Hey guys, I'm running a stock 1968 points distributor on a mostly stock 305 small block Chevy with an 871 blower (1:1 ratio). Yes, I know it's putting out way too much boost, but that's not my problem right now. Every time I step on the gas past 1/4 throttle it backfires like crazy and someties kills the engine. (But believe me, 1/4 throttle is plenty enough to get you in trouble). I've heard blower motors like lots of advance at idle, but only around 31-32 total and advance should come in early. Does anyone know why it's backfiring? The idle timing is set to 8 degrees BTDC.
Sounds like its lean, what carbs are you running? start adjusting accel. pumps and see if that helps. post up some specifics about the motor but im guessing a stock 305 with a 1 to 1 871 aint long for this world. BTW I think my timing is at 20 or 22 degrees at idle on my blown SBC
Dual blower carbs: Holley 600s (there's a chance they are 750cfm, not sure) square bores. The blower was on a drag Nova previously. Good call on the accel pumps, I only set them to minimum thinking it would flood it out since the motor is so small. Ha, true deal about it not lasting very long. I'm "planning" on it blowing sometime soon anyway if I "accidentally" hit it too hard. Oh that much timing? Ok, I'll try advancing it some more then a little at a time. My compression ratio is about 8.3:1. Thanks so much! Ha, I love your quoted saying. I say that sometimes. =)
After you check your accel pumps, recheck your fuel mix screws equally. If you have carburetors from the 80's early 90's may want to check the power valves, if they don't have them install the blow-out preventers available at sumitt, power valves don't like backfires. If you don't have any flooding or rich problems chances are good they are good. How much under driven is that blower?
I think the carbs are from the 70's. I will check the fuel screws. It doesn't seem to flood, only very rarely at idle if I let off too fast. I know it's over-boosted, but it's 1:1 ratio (for now, but it's not going to be driven much at all like this).
You need to put the big accelerator pumps on both carbs to start. Progressive linkage may be something you will need to try also. JC
Was the engine running with no back fire before you put the blower on? Just wondering if the distributor, plugs and wires were in perfect working order prior to the blower install ? Did the carbs work perfectly before mounting them on the blower ?
I wouldnt go progressive linkage on a blower motor,asking for trouble.Something will lean out.I always had them direct so every thing is equal in the manifold.
I just pulled the blower belt off and it still backfires when I hit the gas, even with both accel pumps turned to max? Yes, the plugs, wires... all new. As to the carbs, I'm not 100 percent sure what jets are in them, but I would think it would run plenty fat if it was on a larger motor before. The linkage is set-up direct, both open at once.
Oh, and also, it has 8 accel pump squirters with 2 pumps (one each carb). I wonder if my timing is off again maybe? Although it shouldn't act up from timing by opening the throttle should it?
That blower must be pretty well worn out for the engine to even get enough air through it to be able to run with the blower belt off. Back to the problem at hand, I would start with the ignition on it. Upgrading to a electronic distributor of some sort will probably help solve your problem.
The blower freewheels at anything above idle. I will double check the ignition. My thoughts exactly on the upgrade to pointless style. I am trading some Nova bellhousing and clutch parts hopefully tomorrow for a Pertronix point eliminator kit. =) Thanks a ton guys!
Might want to check your advance mechanism. On my 392 electronic I have the advance locked out. I also have a msd 6btm control box which allows you to retard ignition timing from 1 to 3 degrees per pound of boost http://www.msdignition.com/Products...log/6462_-_MSD_6-BTM_Boost_Timing_Master.aspx
How would I go about locking out my advance before the max limit? I just turned my timing @ idle to 16 degrees and it runs better, but still backfires. Does anyone know which was to turn fuel (if they are indeed fuel) screws on holley blower carbs to richen it? Then again, it backfires at mid-higher rpms, so those screws might not fix it (they're made for idle mixture right?). It does sound a bit leaner than it should be. No black smoke at all.
To make your fuel mix richer you turn counter clockwise do this in small increments equally on all screws, and you are correct they only set idle mixture. I don't think that the adjusting of these will end the backfire but will help you at the stop light and idle, check spark plugs they will give you an indicator of rich lean condition or at idle if you have black smoke or black soot on exhaust tips is a sign of running rich. If you error with a blower always error to the rich side. I believe your problem lies in your ignition. Summitt sells a vac lockout for your distributor, comes with an instruction sheet, involves taking the distributor apart partially. You could cap the vac mechanism on distributor and cap vac source temporarily and bring rpms up and see what this does.
Ok good, because that's what I did. So far, I only did 1/4 turn each one. i'd love to see a tiny bit of black soot, but none so far... I agree, I'd always rather run rich than lean. Yeah, I'm thinking it is ignition too, it pops the hiccup valve on the blower pretty decently, so that's not good, gotta fix it soon! I've already capped the vac advance stuff from the beginning, just in case. Good call on that too, thank you. P.S. Are all the lockout kits the same for the 60-70's points dizzys? (Minus the corvette ones).
