ignition or carb problem???

[playsnmud]

ok 1975 620 with single point dist. stock carb.

 

bought the truck running like poop so i did point,condensor, plugs cap and rotor, wires. runs and idles great now so I drove it for a week and then all the sudden on the freeway it fall flat on its face and starts missings and bucking. so I pull over to the shoulder restart it and it runs and idles great so off i go again down the freeway and again missing and bucking. so I figured out that if I drive like a grandpa it runs drives idles great but once I try to get on it alittle it starts missing but if I just turn the key off and restart it its fine. whats the deal here guys, I think it might be the coil since i had a similar problem a while back with a 510 I had but that one I had to sit on the side of the road for 5-10 minutes before it would restart.

[metalmonkey47]

Clogged jet? Seems as if it's either not getting enough gas, or not getting enough spark.

 

Theres another part that I can't think of the name of...

 

I'm not sure if it has it, but my motorcycle does and it caused a lot of the same problems you seem to be having. I'll have to go read through the manual and find out.

[datsunaholic]

Sounds like a clogged fuel filter to me.

[playsnmud]

I thought it was the fuel filter also so when I got home I bypass the fuel filter and same results

[datzenmike]

I had that once. Drive easy runs good. On the highway under a load it would stutter randomly and run like shit. Stop and shut off count to ten, start and was fine. It was a piece of crap just below the primary jet. At highway speeds or bumps it would stir it up and it would get sucked onto the jet and stay there until you shut it off. Where upon it would settle to the bottom and run fine until next time. 12mm wrench to remove the jet cover and let the gas pore out with the offending clog.

[Chris#51EPGraham]

You're right to try all the basic/obvious stuff first, but old vehicles can have weird problems

with multiple causes. If all the basic items don't fix it:

 

(1) Untape and closely inspect the primary ignition and charge circuits. Look for burned wires

and corroded connections and correct anything that's wrong. Re-make dubious looking connections

with new bare metal splice connectors and/or new wire, reinforce crimped splices with electrical

solder, and insulate splices with heat shrink tubing. Old road race/autocross cars and street rods

with very stiff springs are prone to shaking electrical connections apart. It could happen to a stock

truck too for a variety of reasons.

 

(2) Old and cheap rebuilt alternators can have all kinds of things internally wrong like loose

diodes and armatures. If those parts look bad, replace alt and regulator together with high

quality remanufactured items.

 

This sounds like a battle I had on my '71 510 autox car - irregular misfire under load but runs

fine under light or no load - correcting 1 and 2 in combination finally solved it for good, but it

was hard to diagnose. The car has springs about 4x stiffer than stock and it was my daily driver

at the time the problem appeared.

 

(3) Also...If you've still got a Hitachi carburetor, they don't tolerate dirt from an elderly

gas tank very well. The main metering jets point down into little sumps in such a way as to

draw dirt in rather than let it settle. Weber DGV's are popular replacements because they

tolerate stray dirt in old fuel systems a lot better. The MM jets point upwards and stand

about 3/16" above the float chamber floor in a way that lets stray dirt settle undisturbed.

[playsnmud]

Chris#51EPGraham- thank you I will check out the grounds and alt

 

datzenmike- I think you got the right answer, because it ran beautiful for a week and all the sudden while driving through seattle on I-5 past the tully's brewey with all the bumps, right after that it started all of this. I will try cleaning out the float bowl tomorrow.

[Pumpkn210]

The Pumpkn had this prollem a couple times and it was the Jets! Gotta totally dissas the carb, PITA <_<

[Chris#51EPGraham]

Chris#51EPGraham- thank you I will check out the grounds and alt

 

datzenmike- I think you got the right answer, because it ran beautiful for a week and all the sudden while driving through seattle on I-5 past the tully's brewey with all the bumps, right after that it started all of this. I will try cleaning out the float bowl tomorrow.

