Ok here's the deal. I drove my car to a burger run, and there was another gentleman there with a blown 350 (6-71). He was running Edelbrock carbs and could not get the thing to run right at all. He was amazed at how well my car (started, ran, idled, sounded), so he runs out and buys the same Holley carbs I have (I gave him the name and number of the place I bought them). BDS also told him to ditch the Edelbrocks. He calls me up after he gets the new carbs and still can't get the thing to run right. I offer to help him out (just thought I would give a little background).

It has a mild hyd roller 228/230 @ .050 .474 lift. I pulled the carbs off (Holley blower double pumpers 750CFM) adjust the idle screws set the floats. The carbs come with 8.5 power valves, 70 primary jets, 80 secondary jets. After I set the carbs up they started and idled fine (really had to lean out the idle mixture screws) they are about 1/2 - 3/4 turn out.

Now he is running a Hunt Magneto (I know), with the timing set at (are you sitting down?) 42* I tried backing down the timing some and the car didn't like it (I am assuming the timing tape is correct, he swears it is). Vacuum is 16 inches at idle.

With the carbs set up as mentioned above it idles fine starts fine, he takes it down the street rolls into the throttle and bang backfire, he had changed the jets to 68's, I put the 70's back in and he took it out again, ran a little better but still surged and popped, so I put 72's in it, same thing runs a little better but still popped. So I say lets just jump up to 82's and see what happens, BINGO it runs like a scalded ape (no smoke at all just perfect). This bugs me, there is no way it should need that much jet in the primary side. The power valves are boost referenced. I am thinking he needs a higher rated PV. What do you guys think? Could it be that with that HUGE amount of timing he actually needs that much jet? I think I have him talked into getting rid of the mag (this is a street rod and will never see the track)another project for me but, I don't mind he is an older gentleman and very nice, plus he actually pays even though I refused.

Now for comparison I have a 388 with an 8-71, same carbs as above and WAY more cam, I am currently tuned very conservative 13* initial and 33* total (all mechanical). My car runs fine with the 70's pri and the 80's secondaries. I have the same 8.5 power valves, runs absolutely perfect. More blower, larger engine, more radical cam and needs less jet? Oh I need to double check where his compression is too.

He is a happy camper right now, it runs better than it ever did (actually runs real good). I just don't see why it wanted so much jet on the primary side. The power valve must not be opening. Before I go out and buy a set of higher rated PV's and stick them in, I thought I would run this by you guys.

Sorry for the long post but, I wanted to make sure I included all the details.

Let me know what you think.

I'm no tuning expert but are his current power valves OK. I ask because you mentioned backfiring and popping so I was just wondering if the current power valves could be blown.

Rod

no the power valves aren't blown. If they were it would not idle well at all and would be blowing black smoke at idle. They are free and not sticking or anything.

Does his mag have any advance curve in it? Blown motors don't normally use anywhere near that much timing. I think I would have to verify his timing tape 1st. And who knows...he may have 7:1 compression or something goofy like that. What kind of heads does he have?

NO advance in the mag. What you set it at is what you get. Mags are little different because you can crank the engine over and then flip the mag on and it will fire. The engine was built buy a "pro" and he said he watched when they set up the timing tape. Blown engines do like initial advance but, you usually set them at 14 - 18* and then have the total set from 32-36*. I curved mine so I have 20* of advance in the distributor, I would not think of running anywhere near 42*. I would expect to see my crank in the rear view mirror.

He said when they set the mag, they set the pointer at TDC and then backed up to 35* BTDC, dropped in the mag with the pointer at 35* BTDC. They then adjusted the timing until it started easy, at this point they were between 39 - 43*. I thought no way he could be right, so I took my timing light over there and it was at 42.5*. I backed it off to 38* and the car didn't want to idle at all and ran poorly. I adjusted it to 42* and it runs fine.


I agree the timing tape must be off but, you never know. If/when I get him to dump the mag I will definetly verify true TDC.

Even with the questionable timing why would it want so much jet on the primary side? When I jumped all the way to 82's I figured it would be WAY rich but, the thing runs great now. Doesn't make sense to me. Am I missing something here?

The only other mag I have dealt with on a car is in my buddies race car but, it has big compression, a huge by large solid roller, tunnel ram, etc... I know it likes 38* of advance (it's a small block as well).

The heads on this 350 are Brodix and I think they are -10's.

