kaizenator : Backfiring thru carb on 230, adjustment screw Qs

Backfiring thru carb on 230, adjustment screw Qs
Post by kaizenator on Jul 9, 2006, 7:44pm

The tune-up manual says to back the carb adjustment screw one turn out, and then fine tune it after engine is up to operating temp. How many turns out do other people have their adjustment screws?

My timing is on spec, but maybe I need to move it off the mark a little, or the only other thing I think it would be is the air/ fuel mixr screw. It seemed to get worse as I screwed it in, so I backed it out to 2-4 turns outs. There is not much room to work with that adjustment screw, any suggestions?

It still pops out the carb when mashed, what to do?

Re: Backfiring thru carb on 230, adjustment screw
Post by porkchop on Jul 9, 2006, 7:58pm

When mine was popping out the carb I found that I had a bad valve. It was stuck open. Just giving you something else to look for.

Re: Backfiring thru carb on 230, adjustment screw
Post by brute4c on Jul 9, 2006, 8:08pm

Agreed on the popping....if it pops out the exhaust, look at the carb....popping out the carb, ignition or timing or valve problems...

The mixture adjusting screw should work when the 1 turn out setting is used, to get one started....after it is running, it can be properly adjusted by hooking up a tech and turning the mixture screw slowly to acheive the highest rpm possible...occasionally bumping the throttle while doing so...

At the max rpm, the mixture will be at its best your particular engines needs...this varies from truk to truck.

Re: Backfiring thru carb on 230, adjustment screw
Post by kaizenator on Jul 9, 2006, 8:32pm

I hope it is not a problem in the valvetrain. I do notice that when the engine is started after sitting for about a week, it seems not to want to fire in all of the cylinders until it warms up. It may still be trying to burn out the crude that was fed into the carb (someone else's mistake).

The engine wasn't fired up for abt 2 years. It did start right up, though, and I have used ATF after it warmed up. Maybe all of it hasn't burned out yet, but it has stopped smoking like it did. out of the tailpipe, it now just appears a little rich, not thick smoke. It does backfire out the exhaust also ocationaly when mashed, but not as consistent as out the carb.

Re: Backfiring thru carb on 230, adjustment screw
Post by jeepistdougiowa on Jul 9, 2006, 11:13pm

If you had a bunch of crap make it into the carb, you probably need to tear it down and clean/rebuild it.

Very well could have crap in a jet causing a lean condition, sure hate to see you torch a piston over that.

Re: Backfiring thru carb on 230, adjustment screw
Post by gregolma715 on Jul 9, 2006, 11:16pm

Don't mess with the timing. If you have the timing on and the idle properly set it's not the problem.

Re: Backfiring thru carb on 230, adjustment screw
Post by binfordm715 on Jul 10, 2006, 12:40am


Quote: <TABLE class=bordercolor cellSpacing=1 width="90%"><TBODY><TR><TD width="100%"><TABLE class=quote cellSpacing=0 cellPadding=2 width="100%"><TBODY><TR><TD width="100%">The mixture adjusting screw should work when the 1 turn out setting is used, to get one started....after it is running, it can be properly adjusted by hooking up a tech and turning the mixture screw slowly to acheive the highest rpm possible...occasionally bumping the throttle while doing so...

At the max rpm, the mixture will be at its best your particular engines needs...this varies from truk to truck.</TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE>

Is this the screw at the base--the one pointing right at the valve cover so you can't get at it with a regular screw driver?

It's been a good number of years since I've tuned a vehicle. Ain't quite like riding a bike either....

I did manage to find my timing light and dwell/tach gizmo though.

Re: Backfiring thru carb on 230, adjustment screw
Post by kaizenator on Jul 10, 2006, 8:50am

Yes, that screw pointing to the valve cover is the one.

I have thoroughly cleaned my carb out since all the junk, and rebuilt it with new needle/seat, accl pump, and gaskets.

It runs pretty good and isn't backfiring all the time, just when I try to accelerate hard. It kinds seems sluggish to me in the lower RPM range.

Re: Backfiring thru carb on 230, adjustment screw
Post by elwenil on Jul 10, 2006, 8:53am

Run a compression check. You probably have a stuck valve. If that's ok, I'd look at the plugs and wires.

Re: Backfiring thru carb on 230, adjustment screw
Post by kaizenator on Jul 10, 2006, 9:58am

all cylinders hold good compression, new plugs, good wires, new cap, good rotor, working coil.

Re: Backfiring thru carb on 230, adjustment screw
Post by kaizenator on Jul 10, 2006, 10:18am

...new points with gap right.

Once rpms get past the 800-1000 range, the engine is very strong and smooth, and has nice responsive throttle. From idle to this point, however, I have to ease the pedal, I can't stomp it, or it does pop out of the carb.
hummm? Seems like timing or air/fuel mixr to me, but everything is new or like new, and the timing is set to 5 deg BTC, points set to .020?, plugs gapped right @ .030"..etc...etc.

