Need tuning help on 340 (Part II)

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midnight340

midnight340

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In my original post I was struggling to get timing and fuel worked out. The post generated a number of helpful responses and suggestions of things to take care of. I've done that and want to update and get response. I'll post what I've done + photos of test plugs.

For anyone who wants this info:
.040" over 340, TRW 8.5 forged pistons, I do not know deck/piston height, 70cc heads, conservative Lunati 284 Bracket Master II .458" lift (218 at .050), 1-58" mid length headers, LD340 intake, Edelbrock 650 AVS (1" phenolic spacer). Heads are X heads ported, cylinder pressure is an even 160#.
A couple of times I ran 98+ mph with a weight of about 3400# (13.95) But this was a number of years back and I have no idea how I had timing or jetting set. Have upgraded lots since.

I have a new Ready-to-Run MSD distributor firing a MSD 6A box, new wires, etc. Distr. curve all in at 2500, and I had been advised to sort out the timing first, so have since installed a 10* bushing. (also vac adv limited to 10*) and am currently running 22-32 degrees.

When I began trying to sort out the tuning with the new distributor and a new AEM A/F meter with Bosch sensor, I was struggling to get decent #'s and having WOT detonation problems, seeing silver specks on the plugs. So have been jetting richer and holding back the timing. I am using fresh NGK BPR8ES plugs for testing.

Because of the detonation at WOT, I was told to "get some color on those plugs" before adding any timing. So I've been working on the jetting since.

I was running an Edelbrock 750, I did a lot of research, some saying that model had some design problems making it hard to get rid of lean conditions. After struggling with it, I bought a new 650 AVS. (the secondaries in the 650 AVS are actually larger than those of the 750, and the small primaries help throttle response.)

Trying to cover everything, I taped a fuel pressure gauge to the windshield and found I had fuel pressure dropping as I ran through the power band, so I have replaced the Holley red pump with a new Blue, and it now maintains pressure. So that eliminates any fuel issue I believe.

The advice I got here was to ignore the A/F meter until I got close. Also to throw away the Edelbrock book and pay attention to what the motor wants. So I have been doing that.

With the Edelbrock 650 AVS stock jetting I was very lean and very down on power. So I began moving up in jetting. But in the secondaries I am now running the second largest jet Edelbrock makes!!! I am jetted .104 in the primaries (3 steps up from stock .095) and my secondaries are …seriously, 6 (that's SIX) steps up with .116's. And the motor is finally feeling like it has power. Much more willing to run.

(I suspected a vacuum leak could be messing with things, so replaced all hoses and connections. Also ran raw propane all over hoses and intake. No imdication of any leaks.)

Yesterday I did two back to back WOT runs (probably 3/8 mile) with two fresh NGK's in #1 and #7 shutting it off and coasting to a stop. Only when I hold the test plugs next to a new one in the bright sun do I pick up any color at all, and it is a very faint light tan. (My run around town plugs have a black ring at the very base of the porcelain and a thin black 3/4 of the way around the plug base.)

So I have a hard time believing that I'm only running 32* total, and am six steps up in secondary jetting, and barely getting any color on the plugs. Any time I add more timing I run into detonation.

Possibly my timing light is a slight bit off, but the 340 sounds like it should set by this light. Starts very well, and runs cool. Yes, I have checked TDC at the piston, and I have a fully degreed balancer. I am NOT using a dial back light (which can cause problems with MSD.)

For what it's worth my A/F is now in the 11-11.5 range WOT and the 340 is finally pulling pretty decent through 5500+ (I've held off some due to apparent lean conditions) It used to rev willingly to 7,000 even with this cam. Idle #'s are 13.5-14 so am good there. Cruise is pretty rich at 12.5. This motor seems to want at least a whole number less A/F than what is always advised.

Just can't believe I have to jet this rich to run. AM I MISSING SOMETHING IN ALL THIS? Or am I on the right track and screw the logic?

Here are a couple of photos of the test run plugs (apologies for bad focus but best I could get my cell phone to do):
NGK 5-13-16.JPG
NGK 5-13-16 II.JPG
 
It may rev to 7K, it's not making power there at all with a 218* at 50 camshaft. It probably falls off at 5400-5600 rpm for power. I wouldn't shift any higher than maybe 6K and that would be a stretch.

Make sure the jetting balance of the primaries and secondaries is fairly close. Keeping the primary the same and stepping up ony the rear will cause issues. 11.5 is pretty rich unless the fuel has a bunch of ethanol in it.
 
