Misfire on light load

[MexBug]

I have a problem. The car misfires or 'jumps' or shakes a little when I drive and its under light load and lower speeds up to 70-80kmh.

I have replaced ALL electrical. Plugs, distributor, rotor, cables etc. It's all new and of the best quality brands I could find.

I changed the fuel pump for a brand new one.
I have a new gasoline tank, new filters(three filters before the pump) and adjusted the carburetor twice.
I checked the float level and changed the valve for a new one. I cleaned the carb, changed all seals in the carburetor.

Engine starts easy without pumping or anything. Holds idle.
Ignition is adjusted correctly.

But still this misfiring.

What is wrong? It started about two months ago.

Carburetor is a Solex 34 PICT 3, engine 1600 DP
Frustrating!

Please help!

[FJCamper]

Hi MexBug,

You know the majority of misfires are electrical, not fuel, so let's start with electrical.

I understand you've replaced all the usual electrical stuff and still have the problem, so let's look outside the usual.

1. Semi-loose spade connectors or worn wire to spade connectors on the positive or negative side of the coil.

2. Ignition switch beginning to go bad, shorting out momentairily. You will feel electrical shorts more at low RPM. High RPM (but minor) shorts get masked by the momentum. You can test this with a jumper wire from the battery positive to the ign coil positive once the engine is running. If the short goes away, it was in the switch.

Try these and tell us what happened.

FJC

[MexBug]

Hi FJC!

Thanks for your tips. I did as you said. The connections at the coil are fine. Pulled a cable from the battery to the plus side and took it for a drive of a few kms. Unfortunately it didn't change the misfiring or 'shakyness'.

The car pulls strong without misfire from low speed in any gear, if I floor it.
The absolute worst misfiring takes place at partial throttle, when accelerating. But it's present more or less constantly at even speed. When the engine is cold it appears to misfire less.

[FJCamper]

Hi MexBug,

OK. Process of elimination.

What are your main jet and air correction jet sizes?

Are you using an electrical idle jet? (big solenoid with wire sticking out the carb's side)

What size idle jet do you have?

Have you removed and cleaned your idle jet, either electrical or mechanical?

What air cleaner are you using?

What's your compression ratio?

Do you have a spring between your distributor and distributor drive gear?

FJC

[MexBug]

Okay, very hard to read the brass as they are a bit worn, the main jet has X55 or Y55 stamped. The air correction I cannot read properly. It seem to end with 0Z or 07. The pilot says 40.

The only with wire I have is the air cut-off valve and ofcourse the electric choke on the other side.

Air box is originial from a fuel injected Mexican beetle. The difference is only that there's no connection for preheated air.

Have not been able to check compression. Need go borrow that instrument.
Sparkplugs look fine when I checked about 3weeks ago.

The spring on distributor is there. A very slim one just at the bottom. I use a Mexican electronic injection. Bosch. I changed the ignition module too but it made no difference. I also tested with another Bosch distributor but still error remained.

[MexBug]

That should be electronic IGNITION. Not injection.

[Marc]

Not familiar with that ignition system but if it requires that the pickup module be grounded, there should be a little braided ground strap silver-soldered between the housing and the breaker plate. If that's missing the breaker plate ground can be intermittent as it moves in response to the vacuum advance signal.

[Piledriver]

The Mexican late L-jet ignition is very like early Vanagon, Hall sensor setup, very nice reliable system.
Still needs a drop of oil occasionally.

Does it have the matching coil and have you done any other maintenance lately?

The solenoid type idle jet--- light throttle running is mostly on the idle circuit.
If that connection is loose or solenoid is gimpy, can cause the symptoms you have.

[MexBug]

Coil is the correct one. It worked fine for a long time then in about two months ago it started acting up.

I do frequent maintenance. Oil change about every 1500km. Engine was rebuilt 2012 and worked like a charm after changing from an old old tiny Solex to a new Bocar. Unfortunately that Bocar had a manufacturing error, after a few months the little ball in the accelerator pump, on the body of the Carb, got stuck and no more accelerator pump. Warranties in Mexico ends when you leave the store, so that kind of sucked. Low on funds, I found this used Solex, installed it and the engine ran fine for over 2000km until this misfiring started.

[FJCamper]

Hi MexBug,

It appears you have the solenoid idle device. That has a wire to it. With the ignition, on, engine off, remove the female wire connector and touch it to the male spade. You should hear a distinct click if the solenoid is working. You can replace the electric solenoid with a G55 idle. That idle circuit feeds your carb up to 2000 RPM or so. If your solenoid device is failing and turning on and off, it can be starving the engine for fuel for a second or so.

You need a 127 or a 130 main jet and a 100Z to a 125Z air correction jet. The 100Z will richen up your top end. The 125Z is kind of a general size that does okay, and most people are happy with it.

If you have something other than a plain coil and points and distributor (or Hall-effect module) I'm not the right guy to be giving any ignition advice. By the time I really understand something, it's long obsolete.

FJC

[MexBug]

I must ask, I have a jet kind of above the main jet, in the bowl. It doesn't appear if I Google Solex. It's marked 55. What is that thing?

[MexBug]

The cut-off valve seen to be working properly. Since the car doesn't hesitate at acceleration I think it can be ruled out. I have a 127 main jet, from another Solex. But it doesn't fit. Bigger threads. The two small jets that sits in the bowl might add up to or close to the desired main jet size? The more modern carbs have two main jets in order to save fuel or lower contamination? Speculating.

Oh, the butterfly valve hole is partially plugged with a pop rivet.

[FJCamper]

http://www.geocities.co.jp/Bookend-Hemi ... retor.html

Hi MexBug,

Look at these photos. Anything look familiar? Sounds like you may have one of the emission control "California" carbs, and if so, likely subject to a bag of snakes in getting it to run right.

Just turning any of the variety of adjustment screws on the carb the wrong way can make it run badly.

FJC

[MexBug]

No not really like in that photo. Found a photo of the jet I'm referring to. http://i30.servimg.com/u/f30/13/20/57/78/bocar_11.jpg

[MexBug]

I think I'll just go find a used "normal" carburetor or get a new one asap...