Re: Backfiring thru carb on 230, adjustment screw
Post by hiker on Jul 10, 2006, 11:10am

Backfiring out the carburetor on acceleration is a lean mixture. Somethings wrong with the accelerator pump circuit, most likely. Diaphram torn, missing check ball when rebuilt? With the engine OFF, look down the carb throat and check for a good fuel squirt when you open the throttle.
good luck, Robert- CA

Re: Backfiring thru carb on 230, adjustment screw
Post by kaizenator on Jul 10, 2006, 11:40am

Its got a new accelerater pump and diapham, the check ball was moving nice and freely upon reassembly. There is a nice and steady stream of fuel being pumped into barrel when I rotate the trottle lever.

I am going to re check the timing. would the vaccum advance have anything to do with it or a disconnected vaccum line or leak?

Re: Backfiring thru carb on 230, adjustment screw
Post by brute4c on Jul 10, 2006, 12:00pm

If it is a stock ignition, there is no vacuum advance. Stock has the screw onto the plugs ttype metal braided spark plug wires. Even though there is no vacuum advance on the stock military setup, there are 2 vacuum lines connecting to the ignitor assembly, thats the coil/distributor housing. These are strictly for venting.

Leaving any vacuum line open would cause it to run funny...I would make sure all those lines were hooked up properly before proceeding.

Re: Backfiring thru carb on 230, adjustment screw
Post by jeepistdougiowa on Jul 10, 2006, 12:32pm

I keep thinking "lean pop"

Have you tried partially pulling the choke while accelerating? make any difference?


I know, Intake instead of exhaust, but so hard to tell over the net...LOL

Re: Backfiring thru carb on 230, adjustment screw
Post by kaizenator on Jul 10, 2006, 2:51pm

brute, yes I have a 24v distribr, I typed before thinking, it has no advance, but I need to check over all the vaccum lines. The one going to the master cyclinder may not be sealed well enough, it is makeshift. What should I do with it once I install a different MC that has no vaccum atachment? Can I just plug it?

More choke makes it run better than with no choke! seems lean.

Re: Backfiring thru carb on 230, adjustment screw
Post by jeepistdougiowa on Jul 10, 2006, 2:58pm


Quote: <TABLE class=bordercolor cellSpacing=1 width="90%"><TBODY><TR><TD width="100%"><TABLE class=quote cellSpacing=0 cellPadding=2 width="100%"><TBODY><TR><TD width="100%">

More choke makes it run better than with no choke! seems lean.

</TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE>

Vacuum leak, or crap in the jets...that's my final answer...LOL.

Easiest way I've found to find a vacuum leak is with a small hand held propane torch. Turn it on, but don't ignite it Probe around any possible leaks and you'll here the engine speed change when you find one.

Re: Backfiring thru carb on 230, adjustment screw
Post by hiker on Jul 10, 2006, 3:00pm

I like Doug's idea of slightly pulling out the choke while driving and see if that improves things. Years back when I first got my truck, the mixture was way lean causing some of the problems you discribed. Pulling the choke, temporarily helped it run better. Turning the mixture screw, only helps near the idle range, not midrange or acceleration. I rejetted the carb, in stages, each time test driving to see any improvement. I finally got to the point where performance increased and slightly pulling the choke out didn't help anymore. I was too cheap to pay anyone to put an exhaust analyzer on it to check mixture. Better to be too rich than lean. If you decide to rejet, I'd jump the jet size by at least 4 numbers to see any difference.

You can test your mechanical ignition advance by using a timing light and checking for the marks to advance when increasing engine speed.
Robert-CA

Re: Backfiring thru carb on 230, adjustment screw
Post by hiker on Jul 10, 2006, 3:18pm

Jetting info from a previous posting:
If the float level is correct, then look at your main jet size. I ended up replacing a #58 with a jet #69 (Holley, larger numbers means richer mixture) that did away with the lean mixture . Good Luck, Robert

Also, Vacuum leaks are common on the vacuum windshield wiper lines, or a torn vacuum pump diaphram.

Re: Backfiring thru carb on 230, adjustment screw
Post by brute4c on Jul 10, 2006, 4:25pm

As far as the master cylinder vacuum hose....that is just a vent. It is not, or at least stock it is not, drawing any vacuum. It should just be hooked to the air cleaner snorkel so that it just allows for venting of the master under normal circumstances. If a fording kit is installed on the truck, then all the vents get pressurized when the fording valve is closed....

If the line on the master is hooked to a vacuum source, it is incorrect and should be rerouted.

Re: Backfiring thru carb on 230, adjustment screw
Post by k8icu on Jul 10, 2006, 8:05pm

Its been 10 years since I played with a stock M715, but I know on M151s they are set up to run with a little bit of choke on them. Pull it out a fingers width and your good to go.

I agree to check for a leak in the manifold and vent lines etc. I also wonder if your not having a fuel pump problem. You say the truck runs fine at idle and high rpm, but if you try to go from one to the other it gets jumpy so to speak. You have to ease into it. I'm not wondering if there isn't a small problem with the fuel pump that is causing your wows?

Re: Backfiring thru carb on 230, adjustment screw
Post by kaizenator on Jul 10, 2006, 9:32pm

I don't think there is any prob with the fuel pump, it pumps a lot of fuel out and I have a glass filter and never see any bubbles. I will check for vaccum leaks tommorow and let you guys know what I find.