM, which sux

> > because it makes max power at 5,500 RPM. That being said, I`m replacing

> > the coil because it`s the only thing left. Again, any ideas? I was

> > going to get one of those higher voltage coils from Corvette Central (40

> > KV). The blurb says that it "fits the stock bracket". Does anybody

> > know if it fits under the shielding cover? Thanks gentlemen.

>

> By breaking up, do you mean that it misses at high rpm`s? Seems I recall

> reading something on the lines of it could be weak centrifugal advance

> springs or the weights are too heavy and swinging out prematurely

> throwing off the timing. It may be a distributor problem...

>

> Here`s waving to ya - ||||

>

> Owen

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