Carburetor Problem

[rpgdeity]

So I'm finally getting around to getting my 715 back up and running. Its been torn down for all sorts of different things.

So I've had this carburetor/engine problem ever since I installed the 2-barrel carburetor and intake on the stock 230 engine. I've been messing with trying to fix it for a while now, but nothing has worked.

The engine idles and runs fine at all RPMs, the problem is when I am decelerating, or engine braking. I get massive popping and backfiring out the exhaust. Not like once every now and then, but quit frequently EVERY time I try to slow down while using the engine. Does this make sense? You know, when your going down hill and you go from 4th to 3rd to slow down or keep a constant speed?

The engine timing has been checked several times, and matches what the manual says it should be at. I've messed with fuel ratio, tried different rich/lean mixtures. I have tuned the carburetor to the best of my ability by listening to when the engine sounds and runs the best.

Any suggestions? Is this problem indicative of too rich or lean of a mixture? Is there a more technical way of tuning the carburetor? Any help would be appreciated.

[dave]

you dont have any slight exhaust leaks do you? extra air going in the exhaust system along with a larger amount of air/fuel entering the engine may be attributing to your problem

[randyscycle]

Remember, that whenever you decelerate with a carbureted vehicle, you instanly get a very rich mixture when the throttle plates close. As Dave mentioned, I would look very closely to see that there is no way that air can be getting into the exhaust system and igniting that over-rich fuel in there.

Newer carburetors had a device called a dashpot, that would not let the throttle close completely at once when you took your foot off the accellerator pedal to prevent both the extra fuel and backfire.

Timing, if it were very very far out could cause the problem, but you'd most likely have an issue with performance all across the range if so.

How long is the exhaust system? If it is very short that can also contribute to the problem.

[rpgdeity]

Exhaust system is close to stock length. It dumps out to the rear of the rear axle. I checked all the connections, and it seems alright. I'm still leaning towards carburetor problem, because it never did this when I first had the 715 with the stock single barrel carb. I was doing some reserach, and it appears that this problem is indicative of too rich of a fuel mixture. I should have her back up and running again tomorrow, I'll try to lean her out some more.

Next question: I was told that a good way to determine correct A/F ratio was to run it for a while and check the exhaust temp, too hot is too lean, to cold is too rich, plus I can check the balance between the two barrels. Anyone heard of this? Have any idea what an approximate "correct" exhaust temp should be?

[Kaiserjeeps]

Your right on the temp lean miss fire thing. The only way to be sure of the burn is an exhaust gas sniffer to check the byproducts of combustion.

Do you have a Dougs digital temp gun? You could see what you get with changes. Maybe compare it to some others. I can send some temp data from my stock set up. I need to get the truck out.

[rpgdeity]

No, I don't have one of those infrared heat guns, I am working on trying to aquire/borrow one. Yeah, if I could get any data from you on your temp ranges that would be incredible. What kinda ballpark temp range should I expect to see? I mean plus/minus 100 degrees, are we talking 500, 800, 1000 degrees here?

[Kaiserjeeps]

Just took the truck out today. Dang thing starts right up after sitting. Wish I caught your post. Give me sometime after Monday. Tomorrow's busy helping a friend and It looks like Friday I might be heading north for another Todd's truck marathon weekend. I'll PM them to you.

Al.

[rpgdeity]

Well I finally got one of those infrared heat guns. Took some temperature measurements, I wasn't too concerned as to the exact temp of the exhaust per say, moreoverly concerned with the temps between all 6 cylinders. Since I took an infrared temp of the cast exhaust manifold, its probably a bit colder than the ectual exhaust temp, but the difference in temp should be the same. I started the engine, warmed it up a bit (got the water temp up to where it usually is when running warm), then let it idle and watched until the temps stabilized, then I rounded to the nearest 10 degrees. Here is what I found:

Cylinder Exhaust Temps in Degrees F, with #1 being closest to the firewall:

#1 - 520
#2 - 560
#3 - 560
#4 - 610
#5 - 650
#6 - 570

Anyone have any idea what this means? Too hot, too cold? Whats should be the max difference in temp between the cylinders?

Since I'm running the 2 barrel carb, one thing I notice is that #'s 4,5,and 6 are running a tad bit hotter overal than #1,2,3. So I'll have to play with evening both banks out some more.

[Barrman]

You confused me with your numbering system. #1 should be in front with #6 in the rear next to the firewall.

Anyway, the very front cylinder reading is going to be off some because the cooling fan is blowing air on the manifold. If I understand your numbers correctly, that one was a 570. Assume it was actually higher and you have 1/2 your engine at 600 or higher and the other half below 600.

Doesn't your 2bbl carb split the engine front to back? If so, then I would suggest leaning one venturi down until you feel the engine stumble then out 1/4 turn. Then do the same with the other side of the carb. Then check the temps again.

A vacuum gauge is another good way to do this. Turn the mixture screws until the vacuum peaks and then do the same with the other one.

Also, I have found a more reliable number with my infrared to be had from the spark plug itself. Just aim at the hex where the socket wrench grabs. The numbers will be in the 200-300 range or at least they are on my REO. I don't have a running 230 nearby to give you a range though.