 

My suggestions are definitely for the worst case scenario when all else fails. My car had done this with

an immaculate aftermarket fuel system (fuel cell, Holley pump and filter, Aeroquip, pressure guage and

regulator, Weber DGV) which I had already checked repeatedly.

[datzenmike]

Chris#51EPGraham- thank you I will check out the grounds and alt

 

datzenmike- I think you got the right answer, because it ran beautiful for a week and all the sudden while driving through seattle on I-5 past the tully's brewey with all the bumps, right after that it started all of this. I will try cleaning out the float bowl tomorrow.

 

If you look just below the sight glass cover on the front of the carb you will see two 12mm plugs. The primary is the left one, remove the plug and up inside at an angle is the jet. A screw driver that has been 'trimmed' will unscrew it and have it fall down and out to check. Best to remove the carb for this and have a rag handy to catch all the gas. When I removed the plug the gas washed the offending partical right out. Looked like a flake of paint? Install it before checking the secondary so they don't get mixed up.

[banzai510(hainz)]

a worn out point distributor could possibly do this also. usall it will start poping abd back firing.

 

Ck to see if the dist shaft is so loose that it could open the points.

 

if not then it NOT the proplem.

[playsnmud]

so I had some time to play with the truck last weekend. Changed the coil with a new one and that made no differnce. opened up the site glass to see how dirty it might have been but it was clean so I just blew it out. Now what I notice is that the float when it is in your hand and you push on it gas seems to appear like the the float is a sponge, that is normal right?

 

can you explain they jobs of the primary and and secondary? my thought is the primary does the idle to about half throttle and the secondary overlap with the primary in the cruise to full throttle. I don't know anything about carb's besides to turn screws and clean it and make it run.

 

most likely I will have to remove the carb and clean out the primary and secondary and which ever other ports I come across while I have it apart.

[datzenmike]

Now what I notice is that the float when it is in your hand and you push on it gas seems to appear like the the float is a sponge, that is normal right?

 

Not normal. The float is hollow and empty inside so it will .... float. If it is leaking it will fill and sit too low allowing too much gas into the float chamber.

 

 

can you explain they jobs of the primary and and secondary? my thought is the primary does the idle to about half throttle and the secondary overlap with the primary in the cruise to full throttle. I don't know anything about carb's besides to turn screws and clean it and make it run.

 

 

At idle not much air is flowing through the carb and manifold vacuum is high. When the throttle is opened suddenly there is a huge opening and vacuum drops very low. At the same time air does not instantly start moving so there is a delay and the motor stutters and slowly picks up speed as the air flow increases and can draw in more gas. To avoid this the maximum carb size is broken down into two smaller barrels. The primary is fairly small to avoid this low speed stutter and allow the motor to rev up and get the air moving. If additional power is needed a second barrel is opened for passing or climbing hills. Running on a small properly sized carb is good for economy and drive ability. (smoothness)

 

The primary has almost nothing to do with the idle circuit. There is not nearly enough air flow to draw gas in and mix it. The primary is almost fully closed and some air leaks in. The fuel for this is metered by a small idle adjust screw.

[banzai510(hainz)]

sell me that truck for 500$ and you wont have to worry about it any longer.

 

 

as the float sinks there is a litlle needle valve /maybe ball bearing theing that will let gas in. when the float is at top the valve is closed till the gas level lowers and then gas is let in again.

 

most driving is idle jet and then the main jet.

the 2nd barrel is vaccum is youll have to go alot faster.to get that diaphram to open the 2nd plate on the carb.

 

I would do a valve adjustment justo make sure your not running on 3 cylinders are barely running on 4 cyl. then pull the center plug wire and place near ground. cran over if good snapping spark the eleltrical is OK. Ck the timming. then look at the carb.

 

Run soem carb cleaner thru it.

[playsnmud]

I have it up for sale on craigslist for $2000 and I was hopeing to get it running better for the guy but he wants it now so he knows it runs and drives fine till it messes up and he doesn't care about that since he will be going through the truck. he is picking it up on saturday.

[banzai510(hainz)]

Man I thought we were tite

500$