No ethanol. The 650 AVS comes with larger jets in the secondaries. (.095/.098's)
Yes I know it's probably not making power that high up. Havent run at the track too often (couple of times a year) but have been shifting closer to 6K and that may be too high.
And, crackedback, thanks for the advice to get the 10* bushing!!
So maybe I need to pull back on secondaries and go up on primaries to get them closer.
 
I fully expected plenty of Holley advice!! :)
...and I thought about it, but didn't want to redo my fuel lines and throttle linkage right now. Also, the reviews on the AVS carbs are pretty positive!
 
midnight340 said:
I fully expected plenty of Holley advice!! :)
...and I thought about it, but didn't want to redo my fuel lines and throttle linkage right now. Also, the reviews on the AVS carbs are pretty positive!
Click to expand...

I sure hope you get this figured. I have a 650 AVS on my 340 and I had a hell of a time getting that thing to accept any changes, mainly Air/Fuel mixture changes. Add in that I couldn't solve the hot start problem even with a spacer.

I pulled the motor to paint the engine bay, but thought heavily on selling the AVS and getting a Holley or EFI.
 
No problem on trying to help.

Which carb do you have 1805, 1806?

The factory jet spread on this carb is in the 41% pri 59% sec. At WOT you want to maintain the same basic balance. This is where the metering rods make the math tougher. :) You are pretty close with the factory metering rods to that balance with 104/116 jetting 39%/61%
 
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There was another post on here recently about mods to the secondary boosters that was awesome and also talked about removing tubes. I'll try and find it.....
 
"The factory jet spread on this carb is in the 41% pri 59% sec. At WOT you want to maintain the same basic balance. This is where the metering rods make the math tougher."

So, you are saying that I need to maintain this ratio between the front/back jets?
If I understand you correctly, this is new and helpful information.

I have started computing area of jets minus area of rods. So can maybe handle that when I get time here.
 
And can someone with more experience look at the plug photos and tell me if there is an indication as to timing and plug temp. It's a fairly cold plug, but looked OK to me. I'm mostly wondering about timing.

(Again, when I compare this after WOT runs, to fresh new plugs I see a very slight light tan color over all.) I am trying to learn this (even at my somewhat advanced age!) but don't trust myself yet when looking at the plugs.
 
I saw you are running NGK 8's, That is a VERY cold plug. I'd run something in the 5-6 range.

I did a spreadsheet that can help computing the rod/jet for cruise change and WOT change/balance. If you want it, send me your email address. The Edelbrock chart is good, but people can get lost in it...

I'm not an Edlebrock carb fan... :)

Rob
 
midnight340 said:
And can someone with more experience look at the plug photos and tell me if there is an indication as to timing and plug temp. It's a fairly cold plug, but looked OK to me. I'm mostly wondering about timing.

(Again, when I compare this after WOT runs, to fresh new plugs I see a very slight light tan color over all.) I am trying to learn this (even at my somewhat advanced age!) but don't trust myself yet when looking at the plugs.
Click to expand...


That plug is too cold. I don't use NGK so if it were me, I'd start with an RC12YC Champion. You have to get the plug and timing right before you start anything else. I say timing because they are need a minimum of 18* initial and 35-35 total. That is a given.

Get that stuff right and then tune the carb.
 
So I should maybe set the timing at, say, 24/34 (I do know the motor runs fine and starts good at up to 24 initial)
...and get a hotter plug in there first, then go back to the carb?
 
Hotter plug first. See if things come around a little. I'd leave everything else as is until you get some hotter plugs. If you can get 5's good, 6's would be my next choice.
 
I'll get some different plugs in it tomorrow, and take a look at the relationship between jet sizes and make adjustments there if needed. And I'll keep you posted as I do.
 
I didn't realize these carbs needed so much tuning until I read this. Thanks for bringing this up.
 
Got some BPR5ES NGK's on advice from crackedback. Did a couple of test runs with one fresh plug. Picking up some light color on porcelain and flat of plug, in my limited experience looking better on plug temp, some plating burning back from end of electrode. Purple-ish band at bend of electrode, which seems good. No signs of detonation.

A/F meter reading fairly rich WOT, and cruise. So I can likely jet down. (The spreadsheet program from crackedback is very helpful (thanks!) so will make it easier to figure out jetting and rods.) And you are saying to get the jetting more balanced.

Yellow rose, you are saying get the timing in first once I have the hotter plugs. I don't know whether to add timing back yet or not. I know 32* is pretty low.

So with the hotter plugs, what is next??? Do I work on balancing the jetting and jetting down to a bit leaner? Or start adding in some timing to get to a more normal setting?
 
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