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Discussion Starter #1
I went up to a local chevelle get together and noticed on the way up and back the car seems to have a lot less power at freeway speeds. Specifically going from say 60-70mph the car takes quite awhile to accelerate up to that speed. at 60mph I am turning about 2500rpm. 70mph is about 3000 even. Car is working hard, as if it is in too high of a gear. Around town this isn't noticeable at all. This is a fairly recent build and it seems I am still working the bugs out. Any ideas? Below is my combo

.030 454
049 heads, big valves, roller rockers etc.
Lunati 60204 Hyd. FT Cam 233/241 @.050 276/284 advertised duration 110 LSA
BG Demon 850DP 76/84 jets .035 squirters front and rear.
3.42 gears
TH350 (recently rebuilt to live behind BBC by reputable shop)

Things I have tried:
Checked all electrical, tested coil, replaced cap and rotor (MSD E-curve)
Checked plugs
upped squirters from .031 to .035 (resolved stumble I had at 4500 rpm)
installed fuel pressure guage (needle bounces but is within range)
Carb has heat shield and phenolic spacer underneath it.
 

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Boldly procrastrinating
66 El Camino 57 Chevy pickup 2004 Tahoe
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look up the right jets for that carb, sounds like it has jets for a 750.
 

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I know my 4781 850 DP came with 80's on all 4 corners. Sounds like the fronts need to be at least 78's . I don't know if a power valve is in play when you hit it at that speed or not.
 

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I looked in the Holley tech site
http://www.holley.com/data/TechService/Technical/Carb Numerical Listing.pdf

4781 850DP has 80s in the front and rear 78s or 80s with a 6.5 PV front. (and a 6.5 PV rear). (most would yard the rear PV and jet up 8-10 from stock). Not sure how the BG 850 comes assembled

I did not catch your jetting the other night on FB, too late for me to think...I was mentally running your jetting vs my 800DP that has a smaller front side.

Around town you are on your idle circuit AND your primary circuit until approx 45 mph when you are on your primary side solely. You would not notice lean primaries so much at lower speed cruise. At higher cruise and non-secondary acceleration, you probably are lean...the larger squirt you put in helped cover this a bit...but once the squirt wears off, thinking lean with the other guys.
 

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Discussion Starter #6
Thanks guys, I looked up baseline for this carb on the BG site, and it's 80/85 I believe, I'll jet it up again and go for a run. I had had it jetted there before, but it had backfired out of the exhaust a couple times, so I jetted down thinking I was way rich
 

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Discussion Starter #8
Timing is set to 19* initial with a 15* degree advance for a total of 35*. All in right at 3000 rpm. Vac is advance is connected and set to 10*.
 

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Backfiring exhaust could be late timing and also a plug not firing just dumping fuel. Are all header pipes equally warm? Sure timing mark is right ?
How do the plugs look like when you pull one on the hgw ?
 

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Discussion Starter #10
The backfiring occured before I had the vacuum advance connected, and it only occurred once during a freeway on-ramp launch. I have since added vacuum advance. I also checked continuity of all the wires, checked the coil etc. I haven't checked the plugs since that cruise so I will do that. The weather here is crap, so it may be awhile until I can test drive it again.

Does anyone with a similar combo have a good place for me to start jet wise. BG's site says 80/85 (no PV in the rear) is baseline, but I have found BG tech support posts on other forums saying 80/88.
 

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What about your float levels are they in the middle filled with fuel add some more maybe??
Seems odd that it would fall flat like that under load? I'm running a BG 850 Speed Demon
and seriously jet change point down or up would not cause such a dramatic situation you are experiencing,just my thoughts!!
 

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Discussion Starter #12
I haven't run it in about 2 weeks, I'm going to jet back up to the baseline and then look everything over again...maybe I missed something rushing to head out to that bbq
 

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Discussion Starter #13
So a small update:

I jetted back up to 82/88 and so far power seems to back where it should be. I got the car up to temp and drove up some steep grades and the car didn't feel like it was falling on it's face. So thanks to everyone there.

New problem....some kind of hesitation/sputter at 4500-5000 rpm. The engine noise is too loud for me to really hear what is going on but I can feel it. Car hit's 4500 rpm and almost stops accelerating, then picks back up again. There is a weird noise maybe a sputter that I can't quite feel. I rechecked timing, which is set at:

16 Initial, 20 degree mechanical all in at 3k rpm, 15 vacuum advance. (instructions say to use ported vacuum thoughts?)

Header tubes were all hot and the plugs that I pulled (1 and 7 ) were nice and tan on the strap. I'm going to run it in a dark garage and look for any arcing as well as check the coil again.

I also went through the odd bank valves and found #7 exh rocker was looser than the others (heard a loud tapping at initial fireup but went away after a minute or so)anything else you guys can think of let me know
 

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Discussion Starter #16
Bob, the squiters are. 035 in the front and 0.37 in the rear (I upped the size in the rear to see if that would help any, it was at .035 like the fron)
 

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Discussion Starter #17
Little update on this,

So after upping the jets to 82/88 and upping the rear squirter to .037, I had a rocker tapping pretty loudly. I was hoping this would be the cause of my high rpm noise. I went through all the valves and while it's a bit quieter the 4500rpm sputter is still there. I haven't got a chance to pick up a new coil yet but hopefully I can tomorrow. I tested out again after a email I got from MSD, and my numbers are off a little on the resistance from the + to the - terminal.
 

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Discussion Starter #18
Picked up a new coil and went for a test run, and no change. Still sputters and what sounds like backfiring out of the exhaust.

started jetting down in small increments and still can't get it to stop. I ended up at 82/84 (front PV only) and it still does it, only for about a second as opposed to continually like it did before. No other changes other than jetting, the timing has remained the same.

I'm going to try and borrow a 800 from a friend and see if it happens with different carb on it.
 

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Jeff,
We're having the seasonal rains here. Mason will borrow my 800DP off my Elky once things settle down at my place (daughter and grandbabies here til FRI). I'm going to trade another guy I know my 800 baseplate without kickdown on the throttle arm for one with kickdown so the carb will come off anyway.

Mason,
What is your fuel pressure? Just had to ask. If the pressure is too high you could be blowing past one or both of your needles and seats
 
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