backfire thru carb
i have a new 383 built by T&L engines in NC.
it has dart heads 180cc runners, air gap, roller cam, roler rockers, small cam pertronic's flame thrower with ignitor II, blaster coil, headers of 1 1/2 or 1 5/8, two cats and mufflers all in 2.5" tubing. the truck is an 84 chevy k-10 with 33" tires new rebuilt 700r4 and stock gearing for now i think 340's. the carb is a road demon of 725 size with electric choke.
i have a holley electric fuel pump regulator set at 6.75lbs at carbs, and two filters one before pump and one after.
the pump line to carb goes regulator. then one side of regulator goes to carb and one goes back to tank thru original fuel lines (small one)
its rich at idle ( i can smell it ) gets about 7 mpg. doing a slow increase of engine rpm (100 rpm a sec) the engine will pick up 300 rpm very quickly at 1600 rpm. it does this in park or while driving. most of the backfire thru carb occur prior to it reacing 1600 rpm. i did have backfire thru carb at wot after this rpm point of 1600 but the holley fuel pump fixed that problem.
now now while driving before i hit the 1600 rpm point it acts lean when give a small quick throttle movement, kinda like its not getting a big enough shot.
if i stab it it pops and drops to about 200rpm then comes back to idle of 800 if i let off the throttle once it pops.
the bowls are showing right in the middle, no fuel out of the boosters that i can see or feel.
this engine was dyno broke in and tuned with this carb or so they say. i do know the engine was ran because of the soot on the exhaust ports and the residual oil in oil pan.
it came with auto light 3922's and i have when hotter with the 24's. they seemed to help a bit but not much.
the most vaccume i can get is around 12 indicated. i have the line in the back of the carb going to brakes and and the untimed one going to the a/c / heater controls.
my question is why the rpm change from 1600 to 1900 very quickly?
what to do about the backfire thru carb?
i have been talking to loyd at T&L and he is sending me some larger squirters but i really dont think thats the proble but could be.
also one other problem is that that with the choke on it just pours out black smoke. i now have it fixed in the full open position till i get the other problems fixed.
thanks for all the help in advance
I have found backfire thru carb was caused 80% of the time by timing.
10% of the time by valve issues like sticking intake valve or burn ones.
the last 10% I reserve for the unknown havn't see that yet disclaimer.
Your choke issue sounds very much like shes flooding.
It is possible your getting to much squirt and flooding out rather than leaning on quick throttle bump. <like hitting it with a can of carb cleaner it will bog and recover>
Check the float fuel stops for leak by.
Check the timing , use the back yard method of mark current location and advance it a bit. see if this has an effect. You mark it so you know where you started and can easily return it to that position.
Check the valve train
To get fire in the carb you have to have an intake valve open when there is flame. This is usually timing but, can be fire still in the cylinder as the valve opens or the mixture is hitting a hot cylinder <preignition>.
725 seems like allot of cfm for a 383, but the above link shows a 720 is needed at 6500rpm.
what vacuum is it pulling? Compression ratio? Cam Specs?
A couple of questions for you:
1) Does this backfire happen at a steady RPM, as your slowing increasing the RPM or When you're going to WOT quickly? Different times will have different causes.
2) How large is the camshaft in the engine?
3) How much initial, and total ignition timing are your running?
Normally a backfire through the carburetor is an indication of being too lean. It sounds like you've got a couple of things going on here, rich at idle and lean off idle? You may not have the proper advance curve in the ignition system, it can cause a situation like this.
the roller cam is
224 intake @ .050
232 exhaust @.050
were not going to mess with the road demon they have nothing but trouble with them. so he is going to send a holley 750 out to me.
A Road Demon wouldn't run real well on a combination like this since they are designed for milder cam profiles, up to a maximum of 220 Degrees duration @ .050".
he will be sending a holley 750 double pump that they use on the there small blocks. i like the look of the BG's and the improvments.
So for this engine backfiring thru the intake is because of the cam being 4 degrees more than your recommended specs of 220?
what carb do you recomend for this engine?
i have seen other 383's with bigger cams running close to 460 hp being ran off the 625 road demon with 82 mains.
why does a big cam have an effect on how a carb runs? you would think that if its because of lack of vac. that you could tune for that.
Another question that needs to be asked. Is the distributer vacuum advance, or mechanical. i have run into this problem before when my 69 truck was carbed. Ended going through the entire fuel and ignition system to only find that the advance diaphragm on the distributer had failed. It had hardened up from the fuel vapors. And another thing i have noticed with alot of carbes on the market despite all the power valves and such, if u get 1 up fire through the carb it never runs right again unless u rebuild.
There was two other times this happened to me and found that the advance springs had stretched and were worn out.
Hope this may help.
its not vac advance. it is also a new distributer from pertronics.
its supposed to have backfire protection but who knows.
thanks for trying to help though.
i just went out to mess with engine. i gave the dist. some advance started it and then check the timing. it was dead steady at 11 degrees at 700 rpm cold. the last time i checked it was set at about 11.5-12 degrees at 800. this was with a different gun and about two weeks ago. so anyway i go for a drive. its allot better. i still get a back fire at low rpm with a very quick application of throtle(mash to the floor) but not anywhere near as bad as before.
will different brands of timing lights read different?
i checked it again after i pulled back in parking spot after running it to get it warm and still at 7-800 rpm and 11 degrees. the quick rpm rise from 1600 to 1900 is very small now.
i did rev it up some to an unknown rpm but a guess af around 2800-3000 and saw just about 30 degrees of timing. the builder wants 32 degreesat 4500 rpm.
i will drive to work in am to see if anything has changed or drives about the same.
BG TECH are you just someone working for them and just answering out of a book with common problems? i guess i am just asking for your experience just want to make sure you are truely a tech. i know some companys have so called tech's that are just looking up the problem and then reading the answer back to the person. see what i am getting at. i sent an email to the "tech's" at BG website and no response. but get one smae day here.
Bo we're actually technical support. The 725 Road Demon series carburetors are designed for larger cubic inch milder engines. We're dealing with multiple problems here, a smaller cubic inch engine, with a more radical camshaft, and an airgap intake manifold which acts like an open plenum intake. The decrease in air velocity, and only running 11 degrees of initial timing are causing the rich idle, and lean off idle conditions you're seeing. Increasing the ignition timing a few degrees to about 14 should help some, but you'd need to increase the squirter size if the only time your backfire is happening is when you step on the throttle quickly. We build our carburetors more specifically then other manufactures, this gives us a smaller window where they will work properly. You may be able to crutch the combination to get it to work better with this carburetor, but it will not work as well as it would with a different carburetor. On this combination we would have recommended a 650 Speed Demon carburetor.
I haven't read the whole thread...but your initial post does not refer to the size of carb you have on this engine....
A 383 is still a small engine...
Is the carb jetted correctly? Are you using an appropriate CFM?
Going big on a carb is not always the smart thing to do...
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