Did basic tuning on new build and still running meh. Help with advanced tuning?
 

So I built up my car's previously Mercruiser engine. Replaced heads, cam, etc. Should be getting a theoretical 430ish brake horsepower (Identical build to Hot Rod Mag's budget 440hp Vortec motor but with 1.5 rockers, better heads, and a correctly matched carb.). I'd also like to mention this is my first time actually messing with tuning.

I've got a Holley 670 Street Avenger. Distributor is an MSD Streetfire 8362. Cam is Comp XE274. Heads are Promaxx 2180 64cc aluminum. Intake is an Assault 190cc aluminum intake.

I got my initial set to 12 degrees and according to specs my springs and dizzy should be giving me all in 34 degrees by 3k RPM (Might turn it up to 14 initial to get 36 total later on after getting everything else down). I also set my idle mixture to get best idle vacuum and adjusted my floats.

I'm running a 5.5 power valve (Cam gets about 11" at idle), which I may have blown before the test drive when I backfired through the carb. Plan to replace that tonight. Will also adjust the little arm for the accel pump. I believe I have 65 jets (I'm at 2,000ft and wanna have it set for about 80 degrees average for the time being)


It feels like it bogs and often struggles to get past 3.5-4k RPM (Should be getting up to 6.5k with ease!). It still feels like I'm getting less than 300bhp and my 0-60 mental timing is higher than 7 seconds. It's slower than my Nissan Maxima! Not even able to squeal the tires :ack:


So I have a few questions I'm struggling with finding solid info on.

How do you choose the accelerator squirter nozzle size?
I see there's a standard flow and a high flow 5.5" power valve, which should I choose? Can't even find quite what's different about them
How do you choose the accelerator pump size? (I see this might depend on whether your heads/intake/carb are too big for the engine?)
And finally, how do you determine what accelerator cam you need?
Any other tuning steps I'm missing?

Dyno It
 

The mediocre power and stumbling is likely a combination of things. First, your motor will likely want all 36 degrees of timing, and you'll probably want that all in by 2500rpm or it will feel lazy. I don't know your particular carb, but can say that stumbling can be a lean condition, or incorrect cams/valves, and you can spend a lot of time chasing that combo.

Someone on here may be able to help with carb tune.

If I were you, I would find a local chassis dyno and spend the money for a couple hours of sorting your motor. They can change up timing and carb tune and optimize your A/F ratio which can squeeze a LOT of power out of any combo. My last several motors all got the dyno before Install and it's the best money ever spent on a motor project which isn't cheap to begin with. It'll be 100% worth the $300 give or take or you'll chase issues all summer. You'll also know your real numbers then, if that's important to you.

Are you using the springs that came with it? (Streetfire)

Just about all Holleys in the last 15 yrs or so have Power Valve blow-out protection.

High Flow PW is not needed for street use. Keep the std. 5.5 sounds about right.

Accelerator pump diaphragm size is seldom increased due to interference issues w/o a spacer under the carb. (the 50cc have a thicker housing) The 30cc is just fine.

Holleys plastic cam color choices and which hole for the retaining screw is ususally determined at the track. I wouldn't bother with that or the squirter size, now.

If you can't turn the tires over, I think this is more than a timing & carb issue.

What else could it be? I've got excellent fuel pressure after I installed a new Holley mech fuel pump (4-7psi). Aside from that I've got a finished engine and combo that seems to be running quite well (Aside from the lacking power issue)

Just a couple quick questions, then I'm lost.
Did you take a compression test reading after this rebuild, after break-in? You should have gobs of compression with those heads.

Are you sure the timing chain-sprocket-crank-gear-dots ALL lined up perfect?

I felt the same way after my engine was rebuilt. I had the timing adjusted correctly per Lars' papers and it just didn't feel like it had power. I purchased a wideband A/F meter and once I intalled it I realized how lean I was running. Tuning your carb will teach you how it works and you will be able to see the impact your tuning has on your A/F ratio. Mine really woke up and I am still fine tuning to get it just the way I want. Here is the one I bought - and it has been worth every penny.

Your pv is way too small, put the stock one in go from there, you may end up leaving it or going up in size. (Lucky to see 7-8 in and a 5.5 is about right).
not saying this is the sole problem

You have aluminium heads so leave your total timing between 32-34* @ 3000 rpm all in as a place to start on the ignition side. Also, I would increase initial to 16-18* and limit mech'l to achieve above stated total of 32-34*. Try it and see how it runs. It can always be changed back to 12* initial which I think is too retarded for your combo. Work out your timing curve first before you fool with the carb.

