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1977CJ7 11-13-2002 09:19 AM

Strange detonation problem
 
First here is what I have :

258
MC2100
TR upgrade (duraspark)
Freeflow exhaust (flowmaster)
Timing 4deg. btdc

I am Idleing fine, and have good performance, but in my top gear going up a hill, I get detonation at around 1800 rpm, BUT if I give it more gas (almost WOT) the detonation goes away immediately. What do I do? I can't drive around with my foot in the gas all the time.

Jim_Lou 11-13-2002 09:24 AM

Re: Strange detonation problem
 
I had the same problem with my '78. I tried several things, but I think that what cured it was raising the metering rod one notch.

LEVE 11-13-2002 09:31 AM

Re: Strange detonation problem
 
1. User higher octane gas.
2. Check your distributor parts for wear.
3. Check the timing gears/chain for wear.
4. Recheck the vacuum advance for wear.
5. Is the EGR connected?
6. Has the carbon in the cylinders been cleaned out?
7. Is the gas mixture running lean?
8. Do you downshift when it pings?
9. Pinging ain't good in this condition.. it's not a simple timing issue.


1977CJ7 11-13-2002 09:41 AM

Re: Strange detonation problem
 
MC 2100's don't have metering rods

1977CJ7 11-13-2002 09:45 AM

Re: Strange detonation problem
 
Distributor and timing chain are new, vac advance works properly. My idle mixture is a tad rich, I don't know about carbon in the cylinders. Egr is not hooked up...could this be the problem???

I dont downshift when this occurs, I just give it more gas and the pinging goes away.

AllLockedUp 11-13-2002 10:27 AM

Re: Strange detonation problem
 
Manifold [img]images/graemlins/mad.gif[/img] or ported [img]images/graemlins/wink.gif[/img] vacuum to the distributor?

1977CJ7 11-13-2002 10:30 AM

Re: Strange detonation problem
 
Ported

Jim_Lou 11-13-2002 05:16 PM

Re: Strange detonation problem
 
Sorry; I didn't pay attention to your specs. There goes another star [img]images/graemlins/mad.gif[/img] Now NOBODY will believe me!

Here's my (25-year old) recollection of the way my engine was set up: It had a very lean mixture in the mid-range circuit to reduce unburned hydrocarbons. It had EGR to lower combustion temperatures and consequently lower oxides of nitrogen. It also had an air pump to oxidize more hydrocarbons in the exhaust manifold.

The EGR caused a very flat, maybe negative, throttle response. As throttle opening increased, EGR also increased, resulting in a loss - or a disproportionally small increase - in power. Disconnecting the EGR made the throttle response more linear, but then the engine would ping very easily in that range because the lean mixture was burning so hot. Raising the metering rod one notch took care of that.

If you want to keep the Jeep emissions legal, restoring the EGR might take care of the pinging. If emissions aren't a concern (who said that? [img]images/graemlins/wink.gif[/img] ) bag the EGR and richen the mid-range circuit a little, (by whatever means - I'm not familiar with the MC 2100) and the engine will be a lot happier. Who said THAT? [img]images/graemlins/shocked.gif[/img]

Of course, that's after eliminating the other possibilities everyone has mentioned, such as bad ignition advance, carbon buildup, and so on.

1977CJ7 11-13-2002 07:05 PM

Re: Strange detonation problem
 
Other than dissasembly, how would I remove carbon buildup?

RRich 11-14-2002 12:29 AM

Re: Strange detonation problem
 
Make sure your advance isn't going higher than 35 degrees
total.
It may be reaching it too soon, shouldn't get to 35 until
about 2500. Could be you've put in a dist "spring kit?"
or advance diaphram is weak, or ported vacuum too strong
(older carb, newer dist.)

To SAFELY remove carbon, add a jug of Techron to 1/2 full
gas tank, slow but sure.

Some folks will advise pouring water, ATF, kerosene,
Mystery Oil, chemicals etc, even Grandma's cure for
artritus down the intake.
Cold liquids bend hot valves, agressive chemicals knock big
chunks of carbon loose, possibley doing damage. Sometimes
yes, sometimes no. If you think you'd win big in Vegas
today, go ahead.

But it won't go away until you cure the reason it's building
up - if it is - look at a plug to see.


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