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Subject:
Re: Backfiring
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
From: Diode <me(at)aintgonnatellya.com>
Subject: Re: Backfiring
Lines: 29
Date: Sat, 05 Jun 2004 18:30:06 GMT
NNTP-Posting-Host: 24.47.238.100
________________________________________________
Steve G spoke thusly...
> Ignition timing is extremely unlikely to be the problem (read that as
<snip>
>
Steve:
Coupla things...The car did this when I got it a year ago and never
stopped. The carb was replaced because it needed to be, not to stop the
break up. I was hoping that a side benefit of replacing the carb would
have been fixing the break up as well. The tune up was done by my
friend who has owned a repair shop for over 25 years, is good at what he
does and owns a 67 small block Corvette himself. We changed the plugs,
points, condenser, cap, rotor and wires (and all of the filters too, of
course). We set the dwell angle and the timing and checked the advance
at high RPM. According to the receipts I have, the distributor was
re-curved at some point in the past. I assume that was done to properly
match the cam that was put in. After all of that, it was still breaking
up. When I changed the carb a month or two ago, like I said, I was
hoping it would fix ALL of the problems. It runs a hell of a lot better
with the new carb and makes 100% more power than it did, but alas the
breaking up persists.
--
-|>|- Diode -|<|-
`68 L-79 Coupe
`79 Triumph Bonneville
Shut up, dave.
Professional driver on a closed course. Do not attempt.
Actual mileage may vary.
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