Did it surge and pop in light throttle cruise conditions or just when rolling into the throttle? If the car cruised fine at steady throttle with the smaller jets, the smaller primary jets were adequate. Just a thought...if the surging and popping only occurred while rolling into the throttle, he may need a bigger pump shot to cover the transition. I would think you need a bigger acc. pump shot on a roots blown motor because that shot has to travel a greater distance through the huffer. Just my .02.

Neil B, you could feel it surge under a steady cruise, if you opened it up more (slowly) then you would get the pop. It very well could be a acc. pump problem but, if this was the case once on the main circuit wouldn't you expect to see a VERY rich condition with the 82's? Now I am not saying you are wrong I just want to discuss this and tune the car a little more for the guy.

I have plenty of acc. cams so I can go tune it but, I just don't understand why the 82's aren't running rich. I watched as he got on it and it did not smoke at all. This is what makes me wonder if maybe the power valves are not opening. If the power valves are opening like they should this would mean the primaries are basically jetted at 88, this should be very rich. Am I thinking incorrectly? Sometime you get something in your head and you get one track mind, I am trying to keep all options open.

I think I will order a couple power valves, once I get them, I will go over jet it back down to 70, see if I can tune out the problem with a pump shot adjustment, if not I will then install the higher rated power valves and see how that works.

I am no expert, just throwing some info.
My car ran lean with a 750 sp demon mech sec. 83/92. it also ran lean with a 650 dp holley with 75/80. The fuel sytem was the problem I hope, haven't got to run it again to see if I fixed the problem. I had some trash in my fuel pump. gauge said I had 15lbs fuel pressure, Regulator would not adj down. There was no fuel pouring out of car, plugs were white, car would back fire when letting off. Just an idea, maybe he is having a fuel delivery problem.

The first time we took it out after setting up the carbs, it kind of felt like were running out of gas (surging). We went to the gas station and put fuel in it, no difference. By jetting up that would not sure a fuel delivery problem (if anything I would think it would get worse). He has an electric pump and the pressure is steady.

I will be working on it in the morning.

If it was surging under steady cruise, IMO you did the right thing to jet up on the primary side. I would increase the primary jet size from factory calibration 2 jet sizes at a time until the surge goes away.

With that said, on my normally aspirated 302, I went from a 6.5 to 3.5 power valve (due to a cam change) and picked up a surge at cruise with no other changes. In theory, this cause-effect did not make any sense since the PV should be closed in cruise conditions with either PV, but it happened.

I would get rid of the mag and get a MSD boost retard setup. We find that most blower motors like a lot of lead at cruising conditions but you want to knock the timing back quite a bit under boost.

I agree 100% onovakind. I think I have explained it to him in a way that he now knows a mag is not the best choice on the street. I went with the Crane igniton and boost retard myself but, the MSD is good stuff as well. He doesn't want to do the distributor thing until winter. That might change, it is a major hassle to get the mag out, it is a 29 Ford roadster and the mag is under the cowl.

I think I am going to take my piston stop over there with me this morning to make sure the timing tape is correct. I just can't believe it is running on pump gas with 42* of timing and not knocking or pinging at all.

What are your thoughts on the carb tuning?

Well I went over and hooked up my vacuum gauge and we went for a ride. I can now rule out the power valves. Vacuum dropped well below the 8.5" mark. I didn't get to do mush else to the car, he is all excited since it is running "right" (not right enough if you ask me).

He is like an excited kid right now, he is getting ready for an event next weekend and has to clean the car up and get it ready so we didn't mess with it anymore. I am not done though, I still have to verify that timing (it is driving me crazy). I HIGHLY doubt it is really at 42* on pump gas and not pinging.

The darn thing runs pretty good I must admit, I just don't understand how/why. That is a HUGE amount of timing and jet.

He doesn't have what I would call ideal exhaust for a blower could this have something to do with why it wants so much jet? He has block hugger headers 1 1/2 - 1 5/8" I will make sure next time I see them. They usually recommend a minimum of 1 3/4" headers and you need a VERY free flowing exhaust.

The rest of his system is true dual exhaust with flowmasters, I did not pay enough attention the pipe diameter but I think it was 3".

I'll keep you guys posted.

What about a vacuum leak? Maybe you are compensating for an air leak with extra jet . . .

Eric, that is a good thought. It crossed my mind and I did a quick check (vacuum plugs, busted hose, etc..) I didn't get real involved though. I think I will look closer into that, now that you mentioned it, that could very well be the problem. Thanks