rev limiter, supressor

[JimmyC]

I am having some trouble with my truck and I suspect it is in the ignition. It is a 71o with petronix ignition and civilian cap/rotor/HT leads, and plugs.

It idles fine on all four, but under load it misses and backfires through the intake. At idle I have good spark at all 4 plugs. All plugs except #2 looks good. #2 has black deposits, but nothing horrible. I intend to replace it when I get another one. Timing is good. The coil meters correct resistance. My overspeed light has not come on and has not done so for years. I have never noticed it limiting engine speed. Inside of cap is dry and clean.

I suspect the rev limiter and would like to by-pass it to test. It looks like I can do so by cutting and splicing the black wire and the black/white wire going to the overspeed but was wondering if there is way to do it without cutting wires?

Also they have something between the resistor and the coil. I believe it is some sort of RF suppressor. Do these give trouble? How do you test them? Can I bypass it?

[pcolette]

To eliminate the rev limiter do the following:
-disconnect the 6 conductor plastic plug from the rev limiter.
-prepare a 2-3" wire (14-16 ga) jumper with male spade connectors on each end.
-the plug from the rev limiter has 2 rows of 6 connections.
-using your short jumper wire, plug one end in to the center position of one row and the other end in to the center position of the other row of the plug.
-no connections are made to the rev limiter.

Yes, you can bypass the RF suppressor.

[JimmyC]

Thanks, Paul,

I bypassed the overspeed sensor and it made no difference. I changed my suspicious plug and it ran great for about an hour and started doing the same thing. I pulled the plug and it looked fine. I think it did that just to confuse me. It idles well on all 4 but runs bad under load. Changed the other 3 plugs since I had them and no improvement. Nice blue sparks at distributor.
I am not 100% sure but I think my problem is in #1. Disconnecting it under load seems to make little or no difference compared to the others, but it was hard to tell.

Not sure what I will try next. I might pull the main jets off the side of the carbs and look at them. I guess I could take the top off the carbs and look. It might be a spark plug wire.

Suggestions?

[JimmyC]

If anyone needs to know, the rev limiter jumper needs 1/4" male spade ends.

[pcolette]

As you said, it may be a spark plug wire. I'd try that next. Possibly a hairline crack in the distributor cap?

[pinzinator]

Check the back side of the alternator and make sure all the connections are good. Clean and tighten the battery cables. Check fan belt tension.

[JimmyC]

I think I may have found the problem. I took the spark plug leads off to ohm them and they all ohmed the same at about 1 kOhm, but I noticed some damage on the #1 lead where it snaps into the wire holder at the top of the engine. I wrapped it with electrical tape and everything seems back to normal. Of course it has fooled me once already. I have ordered new leads.

[JimmyC]

Changed the plug wires, the cap and the rotor. I am getting good spark to all four, but I am getting a misfire on #1 under load. I have tried two different spark plugs with gap set correctly. I pulled the valve cover and the valve gaps are normal. I am thinking it is a carburetor problem, mostly by elimination.

I pulled the cover off the side of the front carburetor and looked at the jets. The only thing I thought was unusual was that there were O-rings on the slow jets (700.1.08.405.1/60, 1-40-28) These orings are not shown in my parts book. Does anybody know if they are supposed to be there?

The front one, IE the one that feeds #1, the O-ring was kind of messed up.

I guess I will order a carb kit.

Any thoughts?

[JimmyC]

I took the front carb apart, cleaned it and put it back together and everything is fine again. That is the first time I had a carb problem that was intermittent.