How do you choose the accelerator squirter nozzle size?If you don't have a hesitation off idle and no smoke from the exhaust, your squirter size maybe ok
I see there's a standard flow and a high flow 5.5" power valve, which should I choose? What headsup saidCan't even find quite what's different about them
How do you choose the accelerator pump size? (I see this might depend on whether your heads/intake/carb are too big for the engine?)Again, what headsup said
And finally, how do you determine what accelerator cam you need?Again, what headsup said
Any other tuning steps I'm missing? Again, focus on timing curve first

whether or not the guys issue is carb or other
having an 02 reader is great for tuning.
wish i had one back "in the day"
:lol:

There are about 4 key areas to look for, cruising below 2500 rpm. Your on the idle and transition circuits so try there and see if you have any surging and if so your lean. Then you need to check the power valve opening with a vacuum gauge in cab while driving to see what the reaction is when it opens and your crowding the throttle just to the opening point, you want to see if it bogs when it opens or looses power. Then the next would be to see how the car reacts when the mains start flowing around 2500 to 3000 rpm and finally WO.

Neal

Also, the little arm for the accel pump: Holley says 0.015 clearance using a feeler gauge at wide-open-throttle. That is to prevent rupturing the diaphragm at WOT. However, when the throttle is closed, 0.015 seems too loose. And that could cause a stumble. I have found that on mine (after-market throttle plate) that 0.012 is a little better and will still protect your diaphragm.

And the High Flow PV is a four window design instead of two. Its more for all out racing. Which PV to use? Lots of arguements there. Some say half of your vacuum reading. Others says 2 notches below the vac reading. But this is still not your major issue. And yes, you should be around 400+ horse. Somethings fishy.

The difference in the regular and high flow PV is the window size/stem clearance of the PV and how much fuel it will flow at a given vacuum. Really don't need to worry about this unless you drastically increase the PV restriction.

Neal

This ^^^^^^^

Yes I did and the compression on each cylinder is very good. Timing sprocket and crank gear were aligned 100% perfectly. I have a picture floating around somewhere after someone asked me the same question for a different issue

Guess I'll make another go with the curve. I'll get it around the recommended area, try the vacuum tests, go for a second play with my adjustments, and then do that vacuum reading and will report back!

Thanks for all the advice, hope this finds my issue.

Edit: checked accelerator pump adjustment. It's so far off that It's actually pushing down on the arm at idle it seems? Gonna fix that to .012 and see how that changes things

isnt that the truth especially pre internet. Get one right, there isnt a whole lot of difference between a well set up carb and EFI except for the hot restart thing which is mainly todays crappy gas.

I too have the Streetfire 8362. It should run decent right out-of-the-box. I may have changed to one lighter spring or both. I have yet to change the bushings. But that still doesn't explain your power loss. Basically you have lost 200+ horse with that build / tune and I find it hard to believe its carb / dizzy related. But maybe. Even if the carb was wonky and the dizzy off a little, you should be able to lite 'em up.


Just remembered something about that 8362. Take the dust cover off. You will see the coil and a wire that goes under one of the mounting screws. Using wire brush clean the factory paint coating off under that screw head. Without a "good" ground, issues will haunt you.

Good Luck, things will work out.

"Out of the box", they do not go full advance till 4000RPM. Kinda late for performance.

So as an update I changed my squirters from 31s to 35s and it felt like it was reving higher more easily, but power still lacked. Adjusted the accelerator pump and I see no difference.

Set timing to 16 degrees initial and got my all in at around 3k 32 degrees (Got an MSD kit coming in so I should be able to get 36-38 total with their springs). Messed lightly with my idle mixture, just set it to get highest RPM.

I can floor it at idle and it can't get past 2.5k-3k RPM until I'm running about 15 mph when it finally kicks up to 6k, but even when it's RPMs are climbing, I still feel like she has very underwhelming power and struggles to accelerate quick at all. Still entirely unable to let the tires loose at any RPM. Still slower than the Nissan Maxims as well. As a sidenote, the idle sounds weak as hell :ack:

I'm thinking if I can't figure this out by Monday I'm going to pull my cards and just take her to a tuning shop. I'm a college student so I'm running on a pretty low budget so I was hoping to avoid that, but it's feeling like this might be out of my league.