Surf the msd site on different distributors and instructions alot of valuable info can be found in there. What little I know about blowers is that fuel and ignition are a very big role in your engine life. Looke for some pulleys and get that thing underdriven as much as possible. I have a 671 on the 392 hemi, street driven and I have enough throttle at 1/8 to get me in trouble at 1/4 I will probably have some tow and storage bills. I have the Joe hunt electronic set up for use with the msd 6btm had them set it up, seemed the easiest way for a novice. My research on distributors was so involved advance controls, lock outs etc that I chose this route and right now it is working for me
I read this after I posted so will add what little I know. I am not 100% on all kits being the same but chevy is chevy one of their strong points is interchangeability. What size fuel lines and pump do you have, could be that when you accelerate it is not receiving enough fuel and going lean and at low speeds it has enought to keep up. Those carbs like 5-6 to 6 psi of fuel pressure.
You said you wondered if your timing was off "again" ?????? I'm thinking you have an ignition issue. Whether timing or elsewhere in the ignition system.
I also use the MSD BTM box and would advise it when you can afford it. The ignition,timing and air/fuel ratio are critical. You want to sneak up on the tune. If your distributor is at 16 initial thats gonna be too much total timing so be careful I set my distributor up to only have 12 degrees of mechanical advance so im at 20 or so at idle and 32 total then the BTM can take some out to prevent detination.You might want to run some race gas to help keep from pinging while you get a better tune figured out.
I will look that site up. I'm already well over 6k into this project and it's not even road ready yet... but still, building it is half the fun. And yes, for sure about under-driving it. I took it out for a "test" and almost scared myself, but came back with a big smile. It pulls the front wheels off the ground at less than 1/2 throttle. I wish I could get a magneto, but it's too pricey for me right now. My fuel line is 5/16". I'm glad about the interchagability factor. the pump is so-so, it's not a high pressure/high volume, but it seems to keep fuel pressure up fairly well @ around 7-8 psi. I'll set my pressure down a tad if 5-6 is right then. Yes, because originally the distributor "slipped" (I forgot to tighten it all the way). =P But I tightened it so it shouldn't be off. Yes, I agree, ignition/timing seems to be the deal here. I'll try a mix of both worlds then with the advance unit. I revved it up without the blower belt and it seems to hit about 35 degrees max advance (which is probably too much from what I'm collecting from everyone). *Sigh* back to the garage... again.... and again. =P I keep telling myself it's all part of the fun, but the "fun" starts to wear off @ 3am every night... naw, what am I saying, I love it. Yeah, I probably should as I'm just using premium fuel right now. (race gas).
Just as a note, you can have a back fire which appears to be a lean condition in the upper RPM range, however; it is actually the pop-off valve releasing the pressure from the intake causing a lean condition during the backfire. I had a terrible time last year with the pop-off valve springs on my drag car ! The springs were losing the tensial strength.....
AHG! My computer erased my stuff... Anyways... I will look up MSD. Right-o about underdriving it. I know what you mean because with less than half-throttle, it pulls the front wheels off the ground... I wish I could afford a magneto right now. I'm glad about the interchangability factor. My fuel lines are 5/16" with a so-so fuel pump, it keeps the fuel pressure around 7-8psi, and drops a bit when under hard load. It's not a high volume pump though. Oh, to clarify that, I forgot to tighten the dist bolt the first time, so it retarded itself after it "slipped". it's tight now, I just was wondering if I might have timed it too advanced instead of the other way 'round now. I guess it's too high right now then, because it maxes out at just shy 35 degrees. Think I'll turn it down a tad. Thanks guys!
I hope it's not that, but it is possible. I have double valve springs on my pop-off valve. it pops about 1/2 second after I open the throttle a bunch.
The reason I mention progressive linkage is, 1st it is a mild street motor and 2nd with the progressive linkage on the street it gives you the ability work all 4 accelerator pumps 1 after a nother with a little over lap. The idea is to have a lot of accelerator pump squirt duration. I agree with the guy that didn't like the idea for a race motor. But this is mild street stuff. JC
I had a 502 show up in the shop with a blower on it and the same symptoms. I did all the normal stuff and it would run under 4# boost. After 6# it would just backfire and stop. It turned out to be the intake valves burned up.
305 with 871 1 to 1. To tired to figure it out right now but, that compression ratio has to be at least 15 or 18 to 1 eek
why would you waste time and money on a motor your "planning on blowing up anyway".there are so many variables in your problem that makes it impossible to fix via the internet.just to point out some holley carb idle screws work the opposite way too.and buying a magneto will make your troubleshooting even more difficult.
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